Compound-specific Carbon Isotope Analysis Research Articles

Organic geochemical and compound specific stable carbon isotope analyses of n-alkanes and saturated carboxylic acids extracted from late Paleozoic coals

AbstractIsotopic studies of coal organic matter are often conducted to gain information about past ecosystems and plant communities e.g. under changing climate conditions. While many studies focused on isotope analysis of bulk coal organic matter or extractable aliphatic hydrocarbons, the polar fraction and compounds that are bound to kerogen and not freely extractable have received less attention. The aim of this study was to provide a systematic comparison and evaluation of information gained from different molecular compounds (n-alkanes, n-carboxylic acids) extracted from Paleozoic coals before and after alkaline hydrolysis. Stable carbon isotope analysis of these compounds were used to evaluate a possible application of extractable and ‘bound’ lipids for paleoenvironmental analyses. Coal samples of different stratigraphic age, low thermal maturity (VRr < 1.02%, mostly VRr < 0.8%) and from different paleogeographic locations were selected to investigate organic matter deposited under different paleoenvironmental and paleoclimatic conditions. Molecular distributions show prominent carbon preference suggesting preservation of biologic signatures in coal organic matter. Permian Gondwana coals show similar n-alkane and n-carboxylic acid distribution which may be related to Glossopteris flora. Molecular distributions of Carboniferous coal from Europe are more variable with unimodal or bimodal patterns and maxima in the short- or long-chain range, reflecting higher diversity of paleovegetation and different contribution from algae or bacteria in limnic and paralic environments. The isotope signals of free as well as bound fatty acids show closer response for different depositional environments, whereas signals of paraffins and bulk organic matter are less variable. However, averaged fatty acid isotope signals in coals from different paleovegetational realms, e.g. Euramerica and Gondwana differed only marginally. Graphical abstract

Compound-Specific Carbon, Nitrogen, and Hydrogen Isotope Analysis to Characterize Aerobic Biodegradation of 2,3-Dichloroaniline by a Mixed Enrichment Culture.

Compound-specific isotope analysis (CSIA) is an established tool to track the in situ transformation of organic chemicals at contaminated sites. In this work, we evaluated the potential of multi-element CSIA to assess biodegradation of 2,3-dichloroaniline (2,3-DCA), which is a major industrial feedstock. Using controlled laboratory experiments, we determined, for the first time, negligible carbon (<0.5‰) and hydrogen (<10‰) isotope fractionation and a significant inverse nitrogen isotope fractionation (>10‰) during aerobic 2,3-DCA biodegradation by a mixed enrichment culture. The tentative identification of a glutamate conjugate of 2,3-DCA as a reaction intermediate indicates that the initial multistep enzymatic reaction may be rate-limiting. The formation of the glutamate adduct would increase the bond energy at the N atom, thus likely explaining the observed inverse N isotope fractionation. The corresponding nitrogen enrichment factor was +6.8 ± 0.6‰. This value was applied to investigate the in situ 2,3-DCA biodegradation at a contaminated site where the carbon and nitrogen isotope signatures from field samples suggested similar aerobic processes by native microorganisms. Under the assumption of the applicability of the Rayleigh model in a pilot wetland treating contaminated groundwater, the extent of biodegradation was estimated to be up to 80-90%. This study proposes multi-element CSIA as a novel application to study 2,3-DCA fate in groundwater and surface water and provides insights into biodegradation pathways.

Geochemistry of oils and condensates from the lower Eagle Ford formation, south Texas. Part 6: Carbon isotopes

Compound specific carbon isotope analyses were performed on a well-characterized suite of unconventional oils and condensates produced from the lower Eagle Ford Formation. Thermal maturity is the major influence on the δ13C values of C3–C19 hydrocarbons. From ∼0.8 to ∼1.1 %Ro, the main stage of oil generation, the δ13C values of most measured hydrocarbons remain relatively constant: n-alkanes are ∼ −30 ‰: light aromatic hydrocarbons, cycloalkanes and isoprenoids are ∼ −29 to −28 ‰. Only the C4 to C10 methylalkanes exhibit high variance with carbon number with δ13C values ranging from ∼ −32 to −28 ‰, respectively. Hydrocarbons from samples ∼1.1 to 1.3 %Ro, the late oil generation stage, are 13C-enriched by ∼1–∼2 ‰. This is the range typically assigned to the influence of thermal maturity on oil from many conventional petroleum systems. The onset of significant oil cracking occurs at ∼1.3 %Ro and all measured hydrocarbons systematically become progressively 13C-enriched although the degree varies by compound class and carbon number. The δ13C values of individual hydrocarbons shift by as much as ∼ +9 ‰ for propane and butane to only ∼ +3 ‰ for methylcyclopentane from <1.1 to 1.7 %Ro. The 13C-enrichments observed in the Eagle Ford oils/condensates with increasing maturity are attributed to the kinetic isotope effect (KIE), whereby 12C–12C bonds cleave faster than 12C–13C bonds. No evidence for isotopic equilibrium was found. The consistency of the sums of the weighted δ13C values for C4–C7n-alkane/methylalkane isomers provides support for Mango's (2000a) theory that these hydrocarbons are generated from precursors with a common chemical structure that pass through a similar transition state.Source facies exert only a minor influence on the carbon isotopic composition of Eagle Ford oils/condensates. Small variances in the 13C values of C6+n-alkanes, cyclohexane and methylcyclohexane are consistent with oils produced east of the San Marco Arch being generated from shales with a higher influx of terrestrial high plant matter compared to the shales west of the Arch. Phase fractionation was shown to have a measurable impact on the δ13C values of the most volatile hydrocarbons in evaporated samples and on the least volatile hydrocarbon that underwent phase separation in the subsurface during production.

Dispersive liquid-liquid microextraction as a novel enrichment approach for compound-specific carbon isotope analysis of chlorinated phenols.

Compound-specific isotope analysis (CSIA) via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is a potent tool to elucidate the fate of (semi-)volatile organic contaminants in technical and environmental systems. Yet, due to the comparatively low sensitivity of IRMS, an enrichment step prior to analysis often is inevitable. A promising approach for fast as well as economic analyte extraction and preconcentration prior to CSIA is dispersive liquid-liquid microextraction (DLLME) - a well-established technique in concentration analysis of contaminants from aqueous samples. Here, we present and evaluate the first DLLME method for GC-IRMS exemplified by the analysis of chlorinated phenols (4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol) as model compounds. The analytes were simultaneously acetylated with acetic anhydride and extracted from the aqueous phase using a binary solvent mixture of acetone and tetrachloroethylene. With this method, reproducible δ13C values were achieved with errors ≤ 0.6‰ (n = 3) for aqueous concentrations down to 100 μg L-1. With preconcentration factors between 130 and 220, the method outperformed conventional liquid-liquid extraction in terms of sample preparation time and resource consumption with comparable reproducibility. Furthermore, we have demonstrated the suitability of the method (i) for the extraction of the analytes from a spiked river water sample and (ii) to quantify kinetic carbon isotope effect for 2,4,6-trichlorophenol during reduction with zero-valent zinc in a laboratory batch experiment. The presented work shows for the first time the potential of DLLME for analyte enrichment prior to CSIA and paves the way for further developments, such as the extraction of other compounds or scaling up to larger sample volumes.

Compound-specific carbon isotope analysis of short-chain fatty acids from Pine tissues: characterizing paleo-fire residues and plant exudates

Different types of plant tissues and resin can account for the wax lipids found in sedimentary contexts and archaeological samples. Consequently, there is increasing research to characterize the fatty acid carbon isotope ratios of different plant anatomical parts and their plant exudates (resin). With the aim to explore isotopic differences between plant tissues, state of the fine organic matter, effect of thermal degradation, and to identify plant residues we measured the δ13C values of short-chain fatty acids (δ13C16:0 and δ13C18:0) in: i) dead and fresh (collected and immediately dried) pine needles and branches (Pinus canariensis) and pine resin from laboratory-controlled heating experiments and ii) sediment and charred pine tissue samples from a wild pine forest fire. Our results are compared to previously published experimental open-air fire experiments and pine-fuelled archaeological combustion features. We found that for both fatty acid types, there are differences in δ13C signatures among anatomical parts and initial moisture content. These data allow us to characterize the isotopic signature of pine tissue and the effect of degradation on isotopic biomarkers, as well as to estimate combustion temperatures in pine-fuelled anthropogenic fires.

Compound-specific carbon isotope evidence that the initial carbon isotope excursion in the end-Triassic strata in northwest Tethys is not the product of CAMP magmatism

Compound-specific carbon isotopic analysis of biomarkers and palynological studies from sections across the end-Triassic mass extinction in the northwestern Tethys provide constraints on the origin of the bulk organic carbon ‘initial’ isotope excursion. This excursion is commonly used to correlate among geographically disparate sections that span this pivotal event associated with the rise of modern fauna. Palynological analysis is used to identify intervals of Liassic-equivalent strata in south Wales and northeast England for isotopic study. Compound-specific carbon isotope analysis of n-alkanes and regular isoprenoids from these intervals reveals a trend different than that displayed by bulk organic matter, suggesting the latter signal is not derived from large perturbations in the global carbon cycle but from localized source changes. The most pronounced changes in compound-specific isotope profiles are negative excursions in pristane and phytane, which occur within the positive phase of the bulk carbon isotope excursion and correspond to shifts in total organic carbon and increased heterotrophy. These findings indicate that a re-evaluation of temporal relationships between positive climate feedbacks and their effects is necessary to more accurately understand the likely global nature of the end-Triassic extinction, and more generally, carbon cycle perturbations associated with other mass extinction events.

An optical approach for compound specific carbon isotope analysis of 1,4-dioxane by liquid injection

Gas chromatography coupled to isotope ratio infrared spectroscopy (GC-IRIS) is an emerging tool for compound specific stable isotope analysis (CSIA) of organic pollutants.

Evaluating the accuracy and reliability of compound-specific carbon isotopic analysis by GC/C/IRMS with the addition of a reduction furnace.

Gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) is widely used for compound-specific carbon isotopic analysis. However, current isotopic analysis systems utilize the GC IsoLink combustion reactor, and independent reduction furnaces are not implemented. Therefore, whether this limitation in furnace use affects the precision of compound-specific carbon isotopic analysis needs to be evaluated. We attempted to add a separate reduction furnace to the GC IsoLink interface and compared the δ13 C values of n-alkanes (including C and H elements), fatty acid methyl ester (including C, H and O elements), caffeine (USGS61 and USGS62, including C, H, O and N elements) and 9-Ethylcarbazole (including C, H and N elements) before and after the addition of the reduction furnace using the GC IsoLink combustion reactor. For n-alkanes and fatty acid methyl esters, the δ13 C differences between the measured values and their standard values were basically falling within 0.5 ‰ whether or not an independent reduction furnace was added. However, for the nitrogen-containing compounds (caffeine and 9-Ethylcarbazole), the δ13 C differences between the measured values and their standard values were much larger without the independent reduction furnace (1.0 to 3.71 ‰ for USGS61, 1.78 to 2.19 ‰ for USGS62, and 0.39 to 1.13 ‰ for 9-Ethylcarbazole) than adding a reduction furnace (-0.31 to 0.68 ‰ for USGS61, -0.44 to 0.06 ‰ for USGS62 and -0.04 to 0.25 ‰ for 9-Ethylcarbazole). The adding of an independent reduction furnace had no significant effect on the δ13 C of n-alkanes and fatty acid methyl esters, but had a significant effect on the δ13 C of nitrogen-containing compounds. It is suggested that GC IsoLink need an independent reduction furnace to effectively eliminate the interference of NOx on CO2 isotopic determination to improve the accuracy of δ13 C for nitrogen-containing compounds.

Day-night alternation and effect of sulfate ions on photodegradation of triclosan in water

Photodegradation is an essential process for the in-stream elimination of triclosan (TCS). There are still some knowledge gaps concerning reaction kinetics, pathways, and transformation products, which are important for understanding the environmental fate of TCS. For example, the effects of ionic strength and day-night alternation on TCS photodegradation have been barely considered in previous studies. To fill these gaps, this study assesses the effect of sulfate ions (SO42−) on the photodegradation of TCS under day-night shifts. Compound-specific carbon and chlorine isotope analysis were applied to characterize different bond-cleavage pathways. Moreover, variations in TCS reaction products distribution were analyzed by using Fourier transform ion cyclotron resonance mass spectrometry. A numerical model was developed to describe the concentration and stable carbon isotopic evolution of TCS under different conditions. Based on the observed experimental data, we show that SO42− can affect the kinetics and product distribution during the photodegradation of TCS. In particular, sulfate ions (100 mM) inhibited the yield of the toxic product, 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD). Furthermore, day-night alternations lead to different predominant bond-cleavage pathways depending on sulfate concentration. By so doing, we propose a realistic framework for the direct photodegradation of TCS in sunlit natural surface waters.

Multiple trophic pathways support fish on floodplains of California's Central Valley.

We used compound-specific isotope analysis of carbon isotopes in amino acids (AAs) to determine the biosynthetic source of AAs in fish from major tributaries to California's Sacramento-San Joaquin river delta (i.e., the Sacramento, Cosumnes and Mokelumne rivers). Using samples collected in winter and spring between 2016 and 2019, we confirmed that algae are a critical component of floodplain food webs in California's Central Valley. Results from bulk stable isotope analysis of carbon and nitrogen in producers and consumers were adequate to characterize a general trophic structure and identify potential upstream and downstream migration into our study site by American shad Alosa sapidissima and rainbow trout Oncorhynchus mykiss, respectively. However, owing to overlap and variability in source isotope compositions, our bulk data were unsuitable for conventional bulk isotope mixing models. Our results from compound-specific carbon isotope analysis of AAs clearly indicate that algae are important sources of organic matter to fish of conservation concern, such as Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. However, algae were not the exclusive source of energy to metazoan food webs. We also revealed that other sources of AAs, such as bacteria, fungi and higher plants, contributed to fish as well. While consistent with the well-supported notion that algae are critical to aquatic food webs, our results highlight the possibility that detrital subsidies might intermittently support metazoan food webs.

Insights into amino acid fractionation and incorporation by compound-specific carbon isotope analysis of three-spined sticklebacks

Interpretation of stable isotope data is of upmost importance in ecology to build sound models for the study of animal diets, migration patterns and physiology. However, our understanding of stable isotope fractionation and incorporation into consumer tissues is still limited. We therefore measured the δ13C values of individual amino acids over time from muscle and liver tissue of three-spined sticklebacks (Gasterosteus aculeatus) on a high protein diet. The δ13C values of amino acids in the liver quickly responded to small shifts of under ± 2.0‰ in dietary stable isotope compositions on 30-day intervals. We found on average no trophic fractionation in pooled essential (muscle, liver) and non-essential (muscle) amino acids. Negative Δδ13C values of − 0.7 ± 1.3‰ were observed for pooled non-essential (liver) amino acids and might indicate biosynthesis from small amounts of dietary lipids. Trophic fractionation of individual amino acids is reported and discussed, including unusual Δδ13C values of over + 4.9 ± 1.4‰ for histidine. Arginine and lysine showed the lowest trophic fractionation on individual sampling days and might be useful proxies for dietary sources on short time scales. We suggest further investigations using isotopically enriched materials to facilitate the correct interpretation of ecological field data.

Late Paleocene CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; drawdown, climatic cooling and terrestrial denudation in the southwest Pacific

Abstract. ​​​​​​​Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea level fall. Recent studies confirm that the organic matter in this facies, termed “Waipawa organofacies”, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in 13C. In this study we address the cause of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawke's Bay, paired palynofacies and carbon isotope analysis of heavy liquid-separated density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment may be partly due to lignin degradation. Compound-specific stable carbon isotope analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections display similar trends and further reveal a residual 13C enrichment of ∼ 2.5 ‰ in higher plant biomarkers (long chain n-alkanes and fatty acids) and a ∼ 2 ‰–5 ‰ change in subordinate marine biomarkers. Using the relationship between atmospheric CO2 and C3 plant tissue δ13C values, we determine that the 3 ‰ increase in terrestrial δ13C may represent a ∼ 35 % decrease in atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.5 Ma, which coincides with an interval of carbonate dissolution in the deep sea that is associated with a Paleocene oxygen isotope maximum (POIM, 59.7–58.1 Ma) and the onset of the Paleocene carbon isotope maximum (PCIM, 59.3–57.4 Ma). This association suggests that Waipawa deposition occurred during a time of cool climatic conditions and increased carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial, due to tectonic factors, resulted in short-lived global cooling, growth of ephemeral ice sheets and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep-sea sediment archives may have hidden the evidence of this “hypothermal” event until now.

Molecular and stable isotope analysis (δ13C, δ2H) of sedimentary n-alkanes in the St. Lawrence Estuary and Gulf, Quebec, Canada: Importance of even numbered n-alkanes in coastal systems

Sediments comprise a multitude of inorganic and organic components, with much of the composition of the organics still not fully characterized. Our research targeted n-alkanes, to determine whether compound specific carbon and hydrogen isotope analysis allows for their source identification in coastal sediments. Here, we map the current abundances and sources of straight chain n-alkanes in sediments of the St. Lawrence Estuary and Gulf using molecular (diagnostic ratios) and isotopic fingerprinting (δ13C, δ2H). n-Alkane abundances (117.11 ± 1.61 to 418.64 ± 70.20 µg/g OC), carbon preference index (CPI; 1.95 ± 0.05 to 5.09 ± 0.10), average chain length (ACL; 28.36 ± 0.02 to 28.97 ± 0.01), proportion of aquatic submerged plants and terrestrial plant inputs (Paq; 0.295 ± 0.003 to 0.377 ± 0.002), terrigenous aquatic ratio (TAR; 3.43 ± 0.16 to 7.99 ± 0.05), and n-alkane ratio (NAR; 0.169 ± 0.011 to 0.584 ± 0.011) values varied along the terrestrial-marine continuum. Large differences in the concentration weighted average (WA) δ13C and δ2H for odd and even n-alkanes were found, with WA δ13C ranging from −30.9 ± 0.3 to −33.4 ± 0.09 ‰ and −28.8 ± 0.01 to −32.3 ± 0.2 ‰, respectively, and 165.6 ± 3.6 to −200.8 ± 2.4 ‰ and −96.0 ± 2.8 to −158.7 ± 2.1 ‰ for δ2H. The diagnostic ratios were shown to misrepresent the input sources of organic matter (OM) and were inaccurate as source indicators when more than one OM source was present. With the addition of compound specific δ13C and δ2H analysis of n-alkanes, it was determined that the n-alkanes were predominantly derived from natural, rather than anthropogenic sources, with variations being driven by geographic changes in vegetation type and differing ratios of terrestrial and marine OM inputs. Importantly, compound specific isotope analysis of the even numbered n-alkanes would permit identification and tracking of petroleum-derived contaminants. Molecular data alone are ineffective for this, owing to the similarity in CPI values for petroleum-derived contaminants and highly degraded OM which is discharged by the St. Lawrence River into the estuary.

Tectono-thermal impacts on the formation of a heavy oil in the eastern Tarim Basin (China): Implications for oil and gas potential

Unraveling the accumulation and alteration process of reservoired petroleum is significant for the evaluation of petroleum potential. In this study, a paleo heavy oil from the TD2 well in the eastern Tarim Basin was investigated through integrated geochemical methods including two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS), gas chromatography-mass spectrometry (GC-MS) and compound specific carbon isotope analysis (CSCIA). Oil-source correlation analysis indicates that the TD2 oil accumulation is “self-generation and self-accumulation” within the Cambrian strata. Combined with tectonic evolution analysis, it is suggested that the oil experienced significant impacts due to changes in tectono-thermal process. Abnormally enriched polycyclic aromatic hydrocarbons (62.5 mg/g in total, 4- to 6- ring account for ~41 %) with few substituents suggest oil cracking due to a short-time high-temperature heating during rapid tectonic subsidence under higher geothermal gradient. Phase fractionation, as indicated by the physiochemical features and heavy isotopic values, was induced by severe tectonic uplift and denudation after accumulation, and resulted in the thickening, re-distribution, and destruction of the primary oil pool. After that, the TD2 oil pool was preserved from further alterations under steady tectonic setting with a low geothermal gradient. Four accumulation assemblages in the eastern Tarim Basin were identified and compared, indicating that great oil and gas potential remains including oil/heavy oil in the Cambrian assemblage and light oil/condensate in the Jurassic and Silurian assemblages. Future exploration should focus on locally developed favorable reservoirs with good preservation conditions.

Compound specific stable carbon isotope analysis of aromatic organic contaminants in water using gas chromatography coupled to mid-infrared laser spectroscopy

A mid-infrared laser based analytical approach is newly developed in this study for gas chromatography (GC) separation and carbon compound specific isotope analysis (CSIA) of aromatic organic pollutants in water.

Compound-specific carbon isotope analysis of volatile organic compounds in complex soil extracts using purge and trap concentration coupled to heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry

Compound-specific carbon isotope analysis (CSIA) is a powerful tool to track the origin and fate of organic subsurface contaminants including petroleum and chlorinated hydrocarbons and is typically applied to water samples. However, soil can form a significant contaminant reservoir. In soil samples, it can be challenging to recover sufficient amounts of volatile organic compounds (VOC) to perform CSIA. Soil samples often contain complex contaminant mixtures and gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) is highly dependent on good chromatographic separation due to the conversion to a single analyte. To extend the applicability of CSIA to complex volatile organic compound mixtures in soil samples, and to recover sufficient amounts of target compounds for carbon CSIA, we compared two soil extraction solvents, tetraglyme (TGDE) and methanol, and developed a heart-cutting two-dimensional GC-GC-C-IRMS method. We used purge & trap concentration of solvent-water mixtures to increase the amount of analyte delivered to the column and thus lower method detection limits. We optimized purge & trap and chromatographic parameters for twelve target compounds, including one suffering from poor purge efficiency. By using a 30 m thick-film non-polar column in the first and a 15 m polar column in the second dimension, we achieved good chromatographic separation for the target compounds in difficult matrices and high accuracy (trueness and precision) for carbon isotopic analysis. Tetraglyme extraction was shown to offer advantages over methanol for purge & trap concentration, leading to lower target compound method detection limits for CSIA of soil samples. The applicability of the developed method was demonstrated for a case study on soil extracts from a former manufacturing facility. Our approach extends the applicability of CSIA to an important matrix that often controls the long-term fate of contaminants in the subsurface.

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 Stable Carbon Isotopic Compositions of n-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

Stable carbon isotopic compositions of n-alkanes in surface sediments of the Bohai and North Yellow Seas were investigated to elucidate sources of sedimentary organic matter in these seas. The long-chain n-alkanes in surface sediments are predominantly long-chain C27, C29, and C31 types, with obvious odd carbon predominance. The δ13C values of long-chain n-C27, n-C29, and n-C31 alkanes are −30.8%±0.59‰, −31.9%±0.6‰, and −32.1%±1.09‰, respectively, within the range of n-alkanes of C3 terrestrial higher plants. This suggests that sedimentary n-alkanes are derived mainly from terrestrial higher plants. Compound-specific carbon isotopic analysis of long-chain n-alkanes indicates that C3 terrestrial higher plants predominate (64%–79%), with angiosperms being the main contributors. The n-alkane δ13C values indicate that mid-chain n-alkanes in sediments are derived mainly from aquatic emergent macrophytes, with significant petroleum pollution and bacterial degradation sources for short-chain n-alkanes.

δ 13C compound-specific isotope analysis in organic compounds by GC/MC-ICPMS

The work demonstrates a novel analytical method for compound-specific carbon isotope analysis in organic compounds by gas chromatography hyphenated with MC-ICPMS.

Evidence of a significant marine plant diet in a Pleistocene caribou from Haida Gwaii, British Columbia, through compound-specific stable isotope analysis

The Northwest Coast of Canada was likely an important location for glacial refugia in the Late Pleistocene. Recently, a caribou (Rangifer tarandus) antler fragment was reported from Graham Island dating prior to the Fraser Glaciation (MIS 3; 48,200–45,200 cal yr BP). The high carbon and nitrogen stable isotope values found in the bulk collagen samples of this caribou prompted further investigation into its diet and palaeoecological context. Today, caribou in British Columbia face rapid population and range decline, and have been placed on threatened and endangered species lists by both the U.S. and Canadian governments. Further insights into this species and its responses to changing environmental conditions can both improve our understanding of the Late Pleistocene ecosystems of the Northwest Coast and add relevant data to modern conservation issues. We employed compound-specific carbon and nitrogen stable isotope analysis to the individual amino acids of antler collagen to better understand the isotopic signal of this specimen. Our results suggest a significant marine plant contribution to the caribou's spring–summer diet, potentially reflecting an intensive foraging behavior observed in some modern-day Rangifer tarandus populations under conditions of terrestrial resource restriction.