A biomarker perspective on paleoproductivity variations in two Late Quaternary sediment sections from the Southeast Atlantic Ocean

Organic Geochemistry 30 (1999) 341±366

A biomarker perspective on paleoproductivity variations in two Late Quaternary sediment sections from the Southeast Atlantic Ocean K.-U. Hinrichs a,b,*, R.R. Schneider a, P.J. MuÈller a, J. RullkoÈtter b a

Fachbereich Geowissenschaften, University of Bremen, PO Box 330 440, D-28334 Bremen, Germany Institut fuÈr Chemie und Biologie Des Meeres (ICBM), Carl von Ossietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, Germany

b

Received 6 February 1997; accepted 13 January 1999 (Returned to the author for revision 23 April 1998)

Abstract Sedimentary inventories of selected molecular biomarkers in two sediment sections from the African continental margin below highly productive surface waters in the Southeast Atlantic ocean, each spanning the last 76 kyr, were investigated and compared to each other. A principal component analysis of the stratigraphic data separated groups of biomarkers, representing di€erent assemblages of biological sources and/or di€erent diagenetic histories, related to their chemical structures and their metabolic role in the food chain. The concentrations of long-chain n-alkenones, ndiols, the major n-ketool, and loliolide and its epimer isololiolide, are correlated with paleoproductivity estimates that were calculated from organic carbon contents. This group of relatively refractory compounds very likely represents those planktonic communities, which contributed signi®cantly to surface water productivity and subsequent burial of organic carbon during the considered time interval. Concentrations of (dust-transported) terrigenous long-chain fatty acids (C24 to C28) are interpreted to be related to the intensity of upwelling-driving winds and are also signi®cantly correlated with high contents of organic matter. Concentrations of cholesterol and the major short-chain fatty acids are not related to the marine paleoproductivity signal, most likely as a consequence of their nonspeci®c origin and their metabolic role during heterotrophic feeding processes. Abundances of 24-ethylcholest-5-en-3b-ol and its saturated counterpart, 24-ethyl-5a-cholestan-3b-ol, the major steroid alcohols in both sedimentary environments, appear not to be related to any of the above mentioned components and probably represent the contribution of algal species adapted to nutrient conditions di€erent from those of other major primary producers. Di€erences between both environments, the Angola Dome and the Benguela coastal upwelling, are evident in the relative distribution of major components and the stratigraphic behavior of selected compounds relative to other variables. These di€erences are discussed in relation to speci®c characteristics of both oceanographic environments. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomarker; Late Quaternary paleoproductivity; Principal component analysis; Southeast Atlantic continental margin sediments; Long-chain alkenones; n-diols; n-ketols; Loliolide; Sterols; n-fatty acids

* Corresponding author. Present address: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, USA. 0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 0 0 7 - 8

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1. Introduction The investigation of biomarkers in marine sediments can provide valuable information concerning the sources of organic matter, the early diagenetic conditions in the sediment, and the paleoenvironmental conditions during times of deposition (e.g. Brassell et al., 1987; Prahl and Muehlhausen, 1989). The value of speci®c marine biomarkers to record variations of marine paleoproductivity is limited by uncertainties in the quantitative relationship between algal biomass and the eciency of preservation of speci®c algal lipids in the deep-sea sediment. The burial eciency is controlled by diagenesis in response to changing degrees of both water column and sediment oxygenation (Hartnett et al., 1998) and also to changing sediment texture (e.g. Hedges and Keil, 1995). An increase of sedimentation rate generally has a positive e€ect on sedimentary organic matter preservation (e.g. MuÈller and Suess, 1979) due to decreasing the exposure time to oxygen (Hartnett et al., 1998). However, in a sedimentary setting where the latter conditions can be assumed to have remained suciently constant over the time period considered, variations in sedimentary lipid biomarker concentrations should re¯ect variations of past surface-ocean planktonic communities which have mainly contributed to organic matter burial.

the surface waters of tropical origin in the Angola Basin tend to be only enriched in nitrate and phosphate, whereas silicate concentrations are relatively low (summaries in Summerhayes et al., 1995; Schneider et al., 1997; Kirst et al., 1998). At both sampling locations the ¯uvial transport of terrestrial material can be neglected. Sediments supplied by the Congo River are deposited further north in the Congo Fan where bulk sedimentary organic matter characteristics di€er signi®cantly from those in core GeoB 1016-3 in the Angola Basin (MuÈller et al., 1994). Terrigenous detritus, present in the sediments at both locations was mainly delivered from the African continent by tradewinds. This is supported by pollen analysis which show that Site GeoB-1016 did not receive plant material typical for the Congo out¯ow over the time period considered here (Shi Ning and Dupont, 1997; Dupont et al., in press). For the late Quaternary sediments investigated here, a predominant marine origin of total organic carbon (TOC) is indicated by the bulk characteristics of the organic material. The stable carbon isotopic ratios in samples from both sites range from ÿ21.7 to ÿ19.5PDB; TOC/N ratios range from 5.7 to 11 with lower values in sediments from the Cape Basin (5.7 to 8.7)

1.1. Study area We investigated sediment samples from piston cores underlying two di€erent highly productive surface waters in the Southeast Atlantic (GeoB 1016-3, Angola Margin; GeoB 1710-3, Cape Basin; Fig. 1), well known as Angola Dome oceanic upwelling and the Benguela coastal upwelling areas. Both records span the last 76,000 yr with sample intervals of less than 2000 yr on average, and each sample integrating a time period in the order of 200 yr. The last 76,000 yr were characterized by remarkable changes in marine paleoproductivity in response to trade wind-driven variations of upwelling, displaying the typical pattern of elevated levels of surface water production during glacial climate conditions (e.g. MuÈller et al., 1994; Summerhayes et al., 1995; Schneider et al., 1996; Kirst et al., 1998). The area o€ Walvis Bay represents a classical coastal upwelling regime, where cold nutrient-rich waters are advected to the surface by Ekman transport due to strong trade winds. In the eastern Angola Basin the supply of nutrients to the photic zone is induced by frontal mixing of nutrient-rich waters underlying a shallow thermocline. In contrast to the situation o€ Namibia, where South Atlantic Central Water with high contents of silicate, phosphate and nitrate upwells into the Benguela system, subsurface waters fertilizing

Fig. 1. Core locations of sediment samples analyzed in this study (modi®ed from Schneider et al., 1994).

K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

(MuÈller et al., 1994; and MuÈller, unpubl. data). TOC contents in the Cape Basin core GeoB 1710-3 (0.41 to 3.16 wt%; Kirst et al., 1998) are generally lower than those in the Angola Basin core GeoB 1016-3 (0.67 to 4.35 wt%, MuÈller et al., 1994). Lower contents in the Cape Basin sediments are interpreted to be due to the distance of the core location to the restricted coastal upwelling zone o€ Namibia, as evident from TOC data in a core transect perpendicular to the coast (Kirst et al., 1998). In contrast, the Angola Basin core is located directly under the oceanic frontal mixing system o€ Angola. Sedimentation rates at both sites were similar, ranging from 4.7 to 7 cm/kyr (mean=5.820.8 cm/kyr) at the Cape Basin site and from 3.6 to 8.3 cm/ kyr (mean=5.921.4 cm/kyr) at the Angola Basin site. Several investigators have demonstrated that late Quaternary TOC variations in east-equatorial Atlantic sediments are controlled by changes in marine productivity rather than by changes in the degree of TOC remineralization (Lyle, 1988; Sarnthein et al., 1988; Sarnthein et al., 1992; Westerhausen et al., 1993; MuÈller et al., 1994; Summerhayes et al., 1995). This observation was further supported by the correspondence of the TOC signal to temporal patterns of other proxy parameters that an indicative of ¯uctuations in sea surface temperatures (Schneider et al., 1996; Kirst et al., 1998), nutrient conditions (Holmes et al., 1997),

343

and carbon ¯ux to the sea ¯oor as indicated by benthic foraminiferal assemblages (Schmiedl and Mackensen, 1997). Paleoproductivity, determined from sedimentary TOC concentrations, was signi®cantly elevated during glacial periods. As an example for the core GeoB 1016-3 from the Angola Basin, Fig. 2 illustrates the relationship between paleoproductivity (calculated from TOC contents after MuÈller and Suess, 1979) and, both, alkenone sea surface temperatures (SST) and bulk sedimentary d15 N values (Holmes et al., 1997), respectively. The paleoproductivity record is inversely correlated with SST, exhibiting the relationship between elevated biological production of organic carbon and stronger upwelling intensity as indicated by low SST. Further support for the signi®cance of TOC content as a paleoproductivity indicator is given by the inverse correlation with d15 N values (Holmes et al., 1997). d15 N values are sensitive to nutrient utilization by marine primary producers (Altabet and Francois, 1994). Low d15 N values in sedimentary organic matter indicate a large pool of nitrate and are related to an incomplete nutrient utilization (e.g. Altabet and Francois, 1994). They occur contemporaneously with high paleoproductivity values (Fig. 2). This indicates that at times when enhanced paleoproductivity is indicated by the TOC record, nutrient levels must have been signi®cantly elevated. Based on these relationships, we assume that

Fig. 2. Illustration of the relationship between paleoproductivity (calculated from sedimentary TOC concentrations after MuÈller and Suess, 1979), alkenone sea surface temperatures (note the inverse scale), and sedimentary d15 N ratios in Angola Basin sediments.

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changes in sedimentary TOC content o€ Angola and Namibia have been triggered by paleoproductivity variations rather than by changes in organic carbon degradation in response to non-uniform diagenetic conditions (see also Schneider, 1991; Schneider et al., 1996; Schneider et al., 1997, for the discussion of the TOC signal).

1.2. Scienti®c objectives Downcore studies of abundances of selected biomarkers have gained increasing importance in paleoceanography and paleoenvironmental reconstruction. Several studies were performed in sediments from the Atlantic Ocean (e.g. Poynter et al., 1989; Poynter, 1989; Madureira et al., 1995; Madureira et al., 1997; Martinez et al., 1996; Villanueva et al., 1997). In terms of reconstructing paleoproductivity, the discussion so far was mostly restricted to long-chain alkenones (e.g. Zhao et al., 1995, Martinez et al., 1996; Villanueva et al., 1997) due to their clear relationship to their algal producers (Volkman et al., 1980a), their potential to allow an estimate of past sea surface temperatures (Prahl and Wakeham, 1987), together with their resistance toward degradation (e.g. Wakeham et al., 1997). Because both sedimentary settings are similar in two potential variables a€ecting the early diagenesis of organic matter, i.e. average sedimentation rate and water depth, these sediments are particularly suitable to interpret changes in the concentrations of a suite of sedimentary biomarkers as temporal variations in algal communities or input of land plant remains, which then can be compared to the more general record of marine paleoproductivity established from other proxies. Moreover, assuming that plankton communities are di€erent in response to the type of nutrient supply and mixing of water masses, the biomarker record may re¯ect characteristic di€erences in the transport of nutrients to surface waters between both environments. Especially, for sediments from the Angola Basin this biomarker approach may provide insight into a unresolved problem with regard to their intriguingly low biogenic opal contents (Schneider et al., 1997), indicating only a minor contribution of siliceous phytoplankton to the overall paleoproductivity. Such an interpretation is in contradiction to the common view that diatoms dominate algal communities in highly productive surface waters of pelagic frontal mixing zones (e.g. Kemp and Baldauf, 1993). Thus, the biomarker record may indicate whether low opal contents in the sediments are related to low diatom productivity over the last 80,000 yr or alternatively to a strong dissolution of siliceous tests during settling through the water column and burial in the sediment.

2. Material and methods 2.1. Sediment cores and previous studies Core GeoB 1016-3 (11846,9 S; 11840,9 E, 3411 m water depth) was recovered from the Eastern Angola Basin during R/V ``Meteor'' cruise 6/6 in 1988 (Wefer et al., 1988) while core GeoB 1710-3 (23825,9 S; 11841,9 E, 2987 m water depth) was obtained o€ Namibia in the Cape Basin in 1992 during Meteor cruise 20/2 (Schulz et al., 1992). The chronology for the sediment cores is based on foraminiferal oxygen isotope records (Schneider et al., 1995; Kirst et al., 1998). Amongst the parameters investigated so far were alkenone concentrations in order to determine alkenone-based sea surface temperatures for the entire cores. For this purpose a continuous series of samples at 5 cm depth intervals from each core was extracted and analyzed by gas chromatography (GeoB 1016-3: MuÈller et al., 1994; GeoB 1710-3: Kirst et al., 1998). Extracts were obtained from 0.5 to 1 g aliquots of the freeze-dried sediment by a modi®ed ¯ow-blending technique (Radke et al., 1978) using an Ultra-Turrax T25 at 24,000 rpm and successively less polar solvent mixtures (MeOH, MeOH/CHCl3 1:1, CHCl3), each for 5 min. For this study, we used the remainders of extracts from alkenone analysis, which in the meantime were stored at 08C. SST values were calculated using the calibration of Prahl et al. (1988). The analytical methods applied for elemental analysis and the determination of d13 C ratios of organic carbon were described by MuÈller et al. (1994). 2.2. Analysis of lipid biomarkers Aliquots of total extracts were treated with Nmethyl-N-trimethylsilyl-tri¯uoroacetamide (MSTFA) to transform alcohols into their trimethylsilyl ethers and fatty acids into the corresponding esters. For this purpose, 25 ml dichloromethane and 25 ml MSTFA were added to the extract. The mixture was heated for one hour at 708C. 2 to 4 ml aliquots of this mixture were analyzed by gas chromatography (GC) on a Hewlett Packard 5890 instrument equipped with a fused silica capillary column (HP Ultra 1, length = 50 m, inner diameter = 0.32 mm, ®lm thickness = 0.52 mm) in split injection mode. Helium was used as the carrier gas, and the temperature of the GC oven was programmed from 508C to 3208C (40 min isothermal) at a rate of 58C/min. Quanti®cation of single compounds was performed relative to the amount of the internal standard octacosanoic acid methyl ester (5 mg). Data acquisition and identi®cation of single component peaks was performed with the Nelson analytical integration software. In the case of partly coeluting

K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

peaks, the integration was done by setting baselines manually. Gas chromatography±mass spectrometry (GC±MS) analyses were performed with a Hewlett Packard 5890 series II gas chromatograph, equipped with a fused silica capillary column (J&W DB 5, length = 30 m, inner diameter = 0.25 mm, ®lm thickness = 0.25 mm) and a temperature programmable injector (Gerstel CIS), and coupled to a Finnigan SSQ 710 B mass spectrometer operated at 70 eV. The temperature of the GC oven was programmed from 608C (2 min isothermal) to 3008C (50 min isothermal) at a rate of 38C/ min. Compound identi®cations are based on comparison of relative gas chromatographic retention times and mass spectra with those reported in the literature. 2.3. Principal component analysis In order to distinguish di€erent sources of organic matter and to link these to certain paleoenvironmental conditions, we performed a principal component analysis of the concentrations of selected biomarkers combined with other variables of known environmental signi®cance. The use of principal component analysis (PCA), one of several factor analysis techniques, is an appropriate way to reduce data sets containing a high number of variables. By reducing the number of original variables to a smaller number of independent variables, this approach highlights fundamental di€erences between groups of variables. Factor analysis techniques have been applied occasionally in molecular stratigraphic approaches in Late Quaternary sediments, focusing on paleoenvironmental aspects (e.g. Poynter, 1989; Farrimond et al., 1990; McCa€rey et al., 1991). PCA identi®es dimensions of maximum variation in the original multidimensional data space, resulting in terms of vectors (factors) that are linear combinations of the original variables. The linearly independent factors can be rotated in variable space while remaining orthogonal. A common approach chosen (like in our study) is using the solution obtained with the varimax rotation of the factor axes which leads to the simplest factor composition with either minimum or maximum loadings of each variable on the extracted factors (Mardia et al., 1979). The original data can be represented as coordinates (scores) on the principal component vectors, providing a low-dimensional representation that summarizes the bulk of the variance in the data set. Statistical evaluation of the multivariate data sets was performed using the StatView 4.0 program, applying a factor analysis method with major features of a 1

The complete data set used in this study is available upon request from the ®rst author.

345

principal component analysis. The resulting factors were calculated from correlation matrices of the data set. The solutions presented here were obtained by a Varimax rotation of linearly independent and orthogonal factors. As data input we used the original data set. In addition to biomarker concentrations, TOC/N ratios, d13 C ratios of TOC, d15 N ratios of bulk sediment (Angola Basin only), and alkenone-derived SST data were used in the multivariate analysis (Table 1). The latter two variables with paleoenvironmental signi®cance independent from ¯uxes were chosen to illustrate the relationship between cold or warm water masses with di€erent nutrient concentrations and lipid distributions. This should provide an aid for easier interpretation of the resulting factors. d13 C ratios of TOC and TOC/N values would reveal possible terrigenous contributions to the sedimentary organic matter. 3. Results and discussion 3.1. Investigated biomarkers and their environmental signi®cance In general, the dominant biomarkers in the sediments from both sites are mainly from marine sources, which is in agreement with the stable carbon isotopic composition of the bulk organic matter (MuÈller et al., 1994). However, the presence of compounds from terrestrial plant waxes indicates the contribution of terrigenons material to the sediment. A summary of studied compounds and their potential biological source is provided in Table 2. Average concentrations of the major biomarkers are presented in Fig. 31. Among the analyzed components, long-chain alkenones, the compound pair loliolide and iso-loliolide, and the steroid alcohols dinosterol and 24-methylcholesta-5,22-dien-3b-ol can be speci®cally assigned to phytoplankton sources. Biomarker concentrations (normalized to dry weight of sediment) are generally higher in sediments from the Angola Basin (Fig. 3b). 3.2. Biomarkers from autochthonous sources 3.2.1. Long-chain alkenones Long-chain alkenones are exclusive biosynthetic products of some haptophytes (Volkman et al., 1980a, 1998; Marlowe et al., 1984). High average concentrations of C37 alkenone were observed at both locations (Fig. 3). A detailed discussion of alkenone data from the Angola Basin core was published by MuÈller et al. (1994) and Schneider et al. (1995) while those from the Cape Basin core are described by Kirst et al. (1998). These contributions indicate that alkenone-producing microalgae have always been an important part

upwelling intensity nutrient utilization (nutrient level) ÿ[CO2]aq ÿ(marine/terrestrial mixing indicator?) type of organic matter

Sea surface temperature d15 Nbulk d13 Corganic matter C/N ratio

Schneider et al., 1995; MuÈller et al., 1994 Altabet and Francois, 1994; Holmes et al., 1997 Hayes, 1993; MuÈller et al., 1994 (GeoB 1016-3) Jasper and Gagosian, 1989

Reference 22.321.4 6.420.9 ÿ19.920.4 9.820.8

Average GeoB 1016-3 15.821.4 n.a. ÿ20.320.5 7.320.7

Average GeoB 1016-3

Klok et al. (1984a, 1984b); Repeta (1989)

eustigmatophyceae algae, unknown microalgae eustigmatophyceae algae, unknown microalgae degradation product mainly of fucoxanthin (major carotenoid in diatoms and haptophytes) marine algae (zooplankton) major sterol in many diatoms and haptophytes higher plants, microalgae higher plants, microalgae dino¯agellates marine plankton and bacteria marine plankton marine plankton and bacteria higher plants, land-derived aerosols, marine bacteria marine algae unknown marine algae, major n-alkane in the marine diatom Rhizosolenia setigera unknown higher land plants

C22 n-alkane C29, C31 n-alkane

C18 n-alcohol C20 n-alkane C21 n-alkane

Eglinton and Hamilton (1967)

Blumer et al. (1971)

Gagosian and Nigrelli (1979) Gagosian et al. (1983) Huang and Meinschein (1976); Volkman (1986) Huang and Meinschein (1976); Volkman (1986) Boon et al. (1979) e.g. Wakeham (1995); Gong and Hollander (1997) e.g. Wakeham (1995) e.g. Wakeham (1995); Gong and Hollander (1997) Kollatukudy (1976); Gagosian et al. (1981); Gagosian et al. (1987); Gong and Hollander (1997) Kollatukudy (1976)

Volkman et al. (1992) Volkman et al. (1992)

eustigmatophyceae algae, unknown microalgae eustigmatophyceae algae, unknown microalgae

n-C2±8 1,14-diol n-C30 1,x-diol; Angola Basin: x=15, Cape Basin: x=13, 14, 15 n-C32 1,15-diol n-Triacontan-1-ol-x-one; Angola Basin: x=15, Cape Basin: x=13 Loliolide+isololiolide

Cholesterol 24-Methylcholesta-5,22-dien-3b-ol 24-Ethylcholest-5-en-3b-ol 24-Ethyl-5a-cholestan-3b-ol Dinosterol C16:0 n-fatty acid C18:1o9 n-fatty acid C18:0 n-fatty acid C24+C26+C28 n-fatty acids

Volkman et al. (1980a, 1980b); Marlowe et al. (1984) Volkman et al. (1992) Volkman et al. (1992)

prymnesiophyceae algae

n-C37 alkenones

References

Major sources/signi®cance

Compound

16

7 8 9 10 11 12 13 14 15

6

4 5

2 3

1

Symbol Fig. 3

Table 2 Biomarker compounds, used for principal component analysis of data from sediments from the Cape Basin and Angola Basin (indicated are their major biological sources and corresponding references, n-alkanes were selected according to their signi®cance in n-alkane carbon number distributions Fig. 4)

Environmental signi®cance

Environmental variable

Table 1 Alkenone sea surface temperatures and sedimentary bulk parameters, integrated in principal component analysis of data from cores GeoB 1016-3 and GeoB 1710-3 together with further information concerning environmental signi®cance and mean values in both sediment sections (n.a.: not analyzed)

346 K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

347

Fig. 3. Column plot of average concentrations of selected major components together with standard deviations in sediments from (A) the Angola Basin and (B) the Cape Basin. Compound names are replaced by numbers which are identi®ed in Table 2. Diols concentrations are the sum of coeluting isomers (Table 2). SCFA=short chain fatty acids; LCFA=long chain fatty acids.

of the autochthonous production of organic matter in surface waters of both settings. 3.2.2. Loliolide and isololiolide Loliolide and isololiolide are the end members of the degradation pathways of some major carotenoids (Klok et al., 1984a; Repeta, 1989). In contrast to their

extremely labile precursors, these components appear to be resistant to further geochemical degradation and therefore complement the information obtained by other plankton-derived biomarkers. Loliolide is derived on a mole to mole basis from the degradation of fucoxanthin (Repeta, 1989), the major carotenoid in a suite of phytoplanktonic algae, with diatoms and hap-

348

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tophytes being the major source organisms (Repeta and Gagosian, 1982; Je€rey and Vesk, 1997). The presence of both loliolide isomers was previously reported from sediments underlying highly productive surface waters on the Namibian shelf (Klok et al., 1984b) and the Peruvian shelf (Repeta, 1989) and from Mediterreanean sapropels (ten Haven et al., 1987; Z. Rinna, pers. commun., 1997). Due to the relatively constant ratio between the two isomers in samples from both investigated locations, we only report the summed concentrations (Fig. 3). 3.2.3. Alkanediols and -ketols Another group of well represented planktonic biomarkers are alkanediols and -ketols, although the main planktonic organisms contributing these compounds to the sediment are still unknown (see review of Versteegh et al., 1997b). Volkman et al. (1992) isolated alkanediols from yellow-green microalgae of the class eustigmatophyceae. The authors further suggest that several related microalgal species may be potential biological sources. The presence of these compounds is typical for sediments underlying high-productivity surface waters. For example, high abundances were reported from Peru upwelling sediments (e.g. McCa€rey et al., 1991), the Arabian Sea (e.g. Schulte, 1997) and for Mediterranean sapropels (ten Haven et al., 1987; RullkoÈtter et al., 1998). Concentrations of alkanediols and -ketols are higher in sediments from the Angola Basin than in the Cape Basin (Fig. 3). The distributions of individual compounds are remarkably di€erent between both sites. Di€erences are most likely related to di€ering algal communities as a consequence of di€erent water temperatures and mechanisms of nutrient transport to the ocean surface. The distributions in glacial sediments of the tropical Angola Basin are dominated by triacontane-1,15-diol with average concentrations of about 4.3 mg/g dry sediment which is in a range similar to those of major short-chain fatty acids (Fig. 3, #3). The biosynthetically or diagenetically related ketool triacontane-1-ol-15-one is also present in high concentrations (Fig. 3, #5). The C28-diol only occurs at low concentrations and was therefore not used for statistical evaluation. In contrast to the Angola Basin, the latter compound is the major diol detected in sediments from the Cape Basin (Fig. 3b, #2), closely followed by the C30 compound (Fig. 3b, #3; mixture of three coeluting isomers with the mid-chain hydroxyl group located at positions C-13 to C-15). In Cape Basin sediments, the C32 compound is present in low concentrations (Fig. 3b, #4). A preliminary study of changes in the diol and ketool distribution in sediments of the Southwest African continental margin was performed by Versteegh et al. (1997a), which relates variations this compound group in core GeoB 1016-3 to a shift of

plankton communities typical for the Benguela upwelling system northward into the Angola Basin during glacials. 3.2.4. Steroid alcohols The sterol composition in sediments at both sites is relatively simple. In total extracts, only ®ve compounds were present in considerable concentrations. The simplicity of the composition is most likely related to strong diagenetic degradation during transport through the water column into deep-water sediments under well-oxygenated conditions. This observation is in agreement with high sterol degradation rates in the Southeast Atlantic Ocean reported from analyses of sediment traps and surface sediments (Andersen, 1996). Among the detected steroid alcohols, dinosterol is almost uniquely produced by dino¯agellates (Boon et al., 1979) although exceptional occurrences have been observed in diatoms (Volkman et al., 1993). Average concentrations of dinosterol are low (Fig. 3, #11). 24-Methylcholesta-5,22-dien-3b-ol is the major sterol in a variety of diatoms and also in some prymnesiophytes with Emiliania huxleyi being the most prominent organism (Volkman, 1986). According to Conte et al. (1995) 24-methylcholesta-5,22-dien-3b-ol is the dominant steroid alcohol in water column samples collected after massive blooms of Emiliania huxleyi. This sterol is present in low concentrations in Cape Basin sediments (average 0.2 mg/g dry sediment) whereas in sediments from the Angola Basin (Fig. 3, #8) its presence can be neglected due to concentrations close to the detection limit. Considering the high concentrations of long-chain alkenones produced by haptophytes, this sterol must have been selectively removed by feeding processes in the food chain and during early diagenesis in the sediment. In most of the samples from both sites, 24-ethylcholest-5-en-3b-ol is the most abundant individual sterol (Fig. 3, #9). The saturated analog 24-ethyl-5a-cholestan-3b-ol is also present in signi®cant concentrations (Fig. 3, #10). Although 24-ethylcholest-5-en-3b-ol is the major sterol in terrestrial higher land plants (e.g. Huang and Meinschein, 1976), its presence in highly productive oceanic settings is generally attributed to planktonic sources (Volkman, 1986). This compound is the major D 5-stenol in some algae from the class xantophyceae (Volkman, 1986). With a few exceptions, its occurrence can be neglected in other major phytoplankton classes, and is almost not present in dino¯agellates (Volkman, 1986). There are also existing reports assigning these compounds to the biosynthesis by cyanobacteria (e.g. Volkman, 1986; Kohlhase and Pohl, 1988). However, experiments with cultures of di€erent strains of cyanobacteria (M. Rohmer, pers. commun., 1998) lead to the conclusion that the observation of sterols can be explained by contamination

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problems during cultivation and that these prokaryotes are not an exception to the almost exclusive rule of prokaryotes being unable to synthesize steroids. The saturated analog 24-ethyl-5a-cholestan-3b-ol can be generated by microbial reduction of the D 5 double bond (e.g. Gaskell and Eglinton, 1975; Nishimura and Koyama, 1977; Gagosian and Heinzer, 1979). A primarily origin of this compound from de novo synthesis is considered more likely due to the absence of other 5a-stanols in these sediments, e.g. 5a-cholestan-3b-ol is only present at trace levels. High relative amounts of 24-ethylcholest-5-en-3b-ol were previously reported for other sedimentary environments with mainly marine organisms contributing to the sedimentary organic matter (e.g. Mediterranean Sea: RullkoÈtter et al., 1998; Arabian Sea: Schulte, 1997; Peru upwelling: Volkman et al., 1987; Wakeham, 1987). In sediments from the Cape Basin (GeoB 17103), strikingly high concentrations of 24-ethylcholest-5en-3b-ol of 28.6 mg/g and 12.1 mg/g dry sediment, respectively, occur in two near-surface sediment samples (3 cm and 23 cm depth) of Holocene age. In more deeply buried samples, concentrations are relatively uniform at a lower level and range from 0.26 to 2.96 mg/g dry sediment. This pronounced decrease in concentration may be related to a substantial diagenetic loss of these compounds in more deeply buried sediments and/or to a signi®cantly elevated biological supply to the Holocene sediments. Cholesterol in marine environments originates from a variety of planktonic organisms of all trophic levels (Gagosian and Nigrelli, 1979; Gagosian et al., 1983) with zooplankton being the major source (Volkman et al., 1987). Average concentrations of cholesterol are 1.22 mg/g dry sediment in the Angola Basin sediments and 0.76 mg/g in sediments from the Cape Basin (Fig. 3, #7). 3.3. Compounds from less speci®c and allochthonous sources 3.3.1. Short-chain fatty acids The most abundant single components in the free lipid fractions at both locations are the n-C16:0, n-C18:1 and n-C18:0 fatty acids (Fig. 3, #12±14). Their occurrence in aquatic environments is commonly assigned to planktonic sources (e.g. see review in Wakeham, 1995). Gong and Hollander (1997) reported for sediments from the Santa Monica Basin that heterotrophic bacteria can contribute signi®cantly to the pool of these fatty acids (n-C16:0 and n-C18:0). The n-C18:1og component is often enriched in algae of the class haptophyceae (e.g. Volkman et al., 1981). However, also terrestrial sources have to be considered because these fatty acids represent a signi®cant lipid fraction in con-

349

tinental derived aerosols (Gagosian et al., 1981, 1987). In most samples from both locations, n-hexadecanoic acid is the most abundant fatty acid with average concentrations of 5.1 mg/g dry sediment in the Angola Basin and 1.7 mg/g in the Cape Basin, respectively (Fig. 3, #12). In samples from the Angola Basin, the concentrations of n-octadecenoic acid (C18:1og, assignment of the double bond is based on relative retention time) and n-octadecanoic acid (C18:0) are relatively uniform and vary around 4 mg/g dry sediment with slightly higher concentrations of the monounsaturated compound. In Cape Basin sediments (GeoB 1710-3), the two quanti®ed C18 acids occur at concentration levels about half of that of the saturated C16 compound. With a few exceptions, the concentrations of C16 and C18 fatty acids in Cape Basin sediments are relatively uniform. However, a few samples are characterized by concentrations of more than 10 mg/g per single component. It has to be noted for both sites that the monounsaturated C16:1 fatty acid only occurs in concentrations one to two orders of magnitude lower than those of its saturated analog. This compound is the major fatty acid in the biomass of diatoms (e.g. Volkman et al., 1980b; Smith et al., 1983). The low abundance of this compound in both settings could be related either to a selective loss of this compound in the marine food web (e.g. Harvey et al., 1987) or to low diatom productivity. 3.3.2. Long-chain fatty acids (LCFA) The most abundant fatty acids with more than twenty carbon atoms are the saturated n-C24:0, n-C26:0 and n-C28:0 homologues. In all samples analyzed the distribution is clearly dominated by n-hexacosanoic acid which occurs in twice the concentration of the C24 and C28 homologues. The summed average concentrations of these three compounds are about 4.8 mg/g dry sediment in sediments from the Angola Basin and about 2.6 mg/g in the Cape Basin, respectively. Their average concentration relative to TOC is similar at both sites. In general, concentrations of LCFA are signi®cantly higher in glacial than in Holocene sediments. As long-chain acids occur in higher land plant waxes in high concentrations (e.g. Kollatukudy, 1976), their presence in marine sediments is commonly assigned to terrestrial sources. They were observed in high concentration in eolian dust where they are associated with wax alkanes and wax alcohols (e.g. Simoneit et al., 1977; Gagosian et al., 1981; Gagosian et al., 1987). Prahl (1992) reported high relative concentrations of eolian-transported LCFA in sediments from the central equatorial Paci®c. On the other hand, sediments with almost exclusive terrigenous supply by rivers like those from the Amazon Fan were reported to contain only minor amounts of LCFA (Hinrichs and RullkoÈtter,

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1997). A similar observation was made in sediments from the Ban Bay which receive signi®cant amount of terrestrial organic matter by glacier transport (ten Haven and RullkoÈtter, unpubl. results). However, also marine organisms can be potential sources of LCFA. There is some evidence for their biosynthesis in a variety of marine algal species (Volkman, pers. commun., 1997). Isotopic compositions of these fatty acids in sediments from the Santa Monica indicate their partial derivation from bacteria (Gong and Hollander, 1997).

3.3.3. n-Alkanes The carbon number distributions of n-alkanes in both sediment cores exhibit unusual patterns. Representative examples for the carbon number range from 18 to 33 are illustrated in Fig. 4. The higher land plant contribution in all three examples (Fig. 4a±c) is obvious from an odd-over-even carbon number predominance of long-chain homologues typical for plant waxes, maximizing at n-hentriacontane. Analyses of selected samples with coupled gas chromatography± mass spectrometry did not show any evidence of con-

Fig. 4. Typical n-alkane distributions in the carbon number range from 18 to 33 in samples from the Angola Basin (A) and the Cape Basin (B, C). Relative intensities correspond to those of peaks in the m/z 85 mass chromatograms. Compounds used in the principal component analysis are marked with asterisks.

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tamination of the sediments with hydrocarbons from more deeply buried or eroded rocks containing mature organic matter (e.g. as demonstrated for subrecent sediments from the Santa Barbara Basin by Hinrichs et al., 1995). This is indicated by the absence of diagnostic 17a,21b-hopanes. All investigated samples from the Angola Basin are characterized by a pronounced even-over-odd carbon number predominance in the range from 18 to 24 with a maximum at n-icosane (Fig. 4a). A second maximum occurs at n-nonacosane and is most likely related to a contribution of higher plant waxes to the sediments (e.g. Eglinton and Hamilton, 1967; Prahl and Carpenter, 1984). In sediments from the Cape Basin we observed two typical n-alkane patterns. The two extreme cases are visualized in Fig. 4b and c. One is characterized by a slight even-over-odd carbon number predominance in the carbon number range from 20 to 26 with a maximum at n-tetracosane (Fig. 4b). The second distribution maximizes at n-henicosane (Fig. 4c). n-Henicosane is the major hydrocarbon of the marine diatom Rhizosolenia setigera (Blumer et al., 1971). The predominance of this compound in the lower carbon number range was previously observed in the n-alkane distributions of selected samples from the

351

Arabian Sea (Schulte, 1997). n-Alkane distributions with an even-over-odd carbon number predominance are known from Antarctic zooplankton (Nachman, 1985; Reinhardt and Vleet, 1986a; Reinhardt and Vleet, 1986b) but have not been reported for tropical and subtropical plankton species. Similarly, unusual nalkane patterns with maxima at n-icosane and n-docosane were observed in dissolved organic matter and surface sediments on the Amazon continental shelf in the western tropical Atlantic (Elias and Cardoso, 1996). It is reasonable to assume that the unusual nalkane patterns observed here are related to unknown planktonic sources rather than to contamination during coring or analysis in the laboratory. In order to obtain additional information about potential associations of characteristic n-alkane patterns with other biomarker occurrences, we used selected compounds typical for each type of distribution (Fig. 4, marked with asterisks; selection of n-docosane instead of n-tetracosane due to coelution of the latter compound with an unknown compound) for multivariate analysis. In addition, we included the concentrations of n-nonacosane (Angola Basin) and n-hentriacontane (Cape Basin) to check the signi®cance of terrestrial organic matter supply to the sediments.

Table 3 Compound loadings on three factors explaining 82% of the variance in 43 samples of core GeoB 1710-3, 3±423 cm (2±76 kyr b.p.), Cape Basin Factor

1

2

3

% of total variance Eigenvalue Compound/variable

44.8 8.96

21.8 4.35

15.4 3.08

C37 n-alkenones Loliolide+isololiolide n-C30 1,x-diol (x=13, 14, 15) C21 n-alkane C24+C26+C28 n-fatty acids Dinosterol n-C28 1,14-diol TOC/N ratio 24-Methylcholesta-5,22-dien-3b-ol SST n-Triacontan-1-ol-13-one 24-Ethyl-5a-cholestan-3b-ol 24-Ethylcholest-5-en-3b-ol C22 n-alkane C16:0 n-fatty acid C18:1 n-fatty acid C18:0 n-fatty acid Cholesterol C31 n-alkane d13 Corg

0.906 0.893 0.890 0.834 0.816 0.810 0.789 0.788 0.728 ÿ0.694 0.665 ÿ0.019 ÿ0.193 0.007 0.173 0.09 0.092 0.352 0.448 0.232

ÿ0.154 0.133 0.131 0.069 0.064 0.184 ÿ0.006 ÿ0.304 0.565 0.416 0.665 0.961 0.943 0.863 0.138 0.248 0.321 ÿ0.032 0.03 0.383

0.014 ÿ0.199 0.095 0.387 0.413 0.388 0.271 0.179 0.159 ÿ0.013 ÿ0.08 0.125 0.073 0.455 0.914 0.902 0.877 0.873 0.658 ÿ0.53

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3.4. Molecular stratigraphy ± multivariate analysis PCA results are compiled in Table 3 (Cape Basin) and 4 (Angola Basin), listing factor loadings of each variable for both sites. Fig. 5 (Cape Basin) and 6 (Angola Basin) show plots of Factor 1 vs. Factor 2 loadings for each site. 3.4.1. Factor 1 ± Cape Basin (GeoB 1710-3) In sediments from the Cape Basin (GeoB 1710-3), spanning the last 76 ka, three factors accounting for 82% of the variance in the data set were extracted by Varimax rotation. Factor 1 describes 44.8% of the total variance. High loadings are carried by C37 alkenones, the compound pair loliolide and isololiolide, the C21H44 n-alkane, C28 and C30 diols together with the C30 ketool, and by 24-methylcholesta-5,22-dien-3b-ol and dinosterol. The concentrations of C24 to C28 evencarbon-numbered fatty acids also display a high loading on Factor 1. SST data are signi®cantly negatively loaded on Factor 1, indicating a relationship of increasing abundances of the lipids contributing to Factor 1 with decreasing sea surface temperatures. Especially diols, alkenones, long-chain fatty acids, loliolide and isololiolide, dinosterol and the C21 nalkane are characterized by similar loadings on the ®rst two factors (Fig. 5).

3.4.2. Factors 2 and 3 ± Cape Basin (GeoB 1710-3) Factor 2 explains 21.8% and Factor 3 15.4% of the variance in the data set (Table 3). Factor 2 is comprised of the highly loaded compounds 24-ethyl-5acholestan-3b-ol, the corresponding stenol, 24-ethylcholest-5-en-3b-ol, and the C22 n-alkane. The two sterols display similar loadings on all extracted factors. Also triacontane-1-ol-13-one, already highly loaded on Factor 1, is signi®cantly loaded on Factor 2, mainly due to maximum concentrations coinciding with those of both steroid alcohols. Factor 3 combines the three most abundant short-chain fatty acids (n-C16:0 and nC18:1 and n-C18:0) and the steroid alcohol cholesterol (Tab. 3). The C31 n-alkane is also signi®cantly loaded on Factor 3. 3.4.3. Factor 1 ± Angola Basin (GeoB 1016-3) The compositions of the two ®rst extracted factors in the sample set from the Angola Basin display important similarities with Factors 1 and 3 for sediments from the Cape Basin. The ®rst factor accounts for 43.7% of the total variance in the data set. Factor 1 is comprised of C37 alkenones, triacontan-1-ol-15-one, long-chain fatty acids, C30 and C32 1,15-diols and the compound pair loliolide and isololiolide. These compounds cluster in the correlation diagram of Factor 1 vs. Factor 2 (Fig. 6) in a sharply limited area,

Fig. 5. Compound loadings on Factor 1 vs. Factor 2 for the principal component analysis of stratigraphic biomarker data of sediments from the Cape Basin. Hatched area marks components with signi®cant positive loadings on Factor 1.

K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

suggesting that the processes controlling their accumulation are closely related to each other. Signi®cant negative loadings are carried by the two environmental variables d15 Nbulk and SST, indicating an inverse relationship to concentrations of lipids with high loadings on Factor 1. 3.4.4. Factor 2 ± Angola Basin (GeoB 1016-3) The composition of Factor 2 is similar to that of Factor 3 extracted from the biomarker distribution in core GeoB 1710-3 from the Cape Basin. Factor 2 accounts for 16.4% of the variance in the Angola Basin data set. Compounds with maximum loadings on Factor 2 are the n-C16:0 and n-C18:1 fatty acids as well as the two steroid alcohols cholesterol and dinosterol. In contrast to Cape Basin sediments, the loading of the C18:0 fatty acid is not signi®cant. Similar to the C31 n-alkane in Cape Basin sediments, the C29 n-alkane in Angola Basin sediments displays an elevated loading on the factor associated with cholesterol and the n-C16:0 and n-C18:1 fatty acids (Table 4). 3.4.5. Factors 3, 4, and 5 ± Angola Basin (GeoB 10163) Factor 3 explains 9.7% of the variance in the data set. The two compounds with a signi®cant loading on

353

Factor 3 are the C18 n-alcohol and the C20 n-alkane. The latter compound is characteristic for the unusual distribution of n-alkanes in the lower carbon number range (Fig. 4). As n-alcohols in the lower carbon number range are commonly assigned to marine algae, we suggest that Factor 3 represents a planktonic signal with a non-speci®c signi®cance. Factor 4 explains 6.7% of the variance in the data set. It is comprised of the sedimentary TOC/N ratio and the d13 C ratio of organic matter. Factor 5 describes 5.2% of the variance in the data set and its composition is similar to that of Factor 2 in Cape Basin sediments, with signi®cant loadings of 24-ethyl-5a-cholestan-3b-ol and 24-ethylcholest-5-en-3b-ol. 3.5. Signi®cance of stratigraphic biomarker data The factor composition obtained from PCA can be interpreted in terms of three alternative controlling processes. The lipids combined in a factor either: 1. are derived from marine organisms that have reacted similarly to environmental changes (e.g. marine plankton reacting to changes in hydrographic conditions (upwelling) and nutrient levels) or 2. re¯ect associated processes like increased input of

Fig. 6. Compound loadings on Factor 1 vs. Factor 2 for the principal component analysis of stratigraphic biomarker data of sediments from the Angola Basin. Hatched area marks components with signi®cant positive loadings on Factor 1.

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Table 4 Compound loadings on three factors explaining 81.7% of the variance in 40 samples of core GeoB 1016-3, 6±423 cm (1±77 kyr b.p.), Angola Basin Factor

1

2

3

4

5

% of total variance Eigenvalue Compound/variable

43.7 8.75

16.4 3.28

9.7 1.95

6.7 1.35

5.2 1.04

C37 n-alkenones n-Triacontan-1-ol-15-one C24+C26+C28 n-fatty acid n-C30 1,15-diol n-C32 1,15-diol Loliolide+isololiolide d15 Nbulk SST C16:0 n-fatty acid Cholesterol Dinosterol C18:1 n-fatty acid C18:0 n-fatty acid C18 n-alcohol C20 n-alkane C29 n-alkane d13 Corg C/N ratio 4-Ethyl-5a-cholestan-3b-ol 24-Ethylcholest-5-en-3b-ol

0.928 0.894 0.876 0.863 0.844 0.681 ÿ0.765 ÿ0.698 0.014 0.059 0.199 0.482 ÿ0.205 ÿ0.025 0.177 0.375 0.213 0.388 0.207 0.225

0.035 0.148 0.204 0.023 0.197 0.143 ÿ0.019 ÿ0.137 0.943 0.905 0.808 0.760 0.368 0.157 0.199 0.469 0.162 0.253 0.049 0.363

ÿ0.024 0.050 0.001 ÿ0.066 ÿ0.064 0.384 ÿ0.284 ÿ0.014 0.099 0.069 ÿ0.118 0.117 ÿ0.121 0.887 0.828 0.068 0.073 0.196 0.395 0.228

ÿ0.037 0.225 0.182 0.251 0.024 0.170 ÿ0.473 ÿ0.577 ÿ0.018 0.160 0.238 0.013 ÿ0.408 ÿ0.234 0.141 0.187 0.836 0.726 0.206 0.173

ÿ0.057 0.240 0.113 ÿ0.016 0.100 0.354 0.111 0.051 0.158 0.006 0.182 0.063 0.479 0.034 0.255 0.462 0.259 0.152 0.801 0.699

terrestrial compounds due to stronger trade winds or 3. display a similar diagenetic behavior. Considering the above mentioned mechanisms, we will further discuss the environmental signi®cance of the biomarker data on the basis of PCA results and stratigraphic records of selected biomarkers. Biomarker time series from the Cape Basin are presented in Fig. 7: (a) PCA Factor 1 score and TOC-based paleoproductivity estimates, (b) C37 alkenones and loliolides, (c) long-chain fatty acids and C31 n-alkane, (d) cholesterol and dinosterol. Equivalent records for the Angola Basin are illustrated in Fig. 8a±d. With a few exceptions, the composition of Factor 1 in both sedimentary environments displays important similarities. Alkenones, diols, ketols, loliolides, all compounds originating from marine primary producers, are highly loaded on Factor 1. Abundances of these lipids are negatively correlated with the alkenone SST data, indicating higher accumulation of these lipids under conditions of lower sea surface temperatures and thus responding to intensi®cation of upwelling of cold nutrient-rich waters. In contrast to results from the Angola Basin, dinosterol is signi®cantly loaded in Cape Basin sediments. In addition, the C21 n-alkane

and the 24-methylcholesta-5,22-dien-3b-ol (both compounds were not analyzed in the Angola Basin by PCA due to their low quantitative importance) contribute to Factor 1 in sediments from the Cape Basin. In the case of the Angola Basin data set, d15 N values (only available at this site) display a signi®cant negative loading on Factor 1, also suggesting that the availability of nutrients is closely associated with the abundances of the above-mentioned biomarkers. The close correspondence between time series of Factor 1 and the TOC-based paleoproductivity record at both sites (Fig. 7a, Fig. 8a) indicates that the accumulation of highly loaded biomarkers was closely related to processes that control the accumulation of organic carbon in these sediments. In general, at both sites paleoproductivity estimates and Factor 1 scores are signi®cantly higher in glacial sediment sections of isotopic stages 4 to 2 (66 to 14 kyr b.p.) than in sediments from isotopic substage 5a and the Holocene. At both sites a similar correspondence is evident between C37 alkenone concentrations and those of the compound pair loliolide and isololiolide (Fig. 7b, Fig. 8b). At both sites (Fig. 7b), age trends are signi®cantly correlated with the paleoproductivity record (Cape Basin: both data sets, R=0.85; Angola Basin: loliolides, R=0.81, and alkenones, R=0.73), suggesting that the burial of organic

K. Hinrichs et al. / Organic Geochemistry 30 (1999) 341±366

355

Fig. 7. Time series spanning the last 76 ka from GeoB 1710-3, Cape Basin: (A) paleoproductivity estimates (calculated from sedimentary TOC concentrations after MuÈller and Suess, 1979) and principal component analysis scores of Factor 1, (B) concentrations of loliolide and isololiolide and C37 alkenones, (C) concentrations of LCFA (C24 +C26 +C28 fatty acids) and of the C31 n-alkane, (D) concentrations of cholesterol and dinosterol.

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Fig. 8. Time series spanning the last 76 ka from GeoB 1016-3, Angola Basin: (A) paleoproductivity estimates (calculated from sedimentary TOC concentrations after MuÈller and Suess, 1979) and principal component analysis scores of Factor 1, (B) concentrations of loliolide and isololiolide and C37 alkenones, (C) concentrations of LCFA (C24 +C26 +C28 fatty acids) and of the C31 n-alkane, (D) concentrations of cholesterol and dinosterol.

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carbon was controlled by mechanisms similar to those of accumulation of these lipids. The most straightforward interpretation of the biomarker composition of Factor 1 is a derivation of highly loaded lipids from those planktonic communities which mainly contributed to organic carbon burial. Accordingly, alkenoneproducing haptophytes, diatoms (indicated by loliolide and isololiolide and potentially by the C21 n-alkane) and diol- and ketol-producing microalgae (time series not shown) were possibly the major producers of the buried organic carbon. In addition to highly loaded components assumed to derive exclusively from phytoplanktonic sources, C24 to C28 even-carbon-numbered fatty acids are signi®cantly loaded on Factor 1 in both sedimentary environments. The LCFA concentration pro®les from both sites match closely those of planktonic biomarkers with a high Factor 1 loading and the record of paleoproductivity (Fig. 7c, Fig. 8c). Their abundance in eolian dust (e.g. Simoneit et al., 1977; Gagosian et al., 1981Gagosian et al., 1987) make them potential candidates for recording past variations of eolian-derived material and therefore for wind strength (Prahl, 1992). Prahl (1992) reported high relative concentrations of eolian-transported LCFA in sediments from the central equatorial Paci®c. The LCFA abundance was correlated with the accumulation of total organic carbon that evidently was of marine origin, indicating a causal relationship between production of marine organic carbon and wind intensity, likely driven by the eolian supply of iron. A similar linkage can be considered for sediments o€ Namibia and Angola where trade winds are driving the intensity of upwelling of nutrient-rich waters. Assuming an association of LCFA with wax alkanes in aerosols, the relatively low loading of C29 and C31 n-alkanes in sediments from the Angola Basin and the Cape Basin, respectively, can be explained by their derivation from mixed sources (e.g. hydrocarbons from marine biota) as recently reported for surface sediments from the Arctic (Zegouagh et al., 1998). Such an interpretation agrees with the relatively low odd-over-even carbon number predominance of n-alkanes in various samples (compare data in Fig. 4). Alternatively, the insigni®cant correlation between n-alkanes and fatty acids may be caused by di€erent sorptive interactions of both compound classes in association with changes in mineral dust composition. Dinosterol and 24-methylcholesta-5,22-dien-3b-ol contribute signi®cantly to Factor 1 only in the Cape Basin Fig. 5, whereas in the Angola Basin the loading of dinosterol is not signi®cant and concentrations of 24-methylcholesta-5,22-dien-3b-ol are extremely low. Thus, dino¯agellates probably played only a minor role in the accumulation of organic carbon in sediments of the Angola Basin. On the other hand, the

357

simple composition of sterols in combination with their low concentrations suggests a high degree of alteration of the original surface water sterol signal during particle settling through the water column and subsequent burial in surface sediments. This was probably more pronounced in sediments from the Angola Basin. For example, 24-methylcholesta-5,22-dien-3b-ol which is the major sterol in Emiliania huxleyi (e.g. Volkman, 1986), a marine algae that certainly contributed to the organic carbon burial in both sedimentary environments as indicated by high abundances of longchain alkenones, only occurs in trace amounts in Angola Basin sediments. Similar to other phytosterols, dinosterol which is commonly regarded to be more resistant to degradation by zooplankton grazing than other phytosterols (Harvey et al., 1987, 1989), could have been extensively degraded in oxic surface sediments by benthic communities as observed in abyssal surface sediments in the North Atlantic Ocean (Madureira et al., 1995). Stratigraphic records for dinosterol are presented in Fig. 7d (Cape Basin) and Fig. 8d (Angola Basin). In sediments from the Cape Basin dinosterol concentrations closely match the stratigraphic patterns of estimates of paleoproductivity (Fig. 7a) and concentration pro®les of loliolides and alkenones (Fig. 7b). However, maximum concentrations in Cape Basin sediments are observed in the sediment interval deposited around 48 to 45 kyr b.p. and do not coincide with maxima in the previously mentioned pro®les in Fig. 7a and b. On the other hand, the concentration pro®le of the C31 nalkane (Fig. 7c) displays a pronounced maximum in the same interval. Assuming a terrestrial origin of the n-alkane, this match may indicate a relationship between dino¯agellate abundance in surface waters and supply of nutrients from the continent. In contrast, in the Angola Basin dinosterol is not associated with components highly loaded on Factor 1. Its stratigraphic pro®le (Fig. 8d) displays maximum concentrations in a sediment sample deposited at the last glacial maximum (20 kyr b.p.). Concentration pro®les of cholesterol, another abundant steroid alcohol, are illustrated in Fig. 7d and Fig. 8d. In the multivariate analysis, cholesterol was combined with the major short-chain fatty acids (except C18:0 in the Angola Basin) whose concentration pro®les are highly correlated with cholesterol. Cholesterol is synthesized by almost every planktonic primary producer (e.g. Volkman, 1986). However, its occurrence in deep waters and sediments can be mainly assigned to the contribution of zooplankton (Gagosian et al., 1982) which incorporate cholesterol with their diet and biochemically modify other steroid alcohols with cholesterol being the product of the transformation (Harvey et al., 1987). The photic zone signal of shortchain fatty acids is altered in a similar way (Harvey et

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al., 1987). Such processes have important implications for the use of these biolipids for paleoenvironmental reconstructions. In some sediment sections from the Cape Basin, cholesterol concentrations are weakly correlated with those of other compounds with high loadings on Factor 1. Very high concentrations of 4.1 and 6.3 mg/g dry sediment in two samples, exceeding concentrations in the bulk of the samples by almost an order of magnitude, coincide with maximum concentrations of dinosterol. In the Angola Basin, the situation is similar. An interpretation solely on the basis of PCA results is misleading, because the correlation between concentrations of both components are mainly caused by coinciding maxima (whole data set: R=0.74, excluding sample with maximum concentration: R = 0.2). After exclusion of the sample exhibiting maximum concentrations, dinosterol concentrations are weakly correlated with the record of paleoproductivity (R = 0.57), indicating some relationship between TOC accumulation and dinosterol concentrations. Coinciding maximum concentrations of cholesterol and dinosterol together with maximum concentrations of major short-chain fatty acids in both data sets suggest that early diagenesis has a€ected these components from di€erent sources in a similar way. In PCA results from both sites, 24-ethylcholest-5-en3b-ol is combined with its saturated analog 24-ethyl5a-cholestan-3b-ol which can be either derived from reduction of 24-ethylcholest-5-en-3b-ol in the water column or the sediment or directly from the biomass of algae. The lack of any correlation with other compounds originating from terrestrial higher plant waxes (Tables 3 and 4) supports the assumption that these compounds originate from marine plankton. The isolated behavior of these compounds in the multivariate analysis may indicate an origin of these two C29 steroid alcohols from marine biota responding di€erently to nutrient conditions than other major primary producers at both sites. For example in Cape Basin sediments extremely high concentrations (33.6 and 14.4 mg/ g dry sediment, sum of both compounds) of these compounds were observed in two organic carbon-lean samples of Holocene age. The fact that in the Cape Basin the C22 n-alkane and the C30 ketol are also signi®cantly loaded on Factor 2 is mainly due to their coinciding maxima in two samples. In other sediment intervals they are poorly correlated with the two steroid alcohols. However, these coinciding maxima in the concentrations of these compounds probably indicate particular events when these compounds were buried after periods of high productivity of their producers. 3.5.1. Diagenetic controls on lipid abundance In order to evaluate the information obtained from stratigraphic investigation of biomarkers in the Southeast Atlantic Ocean we have to consider the

impact of diagenesis on the sedimentary lipid abundance. Only a small fraction of the organic material produced in the photic zone reaches the sea ¯oor (e.g. Wakeham et al., 1997). The decrease by a couple of orders of magnitude consequently strongly alters the original signal from the photic zone. This is in particular evident on short time scales in sediment trap studies where comparisons between biomarker signals from the photic zone with those in deep waters and surface sediments often revealed the diculties of reconstructing productivity by sedimentary biomarker and/or organic matter abundances (e.g. Prahl et al., 1993; Andersen, 1996). However, this does not necessarily imply that on longer times scale of a couple of hundred years (time interval roughly integrated by an average sample in this study) relationships between sedimentary abundances of certain components and their initial production in surface waters do not exist. We are aware of the fact that processes determining the amount of a certain biomarker (or organic matter in general) are highly complex and cannot be resolved solely on the basis of our data. High productivity in surface waters increases the net ¯ux of organic components into deeper waters, and also leads to a better preservation of these components during settling through the water column and subsequent burial in surface sediments (various contributions addressing this subject are compiled in Berger et al., 1989). An important issue for the evaluation of our results is the answer to the questions: Does a correlation between a biomarker and paleoproductivity estimates indicate that its source organism was an important contributor to organic carbon burial? Alternatively, does it just indicate independently similar diagenetic characteristics? Most of the lipid compounds displaying a high loading on Factor 1 have been proven to be relatively resistant against degradation under various environmental conditions. Studies of the organic matter composition in sediment traps at di€erent depths and in surface sediments in the central equatorial Paci®c (Wakeham et al., 1997) have shown the longchain fatty acids and alkenones to become selectively preserved in sediments. In a sequence of sediment traps and two sampled sediment depth intervals (surface and 10±12 cm) both compound classes display the highest relative abundance in the deeper sediments. Long-chain diols and ketols have been shown to be of a stability similar to that of alkenones in anoxic sediments from the Black Sea (Sun and Wakeham, 1994) and even more stable than alkenones under oxic conditions (Sinninghe Damste et al., 1997). Loliolide and isololiolide as end members in the degradation pathway of carotenoids also appear to be resistant against further geochemical degradation in the sediment (Repeta, 1989). Dinosterol is considered a refractory phytosterol, mainly because it is not signi®cantly

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altered by zooplankton feeding, which is in sharp contrast to cholesterol (Harvey et al., 1989). However, in oxic sediments, dinosterol appears to be more labile than alkenones (Sinninghe Damste et al., 1997). The generally higher concentrations of both C29 steroid alcohols in each core relative to other sterols are likely related to their selective preservation in the sediment. Like alkenones and long-chain fatty acids, 24-ethylcholest-5-en-3b-ol gets enriched in sediments compared to its ¯ux through the water column (Wakeham et al., 1997). Another feature in PCA results from both sites is a similar behavior of cholesterol and major short-chain fatty acids with the only exception of C18:0 in the Angola Basin core (Tables 3 and 4). Their concentration pro®les are not correlated with the paleoproductivity record obtained from sedimentary TOC concentrations. This is in particular the case for the fatty acids, whereas cholesterol is weakly correlated in some sections of the record obtained from the Cape Basin (Fig. 7c). This lack of correlation is in agreement with the multiple planktonic sources of these components including all trophic levels. Furthermore, feeding processes in the planktonic food web are likely to have substantially altered any SCFA and cholesterol signal related to primary productivity (Harvey et al., 1987). 3.5.2. Alkenone concentrations in relation to loliolide concentrations At both sites concentrations of alkenones remarkably increase relative to those of loliolides in sediments older than 036 kyr. An explanation for changed ratios of loliolides and alkenones is a change of the relative importance of their producers in algal communities. Alkenone-producing haptophytes are also likely to be a major source of fucoxanthin, the major precursor of loliolides, which is also derived from other algae with diatoms being the most important contributors. A simpli®ed approach to resolve the relative importance of di€erent algal classes is the comparison of the ratio of loliolides and alkenones with other available data. Low ratio values then could be interpreted to re¯ect a high importance of alkenone-producing microalgae relative to other major primary producers. The simpli®cations of such an approach are obvious and built on the assumption that alkenone-producing haptophytes contribute loliolides and alkenones in a constant ratio to the sedimentary record. This is unlikely to be the case due to a potential change in species composition of alkenone-producers, which occurred at the transition of isotopic stages 5 and 4 (070 kyr b.p.) at the close-by Walvis Ridge (MuÈller et al., 1997) and signi®cantly a€ected concentrations of alkenones relative to TOC.

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In fact, in sediments from the Cape Basin this ratio is correlated with TOC content and n-triacontan-1-ol13-one and 24-methylcholesta-5,22-dien-3b-ol abundances. Correlation coecients are R= 0.63, 0.77 and 0.70, respectively. Concentrations of other biomarkers are not signi®cantly correlated with the ratio. Stratigraphic patterns of the molecular ratio and concentrations of n-triacontan-1-ol-13-one and 24-methylcholesta-5,22-dien-3b-ol are illustrated in Fig. 9a. The correlation of these data suggests that the ratio of loliolides and alkenones does re¯ect changes in planktonic community composition rather than di€erent rates of degradation of both compounds. Therefore, the most straightforward interpretation is that high values of the (loliolides/(loliolides + C37 alkenones) ratio re¯ect stages when algae other than alkenoneproducers became increasingly important whereas low values potentially characterize periods where alkenoneproducers were dominant contributors to organic carbon burial. This interpretation is supported by the enrichment of loliolides relative to TOC especially in organic carbon-rich sediments with TOC-normalized concentrations correlating with sedimentary TOC concentrations (R=0.77). Low ratios are observed in particular in the early Holocene, between 45 and 40 kyr b.p. and in sediments older than 055 kyr. The low values in the oldest sediments are probably caused by a species change with Gephyrocapsa spp. being the dominant species in these sediments as described by MuÈller et al. (1997) for sediments from the Walvis Ridge. Particularly high ratios occur between 38 and 16 kyr b.p., basically in the interval when paleoproductivity reached maximum values. A better selective preservation of loliolides, 24-methylcholesta-5,22-dien-3bol and n-triacontan-1-ol-13-one relative to alkenones during periods of elevated carbon burial appears unlikely because at least ketols are known to be similarly resistant toward degradation as alkenones (Sun and Wakeham, 1994; Sinninghe Damste et al., 1997). The correlation of 24-methylcholesta-5,22-dien-3b-ol with the molecular ratio suggests that the occurrence of this compound in sediments from the Cape Basin re¯ects the contribution of diatoms rather than that of haptophytes. Producers of n-triacontan-1-ol-13-one are possibly also producers of fucoxanthin or have at least reacted similarly to environmental conditions. Interestingly, at the Angola Basin site, the ratio (loliolides/(loliolides + C37 alkenones) is not correlated with any of the available data. A direct comparison with records from 24-methylcholesta-5,22-dien-3b-ol and n-triacontan-1-ol-13-one is not possible due to their absence in these sediments. For comparative purposes time series of the ratio (loliolides/(loliolides+C37 alkenones) and concentrations of n-triacontan-1-ol-15one are presented in Fig. 9b.

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Fig. 9. Time series of the ratio loliolides/(loliolides+C37 alkenones) together with concentration pro®les for 24-methylcholesta-5,22dien-3b-ol and C30 ketol in sediments from (A) the Cape Basin and (B) the Angola Basin.

3.5.3. TOC/N ratios and d13 C data in relation to the biomarker record The PCA results may highlight hidden linkages between bulk parameters like TOC/N ratios and d13 C with abundances of biomarkers. These linkages can complement the picture derived from an approach solely based on lipid abundances. A detailed discussion of the environmental signi®cance of d13 C and TOC/N ratios is not the focus of this study but was published elsewhere for the entire cores from both locations (MuÈller et al., 1994; Kirst et al., 1998). Sedimentary TOC/N ratios from Cape Basin sediments are highly loaded on Factor 1 (Table 3), suggesting that variations of this parameter are closely linked to paleoenvironmental conditions which determine organic carbon burial. Facing the generally low TOC/N ratios in the Cape Basin, ranging from 5.7 to 8.7, in combination with TOC/N ratios not correlating

with d13 C of TOC, the most plausible explanation is a preferential removal of nitrogen-bearing organic compounds during particle settling in periods of high productivity (Suess and MuÈller, 1980; Fontugne and Calvert, 1992). The fact that d13 C of TOC in the Cape Basin does not display a signi®cant loading on any of the extracted factors indicates that there is no straightforward association of d13 C of TOC to environmental variables which are re¯ected by the lipid abundances. Potential associations include a coupling with d13 C of organic carbon with paleoproductivity due to the relationship between the fractionation of organic material produced by photosynthesis (ep) and growth rate (e.g. Laws et al., 1995), cell geometry (Popp et al., 1998; see Popp et al., 1997 for review), or a coupling between d13 C of organic carbon and terrestrial molecular biomarkers due to mixing of 13 C-depleted terrestrial C3 plant organic carbon and 13 C-enriched marine or-

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ganic matter (e.g. Prahl and Muehlhausen, 1989; Jasper and Gagosian, 1989). Changing sea surface temperatures also a€ect the isotopic fractionation during photosynthesis (Fontugne and Duplessy, 1981; Descolas-Gros and Fontugne, 1990) with a positive gradient of 0.28 to 0.35-/8C for SST below 288C. No linkage between alkenone-derived SST and d13 C is indicated by PCA results from the Cape Basin Table 3. Alkenone-derived SST values display a relatively high negative loading on Factor 4 from the Angola Basin which is composed of highly loaded TOC/N ratios and d13 C Table 4. This even indicates an inverse correlation between d13 C and SST, suggesting that changes of seasurface temperatures were not the major factors which controlled the 13 C content of sedimentary organic matter. A similar relationship was reported by Fontugne and Calvert (1992) for Mediterranean sapropels where authors mainly attributed variations of d13 C to variations of dissolved CO2 concentrations. Similarly, MuÈller et al. (1994) interpreted variations of d13 C in the entire Angola Basin core, spanning the last 200 kyr, to be mainly controlled by concentrations of dissolved CO2. The correlation between d13 C and TOC/N was also discussed in that paper. In contrast, sediments with a signi®cant portion of ¯uvial-derived material at the nearby Congo Fan exhibit the opposite relationship due to a mixing of marine and terrestrial material (MuÈller et al., 1994). An alternative explanation for the correlation between d13 C and TOC/N ratios could be a mixing with isotopically heavy, nitrogen-poor organic material derived from C4 plants. GonÄi et al. (1997) have demonstrated a signi®cant contribution of river-derived C4 plant carbon to sedimentary organic matter in the Gulf of Mexico by analyzing lignin phenols. The authors suggest that in past studies the contribution of C4 plant material to continental margin sediments were underestimated. However, in that study d13 C of bulk organic carbon did not display a systematic relationship with the portion of C3 plant-derived material. At present day, the organic material delivered by the nearby Congo River carries a clear C3 plant signal. It would require a maximum contribution in the order of 20 to 30% organic material, exclusively derived from C4 plants, to explain the 2.5- variation by mixing processes. Due to the contrasting signature of the Congo River, a signi®cant portion of plant debris in the sediments studied must have been associated with eolian-transported particles rather than with river-born particles. However, PCA data from the Angola Basin site (Table 4) do not indicate any correlation between the wind-derived long-chain fatty acids and d13 C, suggesting that other mechanisms caused the correlation between d13 C and sedimentary TOC/N ratios. Therefore, it is reasonable to assume that variations of both parameters are related to complex interactions of

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various paleoenvironmental controls like variations in [CO2]aq (e.g. Hayes, 1993), nutrient levels (e.g. Laws et al., 1995) and their e€ects on algal communities and related consequences on the composition of organic matter. 4. Summary and conclusions The comparative study of time series of abundances of selected biomarkers in Late Quaternary sediments from the Angola Basin and the Cape Basin complements our understanding of both environments which was so far based only on bulk characteristics of sedimentary organic matter. With a few exceptions, the assemblages of major lipids in both sedimentary environments are similar. Di€erences in the relative importance of these components are interpreted to re¯ect speci®c characteristics in biological communities contributing to the burial of sedimentary organic matter and partly also di€erent diagenetic conditions. Important features include the contrast of assemblages of alkanediols and -ketols, the di€erent n-alkane distributions and the absence of 24-methylcholesta-5,22dien-3b-ol in sediments from the Angola Basin. A multivariate analysis divided the studied compounds into di€erent groups according to their biogenic origin and diagenetic fate. In terms of speci®c information, the most important group of components was represented by PCA Factor 1 in both sedimentary environments, including: 1. long-chain alkenones derived from alkenone-producing haptophytes, 2. C28 to C32 alkanediols and a C30 ketool, most likely derived from non-speci®ed microalgae which are common and appear to be generally important in highly productive oceanic waters, 3. loliolide and isololiolide, derived from the degradation of fucoxanthin, the major carotenoid in haptophytes and diatoms, 4. even carbon-numbered n-fatty acids in the carbon number range from 24 to 28, to a major extent derived from wind transport of eolian dust and interpreted to be related to wind strength. Sedimentary abundances of these and other components combined in PCA Factor 1 are signi®cantly correlated with estimates on paleoproductivity, based on sedimentary TOC concentrations. Two plausible explanations account for this observation. 1. Abundances of these biomarkers re¯ect the contribution of the above-mentioned algal groups, which were closely related to the overall production of organic carbon by the entire planktonic community or

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even represent the most important planktonic contributors to organic carbon burial. Concentrations of long-chain fatty acids then re¯ect the atmospheric supply and accordingly wind intensity which is driving upwelling of nutrient-rich waters. 2. All the above mentioned components are known to belong to the most refractory group of biomarkers. This has been shown in comparative studies of sediment traps and surface sediments and in sediments of contrasting redox conditions where these compounds turned out to become enriched in sediments relative to other organic components or to be less subjected to oxic degradation than other lipid biomarkers. Therefore, degradation of this refractory group of components was closely related to degradation of total organic carbon of similar stability. Due to known complex linkages of productivity and preservation of organic matter both explanations are closely associated to each other and cannot be considered in an isolated manner. At both sites, one PCA factor is comprised of the major short-chain fatty acids and cholesterol. Concentrations of these components were not correlated with paleoproductivity estimates or related variables. This behavior can be explained by the heterogeneous origin of these components (phytoplankton, zooplankton, bacteria and even terrigenous dust in case of the fatty acids) and the fact that these components are extensively utilized by organisms during heterotrophic feeding processes on all trophic levels which highly alters any signal related to surface water productivity. A third isolated group of compounds with a stratigraphic pattern di€erent from other major compounds is comprised of 24-ethylcholest-5-en-3b-ol and its saturated counterpart, 24-ethyl-5a-cholestan-3b-ol. Both compounds are the dominant steroid alcohols in most of the samples. The high concentrations are related to the refractory nature of these compounds relative to other phytosterols, but possibly also indicate the importance of their biogenic source organisms in algal communities. In analogy to other highly productive surface water environments with dominantly marine organic matter sources to their underlying sediments, a mainly terrestrial origin of these compounds is considered to be unlikely. Instead, we suggest that the signal of these relatively refractory compounds is related to other planktonic communities, e.g. xantophyceae or other unknown microalgae, that respond di€erently to variations in nutrient conditions than the dominant primary producers. We observed di€erences in stratigraphic patterns of selected components that are potentially related to oceanographic characteristics of each environment. In sediments from the Cape Basin, abundances of 24-

methylcholesta-5,22-dien-3b-ol and the C21 n-alkane were correlated with paleoproductivity estimates, whereas in Angola Basin sediments these components were only present in trace levels. 24-Methylcholesta5,22-dien-3b-ol in the Cape Basin is interpreted to represent the contribution of diatoms rather than of haptophytes. This assumption is supported by the correlation of its concentration with the amounts of loliolides (derived mainly from haptophytes and diatoms) relative to alkenones (representing alkenone-producing haptophytes). In contrast, abundances of 24methylcholesta-5,22-dien-3b-ol and the C21 n-alkane in sediments from the Angola Basin can be neglected, suggesting di€erences in biological communities which contribute to organic carbon burial and/or their selective degradation in the water column and the sediment. Evidence for a strong degradation of 24-methylcholesta-5,22-dien-3b-ol is provided by the fact that this compound is the major sterol in E. huxleyi, an algae certainly growing in surface waters as indicated by high concentrations of alkenones. A less important role of diatoms in the Angola Basin agrees with the speci®c mechanisms of nutrient supply, leading to silicate limitation. Moreover, these ®rst systematic stratigraphic data series of loliolides in Quaternary sediments suggest their high potential for application as biomarkers.

Acknowledgements The authors wish to thank Dr. J.K. Volkman (CSIRO, Hobart) for helpful comments on n-alkane and fatty acid distributions. Comments by J. Rinna (ICBM, Oldenburg) and Dr. S. Pantoja (WHOI) on earlier versions of this manuscript are highly appreciated. Critical reviews of Dr. F.G. Prahl (OSU, Corvallis) and an anonymous reviewer greatly helped to improve the manuscript. Financial support by DFG SFB 261 to K.H. enabled this study and is gratefully acknowledged. This is contribution No. 232 of the Sonderforschungsbereich 261 at Bremen University funded by the Deutsche Forschungsgemeinschaft. Associate EditorÐG. Wol€ References Altabet, M.A., Francois, R., 1994. Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization. Global Biogeochemical Cycles 8, 103±116. Andersen, N., 1996. Biogeochemische Charakterisierung von Sinksto€en und Sedimenten aus ostatlantischen Produktionssystemen mit Hilfe von Biomarkern. Dissertation, UniversitaÈt Bremen, Germany.

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