Journal of Paleolimnology 31: 363–375, 2004. # 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Hydrogen isotope ratios of individual lipids in lake sediments as novel tracers of climatic and environmental change: a surface sediment test Yongsong Huang*, Bryan Shuman, Yi Wang and Thompson Webb III Department of Geological Sciences, Brown University, Providence, RI 02912, USA; *Author for correspondence (e-mail:
[email protected]) Received 13 June 2003; accepted in revised form 17 November 2003
Key words: Climate change, Hydrogen isotope ratios, Lake sediments, Lipids, Modern calibration
Abstract We determined hydrogen isotope ratios of modern lake-waters and individual lipids from surface sediments of 36 lakes in the eastern North America. The lakes selected lie on two transects (south–north transect from Florida to Ontario and east–west transect from Wisconsin to South Dakota) and encompass large temperature and moisture gradients, and a wide range of lake water D values (>100ø). The study allows a rigorous test of the applicability of using D values of sedimentary lipids as paleoclimatic and paleoenvironmental proxies. We examined a range of lipids including C17 n-alkane, straight chain fatty acids, phytol and sterols in both free extracts and ester-bound fractions in the solvent extracted sediments. Useful isotopic indicators are expected to show a linear correlation and constant fractionation factor between their D values in surface sediments and modern lake water. Our results demonstrate that several lipid compounds, free and esterbound palmitic acid (16 : 0), C17 n-alkane, and phytol are useful candidates for paleoclimate reconstructions, in addition to two sterols that have been suggested previously (Sauer et al. 2001a. Compound-specific D/H ratios of lipid biomarkers from sediments as a proxy for environmental and climatic conditons. Geochim. Cosmochim. Acta 65: 213–222). Authigenic or biogenic carbonate in sediments is conventional material for paleoclimatic study using ocean and lake sediments. However, because majority of lake sediments do not contain suitable carbonate materials for isotopic study, hydrogen isotope ratios of these lipids provide invaluable new sources of paleoclimatic and paleoenvironmental information. Introduction Stable isotope analyses of lacustrine carbonates can provide high-resolution records and quantitative estimates of climate and environmental variables (Kelts and Talbot 1990; Anderson et al. 1997; von Grafenstein et al. 1998; Yu and Eicher 1998; Kirby et al. 2001, 2002). However, applications of stable isotope analyses in lake sediments are often hampered by the availability of suitable materials for isotope measurements (Sauer et al. 2001a, b; Huang et al. 2002). For example, many lakes (e.g., lakes in New England) have acidic waters and contain no biogenic/authigenic carbonates in sediments. Bulk organic fractions, such as 18O
(Edwards and McAndrews 1989) of sediment cellulose and D (Krishnamurthy et al. 1995) values of kerogen, have been used to provide past climate information, but are complicated by the nature and multiple origins of bulk sedimentary cellulose and kerogen (Hassan and Spalding 2001; Sauer et al. 2001b). Aquatic photosynthetic organisms in lakes obtain hydrogen from lake water to produce their organic compounds such as cellulose and lipids. The organic hydrogen and oxygen isotope ratios display, relative to the source water, an offset (fractionation) which is shown to be independent of temperature (Estep and Hoering 1980; DeNiro and Epstein 1981). Therefore, the isotope ratios of lipids and cellulose from aquatic organisms carry a
364 signal of oxygen/hydrogen isotope ratios of lake water which is a function of climatic and environmental conditions (Gonfiantini 1986). Compound-specific isotope analysis has become an important approach for paleoenvironmental and paleoclimatic studies from lake sediments (e.g., Huang et al. 1999, 2001, 2002; Andersen et al. 2001; Sauer et al. 2001a). Measuring the hydrogen isotope ratios of individual lipid compounds offers a number of potential advantages over measuring bulk organic fractions: (1) The structure of measured compounds is known, hence the specific compound measured in different sedimentary horizons is identical. Isotopic change due to selective diagenesis of isotopically different components in bulk organic fractions is avoided. (2) Lipids are refractory compounds and can be preserved in sediments for millions and even billions of years (Huang et al. 1995; Brocks et al. 1999). (3) Hydrogen isotope measurements can be done specifically on the carbon-bound (leastexchangeable) hydrogens in lipid compounds and exclusion of exchangeable hydrogens in individual lipid compounds can be achieved accurately using a mass balance approach. (4). Individual lipids in sediments often have restricted origins from individual or groups of organisms: some are so-called biomarkers that are only biosynthesized by certain organisms, others are less specific biosynthetically but have restricted origins due to sediment input mechanisms (Huang et al. 2002). Likewise, local environmental factors (e.g., dominance of aquatic organic inputs) have enabled useful results in the case of some bulk organic measurements (e.g., Edwards and McAndrews 1989). One powerful way to validate a paleoclimate proxy is the surface sediment test. For example, robust transfer functions have been generated from statistical analyses of a transect of lakes between pollen and seasonal climate conditions (Bartlein et al. 1984); between chironomids and water temperature (Walker et al. 1991); diatoms and salinity (Fritz 1990); chrysophytes and pH (Cumming and Smol 1993). Similarly, if lipid compounds in surface sediments from different lakes have hydrogen isotope ratios that show a clear correlation with those in modern lake waters, the compounds are potential paleoclimate indicators (because hydrogen isotope ratios of lake waters are primarily determined by a combination of
climatic and hydrologic conditions (Sauer et al. 2001a; Huang et al. 2002). Such test is highly rigorous because lakes have different morphological, biological and hydrological characteristics. In a core from a single site, many of the local (nonclimatic) factors that add to the isotope variability among lakes today are naturally minimized because the local controls remain relatively constant over time. Lipid compounds are therefore expected to better record lake water isotope ratios over time than noise in the surface sediment calibration may suggest. The goals of the present study are: (1) to conduct a rigorous and comprehensive surface calibration study using a large set of lakes from different climate regimes; (2) to examine common lipid compounds in sediments and the suitability of their D values as paleoclimate proxy; (3) to study both free-extractable and bound lipid fractions because previous studies (Sauer et al. 2001a; Huang et al. 2002) focused on only solvent extractable compounds; (4) to study variability in the distributions of relevant lipids in the lake surface sediments in order to provide a general guide for future hydrogen isotope studies.
Samples and experimental methods Samples We collected surface sediment and lake water samples from sites across eastern North America along two transects (1) from Ontario to Florida, and (2) from South Dakota to Wisconsin (Figure 1). The lakes and reservoirs sampled were approximately 10–500 ha in size, and were high in their respective watersheds. To ensure a locally controlled isotopic signature, lakes and reservoirs with large stream or river inputs were not sampled for this study. The north–south transect of lakes spans a wide range of mean annual temperature (1–23 C). The east–west transect of lakes span a wide range of moisture balance with relatively small mean-annual temperature difference (3 C). Water samples were collected from lake centers, at approximately 50 cm below the surface. A 50-ml high-density polyethylene bottle was filled to its full capacity, and capsealed to avoid evaporation. Surface sediments (0–2 cm) were collected using an Ekman dredge
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Figure 1. Sampling locations of studied lakes.
or a short gravity-corer, and were kept at 0 C immediately after sampling, and frozen once at Brown. The sampling took place during August– September 2000 and August 2001, when phytoplankton productivity is often high in North American lakes, allowing the collection of water samples more representative of those used by aquatic algae to produce their lipid compounds. Extraction and lipid isolation Sediment samples were freeze-dried, and free lipids extracted using an Accelerated Solvent Extractor ASE200 (Dionex) (Huang et al. 2002). Carboxylic acid fraction was isolated from the total extracts using solid phase extraction (Aminopropyl Bond Elute1) (Huang et al. 1999), and then methylated using anhydrous 2% HCl in methanol. Hydroxyl acids were removed using silica gel column chromatography (DCM as solvent), in order to further purify the fatty acid methyl esters and avoid chromatographic coelution. The neutral
fractions were separated into hydrocarbons (hexane), alcohol (hexane : EtoAc 3 : 1) and polar fractions (MeOH) using column chromatography (Yang and Huang 2003). The alcohol functional groups in the molecules were derivatized using pyridine : N, N-bis(trimethylsily1)-trifluoroacetamide (BSTFA) (1 : 1). The bound lipids from sediments were released by saponifying the ASE extracted sediments under reflux using 0.5 N KOH/MeOH (with 2–3% water) (e.g., Meyers and Ishiwatari 1993). The solution was then acidified and extracted with hexane. The combined extracts were separated into various fractions using the procedures described above. If C17 n-alkane is present in adequate abundance in the hydrocarbon fractions, it is purified by two consecutive urea adduction procedures (Pond et al. 2002) prior to hydrogen isotope analysis. Quantification and identification of compounds were carried out using GC and GC-MS, respectively (Yang and Huang 2003). An HP 6840+ GC-pyrolysis system interfaced to a
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Figure 2. Summary of relative lipid concentrations (error bars are 1 standard deviations) normalized to the most abundant compound within the compound series in lake surface sediments. (A) Distribution of major n-alkanes in hydrocarbon fractions. C22 to C32 even carbon number n-alkanes are not shown due to their low concentrations. (B) Distributions of normal fatty acids (n-alkanoid acids). (C) Relative abundances (normalized to cholesterol which is given an arbitrary number of 10) of selected lipid compounds: n-alkanes (C17 and C29), free fatty acids (16 : 0 and 26 : 0), ester bound fatty acids (14 : 0 and 16 : 0), ester-bound phytol, sterols, and hopanoids (including hopanoid acids and alcohols). Bound lipids are designated by a suffix ‘‘B’’. Cholesterol is artificially set as 12 in all the samples. All compounds are then normalized to the cholesterol value (i.e., 12). (D) Distributions of ester-bound normal fatty acids (nalkanoid acids).
Finnigan Delta+ XL stable isotope spectrometer through a high-temperature pyrolysis reactor was used for hydrogen isotopic analysis (Andersen et al. 2001; Wang and Huang 2001; Huang et al. 2002; Pond et al. 2002). The precision (1) of the triplicate analyses was kept
