Relationship between Late Pleistocene sealevel

Sedimentology (2012) 59, 1640–1658

doi: 10.1111/j.1365-3091.2011.01319.x

Relationship between Late Pleistocene sea-level variations, carbonate platform morphology and aragonite production (Maldives, Indian Ocean) ¨ RN FU ¨ RSTENAU, HANNO KINKEL and A NDREAS PAUL*, JOHN J. G.REIJMER*, JO CHRISTIAN BETZLER *Department of Sedimentology and Marine Geology, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands (E-mail: [email protected]) Department of Geosciences, University of Hamburg, Bundesstrasse 55, 20146, Hamburg, Germany Graduate Centre ‘Human Development in Landscapes’, University of Kiel, Neufeldstrasse 10, 24118 Kiel, Germany Associate Editor – Bernhard Riegl ABSTRACT

A piston core from the Maldives carbonate platform was investigated for carbonate mineralogy, grain-size distributions, calcium carbonate content and organic carbon. The sedimentary record was linked to Late Pleistocene sealevel variations, using an age model based on oxygen isotopes obtained from planktonic foramanifera, nannofossil biostratigraphy and 14C age determinations. The correlation between the sedimentary record and Late Pleistocene sea-level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the Maldives carbonate platform. Platform flooding events were characterized by strongly increased deposition of aragonite and mud within the Inner Sea of the Maldives. Exposure events, in contrast, can be recognized by rapid decreases in the values of both proxy records. The results show that sediments on the Maldives carbonate platform contain a continuous record of Pleistocene sea-level variations. These sediments may, therefore, contribute to a better understanding of regional and even global sea-level changes, and yield new insights into the interplay between ocean currents and carbonate platform morphology. Keywords Highstand shedding, Indian Ocean, Late Pleistocene, Maldives carbonate platform, peri-platform ooze, sea-level.

INTRODUCTION Highstand shedding is a term widely used to describe the processes leading to the export of aragonite-rich, muddy sediments from the top of a carbonate platform into an adjacent basin, during periods of high sea-level (Droxler & Schlager, 1985; Glaser & Droxler, 1991; Schlager et al., 1994). Sediment production and export processes are generally initiated immediately after sea-level refloods the top of a carbonate platform following exposure during a lowstand in sea-level. As long as 1640

the water depth on the platform is shallow enough to sustain carbonate production ()30 m of water depth; see Schlager, 1981, for references), sediment export will continue. Basinal sedimentation rates are therefore elevated during sea-level highstands as compared with sea-level lowstands, but may vary with distance to the flat platform top and the platform slope (Rendle & Reijmer, 2002). Signs of local erosion, mass movement and re-deposition events may also be evident within a basinal sedimentary record. In particular, the occurrence of calciturbidites increases during sea-level high-

2011 The Authors. Journal compilation 2011 International Association of Sedimentologists

Sea-level and aragonite on the Maldives stands (Droxler & Schlager, 1985; Reijmer et al., 1988; Andresen et al., 2003); they also might, however, occur at transitions from highstands to lowstands in sea-level and vice versa (Droxler & Schlager, 1985; Rendle et al., 2000; Andresen et al., 2003; Lantzsch et al., 2007). Near carbonate platforms, periods of major sea-level highstands can also be identified as distinct intervals with sediments containing very high percentages of aragonite (Droxler et al., 1983, 1990; Boardman et al., 1986). Such an aragonite record resembles glacial–interglacial variations found in the planktonic oxygen isotope curve of the same sedimentary record, and thus can be used as a stratigraphic tool (Droxler et al., 1983; Reijmer et al., 1988; Glaser & Droxler, 1993) and/or determine flooding and exposure events related to such sea-level variations. This paper presents an approximately 150 000 year long record of basinal carbonate sedimentation from the Maldives carbonate platform. The Maldives carbonate platform is unique in terms of its bathymetric features and its location under the influence of the Asian Monsoon and associated wind and ocean currents. Hence, it represents an exceptional opportunity to study the characteristics of sediment production and export, namely highstand shedding, in the Indian Ocean. The interaction between rates of sea-level rise and the sediment production window are shown to be crucial parameters for sediment export patterns.

STUDY AREA

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water depth of more than 1000 m b.s.l. within only a few kilometres of the reef rims. The slopes facing the Inner Sea, however, reach down to water depths of only 200 m b.s.l. and pass gently over into the subhorizontal sedimentary basin. This basin, which serves as a depocentre for carbonate sediment, exhibits depths of 200 to 500 m b.s.l. in the study area (Fig. 1C), accompanied by a gradual northward-deepening trend. East–west-oriented channels are present between the single atolls, which connect the Inner Sea with the Indian Ocean. The channels exhibit steep walls, can be as deep as 400 m b.s.l. and generally deepen towards the open Indian Ocean.

Climate and oceanography The Maldives carbonate platform lies under the influence of the Asian Monsoon system. Strong south-westerly winds prevail from mid-May to November, accompanied by eastward currents and torrential rainfall (south-west monsoon, wet season; Schott et al., 2007). A reversed pattern of wind and ocean currents is developed from January to March, with very low precipitation rates (north-east monsoon, dry season). Mean annual temperatures lie between 25Æ7C and 30Æ5C, with the highest temperatures observed in April and the lowest in December. Mean annual precipitation is 1924Æ7 mm for the central Maldives, with precipitation maxima occurring from May to December. The Maldives are thus amongst the few carbonate platforms which have developed in a regime of strong annual changes in wind and current patterns.

Local setting and platform geomorphology The Maldives archipelago is an isolated carbonate platform, situated in the Northern Indian Ocean (Fig. 1A). It is one of the largest active carbonate platforms on Earth, with an estimated production area of 22 000 km2. The Maldives consist of a 900 km long and 120 km wide, north–south trending double row of 22 atolls, which are separated by a relatively deep sedimentary basin, the Inner Sea (Fig. 1B). Highly dissected outer reef rims characterize the atolls, while patch reefs and faros populate their inner lagoons. Reefs and reefal lagoons on Male´ atoll exhibit a depth range of 0 to 10 m below sea-level (b.s.l.), whereas water depths in the lagoons between those reef bodies exhibit an average of 45 ± 10 m b.s.l. (depth data from British Admiralty Charts BA 1013 and BA 3323). The platform slopes facing the open Indian Ocean are characterized by rapid increases in

Sea-level history Sea-level is the fundamental driving force behind flooding and exposure of the shallow-water production areas, and thus sediment production and export. Therefore, it is essential to understand the history of sea-level change in order to interpret the sedimentary record on and in the vicinity of carbonate platforms. Knowledge from the Maldives carbonate platform is limited to postglacial to Holocene reef records (Gischler et al., 2008; Fu¨rstenau et al., 2010). Sea-level records from the Red Sea and Arabian Sea, however, may reflect similar amplitudes of sea-level changes as they might have occurred on the Maldives. During the late Marine Isotope Stage 6 (MIS 6; 130 to 150 ka bp), sea-level was as low as -90 ± 15 m (Siddall et al., 2003; Rohling et al., 2009). Siddall et al. (2003) further showed that

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Fig. 1. (A) Location of the Maldives carbonate platform in the Northern Indian Ocean. (B) Position of core M74/ 4-1095 (4Æ2622 N, 73Æ2458 E, 328 m b.s.l.) in the eastern Inner Sea of the Central Maldives carbonate platform. The core is situated approximately 11 km from the closest reef body of Male´ atoll. Contour lines indicate the bathymetric features of this carbonate platform. Blue = reef bodies. (C) East–west cross-section (green line) through the Maldives carbonate platform, demonstrating their mega empty-bucket structure and general submarine geomorphology.

from approximately 130 ka bp onwards, sea-level rose quickly towards its peak of between +9Æ7 m (Rohling et al., 2009) and +17 m (Siddall et al., 2003) during MIS 5e (123 to 126 ka bp). Deposits of this age were found on Rashdoo atoll and described by Gischler et al. (2008). This period was followed by a relatively slow fall in sea-level until ca 115 ka bp, after which it stabilized around an average of -47 m during MIS 5a to 5d (71 to 115 ka bp). Nonetheless, extreme values of ±20 to 30 m sea-level change relative to the average occurred regularly and were most pronounced during stadials 5b and 5d (Siddall et al., 2003; Rohling et al., 2009). Marine isotope stage 4 (57 to 71 ka bp) was characterized by an average sea-level similar to that of MIS 6. Marine isotope stage 3 (29 to 57 ka bp), in contrast, was characterized by a slightly higher sea-level when compared with MIS 4, with an average of -75 m.

During the last glacial maximum (LGM; 26Æ5 to 19Æ5 ka bp; Clark et al., 2009), sea-level reached values as low as -120 m in the Western Indian Ocean (Camoin et al., 2004). Camoin et al. (2004) further showed that from the end of the LGM on, starting at ca 19 ka bp, sea-level rose quickly towards present-day values, with a short interruption between 12Æ5 and 11Æ5 ka bp. Holocene reef growth began around 8Æ5 ka bp (Gischler et al., 2008). These authors identified the base for those Holocene reefs at 14Æ5 m b.s.l. on Rasdhoo atoll. Present-day sea-level was reached at approximately 2Æ5 ka bp.

MATERIALS & METHODS The 9Æ70 m long piston core M74/4-1095 (4Æ2622N, 73Æ2458E), discussed in this study,


Sea-level and aragonite on the Maldives was recovered during leg M74/4 of the German R/V Meteor in December 2007. The core was obtained on the western slope of Male´ atoll, in a water depth of 328 m b.s.l., approximately 11Æ5 km away from the nearest shallow-water reef structure (Fig. 1B). The core description included an estimation of grain size, qualitative content of macrofossils (foraminifera, pteropods, molluscs and echinoderms) and rock classification according to Dunham (1962). The core was sampled with disposable PVC-syringes at 5 cm intervals from 0Æ24 m on downwards, since the first 0Æ24 m of the core were completely disturbed due to the very high water saturation of this sediment. Each sample was freeze-dried for at least 38 h and then split into different portions for analyses. The portion designated for chemical and mineralogical analyses was ground manually using an agate mortar.

Planktonic stable oxygen isotopes Planktonic stable isotope ratios (d18O) were measured on the planktonic foraminifer Globigerinoides ruber (white), at the VU University Amsterdam, The Netherlands. Five to seven specimens were picked from the 250 to 355 lm fraction of each sample. Ethanol (96%) was added to each sample, which was ultrasonified for 30 sec. Excess ethanol was subsequently drawn off with strips of tissue paper and the samples were left to dry. A final ‘clean-check’ under a binocular was carried out in order to confirm a successful cleaning process. The foraminifera, the house-internal standard VICS and the control standards National Bureau of Standards (NBS)-18, NBS-19 and NBS-20, were then dissolved in warm phosphoric acid using a GasBench II. The generated CO2 gas was injected into a Thermo Finnigan DELTA Plus mass spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). Due to the internal machine configuration, the measured isotope ratios were generally offset from the real values. Based on the isotope ratios of the internal standards, an offset-function was computed. The isotope values of the samples were recalculated using this function. The NBSstandards were used to check the successful recalculation. Standard error was between 2h. Values are displayed in & relative to the international standard Vienna Pee-Dee Belemnite (V-PDB).

Nannofossil biostratigraphy The automatic coccolithophorid identification software SYRACO (Beaufort & Dollfus, 2004),

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installed on an Apple Mac Pro workstation, was used to locate the biohorizon marked by the crossover from the Emiliania huxleyi zone to the E. huxleyi global acme zone, between 82Æ0 and 63Æ2 ka bp (Raffi et al., 2006). For this analysis, smear slides of bulk sediment sampled at 0Æ10 m, and occasionally at 0Æ05 m intervals, were scanned using a digital microscope camera in order to produce high-resolution images that were loaded into the software. Subsequently, the amounts of the three most common coccolithophorid species were counted: E. huxleyi, Gephyrocapsa ericsonii and Florisphaera profunda. This procedure allowed for the quick characterization of the main coccolith fauna in the sediments. Analyses were conducted at the University of Kiel, Germany.

Radiocarbon dating Three samples of the grain-size fraction 1 g) of each sample was washed through a 63 lm sieve using a tap water shower. The fine sediment fraction was collected in 5 l buckets and left to settle for at least 24 h. The excess water was decanted, the remaining sediment rinsed with distilled water and dried at 50C. The coarse fraction was washed again with distilled water, and also left to dry at 50C. Bulk sediment was weighed before and the coarse fraction was weighed after the sieving process. The weight for the coarse fraction was then subtracted from the weight of the bulk sediment, which gave the weight of the fine fraction.

Calcium carbonate content and organic carbon A Flash EA-1112 NC analyzer (Thermo Fisher Scientific Inc.) was utilized to determine the content of organic carbon and calcium carbonate at the VU University Amsterdam, The Netherlands. Therefore, the amounts of total carbon (TC) and total organic carbon (TOC) incorporated in the sediments had to be identified. TC was determined by loading 10 to 15 mg of each sediment sample (ground and dried at 50C) into tin capsules, burning these capsules at 900C and measuring the conductivity of the generated gas. Procedures to measure TOC involved weighing 15 to 20 mg of sediment (ground and dried at 50C) into silver capsules, placing the capsules in a desiccator and exposing them to hydrochloricacid fume (from conc. HCl 37%, in a small bowl at the bottom of the desiccator) for at least 72 h in order to remove the bulk of calcium carbonate. This step was followed by placing the samples on a hotplate at 50C, and adding 2 ll of 10% hydrochloric acid to remove the remaining calcium carbonate. The dried silver capsules were then packed into tin capsules and again burned at 900C. The difference between TC and TOC yields the total inorganic carbon (TIC) content. The value for TIC was multiplied with the constant 8Æ33 to obtain the content of calcium carbonate. Double measurements were performed for each of the two analyses. If the standard deviation between two measurements of one sample was higher than 5% (TC) and 10% (TOC), respectively, a sample was measured one

more time and the average of the three measurements was used.

Mass accumulation rates Bulk mass accumulation rates (bulk MARs) for core M74/4-1095 were calculated after Ehrmann & Thiede (1985) using Eq. 1. This equation includes linear sedimentation rates (LSR), which were calculated for a specific time interval as, for example, MIS 6, MIS 5e, etc., and dry bulk density (DBD). The latter was calculated beforehand using Eq. 2, which includes qDB = dry bulk density, qx = density of fraction x and Px = percentage of fraction x in bulk sediment. This approach represents a straightforward method by which the dry bulk density of large amounts of samples can be calculated in a short period of time, using data that is necessary for sedimentological laboratory work. The total flux of calcium carbonate was calculated by simply multiplying the bulk MARs with the average percentage of calcium carbonate within a discrete stratigraphic interval. Finally, carbonate mineral fluxes (aragonite, HMC and LMC) were calculated by multiplying the MARs for calcium carbonate with the average percentage of the respective component over the interval in question: Bulk MAR

cm g g DBD ¼ LSR cm2 ka ka cm3 ð1Þ

qDB ¼ ðqAragonits PAragonits Þ þ ðqCalcite ðPHMC þ PLMC ÞÞ PCaCo3 Þ þ ðqQuartz PResidue Þ þ ðqMOC PCorg Þ

ð2Þ

RESULTS

Core description The sediments analyzed consist of olive to olivegrey peri-platform ooze. The sediments have a wackestone to packstone texture (after Dunham, 1962, modified for loose sediment) and appear to be highly bioturbated throughout the core. A thin (1 cm), light-greyish and sharply confined layer was encountered at 7Æ90 m. Visual estimates of fossil content indicate the dominance of planktonic foraminifera, such as G. ruber, G. sacculifer and Globorotalia menardii (complex). Maxima of the planktonic foraminifera occur between approximately 5Æ0 m and 8Æ0 m along the core, with the exception of Gr. menardii.


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Fig. 2. (A) The reference curves used for comparison with the planktonic stable oxygen isotope curve of core M74/41095 (B). Thick red line at (B) indicates a 3-pt moving average. Blue = Lisiecki & Raymo (2005); Green = Ivanova et al. (1999). Major interglacial periods, their sub-stages, the last glacial maximum (LGM) and the Younger Dryas (YD) cold period are indicated. The three stars in the lower part indicate positions for the radiocarbon datings in core M74/ 4-1095 (Table 2). Nannofossil biostratigraphy in the lowermost part indicates the transition from the Emiliania huxleyi zone towards the E. huxleyi global acme zone (ca 75 ka bp). The core reaches a maximum age of approximately 150 000 years before present. The sediments recorded all Marine Isotope Stages (MIS) within this interval and yield a very good, highly resolved stratigraphic record.

The latter foraminifer appears to be most abundant at depths of between 1 m and 3 m. Secondary components consist of pteropods, foraminiferaaggregates and ostracods. Pteropods are abundant in two intervals, between approximately 1Æ0 to 4Æ0 m and 8Æ0 to 9Æ70 m. All other foraminifera do not seem to exhibit specific trends.

Planktonic oxygen isotopes A 3-pt moving average was used to filter the oxygen isotope record of core M74/4-1095 (Figs 2B and 3A). The curve shows distinct variations with a maximum of )0Æ40& and a minimum of 3Æ34& (range: 2Æ94&; mean: -1Æ60&). The lightest


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values can be observed between 0Æ25 to 0Æ90 m and 8Æ20 to 8Æ70 m, while the heaviest values occur between 1Æ50 to 2Æ50 m and 8Æ90 to 9Æ70 m, respectively. Core-upward transitions from intervals with predominately heavy values towards intervals with light values show a stepwise transition; this is particularly well-developed between 0Æ90 m and 1Æ60 m, but is also visible between 8Æ70 and 9Æ00 m.

Nannofossil biostratigraphy & radiocarbon dating Of the three species analyzed, F. profunda represents the least abundant coccolith species in the sediments which is, however, increasing up-core. The remaining two species, E. huxleyi and G. ericsonii, exhibit higher but equal abundances. Both species show a high negative correlation of R = )0Æ86. The crossover biohorizon, which marks the transition from 82 to 63Æ2 ka bp, could clearly be identified between approximately 4Æ95 and 5Æ10 m (Fig. 2B). Accelerated-mass spectrometry radiocarbon dating of three samples of the fine sediment fraction yielded reliable ages, covering the last glacial and the Holocene (Table 1, Fig. 2B).

Age model The age model of core M74/4-1095 was constructed by linking the planktonic oxygen isotope curve to the global benthonic isotope stack of Lisiecki & Raymo (2005) and the planktonic isotope record from Ivanova et al. (1999), using the free software AnalySeries 2Æ0 (Paillard et al., 1996). The resulting age model (Fig. 2) is complemented by the nannofossil biostratigraphy, and the 14C age determinations. The bottom of the core at 9Æ70 m is dated ca 150 ka bp, which corresponds to middle to late MIS 6; this yields an average temporal resolution of 0Æ154 ka per 1 cm of sediment or 0Æ772 ka per sampling interval. Marine isotope stage 5 suc-

ceeds MIS 6 at 8Æ85 m. The sub-stages of MIS 5 were assigned based on the occurrence of the lightest and heaviest peaks in each of the intervals between 4Æ90 m and 8Æ90 m. This assignment is tentative, however, related to evident noise in this interval. Marine isotope stage 4 (57 to 71 ka bp) seems to be very thin and could not be identified with great certainty, but it probably occurs between 4Æ60 m and 4Æ90 m. This observation is supported by the results of the biostratigraphic analyses, which places the transition from MIS 5 towards MIS 4 at ca 75 ka bp between 4Æ95 m and 5Æ10 m. The correlation between both isotope curves is relatively straightforward up to the end of MIS 3 at ca 2Æ50 m. From 2Æ50 m upwards, however, both curves deviate strongly. While the values of the benthonic isotope stack decrease to present-day values until 10 ka bp, the values of core M74/41095 remain heavy until approximately 12Æ5 ka bp. The M74/4-1095 record reaches the same level as the benthonic stack at around 5Æ5 ka bp. This mismatch can probably be attributed to the fact that the reference stack incorporates benthonic, deep-water records, which only record major changes in ocean water d18O, while short-term variations do not affect the deepwater d18O record. In order to compensate for this apparent mismatch, an additional record from offshore Somalia (Ivanova et al., 1999) was used for comparison with the 0Æ00 to 2Æ50 m interval. This high-resolution record covers the LGM to the Holocene. The match of both curves is good and significantly improves the age model for the upper 2Æ50 m. It agrees well with the three conventional, un-calibrated 14C ages for this interval. However, of three calibrated ages, only two fit into the proposed age model, while the intermediate dating at 1Æ29 m does not. It was therefore decided to integrate the uncalibrated 14C dates into the age model of the core. Based on the age model for the uppermost 2Æ50 m of core M74/4-1095, it becomes clear that the LGM is succeeded by Termination I from

Table 1. Results of AMS 14C dating of the fine sediment fraction (