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1.
《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   

2.
The hydrogen isotope composition of terrestrial plant leaf wax in sediments is increasingly used as a paleoclimatic indicator. Modern calibration studies suggest that paleoclimatic interpretation of leaf wax δD values requires consideration of the differences in the apparent fractionation of hydrogen isotopes among different groups of plants. However, it is not common that paleoecological data are used to help interpret leaf wax δD profiles. Here we assess the relative importance of factors influencing millennial-scale shifts in δD values of n-alkanoic acids at Steel Lake (Minnesota, USA), an extensively studied site with independent records of vegetation composition, δD of input water to the lake, and evaporation. The δD values of the n-C28 alkanoic acid (δDC28) vary between −190 and −168‰, and do not correlate with δD of input water or the extent of evaporation. However, δDC28 is negatively correlated with the δ13C values of the n-C28 alkanoic acid (δ13CC28). The correlation, along with pollen assemblage and carbonate δ13C records, suggests that Holocene shifts between forest and grassland and/or in the water use efficiency of C3 plants influenced the stratigraphic variation in leaf wax δD. Thus, paleoecological information, such as that inferred from pollen assemblages and carbon isotopes of plant-derived compounds, may aid paleoclimatic interpretation of leaf wax δD profiles from lake sediments.  相似文献   

3.
We analyzed hydrogen isotope ratios of high-molecular weight n-alkanes (δDl) and oxygen isotope ratios of α-cellulose (δ18OC) for C3 and C4 grasses grown in the field and in controlled-environment growth chambers. The relatively firm understanding of 18O-enrichment in leaf water and α-cellulose was used to elucidate fractionation patterns of δDl signatures. In the different relative humidity environments of the growth chambers, we observed clear and predictable effects of leaf-water enrichment on δ18OC values. Using a Craig-Gordon model, we demonstrate that leaf water in the growth chamber grasses should have experienced significant D-enriched due to transpiration. Nonetheless, we found no effect of transpirational D-enrichment on the δDl values. In field samples, we saw clear evidence of enrichment (correlating with relative humidity of the field sites) in both δ18OC and δDl. These seemingly contrasting results could be explained if leaf waxes are synthesized in an environment that is isotopically similar to water entering plant roots due to either temporal or spatial isolation from evaporatively enriched leaf waters. For grasses in the controlled environment, there was no enrichment of source water, whereas enrichment of grass source water via evaporation from soils and/or stems was likely for grass samples grown in the field.Based on these results, evaporation from soils and/or stems appears to affect δDl, but transpiration from leaves does not. Further evidence for this conclusion is found in modeling expected net evapotranspirational enrichment. A Craig-Gordon model applied to each of the field sites yields leaf water oxygen isotope ratios that can be used to accurately predict the observed δ18OC values. In contrast, the calculated leaf water hydrogen isotope ratios are more enriched than what is required to predict observed δDl values. These calculations lend support to the conclusion that while δ18OC reflects both soil evaporation and transpiration, δDl appears to only record evaporation from soils and/or stems. Therefore, the δD of n-alkanes can likely be used to reconstruct the δD of water entering a leaf, supporting the soil-enrichment model of Smith and Freeman (2006). In both the field and controlled studies, we found significant photosynthetic pathway effects on n-alkane δD suggesting that biochemical pathways or plant phylogeny have a greater effect on leaf wax δD than leaf-water enrichment in grasses.  相似文献   

4.
The stable carbon and hydrogen isotope composition of higher plant-derived long chain n-alkanes (δ13Cn-alkanes and δDn-alkanes) from 45 surface soil samples (within well characterized vegetation zones) from eastern China (18°N–50°N) are reported. The weighted average δDn-alkanes value for n-C27, n-C29 and n-C31 in the samples and the annual average δD of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations. The δD of n-alkanes shows a gradual depletion in value with increasing latitude. The results demonstrate that, on a large spatial scale, the δD values of long chain n-alkanes derived from higher plants have the potential to record the δD of meteoric water, although many other factors can also influence the isotope values. There appears to be no apparent relationship between the δD of the n-alkanes extracted from the surface soil and the overlying vegetation type (i.e. forest/grassland or C3/C4 composition). Therefore, palaeoenvironmental studies utilizing δDn-alkanes from higher plant-derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle.  相似文献   

5.
We measured hydrogen isotope compositions (δD) of high-molecular-weight n-alkanes (C27-C33) from grasses grown in greenhouses and collected from the US Great Plains. In both cases, n-alkanes from C4 grasses are enriched in D by more than 20‰ relative to those from C3 grasses. The apparent enrichment factor (εC29-GW) between C29n-alkane and greenhouse water is −165 ± 12‰ for C3 grasses and −140 ± 15‰ for C4 grasses. For samples from the Great Plains, δD values of C29n-alkanes range from −280 to −136‰, with values for C4 grasses ca. 21‰ more positive than those for C3 grasses from the same site. Differences in C3 and C4 grass n-alkane δD values are consistent with the shorter interveinal distance in C4 grass leaves, and greater back-diffusion of enriched water from stomata to veins, than in C3 grass leaves. Great Plains’ grass n-alkane isotopic ratios largely reflect precipitation δD values. However, the offset or apparent fractionation between n-alkanes and precipitation is not uniform and varies with annual precipitation and relative humidity, suggesting climatic controls on lipid δD values. The dryer sites exhibit smaller absolute apparent fractionation indicative of D-enrichment of source waters through transpiration and/or soil evaporation. To explore the relationship between climate and n-alkane δD values, we develop three models. (1) The ‘direct analog’ model estimates δDC29 values simply by applying the apparent enrichment factors, εC29-GW, observed in greenhouse grasses to precipitation δD values from the Great Plains. (2) The ‘leaf-water’ model uses a Craig-Gordon model to estimate transpirational D-enrichment for both greenhouse and field sites. The transpiration-corrected enrichment factors between C29 and bulk leaf-water, εC29-GW, calculated from the greenhouse samples (−181‰ for C3 and −157‰ for C4) are applied to estimate δDC29 values relative to modeled bulk leaf-water δD values. (3) The ‘soil- and leaf-water’ model estimates the combined effects of soil evaporation, modeled by analogy with a flow-through lake, and transpiration on δDC29 values. Predictions improve with the addition of the explicit consideration of transpiration and soil evaporation, indicating that they are both important processes in determining plant lipid δD values. D-enrichment caused by these evaporative processes is controlled by relative humidity, suggesting that important climatic information is recorded in leaf wax n-alkane δD values. Calibration studies such as this one provide a baseline for future studies of plant-water-deuterium systematics and form the foundation for interpretation of plant wax hydrogen isotope ratios as a paleo-aridity proxy.  相似文献   

6.
The distributions of n-alkanes and their hydrogen isotopic composition (δD) in surface and core sediments from the saline Qinghai Lake were measured to assess whether or not biological source information was recorded in the δD values of n-alkanes. The results indicate that the n-alkane distributions between shallow water surface and core sediments were similar, and closer to those of terrestrial herbaceous plants from the Qinghai Lake surrounding areas, rather than the aquatic plants living in the lake. The n-alkanes in the surface and core sediments had similar mean δD values, ranging from −185‰ to −133‰ and −163‰ to −142‰, respectively. The mean δD values of n-alkanes in the sediments showed that the even n-alkanes were heavier in D compared with the odd homologues.  相似文献   

7.
Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of Paq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C23, C25 and C27) and high molecular weight (C31) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low Paq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C23, C25 and C27) relative to the long chain n-alkane (C31), together with higher Paq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.  相似文献   

8.
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   

9.
Controls on the D/H ratios of plant leaf waxes in an arid ecosystem   总被引:1,自引:0,他引:1  
The extent to which leaf water D-enrichment (transpiration) and soil water D-enrichment (evaporation) affect the D/H ratio of plant leaf waxes remains a contentious issue, with important implications for paleohydrologic reconstructions. In this study we measure δD values of precipitation (δDp), groundwater (δDgw), plant xylem water (δDxw) and leaf water (δDlw) to understand their impact on the δD values of plant leaf wax n-alkanes (δDwax) in an arid ecosystem. Our survey includes multiple species at four sites across an aridity gradient (80-30% relative humidity) in southern California.We find that many species take up groundwater or precipitation without significant fractionation. D-enriched soil water is a minor source even in species known to perform and utilize waters from hydraulic lift, such as Larrea tridentata (+10‰). Measurements of leaf water isotopic composition demonstrate that transpiration is an important mechanism for D-enrichment of leaf waters (+74 ± 20‰, 1σ), resulting in the smallest net fractionation yet reported between source water and leaf waxes (L. tridentata −41‰; multi-species mean value is −94 ± 21‰, 1σ). We find little change in leaf water D-enrichment or net fractionation across the climatic gradient sampled by our study, suggesting that a net fractionation of ca. −90‰ may be appropriate for paleohydrologic reconstructions in semi-arid to arid environments. Large interspecies offsets in net fractionations (1σ = 21‰) are potentially troublesome, given the observed floristic diversity and the likelihood of species assemblage changes with climate shifts.  相似文献   

10.
Compound specific hydrogen isotope ratios (δD) of long chain sedimentary n-alkanes, which mostly originate from the leaf waxes of higher terrestrial plants, are increasingly employed as paleoclimate proxies. While soil water is the ultimate hydrogen source for these lipids and the isotopic fractionation during biosynthesis of lipids is thought to remain constant, environmental parameters and plant physiological processes can alter the apparent hydrogen isotopic fractionation between leaf-wax lipids and a plant’s source water. However, the magnitude and timing of these effects and their influence on the isotopic composition of lipids from higher terrestrial plants are still not well understood. Therefore we investigated the seasonal variability of leaf-wax n-alkane δD values for two different temperate deciduous forest ecosystems that are dominated by two different tree species, Beech (Fagus sylvatica) and Maple (Acerpseudoplatanus).We found significant seasonal variations for both tree species in n-alkane δD values of up to 40‰ on timescales as short as one week. Also, the isotopic difference between different n-alkanes from the same plant species did vary significantly and reached up to 50‰ at the same time when overall n-alkane concentrations were lowest.Since δD values of soil water at 5 and 10 cm depth, which we assume represent the δD value of the major water source for the investigated beech trees, were enriched in autumn compared to the spring by 30‰, whereas n-alkane δD values increased only by 10‰, we observed variations in the apparent fractionation between beech leaf derived n-alkanes and soil water of up to 20‰ on a seasonal scale. This observed change in the apparent fractionation was likely caused by differences in leaf water isotopic enrichment. Based on mechanistic leaf water models we conclude that changes in the isotopic difference between water vapor and soil water were the most likely reason for the observed changes in the apparent fractionation between n-alkanes and soil water.The large variability of n-alkane concentrations and δD values over time implies a continuous de novo synthesis of these compounds over the growing season with turnover times possibly as short as weeks. The signal to reach the soil therefore represents an integrated record of the last weeks before leaf senescence. This holds true also for the sedimentary record of small catchment lakes in humid, temperate climates, where wind transport of leaf-wax lipids is negligible compared to transfer through soil and the massive input of leaves directly into the lake in autumn.  相似文献   

11.
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   

12.
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   

13.
We measured δD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in δD value with elevation in the wetter region greatly exceeded that in drier region by, −1.9 ± 0.1‰/100 m and −1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation εwax-p values in the wetter region (ca. −164‰) were more negative than those in drier region (ca. −125‰ above 3200 m).We also measured δD values in leaves of six common living trees (values from −287‰ to −193‰) from Gongga Shan, ranging from about 2900-4200 m. The abundance-weighted average values of the n-alkanes (δDwax) show a strong reverse correlation with sample source elevation (R2 0.78 for soils from Gongga Shan; R2 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane δDwax faithfully records the precipitation δD and that the isotopic altitude effect of precipitation controls δDwax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane δD values on elevation for the eastern Plateau, but a complex relationship between n-alkane δD values and elevation for the northwestern Plateau. The δDwax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment δDwax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of δD values with elevation.  相似文献   

14.
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   

15.
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   

16.
To quantitatively analyze the response of distributions and hydrogen isotopic compositions (SD) of plant leaf wax to moisture, and to better understand their implications for paleoclimatic reconstruction, we measured av- erage chain length (ACL) and 8D values of n-alkanes and n-fatty acids (n-FAs) from Orinus kokonorica, a typical and representative plant in Lake Qinghai area, along a distance transect extending from lakeshore to wetland to dry- land in the arid ecosystem. The results showed that the ACL values of n-alkanes and n-FAs were negatively corre- lated with soil water content (SWC) with R2~0.593 and R2=0.924, respectively. This is as a result of plant's response to water loss with more abundance in long-chain n-alkyl lipids under increasing aridity by analyzing relationships between the molecular ratios of long-chain n-alkyl lipids (n-alkanes and n-FAs) from O. kokonorica and SWC. The 8D values of C29 n-alkane and C28 n-FA were also negatively correlated with SWC with R2-0.778 and R2-0.760, respectively, which may due to enhanced D-enrichment in leaf water by evapotranspiration (soil water evaporation and leaf water transpiration) with increasing aridity. Our results demonstrated that moisture exerts a significant con- trol on the ACL and 8D values from O. kokonorica in an arid ecosystem. This preliminary study on a modern single plant (O. kokonorica) sets a foundation for comprehending these values as quantitative proxies for paleo-humidity reconstruction.  相似文献   

17.
Coastal salt marsh ecosystems contain strong environmental gradients that are anticipated to influence the D/H ratios recorded in the leaf waxes of salt-tolerant plants. We characterized the molecular and hydrogen isotopic composition of alkanes in plant and sediment samples as well as the D/H ratios of environmental and plant waters across an elevation and inundation gradient in a southern Californian, coastal salt marsh. We sampled the dominant salt marsh plant species: Salicornia virginica, Arthrocnemum subterminale and Jamuea carnosa (all succulents), as well as Monanthochloe littoralis and Limonium californicum (nonsucculents). Plant xylem water hydrogen isotopic compositions indicate a shift in source waters from meteoric influences at upland sites (δD value −20‰) to seawater dominated values (0‰) at lowland areas. We found leaf water D enrichment relative to xylem water ranging from mean δD values of +54‰ (upland) to +28‰ (lowland), interpreted as a reduction of transpiration with increasing inundation time. This has the effect of increasing the net fractionation between source water and leaf wax product across the environmental gradient from mean values of −101‰ (upland) to −134‰ (lowland), with an attenuated signal recorded in the δD values of plant leaf wax n-alkanes (−122‰ to −136‰). These results constrain the hydrogen isotopic composition of salt marsh organic matter that may contribute to marine carbon budgets of the Santa Barbara Basin, and further indicate the potential for plant leaf waxes to resolve paleoenvironmental change, including sea level change, in sediment cores from salt marsh ecosystems.  相似文献   

18.
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   

19.
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   

20.
《Applied Geochemistry》2005,20(3):455-464
In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in 2H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.  相似文献   

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