Past changes in biologically mediated dissolution of calcite above the chemical lysocline recorded in Indian Ocean sediments

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Quaternary Science Reviews 22 (2003) 1757–1770

Past changes in biologically mediated dissolution of calcite above the chemical lysocline recorded in Indian Ocean sediments Sonja Schulte*, Edouard Bard # de l’Arbois BP 80, F-13545 Aix en Provence, Cedex 4, France CEREGE, UMR6635, CNRS & Universit!e Aix-Marseille III, Europole Received 16 June 2002; accepted 22 May 2003

Abstract In a deep-sea sediment core recovered from a site lying well above the local lysocline, several organic geochemical proxies, and two different calcite dissolution indicators, are compared in order to evaluate the relationship between calcite dissolution and paleoproductivity over the past three glacial-interglacial cycles. The degree of foraminiferal break-up, and the CaCO3 particle size distribution, both point to significant periods of dissolution every 22 kyr during glacial stages and substages. These dissolution events are concomitant with periods of enhanced primary productivity, as indicated by the abundance of several biomarkers (alkenones, cholesterol, brassicasterol, keto-ol), used here to indicate changes in paleoproductivity. Dissolution fluctuations are highly coherent and in phase with the estimated paleoproductivity variations providing strong evidence that the observed dissolution is due to organic matter remineralization within the sediments rather, than to changes in CO2
3 concentration in the overlying water column. r 2003 Elsevier Ltd. All rights reserved.

1. Introduction Dissolution of calcite constituents of deep-sea sediments is generally thought to be controlled by the degree of saturation with respect to calcium carbonate of oceanic bottom waters (e.g. Berger, 1968, 1970, 1973, 1976; Li et al., 1969; Broecker, 1971; Morse and Berner, 1972; Berger and Winterer, 1974; Edmund, 1974; Volat et al., 1980; Broecker and Peng, 1982). Accordingly, variations in the carbonate content of oceanic sediments are often interpreted as reflecting variations of biological calcification in surface waters or changes in the deepwater carbonate chemistry (Farrell and Prell, 1989; Curry and Lohmann, 1990). However, an important additional factor, suggested from recent in situ analyses of deep-sea sediment pore waters (pH, O2, CO2(aq), Ca2+), benthic flux chamber experiments, and models of early diagenesis (Emerson and Bender, 1981; Archer et al., 1989; Berelson et al., 1990; Archer, 1991; Hales et al., 1994; Jahnke et al., 1994, 1997; Hales and Emerson, 1996, 1997; Martin and Sayles, 1996, Adler *Corresponding author. Present address: Institute of Chemistry and Biology of the Marine Environment, Carl von Ossietzky University Oldenburg, P.O. Box 2503, D26111 Oldenburg, Germany. E-mail address: [email protected] (S. Schulte). 0277-3791/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00172-0

. et al., 2001, Wenzhofer et al., 2001), involves the dissolution of carbonate constituents interstitially, driven by metabolic release of CO2 into sediment pore waters during organic matter remineralization. The importance of metabolic dissolution was further emphasized by Archer and Maier-Reimer (1994), who proposed that glacial–interglacial changes in atmospheric CO2 content could be reproduced by slightly altering the ratio of calcite and organic carbon fluxes to seafloor sediments. Understanding carbonate dissolution is also crucial when reconstructing ocean pH variations linked to past changes in atmospheric pCO2. Indeed, there is still a conflict between such reconstructions based on boron isotopes (Sanyal et al., 1995), and those based on the dissolution of planktonic foraminifera (Anderson and Archer, 2002). Furthermore, different foraminiferal-based pH proxies tend to provide contrasting answers for the same glacial periods (e.g. Anderson and Archer, 2002, versus Broecker and Clark, 2002), which suggests that one (or more) underlying assumptions is (are) not fulfilled. In order to evaluate the link between primary productivity, organic matter remineralization, and calcite dissolution, we investigated a 350-kyr sediment record from core MD900963, which was recovered from a site near the Maldives platform in the equatorial

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Indian Ocean (5 040 N, 73 530 E) from a water depth of 2450 m. We determined the concentrations of several biomarkers in this core, and compared those with two foraminiferal-based dissolution proxies. The new records of cholesterol and keto-ol complement our previous studies based on this core (Bassinot et al., 1994b, Rostek et al., 1997; Schulte et al., 1999; Pailler et al., 2002). Taken together, the new and previous results provide key evidence with which to address the question of whether carbonate dissolution occurs above the chemical lysocline, due to organic matter remineralization past deposition. 1.1. Present location of the lysocline The core was taken at a water depth of 2450 m, which is about 2200 m above the local calcite compensation depth (CCD; Berger and Winterer, 1974; Banakar et al., 1998). The depth of the chemical lysocline corresponds to a critical level of carbonate undersaturation, below which calcite dissolution increases dramatically. It is often assumed to correspond to a saturation state (O index) equal to 0.8 (Milliman et al., 1999). The saturation state O for a water depth of 2450 m was computed using the software CO2SYS (Lewis and Wallace, 2000). The necessary input values for salinity, temperature, pressure, total alkalinity (TA) and total inorganic CO2 (TCO2), as well as silicate and phosphate concentration, were obtained from GEOSECS station 418 (6.2N/64.4E), which is closest to the location of MD900963. Two definitions are currently in use for alkalinity, with four different pH scales and several different formulations for Kl and K2 (the first and second dissociation constants for carbonic acid in sea water). We calculated O for all possible combinations (with different pH scales, Kl and K2, as well as different definitions of TA) proposed by the software. The resulting O values vary between 1.10 and 1.02, indicating that the waters overlying MD900963 are clearly supersaturated with respect to calcite. The equilibrium saturation horizon (O value of 1) was found at a water depth of 3684 m. This depth for the saturation horizon is in accordance with the results of Millero et al. (1998), who determined a O of 1 in the northern Arabian Sea, at a water depth of 3400 m, on the basis of measured TCO2, TA, pCO2 and pH. The lysocline depth (O=0.8) was found at about 4000 m, at least 1500 m deeper than the core site. Consequently, site MD900963 is located well above the local saturation horizon, and thus provides an excellent opportunity to investigate supralysoclinal calcite dissolution. Indeed, no clear change in the depth of the lysocline or CCD has been reported for the northwestern Indian Ocean over the many glacial/ interglacial transitions of the Neogene (Peterson and Prell, 1985b; Peterson and Backmann, 1990). Consequently, any dissolution events for this site would be

linked to processes occurring within the sediments rather than sea-water undersaturation.

2. Results and discussion 2.1. Past variations in calcite dissolution A variety of indicators have been used to reconstruct the record of dissolution of carbonate constituents from deep-sea sediments, including carbonate content, percentage of coarse fraction, the ratio of fragments to whole foraminiferal tests, the ratio of foraminiferal species susceptible or resistant to dissolution, the ratio of coarse to fine CaCO3, and the test thickness of foraminifera (e.g., Berger, 1968, 1973; Thunell, 1976; Johnson et al., 1977; Metzler et al., 1982; Peterson and Prell, 1985a; Le and Shackleton, 1992; Bassinot et al., 1994b; Lohmann, 1995; Broecker and Clark, 1999, 2001a–c). In the present study two classical approaches are employed to reconstruct the carbonate dissolution of core MD900963. Firstly, the ratio of fragments to whole foraminifera as observed under the binocular microscope, is the simplest way to determine the extent of break-up, and one of the most reliable indicators of carbonate dissolution (e.g., Metzler et al., 1982; Le and Shackleton, 1992; Broecker and Clark, 1999). Secondly, a less direct method, recently used by Broecker and Clark (1999), is based on measuring the ratio of coarse calcite to total calcite (>63 mm CaCO3/total CaCO3). The ratio of the >63 mm fraction to total CaCO3 is also related to the percentage of whole foraminifera. This is because the proportion of fragments varies as a result of three processes during progressive dissolution: whole foraminifera break into fragments, coarse fragments break into finer fragments, and fine fragments then become completely dissolved (Le and Shackleton, 1992). As shown in Fig. 1, which is based on the data presented by Broecker and Clark (1999), both of these dissolution proxies are linearly related to each other. Our intention in using this simple particle-size based parameter is to follow previous work by Broecker and Clark (1999, 2001a–c, 2002). Nevertheless, it should be remembered that this simple index has some pitfalls and is not always a reliable carbonate dissolution proxy. Since most of the carbonate in deep-sea sediments is composed of coccoliths, this size ratio index expresses predominantly the ratio of whole adult foraminifera to coccoliths and foraminiferal fragments and juveniles, respectively. The simple size index can also reflect changes in productivity in addition to dissolution. Following Wang and McCave (1990), Robinson and McCave (1994) and McCave et al. (1995) a more sophisticated method of sediment characterization would be to divide the fine fraction (o63 mm) into further size fractions (i.e. o2 mm, 2–10 mm, 10–63 mm).

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McCave et al. (1995) showed that the sediment o10 mm is dominantly coccoliths and that the 10–63 mm fraction contains mainly foraminiferal fragments. In contrast to

Fig. 1. Fraction of the CaCO3 in the >63 mm size range for core tops from the Indian Ocean versus percent whole planktonic shells (R ¼ 0:93) (taken from Broecker and Clark, Fig. 8, 1999). Note that both proxies are plotted with reversed axis.

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this, the >63 mm fraction consists of whole foraminifera. A more accurate dissolution index could be derived by using a ratio of the 10–63 mm carbonate fraction (mainly foraminiferal fragments) to the >63 mm carbonate fraction (mainly whole foraminifera). Such data are not yet available for core MD900963 and for the present study we use the ratio of coarse calcite to total calcite (>63 mm CaCO3/total CaCO3) as a parameter for carbonate dissolution, keeping its limitations in mind This will also allow us to compare our study with previous work by Broecker and Clark. As shown in Fig. 2, the percentage of whole foraminifera, as well as the 63 mm CaCO3/total CaCO3 ratio as measured in core MD900963 (Bassinot et al., 1994b) both indicate clearly that significant variations in carbonate dissolution have occurred at this site, even though it is located well above the lysocline. Spectral analyses of both dissolution records reveal a 23-kyr cyclicity (Table 1), with the more intense dissolution events occurring during glacial stages and substages (Fig. 2). Despite both dissolution records are highly correlated with each other during the past 350 kyr, two exceptions of this correlation are observed around 200 and 300 kyr BP. At these times the particle-size based indicator suggests a dissolution event while the foraminifera-based indicator does not. Since both events are matched by peaks in TOC and microfossil derived paleoproductivity records (Fig. 3), this could suggest, that for these times, the particle-size based indicator

Fig. 2. Records of MD900963 versus age: (in blue) percentage of whole foraminifera calculated as the percentage of whole foraminifera to fragment plus whole; (in black) >63 mm CaCO3/TOTAL CaCO3 ratio calculated from percent CaCO3 and percent >63 mm CaCO3. See raw data in (Bassinot et al., 1994a, b). The chronology of core MD900963 based on the measurement of d18O of the planktonic foraminifer G. ruber (white). The d18O curve was then tuned to orbital variations (for full details see Bassinot et al., 1994a).

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Table 1 Summary of main spectral power of all sedimentary records analyzed and of its coherencies at 22–23 kyr, and phase relationships between the time series analyzed and the estimated mean primary productivity index (on the basis of foraminifera and coccolithophorides) Parameter

Main power (kyr)

Coherence at 22–23 kyr

Lead/lag (kyr)

Mean PP Size parameter % whole foraminifera Uraniumcfb C37-alkenones/g sed. Cholesterol/g sed. Brassicasterol/g sed. C30-keto-ol/g sed. C37/TOC Cholesterol/TOC Brassicasterol/TOC C30-keto-ol/TOC

22 23 23 23 23 19 19 22 23 19 19 22

— 0.9971 0.9948 0.9926 0.9857 0.9809 0.9737 0.9814 0.9835 0.9759 0.9727 0.9860

—
1.670.2 1.270.2
1.270.28 0.270.4
1.870.4
4.770.5
0.970.5
0.470.4
1.870.5
4.970.5
1.770.4

Analyzes were made using the program SPECTRUM (Schulz and Stattegger, 1997). Because of an internal algorithm this program does not need equidistant time series. To prove the statistical significance, a Fischer/Siegel test was employed in the subroutine ‘‘Harmonic’’. Identical parameters where used in the calculation procedures for all records (level of significance is 0.05, Oversampling Factor (OFAC) is 5, and High Frequency Factor (HIFAC) is 1). For a more precise evaluation of temporal correspondence between the sedimentary records analyzed and the estimated productivity index, cross spectral analyses were performed. This method does not only include an estimation of coherency, a linear correlation coefficient in the spectral domain, but also estimates the lead or lag reported as a phase angle between records sharing a similar variance at a given frequency. The calculation procedure of the cross-spectral analyses was also kept constant for all records (level of significance is 0.1, OFAC is 4, HIFAC is 1, window type is Welch, and 2 segments with 50% overlap). Coherence is characterized by values >0.963. Errors at the 95% confidence level. cfb=carbonate free basis. Note that the records of size parameter and % whole foraminifera were multiplied by
1 because both proxies decrease with increasing organic remineralization (see Fig. 1).

reflects paleoproductivity variations, rather than carbonate dissolution effects. 2.2. Paleoproductivity variations Past fluctuations in primary productivity at site MD900963 were reconstructed using a multiproxy approach, based on both micropaleontological and geochemical indices (see data in Cayre et al., 1999, for planktonic foraminifera; Beaufort et al., 1997, for coccolithophorida; Rostek et al., 1994, 1997, and Schulte et al., 1999, for total organic carbon; and Pailler et al., 2002, for redox-sensitive trace elements). Fig. 3a (upper) shows the averaged primary productivity based on transfer functions for both foraminifera and coccolithophorida. The record is characterized by well-defined peaks and troughs every 22 kyr, with amplitudes varying by a factor 2 or 3 between extrema

(120–150 g C/m2/yr for minima and about 200–300 g C/ m2/yr for maxima). The paleoproductivity variations inferred from micropaleontological transfer functions (Fig. 3a upper) are also recorded in the measured total organic carbon (TOC) profile for the core (Fig. 3b). However, in comparison with the estimated microfossil paleoproductivity, the TOC record shows a less clear and stable cyclicity, with, for example, a long-term rising trend during the last glacial cycle. Using the total organic carbon content and the physical properties measured in core MD900963 (density and porosity), we have also estimated primary productivity (PP) according to the relationship proposed by Muller . and Suess (1979; Fig. 3a lower). This approach assumes that the terrestrial organic matter supply is negligible, which is likely to be true in the area around the Maldives (Rostek et al., 1994, 1997; Schulte et al., 1999). The estimated level of PP is significantly lower, and often exhibits a reduced amplitude of cyclicity compared with values estimated from microfossils (Fig. 3a upper). Although transfer functions based on microfossils may not be completely accurate, low TOC-based PP values suggest that significant remineralization of organic matter occurred in the sediment at this site during the last 350 kyr. To improve our assessment of the extent of early diagenesis of organic matter in the sediment, we have analyzed a series of biomarkers synthesized by different plankton classes.

2.3. Molecular records of paleoproductivity Sediments contain biomarkers that are chemical compounds with an unambiguous link to specific precursor molecules in living organisms. Biomarker concentrations may be used as qualitative proxies for paleoproductivity (e.g. Brassell et al., 1986; Rostek et al., 1994, 1997; Schubert et al., 1998; Villanueva et al., 1998; Schulte et al., 1999). However, under conditions of lowered redox potential, e.g. during periods of high primary productivity, the preservation of specific lipids is favored relative to total organic carbon (e.g. Harvey et al., 1995; Schulte et al., 2000). Since the variations in amplitude are dramatically enhanced, this means that concentration changes are not linearly correlated with paleoproductivity variations. This complication may actually be an advantage because subtle changes in paleoproductivity can be revealed as large deviations in the ratio of lipid concentrations to total organic carbon. In this study, we have considered concentrations of several planktonic biomarkers in order to evaluate variations in paleoproductivity (see some analytical details in the caption of Fig. 4). These include the following:

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PP estimate based on forams and coccoliths PP estimate based on TOC (after Müller & Suess, 1979)

400 350 PP (gC/m²/yr)

300 250 200 150 100 50 (a)

0 1 0.9

TOC (%)

0.8 0.7 0.6 0.5 0.4 0.3 (b)

0.2 0

50

100

150

200

250

300

350

Age (kyr) Fig. 3. (a) Estimated primary productivity (PP) for core MD900963 based on the relative abundances of coccoliths and foraminifera versus age. The black solid line represents the average PP calculated from estimated PP based on coccoliths and foraminifera (Beaufort et al., 1997; Cayre et al., 1999). The dashed line represents estimated PP based on the relative abundance of coccoliths alone (Beaufort et al., 1997). The blue line shows the paleoproductivity estimated from TOC after the calibration by Muller . and Suess (1979) (in red). (b) Record of total organic carbon for core MD900963 (Rostek et al., 1994, 1997; Schulte et al., 1999).

(i) C37-alkenones, which are specific biomarkers for coccolithophorids (i.e. Emiliania huxleyi and Gephyrocapsa oceanica (Volkman et al., 1998; Volkman, 2000 and references therein), and their concentration in marine sediments is often used as a qualitative proxy for paleoproductivity as illustrated by several studies in different oceanic settings (e.g. Brassell et al., 1986; Rostek et al., 1994, 1997; Schubert et al., 1998; Villanueva et al., 1998; Schulte et al., 1999; Schulte and Muller, . 2001). (ii) Brassicasterol, which in sediments, is probably derived mostly from diatoms (Volkman et al., 1998). Changes in the brassicasterol concentration of marine sediments are related to variations in primary productivity, as recently demonstrated for Indian Ocean and South Atlantic sediments (Schubert et al., 1998; Hinrichs et al., 1999). (iii) Cholesterol, which is synthesized by a wide variety of marine organisms, including most marine zooplankton and many photoautothrophs (Goad, 1981; Volkman, 1986; Barrett et al., 1995). Zooplankton

represents one of the first crucial links in the food chain, forming a link between production and deposition of biolipids from primary producers. Because of this, sedimentary cholesterol contents should reflect changes in primary productivity. (iv) Keto-ols and alkanediols, which constitute another group of planktonic biomarkers represented in marine sediments. The alkanediols are synthesized by freshwater and marine eustigmatophyte microalgae (Volkman et al., 1992, 1999), of which one marine genus (Nannochloropsis gaditana) also produce keto-ols (Me! janelle et al., 2003). Recently, long-chain alkanediols were also detected in rhizosolenoid diatoms belonging to the widespread diatom genus Proboscia (Sinninghe Damste! et al., 2003). However, the main producers of these components still remain unknown (see Versteegh et al., 1997, for a review). These compounds are highly abundant in sediments underlying high-productivity upwelling zones such as off Peru (e.g., McCaffrey et al., 1991), and Oman (Ten Haven . and Rullkotter, 1991), as well as in the southeast

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Fig. 4. Representative gas chromatogramm of the NSO fraction including the chemical structures of cholesterol, brassicasterol and C30-keto-ol, which were detected as TMS-ethers. Concentrations of C37-alkenones, cholest-5-en-3b-ol (cholesterol), 24-methylcholesta-5,22-dien-3b-ol (brassicasterol) and n-triacontane-l-ol-15-one (C30-keto-ol) were determined with the following analytical methods. After freeze-drying and grinding (2–3 g) solvent extractions were performed ultrasonically using dichloromethane plus 1% methanol. After addition of internal standards (squalane, 5a-androstane-17-on, erucic acid), the extract was separated into fractions soluble in n-hexane and dichloromethane, respectively. The hexane-soluble portion was separated by medium-pressure liquid chromatography (MPLC) into fractions of aliphatic hydrocarbons, aromatic hydrocarbons, and polar compounds (NSO fraction). The NSO-fraction was methylated with diazomethane and silylated with N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA, for details see Schulte, 1997). Conditions of gas chromatography and gas chromatography coupled with mass spectrometry are described elsewhere (Sonzogni et al., 1997). Compound identifications are based on comparison of relative gas chromatographic retention times and mass spectra with those reported in the literature.

Atlantic (Hinrichs et al., 1999, Versteegh et al., 2000; Sinninghe Damste! et al., 2003), the Arabian Sea (Schulte, 1997; Sinninghe Damste! et al., 2003), the California continental margin (Mangelsdorf, 2000), and also in sapropels of the Mediterranean Sea (Rullk. otter et al., 1998; Rinna et al., 2002). In southeast Atlantic sediments, the abundances of C28-C32 alkanediols and C30 keto-ol (n-triacontane-1ol-15-one) are significantly correlated with estimates of paleoproductivity and alkenone concentration (Hinrichs et al., 1999). 2.4. Implications of the results of biomarker paleoproductivity indices For all analyzed biomarkers, the highest concentrations are recorded every 19–23 kyr during glacial stages and substages (Fig. 5), indicating that paleoproductivity

was enhanced during these episodes. This agrees qualitatively with other proxy records based on micropaleontological indicators (Fig. 3), as well as with redoxsensitive trace element profiles (see Fig. 6a for authigenic uranium; Pailler et al., 2002). Furthermore, the good correlation between all analyzed biomarkers suggests that the planktonic community structure did not change significantly during the last 330 kyr, which is consistent with the results of Schubert et al. (1998). Haptophytes, diatoms, zooplankton, and the keto-ol producing organisms have all contributed continuously to primary productivity in the equatorial Indian Ocean during the past 350 kyr. Consequently, the observed cyclicity in biomarker concentrations cannot be due to relative changes in the different planktonic populations. It is noteworthy that dinosterol, a biomarker characteristic of dinoflagellates (Boon et al., 1979; Volkman, 1986; Volkman et al., 1998), is found only in trace amounts, in

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Fig. 5. Records of MD900963: (a) d18O stratigraphy on which the age model of the core is based (for full details on the age model see Bassinot et al., 1994a), (b) C37-alkenones in mg/g dry sediment (Rostek et al., 1994, 1997; Schulte et al., 1999), (c) cholesterol in mg/g dry sediment, (d) brassicasterol in mg/g dry sediment (Schulte et al., 1999) and (e) C30 keto-ol in mg/g dry sediment.

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20

(a) authigenic uranium

ppm cfb

15 10 5 0 7

(b) C -alkenones 37

6

µ g/g TOC

5 4 3 2 1 0 10

(c) cholesterol

µ g/g TOC

8 6 4 2 0 5

(d) brassicasterol

µ g/g TOC

4 3 2 1 0 10

(e) C

30

keto-ol

µ g/g TOC

8 6 4 2 0 0

50

100

150

200

250

300

350

Age (kyr) Fig. 6. Normalized records for core MD900963: (a) authigenic uranium in ppm on a carbonate free basis (cfb) (Pailler et al., 2002), (b) C37-alkenones/TOC, (c) cholesterol/TOC, (d) brassicasterol/TOC and (e) C30 keto-ol/TOC.

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contrast to the northern Arabian Sea, where it is abundant (Schubert et al., 1998). As stated above, biomarker concentrations are only qualitative proxies for paleoproductivity because they can be affected by past variations in remineralization. During high-productivity periods, the increased organic matter burial would have led to a decrease in the redox potential (Eh) in the sediment interstitial waters. This is clearly indicated in core MD900963 by the precipitation of redox-sensitive metals such as U, Cd, and Mo (Pailler et al., 2002). Several studies (Teece et al., 1994, 1998; Canuel and Martens, 1996; Hoefs et al., 1998; Sun and Wakeham, 1998) show that low Eh, favors the selective preservation of lipids. Incubation experiments of marine phytoplankton under different redox conditions (oxic/anoxic) have demonstrated that the decay of organic matter follows a first-order kinetic function (Harvey et al., 1995). The observed degradation rates (R%) at the asymptotic limit of such a first-order decay function can be used as an estimation of total degradation R% occurring in sediments. Phytoplanktonic organic matter generally consists of carbohydrates, proteins, lipids and a socalled unidentified refractory fraction, which accounts for 12–87% of the total organic matter (Jewell and McCarty, 1971; Harvey et al., 1995). The individual R% values of these constituents decrease in the order carbohydrates proteins > lipids > total organic matter, and all R% values are reduced by about 20% under anoxic conditions (Harvey et al., 1995). This implies that the lipid/TOC ratio generally increases under suboxic and anoxic conditions. Such an increase of the lipid/ TOC ratios has been observed in surface sediments deposited within the oxygen-minimum zone in the Arabian Sea, demonstrating that suboxic conditions favor the selective preservation of lipids in comparison to TOC (Schulte et al., 2000). Six to eight times higher concentrations of lipids are also observed in core MD900963 during the glacial stages (Fig. 5), indicating a strongly enhanced preservation of these labile compounds relative to TOC, which is only enriched by a factor of three over the same intervals (Fig. 3). This differential preservation is best illustrated by considering the ratio between each biomarker concentration and its associated TOC value (Fig. 6). The excellent correlation between normalized lipids and the uranium concentration record provides strong evidence for a decrease of the sediment redox potential every 22 kyr. An increased flux of organic matter would increase the oxidant demand at the sediment/water interface, thereby steepening the redox gradient in the sediment and increasing the accumulation of authigenic uranium as well as the preservation of lipids. Along with the enhanced preservation of lipids, there is a relative increase in the total remineralization of organic matter, as indicated by the positive correlation

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between calcite dissolution records (Fig. 2), normalized lipids (Fig. 6), and the authigenic uranium profile (Fig. 6). During these periods of high organic matter flux to the sediments, the relative increase in total remineralization lowers the pH of the sediment porewaters which, in turn, favors calcite dissolution (e.g., Hales et al., 1994; Jahnke et al., 1994, 1997; Hales and Emerson, 1996, 1997; Adler et al., 2001). It thus becomes clear that both Eh, and pH decreased during high-productivity events, enhancing relative preservation of lipids but, at the same time, decreasing calcite preservation. Just changing bottom water [CO2
3 ] during episodes of elevated productivity would not produce the contrasted preservation of TOC and lipids as observed in core MD900963. Consequently, metabolic respiration in near-surface sediments is the most reasonable hypothesis for explaining the observed co-variation of proxies for Eh and pH. In a further step, we performed spectral and crossspectral analyses of organic and inorganic proxies using the SPECTRUM software developed by Schulz and Stattegger (1997). As summarized in Table 1, the variations in all geochemical parameters, as well as dissolution proxies, are dominated by a clear 19–23 kyr periodicity (orbital precession). The fluctuations are also in phase, and highly coherent with estimated PP variations in the precessional band (Table 1). This statistical analysis further supports the hypothesis of metabolically driven dissolution, invoked to explain the behavior of the studied parameters. 2.5. A simple model It is not easy to explain quantitatively such a clear positive correlation between lipids/TOC and dissolution records. During high-productivity periods, the degradation rate of organic carbon probably decreases slightly due to the suboxic conditions and the sealing effect of sediment burial. However, calcite dissolution is governed principally by the total breakdown of organic carbon, which itself depends on the product of organic matter accumulation and the rate of degradation. The crucial observation is that lipid/TOC ratios are also positively correlated with productivity fluctuations. This can be explained by the fact that, although lipids belong to the labile fraction and are thus more easily degraded than the bulk organic matter, the relative change between oxic and suboxic degradation rates is larger for lipids than for TOC (Harvey et al., 1995). The best quantitative approach would be to use a full chemical model, including both equilibrium and kinetic aspects (e.g., CoTReM described by Adler et al., 2001), applied to a sediment column accumulating steadily over several millennia. Such a simulation is still beyond present possibilities, not only because of excessive computation time (the time step is 5 min for CoTReM)

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Age (kyr) 0

10

20

30

40

a

productivity (MAR) TOC lipids

1.2

normalized units

1 0.8 0.6 0.4

anoxic degradation

anoxic degradation oxic degradation

0.2 0

1.2

0.2

1

anoxic degradation

anoxic degradation

0.15

0.8

0.6

0.1

0.4

lipids/TOC

dissolution index & degradation coefficient

b

oxic degradation

0.05 0.2

degrad coeff dissol index

lipids/TOC 0

0 0

10

20

30

40

Age (kyr) Fig. 7. Simple model simulation (see text for more details): (a) the green curve represents the input of organic matter to the sea-floor, the black curve shows the remaining total organic carbon fraction and the orange curve the remaining lipid fraction after degradation. For this simulation the initial lipids/TOC is 0.2 and the maximum oxic degradation rates, Rlipids% and RTOC%, are 99% and 95%, respectively. (b) The dotted curve shows the multiplicating factor that varies as a sine wave through time between 1 and Fanox (0.8 in this case) under fully anoxic conditions. The blue curve shows the lipids/TOC ratio after degradation. The pink curve represents the dissolution index assumed to be equivalent to the total organic carbon degradation.

but also because of the sensitivity of redox chemical reactions to very subtle changes in boundary conditions. For example, Adler et al. (2001) showed that great care should be taken to parameterise a diffusive boundary layer at the sediment-water interface. In addition, these authors pointed out that further refinements in modelling were needed incorporating redox reactions other than those involving O2, NO3, and carbon species. The re-oxidation of anoxic species (e.g. manganese) is another problem that also requires some further development of the CoTReM model. All these complications have probably affected the record in core MD900963, and may even have acted with variable intensities through geological time. In particular, the Cd/U ratio profile in core MD900963 shows evidence

for systematic remobilization of Cd after its initial deposition (Pailler et al., 2002). Nevertheless, we attempted to construct a simple conceptual model able to mimic the co-variation between carbonate dissolution and selective preservation of lipids. The model is based on several assumptions—admittedly oversimplified—for which numerical values can be changed. A co-variation between dissolution and lipids/TOC ratio can be reproduced with different combinations of numerical values for the different parameters. Fig. 7 shows the results of the model obtained with a set of degradation parameters in agreement with the literature and with an input productivity curve aimed at reproducing the case observed in core MD900963. In Fig. 7a the primary

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productivity signal (green curve), representing the flux of phytodetritus, reaching the seafloor shows a sinusoidal variations with a period of 22-kyr (the peak to trough amplitude is 3–1). The lipids/TOC ratio for this input function is assumed to be constant (0.2). Different maximal degradation rates are assumed for TOC (RTOC%) and lipids (Rlipids%) under fully oxic conditions (i.e. during a productivity minimum). Rlipids% is assumed to be larger than RTOC% (99% and 95%). This implies that the bulk of organic matter includes a resistant fraction in addition to the more labile fractions such as carbohydrates, proteins and lipids. In Fig. 7b degradation rates for all fractions are assumed to vary regularly in antiphase with the productivity signal: the lowest rates being reached during high productivity periods which create the most intense suboxic or anoxic conditions within the sediments. The decrease in the degradation rate is obtained by multiplying the maximum oxic degradation rates (RTOC% and Rlipids%) by a factor that varies as a sine wave through time, between 1 and Fanox under fully anoxic conditions (Fanoxo1; dotted line). As a consequence the minimum degradation rates for TOC and lipids become Fanox  RTOC% and Fanox  Rlipids%. The important point is that the same Fanox is used for both TOC and lipids, in broad agreement with the observations by Harvey et al. (1995). Because Rlipids%>RTOC%, lipids will always be more degraded than the bulk organic matter. However, Fanox being the same for both TOC and lipids, the resulting decrease in the degradation rates leads to an increase in the ratio between preserved lipids and preserved TOC (for the simulation in Fig. 7b Fanox is 0.8, as calculated from the results of Harvey et al., 1995). A calcite dissolution index (pink curve in Fig. 7b) is then computed and taken directly as equivalent to total organic carbon degradation, that is the difference between TOC in the input material and residual TOC preserved in the sediments (black curve in Fig. 7a). This . assumption is supported by Wenzhofer et al. (2001), who found that the molar ratio between calcite dissolution and organic carbon degradation ranges between 1 and 0.5. A problem is that the link between this dissolution index and the paleoceanographic dissolution indicators given in Fig. 1 remains to be quantified. In this model, we can introduce various values for all numerical parameters. It always indicates a positive correlation between dissolution index and lipids/TOC ratio if Rlipids%>RTOC% and Fanox is kept constant for all fractions. Fig. 8 shows this correlation for the modelling experiment depicted in Fig. 7 and indicates that important information is embedded in the form of the relationship between dissolution index and lipids/ TOC ratio. In this case, the relationship exhibits a reduced general slope when Fanox is decreased. This observation is intuitively understandable because a

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Fig. 8. Dissolution index positively correlated with the lipids/TOC ratio.

decrease in Fanox increases the contrast between lipids and TOC degradation. However, it also decreases the overall remineralization of TOC during high-productivity periods which, in turn, reduces the enhancement of calcite dissolution. Further details of numerical tests of, and results from these modelling experiments will be presented elsewhere.

3. Conclusions By using a multiproxy approach based on dissolution indicators and paleoproductivity reconstructions, as well as organic and inorganic geochemical profiles, we have shown that supralysoclinal dissolution occurs in the equatorial Indian Ocean, and has varied dramatically over the last three glacial cycles. Although the studied site of core MD900963 is located, and has always remained, well above the chemical lysocline, the variations in organic matter flux here have had a direct effect on calcite dissolution rates. The ultimate cause of this variability is cyclic variations in primary productivity, which are driven by orbital precession. High paleoproductivity episodes occur every 22 kyr, increasing the flux of organic matter to the seafloor. The proxy records indicate that significant remineralization occurs within the sediments, thus lowering both pH and Eh of the pore waters. These chemical changes leave very clear signatures in the sediments, i.e. an increase in calcite dissolution (as seen with proxies based on foraminiferal break-up and the >63 mm CaCO3/total CaCO3 ratio), as well as an increase in the ratio between preserved lipids and TOC

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and significant enrichments in redox-sensitive metals such as U, Cd and Mo. It remains difficult to calculate the concomitant increase in calcite dissolution and preferential lipid preservation during these events simply by taking into account the different chemical reactions occurring in sediment pore waters. Nevertheless, we have constructed a simple parameterized model in order to illustrate the mechanisms involved in a semi-quantitative manner. We should stress that paleoproductivity variations driven by orbital precession seem to be a general feature in the inter-tropical zone (Beaufort et al., 1997, 2001; Cayre et al., 1999; Rostek et al., 1994, 1997; Schubert et al., 1998; Villanueva et al., 1998). Although a precise carbon budget cannot be derived from studying a single core, we may expect that biologically mediated postdepositional dissolution of carbonate may have had a significant impact on the global carbon cycle over at least the last three glacial-interglacial cycles.

Acknowledgements S. Schulte benefited from a postdoc fellowship supported by the European Community (FMRXCT96-0046) as well as from a front end financing by the central research pool of the Carl von Ossietzky University, Oldenburg. Paleoclimate work at CEREGE is supported by CNRS (PNEDC) and the EC (HPRN. CT-2002-0221). J. Rullkotter kindly provided laboratory facilities at Institute for Chemistry and Biology of the Marine Environment (ICBM, Oldenburg, Ger. many). K. Mangelsdorf and T. Mohring are thanked for technical assistance during MPCL separation. Data are available at the PANGAEA data base (http:// www.pangaea.de/home/sschulte). We greatly appreciate the reviews of Lloyd Keigwin and Simon Robinson.

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