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
aaDepartment of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji,
Tokyo 192-0397, Japan
bDepartment of Environmental Health, Kanagawa Prefectural Public Health Laboratory, Nakao 1-1-1, Asahi-ku,
Yokohama 241-0815, 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 (d13C) ofn-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 (>C27)n-alkanes, on the basis of the observed high CPI27-33values (range: 8.7±
10.8) typical ofn-alkanes derived from leaf waxes of higher plants. A shift in the abundance ofn-C27alkane relative to n-C31homologue is observed across the late Pleistocene glacial±Holocene interglacial climate change, perhaps
indica-tive of the climate-induced vegetational change previously deduced from palynological analyses. Compound-speci®c isotope analyses indicate remarkably uniformd13C values in the range ofÿ31.0 toÿ33.5%for the leaf-wax C
27±C33n
-alkanes in the entire cored sequence. Such an isotopic compositional range is characteristic forn-alkanes biosynthe-sized by plants utilizing the C3photosynthetic pathway. Our data suggest that the observed13C-enrichment in the bulk
organic matter in the glacial age sediments, relative tod13C values of total organic carbon in the Holocene section, is
therefore unlikely to be attributed to an expansion of C4-type vegetation in the Baikal watershed during the late
Pleistocene glacial interval.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Lake Baikal; Long-chainn-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 mat-ter has been shown to re¯ect variations in limnological factors resulting from paleoclimatic changes (Stuiver, 1975; HaÊkansson, 1985).
The carbon-isotopic composition of sedimentary organic matter is useful in identifying organic material
originating from land plants with dierent metabolic pathways (e.g. Meyers, 1994). Carbon-isotopic analyses of bulk tissues from plants utilizing dierent pathways of carbon ®xation revealed that those plants which employ the Calvin cycle pathway (C3 plants) during
photosynthesis are more depleted in 13C than those
plants which use the Hatch-Slack pathway (C4plants)
(Smith and Epstein, 1971). Indeed, changes in 13C/12C
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 C3
relative to C4 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
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 1 6 4 - 3
www.elsevier.nl/locate/orggeochem
* Corresponding author.
decade in an eort 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 C3 versus C4 vegetation in the
lake watershed (X. Chen, 1992, cited in Qiu et al., 1993). The present study evaluates the input of terrestrial vegetation upon d13C values of bulk organic matter in
Lake Baikal sediments. The approach adopted in this work involves the determination of the isotopic compo-sition of individual odd carbon-numbered long-chain (>C27) normal (n-) alkanes as a proxy indicator of input
from terrestrial plants utilizing dierent metabolic path-ways. Several studies have reported the occurrence of such components, as the majorn-alkanes, in leaf waxes of terrestrial plants (e.g. Eglinton and Hamilton, 1967; StraÂnsky 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 abil-ity to assess the source input of the individual compo-nents in the sedimentary environment (Freeman et al., 1990; Rieley et al., 1991). For example, the distinctive isotopic dierence observed in the bulk tissue between C3
and C4plants is also re¯ected at the molecular level in the 13C-enrichedn-alkanes derived from C
4plants relative to
the isotopically lighter homologues biosynthesized by C3
plants (Collister et al., 1994; Lichtfouse et al., 1994; Yamada, 1997). Such isotopic dierences at the mole-cular level have been successfully exploited in a recent reconstruction of the temporal variation in the relative input of C4versus C3plant 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 com-pound-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 (55.5347N, 109.5213E; 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 esti-mated 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 radio-carbon-based chronology for a suite of similar cores recovered from other sites within Lake Baikal, namely the Selenga Delta area (southern basin) and Academi-cian Ridge region (Carter and Colman, 1994).
2.2. Previous analyses
Elemental analyses and isotopic composition of total organic carbon (d13C
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). Fol-lowing 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 pro-cedures 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 withn-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% H2O w/w) column chromatography.
Alkenes were removed from the hydrocarbon fraction using a column of AgNO3±impregnated (10% w/w)
silica-gel. 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.
2.3. Molecular analyses ofn-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.; 0.25mm ®lm thickness). Helium was used as carrier gas. The GC oven temperature was pro-grammed as follows: injection at 50
C, 30
C/min to 120C, 5C/min to 310C, isothermal for 17 min.
deuterated n-alkane C24D50. 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 spectro-metry (GC±MS). The GC±MS system employed was a Hewlett-Packard 6890 series gas chromatograph ®tted with a split/splitless injector (280C) and interfaced with
a Hewlett-Packard 6890 series mass selective detector (MSD), which was operated in full scan mode. Separa-tion was performed on a DB-5 fused silica capillary column (30 m0.32 mm i.d.; 0.25mm ®lm thickness). Helium was used as carrier gas, with the oven tempera-ture program being the same as that described for the GC analyses.
2.4. Molecular sieve treatment for isolation ofn-alkanes for stable carbon isotope analysis
Saturated hydrocarbon fractions containing sig-ni®cant amounts of branched and cyclic compounds were treated with 5 A molecular sieves (Yamada et al., 1994) in order to isolate then-alkanes for accurate iso-topic measurements. Such an isolation procedure was not observed to aect thed13C values of the individual
n-alkanes (Yamada et al., 1994). Brie¯y, the saturated hydrocarbon fractions dissolved inn-hexane were eva-porated to dryness under nitrogen. Subsequently, the saturated hydrocarbons were re-dissolved in 1 ml of iso-octane 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. Then-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 ofn-alkanes. More-over, a procedural blank carried out during the isolation of n-alkanes from the samples did not indicate the presence of any contamination.
2.5. 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
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 averaged13C
TOCvalues in glacial age sediments
wires. The n-alkanes were separated on an HP-5 trace analysis fused silica capillary column (60 m0.32 mm i.d.; 0.25mm ®lm thickness). Helium was used as carrier gas. The GC oven temperature was programmed from 50 to 120C at 30C/min, from 120 to 310C at 5C/
min, and then held isothermally at 310
C for 23 min. Thed13C values were calibrated by co-injectedn-alkanes
C16D34, C24D50 and C38H78. 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 carbon-isotopic compositions represent averaged values of tri-plicate analyses. Standard deviations were generally 4
0.5%.
3. Results and discussion
3.1. Molecular distribution ofn-alkanes
The gas chromatograms of the saturated hydrocarbon fraction extracted from two sediment samples deposited under dierent climatic conditions (Holocene and LGM) are displayed in Fig. 2. A unimodal distribution of n-alkanes is observed in all samples analyzed, max-imizing at eithern-C27orn-C31. Such a molecular
fea-ture 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 C27to C33 n-alkanes extracted from core 323-PC1 are listed in Table 1, together with other data related to then-alkane distribution.
The long-chainn-alkanes are characterized by a pro-nounced odd-carbon predominance. The carbon pre-ference index (CPI) forn-alkanes in the range C27±C33
varies between 8.7 and 10.8 for the samples analyzed here (Table 1). The extent of predominance of the odd carbon-numberedn-C27 ton-C33homologues is within
the range observed forn-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 CPI27-33 values observed in the lake
sedimentary record and in extracts from a peat bog interval sampled close to the shoreline of Baikal (Brin-cat et al., unpublished data).
As indicated in Fig. 2, the distribution ofn-alkanes in the glacial age sediment is dominated by C31homologue
whereas C27component is the predominantn-alkane in
the Holocene age sediment. An insight into the down-core variation of the dominantn-alkane homologue is provided by evaluation of the C27n-alkane/C31n-alkane
ratio (Fig. 3). Sediments dated to a glacial age are uni-formly dominated by C31n-alkane. However, the
litho-logical boundary at ca. 150 cm depth marks a change in the distribution pattern of n-alkanes, with the C27
component becoming increasingly important towards
Table 1
Concentration data for the long-chain odd carbon-numberedn-alkanes analyzed in this study as a function of depth in Lake Baikal core 323-PC1. The CPI values for the range C27±C33n-alkanes are also provided
Depth (cm) TOC (wt%)a n-C
27mg/g OCb n-C29mg/g OC n-C31mg/g OC n-C33mg/g OC CPI27ÿ33c
9.5 2.09 49.9 34.0 28.8 11.0 9.5
44.5 2.85 30.3 18.3 14.9 6.2 9.6
55.8 2.72 46.8 30.0 27.1 10.7 9.8
74.8 2.41 63.3 43.1 37.6 14.0 10.4
95.3 1.39 221.9 158.0 163.6 63.3 10.8
120.0 1.02 29.0 22.2 20.0 9.2 8.9
139.0 0.63 58.1 48.1 45.6 16.3 9.8
176.0 0.36 39.9 39.6 44.7 12.7 9.2
200.3 0.31 29.2 31.7 36.6 10.7 9.6
218.0 0.29 34.6 41.5 49.6 14.3 10.2
244.3 0.28 43.2 52.4 61.2 17.5 10.4
274.5 0.25 61.5 72.1 84.1 22.2 10.2
291.3 0.29 37.8 41.6 46.8 13.5 10.2
329.8 0.28 64.8 76.6 90.2 25.6 10.1
350.3 0.26 44.9 54.4 64.6 17.1 10.0
378.0 0.25 56.3 67.2 79.8 22.9 10.1
399.5 0.25 63.6 73.4 81.7 23.1 10.0
419.8 0.26 80.3 95.9 111.1 34.4 8.7
443.8 0.23 121.1 147.7 174.1 51.2 9.9
a Total organic carbon data from Ishiwatari et al. (1992). b Concentration expressed asmg/g organic carbon. c CPI
27-33=0.5*C27;29;31;33ÿ1=C26;28;30;321=C28;30;32;34
.
Fig. 3. Depth pro®le of the variation in the abundance of C27relative to C31n-alkanes together with a summary of the palynological
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 (Holo-cene). On the other hand, signi®cantly fewer pollen grains were observed in the lower section, with the pol-len assemblage being indicative of the presence of her-baceous 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 weightn -alkane homologues re¯ect a change in the types of vegetation in the lake watershed.
3.2. Carbon-isotopic composition ofn-alkanes
Compound-speci®c d13C values of the odd
carbon-numbered C27to C33n-alkanes are listed in Table 2. The
downcore average d13C values indicate that the n
-alkanes get systematically more 13C depleted with
increasing chain length, a feature also noted for long-chain n-alkanes in other lake sediments (Rieley et al., 1991; Spooner et al., 1994; Ficken et al., 1998). Down-cored13C pro®les of the individualn-alkanes are shown
in Fig. 4. In spite of the signi®cant change in the pollen record and then-alkane distribution observed across the late Pleistocene glacial±Holocene interglacial climate transition, thed13C pro®les of the high molecular weight n-alkanes are observed to be remarkably homogeneous as a function of core depth, with the maximum down-core range ofd13C values being 1.6, 0.9, 1.2 and 1.5%
for C27, C29, C31and C33n-alkanes, respectively (Table
2).
Moreover, the d13C values of C
27 to C33 n-alkanes
vary fromÿ31.0 toÿ33.5%, well within the range for leaf-wax n-alkanes biosynthesized by C3 plants (Rieley
et al., 1991; Collister et al., 1994; Yamada, 1997). Hence, the observed range of the carbon-isotopic com-position of the long-chain n-alkanes suggests the pre-valence of C3-type vegetation in the Lake Baikal
watershed even during the glacial interval. This infer-ence is corroborated by d13C values of the long-chain n-C29 and n-C31 alkanes extracted from a glacial age
peat bog interval collected from the lakeside of Baikal (d13C range: ÿ30.3 toÿ32.4%; Brincat et al.,
unpub-lished data), which are signi®cantly depleted in 13C
relative to the isotopic composition reported for the correspondingn-alkanes extracted from C4plants (d13C
Table 2
Carbon-isotopic data for the individual long-chain odd carbon-numberedn-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) TOCd13C (%)a n-C
27d13C (%) sb n-C29d13C (%) s n-C31d13C (%) s n-C33d13C (%) s
9.5 ÿ26.2 ÿ31.1 0.2 ÿ31.9 0.1 ÿ32.7 0.2 ÿ32.8 0.6 44.5 ÿ26.6 ÿ31.7 0.0 ÿ32.0 0.1 ÿ32.7 0.2 ÿ32.9 0.2 55.8 ÿ26.3 ÿ32.1 0.2 ÿ32.2 0.4 ÿ32.8 0.6 ÿ32.6 0.9 74.8 ÿ25.6 ÿ31.5 0.2 ÿ32.2 0.4 ÿ33.0 0.6 ÿ33.0 0.7 95.3 ÿ26.1 ÿ31.3 0.2 ÿ32.2 0.4 ÿ33.0 0.5 ÿ33.0 0.3 120.0 ÿ30.3 ÿ32.2 0.2 ÿ32.2 0.2 ÿ32.9 0.4 ÿ32.0 0.8 139.0 ÿ26.1 ÿ32.3 0.3 ÿ32.0 0.1 ÿ32.6 0.3 ÿ33.4 0.2 176.0 ÿ25.4 ÿ32.6 0.3 ÿ31.8 0.5 ÿ32.4 0.4 ÿ33.1 0.7 200.3 ÿ22.8 ÿ31.8 0.2 ÿ32.0 0.1 ÿ32.9 0.2 ÿ32.8 0.5 218.0 ÿ23.7 ÿ31.4 0.2 ÿ32.0 0.1 ÿ32.8 0.0 ÿ32.3 0.7 244.3 ÿ24.9 ÿ31.8 0.1 ÿ32.1 0.2 ÿ32.7 0.1 ÿ32.8 0.1 274.5 ÿ24.9 ÿ31.5 0.1 ÿ31.8 0.1 ÿ32.2 0.2 ÿ32.9 1.0 291.3 ÿ24.9 ÿ31.7 0.3 ÿ31.8 0.3 ÿ32.0 0.5 ÿ32.9 0.5 329.8 ÿ23.6 ÿ32.0 0.3 ÿ32.3 0.3 ÿ32.7 0.2 ÿ33.2 0.1 350.3 ÿ24.0 ÿ31.4 0.3 ÿ31.8 0.2 ÿ31.8 0.4 ÿ32.8 0.5 378.0 ÿ23.9 ÿ31.9 0.3 ÿ32.4 0.3 ÿ33.0 0.5 ÿ33.5 0.3 399.5 ÿ24.6 ÿ31.3 0.3 ÿ31.7 0.2 ÿ31.9 0.2 ÿ32.5 0.4 419.8 ÿ22.2 ÿ31.7 0.2 ÿ32.4 0.2 ÿ32.9 0.1 ÿ33.0 0.2 443.8 ÿ24.4 ÿ31.0 0.1 ÿ31.5 0.1 ÿ31.9 0.1 ÿ33.0 0.2 Averagec ÿ31.7 0.4 ÿ32.0 0.2 ÿ32.6 0.4 ÿ32.9 0.4
d(%)d 8.1 1.6 0.9 1.2 1.5 a Bulk isotopic values from Ishiwatari et al. (1992).
range: ÿ18.4 to ÿ24.5%; Collister et al., 1994; Licht-fouse et al., 1994; Yamada, 1997). Therefore, our d13C
data for the leaf-wax n-alkanes suggest that the observed 13C-enrichment in the bulk organic matter in
the glacial age sediments, relative tod13C
TOCvalues in
the Holocene section (Fig. 1b), is unlikely to be attrib-uted to a spread of C4-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-numberedn-alkanes in the C27±C33range, consistent with a terrestrial plant
epicu-ticular wax origin. Moreover, a systematic change in the most abundantn-alkane was also noted as a function of core depth, with the LGM section being dominated by
n-C31alkane whereas the Holocene age sediments byn
-C27homologue. Such a change in the distribution
pat-tern 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
previously recognized by palynological analyses of the same samples we analyze here.
Compound-speci®c isotopic analyses revealed
remarkably homogeneous downcore d13C pro®les for
the leaf-wax n-alkanes in both Holocene and LGM sediments. The isotopic compositional range ofÿ31.0 to
ÿ33.5% for C27±C33 n-alkanes in the entire cored
sequence represents an input from C3-type plants.
Therefore, the observed 13C-enrichment in the bulk
organic matter in the glacial age sediments is unlikely to be due to an expansion of C4-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 manu-script and providing many helpful suggestions. This work was supported by a research grant from the Min-istry of Education, Science and Culture of Japan.
Associate EditorÐJ.W. Collister
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