Estuarine, Coastal and Shelf Science 67 (2006) 313e320 www.elsevier.com/locate/ecss
Geochemical variations in estuarine sediments: Provenance and environmental changes (Southern Spain) N. Lo´pez-Gonza´lez a,*, J. Borrego a, F. Ruiz b, B. Carro a, O. Lozano-Soria a, M. Abad b a
Departamento de Geologı´a, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus El Carmen, 21071 Huelva, Spain b Departamento de Geodina´mica y Paleontologı´a, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus El Carmen, 21071 Huelva, Spain Received 27 December 2004; accepted 29 November 2005 Available online 19 January 2006
Abstract The geochemical characteristics of the sedimentary infilling in the central basin of the Odiel River estuary have allowed the determination of the sediment provenance and the environmental conditions during the last 10,000 years. Thus, sediment provenance can be divided into three stages: (1) 9060e6500 years BP: the estuarine valley materials became the main sediment source because of intense erosion and sedimentation under transgressive conditions; (2) 6500e3200 years BP: the stabilization of the sea level and the current dynamic conditions involved an erosion of the Neogene and Quaternary formations that acted as sediment sources. At approximately 5705 cal. year BP, a high-energy marine event took place, suggesting that a tsunami could have affected this area; (3) 3200 years BP to present: the high sedimentation rate of the previous stage and the partial closure of the estuary, allowing the acid fluvial discharges from the Tinto and Odiel to reach the estuary and become the present main source of sediments. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: geochemistry; sediment provenance; environmental changes; Odiel estuary
1. Introduction The chemical composition of recent sediment deposits depends on geological, biological, and climatic factors affecting the weathering of both source rocks and soils (Nesbitt and Young, 1996). Numerous investigations that focused on sedimentary rocks have shown the importance of knowledge of the sediment chemistry in the determination of the origin of these rocks (Fralick and Kronberg, 1997; Singh and Rajamani, 2001), but only in recent studies have the usefulness of sediment chemical composition been used in paleogeographic reconstructions and stratigraphical sequence analysis of modern sedimentary environments (Condie et al., 1992; Fralick and Kronberg, 1997; Singh and Rajamani, 2001). Sediment geochemistry can also be very useful as tracers of environmental
* Corresponding author. E-mail address:
[email protected] (N. Lo´pez-Gonza´lez). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.11.028
changes induced by natural and anthropogenic causes (Leblanc et al., 2000; Borrego et al., 2004; Garcia et al., 2004). In most cases, concentrations of major elements are used to classify rocks and sediments (Herron, 1988) and to build up variation charts between elements and compounds. In the latter case, it is common to examine the relationships between the different elements and the geochemical processes that influence their distribution (Rollinson, 1993; Vital et al., 1999), with special attention focused on the weathering processes that affect the source rock of sediments or the determination of the tectonic environment where the source rock was formed (Cox and Lowe, 1995). The Odiel estuary (southwest of Spain) is a coastal environment known worldwide. It is highly polluted and the causes of pollution are: (1) the large amounts of suspended and dissolved trace elements that arise the acid drainage of the Iberian Pyrite Belt, the biggest sulfide ore mining area in Europe and (2) the presence of an industrial complex in the central basin of the estuary, including chemical and basic factories,
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petroleum refineries, and a paper mill (Grande et al., 2000; Leblanc et al., 2000; Borrego et al., 2002). The objective of this paper is the geochemical characterization of the different sedimentary bodies that constitute the infilling of this estuary. The results obtained will be the basis for the recognition of its origin and the definition of Holocene environmental changes. 2. The Odiel estuary The estuary of the Odiel River is a bar-built system (cf. Fairbridge, 1980) located in the southwestern Spanish coast (Fig. 1). The inner part of this coastal environment consists of wide tidal flats and salt marshes developed above the estuarine accretionary bodies of fluvialetidal origin (Borrego et al., 1999). The mouth of this estuarine system consists of three geographical elements (Fig. 1), separated by two channels (Punta Umbrı´a and Padre Santo): (1) the Punta Umbria spit, to the west; (2) the Saltes Island, which comprises a complex system of sandy ridges, subparallel to the coastline; and (3) the Torre Arenillas spit, developed on the eastern margin and directly linked with a Plio-Pleistocene cliff. The sedimentary evolution of this estuary has been analyzed in several papers using sediment cores (Borrego et al., 1999; Dabrio et al., 2000). According to these studies, its
Holocene infilling started at about 9060 cal. years BP (Lario et al., 2002), when it was invaded during the last postglacial transgression and involved continuous rise in the sea level for a long period of time. This transgressive phase was completed approximately 6500 years ago when the sea level reached its present level. Sandy barriers (Punta Umbria and Torre Arenillas spits) were formed later, with the closing of the estuarine mouth and the subsequent development of tidal flats and marshes in the inner areas. The Iberian Pyrite Belt constitutes the main geological substratum of the Odiel River drainage network. It consists of three stratigraphical groups: (1) a Devonian PhylliteeQuartzite formation (PQ), with alternating shales and quartzites that contain local lenses of conglomerate and carbonates; (2) a Carboniferous Volcanic Sedimentary Complex (VSC), represented by subaerial to marine felsic and mafic volcanic rocks and epiclastic volcanic sediments that includes important sedimentaryevolcanic sulfide ores; and (3) the upper Culm group (Carboniferous), composed of a succession of shales, graywackes, and turbidite units (Moreno, 1993). Near the mouth of the estuary, the Holocene sediments were deposited over MioceneePliocene siliciclastic sediments formed in marine and continental environments (Civis et al., 1987). This Tertiary succession consists of basal gray-blue clays and silt (Gibraleon Clay Formation, GCF) and upper
Fig. 1. Geological setting of the Odiel River basin and the Bacuta core situation in the Odiel estuary.
N. Lo´pez-Gonza´lez et al. / Estuarine, Coastal and Shelf Science 67 (2006) 313e320
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fine sands and gray-yellow silt (Huelva Formation). These formations constitute a large system of cliffs distributed along the coastline that surrounds the estuary. 3. Methods A continuous core (50 m in length) was obtained in Bacuta Island, located near the main channel of the Odiel River (Fig. 1). In a first step, the main lithostratigraphic units were delimitated, with the later geochemical analysis of 17 samples collected from the different lithologies. The two samples that were shallowest were not analyzed in this work because they belonged to an artificial infilling carried out by the Huelva Harbor along the first half of the 20th century. The wet samples were air-dried at room temperature and divided in two subsamples. One subsample (20 g) was used for particle size analysis, with the clayesilt contents estimated by a ZM model Coulter particle counter. The subsamples for chemical analyses were sieved (2 mm) to remove large particles and powdered in an agate mortar after drying. Chemical analyses were performed on the bulk samples by X-ray Assay Laboratories, Toronto, Canada. Metal concentrations were determined by X-ray fluorescence (SiO2, Al2O3, Fe2O3, MnO, CaO, TiO2, K2O, Na2O, and P2O5) and inductively coupled plasma mass spectrometry (Ni, Cr, Cu, Zn, Y, Ba, Pb, V, Sc, and Co), with previous nitric aqua regia digestion. Calibration was based on over 40 international standard reference materials. Dating of samples 9 and 15 (Fig. 2) were done at the Geochron Laboratories (Massachusetts, USA) by radiocarbon analysis of mollusk shells (mainly ostreidae). Data were calibrated using CALIB version 4.2 (Stuiver and Reimer, 1993) and the Stuiver et al. (1998) calibration data set. The results correspond to calibrated ages (ca.) using 2s intervals, with a reservoir correction (ÿ440 85 years) as suggested by Dabrio et al. (2000) for this area. Ages discussed below are expressed as the highest probable age of the 2s calibrated range (e.g., Van der Kaars et al., 2001). 4. Results 4.1. Lithological description Sedimentological analysis permits the delimitation of six Holocene lithological units (Fig. 2) disposed over gray-blue mud belonging to the GCF, which constitutes the lower 12 m of the core. The overlying 5 m is made up of gravels and very coarse to medium sands with a matrix of brownish to yellowish silty clays (unit 1). The next 11.3 m consisted of green clayey silts (unit 2) with high percentages (50e70 wt%) of fine- to medium-grained silts. The occurrence of some marine bivalves (Acanthocardia aculeata, Corbula gibba) and gastropods (Hinia reticulata) is frequent near the base of this unit. Unit 3 consists of two subunits. The lower subunit (3A) is formed by 10.5 m of coarse- to medium-grained yellow sands with scattered fragments of mollusks. The upper subunit (3B) constituted of 1.5 m of fine sands that include an accumulation layer of shell fragments with numerous bivalves
Fig. 2. Stratigraphic succession of the Odiel central estuarine basin. It also indicates two dated samples calibrated according to Dabrio et al. (2000).
(Acanthocardia aculeata, Corbula gibba, Chamelea gallina, Ostrea edulis), gastropods (Calyptraea chinensis, Cymbium olla, Hinia reticulata), and fragments of both scaphopods and anthozoans. The thickness of this layer is 37 cm, with a lower limit that is very sharp and erosional. Unit 3 is overlain by 4 m of very bioturbated and laminated silty sands (unit 4) with high proportions of very fine sands and increasing clay contents in the upper samples. The next 2 m (unit 5) zone is formed of gray to black clayey silts with important percentages of fine and very fine silts (40e 55 wt%) and clays (10e15 wt%). The base of this unit presents a strong bioturbation, with numerous burrows filled by very fine sands. Finally, unit 6 is made up of red clayey silts strongly bioturbated by roots, with fine to very fine silts (52e 65 wt%) dominating over clays (14e20 wt%). 4.2. Geochemical analysis Results from geochemical analysis are shown in Table 1, including mean concentrations of the rocks that can be
316 Table 1 Concentrations of major (%) and trace (ppm) elements, LOI and mud content (wt%), and SiO2/Al2O3 ratios in Bacuta core sediments. Mean values of major (%) and trace (ppm) elements from Bacuta core, Gibraleon Clay Formation (GCF), and PQeCulm formations are also displayed SiO2
Al2O3
Fe2O3
Unit 6 3 4 Mean
45.9 52.4 49.2
13.8 18.7 16.3
16.6 7.6 12.1
Unit 5 5 6 Mean
52.9 65.6 59.3
17.2 11.8 14.5
Unit 4 7 8 Mean
72.3 74.2 73.3
Unit 3 9 10 11 12 Mean
CaO
MgO
Na2O
K2 O
MnO
TiO2
P2O5
Ni
Cu
Zn
Pb
Cr
1.4 1.0 1.2
1.6 2.1 1.9
2.2 2.0 2.1
2.0 3.0 2.5
0.10 0.05 0.08
0.8 1.0 0.9
0.2 0.2 0.2
18 28 23
906.0 175.0 540.5
1370.0 412.0 891.0
234 81 158
38 48 43
7.6 5.0 6.3
0.9 2.2 1.6
2.1 1.5 1.8
2.2 2.0 2.1
2.9 2.3 2.6
0.05 0.04 0.05
1.0 0.8 0.9
0.1 0.1 0.1
24 14 19
278.0 18.8 148.4
170.0 47.3 108.7
101 16 59
9.7 8.3 9.0
3.5 3.0 3.3
2.4 3.2 2.8
1.2 1.0 1.1
1.9 1.7 1.8
1.9 1.7 1.8
0.03 0.03 0.03
0.7 0.6 0.7
0.1 0.1 0.1
9 8 9
15.6 8.1 11.9
38.1 29.4 33.8
71.7 86.7 86.9 90.6 84.0
3.8 3.4 4.2 2.6 3.5
1.7 1.2 1.3 0.7 1.2
10.2 3.0 2.0 1.9 4.3
0.4 0.3 0.4 0.2 0.3
0.7 0.7 0.8 0.5 0.7
0.8 1.0 1.3 0.9 1.0
0.02 0.01 0.01 0.05 0.02
0.4 0.3 0.3 0.2 0.3
0.0 0.0 0.0 0.0 0.0
4 2 3
