Organic Geochemistry Organic Geochemistry 37 (2006) 165–176 www.elsevier.com/locate/orggeochem Hydrogen isotope ratios of aliphatic and diterpenoid hydrocarbons in coals and carbonaceous mudstones from the Liaohe Basin, China Jincai Tuo a,* , Mingfeng Zhang a,b , Xianbin Wang a, Chuanlun Zhang c a Key Laboratory of Gas Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 382 Donggang West Road, Lanzhou 730000, Gansu, PR China b Graduate University of the Chinese Academy of Sciences, 100039, PR China c Savannah River Ecology Laboratory and Department of Marine Sciences, University of Georgia, P.O. Box Drawer E, Aiken, SC 29802, USA Received 21 June 2005; accepted 1 October 2005 (returned to author for revision 8 September 2005) Available online 1 December 2005 Abstract Hydrogen-isotope compositions of the aliphatic and diterpenoid hydrocarbons were determined for five coal and carbonaceous mudstone samples collected from drilling cores (1531–1767 m depths) in the Liaohe Basin, China. The bulk organic materials were mainly derived from terrestrial higher plants. dD values for most of the n-alkanes varied from 150& to 220&, and were not significantly different among the samples. Pristane was 34–69& depleted in D relative to phytane; both pristane and phytane, however, had the same trend of variation in dD from sample to sample. Diterpenoids were on average 49–81& depleted in D relative to the n-alkanes. Variations in dD also occurred between different diterpenoids, indicating a different source for these compounds. An enrichment process for the heavy hydrogen isotope was observed as expected when a compound was progressively altered through diagenesis (especially the dehydrogenation process). Overall, dD and d13C showed distinct patterns between structurally different lipid classes, although possible hydrogen exchange cannot be completely excluded during maturation. Our results further support the notion that hydrogen isotopes of lipid biomarkers from ancient sediments can be used to assess the origin of the organic matter, to determine oil-source rock correlation, and perhaps to reconstruct the paleoenvironment under which the organic material was deposited. 2005 Elsevier Ltd. All rights reserved. 1. Introduction Lipid biomarkers are biochemicals that derive from a restricted range of organisms and thus pro* Corresponding author. Tel.: +86 0931 4960854; fax: +86 0931 8278667. E-mail address: jctuo@ns.lzb.ac.cn (J. Tuo). vide a highly selective means of isolating material of specific origin (Volkman et al., 1998; Sauer et al., 2001; Schouten et al., 2001a,b, 2003; Sinninghe Damste´ et al., 2002; Chikaraishi and Naraoka, 2003; Van der Meer et al., 2003; Pancost and Sinninghe Damste´, 2003; Pancost and Boot, 2004). For example, terrestrial plants are characterized by a strong odd predominance in the C25–C35 0146-6380/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2005.10.001 166 J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 n-alkanes (Castillo et al., 1967; Rieley et al., 1991; Collister et al., 1994; Chikaraishi and Naraoka, 2003), whereas aquatic plants are characterized by C23 and C25 n-alkanes (Baas et al., 2000; Ficken et al., 2000; Zhang et al., 2004). Algae and cyanobacteria are characterized by short-chain n-alkanes (C15, C17 and C19) (Han et al., 1968; Gelpi et al., 1970). Tricyclic diterpenoids such as 18-norpimarane, pimarane, dehydroabietane and simonellite are thought to be derived from resins formed by higher plants, primarily gymnosperms (in particular conifers), but also some angiosperms, pteridophytes, and bryophytes (Noble et al., 1985). Tetracyclic diterpenoids such as phyllocladane are primarily derived from conifers (Karrer, 1958; Aplin et al., 1963; Erdtman and Norin, 1966; Karrer et al., 1977; Alexander et al., 1987; Sukh Dev, 1989; Otto et al., 1997; Otto and Simoneit, 2001; Bechtel et al., 2002; Otto et al., 2005). Compound-specific hydrogen isotopes of organic compounds are emerging as a new proxy for paleoclimatic, paleohydrological, and paleoenvironmental studies (Andsersen et al., 2001; Sauer et al., 2001; Huang et al., 2002; Yang and Huang, 2003; Sachse et al., 2004a,b; Sessions et al., 2004; Dawson et al., 2004, 2005; Xiong et al., 2005; Liu and Huang, 2005; Sun et al., 2005). Hydrogen in lipid biomarkers is commonly bound to carbon and nonexchangeable, whereas it is often exchangeable in kerogen, cellulose, and phenolic substances (Schimmelmann et al., 1999; Sauer et al., 2001). Hydrogen isotopes commonly show a large range of fractionation (Bigeleisen, 1965; Li et al., 2001; Chikaraishi and Naraoka, 2003). It is known that n-alkanes from algae are in general depleted in D relative to growth water by 160&, while sterols are depleted in D relative to growth water by 200& (Sessions et al., 1999; Sauer et al., 2001). Current knowledge suggests that the fractionation of H isotopes associated with biosynthesis is constant and mostly controlled by the biochemical pathways used by living organisms (Sessions et al., 1999; Sachse et al., 2004b). The hydrogen-isotope composition of lipids is controlled by three factors: isotope composition of biosynthetic precursors, fractionation and exchange accompanying biosynthesis, and hydrogenation during biosynthesis (Sessions et al., 1999; Xiong et al., 2005). Variations in the natural abundance of the deuterium (D) covalently bound to carbon may record both environmental conditions (Yapp and Epstein, 1982; Sternberg, 1988) and biochemical effects (Estep and Hoering, 1980; Yakir and DeNiro, 1990; Xiong et al., 2005; Sun et al., 2005). Therefore, dD ratios of lipid biomarkers have the potential to record the sources of hydrogen in a particular environment (Sachse et al., 2004b). Most studies involving compound-specific hydrogen isotopes are restricted to samples from Holocene to Miocene (e.g., Xie et al., 2000; Sauer et al., 2001; Yang and Huang, 2003). Recently, however, dD ratios of lipid biomarkers have been reported from source rocks of Oligocene to Upper Permian (Xiong et al., 2005) and from oil samples derived from source rocks of Oligocene to Ordovician (Li et al., 2001; Sun et al., 2005). These studies have proved that older fossil material preserved in sedimentary deposits can retain primary D/H compositions in their lipid biomarkers and thus hydrogen-isotope ratios of lipid compounds from ancient sediments can be applied to paleoecological and paleoenvironmental studies at the geological time scale. Tuo et al. (2003) determined carbon-isotope compositions for n-alkanes and diterpenoids in coals and carbonaceous mudstones, in which the organic materials were mainly derived from terrestrial higher plants. Terrigenous tricyclic and tetracyclic diterpenoid hydrocarbons are about 4–6& enriched in 13C compared to n-alkanes in these samples. In the current study, the hydrogen-isotope compositions were determined for the aliphatic and diterpenoid hydrocarbons in the same samples. The purpose for this study was to identify differences in hydrogen-isotope compositions between different lipid classes, which were derived from similar precursor organisms and to validate the use of hydrogen-isotope signatures for assessing the origin of the organic matter for oil-source rock correlation and paleoenvironmental reconstruction. 2. Materials and methods 2.1. Geological setting The Liaohe Basin, located in Liaonin Province, is one of the most important Cenozoic sedimentary basins in northeastern China. The basin was developed from a rift during the Cenozoic and accumulated lacustrine sediments of the Early Tertiary. Since the discovery of oil from the Xinglongtai Field in 1975, the Liaohe Basin has become the third largest oil field in China. The Liaohe Basin can be divided into seven structural units (Tuo et al., 2003; Tuo and Philp, 2005): western uplift, western depression, central uplift, J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 0.49 0.48 0.48 0.48 0.54 c d a b E, eogene. Bit.‘‘A’’, chloroform soluble extract; Sat., saturated hydrocarbons; Arom., aromatic hydrocarbons; NSO, N, S, O compounds (‘‘resins’’); Asph., asphaltenes. Precent of bitumen ‘‘A’’ extract. H/C, hydrogen/carbon; O/C, oxygen/carbon. 0.32 0.31 0.33 0.31 0.34 1.01 1.10 1.11 1.16 1.07 0.4 2.0 1.2 1.7 1.8 1.5 5.8 3.4 4.8 6.2 42.70 37.84 36.45 36.22 40.34 28.58 26.81 28.96 28.31 30.07 12.24 15.03 12.66 13.90 13.73 16.40 19.92 21.69 21.17 15.47 0.17 0.39 0.s8 0.60 0.66 0.59 1.11 0.52 1.71 2.26 39.37 19.09 15.12 35.71 36.80 Coal Carbonaceous mudstone Carbonaceous mudstone Coal Coal NSO Arom. 1531 1535 1637 1642 1767 E E E E E O/C H/C HC/TOC A/TOC Sat. Lithology Stratuma Xiao13 Xiao13 Xiao12 Xiao12 Ou 15 Methods for extraction, fractionation, and purification of the soluble organic materials have been Depth (m) 2.3. Experimental Well no. The five samples used in this study were selected from the Shahejie Formation of the Eocene age, in the eastern depression of the Liaohe Basin (Wells Xiao 12, Xiao 13, and Ou 15) (Tuo et al., 2003; Tuo and Philp, 2005). The geochemical characteristics of these samples are listed in Table 1. The distribution patterns of the aliphatic, diterpenoid and triterpenoid hydrocarbons and carbon-isotope compositions of aliphatic and diterpenoid hydrocarbons have been discussed in Tuo et al. (2003) and Tuo and Philp (2005). Table 1 Basic geochemical parameters for the samples under study 2.2. Samples Asph. Conversion rate (%) TOC (wt%) Bit.‘‘A’’ (%)b HC (%)c Bitumen ‘‘A’’ composition (%)b Atom ratios of kerogend Ro (%) eastern depression, eastern uplift, Damintun depression, and Shenbei depression. The Paleogene sequence can be divided into three formations: Fangshenpiao, Shahejie and Dongyin. The Shahejie Formation (Es) is widely distributed and contains the most important source rock and reservoir units in the entire basin. Based on lithology and fossil assemblages, it can be further subdivided into four members (Es4, Es3, Es2, and Es1, oldest to youngest) (Huang et al., 2003). Previous studies have shown that the third and fourth members (Es4, Es3) of the Shahejie Formation in the lower Eogene (Eocene-Oligocene) are the main source rocks in this basin. Most of the source rocks are mudstone and sandy mudstone with types II–III kerogen. Coal and carbonaceous mudstone are found mostly in the eastern depression of the basin. They were deposited mainly in fresh water lacustrine bog and flood plain facies. Vertically, coal and carbonaceous mudstone are generally present in the third member of Shahejie Formation and the first member also contains some carbonaceous mudstone. The coal beds are distributed in the southern and central parts of the eastern depression and generally occur in layers from 1 to 5 m thick, with a maximum singlebed thickness of 25 m (Rong 28 well) and total cumulative thickness of 111 m (Rong 60 well). The carbonaceous mudstones are found mostly in the northern parts of the eastern depression with an overall cumulative thickness of 344 m (Long 40 well). Coal and carbonaceous mudstone also constitute the source rocks for the oil and natural gas produced in eastern depression of the Liaohe Basin. 167 168 J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 described earlier (Tuo et al., 2003; Tuo and Philp, 2005). Briefly, the coal and carbonaceous mudstone-samples were powdered to less than 120 mesh and Soxhlet-extracted with chloroform for 72 h. The extracts (chloroform asphalt ‘‘A’’) were concentrated and deasphaltened by addition of excess hexane. Saturated and aromatic hydrocarbons and nonhydrocarbons (resin) were separated from the deasphaltened samples by column chromatography on a column of neutral alumina over silica gel (approx. 4 g of each). Saturated fractions were eluted with hexane (150 ml), aromatic fractions with methylene chloride (150 ml), and nonhydrocarbon fractions with methanol (30 ml). Analyses of hydrogen isotopes of individual compounds were performed on a Delta Plus XP gas chromatography-pyrolysis-isotope ratio mass spectrometer. The gas chromatography was performed using a Thermo Finnigan GC COMBUSTION III system equipped with an HP-5 fused silica capillary column (50 m · 0.32 mm) and helium used as carrier gas with a flow rate of 1 ml/min. The oven temperature was isothermal for 5 min at 80 C and then programmed from 80 to 300 C at 3 C/min. GCseparated compounds passed through an alumina tube, which was heated to 1450 C to convert organic H to H2, which was then introduced to a Delta Plus XP isotope ratio mass spectrometer. The reproducibility and accuracy of the analysis were evaluated routinely using laboratory standards of known dD values (C14, C16, C18, C23, C28, C32 nalkanes). Typically, one injection of laboratory standard was performed for every eight sample analyses. The isotope values are given with respect to the V-SMOW standard. Samples were analyzed four times and the results are presented as the average value with a standard deviation. For most of the lipid compounds, the standard deviations were below 3&. Greater standard deviations were probably caused by coeluting peaks or small peaks. They were included in order to provide an unbiased view of the results. 3. Results and discussion 3.1. Variation of dD in n-alkanes Table 2 displays measured D/H values for the three classes of lipid compounds (n-alkanes, isoprenoid alkanes, and diterpenoids) from five coal- and carbonaceous mudstone-samples. For the n-alkanes, several features are noticeable in Fig. 1, which graphically summarizes data from Table 2. First, for any of the five samples examined, n-alkanes exhibit restricted ranges in dD, with individual lipids in each sample generally falling within a range of <50&. The dD values for most of the measured n-alkanes in all of the analyzed samples vary from 150& to 220&. Similar dD values for the n-alkanes have also been reported in Jurassic lacustrine mudstones inter-bedded with coal (Xiong et al., 2005). Chikaraishi and Naraoka (2003) reported similar results in n-alkanes from C4 plants ( 171 ± 12&) and from aquatic freshwater plants ( 187 ± 16&). The long-chain n-alkanes of higher plant leaf waxes isolated from a Chinese loess profile also have a similar dD range ( 140& to 200&) (Liu and Huang, 2005). The measured dD values for the n-alkanes from all the analyzed samples in this study are about 10–40& depleted in D relative to n-alkanes extracted from C3 plants ( 152 ± 26& for angiosperms and 149 ± 16& for gymnosperms) and from seaweeds ( 155 ± 34&) (Chikaraishi and Naraoka, 2003). Second, C20 to C30 n-alkanes are depleted in D by 30–50& relative to both shorter (<C19) and longer (>C31) chain n-alkanes. Third, in n-alkanes of C20 to C33, the even carbon numbers are more enriched in D than they are in their neighboring odd-numbered carbon homologues. This zigzag distribution pattern for dD value of n-alkanes has been noticed by Yang and Huang (2003) in the n-alkanes extracted from some Miocene lacustrine sediments and plant fossils and by Xiong et al. (2005) in the n-alkanes from some swamp environments. This distribution of dD of n-alkanes has been attributed to waxes from terrestrial higher plants (Xiong et al., 2005). Finally, n-alkanes with the same chain-lengths from the different samples exhibit no substantial difference in dD values for four of the five analyzed samples. The H isotopic compositions of mid-chainlength individual n-alkanes (C19 to C30) from the sample of Xiao 13 (1535 m) are about 10–20& more enriched in D than they are from the other four analyzed samples (Table 2, Fig. 1), but this difference is much smaller than that within each sample for different chain-length n-alkanes. This restricted difference in dD values of n-alkanes with the same chain-lengths from the different samples may be attributed to either different rates of D-exchange due to lithology differences or differences in the sub-types of organic matter in these sediments, e.g., difference in the amount of organic matter derived from angiosperm (Tuo and Philp, 2005). J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 169 Table 2 Hydrogen-isotopic compositions of alkanes and diterpenoids in the coal and carbonaceous mudstone-samples of Liaohe Basin Well no. (depth) Xiao 13 (1531 m) n-Alkanes dD(&) r(&)a dD(&) r(&)a dD(&) nC13 nC14 nC15 nC16 nC17 nC18 nC19 nC20 nC21 nC22 nC23 nC24 nC25 nC26 nC27 nC28 nC29 nC30 nC31 nC32 nC33 150 162 165 192 168 174 184 207 165 187 196 189 213 194 217 199 221 188 201 162 165 NAb NAb 2 2 6 2 3 2 3 2 1 4 3 3 3 4 3 NAb 3 NAb 4 166 158 152 166 177 169 171 175 146 175 177 175 180 168 179 162 181 164 165 ndc 162 NAb 3 3 9 6 2 6 19 4 7 2 1 7 NAb 8 NAb 2 NAb 4 1 ndc 158 174 178 175 162 180 202 ndc 207 209 210 213 202 214 210 209 ndc 175 ndc ndc Isoprenoid alkanes Pristane Pr Phytane Ph 243 209 3 3 217 171 15 12 264 213 305 319 198 233 284 3 5 3 2 1 288 297 152 185 280 6 3 2 NAb 3 186 2 168 267 3 240 Diterpenoids Norpimarane Pimarane Simonellite Dehyaroabietane 16a(H)Phyllocladane Average (nalkanes) Average (diterpenoids) a b c Xiao13 (1535 m) Xiao12 (1637 m) r(&)a Xiao12 (1642 m) dD(&) r(&)a Ou15 (1767 m) dD(&) r(&)a NAb NAb NAb NAb 2 3 3 NAb 2 5 5 3 1 2 2 8 2 NAb 2 168 160 155 170 152 145 ndc ndc ndc 215 206 244 207 196 214 ndc 216 ndc 170 ndc 155 2 4 2 4 2 5 5 158 147 150 165 171 174 192 217 167 200 211 202 206 176 203 185 202 159 173 ndc 160 3 7 245 183 2 14 264 195 3 1 293 296 174 186 273 3 1 2 4 1 292 299 152 234 274 1 1 4 6 3 269 306 176 143 259 4 2 3 2 1 4 192 4 185 3 181 2 3 244 5 250 2 230 2 9 1 3 2 1 5 19 5 1 4 2 2 1 2 3 10 7 4 4 6 4 3 1 2 4 Standard deviation of 4 replicate analyses. Only one analysis available. No data. No substantial differences in dD have been noticed among the n-alkanes from all other analyzed samples. According to Tuo et al. (2003) and Tuo and Philp (2005), all the samples used in this study were collected from similar depositional environments and the organic matter in all the samples was at a similar thermal evolution stage (Table 1). So the generally consistent dD values for the n-alkanes within each sample and from different samples probably reflect similar sources of hydrogen for these saturated hydrocarbons. 3.2. Variation of dD in isoprenoid alkanes (pristane and phytane) Values of dD vary from 217& to 264& in pristane (Pr) and from 171& to 213& in phytane (Ph) (Table 2). Thus, phytane has dD values similar to the n-alkanes, whereas the pristane is about 50& depleted in D relative to the n-alkanes. Interestingly, pristane and phytane follow a similar trend of variation in dD from sample to sample (Fig. 2). The generally lighter dD values of pristane relative to 170 J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 -100 δD(‰ , SMOW) Xiao13(1531) Xiao13(1535) Xiao12(1637) Xiao12(1642) Ou 15(1767) -150 -200 -250 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Carbon number Fig. 1. Hydrogen-isotope ratios of n-alkanes. -150 δD(‰, SMOW) Pr Ph -200 -250 -300 Xiao13(1531) Xiao13(1535) Xiao12(1637) Xiao12(1642) Ou 15(1767) Samples Fig. 2. Comparison of dD for pristane and phytane among different samples. phytane was also noticed by Li et al. (2001) in oil samples derived from source rocks deposited in a variety of environments. The acyclic isoprenoid hydrocarbons pristane and phytane are ubiquitous in sedimentary rocks, crude oils and coals (Koopmans et al., 1999). The phytol side chain of chlorophyll a is known to be the precursor for pristane and phytane. Alternative precursors for pristane may be from tocopherols (Goossens et al., 1984; Koopmans et al., 1999) and lipids from archaea (Rowland, 1990; Navale, 1994), Pr and its mono-, di-, and tri-unsatured counterparts in zooplankton (Blumer et al., 1963, 1969; Blumer and Thomas, 1965), diphytanyl glyceryl ether, S- and O bound Pr, the (unknown) precursor of prist-2-ene (Koopmans et al., 1999), and bound methylated 2-methyl- 2-(4,8,12-trimethyltridecyl) chromans (Li et al., 1995). Other precursors for phytane include ether lipids from archaea (Risatti et al., 1984; Rowland, 1990; Navale, 1994), S-bound Ph (probably originating from phytol or its diagenetic products) and diphytanyl glyceryl ether (Koopmans et al., 1999). The multiple sources for pristane and phytane and associated hydrogen isotope fractionations may be far more complicated than currently understood. However, for samples that formed in similar environments, the precursors for pristane and phytane perhaps can be generally characterized. One such generalization is the similar trend of variation in dD from sample to sample. For example, Li et al. (2001) and Schimmelmann et al. (2004) also observe that phytane is enriched in D relative to J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 pristane in sediments and crude oils. The reason for the different dD values between pristane and phytane may be attributed to either different origins of pristane and phytane or different isotope effects during their derivation from a common phytol precursor (Li et al., 2001; Dawson et al., 2004, 2005). With increasing maturity, pristane and phytane become enriched in D while the n-alkanes generally remain at a constant isotopic composition. The preferential enrichment of D in isoprenoids over nalkanes with increasing maturity suggests that hydrogen isotopic exchange occurs more readily with isoprenoids than with n-alkanes (Dawson et al., 2004, 2005). The offset between the dD values of n-alkanes and isoprenoids has been observed in modern biological samples as well as in ancient oil and sediment samples (Li et al., 2001; Schimmelmann et al., 2004; Dawson et al., 2004, 2005), indicating that their indigenous dD signatures can be preserved in geological samples, although possible hydrogen exchange cannot be completely excluded. 3.3. Variation of dD in diterpenoids Values of dD measured for diterpenoids are listed in Table 2 and graphically summarized in Fig. 3. The major feature is a significant depletion (49– 81&) in D in the diterpenoids relative to the n-alkanes. To the best of our knowledge, no published work has demonstrated the difference of the hydrogen-isotope ratios between n-alkanes and diterpenoid compounds in sediments or modern plants. But comparing lipid classes (Estep and Hoering, 1980) or individual compounds within a single -100 δD(‰, SMOW) -150 171 organism (Sessions et al., 1999; Sauer et al., 2001), polyisoprenoid lipids are generally depleted in D relative to acetogenic (n-alkyl) lipids. Therefore, it is likely that the isotopic difference between n-alkanes and diterpenoids may reflect biosynthetic control of initial values of dD in living organisms that are preserved in the sediments (Sauer et al., 2001). Hydrogen isotopic variations can also occur between different diterpenoid compounds (Table 2 and Fig. 3). Values of dD varied from 292& to 305& for norpimarane and from 296& to 319& for pimarane. The difference in dD (3– 37&) is insignificant between pimarane and norpimarane. This result is expected since previous study has proved that both norpimarane and pimarane are derived from same starting precursor pimaric acid (Simoneit, 1986; Simoneit et al., 1986; Tuo and Philp, 2005). However, simonellite is 120–150& enriched in D relative to norpimarane or pimarane and has dD values ranging from 152& to 198&. These values indicate that simonellite has a different source of hydrogen than norpimarane or pimarane. Dehydroabietane is about 12–82& depleted in D relative to simonellite in the Xiao 13 and Xiao 12 samples but about 33& enriched in D relative to simonellite in the Ou 15 sample (Table 2). Values of dD are also more variable for dehydroabietane than for simonellite, ranging from 143& to 234& in the analyzed samples. The diterpenoid structures encountered in this study may be derived diagenetically from tricyclic precursors based on the abietane and pimarane skeletons (Wakeham et al., 1980; Simoneit, 1986; Simoneit et al., 1986; Ellis Samples Xiao13(1531) Xiao13(1535) Xiao12(1637) Xiao12(1642) Ou 15(1767) -200 -250 -300 -350 Norpimarane Pimarane Simonellite Dehydroabietane 16α(H)- Phyllocladane Fig. 3. Hydrogen-isotope ratios of diterpenoids. 172 J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 et al., 1996; Otto et al., 1997; Otto and Simoneit, 2001; Otto et al., 2005; Tuo and Philp, 2005). Both simonellite and dehydroabietane may be derived from the same starting precursor, abietic acid. During the formation of tricyclic aromatic hydrocarbons from abietane-type diterpenoid precursors, however, dehydroabietane is usually present at a relatively earlier diagenetic stage than simonellite (Wakeham et al., 1980; Simoneit, 1986; Simoneit et al., 1986; Ellis et al., 1996; Li et al., 1990; Otto et al., 1997; Otto and Simoneit, 2001; Tuo and Philp, 2005). If that is the case, our results of hydrogen isotopes for simonellite and dehydroabietane suggest that a D-enrichment process occurred during diagenesis of the diterpenoids with later-derived byproducts such as simonellite being enriched in D relative to the earlier-derived byproduct such as dehydroabietane. Dehydrogenation may be a major pathway for such enrichment, which preferentially removes light hydrogen as diagenesis progresses. Norpimaranes are slightly enriched in D relative to pimaranes, which is probably the result of decarboxylation or dehydrogenation from a common precursor during diagenesis. The dD for 16a(H)-phyllocladane varies narrowly from 259& to 284&. These values are about 10–20& enriched in D relative to norpimarane and pimarane but about 50–100& depleted in D relative to simonellite and dehydroabietane. Such a comparison also indicates a different source for these compounds. Tetracyclic diterpenoids such as phyllocladane are primarily derived from conifers (Alexander et al., 1987), including Podocarpaceae, Araucariaceae, Cupressaceae (Karrer, 1958; Aplin et al., 1963; Erdtman and Norin, 1966; Karrer et al., 1977; Sukh Dev, 1989; Otto et al., 1997), Taxodiaceae (Otto et al., 1997; Bechtel et al., 2002), and probably pteridophytes (Cheng et al., 1997). So a predominance of diterpenoid hydrocarbons with phyllocladane as the major peak suggests that the organic material in five samples originates primarily from higher plants and more specifically, conifers (Tuo et al., 2003; Tuo and Philp, 2005). Phyllocladane has a more limited geological distribution than the tricyclic diterpenoids and is found mostly in Devonian (Sheng et al., 1991; Cheng et al., 1997), Carboniferous (Grantham et al., 1983; Schulze and Michaelis, 1990) and Tertiary (Dai and Mei, 1988) deposits. The phyllocladanes have fewer precursors compared to the tricyclic diterpenoids (Otto and Wilde, 2001; Otto et al., 2005), which may be a reason that phyllocladanes have a relatively con- strained distribution in their hydrogen-isotopic compositions. On the contrary, the broader relative precursor input to the tricyclic diterpenoids (Otto and Wilde, 2001; Otto et al., 2005) may result in a slightly greater variation of their hydrogen-isotopic compositions. This finding is consistent with the carbon-isotopic compositions measured for the same compounds in the same samples (Tuo et al., 2003). It cannot be excluded that the hydrogen-isotope signature has been altered to some extent by diagenetic transformation in the subsurface (Dawson et al., 2004, 2005), even though these sediments are relatively immature. It has been shown that hydrogen exchange occurs between organic matter and water during hydrous pyrolysis (Alexander et al., 1982, 1983; Schimmelmann et al., 1999, 2004; Leif and Simoneit, 2000). Consequently, the dD values of diterpenoids in our samples might have been affected to an unknown extent by hydrogen exchange reactions between organic matter and formation water during maturation (Dawson et al., 2004, 2005). But the results in this study suggest that hydrogen exchange processes have not significantly altered the indigenous isotopic signatures. Furthermore, different compounds can have different hydrogen isotopes even after extensive D-exchange (Dawson et al., 2004, 2005). Theoretical calculations also indicate that, for polyisoprenoid compounds (such as steranes) in immature sediments, the D/H ratio imparted by biosynthesis is largely preserved in spite of significant structural change (Sessions et al., 2004). 3.4. Application of lipid-hydrogen isotopes in geological studies Figure 4 shows the relationship between dD and d13C for the lipid compounds in the same samples (details of carbon-isotope analysis were reported in Tuo et al., 2003). The plot includes average values of structurally-similar compound classes (e.g., n-alkanes) as well as individual compounds (pimarane, norpimarane, 16a(H)-phyllocladane, dehydroabietane, and simonellite). The dD and d13C are distinct among n-alkanes, dehydroabietane and simonellite, and pimarane, norpimarane and 16a(H)-phyllocladane (Fig. 4). The differences in dD and d13C among these groups are likely due to isotope effects associated with different biosynthetic pathways within the organisms that synthesized these molecules. This conclusion is based on the following reasons: J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 173 -21 Simonellite -22 16α(H)-Phyllocladane -24 Pimarane -25 Norpimarane -26 Dehyaroabietane 13 δ C(‰, PDB) -23 -27 -28 -29 -30 -350 Xiao13 (1531) Xiao13 (1535) Xiao12 (1637) Xiao12 (1642) Ou15 (1767) -300 n-Alkanes(average) -250 -200 -150 -100 δ D(‰, SMOW) Fig. 4. dD versus d13C of the same lipid compounds (values of d13C for the lipid compounds are from Tuo et al., 2003). (1) the samples were formed in a similar depositional environment; (2) the biomarkers were mainly derived from terrestrial higher plants; and (3) the organic matter in all the samples is at a similar thermal evolution stage (Table 1) (Tuo et al., 2003; Tuo and Philp, 2005). It has been a concern that hydrogen-isotope exchange occurs between organic molecules preserved in sedimentary rock and the surrounding environment during diagenesis over an extended geological time scale (Andsersen et al., 2001; Sessions, 2001). A systematic study of compound-specific hydrogen isotopes of major genetic oil families in the Western Canada Sedimentary Basin has shown that oils derived from source rocks of Cambrian age still retain a strong signature of the hydrogen isotopic compositions of source organic matter and source water (Li et al., 2001). Based on their study, Li et al. (2001) proposed that dD values are useful for oil-source correlation and for paleoenvironmental reconstructions. A study on the lipidhydrogen isotope ratios in Miocene lacustrine sediments and plant fossils in Clarkia has also proven that an extensive H-exchange between environmental waters and fossils is unlikely and the lipid hydrogen isotopes can be used for paleoecological studies over an extended geological time period (Yang and Huang, 2003). Xiong et al. (2005) characterized the hydrogen-isotopic compositions of individual n-alkanes in terrestrial source rocks. They observed a trend of depletion in dD for n-alkanes from saline water, to freshwater paralic lacustrine, and to swamp deposits. Our results indicate that ancient fossil material preserved in sedimentary deposits can retain large variations in both hydrogen- and carbon-isotope compositions between different lipid biomarkers, which can be applied to assess the origin of the organic matter, for oil-source correlation and for paleoenvironmental reconstructions at the geological time scale. 4. Conclusions Our study on hydrogen-isotope compositions of aliphatic and diterpenoid hydrocarbons has the following conclusions: 1. The n-alkanes exhibit restricted ranges in dD values, which varied from 150& to 220&. No significant difference existed in dD values among the n-alkanes from the analyzed samples. 2. Pristane was 34–69& depleted in D relative to phytane; the variation of dD values for both pristane and phytane follow a similar trend and appear as parallel lines among the analyzed samples. The reason for the different dD values between pristane and phytane may be attributed to either different origins of pristane and phytane or different isotopic effects for their derivation from a common phytol precursor. 3. Values of dD measured for diterpenoid compounds are 49–81& depleted in D relative to nalkanes. Hydrogen isotopic variations also occur between different diterpenoid compounds, indicating a different source for these compounds. Based on the comparison of dD values of the 174 J. Tuo et al. / Organic Geochemistry 37 (2006) 165–176 different classes of diterpenoid compounds, which may be derived from the same biosynthetic precursors, a D-enrichment process (especially dehydrogenation) will be expected when a compound is diagenetically altered from a natural precursor structure to a geological structure. 4. The dD and d13C were distinct between structurally different compound classes. This observation indicates that hydrogen-exchange processes have not altered the indigenous isotopic signature of aliphatic and diterpenoid hydrocarbons to a large extent, although possible hydrogen exchange cannot be completely excluded during maturation. These observations suggest that hydrogen isotopes of lipids can be applied to assess the origin of the organic matter, oil-source correlation, and depositional environments in the geological past. Acknowledgements This research was supported by the China National Natural Science Foundation (Grant Number 40073022), the China 973 National Key Research and Development Program (Grant Number: 2003CB214606), the American Chemical Society Petroleum Research Fund (CLZ), and the US Department of Energy Financial Assistance Award DE-FC09-96SR18546 to the University of Georgia Research Foundation (CLZ). We thank Dr. Maowen Li and an anonymous reviewer for valuable comments that improved an earlier version of the manuscript. Drs. S. Schouten and L.R. Snowdon are acknowledged for their constructive comments and for handling our paper in the review and publication process. Associate Editor—Stefan Schouten References Alexander, G., Hazai, I., Grimalt, J.O., Albaiges, J., 1987. Occurrence and transformation of phyllocladanes in brown coals from Norgad Basin, Hungary. Geochimica et Cosmochimica Acta 51, 2065–2073. Alexander, G., Kagi, R.I., Larcher, A.V., 1982. 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