Monitoring the variations of the Socotra upwelling system during the last 250 kyr: A biogenic and geochemical approach

Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 243 – 259 www.elsevier.com/locate/palaeo

Monitoring the variations of the Socotra upwelling system during the last 250 kyr: A biogenic and geochemical approach He´le`ne Jacot Des Combesa,*, Jean Pierre Cauletb, Nicolas Tribovillardc a

Alfred Wegener Institute for Polar and Marine Research, P.O. Box 120161, Columbusstrasse, 27515 Bremerhaven, Germany b Laboratoire de Ge´ologie, Muse´um National d’Histoire Naturelle, 43 rue Buffon, 75005 Paris, France c UFR des Sciences de la Terre, Universite´ Lille I, 59655 Villeneuve D’ASCQ, France Received 22 October 2002; received in revised form 20 December 2004; accepted 1 April 2005

Abstract A combination of changes in the species composition of the radiolarian populations, and in the sediment chemical composition (content and mass accumulation rates of carbonate, organic carbon, and selected major and trace elements, with special attention paid to Ba) is used to reconstruct the variations in upwelling activity over the last 250 kyr in the Socotra gyre area (Somali–Socotra upwelling system, NW Indian Ocean). In the Socotra gyre (Core MD 962073 at 108N), the variations in upwelling intensity are reconstructed by the upwelling radiolarian index (URI) while the thermocline/surface radiolarian index (TSRI) testifies to productivity variations during nonupwelling intervals. Despite an origin related both to marine and terrigenous inputs, the geochemical records of organic carbon, silica, and trace elements (Ba, P, Cu, and Zn) normalized to Al are controlled by the variations in surface paleoproductivity. The data indicate a continuous increase in upwelling intensity during the last 250 kyr with a maximum activity within the MIS 3, while high productivity periods in between the upwelling seasons occurred both during glacial and interglacial intervals. A comparison of our data with published observations from another gyre of the Somalian upwelling area located at 58N in the Somali gyre area shows differences regarding periods of upwelling activity and their geochemical imprint. Three hypotheses are proposed to explain these differences: (1) changes in the planktonic community, resulting in more silica-rich deposits in the Socotra gyre, and more carbonate-rich deposits in the Somali gyre, that are controlled by differences in the source water of the upwelling; (2) a more important terrigenous input in the southern gyre; and (3) a different location of the sites relative to the geographic distribution of the upwelling gyres and hydrologic fronts. D 2005 Elsevier B.V. All rights reserved. Keywords: NW Indian Ocean; Upwelling; Geochemistry; Barium; Sequential leaching; Radiolarians

* Corresponding author. Tel.: +49 471 4831 1105; fax: +49 471 4831 1923. E-mail addresses: [email protected] (H. Jacot Des Combes), [email protected] (J. Pierre Caulet), [email protected] (N. Tribovillard) 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.04.007

1. Introduction The oceanic circulation in the northwest Indian Ocean is forced by the Indian monsoon, induced by

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a strong seasonal reversal of winds. This atmospheric process deeply influences the circulation of the surface water masses, and leads to the occurrence of several upwelling systems along the western border of the northwestern Indian Ocean, from the Arabian Sea to the Somali coast (Bruce and Beatty, 1985; Anderson and Prell, 1991, 1993; Clemens et al., 1991; Caulet et al., 1992). Several paleoceanographic reconstructions, using biogenic and/or geochemical markers, have already pointed out time variations in these upwelling systems (Clemens and Prell, 1991; Murray and Prell, 1991, 1992; Shimmield and Mowbrays, 1991; Zahn and Pedersen, 1991; Caulet et al., 1992; Anderson and Prell, 1993; Hermelin and Shimmield, 1995; Ve´nec-Peyre´ et al., 1995, 1997; Vergnaud-Grazzini et al., 1995; Tribovillard et al., 1996; Ouahdi, 1997; Ve´nec-Peyre´ and Caulet, 2000). The upwelling system of the Somali Basin is divided into two major distinct gyres: the Somali gyre, located at 58N, and the Socotra gyre, centered at 108N (Du¨ing et al., 1980; Swallow, 1980; Quadfasel and Schott, 1982; Schott and Quadfasel, 1982; Fieux, 1987). Sediments from piston cores recovered from both these areas show that sedimentary records are significantly different under the Somali and the Socotra gyres. According to Ouahdi (1997), the sediments deposited under the Somali gyre are characterized by a significant terrigenous input, whereas the sediments recovered under the Socotra gyre are mostly composed of biogenic material. In both areas, the content in siliceous debris (5–10% diatoms and radiolarians) of the sediment is high when compared to the other pelagic deposits of the NW Indian Ocean. This siliceous fraction can, thus, be related to an enhanced export of surface productivity and can be used for a reconstruction of paleoproductivity changes (Caulet et al., 1992; Ve´nec-Peyre´ et al., 1995, 1997; Vergnaud-Grazzini et al., 1995; Ve´nec-Peyre´ and Caulet, 2000). The accumulation of selected trace metals may also provide information for reconstruction of changes in paleoproductivity (Shimmield, 1992; Calvert and Pedersen, 1993; Francois et al., 1995). Barium is considered to be representative of these trace elements. However, due to its multiple origins, sequential leaching procedures must be applied for a better correlation of its variations in the sediment to paleoproductivity

changes (Dymond et al., 1992; Jacot Des Combes et al., 1999a,b). Until now, the reconstruction of paleoproductivity changes in the NW Indian Ocean, and especially under the Socotra gyre, was limited to short time intervals (less than 168 kyr for the Somali gyre, and 72 kyr for the Socotra gyre Tribovillard et al., 1996; Ouahdi, 1997; Ve´nec-Peyre´ and Caulet, 2000). Due to the high sediment mass accumulation rates recorded in such environments, the length of the piston cores recovered in these areas limited paleoclimatic reconstructions to short time intervals. Core MD 962073 (34 m long) was successfully recovered under the Socotra gyre, and provides a longer detailed record of variations of the Socotra upwelling system. The goal of this paper is to reconstruct recent variations in upwelling intensity in the Socotra upwelling area, using geochemical and biological proxies. The variations in trace element contents are assessed in the bulk sediment. Additionally, a sequential leaching procedure was performed to determine the main Ba-carrier fractions of the sediment. The geochemical record was next compared to the biological markers of the variations in upwelling intensity, such as the variations in the upwelling radiolarian index (URI, Caulet et al., 1992) that characterize the upwelling areas. The thermocline surface radiolarian index (TSRI), previously used by Jacot Des Combes et al. (1999a,b) to monitor recent variations in surface productivity in the pelagic domain of the northwestern Indian Ocean, was also calculated. Combining both these radiolarian indices allowed for comparison of surface productivity with periods of enhanced, reduced or even absent upwelling. These data are compared to previously published data to complement the reconstruction of variations in the Socotra and Somali upwelling systems and their possible interconnection.

2. Oceanographic settings In the NW Indian Ocean, and more particularly along the Somali coast, the hydrological conditions are subjected to climatic variations induced by the summer Indian monsoon. The monsoon winds, blowing from the Southwest from May to August, generate

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the strong Somali current along the eastern coast of Africa. This northeastwardly oriented current is comprised of relatively cold and less saline water masses (Swallow and Bruce, 1966; Warren et al., 1966) that induce two gyres along the coast: the southern bSomaliQ gyre, is located at 58N, and the northern bSocotraQ gyre, is centered at 108N. The upwelling activity in both these gyres depends on the monsoon intensity (Du¨ing et al., 1980; Swallow, 1980; Quadfasel and Schott, 1982; Schott and Quadfasel, 1982; Fieux, 1987). From September to April, the Somali Current loses its strength, changing its direction and the Somali and Socotra gyres disappear. In both upwelling areas linked to these gyres, the surface waters are rich in nutrients (Smith and Codispoti, 1980) and present rather cold temperatures compared to the surrounding surface waters: 18 8C in the Somali gyre, and 17.7 8C in the Socotra gyre (Brown et al., 1980), although the average surface temperatures during upwelling seasons range from 19 to 23 8C (Scott and McCreary, 2001). The intermediate water masses that advect up to the surface originate from a depth of 200 m in the Socotra gyre, and 100 m in the Somali gyre (Smith and Codispoti, 1980). Previous studies have shown that the Socotra gyre is more productive than the Somali gyre because the upwelled waters are richer in nutrients (Smith and Codispoti, 1980). Two main shallow water masses are present in the NW Indian Ocean. The more salty one, the Arabian Sea Water, is formed in the Arabian Sea during the winter monsoon and spreads southward under the surface-mixed layer, while the less salty subtropical subsurface water enters the NW Indian Ocean with the South Equatorial Current at a depth of about 100 m (Scott and McCreary, 2001 and references herein). Previous studies related to water masses in the NW Indian Ocean have shown, however, that the subsurface waters advected at each site are different. These differences result from the influence of the Persian Gulf Water, flowing out from the Persian Gulf at a depth of 250–300 m as far as 5–108N (Quadfasel and Schott, 1982; Schott and Fischer, 2000). The extension of this water farther south of this latitude is limited by bThe Great WhirlQ, a large anticyclonic eddy, located offshore between 58 and 108N (Leetmaa et al., 1982; Fischer et al., 1996). Reaching at least 200 m deep, this gyre limits the exchange of surface

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and subsurface waters between the northern and southern parts of the Somali Basin. The water upwelling in the Socotra gyre is a mixing of the Persian Gulf Water with the Arabian Sea Water and the Indian Ocean Intermediate Water, and presents local differences in physical properties (temperature and salinity) and chemical composition (especially the nutrient content compared to the water that upwells in the Somali gyre). Below the upper 300 m, the water masses are less, or not, affected by the upwelling activity. At depths between 500 and 2500 m, a salty, oxygen-poor water mass flowing from the Red Sea and the Arabian Sea extends southeastward in the Indian Ocean. At the same depth, the Antarctic Intermediate Water enters the area from the south (Scott and McCreary, 2001). Below a depth of 2500 m, a colder oxygen-rich water mass flows northward from the Southern Ocean (Fieux et al., 1986).

3. Materials and methods In 1996, during the MD 104-Pegasom cruise of the R/V Marion-Dufresne, a 34 m long piston core (MD 962073) was recovered at 10.9368N, 52.6168E, 3142 m, under the Socotra gyre and in the vicinity (60 km NE) of Core MD 85682 previously recovered during the MD 44-INDUSOM cruise (Fig. 1). The sediment is a light-gray carbonate ooze containing 5–10% siliceous plankton debris and 5% terrigenous components. A total of 85 samples were analyzed, from a depth of 4 mbsf to the bottom of the core (32.96 mbsf), with a 30 cm sampling interval. The depth of 4 mbsf was chosen for the top of our study in order to provide complementary data to those gathered during the study of Core MD 85682 because the age of the base of the first recovered core roughly corresponds to the 4 mbsf of the longer core. The age model for Core MD 962073 is based on oxygen-isotope analysis of 98 bulk sediment samples, collected every 30 cm, using a mass spectrometer (OPTIMA). Based on the detailed isotope chronology produced for Core MD 85682 (Ouahdi, 1997), a sampling interval of 30 cm was chosen so that the time interval separating two successive samples would be shorter than the precession period (20 kyr). Isotope data were based on bulk sediment analyses because

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Fig. 1. Location map of piston cores in both the Socotra gyre (MD 85682, dot, and 962073, star) and in the Somali gyre (MD 85674). The upwelling gyres are figured as shaded areas. The Somali current represented on the map is an average estimation of its position during the upwelling season.

oxygen-isotope analyses of bulk sediments had previously been successfully used to date sediments from this area of the Indian Ocean (Shackleton et al., 1993; Jacot Des Combes et al., 1999a,b). Total organic carbon (TOC) contents were measured on the bulk sediment through a LECO analysis, and palynofacies samples were prepared according to standard procedures (Combaz, 1980). Carbonate content was measured with a Bernard calcimeter. The amount of major and significant trace elements (Ba, Cu, Ni, P, Ti, and Zn) was measured by ICP AES and ICP MS analyses. The samples were prepared by fusion with LiBO2 and HNO3 dissolution. The analytical precision and accuracy were both

found to be better than 1% (mean 0.5%) for major– minor elements, 5% for Ba, and Zn, and 10% for Ni and Cu. These values were obtained respectively using international standards and by analyzing replicate samples. In order to determine the main carrier-phase of the barium, a sequential leaching procedure was performed on eight samples scattered along the core (at 500, 980, 1160, 1626, 1866, 2166, 2706 and 3186 cmbsf). These samples were chosen to test a wide range of barium contents. The protocol used in this study is derived from that described by Robbins et al. (1984) and Lyle et al. (1984), and was already adapted to a previous study on pelagic sediments from the

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Madingley Rise, the Somali Basin (Jacot Des Combes et al., 1999a) and the Amirante Passage (Jacot Des Combes et al., 1999b). The dried and ground samples were subjected to a pH 5 acetic acid extraction to remove adsorbed and calcite-bound elements, and then to treatment with a pH 5 hydroxyl amine hydrochloride solution to remove elements adsorbed on poorly crystalline ferromanganese oxy-hydroxides. The pH 9 sodium sulfate extraction of organic matter was not carried out in Core MD 962073 because of too low TOC values. An extra treatment with HF was added to dissolve the silicate-bound barium (HF alone does not dissolve barite; Jacot Des Combes et al., 1999a). Residual elements were determined from the difference between the total content (from the bulk analysis) and the sum of the three partial extractions. After each step, element concentrations were measured by ICP spectrometry. Bulk sediment mass accumulation rates (MAR) were calculated by multiplying linear sedimentation rates (LSR) derived from the age model and dry bulk densities (DBD). DBD were obtained by calculating the ratio g dry weight/cm3 wet volume. Radiolarian associations have been used as an independent proxy to monitor variations in surface paleoproductivity. Two different indices based on variations in the specific composition of radiolarian assemblages have been used. The upwelling radiolarian index (URI) corresponds to the ratio of the percent of species characteristic of fertile (upwelling) areas to the total number of specimens counted on a slide (3000 to 8000) multiplied by 1000. Variations in this index are considered indicative of variations in the intensity of the upwelling (Caulet et al., 1992). The thermocline/surface radiolarian index (TSRI) corresponds to the ratio of the abundance of six radiolarian species (Cycladophora davisiana davisiana, Cycladophora d. cornutoı¨des, Cyrtolagena laguncula, Eucyrtidium calvertense, Larcopyle bu¨tschlii and Spongopyle osculosa) living in, or below, the thermocline layer (below 200 m) to the abundance of three radiolarian species (Heliodiscus asteriscus, Lithopera bacca and Siphonosphaera polysiphonia) that are known to be restricted to the upper 50 m of the water column (Kling and Boltovskoy, 1995). The TSRI is considered to be an indicator of the level of fertility of the surface layer of the ocean because higher surface fertility, resulting in an increased

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export of fecal pellets and food towards deeper oceanic levels, allows the thermocline populations to thrive, and increases the proportion of thermocline species in the radiolarian populations accumulated in local sediments (Jacot Des Combes et al., 1999a,b). Since radiolarian skeletons are sensitive to dissolution during their settling through the water column and within the sediment, silica dissolution may modify the composition of fossil radiolarian assemblages. However, since seawater is always silica-undersaturated from the surface to the bottom of the ocean, dissolution begins at the death of the organisms and concerns all radiolarians. Variations in abundances of radiolarian populations at the water/sediment interface are, thus, considered to be linked mostly to variations in surface productivity, rather than to variations in dissolution strength (Berger, 1968). Previous studies have shown that after deposition opal dissolution occurs within the first centimeters below the water/ sediment interface, but in areas subjected to significant export of siliceous debris, this dissolution leads to a silica saturation of the interstitial water and dissolution stops at about 20/30 cm below the interface (Johnson, 1974, 1976). Under fertile water masses, variations in radiolarian content and composition in sediments can, thus, be considered as representative of surface productivity changes. Radiolarian slides were prepared following a standard procedure (Sanfilippo et al., 1985). This technique was preferred to the one used by Moore (1973), which is more time consuming, because the total number of counted radiolarian is far larger (3000 to 8000) compared to the 300 counted by Sanfilippo. In order to record the occurrence of rare bupwellingQ species that represent less than 1% of the total assemblage (Nigrini and Caulet, 1992), it is necessary to count a large number of radiolarians. Reproducibility is usually good, as many counts of the same sample produce results varying within a range of F 1 or 2 individuals. Occurrences of representatives of selected radiolarian species were counted in 145 samples. A sampling interval of every 10 cm in the upper part of Core MD 962073 (0 to 710 cmbsf) was used in order to establish a comparison with Core MD 85682. Below this level, radiolarian species were counted at the same levels as where geochemical analyses were performed.

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4. Chronological framework Due to the length of Core MD 962073 (34 m), and the mass accumulation rate calculated for this area, the sediment record at this site was expected to span the last 400 kyr and, thus, to lengthening the detailed record that is already available for the last 72 kyr on Core MD 85682 (Ouahdi, 1997). Isotope data obtained for the upper part of Core MD 962073 are compared with the SPECMAP reference stack (Fig. 2) and the age model published for Core MD 85682 (Ouahdi, 1997) to control the coherence between both sets of data (Fig. 2). The isotope record of Core MD 85682 was obtained using shells of G. bulloides picked every 10 cm in the 7.16 m long piston core. Comparison between the isotope curves of Core MD 962073 and Core MD 85682 (Fig. 2) shows that the records obtained for levels located above the 490 cmbsf sample (30.9 kyr) present the

same trend, despite significant differences between low and high oxygen isotope values in the isotope record of Core MD 962073. To confirm the consistency between both isotope records, the abundance of the radiolarian species Cycladophora davisiana, considered as a reliable stratigraphic marker for glacial–interglacial transitions (Morley and Robinson, 1986), was measured in each piston core, and the variations in abundance in each core were compared (Fig. 3). As for the isotope record, the difference in the sampling interval leads to some gaps in the interpretation of this marker, but both records can be considered as synchronous from 556 cmbsf level (35.1 kyr) to the top of the cores (Fig. 3). From 716 to 556 cmbsf, a significant discrepancy can be observed. However, peak and mean values being of the same order, differences in the succession of maximum and minimum distribution values suggest that the sedimentation patterns were different

Fig. 2. Comparison of the d 18O curve from Core MD 962073 (this study) with the SPECMAP reference stack. The detail of the d 18O curve corresponding to the upper 10 m of Core MD 962073 (shaded area) is compared to the d 18O curve from core MD 85682 (from Ouahdi, 1997). The first occurrence of the radiolarian Buccinosphaera invaginata is indicated on the d 18O curve of Core MD 962073.

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Fig. 3. Variation of Cycladophora davisiana content in Cores MD 962073 and MD 85682.

at both sites below a depth of 556 cmbsf (prior to 35.1 kyr). The discrepancy observed between the isotope record from both cores, as well as the potential occurrence of rapid changes in the sedimentation rate, prompted us to use other radiolarian stratigraphic markers to control the reliability of the isotope age model of Core MD 962073 based on bulk sediment isotope record. The first occurrence of the radiolarian species Buccinosphaera invaginata, the most recent radiolarian stratigraphic event, was observed in Core MD 962073 at a sediment level dated 170 kyr by the isotopic age model (Fig. 2). This local age is consistent with the absolute age estimated (170 F 10 kyr) for this event in the Central Indian Basin (Johnson et al., 1989). Due to the presence of reworked radiolarian species indicative of a Mio-Pliocene age below 24.46 mbsf, this study is restricted to the time interval from 248 kyr to Present.

5. Results 5.1. Variations of the biogenic markers The upwelling radiolarian index (URI) record presents a trend of increasing values from 250 kyr to Present, suggesting a continuing increase in upwelling intensity during this period (Fig. 4). Maximum upwel-

ling activity occurred during Marine Isotope Stage (MIS) 3, with a maximum value at 26 kyr. Other peak values are recorded during the Holocene and at the transition between glacial and interglacial intervals (7/6, 6/5 and 5/4). No trend is observed in the thermocline/surface radiolarian index (TSRI) record (Fig. 4). High values are located at 110 and 30 kyr (MIS 5.5 and 3), while low values are recorded during isotope stage 7 (245– 185 kyr), from 100 to 45 kyr, and from 20 kyr to Present. Relatively coherent changes in both indexes since 100 kyr suggest that fertility changes could be related to vertical advection. 5.2. Variations of the geochemical markers The carbonate content (Fig. 4) varies from 60 to 90%, with significant fluctuations (~20%) between the isotopic transition 8/7 (248 kyr) and the end of MIS 3 (25 kyr). The CaCO3 content decreased during MIS 7 and reached minimal values in MIS 6 and 5. No direct correlation may be observed between the variations of the CaCO3 content and the radiolarian proxies. The TOC content is comparatively low (varying from 0.1% to 1.8%) in the sediments deposited under this upwelling area with marked changes throughout the sedimentary sequence (Fig. 4). Peak values are usually around 1% with a maximum of 1.8% at 170 kyr (MIS 6). These values are of the same order as those measured under the Somali gyre (Ouahdi,

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Fig. 4. Variations of bulk MAR, URI, TSRI, TOC, CaCO3, Si, Al and Ba contents in Core MD 962073. Glacial isotope stages figured as shaded areas. Data available on http://www.pangaea.de.

1997). A negative correlation is observed between the TOC and the CaCO3 contents, while a positive correlation is recognized between the TOC content and TSRI. However, no correlation is observed between the TOC content and URI. Periods of high TOC content mostly occurred when the Al content is also high (Fig. 4), indicating a potential detrital origin for the organic matter at this site. However, palynofacies analyses performed on samples with a high TOC content show that the organic matter is of dominantly marine (dominating amorphous organic matter, scolecodonts, and foraminifer linings) with few terrigenous debris (higher plant debris, pollen and spores). The Si content varies from 3.3% to 9.9% (Fig. 4), which is lower than the content measured under the Somali gyre (Tribovillard et al., 1996). High values are recorded around 7% and 8%, with the maximum at 187 kyr (transition between MIS 7 and 6). Low values are located between 80 and 40 kyr, and from 250 to 200 kyr. This last interval is also characterized by low TOC values. More generally, periods with high Si

content are also characterized by high TOC content, while a negative correlation is observed between the Si and CaCO3 contents, especially during MIS 7. From the 7/6 transition, a positive correlation is observed between Si content and TSRI. The variations in Al content follow the glacial/ interglacial cycles (Fig. 4), with high values during glacial intervals (MIS 6) and low values during interglacial stages (MIS 7 and 5). The Ti content presents a similar record, confirming that the terrestrial input is more important during glacial periods. Variation in the Ba content is similar to that of the Si content, with peak values at the 7/6 transition and during MIS 5.5 and 3. 5.3. Variations of the normalized contents of geochemical elements As the samples have variable carbonate contents, trace element contents were normalized to the Al content, considered as a reliable tracer of fine-grained aluminosilicate material.

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The TOC/Al ratio varies from 0.2 to 1.4 with wellmarked variations throughout the sedimentological record (Fig. 5). High values are found within interglacial MIS 7, 5, and 3, but also within glacial MIS 6 (169 kyr). These values are significantly higher than those observed in the Somali gyre (0.04 to 0.21; Ouahdi, 1997). Variation in the Si/Al ratio (3.5 to 12) indicates that silica enrichment was significantly higher under the Socotra gyre than at any other site from the NW Indian Ocean, including the Somali gyre (0.45 to 0.75) (Ouahdi, 1997; Jacot Des Combes et al., 1999a,b). Higher values of this ratio are recorded during interglacial MIS 7, 5 and 3, and the simultaneous Si/Al and TOC/Al peak values in these intervals suggest an opal enrichment related to higher production. Other ratios show similar variations, namely Ba/ Al, P/Al, Cu/Al and Zn/Al. This indicates that the enrichment of the sediment in these elements is related to the enrichment in opal rather than to the enrichment in terrigenous material. All these elements may, thus,

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be considered as directly or indirectly related to the surface production. Peak values of the Ti/Al ratio, which is considered as an indicator of wind strength, are recorded at 230–220 kyr, 120 kyr and 85 kyr, while lower values are observed at 200 kyr, 170–150 kyr, 90 kyr and 80–60 kyr (Fig. 5). Ti/Al ratio values are lower at this site than in other sites in the Somali Basin, except on Madingley Rise (Jacot Des Combes et al., 1999a,b). 5.4. Sequential leaching analysis Eight samples were analyzed through the sequential leaching procedure and give a mean distribution of barium in the sediment samples: – 15% of the bulk Ba is linked to the carbonate fraction, – 15% of the bulk Ba is linked to the oxy-hydroxides fraction,

Fig. 5. Variations of the TOC/Al, Si/Al, Ba/Al, P/Al, Cu/Al, Zn/Al, Mn/Al, V/Al, Mo/Al, U/Al and Ti/Al ratios in Core MD 962073. Glacial isotope stages figured as shaded areas. Data available on http://www.pangaea.de.

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– 39% of the bulk Ba is linked to the aluminosilicates fraction, – 31% of the bulk Ba is linked to the residual fraction, and is assumed to be deposited as barite. The distribution of Ba within the different fractions shows, however, some marked differences between samples (Fig. 6), especially in the abundance of barite (19.6–48.7%), and of aluminosilicate (24.8–54.9%) fractions. The sample located at 16.26 mbsf (dated 124 kyr) has a barite content significantly higher than in other samples, whereas samples located at 11.60 and 21.66 mbsf (i.e. 82 and 187 kyr) contain a large amount of terrigenous–Ba. Both barite and the terrigenous supply appear to be the main barium sources at this site. Except for the carbonates-linked Ba, which seems to increase with depth, no trend is observed in the barium distribution (Fig. 6). Moreover, no significant difference in the barium distribution within the different fractions of the sediment is observed in samples corresponding to glacial and interglacial intervals. 5.5. Variations of mass accumulation rates (MAR) In Core MD 962073, the bulk MAR varies from 4 to 14 g/cm2/kyr, with maximum values during isotope

stages 5 (128–72 kyr) and 3 (49–24 kyr). These interglacial intervals are also characterized by strong variations in bulk MAR (Fig. 4). Maximum values are significantly higher than those recorded in the pelagic domain, but these are similar to those observed in the Somali gyre (Ouahdi, 1997; Jacot Des Combes et al., 1999a,b). The bulk MAR shows an increasing trend similar to what is observed in the URI record. However, bulk MAR peak values are not synchronous with URI or TSRI peak values. Hence, the bulk MAR increase may be controlled by a more intense upwelling activity in the long term, but the relation is not so direct in shorter intervals. Data are available on PANGAEA (http://www. pangaea.de).

6. Discussion 6.1. Paleoproductivity proxies In a seasonal coastal upwelling environment such as the Somali–Socotra upwelling system, the accumulation of biogenic material (opal, carbonates, organic matter) and trace metals (Ba, P, Cu, and Zn) in the sediment is controlled by four main processes: – – – –

upwelling-controlled productivity thermocline-controlled productivity terrigenous input early diagenesis in the sediment, and especially changes in the redox conditions.

Each biogenic and geochemical marker is influenced differently by the combined control of these processes and, thus, elucidates a particular aspect of the paleoceanographic conditions in the Socotra gyre. However, the similar variation of trace elements that have a different sensitivity to redox changes, as well as the relatively low TOC content and MAR suggest that early diagenesis is not the main control of the trace elements accumulation at this site. We, thus, focus on the origin of these proxies.

Fig. 6. Distribution of barium within the different fractions of eight sediment samples in Core MD 962073. Data available on http:// www.pangaea.de.

6.1.1. Radiolarian indexes and variations of paleoproductivity As opposed to the organic carbon and trace metals, the upwelling radiolarian index (URI) and

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the thermocline/surface radiolarian index (TSRI) appear to depend only on upwelling-controlled and thermocline-controlled productivity, respectively; the influence of opal dissolution on their variation is considered negligible. We, thus, consider URI as a reliable marker of upwelling activity in the Socotra gyre, while TSRI is indicative of the productivity changes occurring after the summer upwelling season. URI and TSRI variations, thus, appear not correlated because productivity patterns during and after the upwelling season are independent. For example, the increasing trend in upwelling intensity recorded for the last 250 kyr by both URI and the bulk MAR is not recorded by TSRI. Some periods are, however, characterized by high URI and high TSRI (i.e. from 150 to 130 kyr and the 5/4 transition), indicating a continuous increase in productivity. This enhanced paleoproductivity during and after the upwelling-controlled intervals may be indicative of widespread changes in the water composition within the Socotra gyre. It could correspond to the inflow into the northern Somali basin of both surface and subsurface waters richer in nutrients. Similarly, periods when only the TSRI increased (i.e. 175–160 kyr, 90 kyr) may indicate an enrichment in nutrients limited to the surface layer. On the other hand, high URI periods (i.e. 50 kyr) may be indicative of an increased upwelling forcing, an enrichment of the subsurface waters in nutrients, or both. 6.1.2. Organic carbon, trace metals and paleoproductivity Unlike the radiolarians, organic matter, silica and trace metals may be controlled by both productivity and terrigenous input, and are sensitive to early diagenesis in the sediment. Our results show that total organic carbon (TOC), and contents in Si, Ba, P, Cu and Zn vary similarly in Core MD 962073, indicating a common response to paleoproductivity changes. Our data also show evidence that TOC and Ba are mainly of marine origin, despite a small but variable amount of terrestrial debris and that the marine component controls the accumulation of these markers in the sediment. Palynofacies analyses of organic matter in selected samples from the Socotra upwelling show the dominance of amorphous organic matter flakes, interpreted as being of

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marine origin, and, more precisely, derived from both siliceous and calcareous planktonic populations, in addition to non-mineralized-wall phytoplankton (Tyson, 1995) in the organic matter. The distribution of Ba in the sediment is more balanced between the terrigenous and biogenic origins. The sequential leaching procedure data show that terrigenous and biogenic Ba appears to be the main source of Ba in the sediment in Core MD 962073. However, the positive correlation is observed between Ba/Al and both Si/Al and TOC/Al ratios (Fig. 5), suggesting a link between Ba-enrichment and surface productivity. This relationship is coherent with previous observations of barite formation in particular microenvironments, especially in siliceous plankton debris where the decay of the organic matter induces a sulfate saturation that leads to barite formation (Bishop, 1988; Dehairs et al., 1980, 1987, 1991). This is also supported by the positive correlation between the Ba/ Al, P/Al, Cu/Al, Ni/Al and Zn/Al ratios, since the enrichment of the sediment in these trace elements can be related to the increased productivity (Calvert and Pedersen, 1993). All these data suggest that the marine organic matter enrichment under the Socotra gyre is linked to the abundance of opal-producing organisms. This interpretation would confirm the observations previously made on the intensity changes of the Socotra gyre during the last glacial cycle indicating that, at this site, productivity was mainly of siliceous origin (Ouahdi, 1997; Ve´nec-Peyre´ and Caulet, 2000). These data also suggest that the terrigenous input, though partly contributing to the trace elements input in the Socotra gyre, is not important enough, or irregular enough, to deeply disturb the record of paleoproductivity variations, except during MIS 6 (see below). Additional evidence of the direct relationship between the enrichment of the sediment in organic matter and associated elements is offered by the correlations between the TOC/Al and element/Al ratios with URI and TSRI. There is no direct correlation between the radiolarian indexes and the other proxies, but high values of the preceding ratios correspond to URI and TSRI high values (Figs. 4 and 5). These synchronous trends in variations with both radiolarian indices indicate, however, that though the organic matter and associated elements are mostly controlled by surface productivity, it is

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not possible, by using these proxies only, to separate the upwelling-controlled production from the thermocline-controlled production. Hence, in the Socotra gyre, if a global paleoproductivity reconstruction based on the geochemical record is possible, only its combined use with URI and TSRI allows for the establishment of the respective impact of upwellingproduction and thermocline-production on the sedimentary record. The situation is slightly different during MIS 6. Although the relation between TSRI and TOC content is still observed, the Si fraction seems to be more related to the Al content in this interval. Moreover, the trace elements associated with surface productivity (Ba, P, Cu and Zn) are, in this interval, more closely related to the Ti/Al ratio, indicating a stronger wind forcing (Weedon and Shimmield, 1991; Shimmield, 1992). These modifications in the geochemical record may be related either to an increased terrigenous input, or to the dominance of non-siliceous organisms in the planktonic community, especially organic-walled planktonic forms, as the carbonate content also remains low during this interval (Figs. 4 and 5). However, the difficulty in estimating the importance of organic-walled organisms and the occurrence of a maximum Al content in the sediment during this glacial interval leads us to favor the interpretation of an episode locally characterized by an increased terrigenous input, prompted either by a stronger wind forcing, or an increased continental weathering. The geochemical record during MIS 7 presents a different set of conditions. The strong negative correlation observed between the CaCO3 and Si contents, while the Al content remained relatively stable (Fig. 4), may indicate a drastic change in the biological response to the upwelling forcing in the Socotra gyre, with the transition from a calcareous phytoplankton controlled production to a siliceous phytoplankton controlled production. This change in the plankton population could be related to a change in the source water of the Socotra gyre, and especially to the inflow of waters richer in dissolved silica, which allows diatoms to dominate the plankton assemblage associated with the upwelling activity. Although diatoms are of significant importance in the plankton blooms responsible for the enhanced surface productivity recorded in the upwelling systems (Schuette and

Schrader, 1979, 1981; Schrader and Sorknes, 1991; Schrader, 1992), the seasonal character of the Socotra upwelling prevents the formation of diatomites in the sediments, in contrast to what is observed in the California Basin, and on the Peru and Angola Margins (Aplin et al., 1992; Diester-Haas et al., 1992; Meyers, 1992; White et al., 1992). The impact of the diatom production is not recorded by our radiolarian index but may explain the good correlation between the organic matter and the silica enrichment throughout the core. The variations in upwelling intensity recorded off Socotra by both URI and the geochemical proxies are similar to other records of upwelling activity in the NW Indian Ocean. Off Oman, changes in the foraminifer population indicate that upwelling activity increased during Pleistocene (Anderson and Prell, 1991, 1993), and that, due to increased southwest winds related to a stronger monsoon, upwelling was more intense during interglacial intervals (Anderson and Prell, 1991). MIS 3, when highest URI values are recorded off Socotra, is characterized by a large offshore extension off Oman, corresponding to an intense upwelling activity (Anderson and Prell, 1991). The increased upwelling during MIS 3 is also recorded in the Benguela upwelling system (SE Atlantic), and may correspond to a global increase in upwelling activity (Summerhayes et al., 1995; Romero et al., 2003; Jacot Des Combes and Abelmann, submitted for publication). 6.2. Comparing the Socotra and Somali gyres The comparison of our results with other data from the Somali gyre (Caulet et al., 1992; Tribovillard et al., 1996; Ouahdi, 1997) shows several differences between the two sites. Periods with high URI values are not always synchronous at both sites. Despite common high URI values at the end of MIS 6 and in the upper half of MIS 3, the URI records at both sites are significantly different. Moreover, URI values are lower in the Socotra gyre than in the southern Somali gyre while previous studies demonstrated that upwelling-controlled productivity is higher in the northern gyre (Smith and Codispoti, 1980). Finally, the relationship between URI and the sedimentological and geochemical records are different at both sites. In the Somali gyre, URI is strongly

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correlated to the CaCO3 content, but this is not so in the Socotra gyre. Three hypotheses are proposed to explain these differences: – the influence of the composition of the planktonic community – the influence of the terrigenous input – the influence of the location of the core. 6.2.1. Influence of the planktonic community According to Wyrtki (1971) and Smith and Codispoti (1980), the water upwelling in the Socotra gyre is colder, saltier and more nutrient-rich than that the water upwelling in the Somali gyre. The nitrate, phosphate and dissolved silica concentrations are two to three times higher in the northern gyre than in the southern gyre. The upwelling water in the Socotra gyre is, therefore, more favorable to primary productivity than that upwelling in the Somali gyre, especially in the case of the siliceous plankton. This may lead to a siliceous plankton rich surface productivity in the Socotra gyre and a calcareous plankton rich surface productivity in the Somali gyre. The hypothesis is supported by significant differences between the Si/Al values in Core MD 962073 from the Socotra gyre, and Core MD 85674 from the Somali gyre (3.5 to 12 and 0.4 to 4, respectively; Tribovillard et al., 1996; Ouahdi, 1997). There is also a better correlation between URI and the CaCO3 content in Core MD 85674 (Caulet et al., 1992), indicating that the increased productivity related to the upwelling intensity is mostly of calcareous plankton origin. Such a dominant role of the calcareous phytoplankton (coccolithophorids) may also explain the significant differences observed in the TOC/Al values at both sites (0.2 to1.4 in the Socotra gyre and 0.04 to 0.21 in the Somali gyre; Ouahdi, 1997). Due to its composition, and especially to its low lipid content, the organic matter produced by coccolithophorids is known to have a low potential for preservation in the sediment (Noe¨l et al., 1993; Tribovillard et al., 1994, 1996). Hence, an important part of the organic matter produced by the dominant nannofossil flora may not have been preserved in the sediment, thus lowering the TOC/Al ratio in the Somali gyre. The composition of the planktonic community may also have influenced URI by limiting the

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amount of dissolved silica available for the radiolarians in the Socotra gyre, where diatom blooms are very important during the upwelling season. However, the lower URI values observed in this northern gyre may also be related to a shorter duration of the upwelling season in the Socotra gyre (3 months compared to 4 months in the Somali gyre, Fieux, 1987). 6.2.2. Influence of the terrigenous input Important terrigenous input observed during MIS 6 in Core MD 962073 disturbs correlations between radiolarian indices and the geochemical record (TOC and trace elements accumulation). Since the terrigenous input is significantly more important at the Somali site than at the Socotra site (2.5–8.3% of Al; Tribovillard et al., 1996; 0.7–2.2% of Al, respectively), the geochemical record of paleoproductivity is more disturbed in the southern gyre, leading to the observed lack of correlations between URI, TOC/Al and Si/Al ratios observed in Core MD 85674 (Tribovillard et al., 1996; Ouahdi, 1997). 6.2.3. Influence of site location Upwelling systems are known to be made of narrow plumes where primary productivity is locally enhanced by the high nutrient content of the advected waters. Beside these narrow bands, the productivity is of the same order of magnitude as in the pelagic domain. The Somali site (Core MD 85674), being located at the fringe of an upwelling gyre, is, thus, more sensitive to the occurrence of upwelling filaments. When the regional forcing of the upwelling is not very strong, such as in MIS 5, the local upwelling response may be preferentially controlled by the presence of upwelling filaments. The location of the site at the fringe of the upwelling system may also explain the higher URI values observed in the Somali gyre. Fringe environments are characterized by downwelling fronts, where the water upwelling near the coast sinks under the less dense surrounding surface waters. These fronts are considered as an area where the material (including radiolarians) transported offshore within the upwelled water is concentrated before sinking. Such hydrological fronts could, in this way, artificially increase the URI by concentrating the radiolarian species associated with this index.

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7. Conclusions Quantitative analyses of species changes in radiolarian assemblages (URI and TSRI indexes) related to variations of the geochemical composition of the sediments allow a detailed reconstruction of the upwelling activity under the Socotra gyre (Somalian upwelling system) during the last 250 kyr. At this location (108N), oceanic paleoproductivity irregularly increased its intensity with a maximum at 30 kyr. Increased upwelling activity is recorded by the URI index from 50 to 20 kyr. Periods of high productivity are recorded by both radiolarian indexes (URI and TSRI) and geochemical proxies (TOC/Al, Si/Al and Ba/Al variations) at 185 kyr, 132 kyr, and from 95 to 65 kyr, and by the TSRI from 180 to 160 kyr, 145 to 130 kyr, 120 to 100 kyr, 95 to 65 kyr, and 35 to 25 kyr. Geochemical data indicate that the surface productivity during and after the upwelling season was mostly dominated by the siliceous plankton. A predominance of the planktonic siliceous production is recognized during MIS 7. Terrigenous inputs, although relatively low at the site, have disturbed the geochemical record of paleoproductivity when increasing during MIS 6. A comparison with the paleoproductivity reconstructions previously published for another upwelling site located at 58N (Somali gyre) indicates that upwelling activity, although progressively increasing during the same time interval, was not synchronous within all the Somalian upwelling system. Many hypotheses are offered to explain these differences. Some differences, related to planktonic sources being more siliceous at the Socotra than at the Somali gyre, are attributed to the upwelling of different water masses at both sites. Other can be related to a significantly important terrigenous input in the southern Somali gyre that has increased the mixing of marine and terrestrial organic matter and trace elements, and diluted the surface marine productivity. Another explanation directly relates these differences to the physical structure of the upwelling system and the location of the analyzed sites regarding the centers of the gyres. The Somali site is located at the fringe of the present upwelling domain where hydrodynamic patterns (upwelling filaments and local fronts) are different from those prevailing at the center of the northern gyre (Socotra site) and may have, thus, differently modulated the local

response to the regional forcing of the Somalian upwelling by the Indian monsoon. Acknowledgements This study was supported by joined funding from the CNRS and MNHN. M. Tamby (MNHN) is thanked for his technical cooperation. C. Pierre and J.-F. Salie`ges (LODYC) were essential for the oxygen isotope analyses. F. Baudin (Universite´ Pierre et Marie Curie, Paris VI) was very helpful for the measurement of the organic carbon. We also thank J. Morel of the spectrochemical laboratory of the CRPG of Vandoeuvre-les-Nancy for his help in ICP AES and ICP MS analyses. The IFRTP and the Marion-Dufresne crew provided a crucial help in the coring of the giant piston core MD 962073. Thanks are due to C. Nigrini who kindly revised the English. A. Sanfilippo, D. Lazarus, and an anonymous reviewer were an invaluable help for the improvement of this paper. The data are presented in this paper available on http://www.pangaea.de. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.palaeo.2005.04.007. References Anderson, D.M., Prell, W.L., 1991. The coastal upwelling gradient off Oman during the Late Pleistocene. In: Prell, W.L., et al., (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 117, pp. 265 – 276. Anderson, D.M., Prell, W.L., 1993. A 300 kyr record of upwelling off Oman during the late Quaternary: evidence of the Asian southwest monsoon. Paleoceanography 8, 193 – 208. Aplin, A.C., Bishop, A.N., Clayton, C.J., Kearsley, A.T., Mossman, J.R., Patience, R.L., Rees, A.W.G., Rowland, S.J., 1992. A lamina-scale geochemical and sedimentological study of sediments from the Peru Margin (site 680, ODP Leg 112). In: Summerhayes, C.P., et al., (Eds.), Upwelling Systems: Evolution since the Early Miocene, Geological Society Special Publication, vol. 64, pp. 131 – 151. Berger, W.H., 1968. Radiolarian skeletons: solution at depth. Science 159, 1237 – 1238. Bishop, J.K.B., 1988. The barite–opal–organic carbon association in oceanic particulate matter. Nature 332, 341 – 343.

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