Compound-specific δ13C Research Articles

Can stable carbon isotope fingerprints be competent for geographic traceability of rice?

This study aimed to improve the traceability of rice-producing areas to address the increasing demand for accurate methods to confirm food quality and safety. Compound-specific δ13C of fatty acids, δ13C of starch and bulk of rice were measured. PCA, PLS-DA and VIP value analysis of the obtained data were performed to track the source of rice from the six regions. The PLS-DA model established with bulk δ13C, starch δ13C, and fatty acid δ13C, which clearly separated the rice from six regions. The VIP graph showed the value of starch, C18:0 and C18:2 δ13C values (VIP > 1) were important to distinguish the origin of rice. Also, according to loading plots the contribution of starch δ13C was the largest. The findings indicate that the introduction of starch δ13C improves the precision of rice traceability and provides an effective method for identifying rice origin.

Purity Assessment of Honey Based on Compound Specific Stable Carbon Isotope Ratios Obtained by LC-IRMS.

The use of stable carbon isotope ratios (δ13C) of sugar fractions of honey is a powerful tool to detect adulteration with sugar syrups. This is accomplished by calculating differences of the δ13C values between individual honey saccharides and comparing them to published purity criteria. A liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) method for the determination of δ13C values of sugars in honey was previously validated by an interlaboratory comparison, but no further guidance was given how to include the obtained precision figures of the compound-specific δ13C values in the purity assessment of honey. To use existing data to estimate the standard deviation of the repeatability (sr) and reproducibility (sR) of differences (Δ δ13C) between the δ13C values of individual honey saccharides. Previously published δ13C values were used to calculate differences (Δ δ13C values) between δ13C fructose-δ13C glucose, δ13C glucose-δ13C disaccharides, etc. in a honey sample; sr and sR of Δ δ13C values were calculated according to ISO 5725-2:2019. The Δ δ13C sr and sR values were essentially of the same magnitude as the sr and sR values of δ13C values of the sugar fractions. The precision of the Δ δ13C values was used to estimate the critical difference for comparing a test result with a reference value according to ISO 5725-6:1994. This varied between 0.26 and 1.10‰. The estimated critical differences can be used to determine whether a honey test result complies with published Δ δ13C purity criteria. The proposed procedure will increase confidence in decisions based on compound-specific δ13C values regarding the conformity of honey with published purity criteria.

Evaluation of lipid extraction methods for fatty acid quantification and compound-specific δ13C and δ2Hn analyses

Lipids, with fatty acids (FA) as a crucial subset, have become a focal point for diverse medical, physiological, and ecological studies. However, a comprehensive assessment of the various pre-analytical FA extraction methods published in the scientific literature remains lacking. In this study, we examined the efficacy of seven well-established sample preparation methods, specifically focusing on their effectiveness in total lipid and fatty acid extraction and their impact on compound-specific stable hydrogen (δ2H) and carbon (δ13C) isotope values. We also considered the repercussions of FA removal efficacy on residual bulk tissue δ2Hn analysis, because lipids typically have low δ2H values.Our findings showed that in most cases chloroform-based extraction methods outperformed those without chloroform. While discrepancies were not as evident for smaller organisms, such as plankton, marked variations were discernible in the extraction efficiencies for muscle and liver samples, which was also manifested in the residual bulk tissue δ2Hn results. Notably, most extraction methods had little effect on specific δ13C or δ2H isotope values of FA; instead, an emphasis should be on using an extraction method that achieves optimal baseline peak separation of the chromatograms for C and H isotope measurements.

Determination of intermolecular and intramolecular isotopic compositions of low-abundance gaseous hydrocarbons using an online hydrocarbon gas concentration method

The stable isotopic composition of natural gas can be used to identify its origin and source. However, low concentrations of gaseous hydrocarbons in high-mature natural and shale gases hinder accurate determination of their compound- and position-specific isotopic compositions. In this study, an online C2+ hydrocarbon gas concentration system combined with gas chromatography–isotope ratio mass spectrometry (GC–IRMS) or gas chromatography–pyrolysis–gas chromatography–isotope ratio mass spectrometry (GC–Py–GC–IRMS) was developed to determine compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons. The lower limit of the gas concentration required for isotope ratio determination using the online concentration system is 0.001% (0.003%) for compound-specific carbon (hydrogen) isotopes and 0.005% for position-specific carbon isotopes and is thus applicable to most natural gas samples. The online concentration technique does not cause significant isotopic fractionation effects, and the combination with GC–IRMS and GC–Py–GC–IRMS can accurately and precisely determine the compound-specific δ13C and δD values of low-content C2+ gaseous hydrocarbons and the position-specific δ13C values (δ13Ca, δ13Cb, and SP values) of propane in low-content propane samples, respectively. The application of our method to two natural gas samples from the Ordos and Sichuan basins further confirms that the online concentration method allows simple and rapid determination of the compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons.

Botanical characterization and authentication of lavender essential oil using its volatile organic compounds and compound-specific carbon and hydrogen isotope ratio analysis

The determination of authenticity and provenance is one of the main issues in high-value natural products such as lavender (L. angustifolia) essential oil. Due to their high value, natural products are often totally or partially substituted with cheaper alternatives. Therefore, it is necessary to accurately characterize the composition of lavender essential oil and its related species, particularly lavandin (L. intermedia) essential oil, which is frequently used as an adulterant. In this study, we analyzed the most distinctive volatile organic compounds (VOCs) and their relative compound-specific stable carbon and hydrogen (δ13C and δ2H) isotope ratios in lavender, lavandin, and commercial lavender essential oil samples using fast GC-MS/MS and GC-IRMS methods and sparse Partial Least Square Discriminant Analysis (sPLS-DA) to classify and verify their botanical origin. For the first time, concentration ranges for 79 VOCs and the δ13C and δ2H values in 32 VOCs were used to characterize lavender essential oils. sPLS-DA models with either VOCs, δ13C and δ2H, or in combination successfully classify lavender and lavandin with 100% accuracy. VOCs such as camphor, borneol, eucalyptol, α-bisabolol, and α-pinene were found to be the most important variables for differentiating lavender oil from lavandin. Similarly, δ13C in camphor, eucalyptol, trans-β-ocimene and 3-octanone, and δ2H in α-terpineol and trans-β-ocimene and 3-octanone also differentiated lavender and lavandin oil. In addition, we demonstrated that compound-specific δ13C and δ2H detected the adulteration of natural lavender or lavandin oil if more than 15% of linalool, linalyl acetate, eucalyptol, and α-terpineol were of synthetic origin. These preliminary results with sPLS-DA models suggest that VOCs and compound-specific δ13C and δ2H can be a promising tool for the botanical classification of lavender essential oil and the detection of possible adulteration with synthetic compounds.

Aryl isoprenoids from the Lower Paleozoic in the Tarim Basin, NW China: Insight into deep ancient hydrocarbon exploration

The deep ancient hydrocarbon accumulation in the Tarim Basin was extremely complex. It was characterized by large deep burial, multistage accumulations, multistage adjustments, and multistage secondary alterations, affording limited effectiveness of steroid/terpene biomarkers used widely in oil-source correlation. Aryl isoprenoids (AIs), as a series of diagnostic biomarkers, have attracted increasing attention. For a detailed investigation of AIs from the Lower Paleozoic strata and its internal crude oil in the Tarim Basin, various experimental techniques have been employed. The m/z 133 and 134 mass chromatograms of the AIs confirmed their identification. The anomalously heavy compound-specific δ13C (up to ∼ −16‰) indicated the unequivocal precursor of green sulfur bacteria that was only developed under photic zone euxinia (PZE) conditions. AIs were detected in highly mature source rocks and the crude oils suffered secondary alterations, indicating that AIs might have strong stability and anti-secondary-alteration ability. Based on their widespread distribution in the Lower–Middle Ordovician Heituao Formation (O1-2h Fm) and the Lower Cambrian Yuertus Formation (Є1y Fm) shales, the discovery of AIs in almost all crude oils suggests that the O1-2h Fm and Є1y Fm shales were the primary source rocks for the remarkably abundant Lower Paleozoic deep petroleum resources. This reverses the previously held belief that the Middle-Upper Ordovician source rocks were the primary source rocks. In addition, AIs might induce the formation of organic-rich source rocks (the TOC was up to 29.8%) by promoting high-quality preservation conditions (containing H2S). Furthermore, unlike the H2S poisoning mechanism for the end-Permian mass extinction explained by AIs, regionally distributed AIs triggered by H2S from non-volcanic eruptions could explain the biological radiations (such as the early Cambrian radiation). Therefore AIs might provide important insights into the research on deep ancient petroleum exploration and its contemporaneous biological events.

Two-dimensional gas chromatography coupled to isotope ratio mass spectrometry for determining high molecular weight polycyclic aromatic hydrocarbons in sediments

The accuracy of compound-specific isotope analysis (CSIA) of trace-level pollutants in complex environmental samples has always been limited by two main challenges: poor chromatographic separation and insufficient amounts of analytes. In this study, a two-dimensional gas chromatography-isotope ratio mass spectrometry (2DGC-IRMS) system was constructed for compound-specific δ13C analysis of high molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) in estuarine/marine sediments. This construction occurred through hyphenating an extra gas chromatography system (GC) to a conventional GC-IRMS using a commercially available multi-column switching-cryogenic trapping system (MCS-CTS). Compared with the previous 2DGC-IRMS strategy, which utilizes a Deans Switch device, the newly implemented 2DGC-IRMS scheme resulted in online purification of target analytes as well as enriched them online via duplicate injection and cryogenic trapping in CTS; this resultingly lowered the limits of detection (LOD) of CSIA. To improve the sample transfer efficiency to the IRMS, a broader-bore and longer fused-silica capillary was utilized to replace the original sample capillary running from the sample open split to the IRMS. A ẟ13C analysis of PAH standards showed accurate ẟ13C values, and high precisions (standard deviations 0.13–0.37%) were achieved, with the LOD of HMW-PAHs reduced to at least 1.0 mg/L (i.e., 0.07 to 0.09 nmol carbon per compound on-column). The successful application of this newly developed 2DGC-IRMS scheme provides a practical solution for the reliable CSIA of trace-level pollutants in complex environmental samples that cannot be measured using the conventional GC-IRMS system.

Late-season biosynthesis of leaf fatty acids and n-alkanes of a mature beech (Fagus sylvatica) tree traced via 13CO2 pulse-chase labelling and compound-specific isotope analysis.

Leaf cuticular waxes play an important role in reducing evapotranspiration via diffusion. However, the ability of mature trees to regulate the biosynthesis of waxes to changing conditions (e.g., drought, light exposition) remain an open question, especially during the late growing season. This holds also true for one of the most widely distributed trees in Central Europe, the European beech tree (Fagus sylvatica L.). In order to investigate the ongoing formation of wax constituents like alkanes and fatty acids, we conducted a 13CO2 pulse-chase labelling experiment on sun-exposed and shaded branches of a mature beech tree during the late summer 2018. The 13C-label was traced via compound-specific δ13C isotope analysis of n-alkanes and fatty acids to determine the de-novo biosynthesis within these compound classes. We did not observe a significant change in lipid concentrations during the late growing season, but we found higher n-alkane concentrations in sun-exposed compared to shaded leaves in August and September. The n-alkane and fatty acid composition showed ongoing modifications during the late growing season. Together with the uptake and following subsequent decrease of the 13C-label, this suggests ongoing de-novo biosynthesis, especially of fatty acids in European beech leaves. Moreover, there is a high variability in the 13C-label among individual branches and between sun-exposed and shaded leaves. At the same time, sun-exposed leaves invest more of the assimilated C into secondary metabolites such as lipids than shaded leaves. This indicates that the investigated mature beech tree could adjust its lipid production and composition in order to acclimate to changes in microclimates within the tree crown and during the investigated period.

Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents sea

The origin of modern seafloor methane emissions in the Barents Sea is tightly connected to the glacio-tectonic and oceanographic transformations following the last ice age. Those regional events induced geological structure re-activation and destabilization of gas hydrate reservoirs over large areas of the European continental margins, sustaining widespread fluid plumbing systems. Despite the increasing number of new active seep discoveries, their accurate geochronology and paleo-dynamic is still poorly resolved, thus hindering precise identification of triggering factors and mechanisms controlling past and future seafloor emissions. Here, we report the distribution, petrographic (thin section, electron backscatter diffraction), isotopic (δ13C, δ18O) and lipid biomarker composition of methane-derived carbonates collected from Leirdjupet Fault Complex, SW Barents Sea, at 300 m depth during an ROV survey in 2021. Carbonates are located inside a 120 x 220 m elongated pockmark and form <10 m2 bodies protruding for about 2 m above the adjacent seafloor. Microstructural analyses of vein-filling cements showed the occurrence of three–five generations of isopachous aragonitic cement separated by dissolution surfaces indicative of intermittent oxidizing conditions. The integration of phase-specific isotopic analysis and U/Th dating showed δ13C values between −28.6‰ to −10.1‰ and δ18O between 4.6‰ and 5.3‰, enabling us to track carbonate mineral precipitation over the last ∼8 ka. Lipid biomarkers and their compound-specific δ13C analysis in the bulk carbonate revealed the presence of anaerobic methanotrophic archaea of the ANME-2 clade associated with sulfate-reducing bacteria of the Seep-SRB1 clade, as well as traces of petroleum. Our results indicate that methane and petroleum seepage in this area followed a similar evolution as in other southernmost Barents Sea sites controlled by the asynchronous deglaciation of the Barents Sea shelf, and that methane-derived carbonate precipitation is still an active process at many Arctic locations.

N-Alkanes and their carbon isotopes (δ13C) reveal seasonal foddering and long-term corralling of pastoralist livestock in eastern Mongolia

The compound-specific δ13C values of n-alkanes from pastoralist winter campsites in Mongolia have great potential to reconstruct dietary seasonality of livestock and local vegetation. We analysed leaf wax n-alkanes and their carbon isotope ratios in common fodder plants (Artemisia mongolica, Chenopodium album, Elymus dahuricus, Elymus sibiricus and Stipa sibirica) fed to domesticates during the winter months, and also in topsoils (0–8 cm) from four livestock corrals in northern and southern Mongolia. n-Alkanes identified in fresh plant parts reflect signatures with mean δ13C stable isotope values of individual n-alkanes typical for C3 plants (−34.1‰). Plants from forest- and desert-steppe exhibit similar mean n-alkane δ13C values but significantly wider isotopic variation in the desert steppe flora. The non-anthropogenic control samples exhibit δ13C mean values significantly higher from that of dung deposits and fresh plants for the same regions. Our data support previous research that n-alkanes profiles and their carbon isotopic values vary depending on the local vegetation and environment, while simultaneously demonstrating how anthropogenic activity, such as corralling of livestock, is recorded in the n-alkane of domesticates dung validating the use of biomarker studies in pastoralist archaeological contexts for distinguishing seasonal foddering, local environmental data and to an extent grazing habitats.

Fingerprinting macrophyte Blue Carbon by pyrolysis-GC-compound specific isotope analysis (Py-CSIA).

There is a need for tools to determine the origin of organic matter (OM) in Blue Carbon Ecosystems (BCE) and marine sediments to (1) facilitate the implementation of Blue Carbon strategies into carbon accounting and crediting schemes and (2) decipher changes in ecosystem condition over decadal to millennial time scales and thus to understand and predict the stability of BCE in a changing world. Pyrolysis-GC-compound specific isotope analysis (Py-CSIA) is applied for the first time in marine environments and BCE research. We studied Australian mangrove, tidal marsh and seagrass sediments, in addition to potential sources of OM (Avicennia, Posidonia, Zostera, Sarcocornia, Ecklonia and Ulva species and seagrass epiphytes), to identify precursors of different biomacromolecule constituents (lignin, polysaccharides and aliphatic structures). Firstly, the link between bulk δ13C and δ13C reconstructed from compound-specific δ13C showed that the pyrolysis approach allows for the isotopic screening of a representative portion of the OM. Secondly, for all samples, the C isotope fingerprint of the carbohydrate products (plant polysaccharides) was the heaviest (13C enriched), followed by lignin and aliphatic products. The differences in δ13C among macromolecules and the overlap in δ13C among putative sources reflect the limitations of bulk δ13C analyses for deciphering OM provenance. Thirdly, phanerogams specimen had the heaviest carbohydrate and lignin, confirming that seagrass-derived lignocellulose can be traced based on δ13C. Consistent differences for individual compounds were identified between seagrasses and between Avicennia and Sarcocornia using Py-CSIA. Fourth, ecosystem shifts (colonization of seagrass habitats by mangrove) on millenary time scales, hypothesized in previous studies on the basis of bulk δ13C and Py-GC-MS, were confirmed by Py-CSIA. We conclude that Py-CSIA is useful in Blue Carbon research to decipher OM sources in marine sediments, identify ecosystem transitions in palaeoenvironmental records, and to understand the role of different OM compounds in Blue Carbon storage.

Kinetic and equilibrium reactions on natural and laboratory generation of thermogenic gases from Type II marine shale

The phenomenon that laboratory pyrolysis experiments produce much wetter gases than those in natural reservoirs is a long-recognized and debated problem in the investigation of natural gases in sedimentary basins. In this study, we explore the discrepancy by pyrolyzing a type II kerogen from the Woodford Shale in Oklahoma, compared with the previous results on the produced natural gases from the Arkoma Basin generated from the same source rock (Liu et al., 2019) with the discussion of gas and isotopic compositions at bulk and position-specific (PS) levels. An improved GC-pyrolysis-GC IRMS method is applied for the determination of PS δ13C of propane produced in the pyrolysis of the Woodford Shale at Easy %Ro from 0.76 to 3.27. Kinetic and thermodynamic considerations of the chemical and isotopic compositions of the natural and laboratory pyrolysis gases suggest that the generation of light hydrocarbons involves uni-directional cracking reactions, exchange reactions with water, and likely reversible reactions among light hydrocarbons and other H-containing volatiles. After the gas generation in the unconventional Woodford Shale reservoirs, the C1-C4 gases might have approached close to chemical equilibrium of C1-C3 and isotope equilibrium of C2-C1 and C3-C1 pairs at their peak temperatures. The capping H for the generation of C1-C4 in the Woodford Shale gases appears to have experienced at least partial exchange with the water, while that in the pyrolysis gases is only originated from organic-bound compounds with large kinetic isotope effects (KIE). Our findings indicate that elevated compound-specific and PS δ13C values of propane in the wet-gas cracking stage are significantly influenced by the breakdown of the thermally stable compounds (e.g., remaining kerogen, residues). A first synthesis of PS δ13C and δ2H isotopic compositions of propane from this study and the literature data suggests relatively similar isotopic structures of propane precursors in kerogens. This study demonstrates that PS isotope analysis of propane can contribute to identifying various geological (e.g., maturation, wet-gas cracking, H exchange, diffusion) and biodegradation processes.

Assessment of Compound-Specific Fatty Acid δ13C and δ2H Values to Track Fish Mobility in a Small Sub-alpine Catchment.

Methods for identifying origin, movement, and foraging areas of animals are essential for understanding ecosystem connectivity, nutrient flows, and other ecological processes. Telemetric methods can provide detailed spatial coverage but are limited to a minimum body size of specimen for tagging. In recent years, stable isotopes have been increasingly used to track animal migration by linking landscape isotope patterns into movement (isoscapes). However, compared to telemetric methods, the spatial resolution of bulk stable isotopes is low. Here, we examined a novel approach by evaluating the use of compound-specific hydrogen and carbon stable isotopes of fatty acids (δ2HFA and δ13CFA) from fish liver, muscle, brain, and eye tissues for identifying site specificity in a 254 km2 sub-alpine river catchment. We analyzed 208 fish (European bullhead, rainbow trout, and brown trout) collected in 2016 and 2018 at 15 different sites. δ13CFA values of these fish tissues correlated more among each other than those of δ2HFA values. Both δ2HFA and δ13CFA values showed tissue-dependent isotopic fractionation, while fish taxa had only small effects. The highest site specificity was for δ13CDHA values, while the δ2H isotopic difference between linoleic acid and alpha-linolenic acid resulted in the highest site specificity. Using linear discrimination analysis of FA isotope values, over 90% of fish could be assigned to their location of origin; however, the accuracy dropped to about 56% when isotope data from 2016 were used to predict the sites for samples collected in 2018, suggesting temporal shifts in site specificity of δ2HFA and δ13CFA. However, the predictive power of δ2HFA and δ13CFA over this time interval was still higher than site specificity of bulk tissue isotopes for a single time point. In summary, compound-specific isotope analysis of fatty acids may become a highly effective tool for assessing fine and large-scale movement and foraging areas of animals.

Tree organ growth and carbon allocation dynamics impact the magnitude and δ13C signal of stem and soil CO2 fluxes.

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and δ13C of shoot, stem and soil CO2 fluxes (Ashoot, Rstem and Rsoil), (ii) concentration and δ13C of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted Rstem. Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on Rsoil. Time series analysis between δ13C of Ashoot and δ13C of Rstem or δ13C of Rsoil revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between δ13C of Rstem and δ13C of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and δ13C analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.

Assessment and Implication of PAHs and Compound-Specific δ13C Compositions in a Dated Marine Sediment Core from Daya Bay, China.

PAHs in a sediment core covering ~120 years from Daya Bay in South China Sea were extracted using Soxhlet and high performance thin layer chromatography, and the compound-specific δ13C were analyzed using gas chromatography–combustion–isotopic ratio mass spectrometry. The concentrations of PAHs ranged from 99.3 to 676 ng g−1, with high molecular weight PAHs as a key component. PAHs’ compound-specific δ13C ranged from −35.02‰ to −16.14‰. The patterns of 16 PAHs, molecular ratios, and compound specific δ13C compositions indicate important pyrolytic and petrogenic sources: PAHs derived predominantly from pyrogenic sources (including coal and wood incomplete combustion) before the 1960s, while after the 1960s, they derived predominantly from mixed pyrogenic and petrogenic sources (including automotive exhaust emissions, oil spills, and coal and wood incomplete combustion). Our results can provide important insights into organic pollution emissions influenced by human activities and the urbanization of Daya Bay.

FRUITS of the sea? A cautionary tale regarding Bayesian modelling of palaeodiets using stable isotope data

A recent paper by Bownes et al. (2017, Radiocarbon 59(5): 1275–1294) used the Bayesian modelling software package FRUITS (Fernandes et al. 2014, PLoS ONE 9(2): e87436) to argue that Neolithic individuals from Carding Mill Bay on the west coast of Scotland obtained up to ca. 21% dietary protein from marine sources. This is in contrast to previous interpretations of the same isotopic (δ13C and δ15N) data, which concluded that these individuals showed little if any use of marine resources (Schulting and Richards 2002, European Journal of Archaeology 5(2): 147–189). Resolving this discrepancy is important for our understanding of the nature of the Mesolithic–Neolithic transition not only on the west coast of Scotland, but along all of Atlantic Europe, since similar isotopic results to those obtained at Carding Mill Bay are widespread throughout the Neolithic and indeed later periods. We suggest that greater caution needs to be excercised in the interpretation of the output of Bayesian palaeodietary modelling, which can be very useful heuristically, but should not always be taken at face value. Given the large number of parameters employed in such models (diet isotopic values and nutrient concentrations, trophic level and tissue fractionations, etc.) and the uncertainty involved in almost all of them, a wide range of outcomes are possible, as we demonstrate in this paper. We reaffirm the overwhelmingly terrestrial nature of diet at Carding Mill Bay both through FRUITS modelling and through the new application of compound-specific δ13C analysis of single amino acids of humans and fauna from the site.

Controls on the distributions of GDGTs and n-alkane isotopic compositions in sediments of the Amazon River Basin

Lipid biomarker proxies from terrigenous sediments have been extensively used to understand variations in paleoenvironmental conditions, but many of the mechanisms affecting these proxies during riverine transport are still poorly understood. Here, we analyze glycerol dialkyl glycerol tetraether (GDGT) distributions and n-alkane isotopic compositions of soils and sediments from the Amazon River Basin. Our dataset includes suspended sediments of the Amazon River and its main tributaries, as well as soils and sediments of the Xingu River, a large clearwater tributary draining the easternmost part of the Amazon River Basin. Our sampling design aimed at understanding the processes behind spatially distinct GDGT distributions and n-alkane isotopic signatures across lowland Amazonia. Gradual changes in the fractional abundances of isoprenoid GDGTs and in 5- and 6- methyl branched GDGT ratios in suspended sediments of the Amazon River towards its mouth suggest that riverine production is an increasingly important control on the distribution of GDGTs in the lower parts of the system, while values from the western parts are more in line with a dominant soil sourcing. In the Xingu River, indices based on the fractional abundances of branched GDGTs and long-chain n-alkanes demonstrate a strong contribution of terrestrial organic material during the high-water season and an important aquatic component during low-water season. Meanwhile, average stable carbon (δ13C) and hydrogen (δD) isotopic signatures of long-chain n-alkanes in soils, riverbed and suspended sediments of the Xingu River are similar and reinforce the relatively conservative behaviour of these proxies within large river systems. The average compound-specific δ13C signatures of sediments in the Xingu River are within the expected range for C3 vegetation and do not seem to capture the signals from the nearby deforested areas. n-Alkanes δD signals in the Xingu Basin are similar to values obtained in the Amazon River mouth and indicate that n-alkanes sourced from easternmost Amazonian lowlands may predominate over signals from western areas.

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.

Antibiotics as a silent driver of climate change? A case study investigating methane production in freshwater sediments

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2) and is inter alia produced in natural freshwater ecosystems. Given the rise in CH4 emissions from natural sources, researchers are investigating environmental factors and climate change feedbacks to explain this increment. Despite being omnipresent in freshwaters, knowledge on the influence of chemical stressors of anthropogenic origin (e.g., antibiotics) on methanogenesis is lacking. To address this knowledge gap, we incubated freshwater sediment under anaerobic conditions with a mixture of five antibiotics at four levels (from 0 to 5000 µg/L) for 42 days. Weekly measurements of CH4 and CO2 in the headspace, as well as their compound-specific δ13C, showed that the CH4 production rate was increased by up to 94% at 5000 µg/L and up to 29% at field-relevant concentrations (i.e., 50 µg/L). Metabarcoding of the archaeal and eubacterial 16S rRNA gene showed that effects of antibiotics on bacterial community level (i.e., species composition) may partially explain the observed differences in CH4 production rates. Despite the complications of transferring experimental CH4 production rates to realistic field conditions, the study indicated that chemical stressors contribute to the emissions of greenhouse gases by affecting the methanogenesis in freshwaters.

Benchmarking source specific isotopic ratios of levoglucosan to better constrain the contribution of domestic heating to the air pollution

We report source specific isotope ratios of levoglucosan, the specific biomass burning tracer, in aerosol particles from the combustion of selected woods used for domestic heating in Europe, of coals containing cellulose (lignites) as well as of corn, a C4 plant. Here, we combine compound specific δ13C measurements of levoglucosan with total carbon δ13C of parent materials, to assess isotopic fractionations due to biosynthetic pathways or combustion processes. Levoglucosan formed during the combustion of cellulose from coals shows with δ13C of −21.1‰ and −18.6‰ a moderate enrichment in the heavier isotope compared to the C3 plant samples. Contrarily, observed levoglucosan isotope ratios of −25.0 to −21.5‰ for C3 plant samples are significantly lower than for the C4 plant sample (−12.4‰), as expected from the stronger 13C discrimination during the carbon fixation process by C3 compared to C4 plants. Overall, the C4 plant sample shows a 13C enrichment in all bulk measurements, on average by 12.2‰, 14.2‰ and 14.2‰ for total carbon (TC) in aerosol particle, whole plant/coal material and cellulose samples, respectively. Further, δ13C measurements of levoglucosan and TC for biomass burning aerosol particles, bulk plant/coal and cellulose in C3 plant samples agree well with the published observations. The combined levoglucosan/TC isotopic analyses can be used to differentiate among C3/coal/C4 origin of the smoke emissions from the cellulose-containing-fuel combustion. Noticeably, there is a consistent δ13C offset between C3 plant material and levoglucosan, which allows deriving emission levoglucosan isotope ratios when the combusted plant types are known.