Compound-specific Carbon Isotopic Compositions Research Articles

Two-phase hydrological changes during the mid-to late Holocene transition in Southwest China

The Indian summer monsoon (ISM) rainfall feeds a vast majority of the densely populated Asian continent. A deep understanding of the ISM rainfall variations and the underlying driving mechanisms during the Holocene is essential to predict its future behaviors and formulate improved mitigating strategies under the context of global warming. It has been widely recognized that the ISM intensity declined from the early to late Holocene, with a mid-to late Holocene transition. However, the nature of hydrological changes during this critical transition still remains vague. In this study, hydrological changes in Southwest China during the mid-to late Holocene transition are reconstructed based on the molecular and compound-specific carbon and hydrogen isotopic compositions of leaf waxes (δ13C and δD of n-alkanes) from the Yanjiang peat deposit in western Yunnan. Results reveal two-phase changes of hydroclimate during the mid-to late Holocene transition in Southwest China. During the first phase (5.5–4.2 cal kyr BP), Southwest China experienced an abrupt drying as indicated by the pronounced increases in both δ13C and δD values of the C29n-alkane (δ13CC29 and δDC29) by 2.2% and 26‰, respectively. The second phase (4.2–2.6 cal kyr BP) witnessed shortening in the duration of the ISM season without further aridification, as demonstrated by an increase up to 35‰ in the δDC29 values and moderate oscillations in the δ13CC29 values. Such a two-phase hydrological transition is likely to have been triggered by the changes in the summer solar insolation and amplified by the ocean-atmosphere interactions. The first phase is primarily modulated by the positive Indian Ocean Dipole mean state, while the second phase is mainly linked with a more El Niño-like mean state over the Pacific Ocean.

Biodegradation of dissolved organic matter and sedimentary organic matter in high arsenic groundwater system: Evidence from lipid biomarkers and compound-specific carbon isotopes

Organic matter plays important roles in arsenic mobility. Whether sedimentary or dissolved organic matter is the predominant electron donors is still on debate. To fill this gap, both sediments and groundwater were collected from the Hetao Basin, a typical arid-semiarid area hosting high arsenic groundwater, to characterize diagnostic biomarkers and compound-specific carbon isotopes of lipids in dissolved organic matter (DOM) and its depth-matched sedimentary organic matter (SOM). Results showed that SOM in both clay and sand layers were mainly sourced from terrestrial higher plants, but sand SOM had additional sources from microbial and petroleum OM. Compound-specific carbon isotope compositions of DOM and SOM showed that δ13C of low molecular weight (LMW) n-alkanes (δ13CL) were more positive than those of high molecular weight (HMW) n-alkanes (δ13CH). The δ13CH in groundwater (δ13CH-DOM) and sediments (δ13CH-SOM) fall in the range of C3 plants, indicating that DOM and SOM had similar origins. More positive δ13CL than δ13CH suggests preferential degradation of LMW compounds in both sediments and groundwater. In comparison with SOM, more microbial-derived OM was observed in groundwater DOM, especially in high‑arsenic groundwater. High arsenic groundwater was accompanied by the low ratio of n-alkanes to hopanes in depth-matched SOM, and the great difference between δ13CL and δ13CH and low terrestrial/microbial ratios (TAR) in groundwater DOM. This indicates that SOM and DOM should be utilized by indigenous bacteria to fuel the reductive dissolution of Fe oxides for arsenic release into groundwater. Furthermore, more abundant LMW compounds and weaker odd-to-even predominance in DOM than those in SOM support that DOM encountered stronger biodegradation. This study highlights that preferential biodegradation of DOM (especially LMW components and petroleum sourced OM) would provide electrons for the reductive dissolution of arsenic-bearing Fe/Mn oxides and thus induce arsenic enrichment in groundwater.

Compound specific isotope evidence points to use of freshwater resources as weaning food in Middle Neolithic Paris Basin

ObjectivesA clear understanding of past weaning practices can provide invaluable insights into social issues such as infant care, fertility rate, and demographic patterns in past societies. This study presents the first archeological research employing compound specific isotope analysis (CSIA) for the reconstruction of past weaning practices.MethodsWeaning practices of two Middle Neolithic communities in the Paris Basin region: Balloy (BLR) and Vignely (VPB), are evaluated by combining previously published bone collagen stable carbon, nitrogen, and sulfur (n = 66) isotope analysis with new compound specific carbon and nitrogen isotope compositions of bone collagen (n = 10).ResultsOur results demonstrate that the diets of individuals from BLR and VPB likely incorporated freshwater resources. The signals of freshwater resources consumption are even stronger among subadults, suggesting that freshwater resources were used as weaning food at these sites.ConclusionsThe implications of our result are threefold. Currently many CSIA studies in archeology only involve either carbon or nitrogen. Our data shows that it is important to conduct CSIA on both carbon and nitrogen for a more integrated picture. Secondly, our data demonstrates that the use of a protein‐based weaning food—instead of a starch‐based weaning food (such as cereal gruel)—was likely more prevalent among the Middle Neolithic communities in the Paris Basin Region than previously thought. The finding thus prompts a rethinking of the role of protein‐based weaning food in other archeological contexts. Lastly, the common assumption that weaning foods and adult diets share similar isotopic compositions can be problematic, as the use of protein‐based, high trophic‐level weaning foods can skew the δ 15N weaning curve and produce an erroneously late estimation for weaning ages.

Biomarker Records From Eocene Lacustrine Sequence in the Eastern Tibet Plateau and Its Implication for Organic Matter Sources

The Eocene is the initial stage of the Cenozoic global cooling. Compared with the abundant marine records, the continental records of Eocene are scarce. Throughout the Eocene, a series of continuous deposition of gypsum and volcanic tuff-bearing red clastic sediments have developed in the Nangqian Basin (NB). In this work, representative sediments were collected from the NB, and lipid biomarkers and compound-specific carbon isotopes of n-alkanes were analyzed. Based on the robust paleomagnetic age–depth model, from the early to the late Eocene, the compound-specific carbon isotopic compositions (δ13C23 and δ13C25) increased with the sedimentary facies changed. At the same time, the relative proportion of mid-chain length to the long-chain length homologs (Paq) decreased, and the peak carbon number (Cmax) shifted from nC21, nC22 or nC23 to nC16, nC25, nC27, or nC31. We ascribed these variations to the climate drying and water level turning high as indicated by the lithology change from fluvial to lacustrine facies and the terrestrial inputs from neighboring mountain belts in the middle–late Eocene. Moreover, we compared our n-alkane results with other records from the TP and the global sea level and marine benthic δ18O. We found that a nearly synchronous deformation and drying of the eastern TP caused by the India–Asia collision in the early Eocene was closely related to the arid conditions and topographically changed in the northern TP. The climate variations in the Eocene in the NB were mainly controlled by the global climate change and the uplift of the TP and affected by the Paratethys Sea on a long scale.

Determination of carbon isotopic composition of crocetane in sediments by heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry

Compound-specific isotope analysis of crocetane in sediments can be used to determine its origin. However, the co-elution of crocetane with phytane and complex substrates hinders the accurate determination of its isotopic composition. A heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry (2D-GC–IRMS) method was developed to determine δ13C values of crocetane. Full chromatographic separation of crocetane was achieved with a 30 m CycloSil-B chiral capillary first column and a 60 m Chiraldex B-pH capillary second column (two series-connected 30 m Chiraldex B-pH capillary columns). Analyses of standard mixtures confirmed that the new method has satisfactory accuracy (deviations from the authentic values <0.5‰) and precision (RSDs <5%) for carbon isotope analyses of crocetane and phytane. The application of the method to two sediment samples indicates that baseline resolution is achieved (R >1.5) for crocetane and phytane, with 13C-depleted crocetane (δ13C values of −119.5‰ and −111.5‰ in the two natural samples, respectively) being an indicator of anaerobic oxidation of methane in cold seep areas. The new 2D-GC–IRMS method allows simple and efficient determination of the compound-specific carbon isotopic compositions of crocetane.

Simultaneous determination of compound-specific isotopic compositions of acyclic isoprenoids and n-alkanes in light crude oil.

Stable carbon and hydrogen isotope ratios of individual n-alkanes and acyclic isoprenoids are important tools in petroleum geochemistry. However, the analysis requires baseline separation and peak profiles using gas chromatography-isotope ratio mass spectrometry to obtain accurate compound-specific isotope data. Time-consuming isolation or purification is typically conducted to separate the compounds to avoid co-elution with other compounds or matrices in crude oils. We developed a simple gas chromatography separation method to simultaneously measure the compound-specific carbon or hydrogen isotope compositions of n-alkanes and acyclic isoprenoids. It was achieved by direct injection of the whole crude condensate and light oil or the saturated fractions of different types of crude oils using a 60m DB-17ms column. This method simplifies the pre-treatment of compound-specific isotope analysis, saves manpower and time, and reduces the use of organic solvents to be more environmentally friendly.

Organic carbon source variability in Arctic bivalves as deduced from the compound specific carbon isotopic composition of amino acids

In this study we used compound-specific carbon isotope analysis of amino acids (δ13CAA) to determine organic carbon sources utilized by the dominant benthic bivalve species collected along a latitudinal gradient in the northern Bering and Chukchi Seas, specifically at productivity hotspots identified within the Distributed Biological Observatory (DBO) program, and over Hanna Shoal in the northern Chukchi Sea. The recent shift to earlier sea-ice melt is one of the climate change consequences influencing Pacific Arctic ecosystems, which we integrate within our observations. Our goals included investigating the utilization of organic matter resources by several dominant Arctic bivalves and their trophic elasticity to changes in primary productivity patterns following changes in the onset of the annual productive season. Based upon δ13CAA patterns observed, these species utilized different carbon sources along the latitudinal gradient, including a strong input of bacterially reworked material and microalgae, mainly in particulate organic matter mixtures. Species type and the sampling location both played roles in δ13CAA variability, suggesting the influence of local production and decomposition processes. Macoma calcarea and Ennucula tenuis were shown to utilize organic matter of different quality, suggesting they may switch their feeding preferences to more detrital sources on a seasonal basis, but this was also affected by geographical location. These observations may have important implications for the benthic populations as microbial reworking of organic material is expected to increase with climate warming and likely shifts in food web structure.

The leaf wax biomarker record of a Namibian salt pan reveals enhanced summer rainfall during the Last Glacial-Interglacial Transition

Conventional continental geoarchives are rarely available in arid southern Africa. Therefore, palaeoclimate data in this area are still patchy and late Quaternary climate development is only poorly understood. In the western Kalahari, salt pans (playas, ephemeral lakes) are common and can feature quasi-continuous sedimentation. This study presents the first climate-related biomarker record using sediments from the Omongwa Pan, a Kalahari salt pan located in eastern Namibia. Our approach to reconstruct vegetation and hydrology focuses on biogeochemical bulk parameters and plant wax-derived lipid biomarkers (n-alkanes, n-alkanols, and fatty acids) and their compound-specific carbon and hydrogen isotopic compositions. The presented record reaches back to 27 ka. During the glacial, rather low δ2H values of n-alkanes and low sediment input exclude a strong influence of winter rainfall. n-Alkane and n-alkanol distributions and δ13C values of n-hentriacontane (n-C31) indicate a shift to a vegetation with a higher proportion of C4 plants at the end of the Last Glacial Maximum until the end of Heinrich Stadial I (ca. 18–14.8 ka), which we interpret to indicate an abrupt excursion to a short wetter period likely to be caused by a temporary southward shift of the Intertropical Convergence Zone. Shifts in δ2H values of n-C31 and plant wax parameters give evidence for changes to drier conditions during early Holocene. Comparison of this dataset with representative continental records from the region points to a major influence of summer rainfall at Omongwa Pan during the regarded time span and demonstrates the potential of southern African salt pans as archives for biomarker-based climate proxies.

Separation of crocetane and phytane and measurement of their compound-specific carbon isotopic compositions

Crocetane and phytane are two isoprenoids isomers with similar molecular structures and often present together in methane-seep sediments and some Palaeozoic crude oils. Their commonly co-elution on gas chromatography is challenging for quality and quantity analysis, making it impossible to determine their compound-specific isotopic composition, and thus, insight their geological and geochemical significance. A new gas chromatography method is reported here using a DB-17MS column (50%-phenyl-methyl polysiloxane as the stationary phase) that successfully achieved baseline separation of crocetane and phytane and can be used to accurately identify and quantify them on gas chromatography and gas chromatography–mass spectrometry. Routine steroids and terpenoids biomarkers can also be analysed simultaneously. Additionally, their compound-specific carbon isotopic compositions were also measured without matrix influence using this method. This is the first time that a simple chromatographic method for direct determination of compound-specific carbon isotopic composition of crocetane has been reported publicly.

Compound-Specific Carbon Isotopic Composition of Ethanol in Brazil and US Vehicle Emissions and Wet Deposition.

Global atmospheric ethanol budget models include large uncertainties in the magnitude of ethanol emission sources and sinks. To apply stable isotope techniques to constrain ethanol emission sources, a headspace solid phase microextraction gas chromatograph-combustion-isotope ratio mass spectrometry method (HS-SPME-GC-C-IRMS) was developed to measure the carbon isotopic composition of aqueous phase ethanol at natural abundance levels (1-30 μM) with a precision of 0.4‰. The method was applied to determine the carbon isotope signatures (δ13C) of vehicle ethanol emission sources in Brazil (-12.8 ± 2.4‰) and the US (-9.8 ± 2.5‰), and to measure the carbon isotope composition of ethanol in wet deposition (-22.6 to -12.7‰). A two end-member isotope mixing model was developed using anthropogenic and biogenic end members and fractionation scenarios to estimate ethanol source contributions to wet deposition collected in Brazil and US. Mixing model results indicate anthropogenic sources contribute two and a half to four times more ethanol to the atmosphere than previously predicted in modeled global ethanol inventories. As established and developing countries continue to rapidly increase ethanol fuel consumption and subsequent emissions, understanding the magnitude of all ethanol sources and sinks will be essential for modeling future atmospheric chemistry and air quality impacts.

Compound-Specific Carbon Isotope Compositions of Aldehydes and Ketones in the Murchison Meteorite.

Compound-specific carbon isotope analysis (δ13C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker-cyanohydrin synthesis in the early solar system. Previous δ13C investigations have targeted α-amino acid and α-hydroxy acid Strecker products and reactant HCN; however, δ13C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification and δ13C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde and acetone, were detected in the sample. δ13C values, ranging from -10.0‰ to +66.4‰, were more 13C-depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar 12C/13C ratios. These relatively 13C-depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g. oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ13C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration-driven chemical pathways.

Eastern Mediterranean hydroclimate reconstruction over the last 3600 years based on sedimentary n-alkanes, their carbon and hydrogen isotope composition and XRF data from the Gialova Lagoon, SW Greece

Understanding past hydroclimate variability and related drivers is essential to improve climate forecasting capabilities especially in areas with high climatic sensitivity, such as the Mediterranean. This can be achieved by using a broad spectrum of high resolution, multiple proxy records which can also allow us to assess linkages between regional hydroclimate variability and shifts in the large-scale atmospheric patterns. Here, we present a multiproxy reconstruction of the central-eastern Mediterranean hydroclimate changes over the last 3600 years based on a sediment core from the Gialova Lagoon, a shallow coastal ecosystem in SW Peloponnese, Greece. Our combined dataset consists of the distribution and compound-specific carbon and hydrogen isotope (δ13C and δD) composition of n-alkanes, bulk organic matter properties and X-ray fluorescence (XRF) core scanning data. This approach was complemented with a semi-quantitative analysis of plant remains in the core. The results indicate a high contribution of local aquatic vegetation to organic matter. Large δ13C variations in predominantly aquatic plant-derived mid-chain alkanes (C23-25) mainly reflect changes in the aquatic plant abundance and their carbon source. Our data suggest that higher δ13C23-25 values (up to −19‰) largely correspond to expansion of aquatic vegetation during wet and/or cold periods causing carbon-limiting conditions in the water and assimilation of isotopically-enriched bicarbonate by the plants. The δD records of the individual n-alkanes (C17 to C31) exhibit a nearly identical pattern to each other, which implies that they all reflect changes in the source water isotope composition, driven by hydroclimate variability. In addition, the δD profiles are consistent with the XRF data with both proxies being driven by a common hydroclimate signal. We observe two major shifts from dry and/or warm periods at ca 3600-3000 cal BP and ca 1700-1300 cal BP to wet and/or cold episodes at ca 3000-2700 cal BP and ca 1300-900 cal BP. The period ca 700-200 cal BP is the wettest and/or coldest in our record and coeval with the Little Ice Age. The climatic fluctuation reported in this study can be explained by the relative dominance of high-latitude (e.g. North Atlantic Oscillation during winters) and the low-latitude atmospheric patterns (Intertropical convergence zone, Subtropical High and the effects of Asian monsoons during summers) which suggests an Atlantic-Mediterranean-Monsoon climate link in this area for the late Holocene.

The Nitrogen Heterocycle Content of Meteorites and Their Significance for the Origin of Life.

Carbonaceous chondrites are very primitive meteorites that are rich in carbon. They contain many soluble organic compounds, including nitrogen heterocycles. These play a crucial role in present-day living organisms as they are components of the genetic material and of the co-factors of enzymes. This review outlines the nitrogen heterocycle content of carbonaceous meteorites. The potential mechanisms of formation of these molecules are also described. Measurements of the compound-specific carbon and hydrogen isotopic compositions are mentioned as a way of establishing the origin of the nitrogen heterocycles detected in meteorites.

Characteristics and compound-specific carbon isotope compositions of sedimentary lipids in high arsenic aquifers in the Hetao basin, Inner Mongolia

Organic matter, as an electron donor, plays a vital role in As mobilization mediated by microorganisms during reductive dissolution of Fe/Mn oxides in shallow aquifers. However, the specific types and sources of organic matter involved in biogeochemical processes accelerating As mobilization are still controversial. Both sediment and groundwater samples were collected at different depths from aquifers of the Hetao Basin, a typical inland basin hosting high As groundwater. Sedimentary lipids and their compound-specific carbon isotope ratios were analyzed to evaluate characteristics and sources of organic matter. Results show that sedimentary As were well correlated with Fe and Mn oxides, suggesting that As exist as Fe/Mn oxide bound forms. Groundwater As far exceeded the drinking water guide value of 10 μg/L. Moreover, As concentrations in shallow groundwater were relatively higher. Lipids in clay were mainly originated from terrestrial higher plants, while that in fine sand samples were derived from terrestrial higher plants, microorganism and petroleum. Shallow fine sand samples were also characterized by evident in-situ biodegradation. Compound-specific carbon isotope compositions of sedimentary lipids showed that short-chain n-alkanes and n-alkanoic acids had more positive δ13C values compared to long-chain compounds, especially in shallow fine sand samples. δ13CTOC were also low in shallow fine sand samples. These results jointly indicate that these lipids in shallow fine sand samples acted as carbon source for indigenous microorganism and the short-chain components were particularly more vulnerable to biodegradation, which may contribute to high As concentrations in shallow groundwater. The new findings provide the first evidence that short chain length n-alkyl compounds afforded a source of potential electron donors for microbially mediated As mobilization process in the shallow aquifers.

Research Status and Progress of Genetic Mechanism of Abnormal Regular Sterane Distribution in Geological Bodies

Regular steranes occur ubiquitously in petroleum and hydrocarbon source rocks as an important composition part of biological markers. Abnormal regular sterane distribution occurring in hydrocarbon source rocks and crude oils has been widely reported in recent years, but their origin and genetic mechanism are unclear. This research gives an overview of regular steranes in sediments and crude oils, and states its possible development directions in the future. Based on systematic research on compound-specific carbon isotope compositions of regular steranes in sediments and crude oils using separation, enrichment, and purification of regular steranes in sediments and crude oils, combined with organic petrological, micropaleontological, and sedimentary geochemical characteristics of hydrocarbon source rocks with abnormal distribution of regular steranes, the occurrence, variation and main controlling factors of these compounds will be discussed, thus further uncovering their genetic mechanism and probing into their application on assessment of organic matter source, depositional environment and thermal maturity, as well as oil and source correlation. It not only enriches and improves the approaches and theories on biomarker study, but also will be the breakthrough for the new theory and application in oil and gas exploration.

Defining pottery use and animal management at the Neolithic site of Bylany (Czech Republic)

Archaeological potsherds have become a valuable source of information about diet and the wider economies of ancient communities, especially through the analysis of lipids preserved in the microporous matrix of the ceramic vessels. This study investigated >160 potsherds recovered from settlement phase 19 dated to 5160–5100cal. BC from the Neolithic site of Bylany, one of the largest Linearbandkeramik (LBK) settlement in Central Europe. The aim was to investigate vessel use and animal management at the site and explore variations in organic residue composition and thus human activity at the household level. Pottery technology was also studied revealing a predominance of micro- and mesopores, indicating an advanced level of pottery production technology. >70% of the analysed potsherds yielded appreciable amounts of lipids dominated by C16:0 and C18:0 fatty acids, with compound-specific carbon isotope compositions indicating origins predominantly from ruminant and non-ruminant animal fats. Detection of very long fatty acids, fatty alcohols and traces of terpene compounds originating from plants suggested a combination of meat- and plant-based diet components and specialised use of some vessels. However, evidence of the use of vessels for milk collection or processing was not detectable at Bylany, at least during the settlement phase investigated herein.

Climate conditions and relative abundance of C3 and C4 vegetation during the past 40 ka inferred from lake sediments in Wudalianchi, northeast China

Paleoclimate and paleovegetation changes over the last 40 cal ka were recorded by multiple variables in a sediment core from Qingshi, Wudalianchi City, northeast China. The history of vegetation types inferred from n-C27/n-C31 and average chain length of n-alkanes indicates the paleovegetation went through several distinct stages, consistent with pollen records from the study area. Compound-specific carbon isotope composition was also determined for C27, C29 and C31 n-alkanes in the Qingshi core sediments. The relative abundance of C3 and C4 plants was calculated using a binary model and indicates that C3 plants were the dominant input during the last glacial and Holocene. There were, however, shifts in the ratio of C3 to C4 vegetation abundance that correspond to changes in climate conditions. Generally, the long-term trend towards greater C4 plant abundance from the last glacial to Holocene correlated with an increase in pCO2, higher temperature, greater precipitation and more growing season precipitation. Our results suggest that temperature and seasonality of precipitation played a strong role in altering the relative abundance of C3 and C4 plants in the study area. These results provide information for predicting future vegetation changes in response to on-going global warming.

Mid- to late-Holocene paleoenvironmental changes inferred from organic geochemical proxies in Lake Tangra Yumco, Central Tibetan Plateau

Investigations of the paleoclimate and paleoenvironmental changes in the Tibetan Plateau in recent years have led to many significant achievements. However, high-resolution lake records from the mid- to late-Holocene have not been determined for the Central Tibetan Plateau, which is in the transition zone between the Indian summer monsoon (ISM) system and the westerlies. In this study, we analyzed the distribution and compound-specific carbon isotope compositions ( δ13C) of sedimentary n-alkanes and the bulk sediment total organic matter (TOC), total nitrogen (TN), and C/N ratios to reconstruct the paleoenvironmental changes over the past 6 cal. ka BP using a 376-cm sediment core from the southern basin of Tangra Yumco, Central Tibetan Plateau. Our results indicated that clear stepwise decreases in lake productivity and lake level occurred during the mid- to late-Holocene based on decreases in δ13C values of mid-chain n-alkanes and Paq (an n-alkane-based proxy). Additionally, the variations in the TOC, TN, and C/N ratios revealed generally increased inputs of allochthonous organic matter over the past 6 cal. ka BP. When our findings at Tangra Yumco were compared with records from the Central Tibetan Plateau and adjacent regions, we found generally consistent patterns of lake environmental changes over the past 6 cal. ka BP, suggesting that the climate of the Central Tibetan Plateau was primarily influenced by the ISM system at the millennial time scale. Therefore, our results depict the evolution of the ISM over the past 6 cal. ka BP in the Central Tibetan Plateau, which corresponds to summer solar insolation and the migrations of the Intertropical Convergence Zone (ITCZ). We also propose that the climate of the North Atlantics influenced the climate of the Central Tibetan Plateau through some indirect climatic mechanisms.

Effects of early diagenesis on molecular distributions and carbon isotopic compositions of leaf wax long chain biomarker n-alkanes: Comparison of two one-year-long burial experiments

Two burial experiments were performed to compare and contrast the effects of degradation and early diagenesis on the molecular distributions and compound specific carbon isotope compositions of leaf wax n-alkanes under different conditions. Leaves of the bamboo Neosinocalamus affinis and the shrub Osmanthus fragrans were packaged in perforated aluminum packets and buried in soil at two different subtropical locations in China. The packets were recovered after successive intervals totaling ∼1year, and the n-alkane properties of their leaves compared to those of the fresh leaves and the surrounding soil. Concentrations of total leaf n-alkanes decreased in both experiments. The waxes of N. affinis exhibited a fairly rapid decrease in their proportions of C23–C27n-alkanes and increases of less than 1‰ in n-alkane δ13C values during the burial time. In contrast, the leaves of O. fragrans showed little change in either their molecular distributions or isotopic compositions during burial. The results of the two experiments indicate that early diagenesis can lead to losses of leaf wax biomarker n-alkanes during burial in soil but has little to no effect on their stable carbon isotopic signatures, validating the reliability of compound specific n-alkane δ13C values as proxies for paleoecological and paleoenvironmental reconstructions.

Enantiomer signature and carbon isotope evidence for the migration and transformation of DDTs in arable soils across China.

Due to the adverse impact of DDTs on ecosystems and humans, a full fate assessment deems a comprehensive study on their occurrence in soils over a large region. Through a sampling campaign across China, we measured the concentrations, enantiomeric fractions (EFs), compound-specific carbon isotope composition of DDT and its metabolites, and the microbial community in related arable soils. The geographically total DDT concentrations are higher in eastern than western China. The EFs and δ13C of o,p’-DDT in soils from western China show smaller deviations from those of racemic/standard compound, indicating the DDT residues there mainly result from atmospheric transport. However, the sources of DDT in eastern China are mainly from historic application of technical DDTs and dicofol. The inverse dependence of o,p’-DDT and p,p’-DDE on temperature evidences the transformation of parent DDT to its metabolites. Initial usage, abiotic parameters and microbial communities are found to be the main factors influencing the migration and transformation of DDT isomers and their metabolites in soils. In addition, a prediction equation of DDT concentrations in soils based on stepwise multiple regression analysis is developed. Results from this study offer insights into the migration and transformation pathways of DDTs in Chinese arable soils, which will allow data-based risk assessment on their use.