13C Depletion Research Articles

The methylation of aromatic nuclei — I: Implications for the geochemical evolution of gas

Aromatic hydrocarbons are an important component in crude oil and sediments. Furthermore, the structure of macromolecular organic matter (such as kerogen) is also based on aromatic nuclei. The earlier methylation and later demethylation of various aromatic nuclei would affect the chemical components and carbon isotopes of aromatics as well as saturated hydrocarbons and gaseous hydrocarbons. The scope of the present study was to investigate the role of aromatic nuclei in the formation and geochemical evolution of natural gas in complex geological systems. The use of model compounds could simplify the complex system, and n-tricosane (n-C23), n-tetracosane (n-C24), and naphthalene (N) were selected as model compounds for paraffins and aromatic hydrocarbons and subjected to pyrolysis experiments. The results showed that aromatic nuclei had a slight influence on the gas generation without using a catalyst. However, the gaseous components were substantially affected by aromatic nuclei under the catalysis of clay, when the temperature was lower than 400 °C. The ratios of isoparaffin to n-alkane of gaseous hydrocarbons were reduced by the aromatic nuclei. As carbon isotopically lighter methyls preferred to combine with the aromatic nuclei, the gas productions were reduced, and there was a 13C depletion of methane with increasing temperature, which is contrary to the case of paraffin cracking alone. This trend can also be observed in the pyrolysis gases formed from the type II and III kerogens. The macromolecular aromatic nuclei played a similar role with the small aromatic hydrocarbons. These effects would mainly occur in the mature to highly mature stage of the organic matter (the paleo-geothermal temperature is ≤ 150 °C). The above explorative experiments suggested that a large number of aromatic nuclei and their methylation reactions could exert a significant influence on the distributions and the carbon isotopic compositions of gaseous hydrocarbons in the evolution of sedimentary organic matter. This research also provided a new explanation for the carbon isotopic abnormality (the 13C-depleted methane and the increasing δ13C2-δ13C1 values) of deep natural gas of the Tarim Basin, in northwest China.

Characteristics of desorption gas of lacustrine shale in the Ordos Basin, China

Desorption gases of four lacustrine shale cores from the Triassic Yanchang formation of the Ordos Basin, China were collected, and their molecular and carbon isotope compositions were analyzed in this study. Results show that hydrocarbon gases of desorption gas are dominated by methane with relatively low dryness coefficient ranging from 0.45 to 0.82. The 513C values for hydrocarbon gases increase with increasing carbon number, and which for methane ranges from -57.15%« to -53.71%«. The hydrocarbon gases generally derived from a mixed origin with primary contribution from thermogenic processes. Desorption gases of sample YK-340.7 could experience biodegradation for its relatively high dryness coefficient and (iC4H10 + iC5H12) / (nC4H10 + nC5H12) value. A significant depletion of13C for methane was observed during desorption process, the reason for which could be differences in molecular mobility of the12CH4 and13CH4. Changes in the 513Cethane and 513Cpropane during desorption process are week.

Radiocarbon Calibration around AD 1900 from Scots Pine (Pinus Sylvestris) tree rings from Northern Norway

ABSTRACTTo resolve an inconsistency around AD 1895 between radiocarbon (14C) measurements on oak from the British Isles and Douglas fir and Sitka spruce from the Pacific Northwest, USA, we measured the 14C content in single-year tree rings from a Scots pine tree (Pinus sylvestris L.), which grew in a remote location in Saltdal, northern Norway. The dataset covers the period AD 1864–1937 and its results are in agreement with measurements from the US Pacific coast around 1895. The most likely explanation for older ages in British oak in this period seems to be 14C depletion associated with the combustion of fossil fuels.

Environmental stress and eutrophication in freshwater wetlands: evidence from carbon and nitrogen stable isotopes in cattail (Typha domingensis Pers.)

BackgroundFlorida’s Everglades is a vast freshwater peatland that has been impacted by the alterations of hydrological pattern and water quality which led to changes in plant species composition and biodiversity. In this study, carbon and nitrogen stable isotopes (δ13C and δ15N) in cattail (Typha domingensis) are evaluated as indicators of environmental changes in the Everglades wetlands along nutrient and hydrological gradients represented by reference sites with total phosphorus (TP) < 10 μg L−1, transition sites with TP ≥ 10 μg L−1 < 20 μg L−1 and impacted sites with TP ≥ 20 μg L−1 which differed in hydrology or habitats (marsh and canal).ResultsCattail δ13C values decreased significantly from reference (–24.1‰), transition (–26.8‰) to the impacted sites (–28.2‰). In contrast, δ15N values increased significantly from reference (–5.2‰), transition (2.4‰) to the impacted site (5.9‰). In response to a poor hydrological condition, cattail in marsh area displayed 13C enrichment (–26.0‰) and 15N depletion (0.2‰). By contrast, cattail grown in the canal sites with favorable hydropattern displayed 13C depletion (–27.6‰) and 15N enrichment (5.9‰) from the canal sites with more favorable hydrological condition.ConclusionsThe different patterns for the changes in δ13C and δ15N in cattail suggested that increased nutrients led to increased stomatal conductance and 13C fractionation during carbon uptake and decreased 15N fractionation with increasing nitrogen demand while poor hydrological condition coupled with low nutrients led to reduced plant growth indicated by higher δ13C and lower δ15N values. Findings from this study suggested that δ13C in emerged macrophytes such as cattail can be used as an indicator for environmental stress while δ15N is a robust indicator for wetland eutrophication.

Carbon and hydrogen isotope fractionation for methane from non-isothermal pyrolysis of oil in anhydrous and hydrothermal conditions

In this study, to ascertain the carbon and hydrogen isotope fractionation of oil cracking gas (secondary gas) in hydrothermal conditions, non-isothermal pyrolysis of oil with and without water was carried out by a gold-tube system. By determination of the yields of individual gas products, it is found that the presence of water enhanced the yields of hydrocarbon gases. However, kinetic calculations indicate that Ea for the generation of methane and C2–5 in pyrolysis in hydrothermal conditions are essentially identical with those in anhydrous pyrolysis. The yields of carbon dioxide (CO2) and alkene gases in pyrolysis in hydrothermal conditions are evidently higher than those in anhydrous pyrolysis. It is reasonable that water–hydrocarbon reactions occurred and contributed to the generation of secondary gas in hydrothermal conditions. Meanwhile, the presence of water resulted in a slight depletion of 13C for methane and an evident depletion of 13C for CO2. Thermodynamic calculations suggest that water–hydrocarbon reactions in non-isothermal pyrolysis are dominated by free radical mechanism rather than ionic mechanism. Moreover, δ2H values of methane are apparently different in pyrolysis involving water with different δ2H. This result demonstrates that water provided hydrogen for hydrocarbon gas generation. Finally, we established mathematical models based on isotope fractionation to quantitatively determine the contribution of water–hydrocarbon reactions for gas generation in both experimental and geological conditions.

Different particle sources in a bivalve species of a coastal lagoon: evidence from stable isotopes, fatty acids, and compound-specific stable isotopes

The trophic fate of various food sources is of central interest for ecologists, yet not well understood in coastal lagoon food webs. In this field study, fatty acids (FA), stable isotopes (SI), and compound-specific isotopic analysis (CSIA) on FA were used to investigate how diets from oceanic and local sources are retained in a bivalve species (Spondylus crassisquama; Lamarck 1819) along a transect in the Ojo de Liebre lagoon (BCS, Mexico). Results from SI and FA indicated the contribution of oceanic diatoms at the entrance of the lagoon, through 15N enrichment, and higher proportions of 16:1n-7 and 20:5n-3 in digestive glands. In the inner bay, higher abundance of 18-carbon FA (18:1n-9, 18:3n-3, 18:4n-3) suggested a higher contribution of microheterotrophs, including (dino)flagellates and ciliates, to the diet of this bivalve derived from local production. Significant spatial differences for the δ13C of FA highlighted changes in the origin of food sources. Indeed, a 13C depletion was observed in the δ13C of heterotrophic flagellates biomarkers in individuals from the innermost station, revealing that their origin in the diet of bivalves differs within the lagoon, highlighting the importance of local processes (sediment resuspension, remineralization) in the trophic functioning of the lagoon. The δ13C values of FA considered as diatoms biomarkers (16:1n-7 and 20:5n-3) were consistent, which suggests that diatoms assimilated have very similar origins throughout the lagoon. The complementary of the tracers used here allowed for a better understanding of the trophic functioning of this coastal lagoon submitted to oceanic influences.

Parasites and stable isotopes: a comparative analysis of isotopic discrimination in parasitic trophic interactions

Stable isotopes are widely used to identify trophic interactions and to determine trophic positions of organisms in food webs. Comparative studies have provided general insights into the variation in isotopic composition between consumers and their diet (discrimination factors) in predator–prey and herbivore–plant relationships while other major components of food webs such as host–parasite interactions have been largely overlooked. In this study, we conducted a literature‐based comparative analysis using phylogenetically‐controlled mixed effects models, accounting for both parasite and host phylogenies, to investigate patterns and potential drivers in Δ13C and Δ15N discrimination factors in metazoan parasitic trophic interactions. Our analysis of 101 parasite–host pairs revealed a large range in Δ13C (–8.2 to 6.5) and Δ15N (–6.7 to 9.0) among parasite species, with no significant overall depletion or enrichment of 13C and 15N in parasites. As previously found in other trophic interactions, we identified a scaling relationship between the host isotopic value and both discrimination factors with Δ13C and Δ15N decreasing with increasing host δ13C and δ15N, respectively. Furthermore, parasite phylogenetic history explained a large fraction (&gt;60%) of the observed variation in the Δ15N discrimination factor. Our findings suggest that the traditional isotope ecology framework (using an average Δ15N of 3.4‰) applies poorly to parasitic trophic interactions. They further indicate the need for a scaled rather than a fixed trophic discrimination factor framework along gradients of host δ15N. We also identified several conceptual and methodological issues which should to be considered in future research to help integrate parasitic interactions into a holistic isotope ecology framework across diverse trophic interactions.

δ13C stratigraphy of a Turinian-Chatfieldian (Upper Ordovician) foreland succession, Ottawa Embayment (central Canada): resolving local and inter-regional isotope excursions in a tectonically active basin

Positive δ13Ccarb excursions are correlated through an upper Turinian to lower Chatfieldian carbonate-platform succession along the axis of the Ottawa Embayment and into outliers of the northern Ottawa-Bonnechere graben in central Canada. Successive Turinian excursions (E1 and E2) are lithostratigraphically constrained by erosional surfaces and hosted within the Watertown and overlying L'Orignal formations, respectively, the latter coeval with the Selby Formation in the adjacent northern Appalachian Basin. The excursions coincide with periods of regional transgression, but geographic patterns of 13C depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions, respectively. Differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas, with preferential Corg recycling with stratigraphic condensation. By early Chatfieldian time, segmentation of the once regional carbonate platform (L'Orignal Formation) produced a regional mosaic of low-energy muddy carbonate banks (Rockland Formation) and a deeper water platform (lower Hull Formation) settings subject to fluctuating high to low energy current flow. Excursion E3 occurs in both successions, but 13C enrichment is associated only with the bank-top muddy facies. This may identify preferential photosynthetic drawdown of 12C across the bank tops due to limited seawater exchange across the bank-deeper platform boundaries. Excursions E1 to E3, and a younger excursion (E4) in the Hull Formation, are correlated with varying confidence with excursions across southern Laurentia, excursion E3 being the local expression of the Guttenberg δ13C excursion. Our study supports local modulation of regional, if not global, δ13C excursions arising from structurally controlled changes in oceanography and productivity.

Variation of isotope composition in kelps from Kongsfjorden (Svalbard)

Kelp-derived detritus is a potential food item at the base level of marine food webs. A good knowledge of the factors that influence stable isotope composition in kelps is essential for reliable food web models. The variation of stable isotope signatures was analysed in three species of kelp in Kongsfjorden, Svalbard. Between 15 and 2.5 m depth, towards higher photon fluence rate, Alaria esculenta was gradually enriched in 13C while 15N did not change. The carbon/nitrogen ratio decreased with depth. 2- and 3-year-old kelp individuals had significantly higher δ15N values in their blades than 5- and 6-year-old ones. C/N ratio in blades remained statistically the same over the range of 2–6 years old plants. A two-way ANOVA did not show any interactive effects between depth and age group. A. esculenta as well as Saccharina latissima and Laminaria digitata exhibited enrichment in heavy carbon isotopes in the blades compared to the stipes, while within each species, δ15N values remained the same between blades and stipes. A more detailed analysis of kelp blades showed that independent of location in the fjord, year or season, and species, young tissue was more enriched in 13C than meristem and that decaying apical tissue was depleted relative to young tissue. The degree of this 13C depletion varied with kelp species. Meristem tissue appeared most uniform in the blades compared to young and decaying tissues and its δ13C signature was characteristic for each species with no significant difference in δ15N values. Decaying tissue showed no difference in δ13C between species. Carbon and nitrogen contents along the blades were species-specific and varied with growing location and year or season. Some advice is given on the temporal, spatial, and tissue-related choice of samples for different research objectives.

A sensitive λ 3 mm line survey of L483: A broad view of the chemical composition of a core around a Class 0 object.

An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had their molecular composition characterized in detail. Here we present a λ 3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30m telescope in the 80-116 GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines (FWHM ~0.5 km s-1) and low rotational temperatures (≲10 K). Of particular interest among the detected molecules are the cis isomer of HCOOH, the complex organic molecules HCOOCH3, CH3OCH3, and C2H5OH, a wide variety of carbon chains, nitrogen oxides like N2O, and saturated molecules like CH3SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH+, CNCN, NCO, H2NCO+, and NS+) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293-2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C3S, SO2/C2S, and CH3SH/C2S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving 34S and 33S), while there are also several isotopic anomalies like an extreme depletion in 13C for one of the two isotopologs of c-C3H2, a drastic enrichment in 18O for SO and HNCO (SO being also largely enriched in 17O), and different abundances for the two 13C substituted species of C2H and the two 15N substituted species of N2H+. We report the first detection in space of some minor isotopologs like c-C3D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.

Influence of lipids on stable isotope ratios in mammal hair: highlighting the importance of validation

AbstractCarbon (δ13C) and nitrogen (δ15N) stable isotope ratios are increasingly used in ecological studies to evaluate diet composition and trophic relationships. However, lipids may influence stable isotope ratios due to the depletion of 13C in adipose tissues relative to proteins and carbohydrates. δ13C values can be corrected by lipid extraction or normalization models. The aims of our study were to evaluate the effects of lipid extraction on stable isotope ratios in a terrestrial mammal, the caribou (Rangifer tarandus caribou), and to propose relevant lipid normalization models that are method‐ and tissue‐specific for δ13C values. We also evaluated whether four δ13C lipid normalization and correction models proposed in the literature were applicable to our study species. Stable isotope ratios were obtained for hair, plasma, and red blood cell samples of 44 caribou in the Gaspésie National Park (Québec, Canada). The effects of lipid extraction on stable isotope ratios were tested using a paired t‐test. A simple linear model was used to correct for the effects of lipid extraction and to assess its performance compared to that of published equations. Lipid content significantly influenced δ13C values in caribou hair. The four lipid normalization equations commonly used in the literature did not accurately predict δ13Clipid‐free values of caribou hair. Based on our results, we recommend controlling systematically for lipids in terrestrial systems and analyzing δ13C (lipid‐free) and δ15N (bulk) from two separate aliquots to reach a greater precision. We also recommend controlling for lipids in hair tissue. If not possible, we recommend using a lipid normalization model that is tissue‐, method‐, and species‐specific or applying a model that has been previously validated for the tissue and species of interest.

Reconstruction of Atlantic herring (Clupea harengus) recruitment in the North Sea for the past 455 years based on the δ13C from annual shell increments of the ocean quahog (Arctica islandica)

AbstractUnderstanding the recruitment variability of the Atlantic herring North Sea stock remains a key objective of stock assessment and management. Although many efforts have been undertaken linking climatic and stock dynamic factors to herring recruitment, no major attempt has been made to estimate recruitment levels before the 20th century. Here, we present a novel annually resolved, absolutely dated herring recruitment reconstruction, derived from stable carbon isotope geochemistry (δ13C), from ocean quahog shells from the Fladen Ground (northern North Sea). Our age model is based on a growth increment chronology obtained from fourteen shells. Ten of these were micromilled at annual resolution for δ13C analysis. Our results indicate that the anthropogenically driven relative depletion of 13C, the oceanic Suess effect (oSE), became evident in the northern North Sea in the 1850s. We calculated a regression line between the oSE‐detrended δ13C results (δ13CṠ) and diatom abundance in the North Sea, the regression being mediated by the effect of phytoplankton on the δ13C of the ambient dissolved inorganic carbon. We used this regression to build an equation mediated by a nutritional link to reconstruct herring recruitment using δ13CṠ. The reconstruction suggests that there were five extended episodes of low‐recruitment levels before the 20th century. These results are supported by measured recruitment estimates and historical fish catch and export documentation. This work demonstrates that molluscan sclerochronological records can contribute to the investigation of ecological baselines and ecosystem functioning impacted by anthropogenic activity with implications for conservation and stock management.

Flipping for Food: The Use of a Methane Seep by Tanner Crabs (Chionoecetes tanneri)

Methane seep habitats are widespread chemosynthesis-based ecosystem that span continental margins and interact with surrounding marine systems. With many of these habitats occurring below 200m, seeps can serve as an important source of nutrients in otherwise food limited deep-sea environments. However, the potential for marauding megafauna to assimilate seep derived nutrition has been difficult to quantify. Here, we provide the first evidence of a commercially harvested species, Chionoecetes tanneri (tanner crab), assimilating chemosynthetic production. Although bulk isotope analysis of C. tanneri tissue indicated no quantifiable incorporation of seep-derived carbon or sulfur (mean δ13C, -18.5‰; mean δ34S, 19.5‰), depletions in 13C (δ13C as light as -38.8 ‰) were noted in fatty acid (FA) compounds. In addition, diagnostic biomarkers for seep bacteria, including 16:16 and 18:18c FA’s, were found to have been assimilated by C. tanneri. Futher supporting a trophic link between the seep and the C. tanneri, seep associated bacteria and archaea were, in certain cases, the dominant taxa in the gut contents of the crab. This work provides the first insights into a link between seep production and deep-sea ecosystem services, specifically fisheries production. In addition, it reveals a methodological bias that could exist in some trophic studies where bulk isotopes under-represent the role of seep nutrition in the diet of marauding animals.

Lipid Biomarkers and Their Stable Carbon Isotopes in Ancient Seep Carbonates from SW Taiwan, China

AbstractFour massive brecciated, chimney‐like, and slender pipe network carbonate samples (JA‐4, JA‐5, JX‐8 and BG‐12) were collected from southwestern Taiwan, which were suggested to have formed as a result of anaerobic oxidization of methane (AOM). Considering that the environmental conditions of the carbonates precipitation and the sources of carbon and organic matter need to be further declared, molecular fossils and compound‐specific carbon isotopic investigations of the carbonates were conducted in this study. According to lipid biomarkers of 2,6,10,15,19‐pentamethyleicosane (PMI) and squalane diagnostic to methanotrophic archaea, as well as the extremely low δ13C values (as low as –113.4‰) detected in samples JA‐4, JA‐5 and JX‐8, these carbonates were revealed to be a result of AOM. Based on the varied δ13C values of characteristic archaea biomarkers in specific samples, biogenic methane was proposed to be responsible for the formation of samples JA‐4 and JA‐5, whereas a mixed carbon source of 13C‐depleted methane and 13C‐enriched residual CO2 from methanogenesis was suggested for the carbonate of JX‐8 due to the co‐occurrence of a highly positive δ13Ccarb value (+8‰) and a moderate 13C depletion of PMI. The low content of AOM‐related biomarkers and the absence of indicators for ANME‐2 suggested that these carbonates were formed in weak seep settings. By comparison, no typical lipid biomarkers for methanotrophic archaea was detected in carbonate BG‐12. The short‐chain and long‐chain n‐alkanes accounted for 30% and 45% of all hydrocarbons, respectively, with a CPI value of 1.2, suggesting that the n‐alkanes were derived from both marine organisms and terrestrial inputs. A low thermal maturity could be revealed by the incomplete equilibrium value of the C31αβ 22S/(22S+22R) ratio (0.5), and the carbonate BG‐12 was probably deposited in a suboxic condition indicated by a value of Pr/Ph ratio (2.5).

Stable carbon isotopic composition of dissolved inorganic carbon (DIC) as a driving factor of aquatic plants organic matter build-up related to salinity

Stable isotopes probing is among the most used methods applied in the studies of paleoenvironment. Isotope signatures of sediments are influenced by different environmental conditions such as climate, salinity, and plant coverage at the time those sediments were deposited, representing an inestimable value for paleoenvironmental reconstruction and interpretation. In this study the carbon stable isotopic composition of organic matter (δ13CORG) and lignin monophenols (δ13CVSC) extracted from this organic matter of submerged vascular plant Stuckenia pectinata (L.) Böerner 1912 syn. Potamogeton pectinatus L 1753 was investigated. Samples were collected from five different environments along a gradient of salinity from brackish Baltic Sea and freshwater lake in Estonia. The salinity influenced δ13CDIC of ambient water, in the freshwater 13C depletion was observed compare to brackish sites. S. pectinata have an intraspecific variation of the carbon stable isotope signature of organic matter depending on the environmental conditions, samples were 13C enriched in the more brackish sites. Strong positive correlation between δ13CORG and C/N ratio for both leaves and stems were found. Lignin monophenols were 13C depleted if compared with bulk organic matter, reflecting a common biochemical pattern of C fixation during lignin synthesis as for terrestrial plants. To conclude, organic matter isotopic signature and lignin composition of aquatic plants responded to environmental salinity. The innovative combination of these analyses provides a more refined interpretation of δ13CORG recorded in sediment deposited in the zones colonized by those plants.

Sources of carbon to suspended particulate organic matter in the northern Gulf of Mexico

Suspended particulate organic carbon (POCsusp) in the Gulf of Mexico is unique compared to other seas and oceans. In addition to surface primary production, isotopic analysis indicates that microbial cycling of oil and riverine inputs are primary sources of carbon to POCsusp in the Gulf. To characterize POCsusp from seep sites and non-seep north central Gulf (NCG) sites potentially affected by the Deepwater Horizon (DWH) spill, we analyzed 277 and 123 samples for δ13C and Δ14C signatures, respectively. Depth, partitioned into euphotic (&amp;lt;300 m) and deep (&amp;gt;300 m), was the main driver of spatial δ13C differences, with deep depths exhibiting 13C depletion. Both deep depths and proximity to sources of natural seepage resulted in 14C depletion. A two-endmember mixing model based on Δ14C indicated that sources to POCsusp were 14–29% fossil carbon at NCG sites and 19–57% at seep sites, with the balance being modern surface production. A six-component Bayesian mixing model MixSIAR, using both 13C and 14C, suggested that riverine inputs were an important carbon source to POCsusp contributing 34–46%. The influence of seeps was localized. Below the euphotic zone at seep sites, 46 ± 5% (n = 9) of the carbon in POCsusp was derived from environmentally degraded, transformed oil; away from seeps, transformed oil contributed 15 ± 4% (n = 39). We hypothesized that, at NCG sites removed from hydrocarbon seep sources, isotopic signatures would be depleted following the spill and then shift towards background-like enriched values over time. At deep depths we observed decreasing Δ14C signatures in POCsusp from 2010 to 2012, followed by isotopic enrichment from 2012 to 2014 and a subsequent recovery rate of 159‰ per year, consistent with this hypothesis and with biodegraded material from DWH hydrocarbons contributing to POCsusp.

Authigenic carbonate formation revealed by lipid biomarker inventory at hydrocarbon seeps: A case study from the Okinawa Trough

Abstract Authigenic carbonates were recovered from the northern Okinawa Trough at 540–700 m water depth. Development of microbial communities and seepage dynamics driving the precipitation of authigenic carbonates remains poorly constrained, even though the source of methane-rich fluids, formation of Fe-rich carbonates, and the potential driving forces were previously reported. Here, petrologic observations, stable carbon and oxygen isotopic compositions, mineralogy, and lipid inventories of authigenic carbonates from the northern Okinawa Trough were analyzed. The carbonate minerals were comprised predominantly of aragonite, high-magnesium calcite, and siderite. The presence of molecular fossils diagnostic for anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with pronounced 13C depletions (as low as −134‰), together with highly negative δ13Ccarb values (as low as −55.8‰), suggest that the carbonates precipitated from anaerobic oxidation of methane (AOM) with microbial methane as the predominant carbon source. The difference between measured and calculated δ18Ocarb values (based on 0‰ of sea water δ18O vs V-SMOW) was higher than +3‰ in one of the carbonates (GGD16), reflecting an origin from gas hydrate dissociation. Most of the carbonates revealed δ18O offsets between measured and calculated values within +3‰, or a lower measured δ18O than calculated value, suggesting a mixture of methane-derived carbonates (low δ13C/high δ18O) and detrital origin carbonates (high δ13C/low δ18O). Biomarker patterns varied significantly among samples. A suite of 13C-depleted biomarkers indicated the predominance of an ANME-1 assemblage for all samples, suggesting carbonates formed at low to medium methane flux. The predominance of ANME-1, abundant detrital minerals, and allochthonous biomarkers, as well as low carbonate contents indicate that most samples formed at greater depth within the sediment column. Only one carbonate sample in this study is interpreted to have precipitated at relatively shallower depth, as indicated by the occurrence of crocetane, dominance of aragonite, and higher overall carbonate content.

Lipid biomarker patterns reflect different formation environments of mussel- and tubeworm-dominated seep carbonates from the Gulf of Mexico (Atwater Valley and Green Canyon)

Mussels and tubeworms thriving at many methane seeps typically live in symbiosis with chemosynthetic, chiefly methanotrophic or thiotrophic bacteria. It has been shown that the activities of chemosymbiotic animals can result in large differences in the sedimentary environments of their habitats. Here, we put forward the concept that such environmental variability can be archived in the lipid biomarker inventories of authigenic carbonates forming in different, locally confined environments at seeps, mussel beds and tubeworm bushes in this case. To test this hypothesis, lipid biomarker patterns of carbonates from mussel and tubeworm environments from two seep sites (Atwater Valley 340 and Green Canyon 852) of the Gulf of Mexico were analyzed. Previous work revealed stronger carbon isotope fractionation between the methane source and biomarkers of anaerobic methane oxidizing archaea-2 (ANME-2)/sulfate-reducing Desulfosarcina/Desulfococcus (DSS) consortia than for ANME-1/DSS consortia, both performing anaerobic oxidation of methane (AOM). Similar δ13Cmethane values were found at the mussel and tubeworm sites from the same seeps and the local microbial consortia also appear to be largely similar based on the observed AOM biomarker inventories. Yet, a large average offset of 32‰ between the δ13C values of molecular fossils of sulfate-reducing bacteria (SRB) involved in AOM was observed, with lower values typifying tubeworm carbonates than mussel carbonates. This pattern is interpreted to reflect local effects on isotope fractionation caused by the chemosymbiotic metazoans at mussel- and tubeworm-dominated sites. At tubeworm-dominated sites, the excess sulfate produced by thiotrophic symbionts of tubeworms and pumped down into the sediment results in persistent production of AOM-derived bicarbonate and the enrichment of 12C in sub-surface sediments. Interestingly, tubeworm carbonates also contain high amounts of non-isoprenoidal dialkyl glycerol diethers (DAGEs) with extreme 13C depletions, representing compounds that derived from non-DSS cluster SRB. Most likely, 13C-depleted AOM-derived organic intermediates were used as carbon sources by the DAGE-producing non-DSS cluster SRB, possibly performing organoclastic sulfate reduction. Our study identifies significant variation in biomarker patterns between mussel and tubeworm carbonates at two seep sites in the Gulf of Mexico. Such variation allows to characterize different habitats at seeps, which are shaped by the interaction of chemosymbiotic seep metazoans and their symbionts with the local environment. Metazoan community composition apparently controls geobiological interaction in seep ecosystems to a large degree, which may allow tracing of the effects of chemosymbiosis into the rock record.

Determination of in situ biodegradation rates via a novel high resolution isotopic approach in contaminated sediments

A key challenge in conceptual models for contaminated sites is identification of the multiplicity of processes controlling contaminant concentrations and distribution as well as quantification of the rates at which such processes occur. Conventional protocol for calculating biodegradation rates can lead to overestimation by attributing concentration decreases to degradation alone. This study reports a novel approach of assessing in situ biodegradation rates of monochlorobenzene (MCB) and benzene in contaminated sediments. Passive diffusion samplers allowing cm-scale vertical resolution across the sediment-water interface were coupled with measurements of concentrations and stable carbon isotope signatures to identify zones of active biodegradation of both compounds. Large isotopic enrichment trends in 13C were observed for MCB (1.9–5.7‰), with correlated isotopic depletion in 13C for benzene (1.0–7.0‰), consistent with expected isotope signatures for substrate and daughter product produced by in situ biodegradation. Importantly in the uppermost sediments, benzene too showed a pronounced 13C enrichment trend of up to 2.2‰, providing definitive evidence for simultaneous degradation as well as production of benzene. The hydrogeological concept of representative elementary volume was applied to CSIA data for the first time and identified a critical zone of 10–15 cm with highest biodegradation potential in the sediments. Using both stable isotope-derived rate calculations and numerical modeling, we show that MCB degraded at a slower rate (0.1–1.4 yr−1 and 0.2–3.2 yr−1, respectively) than benzene (3.3–84.0 yr−1) within the most biologically active zone of the sediment, contributing to detoxification.

Relationships between grain size and organic carbon 14C heterogeneity in continental margin sediments

The deposition and long-term burial of sedimentary organic matter (OM) on continental margins comprises a fundamental component of the global carbon cycle. A key unknown in interpretation of carbon isotope records of sedimentary OM is the extent to which OM accumulating in continental shelf and slope sediments is influenced by dispersal and redistribution processes. Here, we present results from an extensive survey of organic carbon (OC) characteristics of grain size fractions (ranging from <20 to 250 μm) retrieved from Chinese marginal sea surface sediments in order to assess the extent to which the abundance and isotope composition of OM in shallow shelf seas is influenced by hydrodynamic processes. Our findings show that contrasting relationships exist between 14C contents of OC and grain size in surface sediments associated with two different hydrodynamic modes, suggesting that transport pathways and mechanisms imparted by the different hydrodynamic conditions exert a strong influence on 14C contents of OM in continental shelf sediments. In deeper regions and erosional areas, we infer that bedload transport exerts the strongest influence on (decreases) OC 14C contents of the coarser fraction, while resuspension processes induce OC 14C depletion of intermediate grain size fractions in shallow inner-shelf settings. We use the inter-fraction spread in Δ14C values, defined here as H14, to argue that the hydrodynamic processes amplify overall 14C heterogeneity within corresponding bulk sediment samples. The magnitude and footprint of this heterogeneity carries implications for our understanding of carbon cycling in shallow marginal seas.