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1.
During the last decade, compound-specific hydrogen isotope analysis of plant leaf-wax and sedimentary n-alkyl lipids has become a promising tool for paleohydrological reconstructions. However, with the exception of several previous studies, there is a lack of knowledge regarding possible effects of early diagenesis on the δD values of n-alkanes. We therefore investigated the n-alkane patterns and δD values of long-chain n-alkanes from three different C3 higher plant species (Acer pseudoplatanus L., Fagus sylvatica L. and Sorbus aucuparia L.) that have been degraded in a field leaf litterbag experiment for 27 months.We found that after an initial increase of long-chain n-alkane masses (up to ∼50%), decomposition took place with mean turnover times of 11.7 months. Intermittently, the masses of mid-chain n-alkanes increased significantly during periods of highest total mass losses. Furthermore, initially high odd-over-even predominances (OEP) declined and long-chain n-alkane ratios like n-C31/C27 and n-C31/C29 started to converge to the value of 1. While bulk leaf litter became systematically D-enriched especially during summer seasons (by ∼8‰ on average over 27 months), the δD values of long-chain n-alkanes reveal no systematic overall shifts, but seasonal variations of up to 25‰ (Fagus, n-C27, average ∼13‰).Although a partly contribution by leaf-wax n-alkanes by throughfall cannot be excluded, these findings suggest that a microbial n-alkane pool sensitive to seasonal variations of soil water δD rapidly builds up. We propose a conceptual model based on an isotope mass balance calculation that accounts for the decomposition of plant-derived n-alkanes and the build-up of microbial n-alkanes. Model results are in good agreement with measured n-alkane δD results. Since microbial ‘contamination’ is not necessarily discernible from n-alkane concentration patterns alone, care may have to be taken not to over-interpret δD values of sedimentary n-alkanes. Furthermore, since leaf-water is generally D-enriched compared to soil and lake waters, soil and water microbial n-alkane pools may help explain why soil and sediment n-alkanes are D-depleted compared to leaves.  相似文献   

2.
Various aquatic plants from Lake Qinghai, the largest inland saline lake in China, and terrestrial plants from the surrounding area were investigated for the distribution of n-alkanes and their δD values. The n-alkanes in the samples range from C15 to C33 with C preference index (CPI) values of 4.0–29.7. The n-C23 or n-C25 alkane is the dominant compound in the aquatic submerged plants. The aquatic emergent and terrestrial plants have an abundance maximum at n-C27, n-C29 or n-C31. The average chain length (ACL) values, ranging from 26.0 to 29.6, are closely related to the plant species. The n-alkanes from the aquatic plants have mean δD values of −169‰ to −121‰ and those from the terrestrial plants values of −173‰ to −109‰. The H isotopic composition (δD) and fractionation differ significantly among the plants studied. Comparison shows that additional evaporative enrichment of the lake water associated with saline lakes and humidity influence the δD values of the n-alkanes in aquatic and terrestrial plants, respectively. The mean δD values of n-alkanes in the plants decrease with increasing ACL value. The n-alkanes from the different types of plants are more depleted in D relative to environmental water and those from aquatic plants (with a mean value of −143‰) have a greater isotopic fractionation than terrestrial plants (mean value −113‰).  相似文献   

3.
We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ13C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ13C values of odd carbon numbered C23-C33n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ13C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C25-C31n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.  相似文献   

4.
Leaf wax n-alkanes from barley (Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water varied by 40‰. Additionally the seasonal variability in δD values of leaf wax n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.  相似文献   

5.
Environmental parameters such as rainfall, temperature and relative humidity can affect the composition of higher plant leaf wax. The abundance and distribution of leaf wax biomarkers, such as long chain n-alkanes, in sedimentary archives have therefore been proposed as proxies reflecting climate change. However, a robust palaeoclimatic interpretation requires a thorough understanding of how environmental changes affect leaf wax n-alkane distributions in living plants. We have analysed the concentration and chain length distribution of leaf wax n-alkanes in Acacia and Eucalyptus species along a 1500 km climatic gradient in northern Australia that ranges from subtropical to arid. We show that aridity affected the concentration and distribution of n-alkanes for plants in both genera. For both Acacia and Eucalyptus n-alkane concentration increased by a factor of ten to the dry centre of Australia, reflecting the purpose of the wax in preventing water loss from the leaf. Furthermore, Acacian-alkanes decreased in average chain length (ACL) towards the arid centre of Australia, whereas Eucalyptus ACL increased under arid conditions. Our observations demonstrate that n-alkane concentration and distribution in leaf wax are sensitive to hydroclimatic conditions. These parameters could therefore potentially be employed in palaeorecords to estimate past environmental change. However, our finding of a distinct response of n-alkane ACL values to hydrological changes in different taxa also implies that the often assumed increase in ACL under drier conditions is not a robust feature for all plant species and genera and as such additional information about the prevalent vegetation are required when ACL values are used as a palaeoclimate proxy.  相似文献   

6.
Soil n-alkane δD vs. altitude gradients along Mount Gongga, China   总被引：1，自引：0，他引：1  
The altitude effect on the isotopic composition of precipitation and its application to paleoelevation reconstruction using authigenic or pedogenic minerals have been intensively studied. However, there are still no studies on variations in biomarker δD along altitude transects to investigate its potential as a paleoelevation indicator, although it has been observed that δD of higher plant lipid may record changes in precipitation δD (δDp). Here, we present δD values of higher plant-derived C27, C29, and C31n-alkanes from surface soil along the eastern slope of Mount Gongga, China with great changes in physical variables and vegetation over a range from 1000 to 4000 m above sea level. The weighted-mean δD values of these n-alkanes (δDwax) show significant linear correlations with predicted δDp values (R2 = 0.76) with an apparent isotopic enrichment (εwax-p) of −137 ± 9‰, indicating that soil δDwax values track overall δDp variation along the entire altitudinal transect. Leaf δDwax is also highly correlated with mountain altitude by a significant quadratic relationship (R2 = 0.80). Evapotranspiration is found declining with altitude, potentially lowering δDwax values at higher elevations. However, this evapotranspiration effect is believed to be largely compensated by the opposing effect of vegetation changes, resulting in less varied εwax-p values over the slope transect. This study therefore confirms the potential of using leaf δDwax to infer paleoelevations, and more generally, to infer the δD of precipitation.  相似文献   

7.
Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ13C values. The δD and δ13C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C29 and n-C31 are mainly originated from terrestrial higher plants; n-C28 and n-C30 may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C23 being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δDLW and δDMW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδDLW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδDC23 ‐ long (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδDmid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of εwater/lipid values. The coupling of ΔδDC23 ‐ long, ΔδDmid-long and ΔδDLW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.  相似文献   

8.
We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (εa) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative εa values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum εa values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.  相似文献   

9.
Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the δ13C values of the two dominant n-alkanes in the aerosols, n-C29 and n-C31 alkane, is, however, insignificant. Their δ13C values were translated into a percentage of C4 vs. C3 plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C4 type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C4 plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols.  相似文献   

10.
Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ13C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (εlipid) varies by as much as 10‰ among different chain lengths. Overall, εlipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δleaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3-5‰ more depleted in 13C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and εlipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene-Eocene Thermal Maximum (55.8 Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.  相似文献   

11.
《Organic Geochemistry》2012,42(12):1269-1276
This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ13C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C3 photosynthesis and indicate a wide range of CAM use, with δ13C values ranging from −33.01‰ to −18.54‰ (C27–C33 n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ13Cbulk and δDC31 values with R2 = 0.60 (δ13CC31 and δDC31 values with R2 = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.  相似文献   

12.
We analyzed D/H ratios of common terrestrial leaf wax lipids in a 1400 year sediment core from the Santa Barbara Basin (SBB) to test whether they accurately record terrestrial climate in Southern California. The D/H ratios of long chain n-alkanes vary substantially with depth, but are poorly correlated with other terrestrial climate proxies. Interference from fossil hydrocarbons may be at least partly responsible. Long chain n-alkanoic acids exhibit nearly constant downcore D/H ratio values. This constancy in the face of known climatic shifts presumably reflects a substantial residence time for leaf wax compounds in terrestrial soil and/or on the basin flanks. Alternatively, the isotopic composition of meteoric waters in Southern California may not covary with climate, particularly aridity. However, the δD values of n-C22 and n-C24 fatty acids, commonly attributed to terrestrial aquatic sources, are partially correlated with Southern California winter Palmer Drought Severity Index, a tree ring-based climatic proxy (R2 0.25; p < 0.01) on multi-centennial scales with an inferred ca. 215 year time lag. The improved correlation of these biomarkers can be explained by the fact that they are not stored in terrestrial soil nor are subject to interference from fossil hydrocarbons. Our study indicates that the SBB is unlikely to preserve high resolution leaf wax D/H records that can serve as quantitative paleoclimate proxies, though some qualitative information may be retained. More generally, the sources of lipids in marginal marine basins need to be carefully evaluated prior to attempting paleoclimate reconstruction based on the leaf wax D/H proxy.  相似文献   

13.
The carbon isotope composition (δ13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C29) similar to that of terrestrial C4 plants. Thus, the use of δ13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the δ13C values for C31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ13C values of sedimentary C27 and C29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the δ13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the δ13C values of C27 and C29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.  相似文献   

14.
We present a systematic study of chain-length distributions and D/H ratios of n-alkyl lipids (both n-alkanes and n-alkanoic acids) in a wide range of terrestrial and aquatic plants around and in Blood Pond, Massachusetts, USA. The primary goal is to establish a model to quantitatively assess the aquatic plant inputs of the mid-chain length n-alkyl lipids to lake sediments and to determine the average hydrogen isotopic ratios of these lipids in different plants. Our results show that middle-chain n-alkyl lipids (C21-C23n-alkanes and C20-C24n-alkanoic acids) are exceptionally abundant in floating and submerged aquatic plants, in contrast to the dominance of long-chain n-alkyl lipids (C27-C31n-alkanes and C26-C32n-alkanoic acids) in other plant types, which are consistent with previously published data from Mountain Kenya and the Tibetan Plateau. Combining available data in different environmental settings allows us to establish statistically robust model distributions of n-alkyl lipids in floating/submerged macrophytes relative to other plant types. Based on the model distributions, we established a multi-source mixing model using a linear algebra approach, in order to quantify the aquatic inputs of mid-chain n-alkyl lipids in lake sediments. The results show that ∼97% of the mid-chain n-alkyl lipids (C23n-alkane and C22n-acid (behenic acid)) in Blood Pond sediments are derived from floating and submerged macrophytes. In addition, D/H ratios of C22n-acid and C23n-alkane in the floating and submerged plants from Blood Pond display relatively narrow ranges of variation (−161 ± 16‰ and −183 ± 18‰, respectively). Our study demonstrates that mid-chain n-alkyl lipids such as C23n-alkane and C22n-acid could be excellent recorders of past lake water isotopic ratios in lakes with abundant floating and submerged macrophyte inputs.  相似文献   

15.
Lake sediments generally contain a mixture of terrestrial and aquatic source inputs, and determining the major inputs is important for understanding geological records in paleoenvironment and paleoclimate research. In this study we describe the distribution of n-alkanes and n-fatty acids (FAs) in representative modern plants from around Lake Qinghai. We found a significant difference in the average length of n-FA carbon chains (ACL Fa16–32) in terrestrial (23.3) and aquatic plants (18.6). The results reveal that ACL Fa16–32 may essentially serve as a proxy for evaluating the major source inputs to lake sediments. Assessment of surface sediments from the lake showed that the FAs originated from a mixture of inputs, with the aquatic source input predominant at most sites. Additionally, the δD values of sediment mid-chain n-acids (C22) showed a relationship with the ACL Fa16–32 proxy: an increased Fa ACL corresponded to more negative hydrogen isotope ratio values. We suggest that different sources should be considered and ACL Fa16–32 could be a potential calibration proxy before using δD values to extract reliable isotopic information from lake water. More attention should be paid to source inputs and their relationship to other geochemical proxies in future studies of lake sediments.  相似文献   

16.
A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ13C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. 210Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ13C values of individual hydrocarbons, especially the C23n-alkane, a major compound in certain Sphagnum spp. The δD values of the C29 and C33n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C23n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ13C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced.  相似文献   

17.
Palaeoenvironmental assessment of past C3 and C4 vegetation distributions relies on end member data from plant analyses. In southwestern Africa, end member data of the carbon number distribution of n-alkanes from leaf waxes and their carbon isotopic composition were available for the rainforest and the savannah. To complement this, we analysed the n-alkane parameters of 41 C3 plants and 11 C4 plants from the transition region, i.e., the wood- and shrubland of Angola. The combined results for the rainforest, the wood- and shrubland and the savannah show an increase in the average chain length (ACL) of C3 and C4 plants and an increasingly enriched carbon stable isotope composition for the C3 plants from the equator towards southern Africa. The enlarged database was applied to the data of a north–south transect of deep-sea surface sediments already used in a previous study, which resulted in the proxies showing a good reflection of the vegetation on the adjacent southwest African continent in terms of %C4 plant cover. Applying end member values for ACL and δ13C obtained from the enlarged database by two different averaging methods (arithmetic average and median) to the n-alkane data from the sediment transect yielded similar vegetation reconstructions. In addition, a correlation between ACL and growth height of the plants is discussed, indicating that the ACL may be useful as a tree abundance parameter. Thus, the enlarged end member database strengthens the n-alkane parameters as tools for palaeoenvironmental studies.  相似文献   

18.
Carbon isotope ratio (δ13C) values of lipid biomarkers from plants can be used to assess water use efficiency and to reconstruct environmental conditions in the past. We assessed the effect of salinity on the δ13C values for leaf wax n-C31 and n-C33 alkanes, bulk leaf matter and leaf total lipid extracts from Avicennia marina (gray mangrove) trees growing along the Brisbane River estuary in Queensland, Australia. We observed an increase in 0.19 ± 0.053‰ (R2 0.61, p 0.008) and 0.16 ± 0.052‰ (R2 0.55, p 0.01) per salinity unit for the two n-alkanes, respectively, and of 0.087 ± 0.028‰ (R2 0.41, p 0.009) for whole leaves per salinity unit, indicating that water use efficiency of A. marina increased with the salt content of water. There was no correlation between δ13C values of total lipid extracts and salinity, perhaps because of a decrease in lipid concentration at higher salinity or because of varying contributions of different lipid classes to the extract. The robust relationship between salinity and δ13C values of leaf wax lipids provides a means of quantitatively reconstructing past salinity from carbon isotope ratios of mangrove lipid biomarkers in sediments. When paired with measurements of the hydrogen isotope ratio values of the same compounds, the approach should facilitate quantitative reconstruction of the hydrogen isotope composition of environmental water. In order for the method to successfully reconstruct past salinity and water isotopes, a mangrove source for leaf wax would need to be confirmed by palynological or other evidence, or the isotopic composition of a more source specific biomarker, such as taraxerol, would need to be measured.  相似文献   

19.
A quantitative kinetic model has been proposed to simulate the large D and 13C isotope enrichments observed in individual n-alkanes (C13-C21) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane δ13C values increase by ∼4‰ and δD values increase by ∼50‰ at an equivalent vitrinite reflectance value of 1.5%. While the observed 13C-enrichment shows no significant dependence on hydrocarbon chain length, thermally induced D-enrichment increases with increasing n-alkane carbon number. This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight, n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This carbon-number-linked hydrogen isotopic fractionation behavior could form the basis of a new maturity indicator to quantitatively assess the extent of oil cracking in petroleum reservoirs. Quantum mechanical calculations of the average change in enthalpy (ΔΔH‡) and entropy (ΔΔS‡) as a result of isotopic substitution in n-alkanes undergoing homolytic cleavage of C-C bonds lead to predictions of isotopic fractionation that agree quite well with our experimental results. For n-C20 (n-icosane), the changes in enthalpy are calculated to be ∼1340 J mol-1 (320 cal mol-1) and 230 J mol-1 (55 cal mol-1) for D-H and 13C-12C, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in δD values for individual long-chain hydrocarbons provide a highly sensitive measure of the extent of thermal alteration experienced by the oil. Extrapolation of the kinetic model to typical geological heating conditions predicts significant enrichment in 13C and D for n-icosane at equivalent vitrinite reflectance values corresponding to the onset of thermal cracking of normal alkanes. The experimental and theoretical results of this study have significant implications for the use of compound-specific hydrogen isotope data in petroleum geochemical and paleoclimatological studies. However, there are many other geochemical processes that will significantly affect observed hydrogen isotopic compositions (e.g., biodegradation, water washing, isotopic exchange with water and minerals) that must also be taken into consideration.  相似文献   

20.
We investigated the latitudinal changes in atmospheric transport of organic matter to the western Pacific and Southern Ocean (27.58°N-64.70°S). Molecular distributions of lipid compound classes (homologous series of C15 to C35n-alkanes, C8 to C34n-alkanoic acids, C12 to C30n-alkanols) and compound-specific stable isotopes (δ13C of C29 and C31n-alkanes) were measured in marine aerosol filter samples collected during a cruise by the R/V Hakuho Maru. The geographical source areas for each sample were estimated from air-mass back-trajectory computations. Concentrations of TC and lipid compound classes were several orders of magnitude lower than observations from urban sites in Asia. A stronger signature of terrestrial higher plant inputs was apparent in three samples collected under conditions of strong terrestrial winds. Unresolved complex mixtures (UCM) showed increasing values in the North Pacific, highlighting the influence of the plume of polluted air exported from East Asia. n-Alkane average chain length (ACL) distribution had two clusters, with samples showing a relation to latitude between 28°N and 47°S (highest ACL values in the tropics), whilst a subset of southern samples had anomalously high ACL values. Compound-specific carbon isotopic analysis of the C29 (−25.6‰ to −34.5‰) and C31n-alkanes (−28.3‰ to −37‰) revealed heavier δ13C values in the northern latitudes with a transition to lighter values in the Southern Ocean. By comparing the isotopic measurements with back-trajectory analysis it was generally possible to discriminate between different source areas. The terrestrial vegetation source for a subset of the southernmost Southern Ocean is enigmatic; the back-trajectories indicate eastern Antarctica as the only intercepted terrestrial source area. These samples may represent a southern hemisphere background of well mixed and very long range transported higher plant organic material.  相似文献   

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