Molecular-isotopic stratigraphy of long-chain n-alkanes in Lake Baikal Holocene and glacial age sediments

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1 Organic Geochemistry 31 (2000) 287±294 Molecular-isotopic stratigraphy of long-chain n-alkanes in Lake Baikal Holocene and glacial age sediments David Brincat a, *, Keita Yamada a, Ryoshi Ishiwatari a, Hitoshi Uemura b, Hiroshi Naraoka a a Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo , Japan b Department of Environmental Health, Kanagawa Prefectural Public Health Laboratory, Nakao 1-1-1, Asahi-ku, Yokohama , Japan Received 8 January 1999; accepted 16 December 1999 (returned to author for revision 1 May 1999) Abstract The molecular distribution and the carbon-isotopic composition (d 13 C) of n-alkanes extracted from a Lake Baikal core spanning the last 20 kyr of sediment accumulation have been investigated. A terrestrial origin has been inferred for the odd carbon-numbered long-chain (>C 27 ) n-alkanes, on the basis of the observed high CPI values (range: 8.7± 10.8) typical of n-alkanes derived from leaf waxes of higher plants. A shift in the abundance of n-c 27 alkane relative to n-c 31 homologue is observed across the late Pleistocene glacial±holocene interglacial climate change, perhaps indicative of the climate-induced vegetational change previously deduced from palynological analyses. Compound-speci c isotope analyses indicate remarkably uniform d 13 C values in the range of 31.0 to 33.5% for the leaf-wax C 27 ±C 33 n- alkanes in the entire cored sequence. Such an isotopic compositional range is characteristic for n-alkanes biosynthesized by plants utilizing the C 3 photosynthetic pathway. Our data suggest that the observed 13 C-enrichment in the bulk organic matter in the glacial age sediments, relative to d 13 C values of total organic carbon in the Holocene section, is therefore unlikely to be attributed to an expansion of C 4 -type vegetation in the Baikal watershed during the late Pleistocene glacial interval. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Lake Baikal; Long-chain n-alkanes; Carbon-isotopic compositions; Lacustrine sediments; Paleoclimate 1. Introduction Organic matter preserved in lake sediments represents an input from the remains of aquatic organisms as well as plants which inhabited the surrounding land (Meyers and Ishiwatari, 1995). The characterization of the stable carbon-isotopic composition of lacustrine organic matter has been shown to re ect variations in limnological factors resulting from paleoclimatic changes (Stuiver, 1975; HaÊ kansson, 1985). The carbon-isotopic composition of sedimentary organic matter is useful in identifying organic material * Corresponding author. address: davidbrincat@netscape.net (D. Brincat). originating from land plants with di erent metabolic pathways (e.g. Meyers, 1994). Carbon-isotopic analyses of bulk tissues from plants utilizing di erent pathways of carbon xation revealed that those plants which employ the Calvin cycle pathway (C 3 plants) during photosynthesis are more depleted in 13 C than those plants which use the Hatch-Slack pathway (C 4 plants) (Smith and Epstein, 1971). Indeed, changes in 13 C/ 12 C ratio of organic matter in lake sediments were partly attributed to a climate-forced change in the type of watershed ora, re ecting varying detrital input of C 3 relative to C 4 plants (Talbot and Johannessen, 1992; Street-Perrott et al., 1997). Lake Baikal, located in southeastern Siberia (Russia), has been the target of drilling activities during this /00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S (99)

2 288 D. Brincat et al. / Organic Geochemistry 31 (2000) 287±294 decade in an e ort to recover sediment cores suitable for paleoclimatic studies of this high latitude, continental interior location (Colman et al., 1992). Variation in the isotopic composition of the organic carbon in a short core recovered from the northern basin of Lake Baikal (Qiu et al., 1993) has been attributed to a change in the relative abundance of C 3 versus C 4 vegetation in the lake watershed (X. Chen, 1992, cited in Qiu et al., 1993). The present study evaluates the input of terrestrial vegetation upon d 13 C values of bulk organic matter in Lake Baikal sediments. The approach adopted in this work involves the determination of the isotopic composition of individual odd carbon-numbered long-chain (>C 27 ) normal (n-) alkanes as a proxy indicator of input from terrestrial plants utilizing di erent metabolic pathways. Several studies have reported the occurrence of such components, as the major n-alkanes, in leaf waxes of terrestrial plants (e.g. Eglinton and Hamilton, 1967; Stra nskyâ et al., 1967; Tulloch, 1976). The measurement of the isotopic composition at the molecular level, using gas chromatography±isotope ratio mass spectrometry (GC± IRMS; Hayes et al., 1990), has greatly enhanced the ability to assess the source input of the individual components in the sedimentary environment (Freeman et al., 1990; Rieley et al., 1991). For example, the distinctive isotopic di erence observed in the bulk tissue between C 3 and C 4 plants is also re ected at the molecular level in the 13 C-enriched n-alkanes derived from C 4 plants relative to the isotopically lighter homologues biosynthesized by C 3 plants (Collister et al., 1994; Lichtfouse et al., 1994; Yamada, 1997). Such isotopic di erences at the molecular level have been successfully exploited in a recent reconstruction of the temporal variation in the relative input of C 4 versus C 3 plant detritus in lake sediments from eastern Africa (Street-Perrott et al., 1997). However, to the best of our knowledge, this is the rst report of compound-speci c isotope analyses of individual lipids extracted from Lake Baikal sediments. 2. Materials and methods 2.1. Sediment samples The piston core used in this study (Core 323-PC1) was recovered by a team of Russian and American scientists (Colman et al., 1992) from the northern basin of Lake Baikal ( N, E; water depth 710 m; core length 461 cm). The cored sequence consists of a massive clay±silty clay deposit with intercalations of ne±medium sand layers (Takemura et al., 1992). A distinctive change in lithological properties is present at a depth of ca. 150 cm, with the upper layer consisting of silt size sediments enriched in diatom fossil remains relative to the lower, more ne-grained clayey sediments, where very few diatom remains have been observed (Takemura et al., 1992). The cored interval has an estimated basal age of 19.8 kyr (Ogura et al., 1992). On the basis of the available age determinations, it can be inferred that the upper diatomaceous section is of Holocene age whereas the lower clayey sequence was deposited during the late Pleistocene glacial regime representing the last glacial maximum (LGM; 14±22 kyr B.P.; Crowley and North, 1991). The age estimates for the uppermost sediments, characterized by contrasting lithologies, compare very favorably with the radiocarbon-based chronology for a suite of similar cores recovered from other sites within Lake Baikal, namely the Selenga Delta area (southern basin) and Academician Ridge region (Carter and Colman, 1994) Previous analyses Elemental analyses and isotopic composition of total organic carbon (d 13 C TOC ) for 27 sampled intervals from core 323-PC1 have already been reported (Ishiwatari et al., 1992), and are displayed here in Fig. 1 together with the estimated age determinations of Ogura et al. (1992). Following the initial bulk analyses, nineteen sediment samples were further selected for more detailed molecular studies (Ishiwatari et al., 1993, 1995). These samples are indicated with a lled circle in Fig. 1. Details of the extraction procedures have been described elsewhere (Ishiwatari et al., 1993) and can be summarized as follows. Wet sediment samples were saponi ed with 0.5M KOH in methanol under re ux for 2 h. Neutral compounds were recovered with n-hexane: diethylether (9:1, v/v) and separated into lipid classes (saturated and unsaturated hydrocarbons, aromatic hydrocarbons, ketones and alcohols) by silica-gel (deactivated with 5% H 2 O w/w) column chromatography. Alkenes were removed from the hydrocarbon fraction using a column of AgNO 3 ±impregnated (10% w/w) silicagel. The saturated hydrocarbon fractions obtained by these procedures were made available for the present study for the determination of the carbon-isotopic composition of individual compounds Molecular analyses of n-alkanes The saturated hydrocarbons of all 19 samples were analyzed by gas chromatography (GC) using a Hewlett- Packard 5890 series II gas chromatograph equipped with an on-column injector and a ame ionization detector. The n-alkanes were separated on a J & W Scienti c DB-5 fused silica capillary column (30 m 0.32 mm i.d.; 0.25 mm lm thickness). Helium was used as carrier gas. The GC oven temperature was programmed as follows: injection at 50 C, 30 C/min to 120 C, 5 C/min to 310 C, isothermal for 17 min. Concentrations of the various n-alkane homologues were calculated by comparing the peak area of the appropriate compound relative to that of the co-injected

3 D. Brincat et al. / Organic Geochemistry 31 (2000) 287± Fig. 1. Depth pro les of (a) abundance of total organic carbon (wt%); and (b) isotopic composition of total organic carbon for sediment samples from Lake Baikal core 323-PC1 (after Ishiwatari et al., 1992). Samples represented by a lled circle were previously selected for molecular studies. Estimated ages are from Ogura et al. (1992). Lithological description is after Takemura et al. (1992), with the horizontal line at 150 cm depth representing a climate-related lithological boundary, separating an upper diatomaceous clay section from the lower clayey sediments. The vertical broken lines in Fig. 1b indicate average d 13 C TOC values in glacial age sediments ( 24.1%) and Holocene section ( 27.3%). deuterated n-alkane C 24 D 50. The response factor of individual n-alkanes relative to the standard was assumed to be unity. Compound identi cation was based on data from electron±impact gas chromatography±mass spectrometry (GC±MS). The GC±MS system employed was a Hewlett-Packard 6890 series gas chromatograph tted with a split/splitless injector (280 C) and interfaced with a Hewlett-Packard 6890 series mass selective detector (MSD), which was operated in full scan mode. Separation was performed on a DB-5 fused silica capillary column (30 m 0.32 mm i.d.; 0.25 mm lm thickness). Helium was used as carrier gas, with the oven temperature program being the same as that described for the GC analyses Molecular sieve treatment for isolation of n-alkanes for stable carbon isotope analysis Saturated hydrocarbon fractions containing signi cant amounts of branched and cyclic compounds were treated with 5 A molecular sieves (Yamada et al., 1994) in order to isolate the n-alkanes for accurate isotopic measurements. Such an isolation procedure was not observed to a ect the d 13 C values of the individual n-alkanes (Yamada et al., 1994). Brie y, the saturated hydrocarbon fractions dissolved in n-hexane were evaporated to dryness under nitrogen. Subsequently, the saturated hydrocarbons were re-dissolved in 1 ml of isooctane and ca. 200 mg of preheated (350 C for 5 h) 5 A molecular sieve pellets were added to the vial, which was kept at room temperature for 12 h. The n-alkanes were then recovered with n-hexane after dissolution of the molecular sieves with 47% hydro uoric acid solution. GC analyses of the non-adduct (branched and cyclic) fraction did not reveal the presence of n-alkanes. Moreover, a procedural blank carried out during the isolation of n-alkanes from the samples did not indicate the presence of any contamination Gas chromatography±isotope ratio mass spectrometry The carbon-isotopic values of individual n-alkanes were determined using a gas chromatography±isotope ratio mass spectrometry (GC±IRMS) system. A Hewlett- Packard 5890 series II gas chromatograph was used, equipped with an on-column injector, and interfaced with a Finnigan MAT delta-s mass spectrometer via a combustion furnace (840 C) packed with CuO and Pt

4 290 D. Brincat et al. / Organic Geochemistry 31 (2000) 287±294 wires. The n-alkanes were separated on an HP-5 trace analysis fused silica capillary column (60 m 0.32 mm i.d.; 0.25 mm lm thickness). Helium was used as carrier gas. The GC oven temperature was programmed from 50 to 120 Cat30 C/min, from 120 to 310 Cat5 C/ min, and then held isothermally at 310 C for 23 min. The d 13 C values were calibrated by co-injected n-alkanes C 16 D 34,C 24 D 50 and C 38 H 78. All carbon isotope ratios are expressed as per mil (%) relative to the Pee Dee Belemnite (PDB) standard. Data were acquired and processed using ISODAT software. Reported carbonisotopic compositions represent averaged values of triplicate analyses. Standard deviations were generally 4 0.5%. 3. Results and discussion 3.1. Molecular distribution of n-alkanes The gas chromatograms of the saturated hydrocarbon fraction extracted from two sediment samples deposited under di erent climatic conditions (Holocene and LGM) are displayed in Fig. 2. A unimodal distribution of n-alkanes is observed in all samples analyzed, maximizing at either n-c 27 or n-c 31. Such a molecular feature in sediments from the strongly oligotrophic Lake Baikal (Weiss et al., 1991) has been previously observed in sediments deposited in other oligotrophic lakes (Cranwell, 1982; Kawamura and Ishiwatari, 1985). The concentrations of the odd carbon-numbered C 27 to C 33 n-alkanes extracted from core 323-PC1 are listed in Table 1, together with other data related to the n-alkane distribution. The long-chain n-alkanes are characterized by a pronounced odd-carbon predominance. The carbon preference index (CPI) for n-alkanes in the range C 27 ±C 33 varies between 8.7 and 10.8 for the samples analyzed here (Table 1). The extent of predominance of the odd carbon-numbered n-c 27 to n-c 33 homologues is within the range observed for n-alkanes derived from terrestrial plant epicuticular leaf waxes (Eglinton and Hamilton, 1967; Tulloch, 1976; Collister et al., 1994), thereby suggesting a higher plant origin for the long-chain n-alkanes. This source inference is supported by the similarity in the CPI values observed in the lake sedimentary record and in extracts from a peat bog interval sampled close to the shoreline of Baikal (Brincat et al., unpublished data). As indicated in Fig. 2, the distribution of n-alkanes in the glacial age sediment is dominated by C 31 homologue whereas C 27 component is the predominant n-alkane in the Holocene age sediment. An insight into the downcore variation of the dominant n-alkane homologue is provided by evaluation of the C 27 n-alkane/c 31 n-alkane ratio (Fig. 3). Sediments dated to a glacial age are uniformly dominated by C 31 n-alkane. However, the lithological boundary at ca. 150 cm depth marks a change in the distribution pattern of n-alkanes, with the C 27 component becoming increasingly important towards Fig. 2. Gas chromatograms of the saturated hydrocarbon fraction extracted from Lake Baikal sediment samples deposited during (a) Holocene and (b) last glacial maximum. The peak marked with (*) denotes the deuterated n-alkane internal standard C 24 D 50.

5 Table 1 Concentration data for the long-chain odd carbon-numbered n-alkanes analyzed in this study as a function of depth in Lake Baikal core 323-PC1. The CPI values for the range C 27 ±C 33 n-alkanes are also provided Depth (cm) TOC (wt%) a n-c 27 mg/g OC b n-c 29 mg/g OC n-c 31 mg/g OC n-c 33 mg/g OC CPI c a Total organic carbon data from Ishiwatari et al. (1992). b Concentration expressed as mg/g organic carbon. c CPI =0.5*C 27;29;31;33 1=C 26;28;30;32 1=C 28;30;32;34. D. Brincat et al. / Organic Geochemistry 31 (2000) 287± Fig. 3. Depth pro le of the variation in the abundance of C 27 relative to C 31 n-alkanes together with a summary of the palynological analyses of Fuji (1992) for Lake Baikal core 323-PC1. The change in lithology described by Takemura et al. (1992) is represented by the horizontal line at 150 cm depth.

6 292 D. Brincat et al. / Organic Geochemistry 31 (2000) 287±294 the top of the core. Palynological analyses of these sediments (Fuji, 1992) revealed abundant pollen grains typical of present-day forest vegetation surrounding Lake Baikal in the upper sedimentary section (Holocene). On the other hand, signi cantly fewer pollen grains were observed in the lower section, with the pollen assemblage being indicative of the presence of herbaceous vegetation in the Lake Baikal watershed during the LGM (Fuji, 1992). Therefore, on the basis of the pollen data, it is suggested that the depth variations in the relative abundance of the high molecular weight n- alkane homologues re ect a change in the types of vegetation in the lake watershed Carbon-isotopic composition of n-alkanes Compound-speci c d 13 C values of the odd carbonnumbered C 27 to C 33 n-alkanes are listed in Table 2. The downcore average d 13 C values indicate that the n- alkanes get systematically more 13 C depleted with increasing chain length, a feature also noted for longchain n-alkanes in other lake sediments (Rieley et al., 1991; Spooner et al., 1994; Ficken et al., 1998). Downcore d 13 C pro les of the individual n-alkanes are shown in Fig. 4. In spite of the signi cant change in the pollen record and the n-alkane distribution observed across the late Pleistocene glacial±holocene interglacial climate transition, the d 13 C pro les of the high molecular weight n-alkanes are observed to be remarkably homogeneous as a function of core depth, with the maximum downcore range of d 13 C values being 1.6, 0.9, 1.2 and 1.5% for C 27,C 29,C 31 and C 33 n-alkanes, respectively (Table 2). Moreover, the d 13 C values of C 27 to C 33 n-alkanes vary from 31.0 to 33.5%, well within the range for leaf-wax n-alkanes biosynthesized by C 3 plants (Rieley et al., 1991; Collister et al., 1994; Yamada, 1997). Hence, the observed range of the carbon-isotopic composition of the long-chain n-alkanes suggests the prevalence of C 3 -type vegetation in the Lake Baikal watershed even during the glacial interval. This inference is corroborated by d 13 C values of the long-chain n-c 29 and n-c 31 alkanes extracted from a glacial age peat bog interval collected from the lakeside of Baikal (d 13 C range: 30.3 to 32.4%; Brincat et al., unpublished data), which are signi cantly depleted in 13 C relative to the isotopic composition reported for the corresponding n-alkanes extracted from C 4 plants (d 13 C Table 2 Carbon-isotopic data for the individual long-chain odd carbon-numbered n-alkanes as a function of depth in Lake Baikal core 323- PC1. The isotopic values represent the average of triplicate analyses. Also shown is the isotopic composition of the corresponding total organic carbon (after Ishiwatari et al., 1992) Depth (cm) TOC d 13 C(%) a n-c 27 d 13 C(%) s b n-c 29 d 13 C(%) s n-c 31 d 13 C(%) s n-c 33 d 13 C(%) s Average c d (%) d a Bulk isotopic values from Ishiwatari et al. (1992). b Represents 1 standard deviation from the mean of triplicate analyses. c Average=downcore averaged isotopic composition for individual n-alkanes. d d(%)=maximum observed range of d 13 C values.

7 D. Brincat et al. / Organic Geochemistry 31 (2000) 287± previously recognized by palynological analyses of the same samples we analyze here. Compound-speci c isotopic analyses revealed remarkably homogeneous downcore d 13 C pro les for the leaf-wax n-alkanes in both Holocene and LGM sediments. The isotopic compositional range of 31.0 to 33.5% for C 27 ±C 33 n-alkanes in the entire cored sequence represents an input from C 3 -type plants. Therefore, the observed 13 C-enrichment in the bulk organic matter in the glacial age sediments is unlikely to be due to an expansion of C 4 -type vegetation in the Baikal watershed during the late Pleistocene glacial interval. Acknowledgements We are grateful to Prof. S. Horie for providing access to the Lake Baikal samples from core 323-PC1. The authors would also like to acknowledge Dr. James W. Collister and Dr. Fred Prahl for reviewing the manuscript and providing many helpful suggestions. This work was supported by a research grant from the Ministry of Education, Science and Culture of Japan. Fig. 4. d 13 C pro les of the leaf-wax C 27 to C 33 n-alkanes as a function of depth in Lake Baikal core 323-PC1. The horizontal line represents a change in lithology at 150 cm depth (Takemura et al., 1992). range: 18.4 to 24.5%; Collister et al., 1994; Lichtfouse et al., 1994; Yamada, 1997). Therefore, our d 13 C data for the leaf-wax n-alkanes suggest that the observed 13 C-enrichment in the bulk organic matter in the glacial age sediments, relative to d 13 C TOC values in the Holocene section (Fig. 1b), is unlikely to be attributed to a spread of C 4 -type vegetation in the Baikal watershed during the glacial interval. 4. Conclusions Molecular characterization of n-alkanes extracted from Lake Baikal sediments, with an estimated basal age of ca. 20 kyr for the cored interval, revealed a high predominance of odd carbon-numbered n-alkanes in the C 27 ±C 33 range, consistent with a terrestrial plant epicuticular wax origin. Moreover, a systematic change in the most abundant n-alkane was also noted as a function of core depth, with the LGM section being dominated by n-c 31 alkane whereas the Holocene age sediments by n- C 27 homologue. Such a change in the distribution pattern of the long-chain n-alkanes recorded in the lake sediments could be indicative of the climate-induced change in the type of terrestrial vegetation Associate EditorÐJ.W. Collister References Carter, S.J., Colman, S.M., Biogenic silica in Lake Baikal sediments: results from 1990±1992 American cores. Journal of Great Lakes Research 20, 751±760. Chen, X., Paleolimnological and paleoclimatic signals from Asian lakes: Lake Baikal, Russia, and lake systems of China. Ph.D. thesis, University of South Carolina, USA. Collister, J.W., Rieley, G., Stern, B., Eglinton, G., Fry, B., Compound-speci c d 13 C analyses of leaf lipids from plants with di ering carbon dioxide metabolisms. Organic Geochemistry 21, 619±627. Colman, S.M., Karabanov, E.B., Williams, D.F., Hearn, Jr., P.P., King, J.W., Orem, W.H. et al., Lake Baikal paleoclimate project, southeastern Siberia: initial dating and paleoenvironmental results. In: Horie, S., Toyoda, K. (Eds.), International Project on Paleolimnology and Late Cenozoic Climate Newsletter No. 6. pp. 30±39. Cranwell, P.A., Lipids of aquatic sediments and sedimenting particulates. Progress in Lipid Research 21, 271± 308. Crowley, T.J., North, G.R., Paleoclimatology. Oxford University Press, New York. Eglinton, G., Hamilton, R.J., Leaf epicuticular waxes. Science 156, 1322±1335. Ficken, K.J., Street-Perrott, F.A., Perrott, R.A., Swain, D.L., Olago, D.O., Eglinton, G., Glacial/interglacial variations in carbon cycling revealed by molecular and isotope stratigraphy of Lake Nkunga, Mt. Kenya, East Africa. Organic Geochemistry 29, 1701±1719.

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