Present denudation rates on the island of R6union ... - Science Direct

Geochimica et Cosmochimica Acta, Vol. 61, No. 17, pp. 3645 3669, 1997 Copyright © 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97)00180-4

Present denudation rates on the island of R6union determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions PASCALE LOUVAT and CLAUDE JEAN ALLI~GRE Laboratoire de G6ochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universit6 Paris VII, URA-CNRS 1758, 75252 Paris Cedex 05, France (Received January 14, 1997; accepted in revisedform April 30, 1997) Abstract--Dissolved and suspended loads of the main streams on the island of R6union have been analysed for their major and trace element contents in order to characterise both chemical and mechanical erosion products. The chemical composition of R6union surface waters is controlled by partial dissolution of the basaltic rocks they interact with, by atmospheric input and, in some cases, by thermal spring inflow. The elemental contributions arising from these three processes have been calculated using typical concentration ratios for each endmember. The contribution from oceanic type rains is important for C1 and Na and minor for Mg, Ca, K, and Sr. Some thermal springs join their nearest rivers in Piton des Neiges calderas and influence the sulphate, Li, and B concentrations, and to a lesser extent Ca, Sr, and Rb. For all the rivers that are not affected by the thermal springs, the contribution from basalt weathering is higher than 70% of the total dissolved solids. The chemical composition of the suspended load in Rdunion rivers reflects a low weathering state for basalt in each drainage basin, implying that mechanical erosion is more important than chemical erosion. Assuming a steady-state between these two complementary erosion processes, we propose a mass budget between the dissolved and suspended erosion products and the local unweathered basalt for each of the main fiver catchments. Given the lack of reliable long-term measurements of river suspended load at R6union, this mass budget enables us to calculate the suspended load concentration that is required to counterbalance the concentration of total dissolved solids due to chemical weathering. The calculated suspended load concentration is much higher than that measured which implies that equilibrium between mechanical and chemical erosion was not attained at the time of sampling. However, we suggest that in such a tropical climate, most of the suspended load will be carried during cyclonic events, these short periods of time providing about 50% of the annual amount of river water. Thus, erosional steady-state must be achieved on the scale of the annual hydrologic cycle. From the total dissolved solid concentrations induced by basalt weathering, and given the annual discharge from R6union rivers, we infer specific chemical erosion rates of 63-170 t/km2/yr and specific atmospheric CO2 consumption rates, during basalt erosion, of 1.3-4.4 x 106 mol/kmZ/yr. Mechanical erosion rates deduced from the calculated suspended load concentrations range from 1200 to 9100 t/ kmZ/yr. Total erosion (chemical plus mechanical) give basalt denudation rates of 470-3430 mm/kyr. All these erosion rates (chemical, mechanical, total, and atmospheric CO2 consumption) are among the highest global estimates and are due to high runoff, steep slopes, active volcanics, and related active tectonics, and young basaltic lithologies, which characterise Rdunion. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

dertaken within a more global geochemical study of rivers. Previous works on the Congo and Amazon Basins (Ndgrel et al., 1993; Dupr6 et al., 1996; Gaillardet et al, 1995; All~gre et al., 1996; Gaillardet et al., 1997) highlighted the difficulty of deciphering the mixed information acquired by rivers draining multilithologic terrains. Thus, studies on smaller and monolithologic catchments have also been undertaken to better constrain the influence on erosion of factors other than lithology, such as morphology and climate. As the primary rocks of the early Earth were volcanogenic, understanding the processes that control the alteration of volcanic terrains can also potentially provide valuable information on the formation and evolution of the early continental crust. The tropical climate of R6union and its high relief offer an opportunity to study basalt erosion processes under particular conditions. In this paper we characterise the chemical weathering of R6union basaltic rocks, taking into account input

Rock weathering and subsequent transport by flowing river water to the ocean is one of the most important phenomena that controls the morphology of the Earth surface. This acts through two complementary processes: the chemical weathering partially dissolves rocks and minerals and transports erosion products in a soluble form; fracturing of rocks provides fine grained material that can be more easily transported by river waters. Chemical and mechanical erosion processes enhance each other in that crushed rock provides chemically reactive surfaces and weathered rocks are easily fractured. Global erosion will depend a priori on many factors, such as lithology, climate, relief, tectonics, and vegetative cover. As silicate rock weathering consumes atmospheric CO2, riverine erosion is likely to have an important effect on long-term climate evolution (e.g. Berner, 1992). Our study of riverine erosion processes at R6union is un3645

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P. Louvat and C. J. Allbgre

from rain and thermal springs. This enables us to calculate chemical erosion rates and atmospheric CO2 consumption rates. From the analysed suspended load chemical compositions, we re-estimate a local basalt composition for each of the studied drainage basins. Finally, we assess the steadystate between chemical and mechanical erosions, establishing a mass budget between the fiver dissolved and suspended loads and the local basalt chemical compositions, and calculate mechanical erosion rates at Rrunion. 2. GENERAL SETTINGS OF THE ISLAND OF RI~UNION The island of R6union is located in the southern Indian Ocean, 700 km east of Madagascar. It belongs to the Mascareignes archipelago (with Rodriguez and Maurice islands) and is the youngest, largest, and today the only island in this group with active volcanism (Fig. 1 ). Rrunion is formed by two distinct volcanic massifs. The oldest (2-0.35 My) is the now extinct volcano of Piton des Neiges (3069 m high) which dominates three very deep amphitheaters which result from the combined effects of tectonic processes (such as volcano summit collapse forming calderas) and long-term erosion: the Cirque de Cilaos, the Cirque de Salazie, and the

Cirque de Mafate. The still active Piton de la Fournaise volcano (2600 m high) commenced formation some 0.53 My ago (Gillot and Nativel, 1989). Eruption dynamics has formed three large calderas, but erosion has, as yet, not significantly modified the volcanic morphology. The climate of Rrunion is both tropical and oceanic and is characterised by torrential rains during the summer (January-April) which favour high erosion rates at that time. Annual precipitation can reach 7000 mrn/yr on the eastern part of the island. The prevailing winds are from east to west. Temperatures show no strong inter-seasonal variations with annual means of 20-26°C on the coast and 8-14°C at altitude. Rrunion is built of volcanic rocks of basaltic composition (intermediate between tholeiitic and alkaline magmas). The nature of the rocks in each of the volcanic massifs is similar, and they differ essentially only in age. During the two first eruptions of the Piton des Neiges volcano (3-0.43 My), the rocks are primarily of oceanite type, whereas during the latter stages (0.43-0.03 My) basaltic rocks are increasingly differentiated. However, the volume of lava from the later stages is small compared to the total volume, and these differentiated rocks have only been identified in the centers of the three cirques of Piton des Neiges. For the Piton de la

Fig. 1. Map of the island of Rrunion showing the main streams and the sampling locations.

Denudation rates at Rfiunion Fournaise volcano, the lavas are similar to the early oceanites and basalts of the Piton des Neiges volcano and give an insight into the early stages of volcano building. As a result of this age difference, there is also a difference in the alteration state for each volcanic massif, the Piton des Neiges rocks often being hydrothermally altered and more eroded. The major rivers of R6union drain its main tectonic and erosional structures. They have a general radial drainage centered about Piton des Neiges and Piton de la Fournaise, are relatively short ( 10s k m ) , and experience both torrential flows and low water levels, with water discharge ranging from 10 m3/s to 2500 m3/s in the rainy season. The hydrogeologic characteristics of the two volcanic massifs are quite different. First, the annual rainfall is about seven times higher in the southeastern massif than on the western part of the island. In addition, the younger, unaltered, and porous basaltic rocks of the Piton de la Fournaise volcano, taken with the absence of high relief on this massif, results in more rapid hydrologic circulation than on the Piton des Neiges volcano. On the Piton de la Fournaise volcano, nineteen drainage basins have been recognised (Robert, 1986) of which only three have a tectonic origin (corresponding to the external limits of the volcano collapse calderas) and perennial flow. The Rivibre des Remparts, Rivi~re Langevin, and Rivi~re de l'Est rivers are fed by springs with elevated discharge. The inner part of Piton de la Fournaise can be likened to a real water tower, the very permeable young basaltic rocks allowing a rapid infiltration of the rain waters. On the eastern-most part of the volcano, no drainage basins can be defined because of the permeability of the youngest basaltic rocks, but some coastal resurgence have important discharges (for example, the Anse des Cascades springs). The Bras de la Plaine river is situated on the boundary between the two volcanic massifs, and thus water drains from both massifs. On the Piton des Neiges volcano the three main rivers, Rivibre du M~t, Rivi~re des Galets, and Bras de Cilaos rivers, drain three wide tectonic cirques (Salazie, Mafate, and Cilaos). The Rivi6re des Marsouins also has h i g h water discharge and drains the Forat de B6bour area (the supposed fourth cirque of Piton des Neiges). T w o rivers with lower water discharge, the Rivibre des Pluies and the Rivi~re St Denis, drain the northern part of the massif. No perennial flows exist on the western part of the island because of its drier climate. Infiltration of meteoric water in highly faulted and fissured volcanic areas can induce hydrothermal circulation, and thus mineral water resurgences. Most of the recognised thermomineral springs of R6union are located in the three cirques of the Piton des Neiges Massif. The For~t de Bdbour excavation also possess a thermal spring (Sources du Bras Cabot). Unlike the older massif, the still active volcano of Piton de la Fournaise does not show any sigh of hydrothermal resurgences, probably because its structure is still too sealed (Marty et al., 1993). 3. SAMPLING AND ANALYSIS River samples were collected in August 1993, January 1994, and February 1995 (Fig. 1 ). During the first (dry season ) and second (rainy season) field trips,

3647

we sampled springs and small streams rather than the major rivers, in order to assess the seasonal climatic influence upon the chemical characteristics of Rdunion's surface waters, without the risk of anthropogenic contamination. During those two field trips, most of the waters were sampled at the same spot (see Fig. 1 and Table 1 ). The aim of the third sampling trip was focused on the study of riverine erosion. We principally sampled the major rivers of the island at their drainage basins outlets in order to study their erosion fluxes to the ocean. We thus sampled at the same time waters, suspended loads, and river bed sands. The distinction between the suspended and dissolved phases of a natural water is not clear as some species can be transported in a colloidal form. In this study, we used Sartorius 0.2 #m cellulose acetate filters to separate these two phases and what we term the dissolved phase is that finer than 0.2 #m and the particulate or suspended phase is that which is coarser. For the suspended load we also used tangential filtration (Sartorius cartridges of 0.2 #m cutoff size) to concentrate the river particles. In the field we measured pH, temperature, and HCO3 concentration (by acido-basic titration and end point determination by the Gran plot method). Waters were filtered and filtrate aliquots were acidified at pH 2 with distilled 16 N nitric acid for storage and further laboratory analysis. We used unacidified samples for analysis of anions(C1 , S O l , N O 3 , F , B r ). For the August 1993 and January 1994 samples, analysis was undertaken at the Laboratoire de G6ochimie et M6tallog~nie of University Pierre et Marie Curie (Paris). CI , SO] , and NO3 were measured by high pressure liquid chromatography (HPLC Waters Millipore 510), and F - by specific electrode, with precision better than 5%. Calcium, sodium, magnesium, and potassium concentrations were determined by atomic absorption spectrometry, with a precision of 1-3%. Trace elements (Li, B, Rb, St, Ba) were measured by ICP-MS (VG PlasmaQuad II+), with precision of 5%. For the February 1995 water samples, we measured anions (C1 , SO]-, NO3, F , Br ) and major cations (Ca, Na, Mg, K) by HPLC Dionex 300, with precision better than 5%. Trace element concentrations (Li, B, Rb, Sr, Ba, and U, Th, REEs, transition metals for the suspended loads) were determined by ICP-MS (VG PlasmaQuad II+ ). Finely crushed sand and river particle samples were dissolved by acid attacks (HF, HNO3, HC104). Indium internal standard solution was added in each sample before the ICP-MS analysis. For water samples, the accuracy of the analysis was assessed by running the SLRS-3 riverine standard after every ten samples ( Table 1 ). For the suspended load, we ran the BR and BE-N rock standards (Table 2 ), dissolved under the same conditions as the suspended load samples. The strontium isotopic ratios of dissolved, suspended, and sand loads were measured by thermal ionisation mass spectrometry (TIMS) after specific separation on Sr-Spec resin (Birck, 1986). For fifteen measurements of the NBS 987 standard, we obtained a mean value of 0.710243 _+ 0.000025 ( 2or ). Strontium concentrations of the dissolved loads were also determined by ID-MS (Isotopic Dilution Mass Spectrometry), with a precision of 2%. 4. RESULTS AND COMMENTS 4.1. C h e m i c a l C h a r a c t e r i s t i c s o f W a t e r S a m p l e s Concentrations of the different major and trace chemical species in surface waters, rains, and thermal springs of R6union are given in Table 1.

4.1.1. Surface waters Rivers of R6union are characterised by relatively high pH ( f r o m 6.5 to 8.9), the Piton de la Fournaise rivers being more acidic ( p H < 8.0) than the Piton des Neiges rivers ( p H > 8.0). Overall values for total dissolved solids ( T D S ) are very high ( f r o m 20 to 580 m g / L with a m e a n value of 150 m g / L ) c o m p a r e d to m a j o r global rivers, for example TDS is only 2 8 - 4 9 m g / L for the C o n g o basin rivers (Dupr6 et al., 1996) and 6 - 5 5 m g / L for the A m a z o n basin rivers

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(Gaillardet et al., 1997). Rivers of Piton de la Fournaise volcano are less mineralised (all solutes concentrations are lower) than the rivers draining Piton des Neiges. The concentrations of dissolved silica are also high in the waters of REunion (200-800 #mol/L with a mean of 440 #mol/L), relatively homogeneous on the whole island and close to saturation (100 #mol/L for quartz and 2000 #mol/L for amorphous silica at 25°C, Michard, 1989). By comparison, concentrations of dissolved silica in the Congo basin are between 140 and 210 #mol/L (DuprE et al., 1996), and 6 0 160 #mol/L in the Amazon basin (Gaillardet et al., 1997). Silica represents about 30% of the total dissolved solids (TDS) for the rivers of Piton de la Fournaise volcano and 15% of the TDS for the rivers of Piton des Neiges. Bicarbonates are the dominant anions for all the waters (250-3560 #mol/L). Chloride contents rarely attain 200 #mol/L, and the highest concentrations correspond to samples located close to the sea (Anse des Cascades) and to two samples where anthropogenic pollution is suspected (Les 3 Sources and Source Canal, in the Rivi~re des Remparts basin). These two contaminated samples also have high concentrations of nitrates (25-105 #mol/L), whereas nitrate concentrations are lower than 15 #mol/L in all other samples, consistent with anthropogenic pollution. Excluding these two samples we obtain a mean chloride concentration of 57 #mol/L which is higher than the highest concentration of rain samples (44 #mol/L). Given the low chloride contents of the basaltic rocks (for example lower than 200 mg/ kg in Icelandic basalts, in Gislason et al., 1996), we assume that the chloride content of the river waters is wholly derived from atmospheric inputs, their over-concentration compared to rains being due to the partial re-evaporation of the rains under tropical climate conditions (mean annual evapotranspiration at REunion is close to 1200 mm/y, representing about 25% of the precipitations in the eastern part of the island and up to 80% in the western part). For most of the rivers, sulphate concentrations are lower than 60 #mol/L but for those draining the cirques of Piton des Neiges they can be as high as 2 mmol/L, and this reflects a contribution of some thermal springs in these cirques, as will be discussed later. In these rivers higher concentrations are also found for most of the cations. If we exclude these particular thermal spring admixed river waters, the cations concentrations show the following characteristics. Cations always show the same relative distribution with the following decreasing order: Na > Ca ~ Mg > K ~> Sr > Rb ~ Li > Ba. Sodium concentrations range between 90 and 1940 #mol/L with a mean of 505 #mol/L. Calcium and magnesium are present in similar proportions (Ca/Mg from 1 to 2) with concentrations of 5 0 700 #mol/L and 35-460 #tool/L, respectively. Potassium concentrations range between 10 and 100 /.tmol/L. Strontium, rubidium, lithium, and barium concentrations are of 26-500 nmol/L, 5 - 2 4 0 nmol/L, 1-110 nmol/L, and 0.38 nmol/L, respectively. The cation concentrations are thus highly variable and also differ between each volcanic massif. For all the samples, the inorganic charge balance between the major anion, and cation species is checked within a range of less than 10% (Table 1). Bicarbonates always represent more than 40% of that charge balance in samples which are not affected by thermal springs contributions, while for the

admixed samples, sulphates represent 5-15% of the charge balance. Although differences exist among the chemical compositions of river waters sampled during the dry vs. humid seasons, no systematic trend is observed, and in particular the expected dilution effect during the rainy season is not perceptible (probably because it did not rain very much during this sampling period of the rainy season). In the same way, the annual hydrographs of the principal rivers (Observatoire REunionnais de l'Eau, 1994) do not show large differences in the river discharge between the two seasons, except the enormous values observed during torrential rains and cyclones. Daily measurements on small rivers (Observatoire REunionnais de l'Eau, 1994) confirm that these exceptional rain events do not significantly modify the annual low-water level of the spring-fed rivers. In the following sections we will focus our discussion on concentration ratios, which are not modified from one season to the other, rather than on absolute concentrations. The 878r/86Sr isotopic ratios of the main rivers of REunion vary between 0.70418 and 0.70464 with a mean value of 0.70432. All these values are close to the REunion basalt 87Sr/86Sr isotopic ratio around 0.70420 (Albarrde and Tamagnan, 1988) but slightly higher, reflecting the fact that Sr arises principally from the basalt-water interaction but also from ocean derived rains (the oceanic 87Sr/86Sr isotopic ratio being 0.70907, Burke et al., 1982). The highest value is found for the Anse des Cascades sample where the marine influence is the greatest. 4.1.2. Rains

Understanding the chemical composition of rain and its variations is beyond the scope of this article. The key focus here was to obtain an estimate of the initial atmospheric water input into the island's hydrologic systems, in order to correct the chemical compositions of surface waters and thus quantify that which arises from the basalt chemical weathering only. Our rain sampling is very limited and unlikely to be representative. Nevertheless, these samples can be considered in the context of the wider study of Grunberger (1989), who has shown that the spatial distribution of chloride, Na, Ca, and Mg concentrations in rain on REunion were principally controlled by marine aerosols and by the main Westerly direction of the wind. The concentration of these elements decreases both with altitude and distance to the sea. Grunberger (1989) also suggested that solid and gaseous particles from the active volcano of Piton de la Fournaise and atmospheric erosion suspensions could be washed out and contribute to the rain compositions for elements as K, Ca, and Mg. Excluding the sampling stations closest to the sea, Grunberger (1988) gives the following concentrations for the rains on REunion: C1 = 61 _ 25 #mol/L, Na = 20 _+ 13 #mol/L, K = 5.6 _+ 5.1 /a.mol/L, Ca = 16.5 _+ 15.0 #mol/ L, and Mg = 5.3 _+ 4.1 #mol/L (these values are the means and standard deviations of twenty samples taken between 1985 and 1987 at different locations). Our rain samples were all taken at the same locality La Plaine des Cafres, at the centre of the island (Fig. 1 ), and at a relatively high altitude ( 1550 m). Our chloride concert-

Denudation rates at R6union trations are lower than those of Grunberger (1989), they range from 1 to 44 #mol/L. Calcium and magnesium concentrations are also lower (respectively, 0.1-2.5 #mol/L and 0.1-3.4 #tool/L). The Na ( 1 0 - 3 8 #tool/L) and K (0.5-7 #mol/L) concentrations are in agreement with these of Grunberger (1989) for rains of this altitude. These discrepancies in the two sets of concentrations could be explained by the different sampling methods used. Grunberger (1989) used cumulative rain collectors (collection lasted 4 6 months) and metallic containers whereas we sampled rains on a event basis and in polypropylene bottles. Thus, the rains sampled by Grunberger (1989) integrate a large variety of rain events and are more likely to be affected by atmospheric dry solid deposits. We also analysed sulphate (0.9-13.5 #mol/L), Sr (2.07.3 nmol/L), Rb (0.5-3.5 nmol/L), Ba ( 1.2-5.8 nmol/L), and Li (0.1-14.9 nmol/L). Barium concentrations are of the same order of magnitude than these of the island surface waters. With regard to the X/C1 molar concentrations ratios of the different species, we see that, for most of the rain samples, SO4/C1, Na/C1, Ca/C1, Mg/C1, and Sr/C1 ratios are close to the oceanic ratios (0.052, 0.857, 0.019, 0.097, and 0.00023, respectively) and confirm an oceanic composition for the rains of R6union. But K/CI, Rb/C1, and Ba/C1 ratios of these oceanic type rains are higher than the oceanic ratios, and another input must be involved. Such enrichments of K, Rb, and Ba (by extension) have been noticed by Crozat (1979), Artaxo and Maenaut (1988), and explained as due to biogenic aerosols emitted by vegetation in the tropical forests. Although R6union does not possess a large tropical forest comparable to the Amazonian rainforest, the high density of vegetation under this type of tropical humid climate could be a source of such biogenic aerosols and may explain the high concentrations observed. Leaching of atmospheric particles could also contribute to the chemical compositions of the rains, however, given the long distances particles can travel during atmospheric circulation, it is quite impossible to determine a priori the origin of these particles. Thus, we can suspect particles to be derived both from local erosion as well as those of distant origin (which could, for example, come from Australia given the global atmospheric circulation in the Southern hemisphere).

4.1.3. Thermal springs The thermal springs sampled in the Cirque de Cilaos and the Cirque de Salazie are all bicarbonated types and show calcite and ferrous oxide deposits at their resurgences. Their temperatures are not very high (20-50°C), the hottest spring being the Source Jacqueline located near to the Bras Rouge river in the Cirque de Cilaos. Their pH values range between 6.2 and 6.7 and their bicarbonate contents between 15 and 32 mmol/L. Given the equilibrium constants of the carbonate species (Michard, 1989) and knowing the bicarbonate content and the pH, we can calculate the concentrations of carbonate and carbonic acid and then that of total dissolved CO2. We obtain ZCO2 of 2 2 - 6 8 mmol/L in good agreement with the gas extraction measurements of Marty et al. (1993). The thermal springs also have very high concentrations

3653

of sulphate, alkali, and alkaline-earths with respect to surface waters. The enrichment factors relative to the mean concentrations of the surface waters can be as high as 900 for Li (950-26400 #mol/L), 90 for Ba ( 3 - 2 2 0 nmol/L), 50 for sulphate (40-2140 #mol/L). 45 for Na ( 4 - 1 7 mmol/L), 30 for Mg (1.7-4.2 mmol/L) and bicarbonates, 20 for Ca (1.5-4.2 mmol/L), 15 for Rb (235-1120 nmol/L), and 5 for K ( 140-300 #mol/L). Chloride contents ( 7 8 - 9 7 mmol/ L) are not significantly higher than those of the rivers. Borate concentrations are very high ( 8 - 1 8 0 #mol/L) and give an enrichment factor compared to mean surface waters of 115. Although their discharges are 100 to 1000 times lower than those of the rivers draining the three cirques of Piton des Neiges, the thermal spring contribution to the rivers is likely to be significant for the most enriched elements. These contributions can be identified using elements such as sulphate, Li, or borate which are distinctive for the thermal springs. For example, the C1/B ratio, as utilised by Arnorsson and Andresdottir ( 1995 ) in Icelandic geothermal waters, is a useful criterion. The C1/B ratio is about 1330 in the ocean, but it can be as low as 1 in Icelandic thermal springs and boron concentrations can attain 1 mmol/L. For our R6union thermal springs we measure C1/B ratios of 16-0.5. Lithium is also a good tracer for hydrothermal activity as it is released in solution during high temperature water-rock interaction, like most alkali elements (Thompson, 1983). Sulphur species should result from alteration of sulphide mineralisations present in the basalts, but sulphate is probably not the dominant species, as high oxidation conditions are not attained. Very high concentrations of sulphur species (both $042 and Y~HzS)have also been measured in Icelandic geothermal springs (Arnorsson et al., 1983). We made approximate calculations of mineral saturation indices for these thermal waters, in order to have a qualitative view of the secondary minerals which may deposit. All the thermal springs we have analysed are over-saturated with calcite. Most of them are also over-saturated with strontionite (SrCO3) and, to a lesser extent, with witherite (BaCO3). They are all close to saturation with magnesite (MgCO3). The Source Jacqueline is the only spring close to saturation with barytine (BaSO4). All these secondary mineralisations, if deposited (and there is field evidences for that), are likely to be redissolved in contact with less mineralised surface waters and to contribute more or less directly to the thermal spring admixed waters of some of the Piton des Neiges rivers.

4.2. Chemical Characteristics of Suspended Particles and Sands Concentrations of major and trace elements of suspended load and river bed sand samples are given in Table 2. Mean concentrations of R6union basaltic rocks (Albar~de and Tamagnan, 1988; Nativel et al., 1978) are also quoted for direct comparison. For most elements the concentrations in river suspended loads and sands are in the range of those of the basalts. In order to more easily compare the particle and sand chemical

3654

P. Louvat and C. J. All~gre

compositions with those of REunion basalts, we represent their primitive mantle normalised patterns (Fig. 2). The two basalt patterns for Piton de la Fournaise (from Albarbde and Tamagnan, 1988; Nativel et al., 1979) and Piton des Neiges (from Nativel et al., 1979) are very similar. The Piton des Neiges pattern is a little wider than that of the Piton de la Fournaise, reflecting a larger variability probably due to the longer temporal evolution of this volcano. The river suspended load and sand patterns show the same general trends as the basalt patterns (Fig. 2). The absolute trace element concentrations of our samples also show that suspended loads and sands are chemically very similar to the basaltic rocks they drain. This implies that weathering of the rocks on REunion does not significantly fractionate elements in the time between the formation of a regosol and its physical removal to the ocean as suspended river particulate material. Otherwise we would expect to see a depletion in the most soluble elements (Na, Ca, Mg, Sr, Rb, Ba, and, to a lesser

extent, U ) during chemical weathering, as is seen for the Congo and Amazon basin rivers (Dupr6 et al., 1996, Gaillardet et al., 1997). Given the variability of the chemical compositions of Rdunion basaltic rocks, compared to the chemical compositions of our sand and suspended load samples and the low weathering state of these samples, it is impossible to distinguish depletions in the soluble elements concentrations that are characteristic of rock partial dissolution. Thus, it is necessary to determine precisely the local basalt chemical composition for each drainage basin in order to quantify the weathering state of the river suspended loads and then to compare them with the dissolved load concentrations of the rivers. Patterns for Bras de Cilaos (and Bras Rouge for the sandy sample), and, to a lesser extent, for the Rivi~re du Mat and Rivi6re St l~tienne rivers, are always different from the other samples. They are slightly enriched in elements from Rb to Er (in the Fig. 2 element order), and depleted in Sc, Co, Mg,

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°'°J,

Ni

_o .0.,

100

.__

E

lo

0

c M

E

0.1

._> , ~

E o.ol. 0.001 ' "~Rb

S-Est - - - - S-Mat ........ S-Rouge .... S-Galets/L

basalts @Nelges(2) ~ bMmlts @Fournalse (1) B B Trachytes (2)

t,Ii

Fig. 2. Extended patterns of trace and major elements for the suspended loads and sands of four major rivers of REunion. The patterns are normalised to the primitive mantle (Hofmann, 1988). The shaded areas represent the ranges of the patterns for the basaltic and trachytic rocks of REunion, with distinction of the basalts from Piton des Neiges and Piton de la Fournaise massifs (analyses from: (1) Albar~de and Tamagnan, 1988, and (2) Nativel et al., 1979). The sequence of the elements has been chosen in order to have a monotone decreasing pattern for the mean basalt compositions.

Denudation rates at R~union Cr, and Ni. This probably reflects the participation of more differentiated basaltic rocks (an extreme type being trachytes) that have been observed in both the Cirque de Cilaos and the Cirque de Salazie. These trachytic rocks (analysed by Nativel et al., 1979) have distinct patterns compared to basalts (Fig. 2), with elevated concentrations of alkali elements (Na, K, and Rb) and of Ba, U, and Th. They also have very low concentrations of Ni, Co, Sc, and Cr and lower concentrations of Ca and Mg. The rare earth element (REE) patterns for trachytes show an enrichment in light REEs compared to the heavy ones. The samples from the Cirque de Cilaos (P-Cilaos and S-Rouge) are obviously intermediate between these two types of trachytic and basaltic patterns. The 87Sr/86Sr isotopic ratios of the suspended particles are very homogenous, ranging from 0.70423 to 0.70432. The sand 87Sr/86Sr isotopic ratios are also very constant, from 0.70410 to 0.70429. For a given river, the 878r/86Sr isotopic ratio of the suspended load is always higher than the sand ratio. Such differences could be explained by the differential weathering of minerals in the sediment sources (e.g., Douglas et al., 1995; Blum et al., 1994). 5. CONTRIBUTION OF RAIN, BASALT WEATHERING, AND THERMAL SPRINGS TO THE RIVER WATER CHEMICAL COMPOSITIONS 5.1. River W a t e r C o m p o s i t i o n s as a M i x i n g B e t w e e n Three Reservoirs

Following NEgrel et al. (1993) and Gaillardet et al. ( 1997 ), we will focus the discussion on concentration ratios rather than absolute concentrations in order to circumvent variations caused by dilution (during rainy periods) or concentration (through evaporation) on the chemical characteristics of the rivers. Furthermore, we make use of the X/Na ratios (X being a dissolved species) because Na has been measured for all the samples and is the dominant cation for most and particularly for the oceanic rains. In Fig. 3, we represent some of the river water characteristics in X/Na vs. Y/Na diagrams. In such diagrams, a linear trend can be interpreted as a mixing between two endmembers of characteristic X/Na and Y/Na ratios. We observe linear correlations (Fig. 3a and 3b) between the alkaline earths Ca/Na, Mg/Na, and Sr/Na ratios except for a few rivers (these draining the cirques of Cilaos and Salazie and situated close to thermal springs) which show systematically higher Ca/Na and Sr/Na ratios and lower Mg/Na ratios. We also observe linear correlations between the K/Na, Rb/ Na, and Ba/Na ratios (Fig. 3c and 3d). In Fig. 3e and 3t", we observe that all the samples have very low and constant Li/Na and S O J N a ratios, except for some samples draining the Cirque de Cilaos and Cirque de Salazie and where thermal springs inflow. For all the elements, the X/Na vs. Y/ Na diagrams show the same trends with these particular thermal spring contaminated rivers being well distinguished, in particular with elevated Li/Na, B/Na, and SO4/Na ratios. Thus, we explain the chemical characteristics of the surface waters of REunion as a mixing between three reservoirs. The first is the rains, characterised by low X/Na ratios close to the oceanic values (Table 3). The second is a basalt weathering endmember with relatively high X/Na ratios

3655

(which are not the ratios of the basaltic rocks but those of the waters having interacted with the basaltic rock). The third endmember corresponds to input by thermal springs with high CafNa, Sr/Na, and Ba/Na ratios. This third endmember is quite difficult to define because its X/Na ratios do not always correspond to the ratios measured in the filtered thermal spring samples. In particular, the Ca/Na and Sr/Na ratios are higher than those of the measured thermal springs. The thermal springs show a large heterogeneity of chemical compositions which may be due to the fact that the waters did not attain an equilibrium with the local rocks and minerals they interact with (high Mg concentrations support this hypothesis). Thus, it is not possible to estimate the endmember of the high temperature waterbasalt interaction for these springs. Furthermore, as geothermal water changes its characteristics between resurgence and arrival in the river (by cooling and reequilibration with atmosphere, leading to deposition of secondary minerals), we believe that at least part of the additional Ca and Sr concentrations, observed for the thermal spring endmember, arise from the redissolution of the deposited secondary minerals. The thermal springs, from which secondary minerals have already been deposited before the resurgence, show Ca/Na, Mg/Na, and Sr/Na ratios that are in the range of those of the rivers. This interpretation is supported by the fact that the alkali/Na ratios of the geothermal endmember are similar to the measured alkali/Na ratios of the most evolved thermal springs (the alkali elements are among the most mobile in hydrothermal systems and, but for a few, are not involved in secondary mineralisations). For the alkali/Na ratios vs. the Ca/Na, Mg/Na, and Sr/ Na ratios, the relationship is more complex, and we have to distinguish two sets of rivers that drain slightly different basaltic rock types. The rivers draining the slopes of Piton de la Fournaise massif show a different trend, compared to Piton des Neiges rivers, with higher K/Na, Rb/Na, and Ba/ Na ratios and slightly lower Ca/Na, Mg/Na, and Sr/Na ratios. This difference of the river characteristics for the two massifs is likely to be related to the most rapid circulation of waters in young, little altered, and very permeable rocks of Piton de la Fournaise, and to the absence of well defined drainage basins on this massif (Coudray et al., 1990). Despite their high discharges, the rivers of Piton de la Fournaise have a short residence time (a few days to a few months, Coudray et al., 1990) which explains the different weathering kinetics and thus different weathering characteristics. The fact that basaltic rocks in the Piton de la Fournaise's massif are younger and thus less weathered than these of the older volcano, and consequently not hydrothermally altered, should also influence the X/Na ratios of the waters, although the chemical compositions of the undifferentiated basaltic rocks are not significantly different from one massif to the other (Nativel, 1978; Nativel et al., 1979; Albar~de and Tamagnan, 1988). The kinetics of dissolution of a fresh basalt compared to an altered basalt are likely to be different. Laboratory basalt dissolution by Gislason and Eugster (1987a) showed that basaltic glass dissolved about ten times faster than crystalline basalt and that dissolution of basaltic glass was quite stoichiometric, the reactions on the surfaces of the solids being rate-determining. Comparing these experimental

3656

P. Louvat and C. J. All~gre

5 •

~

v/

~

Oe~

Ca/Na

CaY@a~

~d

'~

t

o

0~

~3

~ a

IJ

*°I m

m

el21

m e e

g

0,1

II

) w Oc~e~

ee - ~

a6

l) •

~J

Ca/N8

Io

~,~

~4

Main streams (Febr. 95) Piton de Is FoUm~l|se alibi)lima (Jan. 94 and Aug. 93) Piton des Nelges streams (Jan. 94 and Aug. 93)

1

=

~

10e3 Sr/Na

J

/ ]

~:~ Rains ~ "rr,ermal springs

Fig. 3. Correlations between different characteristic molar concentration ratios ( X / N a ) of the water samples. Rains, river waters, and thermal springs are represented with different symbols. We also distinguish the main streams from the samples of the 93 and 94 field trips around Piton des Neiges and Piton de la Fournaise. The shaded areas represent the endmembers chosen for the mixing model (see Table 3 ), oceanic ratios (from Michard, 1989) are shown as a star. We utilise two different endmembers for basalt weathering, one for the rivers massif of draining the Piton des Neiges, and one for these draining the massif of Piton de la Fournaise.

i

Denudation rates at Rrunion

3657

Table 3. A priori values and errors (see text and Appendix l) for the characteristic X/Na ratios of the three endmembers, chosen for the resolution of the mixing model by an inverse method, and mean X/Na ratios of Piton de la Fournaise basalts (from Albar~de and Tamagnan, 1988). Oceanic ratios are from Michard (1989). Oceanic ratios C1/Na Ca/Na Mg/Na K/Na Sr/Na

878r/a6Sr Ba/Na Rb/Na HCO3/Na SiOjNa SO4/Na Li/Na

1.17 0.022 0.1l 0.022 2.0E-4 0.70907 2.0E-7 3.0E-6 0.10 2.0E-4 0.06 8.0E-8

_+ 0.20 _+ 0.005 + 0.02 _+ 0.005 + 0.5E-4 + 0.00004 ± 1.0E-7 _+ 1.0E-6 _+ 0.02 ± 0.5E-4 _+ 0.02 +_ 2.0E-8

Basalt weathering @ Fournaise 0.4 0.4 0.3 0.5E-3 0.70420 2.0E-5 6.0E-4 3.0 2.0 0.10 1.0E-4

_+ 0.2 _+ 0.2 ± 0.1 _+ 0.2E-3 _+ 0.00010 _+ 1.0E-5 _+ 2.0E-4 + 1.0 _+ 1.0 -+ 0.05 + 0.5E-4

observations with the chemical characteristics of the rivers of the m a s s i f of Piton de la Fournaise, we observe that the stoichiometric dissolution of basaltic rocks is not attained, the X / N a ratios of the rivers being different from those of the rocks (as analysed by Albarbde and T a m a g n a n , 1988; Table 3 ). A l t h o u g h dissolution rates are higher for fresh glassy basaltic rocks, the short residence time of the waters in contact with the rocks inhibits water-rock equilibration. Given the different characteristics of basalt weathering in the two volcanic massifs, we must use two basalt weathering e n d m e m b e r s in our mixing model (Table 3).

5.2. Proportions of Chemical Species Arising from Rains, Basalt Weathering, and Thermal Springs With the three previously defined e n d m e m b e r s (Table 3 ), we now quantify the contribution of each of them. In this way we can estimate the proportion arising from basalt weathering only and then calculate the chemical erosion rates for the m a i n rivers of Rrunion. The mixing model we present here is similar to that of Ndgrel et al. ( 1 9 9 3 ) and Gaillardet et al. ( 1 9 9 7 ) for the Congo and the A m a z o n drainage basins. In these studies, the aim was to quantitatively determine the weathering of the silicate continental crust in the two largest drainage basins of the world. G i v e n the large areas they drain, these rivers mix the signatures of all the different lithologies they encounter, and the major difficulty was the deconvolution of this composite signal in order to quantify the extent of weathering of different rock types. However, for Rdunion the problem is simplified as all rocks are of basaltic type. ( T h e mixing model equations, inverse method of resolution, and choice of the e n d m e m b e r ratios are detailed in A p p e n d i x A. Only results are discussed here.) The contribution of each e n d m e m b e r for each of the dissolved species to the rivers are listed in Table 4, and shown in the histograms of the Fig. 4. The rain inputs are significant for Na and SO4 in all samples, and the most influenced is the Anse des Cascades sample, located close to the sea. The oceanic influence ( b y rains) decreases with the distance of the sample site from the sea, as do the chloride concentrations. For Ca, Mg, K, and St, the rain contributions are small

Basalt weathering @ Neiges 0.8 0.8 0.15 1.5E-3 0.70420 0.5E-5 1.5E-4 4.0 1.5 0.02 0.2E-4

± 0.3 _+ 0.3 _+ 0.10 -+ 1.0E-5 _+ 0.00010 _+ 0.2E-5 _+ 1.0E-4 _+ 1.5 + 1.0 +_ 0.01 _+ 0.1E-4

Thermal spnngs 1.5 0.4 0.03 60E-4 0.70420 0.10E-5 0.5E-5 2.0 0.05 1.5 20E-4

± 0.5 +_ 0.1 _+ 0.02 +_ 20E-4 _+ 0.00010 _+ 0.05E-5 ± 0.2E-5 _+ 0.5 ± 0.02 _+ 1.0 _+ 10E-4

Fournaise basalts (_+std deviat°) 2.4 2.1 0.20 50E-4 0.70420 30E-5 14E-4

_+ 0.1 ± 0.7 _+ 0.01 +_ 2E-4 ± 0.00005 _+ 2E-5 ± 1E-4

9.7 _+ 0.5

and of similar proportions for each element. Rain Contributions are negligible for all other elements. The thermal spring contribution is d o m i n a n t for the Bras Rouge sample, the Source Jacqueline spring being in the river bed. Marty et al. ( 1993 ), on the basis of a helium isotopic study of R r u n i o n ' s thermal springs, suggested that the Bras R o u g e area was the most active in the supposedly extinct Piton des Neiges massif. The thermal spring supply is also important for the Bras de Cilaos and Rivi~re des Fleurs Jaunes but is negligible (in terms of percentage of the total dissolved solids) at the outlets of the Piton des Neiges cirques. The thermal spring influence on dissolved species is in decreasing order: SO4, Li, Sr, Ca, Rb, H C O 3 , Mg, Na, Ba, and K. The rivers of the Cirque de Cilaos are the most admixed samples; this is consistent with the fact that most of the thermal springs are located in that cirque. For all the samples, dissolved silica is derived only from basalt weathering. Whatever the contributions of rains and thermal springs, there still exists a wide variability among the basalt weathering characteristics of the river waters even at the scale of a small volcanic island, thought to be lithologically homogeneous (Fig. 5). Thus, the chemical weathering characteristics must be very dependent on both the hydrologic flow type of the rivers and the nature of the basaltic rocks, in terms of its mineralisation, alteration, and, therefore, age. This variability also exists for the Massif Central rivers in France (Meybeck, 1986), for the Icelandic rivers (Gislason and Eugster, 1987b, and Gislason and Arnorsson, 1993) and for the Parana Basin rivers (Benedetti et al., 1994; Fig. 5). W e can nevertheless recognise a general trend for these basaltic rivers, some ratios (such as Ca/Mg, Ca/HCO3, M g / H C O 3 , SiO2/K, S i O J HCO3) being quite constant for all rivers.

6. RELATIONS BETWEEN THE SUSPENDED PARTICLES, BASALTIC ROCK, AND DISSOLVED LOAD CHEMICAL COMPOSITIONS 6.1. Definition of a Local Basalt for Each Drainage Basin In order to quantitatively define the weathering state of the river suspended loads, we have to compare the concentra-

3658

P. Louvat and C. J. All6gre

Table 4. Proportion (in %) of each endmember in the chemical compositions of R6union main streams, deduced from the resolution of the mixing model by an inverse method. (r = rain; bw = basalt weathering; th.s. = thermal spring Na

Rivi6re Langevin Anse des Cascades Rivi~re de I'Est Rivibre des Marsouins Rivibre des Fleurs Jaunes Rivi~re du Mgt Bras de Cilaos Bras Rouge Bras Benjouin Rivi~re St Etienne Rivibre des Galets/B Rivibre Ste Suzanne Rivi~re des Galets/L

Ca

K

Sr

Ba

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

8 47 27 16 3 10 4 2 4 18 4 13 6

92 53 73 74 76 77 84 63 7 8 1 1 1

0 0 0 11 21 13 12 35 89 74 95 86 94

1 2 1 1 0 0 0 0 0 1 0 0 0

99 98 99 75 62 71 60 23 72 83 96 97 97

0 0 0 25 38 28 40 77 27 17 4 3 2

3 10 5 3 1 2 2 0 1 4 3 2 3

97 90 95 91 83 88 79 53 91 91 95 97 96

0 0 0 7 17 10 19 46 8 6 2 1 1

1 16 4 3 1 3 2 1 1 4 2 4 3

99 84 96 95 90 92 89 74 96 94 97 96 97

0 0 0 3 9 5 9 25 3 2 1 1 0

4 9 6 3 0 2 1 0 2 4 2 4 4

96 91 94 35 23 32 38 4 38 40 86 76 85

0 0 0 62 77 66 61 96 60 56 12 20 11

0 2 1 0 0 0 1 0 0 1 1 2 I

100 98 99 98 94 96 83 70 97 98 99 97 99

0 0 0 1 6 3 15 30 3 2 1 1 0

Rb

Rivi~re Langevin Anse des Cascades Rivibre de I'Est Rivi~re des Marsouins Rivi~re des Fleurs Jaunes Rivi~re du M~t Bras de Cilaos Bras Rouge Bras Benjouin Rivi~re St Etienne Rivibre des Galets/B Rivi~re Ste Suzanne Rivi~re des Galets/L

Mg

HCO3

SiO2

SiO]

Li

TDS

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

r.

b.w.

th.s.

0 1 0 0 0 0 0 0 0 0 0 1 1

100 99 100 98 90 92 81 47 97 96 98 97 98

0 0 0 2 10 7 18 53 3 3 1 2 1

0 2 1 0 0 0 0 0 0 1 0 0 0

100 98 99 93 86 91 84 49 92 94 99 99 99

0 0 0 7 14 9 15 51 8 5 1 1 1

0 0 0 0 0 0 0 0 0 0 0 0 0

100 100 100 100 98 99 98 93 100 99 100 100 100

0 0 0 0 2 1 2 7 0 1 0 0 0

7 72 47 36 0 4 1 0 1 7 7 28 9

93 28 53 53 4 11 5 1 9 10 58 46 64

0 0 0 11 95 84 94 99 90 84 35 27 27

0 1 0 0 0 0 0 0 0 0 0 0 0

100 99 100 10 10 0 14 3 45 48 69 57 74

0 0 0 90 90 91 86 97 55 52 31 42 26

1 7 4 2 1 1 1 0 1 2 1 1 1

99 93 96 91 74 86 69 32 74 85 85 93 87

0 0 0 7 25 13 30 68 25 13 14 6 12

tions o f the e l e m e n t s that are the m o s t likely to have been d i s s o l v e d during basalt w e a t h e r i n g b e t w e e n the s u s p e n d e d loads and the u n w e a t h e r e d basaltic rock. T h e s e e l e m e n t s are, o f course, those w e m e a s u r e in the d i s s o l v e d loads: principally silica, alkalis (Na, K, R b ) , and alkaline earths (Ca, M g , Sr, and B a ) . E v e n if the c h e m i c a l c o m p o s i t i o n s o f the basaltic rocks are relatively h o m o g e n o u s , the c o n c e n trations w e m e a s u r e d in the s u s p e n d e d load s a m p l e s are included in the variabilities o f the basaltic rock c o n c e n t r a tions ( m e a s u r e d by A l b a r r d e and T a m a g n a n , 1988; Nativel et al., 1979) for all the e l e m e n t s , and also for the e l e m e n t s previously d e f i n e d as soluble. W e thus have to redefine, for each o f the drainage basins, a m e a n c h e m i c a l c o m p o s i t i o n o f the basaltic rock before c h e m i c a l w e a t h e r i n g . This is ass u m i n g that the m o s t insoluble e l e m e n t s ( f o r e x a m p l e the R E E and T h ) stay in the s u s p e n d e d load during the weathering, and then have the s a m e relative c o n c e n t r a t i o n s in the s u s p e n d e d load and in the u n w e a t h e r e d basaltic rock. F r o m the o b s e r v a t i o n that the s u s p e n d e d load and sand s a m p l e s s h o w primitive m a n t l e n o r m a l i s e d patterns very similar to the basaltic rock patterns (Fig. 2 ) , w e can c o n s i d e r that the c o n c e n t r a t i o n ratios for these m o s t insoluble elem e n t s are the s a m e in the s u s p e n d e d load s a m p l e s as for the basaltic rocks. O n c e again w e use c o n c e n t r a t i o n ratios rather than absolute c o n c e n t r a t i o n s , the local basalt c o n c e n t r a t i o n s b e i n g likely to b e a little h i g h e r than the particles c o n c e n t r a -

tions b e c a u s e o f a dilution effect on the s u s p e n d e d load due to the contribution o f organic matter or o f m o r e abundant silica. A s m o s t o f the soluble e l e m e n t s have been partitioned during the basalt w e a t h e r i n g , their c o n c e n t r a t i o n s in the particle s a m p l e s are l o w e r than e x p e c t e d from just a dilution effect. W e thus also calculate the c o n c e n t r a t i o n s that soluble e l e m e n t s w o u l d h a v e in the s u s p e n d e d load if there had not been a partial dissolution o f the basalt during weathering. ( T h e m o d e l e q u a t i o n s for the d e t e r m i n a t i o n o f the local basalt c h e m i c a l c o m p o s i t i o n s and the details o f the c o m p u t a tion are given in A p p e n d i x B.) With these local basalt c o m p o s i t i o n s (Table 5 ) , the soluble e l e m e n t s d e p l e t i o n s in the s u s p e n d e d load are m o r e clearly seen, as in the Fig. 7. It is also o b v i o u s from this figure that, for s o m e o f the rivers ( R i v i r r e des Galets and R i v i r r e du Mfit), the s u s p e n d e d load is diluted c o m p a r e d to the local basalt c o m p o s i t i o n ( w i t h particle to local basalt c o n c e n t r a t i o n ratios m u c h l o w e r than 1 ). The origin o f this dilution can not be constrained, as w e did not m e a s u r e SiO~ or organic m a t t e r c o n c e n t r a t i o n s in the s u s p e n d e d loads.

6.2. The Steady-State Model of Erosion It is r e a s o n a b l e to c o n s i d e r that on a drainage basin scale the t w o m a i n p r o c e s s e s o f erosion, w e a t h e r i n g ( c h e m i c a l e r o s i o n ) and t r a n s p o r t ( m e c h a n i c a l e r o s i o n ) , are c o m p l e -

Denudation rates at R~union

3659

Rivibre du Mat

Rivi6re de I'Est

Na

Ca

Mg

K

Sr

Ba

Rb BICO=SiO2 SO4 Li

Na

Ca

Mg

K

Rivibre des Galets

lf-

Na

Ca

Mg

K

Sr

Ba

~

Ba

Rb HCO3 SiO2 SO4 Li

Bras de Cilaos

Rb HCOs SiO2 SO4 Li

Rains

Sr

Na

Basaltweathering

Ca

Mg

~

K

Sr

Ba

Rb HCO3 SiO2 SO4 Li

Thermalsprings

Fig. 4. Stack column diagrams for Na, Ca, Mg, K, Sr, Ba, Rb, HCO3, SiOz, SO4 , and Li, showing the proportions arising from each endmember in the chemical composition of four major rivers of R~union.

mentary. In simple terms, the weathering water-rock interaction is achieved by incongruent dissolution of the rock, that fractionates elements according to their solubilities. Thus, the residual weathered rock is depleted in the most soluble elements that are the alkali and alkaline earths, whereas the most insoluble elements are present in very small amounts in the river waters (Figs. 6 and 7). Transport by the river results in the partial or total removal of the regolithic soil formed. In the steady-state erosion hypothesis, we assume that the soil formed by rock weathering (chemical erosion) and removed by river transportation (mechanical erosion) conserve a constant thickness. Such an approach is taken from previous studies (Martin and Meybeck, 1979; Gaillardet et al., 1995; Stallard, 1995; Gaillardet et al., 1997). The model is here applied to the R6union rivers, the essential difference in this study is that we have more precise information on the initial unweathered rocks. Such details were unavailable in previous work of Gaillardet et al. (1995, 1997) because of the large size of the Congo and Amazon basins and their multilithologic terrains. Noting the qualitative complementarity between the sus-

pended and dissolved loads (the most soluble elements are depleted in the suspended load, compared to the local basalt chemical composition, and enriched in the dissolved load; Fig. 6), we can establish a mass budget between these two phases. Given a mass of unweathered basaltic rock and a mass of pure water in contact with that rock in a supposed closed-system, after a certain time, the water will have partially dissolved the basaltic rock and be enriched in some elements arising from the basaltic rock. Knowing the mass of water that has been in contact with the basaltic rock (which is given by the river discharge), the soluble element enrichments of the dissolved phase (the now unpure water) and their depletions in the suspended phase (the weathered basalt), we can recalculate the concentration of basaltic particulate matter that is necessary to counterbalance the fiver dissolved load concentrations (both suspended and dissolved loads arising from the local basalt weathering, and lack of soluble elements in the suspended load having necessarily gone in the dissolved load). (The mass-budget equations proposed for the steady-state model of erosion are given in Appendix C.)

3660

P. Louvat and C. J. All~gre 2~5 i...............................................................................................

,.0i 3 O~2

0.s!

i

0.0i 0,0

0.5

1

................................................................... i 0 1.0 1,5 2.0 2,5 3.0 3,5 0,0

0,1 0.2

Ca/Na

0,3 0~4 0~5 0.6 0.7 K/Na

6 ~ ...............................................................................................................

a lo ;

0 o 2 0

ij

..........................................................................................

0.0 14 12

1.0

1.5 2.0 2.5 Ca[Na

i

3.0 3.5

0.0 0.5 14

.....................................................................

2,5

3,0 3,5

.....................................................................................................

!:

3:4L

,! 0.0

1,0 1.5 2.0 Ca/Na

mlO

'"i"i 8

,r

0.5

1! ~o

i

2 0

0.5

1.0

1.5

2.0

2.5

M~a * R~union island ~

Iceland

m

0

1

Massif Central I

....................................

2 3 4 SIO2/Na

5

6

Parana Basin

Fig. 5. Comparison of the X/Na vs. Y/Na (molar ratios ) trends for different basaltic rivers. For the Brazilian Parana basin (Benedetti et al., 1994), French Massif Central (Meybeck, 1986) and Iceland (Gislason and Arnorsson, 1993, and, Gislason and Eugster, 1987b), the chemical compositions of the river waters have been corrected from atmospheric inputs, using CI concentrations and oceanic X/C1 ratios. For Rtunion, the reported points are the concentration ratios arising from the basalt-weathering endmember only, as determined by the inverse method of resolution for the mixing model we proposed.

The calculated suspended load concentrations are very high, in agreement with the qualitatively low weathering state of the river suspended loads, and vary from 1300 m g / L to 1900 m g / L . A major problem of this steady-state model of erosion is that we have only a punctual sampling for each of the studied rivers, whereas the erosion steady-state is likely to be attained over a longer timescale, particularly in regions where hydrologic inter-seasonal variations are important, as is the case for the Rtunion. Given a lack of long-term measurement of suspended load concentrations for the rivers of Rtunion, we cannot accurately test the steady-state erosion hypothesis. However, this steady-state model of erosion gives us a good estimate of the river suspended load concentrations ( P ) which are very difficult to measure, given their variability through time and in a river water column. In this way, we assume that the erosion steady-state is attained over sufficiently long timescale, dissolved species that are transported by the rivers being necessarily issued from chemical basalt weathering and altered basaltic rocks, sands, and particles

being removed by the violent rains and cyclones that occur annually on Rtunion. The onfield suspended load concentration measurements we made, by direct filtration of a one liter volume and weighing of the particle masses on the filters, only allow us to note that the steady-state was not attained at the time of sampling. The measured suspended load concentrations, poorly representative as they may be, are always very much lower than those calculated (Fig. 8). Given that the suspended load concentrations we measured are the lower limit of these concentrations during the low water level of Rtunion rivers (we can take a mean concentration of 0 . 1 - 0 . 2 g / L for the low water levels), and assuming that the water volume of the cyclonic events and violent rains represent about 50% of the annual water volume (from the discharge measurements by Observatoire Rtunionnais de l'Eau, 1994), we obtain suspended load concentrations of about 3 g / L during the rainy events for a mean annual concentration of 1.5 g/L. During our sampling, all the rivers had very low suspended load concentrations, but large deposits of very fine

Denudation rates at Rdunion

3661

Table 5. A priori and a posteriori values and errors (see text and Appendix B and C) of the parameters used in the resolution by an inverse method for the erosion steady state model and the determination of a local basalt chemical composition for each of the studied drainage basins. Rivi~re de I'Est a priori

a posteriori

suspended load ( p p m )

Na Ca Sr Ba Rb K Mg La Th Ce Nd Sm Dy Yb Co Er

19050 69850 368 165 18.3 6250 48800 21.8 2.41 49.1 31.1 6.55 5.73 2.32 53.3 2.82

± ± ± ÷ ± ± ± ± ± ÷ ± ± ± ± + ±

1900 7000 37 17 1.8 625 4900 2.2 0.24 4.9 3.1 0.66 0.57 0.23 5.3 0.28

18322 70039 373 166 18.0 6163 48271 21.8 2.44 48.6 30.4 6.54 5.75 2.33 52.3 2.83

dissolved load

Na Ca Sr Ba Rb K Mg

4014 4226 15.5 0.30 5.56 1450 3035

÷ ± ÷ ± ± ± ÷

400 400 1.6 0.03 0.56 145 300

Na Ca Sr Ba Rb K Mg La Th Ce Nd Sm Dy Yb Co Er

18000 76000 450 175 20.0 6750 48000 22.0 2.70 47.0 28.0 6.60 5.90 2.40 45.0 2.90

± ± ± ± ± ± ÷ ± ± ± ± ± ± ± ± ±

Na Ca Sr Ba Rb K Mg

18000 76000 450 175 20.0 6750 48000

Na Ca Sr Ba Rb K Mg

0.50 0.50 0.50 0.50 0.50 0.50 0.50

Ippb)

local basak (ppm)

Suspended load before weathering (ppm)

Cp/Cp*

Cp(Th)/Cb(Th)

alpha

a

susp. load ( m g / L )

Priv

Rivi~re des Galets a priori

Rivi~re du M~t

a posteriori

a priori

Bras de Cilaos

a posteriori

a priori

a posteriori

÷ 1762 ± 6620 ± 35 ± 16 ÷ 1.7 + 593 ± 4568 _+ 2.0 ± 0.23 ± 4.5 ± 2.8 ± 0.60 ± 0.52 ± 0.21 ± 5.0 ± 0.25

7450 25600 236 118 19.8 4900 59500 16.8 1.77 37.9 22.6 3.68 2.88 1.13 53.0 1.41

± ± ± ÷ ± ± ± ± -± ± ± ± ± ÷ ±

750 2600 24 12 2.0 490 6250 1.7 0.18 3.8 2.3 0.37 0.29 0.12 5.3 0.14

7320 26256 242 119 18.9 4866 52697 16.8 1.80 37.6 22.4 3.83 3.03 1.19 49.7 1.48

± 742 ± 2568 ± 23 ± 12 ÷ 1.8 ± 464 ± 5384 ± 1.6 ± 0.17 ± 3.5 ± 2.1 ± 0.35 ± 0.28 _+ 0.11 ± 4.9 ± 0.13

1220 ÷ 1200 44500 ± 4500 333 ± 33 181 ± 18 25.1 ± 2.5 6350 ± 650 51700 ± 6200 2 2 . 6 _+ 2.3 2.51 ± 0.25 4 9 . 8 ÷ 5.0 3 1 . 0 ± 3.1 5.23 ± 0 . 5 2 4.74 ± 0.47 2 . 0 9 ± 0.21 4 6 . 9 ÷ 4.7 2 . 4 5 ± 0.25

12034 45489 344 180 23.4 6222 50181 22.4 2.52 49.1 30.2 5.38 4.88 2.13 46.6 2.51

± 1172 + 4391 ± 31 _+ 17 ± 2.3 _+ 611 + 4809 ± 2.1 ± 0.24 _+ 4 . 6 + 2.8 ± 0.49 + 0.44 ± 0.19 ± 4.5 ± 0.23

22100 59000 639 395 52.9 13500 26500 41.1 5.79 90.5 51.2 8.58 6.29 2.83 30.0 3.33

± ± ± ± ± ± ± ± ± ± ± ± ± + ± ±

2200 6000 64 18 5.3 1400 2700 4.1 0.58 9.1 5.1 0.86 0.63 0.28 3.0 0.33

21557 56747 621 395 53.7 13636 26715 41.5 5.88 89.5 50.6 8.62 6.30 2.83 30.3 3.33

± 2161 + 5627 ± 61 ± 18 ± 5.1 _+ 1374 + 2589 ± 3.8 ± 0.56 ± 8.4 ± 5.0 ÷ 0.85 ÷ 0.62 ± 0.28 ± 2.9 ± 0.33

3997 4230 15.6 0.30 5.55 1449 3036

± ± ± ± ± ± ±

399 400 1.6 0.03 0.56 145 300

19800 12500 18.8 0.14 1.76 1435 4870

± ± ± ± ± ± ±

2000 1250 1.9 0.02 0.18 145 490

19280 12609 18.9 0.14 1.76 1436 4852

± ± ± ÷ ± ± ±

1970 1248 1.9 0.02 0.18 145 490

11270 11910 20.2 0.32 4.59 1750 7030

1130 1200 2.0 0.03 0.50 175 700

11192 11982 20.3 0.32 4.55 1748 7021

± ± ± ± ÷ ± ±

1121 1199 2.0 0.03 0.50 175 700

37460 17160 40.0 0.16 4.21 2690 8570

± ± ± ± ± ± ±

3800 1700 4.0 0.02 0.42 270 857

36629 17099 40.0 0.16 4.22 2697 8599

± ± ± ± ± ÷ ±

3753 1697 4.0 0.02 0.42 270 856

5000 25000 150 70 6.0 2000 14000 5.0 0.70 11.0 6.0 1.40 1.20 0.50 15.0 0.60

21699 74287 390 168 22.6 7381 51294 22.1 2.47 49.3 30.8 6.63 5.82 2.36 53.0 2.87

± ± ± ÷ ± ± ± ± ± ± ± ± ± ± ± ±

2400 7767 41 19 2.8 855 5306 2.3 0.26 5.1 3.1 0.68 0.59 0.24 5.7 0.29

18000 76000 450 170 20.0 6750 48000 22.0 2.70 47.0 28.0 6.60 5.90 2.40 45.0 2.90

± ± ± ± ± ± ÷ ± ± ± ± ± ± ± ± ±

5000 25000 150 70 6.0 2000 14000 5.0 0.70 11.0 6.0 1.40 1.20 0.50 15.0 0.60

22768 43046 330 156 26.0 7362 72424 22.0 2.36 49.2 29.3 5.02 3.97 1.56 65.2 1.94

± ± ± ± ÷ ± ± ± + ± ± ± ÷ ± ± ±

3757 4863 38 19 2.8 783 7968 2.5 0.27 5.5 3.2 0.57 0.45 0.18 7.5 0.22

18000± 76000 ± 450 ± 175 + 20.0 ± 6750 ± 48000 ± 22.0 ± 2.70 ± 47.0 ± 28.0 ± 6.60 ± 5.90 ÷ 2.40 ± 45.0 ± 2.90 ±

5000 25000 100 70 6.0 2000 14000 5.0 0.70 11.0 6.0 1.40 1.20 0.50 15.0 0.60

20117 55120 368 186 27.2 7614 56530 23.0 2.60 50.5 31.1 5.54 5.02 2.19 48.0 2.58

÷ ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3098 6172 39 21 2.8 832 5979 2.5 0.29 5.5 3.3 0.60 0.54 0.24 5.5 0.28

33000 51000 450 390 64.0 19000 34000 44.0 6.90 84.0 41.0 10.00 6.50 3.00 34.0 3.50

± ± ÷ ± ± ± + ± ± ± ± ÷ ± ± ± ÷

10000 20000 300 150 20.0 10000 11000 10.0 2.00 20.0 20.0 5.00 3.50 2.00 10.0 2.00

40647 65455 639 393 55.7 14989 31102 41.4 5.85 89.1 50.4 8.58 6.27 2.82 30.2 3.32

+ ± ± ± ± ÷ ± ± ± ± + ÷ ± ± + ÷

6498 7915 83 41 7.0 1917 3774 5.1 0.76 10.8 6.7 1.16 0.85 0.38 3.9 0.45

± ± ± ± ± ± ±

9000 35000 220 100 10.0 4000 28000

21414 73311 385 166 22.3 7284 50620

± ± ÷ ± ± ± ±

2303 6840 36 16 2.8 823 4728

1800 0 76000 450 170 20.0 6750 48000

± ± ÷ ± ± ± ±

9000 35000 220 100 10.0 3500 28000

17361 32823 252 119 19.8 5614 55225

± ± ± ÷ ± ± ±

2994 3301 24 12 1.8 516 5454

18000± 76000 ± 450 ± 175 ± 20.0 ± 6750 ± 48000 ±

9000 35000 220 100 10.0 3500 28000

19532 53516 357 180 26.4 7392 54885

± ± ± ± ÷ ÷ ±

3138 5491 32 17 2.5 757 5148

33000 51000 450 390 64.0 19000 34000

± ± ± ± ± ± ±

15000 25000 300 200 30.0 10000 15000

40825 65741 642 395 55.9 15054 31239

± + ± ± ± ± +

6575 6342 62 18 5.1 1460 3003

± ± -± ± +_ ±

0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.86 0.96 0.97 1.00 0.81 0.85 0.95

± ± ÷ ± ± ± ±

0.07 0.02 0.02 0.00 0.09 0.08 0.03

0.50 0.50 0.50 0.50 0.50 0.50 0.50

± ± -± _+ ± ±

0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.42 0.80 0.96 1.00 0.95 0.87 0.95

± ± -± -± ±

0.08 0.05 0.01 0.00 0.01 0.04 0.01

0.50 0.50 0.50 0.50 0.50 0.50 0.50

± 0.10 _+ 0 . 0 6 ± 0.02 -- 0 . 0 0 4-_ 0.05 ÷ 0.06 _+ 0 . 0 3

0.50 0.50 0.50 0.50 0.50 0.50 0.50

± ÷ ± ± ± -+

0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.53 0.86 0.97 1.00 0.96 0.91 0.86

--± ± ± ± ÷

0.09 0.04 0.01 0.00 0.01 0.03 0.05

± ± ± ± ± ± ±

± ± + ± ± ± ---

0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.62 0.85 0.96 1.00 0.88 0.84 0.91

0.50 - 0.50

0 . 9 9 +_ 0 . 0 6

0 . 5 0 -- 0 . 5 0

0.76 ± 0.06

0 . 5 0 -+ 0 . 5 0

0.97 ± 0.07

0.50 ± 0.50

1.00 + 0 . 1 0

0.500 ± 0.500

0.031 ± 0 . 0 1 7

0.500 ± 0.500

0.039 ± 0.013

0 . 5 0 0 +_ 0 . 5 0 0

0.038 ± 0.016

0.500 ± 0.500

0.033 ± 0.012

1293 ± 721

1000 ± 1000

1920 ± 586

1000 ± 1000

1493 ± 6 1 8

1000 ± 1000

1901 + 651

1000 -

1000

sand were visible on each side of the river's beds, indicating that, during high water level, the mechanical transport of the erosion products is important. In addition, the steep slopes of the volcanic massifs are responsible for torrential flows and the existence of small retention basins, where suspended

particulate material and sand accumulate, which are only emptied during higher discharge periods. Most of the volumetric annual hydrologic budget of R~union is due to the cyclonic and exceptional rain events during which highest discharges of the rivers take place (up to 180 m3/s for the

3662

P. Louvat and C. J. All~gre

10~+2 10e+1 ¢: 10e0 :~ 10e-1 a

i

I dissolved load

1~e-2

10e4

E .¢

G.

10e-5

Me'"'u"'°"

10e.6 10e-7

~

K

p...m I

Sm su ~T,, Oyml Er ~. cuCa scco ~cr NI

Nd Sr

Fig. 6. Complementary relationship between the suspended and dissolved loads. These patterns are the theoretical primitive mantle normalised diagrams of both loads we would observe if the water-basalt interaction was more pronounced at Rrunion. This schematic diagram gives a good visual representation of the mass budget between chemical and mechanical erosion products. The shaded lines represent the mean basalt pattern, undiluted for the suspended load diagram and very diluted for the dissolved load diagram. The soluble elements are enriched in the dissolved load (peaks) and depleted in the suspended load (troughs), whereas insoluble elements show identical patterns in the dissolved load (but with weaker concentrations), the suspended load and the mean basaltic rock.

1.2

t l

m o.~Ooooo=oo o=,

g

1.0

.

OI

,P

o,, •

.

.

~'l

oo,.,,~ %,

.

~

g

i%

i

t

0.6

-,° ~ 0.4 "~

"

U)

0.2

~

S:

p,D

:

•

y

I

1

~

i

i

i

I

1

i

~

i

I

I

i

i

i

Rb La Ce Th Ba

ql, i

i

' ii

!

Rivi~reRivide ~re I'Est ,1~i ----du Mat I ........... Brasde Cilaos I ---Rivi~redes Galets/L I t

\ t

!

1

o.

.. ti:"

=,: ooo. °,=4=o,loo,l ., ' "-i ,o,."

tlm

V

!

, l

K

i

i

i

.

i

Nd Sr Sm Dy Na Er Yb Ca Co Mg

Fig. 7. Suspended load vs. local basalt concentration ratios for the four major rivers of Rrunion, after recalculation of the local basalt composition (see Table 5 and Appendix B and C ). Depletions of the most soluble elements in the suspended loads appear more obviously (only the elements which are used in the inversion calculation are represented ).

Denudation rates at Rfunion 2600 U~ e,O m

2400

e-

¢o

3663

assume that the erosion rates for this period correspond to the annual estimates, within the uncertainties of the calculations.

t

iI

Bras

Rivi~re . des | Galets

2000

0 0 "0

7. EROSION RATES 7.1 C h e m i c a l E r o s i o n R a t e s

m p., 1600

"o C

1200

O.

=

U)

800

Rivi~re du Mat

i Rivibre de rEst

"O -¢

u

400

1:1

0

i

0 50 100 Measured suspended load concentrations (rag/I)

Fig. 8. Calculated vs. onfield measured suspended load concentrations for the four studied rivers of Rfunion island. The measured concentrations are not representative of the annual mean concentrations.

Rivibre Langevin and the Rivibre du Mat rivers, during the Hollanda 1994 cyclone, Observatoire Rfunionnais de l'Eau, 1994, but also discharges up to 2600 m3/s have been historically observed at Rfunion island during exceptional floods), the river waters carrying then the highest mass of annual suspended material (of more or less coarse grain size). It was of course not possible to sample river waters during cyclonic rains, but it is quite simple to imagine that during these periods, the total dissolved solid concentrations should be slightly lower than during the rest of the hydrologic season because of the effect of dilution (of course the dissolved solid fluxes are higher). Thus, because the calculation of the suspended load concentrations is directly proportional to the dissolved load concentrations (and not to dissolved fluxes) ( see the equation for P in Appendix C), during the cyclonic periods, the erosion steady-state is likely to show an inverse trend, with calculated suspended load concentrations lower than those measured. This climatic and hydromorphologic evidence does not prove that erosion steady-state is attained but does indicate that, on an annual timescale, cyclonic periods (with high water levels, high concentrations of suspended load, and lower dissolved load concentrations) and long low water periods (with very lower suspended load concentrations and water levels) could compensate each other, and that the erosion steady-state and the calculated concentrations of suspended load are realistic. It is important to note that during the period of the sampling, February 1995, the monthly mean discharges of the rivers are very close to the annual mean. As the suspended and dissolved load concentrations are correlated with runoff at a global scale (e.g., Chorley et al., 1984) and because the dissolved load concentrations do not vary significantly between the dry and rainy seasons, we can

Knowing for each dissolved species the proportion arising from the weathering of the basaltic rocks, we can calculate for each river the total dissolved solids due to the basaltic rocks chemical erosion (TDSbasalt) by summing all the dissolved species concentrations (except HCO 3 which does not directly come from the rock) weighted by their respective basalt weathering endmember proportions (Table 6). We thus calculate TDSbasalt values of 30-100 mg/L which are comparable to the values for Icelandic rivers ( 5 0 - 8 0 rag/L, Gislason et al., 1996), French basaltic rivers in Massif Central ( 2 0 - 6 0 rag/L, from the analysis of Meybeck, 1986), and Parana basin rivers in Brazil (20 to 100 rag/L, from the analysis of Benedetti et al., 1994). We have corrected the published values from atmospheric inputs assuming that all the fiver chloride content arise from rains and that rains have oceanic concentrations ratios. Using the annual discharges of the Rfunion rivers (Observatoire Rfunionnais de l'Eau, 1994) and their drainage basin surfaces (Table 6), we estimate specific chemical erosion rates of 63-170 t/km2/yr, with a mean value of 105 t/km2/yr for the island (Table 6). It is important to note that the highest erosion rates (Rivibre de l'Est and Rivibre Langevin) are correlated with the highest runoffs in the eastern part of the island and correspond also to the youngest part of the island. The influence of the age of the rocks on erosion rates may thus be hidden by that of the runoff. At the local scale of the island, as all the drainage basins have similar mean elevations, the effect of altitude on erosion rates does not appear. The specific chemical erosion rates we have determined, which take into account the size of the drainage basins, can be compared to those of the major worldwide drainage basins (Gaillardet et al., 1995, for the Congo basin, Gaillardet et al., 1997, for the Amazon basin, and the compilations by Pinet and Souriau, 1988 and Summerfield and Hulton, 1994). Our chemical denudation rates for Rfunion have been calculated in the same way as Gaillardet et al. (1995, 1997) and give very much higher rates these authors determined for the Congo ( 5 - 6 t/km2/yr) or Amazon (0.6-20 t/km2/yr) basins. The chemical denudation rates given by Summerfield and Hulton (1994) are corrected from estimated atmospheric and recycled components inputs (from the estimated percentages of Meyheck, 1979 and Berner and Berner, 1987). None of the large rivers of the world shows specific chemical denudation rates as high as those of the relatively small rivers of Rfunion, except where more carbonate lithologies are involved (for example, the Chiang Jiang: 72 t/km~-/yr) and some mountainous rivers (Pinet and Souriau, 1988; Summerfield and Hulton, 1994). Even for the Indus and Ganges, which drain high mountainous areas, the specific chemical denudation rates are only of the order of 42 t/km~-/yr (Summerfield and Hulton, 1994). However, there is a lack of data for many rivers, such as those in southeast Asia which should

P. Louvat and C. J. All6gre

3664

Table 6. Drainage basin surfaces, runoffs, erosion rates, and rates of atmospheric CO2 consumption by chemical weathering for the main rivers of Rfiunion. Surface (km:) Bras de Cilaos Rivi~re du Mat Rivibre des Galets Rivi6re Langevin Rivi~re de I'Est Rivi~re des Marsouins

95 145 114 33 32 109

Runoff TDSbasalt Chem. erosion (mm/yr) (mg/L) (t/km2/yr) 635 1300 1250 2470 7060 1880

100 58 69 59 30 34

63 75 86 145 169 87

have very elevated rates given their high mechanical denudation (Milliman and Syvitski, 1992). The chemical erosion rates of southwestern Iceland are similar to these of R6union rivers, and sometimes higher ( 3 0 - 1 5 0 t/km2/yr, after correction of the geothermal contribution for some of the rivers, Gislason et al., 1996), with a mean for southwestern Iceland of 55 t/km2/yr. Thus, we can reaffirm the important influence of the lithology of the drainage basins upon the chemical erosion rates, as noted previously by Meybeck (1986) and Bluth and Kump (1994): the susceptibility of basaltic lithologies to erosion is very high. In view of the rates obtained for R6union, the role of runoff and high relief also seems dominant.

7.2 Atmospheric CO2 Consumption by Basalt Weathering As bicarbonates are the dominant anion species (which counterbalance the cation species) in all the rivers of R6union island, and as no carbon arises from basaltic rocks (e.g., Gislason et al., 1996), the consumption rates of atmospheric CO2 during basalt weathering are very high, ranging from 1.3 to 4.4 × l06 mol/km2/yr (Table 6) with a mean of 2.3 × 106 mol/kmZ/yr for the whole island. These rates are in agreement with those measured for Icelandic rivers (mean for southwestern Iceland of 1.1 × 106 mol/kmZ/yr, Gislason et al., 1996), but are ten to thirty times higher than those defined by Bluth and Kump (1994) for Hawaiian catchments (runoff ranges are comparable in Iceland, Hawaii, and R6union). The reason given by Bluth and Kump (1994) for the differences between their CO2 consumption rates and those for Iceland was that soil formed in the drainage basins were thicker in Hawaii and of a different type, due to the wet tropical climate of the Hawaii archipelago. The problem is that R6union and Hawaii have similar climates but do not have similar chemical erosion rates. However, as we will see later, the erosion regime at R6union is weathering-limited, the mechanical erosion rates being higher than the chemical erosion rates, whereas, according to Bluth and Kump (1994), it is transport-limited in Hawaii. To go further in that comparison, we would need a more precise study of the fiver chemistry in Hawaiian islands (the data used in the study of Bluth and Kump (1994) are not published) and of the soils of R6union. One unanswered question concerning these elevated atmospheric CO2 consumption rates is whether magmatic CO_, fluxes (and also other volatile elements fluxes) could con-

CO2 cons. Susp. load (mol&m2/yr) ( m g / L ) 1.5 2.2 2.1 2.1 4.4 1.3

10e6 10e6 10e6 10e6 10e6 10e6

Mech.erosion Totalerosion (t/kmZ/yr) (mm/kyr)

1900 ± 650 1490 ± 620 1920 ± 590

1200 _+ 410 1930 ± 800 2400 ± 740

1290 _+ 720

9100 ± 5100

470 ± 160 745 _+ 305 925 ± 280 3430 _+ 1890

tribute to the fiver waters, thereby enhancing (by acidification) the chemical erosion of the basaltic rocks. Such fluxes have been recognised in the most active areas of R6union (in thermal springs of the three cirques of Piton des Neiges and also in fumaroles of the Piton de la Fournaise inner caldera, Marty et al., 1993), it is difficult to constrain such input as atmospheric CO2 contribution should be dominant for mountainous streams with torrential flows, due to the very efficient exchange with atmospheric gases.

7.3. Mechanical and Total Erosion Rates If erosional equilibrium is reached on an annual timescale, we can infer rates of mechanical erosion from the calculated concentrations of suspended load, knowing the annual discharge of the rivers and their basin drainage areas, and assuming that these concentrations are representative of the annual hydrologic scale. We have thus estimated mechanical erosion rates of 12009100 t/km2/yr (Table 6), the highest rate being that of Rivibre de l'Est which also has the highest runoff, and drain the youngest rocks of the island. These specific mechanical erosion rates are all very high, among the highest measured in the world. The highest specific denudation loads reported by Summerfield and Hulton (1994) for large rivers are for Indian rivers (Brahmaputra, Ganges, and Indus: 1808, 694, and 323 t/kmZ/yr) which are sourced in the Himalayan mountains. But the study of Milliman and Syvitski (1992) has shown the importance of small mountainous rivers in the global budget of suspended load supply to the ocean and in particular the rivers of southern Asia and Oceania for which the specific yield can be as high as 26000 t/km2/yr. For comparable runoffs (1000s mm/yr), the specific yields of R6union rivers and of these Asian mountainous rivers are in the same range. More than the topographic relief, Milliman and Syvitski (1992) assume that the sediment load is mainly correlated with tectonics. As a consequence, rivers that drain mountainous islands, active edges of continental margins, and collision margins are smaller but collectively they may transport similar amounts of sediments as passive margin rivers (Milliman and Syvitski, 1992). The tectonically active areas that are active volcanic islands may thus also have rivers with high sediment loads, as have R6union and Iceland. For the Jokulsa glacierised river in southern Iceland, Lawler (1991) obtained a mean annual sediment yield of 12700 +_ 1800 t/kme/yr, based on a 16 year period of monthly measurements. He also noted a good correlation

Denudation rates at REunion between discharge and sediment yield. These two parameters are highly variable from one season to the other, the discharge ranging from 1 to 103 m3/s and sediment concentrations from 34 to 6358 m g / L . Glacierised Icelandic rivers transport larger amounts of sediments than the rivers of REunion because they drain large areas of fine grained material deposited from the glacier moraines. If we consider both estimated chemical and mechanical erosion rates from REunion rivers, and assume a mean density for the basalt of 2.7, we obtain a total denudation rate of 4 7 0 - 3 4 3 0 mm/kyr. To check if these high erosion rates are consistent with the relief of REunion, we can make a calculation of a first order approximation: assuming constant denudation rates, integration over 150,000 yr (age of the Piton des Neiges summit caldera formation) would lead to the removal of 7 0 - 5 1 5 m of material. Such relief is smaller than the deep walls of more than 1000 m observed in the cirques, implying that, although high, present day fluvial erosion rates cannot on their own explain such surface depression, and underlines the importance of tectonic processes, such as volcano summit collapse forming calderas, but also faulting and landslides, in the shaping of REunion. The mechanical denudation rates are nineteen to fifty-four times higher than the chemical erosion rates, and the highest mechanical denudation rates correspond to the highest chem-

3665

ical denudation rates (Fig. 9). High mechanical/chemical erosion ratios are characteristic of mountainous rivers (Gaillarder et al., 1997), and are also correlated with high runoff (Fig. 9), consistent with the global study of Pinet and Souriau (1988). Their multicorrelation analysis revealed that the suspended yield is correlated with relief and less so with climate, while the dissolved yield is lesser correlated with relief than with climate conditions (runoff and precipitation). For Milliman and Syvitski (1992), the sediment yields are more likely controlled by relief and tectonics (in particular in the active margin areas). For the single study of REunion rivers, erosion rates are more influenced by runoff than by relief, but this is because the slopes of the drainage basins we study are all similar. However, on a global scale, the REunion rivers fall into the same category as those in mountainous areas with high runoff. With this single study, we are not able to discuss the influence of factors as temperature on erosion rates. To understand the role of temperature, we have to compare our results with many other studies of riverine erosion on basaltic lithologies, in order to decorrelate this factor from the other influencing factors (runoff, relief, glaciers, volcanic activity, tectonics, age of the rocks, vegetation, etc.). This decorrelation will only be possible if we have a sufficiently thick database.

4.5

240 E

4.0 o E 3.5

~'200 E

%

~160 c .2 120

I ,i-o--

.F?*

80

.~= J::

3.0 tO 2.5 Q. 2.0 t-

40

8

0

i

,

i

r

0

t

i

i

i

5000

|

i

a

15000

mechanical

~

O

1.0

i

10000

1.5

m

i

I I

I

em i

60

40

0

t i

i

i

i

i

I

i

80 100 120 140 160 180 c h e m i c a l e r o s i o n (t/km2/y)

e r o s i o n (t/km2/yr)

200

220

90

9000

I I

m

o= .2

E 60

6000

E E

co

¢:

o

em

¢-

o c

E 3ooo

¢.)

.II

30

E

--%..-._ =0=

0

1

0

i

2000

.

.

.

.

I

4000

total e r o s i o n ( m m / k y r )

I

1

1

1

0

6000

~

0

'

~

I

1

.

1

I

1

3000

I

l

6000

runoff (mm/yr)

Fig. 9. Relationships between chemical and mechanical denudation rates, atmospheric CO2 consumption rates and runoffs for the major rivers of REunion.

9000

3666

P. Louvat and C. J. Allrgre 8. CONCLUSION

The tropical and oceanic climate (with high r u n o f f ) , the high relief, the volcanic activity, and the basaltic lithology are all conditions which favour very high chemical and mechanical erosion rates at R r u n i o n , even on the scale of such small drainage basins. Chemical compositions of the fiver waters principally reflect local basalt weathering. However, as it is an island, the chemistry of derived oceanic rains influences some elements, such as Na, Mg, Ca, and K. For some of the rivers draining the ancient Piton des Neiges volcano, a contribution of thermal springs to the water characteristics has also been recognised. This contribution is important for the elements which are the most enriched in the thermal springs compared to rivers, namely SO4, Li, Sr, Ca, and Rb. Calculation of the element proportions provided by each of these three endm e m b e r s (rain, basalt weathering, and thermal springs), which contribute to the river water's chemical signature, is realised using an inverse method for the resolution of the mixing model. This allows us to constrain the concentrations ratios X / N a , characteristic of basalt weathering at Rrunion. These ratios show that for small, monolithologic rivers, there still exists some variability in the weathering of each drainage basin local basalt, similar to that observed for other draining basaltic terrains. The river suspended loads analysed are very similar to the R6union basaltic rocks compositions, reflecting a low weathering state and immature erosion material. Within the assumption that an erosion steady-state exists during the basalt chemical weathering and the transport of its erosion products by the river flows, we propose a mass budget between the dissolved load, the suspended load, and the local basalt, for each drainage basin. This allows us to calculate river suspended load concentrations of 1 1 5 0 - 1 8 0 0 m g / L , necessary to counterbalance the elevated dissolved load concentrations of 3 0 - 1 0 0 m g / L . These suspended load concentrations are m u c h higher than those measured on the field and imply that the equilibrium between the chemical erosion (solute transport resulting from basalt partial dissolution) and the mechanical erosion (river sediment transport) was not attained at the time of sampling. However, given the tropical climate, we suspect that most of the mechanical transport of erosion products occurs during the torrential rains of the cyclonic period and that our suspended load concentration measured during the sampling is not representative on an annual hydrologic cycle scale. The calculated suspended load concentrations are then more likely representative of the annual means. F r o m the determination of the river water chemical compositions due to basalt weathering only, we have estimated specific chemical erosion rates of 6 3 - 1 7 0 t / k m 2 / y r and atmospheric CO2 c o n s u m p t i o n during basalt weathering of 1.3 to 4.4 × 10 6 m o l / k m 2 / y r . Such rates are very high but are comparable to those obtained for Iceland by Gislason et al. ( 1 9 9 6 ) . They are a m o n g the highest global estimates c o m p a r e d to m o u n t a i n o u s and tropical river catchments. The mechanical erosion rates we infer from suspended load concentrations ( 1 1 0 0 - 2 8 5 0 t / k m Z / y r ) are very high compared to other riverine erosion studies and are also related to the

high runoffs ( 6 5 0 - 7 0 0 0 m m / y r ) and relatively high relief slopes ( s u m m i t altitudes of 2600 and 3070 m for a 200 k m island circumference) of the R6union. Total erosion rates (chemical plus mechanical erosion) range between 430 and 3430 m m / k y r and reflect also the enhanced erosion in tectonically active areas. Acknowledgments~This work was supported by the INSU/PIRAT/

DBT Programme (for the 1995 sampling) and by the CEE Rrunion Programme (for the 1993 and 1994 samplings). We are grateful to J. Gaillardet, B. Duprr, G. Michard, and S. Roy for helpful discussions. The computer program used to perform the inversion computations is from E. Lewin. S. Roy, J. Boul~gue, M. Castrec, and F. Jung are thanked for their contribution to fieldwork. K. Burton is acknowledged for English corrections. The 1993 and 1994 samples were analysed at the Laboratoire de Grochimie et Mrtallog6nie, Universit6 Paris VI (Prof. J. Boulbgue). The manuscript was greatly improved by the constructive comments of three anonymous reviewers. This is IPGP contribution no. 1468 and CNRS/INSU contribution no. 80. REFERENCES

Albarbde F. and Tamagnan V. (1988) Modelling the Recent Geochemical Evolution of the Piton de la Fournaise Volcano, Rrunion Island, 1931-1986. J. Petrol. 29, 997-1030. All~gre C. J., Hart S. R., and Minster J. F. (1983a) Chemical structure and evolution of the mantle and continents determined by inversion of neodymium and strontium isotopic data, I. Theoretical methods. Earth Planet. Sci. Lett. 66, 177-190. All~gre C. J., Hart S. R., and Minster J. F. (1983b) Chemical structure and evolution of the mantle and continents determined by inversion of neodymium and strontium isotopic data, II. Numerical experiments and discussion. Earth Planet. Sci. Lett. 66, 191-213. All~gre C. J., Dupr6 B., Nrgrel P., and Gaillardet J. (1996) Sr-NdPb isotopes systematics in Amazon and Congo river systems. Constraints about erosion processes. Chem. Geol. 131, 93-112. Arnorsson S. and Andresdottir A. ( 1995 ) Processes controlling the distribution of boron and chloride in natural waters in Iceland. Geochim. Cosmochim. Acta 59, 4125-4146. Arnorsson S., Gunnlaugsson E., and Svarvarsson H. (1983) The chemistry of geothermal waters in Iceland. II. Mineral equilibria and independant variables controlling water compositions. Geochim. Cosmochim. Acta 47, 547-566. Artaxo P. and Maenaut W. (1988) Composition and sources of aerosols from the Amazon Basin. J. Geoph. Res. 93, 1605-1615. Benedetti M. F., Menard O., Noack Y., Carvalho A., and Nahon D. (1994) Water-rock interactions in tropical catchments: field rates of weathering and biomass impact. Chem. Geol. 118, 203-220. Berner R. A. (1992) Weathering, plants, and the long-term carbon cycle. Geochim. Cosmochim. Acta 56, 3225-3231. Birck J. L. (1986) Precision K-Sr-Rb isotopic analysis: Application to Rb-Sr chronology. Chem. Geol. 56, 73-83. Blum J. D., Erel Y., and Brown K. (1994) 87Sr/~'Sr ratios of Sierra Nevada stream waters: Implications for relative mineral weathering rates. Geochim. Cosmoehim. Acta 58, 5019-5025. Bluth G. J. S. and Kump L. R. (1994) Lithologic and climatic controis of river chemistry. Geochim. Cosmochim. Acta 58, 23412359. Burke W. H., Denison R. E., Hetherington E. A., Koepnick R. B., Nelson H. F., and Otto J. B. (1982) Variation of seawater 878r/ 86Sr throughout Phanerozoic time. Geology 10, 516-519. Chorley R. J., Schumm S. A., and Sugden D. E. (1984) Geomorphology. Methuen. Coudray J., Mairine P., Nicolini E., and Clerc J.M. (1990) Approche Hydrogrologique. In Le Volcanisme de la Rdunion, Monographie (ed. J.-F. Lenat), pp. 307-355. Ctr. Rech. Volcanol. Crozat G. (1979) Sur l'rmission d'un a@osol riche en potassium par la forSt tropicale. Tellus 31, 52-57. Deniel C. (1990) Le magmatisme du Piton des Neiges. In Le Volean-

Denudation rates at Rrunion isme de la Rdunion, Monographie led. J.-F. Lenat), pp 115-143. Ctr. Rech. Volcanol. Douglas G. B., Gray C. M., Hart B. T., and Beckett R. (1995) A strontium isotopic investigation of the origin of suspended particulate matter ( SPM ) in the Murray-Darling River system, Australia. Geochim. Cosmochim. Acta 59, 3799-3815. Dupr6 B., Gaillardet J., Rousseau D., and All6gre C. J. (1996) Major and trace elements of fiver-borne material: The Congo Basin. Geochim. Cosmochim. Acta 60, 1301-1321. Gaillardet J., Dupr6 B., and Allbgre C. J. ( 1995 ) A global geochemical mass budget applied to the Congo Basin rivers: Erosion rates and continental crust composition. Geochim. Cosmochim. Acta 59, 3469-3485. Gaillardet J., Dupr6 B., Allbgre C. J., and Ndgrel P. ( 1997 ) Chemical and physical denudation in the Amazon River Basin. Chem. Geol. Gillot P. Y. and Nativel P. (1989) Eruptive history of the Piton de la Fournaise volcano, Rrunion island, Indian Ocean. J. Volcanol. (in press) Geotherm. Res. 36, 35-42. Gislason S. R. and Eugster H. P. (1987a) Meteoric water-basalt interactions. I: A laboratory study. Geochim. Cosmochim. Acta 51, 2827-2840. Gislason S. R. and Eugster H. P. (1987b) Meteoric water-basalt interactions. II: A field study in N.E. Iceland. Geochim. Cosmochim. Acta 51, 2841-2855. Gislason S. R. and Arnorsson S. (1993) Dissolution of primary basaltic minerals in natural waters: Saturation state and kinetics. Chem. Geol. 105, 117-135. Gislason S. R., Arnorsson S., and Armannsson H. (1996) Chemical weathering of basalt as deduced from the composition of precipitation, rivers, and rocks in SW Iceland. Amer. J. Sci. 296, 837907. Grunberger O. (1989) l~tude gdochimique et isotopique de Finfiltration sous climat tropical contrastr, Massif du piton des Neiges, ~le de la Rrunion. Thesis, Univ. Paris-Sud. Hofmann A. W. (1988) Chemical differentiation of the Earth: The relationship between mantle, continental crust, and oceanic crust. Earth Planet. Sci. Lett. 90, 297-314. Lawler D. t 1991) Sediment and solute yield from the Jokulsa A Solheimasandi glacierised river basin, southern Iceland. In Environmental change in Iceland: past and present led. J. K. Maizels and C. Caseldine), pp. 303-332. Kluwer. Martin J. M. and Meybeck M. (1979) Elemental mass-balance of material carried by major world rivers. Mar. Chem. 7, 173-206. Marty B., Meynier V., Nicolini E., Griesshaber E., and Toutain J. P. ( 1993 ) Geochemistry of gas emanations: A case study of Rdunion Hot Spot, Indian Ocean. Appl. Geochem. 8, 141-152. Meybeck M. (1979) Concentrations des eaux fluviales en elements majeurs et apports en solution aux oceans. Rev. Ggol. Dyn. G~og. Phys. 21, 215-246. Meybeck M. (1986) Composition chimique des ruisseaux non pollurs de France. Sci. Geol. Bull. (Strasbourg) 39, 3-77. Michard G. (1989) IEquilibres ehimiques dans les eaux naturelles. Publisud. Milliman J.D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: The importance of small mountainous rivers. J. Geol. 100, 525-544. Minster J.F., Minster J.B., Treuil M., and Allbgre C.J. (1977) Systematic use of trace elements in igneous processes. Part II: Inverse problem of the fractionnal crystallisation process in volcanic suites. Contrib. Mineral. Petrol. 61, 49-77. Nativel P. (1978) Volcans de la Rrunion, Pdtrologie, Facibs zdolite (Piton des Neiges), Sublimrs (La Fournaise). Thesis, Universit6 Paris-Sud. Nativel P., Joron J.-L., and Treuil M. (1979) l~tude p&rographique et grochimique des volcans de la Rdunion. Ball. Soc. Gdol. France 21, 427-440. Nrgrel P., Allbgre C. J., Dupr6 B., and Lewin E. (1993) Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: The Congo Basin case. Earth Planet. Sci. Lett. 120, 59-76. Observatoire Rrunionnais de l'Eau (ORE) (1994) Annuaire I4ydrologique. Obs. Rrunionnais de l'eau.

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Pinet P. and Souriau M. (1988) Continental erosion and large-scale relief. Tectonics 7, 563-582. Robert R. (1986) Climat et hydrologie a' la Rrunion. Th~se d'rtat, Univ. Montpellier. Stallard R.F. (19957 Relating chemical and physical erosion. In Chemical weathering rates of silicate minerals led. A. F. White and S. L. Brantley), pp. 543-564. MSA. Summerfield M. A. and Hulton N. J. (1994) Natural control of fluvial denudation rates in major world drainage basins. J. Geoph. Res. 99, 13871-13883. Thompson G. ( 1983 ) Basalt-seawater interactions. In Hydrothermal Processes at Seafloor Spreading Centers led. P. A. Rona et al.), pp. 225-278. NATO Conf. Ser. IV: Mar. Sci.

APPENDIX A The Model of Mixing Between Rain, Basalt Weathering, and Thermal Springs Inputs For each X dissolved species of a river, we can write the following mixing equations, in terms of the concentration ratios:

-t- afbasa 1 4- O/thermalsprings(Na) X ( ~ a )

(AI) thermalsprings where a#(Na) is the Na proportion provided by # = rain, basalt weathering, or thermal springs with araln(Na) + O/basaltweathering(Na) + Ofthenna1~p~ing(Na) = 1. For Cl, only rains contribute to the chemical compositions of the rivers, thus:

( NC~al)a ri........ = O/rain(ma) X (NC~la)rain

(A2)

We also can write a mixing equation for the strontium 87Sr/86Sr isotopic ratio (noted R#):

R1i......... X

~aa

= o~(Na)~aln × Rraln X

~

k

(A3) + oz(ma)th......... ings X Uthemlalsprings x ( S z ) \ ~ , - / thermalsprings From the determination of the proportions of Na arising from each endmember (a#(Na)), we can deduce these proportions for all the other X dissolved species involved, writing the equations

Using the previous mixing Eqns. A1, A2, and A3 for all the X dissolved species (Ca, Mg, Sr, K, Rb, Ba, Li, SO4, SiO2, and HCO 3), we obtain thirteen equations and forty-nine parameters from which the three a~(Na) are the unknowns. Thus, we have an over-constrained system, the number of equations being much greater than the number of unknowns. For such systems, an adequate method of resolution is to use inverse technique (introduced in geochemistry by Minster et al., 1977). In the used formalism, data and unknowns are all a priori more or less known parameters (with a priori errors). Given these parameters and their uncertainties, the idea is simply to find the best set of parameters (the a posteriori parameters), among these that verify the model equations. Technically, an optimisation program is used to compute

3668

P. Louvat and C. J. All6gre

the a posteriori set of parameters by minimisation of the distances between the a priori set of parameters and the solution, and by propagation of the errors (All~gre et al.. 1983a, 1983b). In the resolution method by inversion, the choice of the a priori parameters and their errors is the determining step, and this must be done with the upmost care. The ten X/Na ratios and the 87Sr/86Sr isotopic ratio of each fiver water are well known, their uncertainties being those of the analytical measurements. The ten X/Na ratios of each of the three endmembers (Table 3) are known with less precision, being estimated from the previous study of the chemical composition of the fiver waters (see Section 5.1 ). The 87Sr/86Sr isotopic ratio of basaltic rocks is very homogeneous at R6union island (0.70420 _ 0.00005, Albar~de and Tamagnan, 1988). We also use this ratio for the thermal spring endmember, the signature of the basaltic rock being preserved during the high temperature water-rock interaction. For the 87Sr/86Sr isotopic ratio of rain water, we take the mean modern oceanic 87Sr/86Sr ratio of 0.70907 _+ 0.00004 (Burke et al., 1982). Finally, the three a#(Na) are the most poorly known parameters, they range between 0 and l, and their sum must be equal to 1. From the C1/Na ratios of the river water we can also infer that, for most of the samples. ~ra~n(Na) should be lower than 30% (o/~am(Na) is thus 0.3 -2--0.3). The thermal spring contribution also cannot be dominant for Na, so O/rbcm~al~p~i~g(Na) nor should exceed 20% (0.2 _+ 0.2). Finally, the remaining 50% is attributed to basalt weathering, thus the O/basalt wo,,bo~i.g(Na) a priori value is 0.5 _+ 0.5. For the Rivi6re des Marsouins, Rivi6re des Fleurs Jaunes, Rivi~re du Mfit, Bras de Cilaos, Bras Rouge, Bras Benjouin, Rivibre St l~tienne, Rivi~re des Galets, and Rivi6re Ste Suzanne, we used the Piton des Neiges basalt weathering endmember. For the Rivibre Langevin, Anse des Cascades, and Rivi~re de l'Est, we computed a mixing between the Piton de la Fournaise basalt weathering endmember and the rain endmember only. no thermal springs having been recognised for that massif. We calculated the a posteriori sets of parameters for these thirteen streams sampled in February 1995 (the results are given in Table 4, represented in Fig. 4 and discussed in Section 5.2). We generally have a good agreement between the a priori and the a posteriori sets of parameters, the calculated parameters being in the range defined by the a priori parameters _+ their a priori errors. For the a priori most poorly known parameters we, of course, gain precision.

elements we write nine equations that correspond to each insoluble elements), one equation (B1) (for Th) and the seven equations (B3). As for the inverse resolution of the dissolved load mixing model, the definition of the a priori set of parameters is of crucial importance. These a priori parameters and their errors are reported in Table 5. The a priori suspended load element concentrations we choose are the measured ones with an uncertainty of 10%. For the soluble elements, the a priori [X ] s~uspendedload values we chose are the mean basaltic rock concentrations (from Albar~de and Tamagnan, 1988, and Nativel et al., 1979) plus or minus 50%. The a priori element concentrations of the basalt are also these mean concentrations, and their a priori errors are their standard deviations. They are, however, slightly different for the samples of the Cirque de Cilaos (Bras de Cilaos river) and Cirque de Salazie (Rivi6re du Mfit river) for which we suggest a mixing of trachytic type rocks within the local basalts. The proportions of trachytic type rocks are calculated on the basis of the most differentiated insoluble element (La, Th, Ce, and Co) concentrations in the suspended load of the rivers, and in the mean basalts and mean trachytes. These calculated proportions are 40 _+ 10% for the suspended load of Bras de Cilaos River, 10 _+ 10% for the suspended load of Rivibre St t~tienne river, and 5 _+ 5% for the suspended load of Rivi~re du Mfit river. The proportion for the Bras de Cilaos river seems relatively high if we compare it to the 10% global proportion of differentiated rocks for the whole Piton des Neiges massif (Deniel, 1990). However, the differentiated rocks of the Piton des Neiges principally expose in the Cirque de Cilaos and Cirque de Salazie. One of the tributary of the Bras de Cilaos river drain the area of an important syenitic intrusion, the Pain de Sucre, in the Cirque de Cilaos. The resolution of the set of equations by an inverse method is achieved in the same way as was discussed previously, by iterative minimisation of the distances between the a priori set of parameters and an a posteriori set of parameters that best verify the model's equations. The obtained chemical compositions of the local basalt for each river's drainage basin and [X ] *s~,,dedIt,d are listed in Table 5. The a posteriori values for all the model parameters are in the range of their a priori values _+ their a priori errors. The a priori most unknown parameters ( local basalt concentrations and suspended load soluble element concentrations before weathering) gain precision during the resolution by an inverse method, when the a priori better known parameters (suspended load element concentrations) keep their a priori uncertainties.

APPENDIX B APENDIX C Determination of the Chemical Composition of the Local Basalt for Each of the Drainage Basin

The Steady-State Model of Erosion

For Y and Z, two insoluble elements (La, Th, Ce, Nd, Sm, dy, Er, Yb, Co). we can write the following equations:

We can write a mass budget equation for each X chemical element that constitutes the rock:

[Y]susvended [Z ]~p~od~dh,.d h,ad

__

[Ylb,~h and [Z ]bas.l~

[Y]~u~p~.d~d Io,d --< 1 [ Y ]basalt

gbasah X [X]basah = Msuspendedloa d X [ X ]~uspended load

(B1)

For the X soluble elements (Na, K, Ca, Mg. Sr, Rb. Ba), we can also deduce the local basalt concentrations using soluble vs. insoluble element concentration ratios: I X ] s~'uspendedload -- [ X ]basalt [Ylsus~ndedload I X ] ~uspended hlad

(B2)

[Y]ha~a,~

1

basalt weathering

+ M~i........ × [X ]d. . . . . ,c~

(B3)

I X ]basalt

where Mb.~a~, is the mass of basalt subjected to the weathering, M~.~po.ded k,~d is the mass of suspended sediments carried by the river, and Mr~...... ,¢r is the mass of water in the river. [X]ba~d~ and [X]~o~p~.d~d~o~dare the concentrations of an element X in the basalt and the suspended load. and [X ] ~i~w'~[~he~mgis the concentration of this element in the dissolved load of the river, arising only from basalt weathering (after correction of the rain inputs and eventually of the hydrothermal spring supply). Dividing by Mr~....... ,~r, we can refer every mass to one liter of river water: f X ] [~asaltweatbering "/-basalt X [X]basal t = P × [X L,*p~ndedload + t J ...........

where [X] s~uspendedload is the concentration that should have X in the suspended load if there has not been a partial dissolution of the basalt. This results in an over-determined system with more equations and parameters than unknowns, and once again an inverse method seems to be the easiest way to resolve it. The set of equations we use are the sixty-three equations (B2) (for each of the seven soluble

where "/"basaltrepresents a total denudation flux (mass of basalt chemically and mechanically eroded per volume of water) and P is the suspended sediment concentration in the river. As the most soluble elements (Na, K, Ca, Mg, Sr, Rb, Ba) are the most affected by fractionation between dissolved and suspended loads during basalt weathering, and as the Sr concentration is well

Denudation rates at R~union determined, we can write the following concentration ratio equations: [X]b~alt

[ Sr ]ba,ah

-- O~ X

basalt weathering [X]ri ........ ~basalt weathering [ S r ] ri. . . . . . . .

(l - ~) ×

[ X ]suspended load

[ Sr ] suspended

(C1)

load

For insoluble elements (La, Th, Ce, Nd, Sm, Dy, Er, Yb, Co), these equations can be simplified: Ybasalt

__ (1 - o~) X

[ Sr ]basalt

[Y]suspendedload

(C2)

[ Sr ]suspendedIoed

The definition of ~ is: 1

[ Sr ] biaSarltwW~athe r in g

"/-basalt

[ Sr]~u~peeded

o~ is representative of the chemical weathering efficacy. If oL is close to 0, the basalt is weakly weathered, and if c~ is close to 1, the chemical weathering is very efficacious. However, c~ is defined relative to the Sr concentrations in both the dissolved and suspended loads and would be different if defined relative to another soluble

3669

element because of their different solubilities during basalt weathering. Given the definition of c~, the concentration of the suspended load in the river is given by: ( 1 - O/) [Sr],.~basalt . . . . . .weathering .. P - - x (C3) a [ Sr ] suspended load In practice, we compute the a and P parameters and the local basalt chemical compositions using a single inversion method. We add the six equations (C1) (for the soluble elements Na, K, Ca, Mg, Rb, Ba), the nine equations (C2) (for the insoluble elements La, Th, Ce, Nd, Sm, Dy, Er, Yb, Co) and the equation (C3) to the previous seventy-one equations used for the determination of the local basalt chemical composition of each drainage basin. The a priori and a posteriori parameters and their errors are given in Table 5. The a and P parameters, which are the most poorly known in the model, gain much precision but their a posteriori errors remain high, due to the variability of the a priori chemical compositions of the local basaltic rock used in this model. Values of a are very low, ranging between 0.031 for the Rivi~re de l'Est river to 0.039 for the Rivi~re des Galets river, and are a good reflection of the low weathering state of the suspended load of the rivers.