Carbon Isotopic Composition Research Articles

Geochemical Characteristics and Origin of Natural Gas in the Middle of Shuntuoguole Low Uplift, Tarim Basin: Evidence from Natural Gas Composition and Isotopes

Multiple types of reservoirs, including volatile oil reservoirs, condensate gas reservoirs, and dry gas reservoirs, have been discovered in ultra-deep layers buried at depths greater than 7500 m. Understanding the genetic types of natural gas is of utmost importance in evaluating oil and gas exploration potential. The cumulative proved reserves of the super deep layer in the Shuntuoguole low uplift area of the Tarim Basin exceed 1 × 108 t (oil equivalent). The origin, source, and accumulation characteristics of natural gas still remain a subject of controversy. By analyzing the composition and carbon isotope of natural gas, a detailed investigation was conducted to examine the unique geochemical and reservoir formation characteristics of the Ordovician ultra-deep natural gas within different fault zones in the middle region of the Shuntuoguole low uplift. It was determined that most of the natural gas in this area is displaying a characteristic of wet gas with a drying coefficient ranging from 0.41 to 0.99. The carbon isotope composition of methane in the gas reservoir shows relatively light values, ranging from −49.4‰ to −42‰. The carbon and hydrogen isotopes of the components are distributed in a positive order. The natural gas is oil type gas, which is derived from marine sapropelic organic matter and has a good correspondence with the lower Yuertusi formation. The maturity of natural gas in Shunbei No. 1 and No. 5 fault zones is about 1.0%, which is the associated gas of normal crude oil, while the maturity of No. 4 and No. 8 fault zones is higher than 1.0%, which is the mixture of kerogen pyrolysis gas and crude oil pyrolysis gas. The variations in the drying coefficient and carbon isotope composition of the natural gas provide evidence for the migration patterns within the Shuntuoguole low uplift central region. It indicates that the Shunbei No. 5 and No. 8 fault zones have likely migrated from south to north, while the No. 4 fault zone has migrated from the middle to both the north and south sides. These migration patterns are primarily controlled by high and steep strike-slip faults, which facilitate the vertical migration of natural gas along fault planes. Consequently, the gas accumulates in fractured and vuggy reservoirs within the Ordovician formation.

Helium and carbon isotope compositions of thermal fluids in the northeastern Anatolia: Implications for the heat source and volatile flux

In this study, we make a quantitative assessment on the volatile flux of mantle-derived fluids and source of volatiles to explain the geologic controls on the transport of volatiles within thermal fluids of northeastern Anatolia. In line with this objective, we collected 22 samples (gas and water phase) from 16 geothermal fields in NE Turkey covering an area of nearly 100,000 km2 that extends from the eastern termination of North Anatolian Fault towards the Georgian and Armenian borders. The 3He/4He ratios of the samples (R) normalized to the atmospheric 3He/4He ratio (Ra = 1.4 × 10−6) vary from 0.31 to 7.15, and are considerably higher than the crustal value of 0.02Ra. Regarding spatial distribution of helium isotope composition, samples collected from areas of tectonic unrest around the Erzincan and Erzurum pull-apart basins in the southern part are represented by a higher range of 3He/4He ratios (>5 Ra) than those in volcanic areas to the east and northeast of the region. δ13CCO2 values of the gas samples varying from −20.76 to 5.43 ‰ are about 4–5 ‰ lower than δ13CDIC values of the water samples that range from −16.90 to 8.85 ‰, indicating CO2 removal from waters by degassing. CO2/3He ratios of gas samples falling in the range of 3.81 × 109 to 2.83 × 1012 imply that carbon is derived from the mixing between the crustal lithologies and mantle, the latter having a contribution up to 40 %. The maximum flux of mantle-derived fluids is found 88 mm/a at Erzincan (eastern part of the North Anatolian Fault Zone). This value is higher by a factor of about 10 than the western part of the fault zone. Calculations show that degassing from magmatic bodies is the sole mechanism to explain the high helium isotope compositions in the region. Therefore, we suggest that mantle-derived He contents might be due to extensive melting associated with active tectonism. The 3He/Heat ratios of thermal waters in northeastern Turkey were calculated in range of 0.25 to 29.7 × 10−15 cm3(STP)/J, consistent with the data reported for waters along the North Anatolian Fault, indicating a crustal heat source for the geothermal systems in northeastern Turkey.

A multiproxy approach for the feeding ecology of Late Miocene ground sloths from the southwestern Amazonia, Upper Solimões Formation, Brazil

A rich assemblage of fossil sloths is recognized in the southwestern Brazilian Amazon, Upper Solimões Formation (Acre Basin), and represents some of the most diverse Folivora fauna from the Late Miocene of tropical South America. Here, we have used a multiproxy approach using carbon (δ13Csc) and oxygen (δ18Osc) isotope compositions of bioapatite structural carbonate, stereomicrowear, ecomorphological indices (relative muzzle width, RMW and occlusal surface area, OSA), and body mass estimates in order to suggest the habitats and diet of the mylodontids (Octodontobradys puruensis, Urumacotherium campbelli, Pseudoprepotherium venezuelanum and Mylodontinae gen. et. sp. nov.) and the nothrotheriid (cf. Mionothropus) ground sloths from localities along the banks at the Purus and Acre rivers in Amazonas and Acre states. The isotopic results (δ13Csc = −13.2 ± 1.2 ‰; δ18Osc = +24.7 ± 0.8 ‰) indicated a consistently C3 plant diet in open-canopy woodland and closed-canopy forest habitats during warm conditions across the Tortonian ages (∼10 and 8.5 Myr). U. campbelli (Urumacotherium sp., bm: 605 kg; Niterói Site) and cf. Mionothropus (bm: 163 kg; Acre 5 Site) possibly had specialized browsing habits (folivorous/frugivorous), due to the few coarse scars in relatively polished tooth surfaces and elongate skull with narrow muzzle, respectively. Both taxa, as also P. venezuelanum (bm: 733 kg; Niterói Site), showed a low OSA values suggesting limited oral food processing offset by higher fermentation capacity and/or lower metabolic requirements. Efficient capacity for oral food processing and a generalized browsing habit with consumption of high intrinsic toughness and abrasiveness items were inferred for O. puruensis (bm: 526 kg; Talismã Site), resembling the pattern observed for the Pleistocene megathere Eremotherium laurillardi from the southwestern Amazon (Acre State). A non-selective browser diet was also supported for the Mylodontinae gen. et. sp. nov. specimen from Cavalcante Site; its similar dentary morphology to the bulk-feeder taxa implies that this individual could occasionally eat C3 herbaceous/grasses.

Source apportionment of organic carbon and nitrogen in sediments from river and lake in the highly urbanized Changjiang Delta

While the impact of human activities on organic matter pollution is recognized, how these impacts vary seasonally in the Changjiang Delta needs further investigation. This study addresses this gap by investigating seasonal variations in organic matter sources and ecological responses to human activities in Changjiang Delta sediments. Total organic carbon (TOC), total nitrogen (TN), and carbon (δ13C) and nitrogen (δ15N) isotopic compositions of surface sediments collected from the Taipu River and Dalian Lake wetland were analyzed. Both water bodies exhibited similar seasonal trends for TOC and TN, with the Taipu River containing an average of 0.46% and 0.03% higher concentrations of TOC and TN, respectively, compared to Dalian Lake. Additionally, the organic index (OI) and organic nitrogen (ON) index were elevated in both water bodies during the wet season. Sediments from Dalian Lake remained uncontaminated to moderately contaminated, while those from the Taipu River were generally classified as moderately to heavily contaminated. Stable isotope analysis identified terrestrial C3 plants (averaging 25.5%), C4 plants (averaging 16.0%), and municipal wastewater (averaging 16.0%) as the main contributors to organic matter in the sediments. These findings suggest that terrestrial plant material and municipal wastewater are key targets for managing organic matter contamination in the Changjiang Delta. Implementing strategic land-use planning and targeted interventions to minimize these inputs can significantly improve water quality and ecosystem health.

Thermally induced release and generation of CO2 and C1–C4 hydrocarbons in Opalinus Clay from Mont Terri (Switzerland)

Claystone formations are candidate host rocks for high-level heat-emitting nuclear waste (HLW). Temperatures from 90 to 150 °C at the canister surface are discussed internationally as potential emplacement and storage conditions. The thermal energy emitted from waste containers will be transported into the host rock formation, accelerating chemical reactions including the release of sorbed and dissolved gases and the generation of new gases. This study investigated gas release and generation in Opalinus Clay from Mont Terri (Switzerland) at elevated temperature and pressure conditions relevant for HLW storage and beyond. Hydrous pyrolysis experiments were conducted in Dickson-type flexible gold-titanium reaction cells and gold capsules in the temperature range of 80–345 °C and at 20 MPa. CO2(g) was the predominant product, followed by C1–C4 hydrocarbons, which decrease in abundance with increasing carbon atom number. Neither CO nor H2S was detected. H2 was generated only in high temperature experiments at 315 °C and 345 °C, respectively. A combination of CO2(g) quantification, stable carbon isotopic composition data, thermodynamic calculations and aqueous fluid composition (dissolved ions, pH) demonstrated that ≥80 % of the measured CO2(g) originated from carbonate mineral dissolution. The model calculations also suggest that the fraction of CO2(aq) in DIC increases from ∼50 % at 80 °C to nearly 100 % at higher temperatures. Thermal transformation of organic matter represented an additional source for CO2(g) and was the predominant process yielding the C1–C4 hydrocarbons. Our findings stress the importance of quantitative geochemical data for the safety assessment of potential host rocks for HLW storage. We demonstrated that two sources are involved in gas release and generation at temperatures relevant for HLW storage, e.g., in the Opalinus Clay – organic matter and carbonate minerals. Our data will contribute to numerical modelling studies and the refinement of feature, events, and processes (FEP) catalogues.

End-Member Mixing Model for Methane Carbon Isotope Fractionation During Shale Gas Desorption and Its Application.

Unlike conventional natural gas reservoirs, shale gas development involves systematic changes in methane carbon isotopes that cannot be effectively described by existing isotope fractionation models and mechanisms. Therefore, based on fundamental theories such as Rayleigh fractionation, mass transfer flow, and mass conservation, this study established isotopic fractionation equations for methane in adsorbed and free gas. By considering adsorbed and free gases as two end-members and using an isotope mixing model, a fractionation model for methane carbon isotopes during shale gas desorption was constructed. This model quantifies the isotopic fractionation effects during shale gas desorption and elucidates the mechanism of methane carbon isotope fractionation. Using on-site desorbed gas content and isotope data, parameter fitting and model calculations were conducted to characterize methane carbon isotope variations throughout the process of shale core field desorption. The results show a pattern of "initially negative and then turning positive," consistent with those of physical simulation experiments. It was clarified that differences in mixing the two end-members and isotopic fractionation play key roles in the variation of methane carbon isotopic composition in shale gas. By applying the methane carbon isotope fractionation model, the contribution of adsorbed gas during shale gas production was explored. It was found that in the early stage of development, the adsorbed gas in Well JY 1 was negligible. After nearly seven years of development, the contribution of adsorbed gas in the later stage has only reached nearly 15%, indicating that the production contribution of adsorbed gas is still less than 0.3 million cubic feet per day. The open flow of Well JY 6-2 is more conducive to the production of adsorbed gas, but the production capacity is still mainly contributed by free gas, indicating that the shale gas production capacity in the later stage in the Jiaoshiba gas field is still primarily dominated by free gas.

Minimally Invasive Vacuum-Aided Extraction Technique for the Lipid Analysis of Historic Parchment.

Parchment is an ancient writing support formed from dehaired animal skins. Its manufacture comprises a series of liming and scraping steps before being stretched and dried under tension. Historical parchment represents a valuable source of cultural heritage which, until now, has limited investigations to noninvasive analyses to infer ink composition, degradation, or physical changes over time. We highlight the prospect of the molecular and isotope compositions of animal lipids from parchment as an untapped record of its production and the animal's diet and environment. We report a minimally invasive, total lipid extraction aided by a vacuum for historical parchments. The quantitative and qualitative compositions of lipid extracts obtained using this method are compared with those obtained using invasive sampling for nine sacrificial membranes dated 1765-1825 CE. This extraction method is then applied to membranes from the Chancery Parliament Rolls (1814-1820 CE) held by The National Archives, UK to obtain lipids and derive taxonomic and dietary information using their stable carbon isotope compositions. This novel vacuum-aided extraction allows, for the first time, animal lipids to be obtained from parchment minimally invasively, paving the way for dietary and paleoclimate studies using this well-dated and common material.

Impacts of wastewater treatment plant effluents on dissolved inorganic carbon in highly urbanized karst rivers, Southwest China

Wastewater treatment plants (WWTPs) are an important manifestation of the impact of urbanization on aquatic ecosystems, and the influences of effluents discharge from WWTPs on receiving river carbon biogeochemical cycle processes has been a research hotspot. However, our knowledge about potential impacts of WWTP effluents on dissolved inorganic carbon (DIC) cycle in karst rivers is rather limited. This study investigates the water chemistry and carbon isotope composition (δ13CDIC) in the urbanized reach of a river that drains the world’s largest continuous karst region in Southwest China, in an attempt to determine the effects of WWTP effluents on karst river DIC cycle and source discrimination. The results reveal: (1) Partial pressure of dissolved carbon dioxide (pCO2) levels in the river increased noticeably during the dry and normal seasons after receiving WWTP effluents; (2) There existed an ionic ratio overlap between end-members of silicate rock weathering and WWTP effluents which requires caution in identifying end-members of rock weathering in urbanized karst rivers; (3) DIC in the highly urbanized karst rivers was affected by complex geochemical processes and these processes varied in different seasons; (4) The contribution of WWTP effluents to the riverine DIC pool was 21±13 %, 58±21 % and 66±33 % in the wet, dry, and normal seasons, respectively. These findings highlight the interference of WWTP effluents with DIC carbon cycling in the receiving karst rivers. Since DIC from WWTPs do not relate with consuming atmospheric or soil CO2, it is important to exclude their contribution from the watershed’s rock weathering carbon sink budget. Based on these findings, we insist that the role and effect of WWTP effluents should be carefully considered when examining DIC carbon source and cycling in rivers that drain urbanized watersheds with abundant wastewater treatment plants.

Organic Matter Enrichment in Jurassic Hydrocarbon Source Rocks from the Turpan-Hami Basin: Insights from Organic and Elemental Geochemistry.

Petroleum can be generated by thermal cracking of organic matter within sediments, and the organic matter within sediments plays the dominant role in determining oil and gas generation. Organic matter within sediments is characterized by various sources, such as sapropelic organic matter from algal, microbial, and planktonic organisms and humic organic matter from higher plants. Paleo-productivity, terrestrial influx, and depositional environments could obviously influence the enrichment processes of the organic matter within sediments. Organic and elemental geochemical proxies can investigate the sedimentary process and reflect the enrichment characteristics of organic matter. In this study, hydrocarbon source rocks from the Shuixigou Group were collected from the Taibei Sag of the Turpan-Hami Basin, rock-eval pyrolysis was conducted, and stable carbon isotope composition of organic carbon and major and trace element distributions were measured. Based on this, the type and maturation of organic matter, paleo-productivity, terrestrial influx, and depositional paleo-environments were investigated. The results show that the Jurassic hydrocarbon source rocks are characterized by type III kerogen and are in the oil-window stage of maturation. The depositional paleo-environments of hydrocarbon source rocks in different formations are not remarkably different. The water bodies have freshwater oxidizing environments, and the paleo-climatic characteristics are warm and humid. However, the paleo-productivity of samples from the lower Jurassic Sangonghe Formation is higher than samples from other formations. Overall, the organic matter enrichment in Jurassic hydrocarbon source rocks of the Turpan-Hami Basin could be mutually controlled by the paleo-productivity and depositional paleo-environments. The results of this study could provide theoretical insight into deep petroleum exploration and resource evaluation in the Turpan-Hami Basin.

Paleoclimatic significance of the organic carbon isotopes of the Tajikistan loess in arid Central Asia, on orbital timescales since the last interglacial

During the past few decades, the stable carbon isotope composition of bulk organic matter (δ13Corg) in loess sediments has been increasingly used for reconstructing changes in the terrestrial vegetation and climate in several regions, including Europe, Xinjiang (the eastern part of arid central Asia, ACA), and the Chinese Loess Plateau (CLP). However, previous research indicates significant differences in the climatic significance of δ13Corg between the loess-paleosol sequences of Europe and the western and eastern ACA, compared to the loess of the central and southern CLP in monsoonal Asia. The δ13Corg values of loess-paleosol sequences in western and eastern ACA on different timescales exhibit consistent patterns of variation, suggesting that loess δ13Corg in these areas may be influenced by similar climatic factors and have similar mechanisms. However, the lack of detailed δ13Corg studies in central ACA limits our understanding of their applicability to long loess sequences within this region. This limitation not only constrains our understanding of the paleoclimatic significance of loess δ13Corg in central ACA, but it also affects the practical application of loess δ13Corg as a reliable paleoclimate proxy in this region. Consequently, it impedes a comprehensive understanding and in-depth analysis of loess δ13Corg across the mid-latitude Eurasian continent. To address this deficiency, we conducted a δ13Corg study of the Darai Kalon (DK) loess section, in southern Tajikistan, central ACA, since the last interglacial, with the aim of exploring the environmental significance of loess δ13Corg and the origin of the climatic record of this section, on orbital timescales. The results show that the ecosystem in southern Tajikistan, and throughout ACA, was dominated by C3 vegetation since the last interglacial. Loess δ13Corg records can be used as a precipitation proxy in ACA, at least since the last interglacial. However, regional comparisons revealed negative correlations between loess δ13Corg values and pedogenic intensity in Europe, ACA, and the western CLP, which is the opposite to the positive correlations observed in the central and southern CLP. Comparison of the paleoclimate record of the DK section with those of loess-paleosol sequences from monsoonal Asia indicated that warm, humid conditions occurred during past interglacials (MIS5, MIS1) and interstadials (MIS3), and cold, arid conditions occurred during the glacial/stadial periods (MIS4, MIS2) in both ACA and monsoonal Asia. We propose that changes in Northern Hemisphere insolation were responsible for this in-phase pattern of climate change between these two regions on orbital timescales since the last interglacial.

Hidden magma reservoirs: Insights from helium and carbon isotopic compositions in shallow groundwater beneath the Wudalianchi volcanic field, Northeastern China

The lack of strong geothermal manifestations and limited fumarolic activity has sparked widespread debate about the presence of active magma reservoirs beneath the Wudalianchi volcanic field (WVF). Helium (He) in hydrological systems is sensitive and responsive to the mixture of mantle-derived He, making it an excellent tool for deciphering mantle-derived magmatic processes. Through investigating dissolved He and inorganic carbon (DIC) in shallow groundwater of the WVF, we found that He captures mantle characteristics with R/Ra ratios (R equals to 3He/4He ratios, Ra is atmospheric 3He/4He ratio of 1.39 × 10−6) ranging from 0.07 to 2.27 and the highest proportion of mantle-derived He is 30.38 %. In contrast, DIC shows minimal mantle influence, with low concentrations (1.91–7.39 mmol/L) and depleted δ13C-DIC values ranging from −17.9 ‰ to −13.6 ‰. The calculated mantle 4He fluxes in groundwater are 2–3 orders of magnitude higher than that in stable continents and show the signature of volcanic degassing. The high mantle He fluxes and high upward flow rates suggest the efficient release and transfer of mantle-derived He into shallow groundwater through magma degassing. Magma-aging model effectively constraints the period of magmatic activity to within the last 47 ka. These findings, consistent with seismic noise imaging results, support the presence of magma reservoirs in the upper–middle crust. The absence of magmatic carbon signature in groundwater suggests the decoupling of magmatic He and magmatic CO2. Magmatic CO2 may be removed during ascent, possibly being trapped through dissolution or precipitation processes within the low-resistivity bodies identified at ∼2.5 km depth by magnetotelluric imaging. These findings not only contribute to resolving the controversy surrounding the existence of magma reservoirs beneath the WVF but also have broader implications in guiding the exploration and validation of hidden magmatic systems.

Impact of paleohydrological conditions on wetland organic carbon accumulation in the monsoon-arid transition zone of North China during the Holocene

Wetlands rich in carbon are important ecosystems for storing organic carbon, with their carbon storage capacity closely linked to water levels. Therefore, it is valuable to explore the relation between paleohydrological conditions and the organic carbon accumulation in carbon-rich deposits. This study examined the total organic carbon concentration and the molecular and carbon isotope compositions of n-alkanes in Ganhai, a dried up lake located in a region transitioning from a monsoon to an arid climate in north China. The results showed that the apparent carbon accumulation rates (CAR) increased as the East Asian summer monsoon intensified (averaged at 26.5 g C m−2 yr−1, 1 σ = 8.6), but decreased when precipitation levels peaked (averaged at 15.8 g C m−2 yr−1, 1 σ = 2.2). The above relation between precipitation and CAR in Ganhai is supported by the correlation analysis (r = −0.35, p = 0.003). This suggests a threshold relation between wetland carbon accumulation and precipitation in this region. The output of a Bayesian multi-source mixing model, based on the molecular and carbon isotope compositions of long-chain n-alkanes, underscores the significant role of aquatic plants in influencing carbon accumulation in Ganhai during the mid-Holocene, with submersed plants dominating during high CAR periods and floating plants being more prevalent during peak precipitation. The study emphasizes the importance of understanding the hydrological threshold for carbon sequestration and vegetation restoration in carbon-rich wetlands.

Carbon isotope fractionation during methane transport through tight sedimentary rocks: Phenomena, mechanisms, characterization, and implications

The phenomenon of carbon isotopic fractionation, induced by the transport of methane in tight sedimentary rocks through processes primarily involving diffusion and adsorption/desorption, is ubiquitous in nature and plays a significant role in numerous geological and geochemical systems. Consequently, understanding the mechanisms of transport-induced carbon isotopic fractionation both theoretically and experimentally is of considerable scientific importance. However, previous experimental studies have observed carbon isotope fractionation phenomena that are entirely distinct, and even exhibit opposing characteristics. At present, there is a lack of a convincing mechanistic explanation and valid numerical model for this discrepancy. Here, we performed gas transport experiments under different gas pressures (1–5 MPa) and confining pressures (10–20 MPa). The results show that methane carbon isotope fractionation during natural gas transport through shale is controlled by its pore structure and evolves regularly with increasing effective stress. Compared with the carbon isotopic composition of the source gas, the initial effluent methane is predominantly depleted in 13C, but occasionally exhibits 13C enrichment. The carbon isotopic composition of effluent methane converges to that of the source gas as mass transport reaches a steady state. The evolution patterns of the isotope fractionation curve, transitioning from the initial non-steady state to the final steady state, can be categorized into five distinct types. The combined effect of multi-level transport channels offers the most compelling mechanistic explanation for the observed evolution patterns and their interconversion. Numerical simulation studies demonstrate that existing models, including the Rayleigh model, the diffusion model, and the coupled diffusion-adsorption/desorption model, are unable to describe the observed complex isotope fractionation behavior. In contrast, the multi-scale multi-mechanism coupled model developed herein, incorporating diffusion and adsorption/desorption across multi-level transport channels, effectively reproduces all the observed fractionation patterns and supports the mechanistic rationale for the combined effect. Finally, the potential carbon isotopic fractionation resulting from natural gas transport in/through porous media and its geological implications are discussed in several hypothetical scenarios combining numerical simulations. These findings highlight the limitations of carbon isotopic parameters for determining the origin and maturity of natural gas, and underscore their potential in identifying greenhouse gas leaks and tracing sources.

Stable hydrogen isotope evidence of late-Holocene precipitation variability on the Caribbean slope of the Cordillera Central, Costa Rica

In Central America and the Caribbean, periods of increased aridity that correspond to the Terminal Classic Drought (TCD; 1200–850 cal yr BP) and the Little Ice Age (LIA; 500–100 cal yr BP) have been documented in many paleoclimate records. Compound-specific hydrogen (δDalkane) and carbon (δ13Calkane) isotopic compositions of n-alkanes in lake sediment can be used to interpret changes in paleoprecipitation and terrestrial paleovegetation, respectively. To assess the climate forcing mechanisms that drove the TCD and LIA, we established a multidecadal to centennial-scale late-Holocene reconstruction of precipitation variability and vegetation change at mid-elevation on the Caribbean slope of Costa Rica, developed from new δDalkane, δ13Calkane, and geochemical analyses and previous pollen and microscopic charcoal analyses from a sediment core from Laguna María Aguilar. Laguna María Aguilar is a freshwater lake located at 770 m elevation on the Caribbean slope of the Cordillera Central. During the TCD, δDalkane data for María Aguilar indicate relatively wet conditions compared to the mean δDalkane value for the entire record. Other proxy records for the TCD indicate that the Pacific slope of Costa Rica and sites above 3400 m near the continental divide experienced generally drier conditions than mid- and low-elevations on the Caribbean slope. We conclude that the TCD may have been driven by both Pacific and Atlantic climate-forcing mechanisms. During the LIA, the Laguna María Aguilar δDalkane record indicates an increase in hydroclimate variability, with some of the highest recorded δDalkane values (driest conditions) during the earliest portions of the LIA, but conditions were not persistently dry for the entirety of the LIA. Based on regional paleoclimate records overall, the LIA drought appears to be more clearly expressed on the Caribbean slope than on the Pacific slope of Costa Rica, indicating that the LIA was perhaps driven primarily by Atlantic Ocean conditions and climate dynamics.

Trophic ecology of the African riverine elephant fishes (Mormyridae).

Multiple species of the elephant fishes (Mormyridae) commonly coexist in sympatry in most African tropical rivers and lakes. In this study, we investigated the trophic ecology and potential trophic niche partitioning of eleven mormyrid fish species from the Sanaga River system in Cameroon using the stable isotope composition of carbon and nitrogen in the muscle samples. Albeit most mormyrids mainly feed on invertebrates, we found differences in isotope ratios, and we report signs of the trophic niche partitioning among species. We further found significant differences in isotopic signatures within the Mormyrus genus, suggesting ecological niche diversification among three closely related species. We have also evaluated differences in the isotopic signals between seasons in four species, which could be possibly caused by species migration and/or anthropogenic agricultural activities. To evaluate body shape, we applied geometric morphometric analyses, and we show that most of the species are clearly morphologically separated. We focused on the mormyrid ecomorphology to identify a possible interaction between shape and ecology, and we found a relationship between the δ13C (but not δ15N) isotopic signal and morphology, suggesting their interplay during mormyrid evolution. Overall, we present robust evidence of the trophic niche partitioning within the mormyrid species community, and we integrate trophic ecology with morphometrics, shedding light on the enigmatic evolutionary history of these fascinating African fishes.

Geochemical fingerprints of Dongpu Depression crude oils in Bohai Bay Basin, northern China: Insights from biomarkers and isotopic compositions of selected alkylnaphthalenes and alkylphenanthrenes

The Dongpu Depression is a rift lake in the Bohai Bay Basin of eastern China, where abundant oil and gas resources were discovered. Previous geochemical investigations of Dongpu Depression oils have revealed a notable lack of data regarding the isotopic compositions of individual polycyclic aromatic hydrocarbons (PAHs) from the discovered Cenozoic crude oils. Using gas chromatography-mass spectrometry (GC–MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), a novel approach combining molecular parameters and stable carbon isotope compositions of individual PAH was applied to study the depositional environment conditions, the different source inputs, the maturation stage and to identify the potential family of five crude oils selected from Dongpu Depression.A decrease of Ga/C30H together with low Pr/Ph ratios and changes in isotopic compositions indicate that the five Dongpu Depression oils were derived from OM deposited in an anoxic saline environment with a stratified water column where mixing of saline water and freshwater occurred. The low isoprenoid, tricyclic and tetracyclic terpane ratios in combination with the δ13C values of 1,6-DMN, 1,2,5-TMN, 1,3,6,7-TeMN, 9-MP and 1-MP attest that their sources have a mixed origin with high aquatic organism (planktonic) input and a lower land plant (C3 plant) contribution. The δ13C values of phenanthrene between −30.0 ‰ and −20.0 ‰ indicate that the studied Dongpu Depression oils belong to a single family generated from the peak to the late oil generation stage (Rc-TNR-2 between 0.85 and 1.15 %). A correlation with various basins indicates that oil samples featured by δ13C values of Phen between −30.0 ‰ and −20.0 ‰ correspond to crude oils derived from mixing of aquatic and high plant contributions with different degrees of mixtures, which have not yet reached their cracking stage (Rc < 2.0 %). Crude oils from the Carboniferous and Mesozoic-Cenozoic Tarim rocks, Australian petroleum systems, Termit Basin, Bongor Basin, Fushan Depression and the studied Dongpu Depression oils belong to that group of oils. Crude oils characterized by lighter phenanthrene isotopic compositions (δ13C values of Phen < −30.0 ‰) are mainly derived from marine/aquatic input and have already entered the cracking window (Rc > 2.0 %). This group of oils is represented in this study by the Cambrian-Ordovician Tarim Basin oil samples. The research shows the importance of the aromatic isotopic compositions in petroleum system characterization and could be used as a reference for a practical exploration campaign of petroleum.

Effect of light-induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for CO2 in Fagus sylvatica.

Mesophyll resistance for CO2 diffusion (rm) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower rm requires knowledge of its distinct components. We tested new method for estimating the relative drawdowns of CO2 concentration (c) across hypostomatous leaves of Fagus sylvatica. This technique yields values of the ratio of the internal CO2 concentrations at the adaxial and abaxial leaf side, cd/cb, the drawdown in the intercellular air space (IAS), and intracellular drawdown between IAS and chloroplast stroma, cc/cbd. The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree-canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of rm into its inter- (rIAS) and intracellular (rliq) components. Validity of the new method was tested by independent measurements of rm using conventional isotopic and gas exchange techniques. 73% of investigated leaves had adaxial epicuticular wax enriched in 13C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of cd/cb and cc/cbd, respectively. The rIAS to rliq proportion were 5.5:94.5% in sun-exposed and 14.8:85.2% in shaded leaves. cc dropped to less than half of the atmospheric value in the sunlit and to about two-thirds of it in shaded leaves. This method shows that rIAS is minor but not negligible part of rm and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.

Hydroxylated GDGTs-0 in marine methane seep environments: A putative indicator for archaeal methanogenesis

Marine methane seeps are environments with a high microbial diversity, but are known for one biogeochemical key process, the sulfate-driven anaerobic oxidation of methane (SD-AOM) performed by anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). SD-AOM is also the dominant process at methane seeps in the South China Sea based on the lipid biomarker inventory of authigenic seep carbonates characterized by crocetane, a high sn2-hydroxyarchaeol over archaeol ratio, low contents of glycerol dibiphytanyl glycerol tetraethers (GDGTs), and δ13Clipid-methane offsets of −52 ± 4 ‰. Combined with the dominance of aragonite over other carbonate minerals, such pattern suggests high seepage intensity with a predominance of ANME-2/SRB consortia. Interestingly, the studied seep carbonates also yielded some uncommon biomarkers, which may derive from methanogenic archaea. Methylotrophic methanogenesis has been shown to be the dominant mode of methanogenesis in seep environments where non-competitive substrates like methanol or trimethylamine are abundant. The presence of methylotrophic methanogens is possibly indicated by high contents (more than 50 % of all GDGTs) of hydroxylated GDGTs-0 (OH-GDGT-0 and 2OH-GDGT-0) with extreme 13C-depletion (−128 ‰ to −116 ‰); this unique pattern is recognized in only some of the studied seep carbonates, while other carbonates are dominated by typical distributions of ANME-2 lipids, also comprising GDGTs with 0 to 4 rings, but lacking high contents of OH-GDGTs. The overall lack of crenarchaeol, the specific biomarker of planktonic Thaumarchaeota, agrees with the tentative assignment of the highly abundant OH-GDGTs-0 to methanogenic archaea. Such interpretation is necessarily circumstantial considering that the compound and carbon isotope composition of the membranes of ANMEs and methanogenic archaea is similar. Although production of OH-GDGTs has been previously reported for planktonic Thaumarchaeota and ANMEs, OH-GDGT-0 as sole and highly abundant OH-GDGT has only been recognized in one culture of methanogenic archaea. Therefore, the high abundance of 13C-depleted hydroxylated GDGTs-0 compared to only minor contents of regular, ANME-derived GDGTs 1–4 with similar 13C-depletions can possibly be used as an indicator for methylotrophic methanogenesis in seep environments. Future experimental work is needed and should test if 13C-depleted hydroxylated GDGTs-0 are indeed biomarkers of methylotrophic methanogenesis at seeps.

Microfacies and Sedimentary Environments of Nfayil Formation (Middle Miocene) in Karbala Using Stable Isotope

A surface section of the Nfayil Formation (Middle Miocene) was studied approximately 80 km from the holy city of Karbala. The study area includes the Malutiyat area's Nyfail Formation (Middle Miocene age). The Nfayil Formation contains marls interbedded with limestones, as well as marly limestone. Fieldwork and petrographic analysis are being conducted on Miocene sediments from the Karbala region. Details obtained from textural and faunal properties aided in the designation of microfacies zones, which led to environmental subdivisions as a ramp. The carbonate lithic arenite sandstone has a significant amount of carbonate fragments. Microfacies characteristics of the whole successions showed skeletal and non-skeletal grains. The oysters, an index fossil of the Nfayil Formation are found in the lower part of the Nfayil Formation. The stable isotopes 18O/16O and 13C/12C in carbonates are analyzed to define the depositional characters and the basin paleo depth. The studied samples have negative δ18O isotope values (average -11.79 %). The carbon isotope composition is of positive values (average 2.16‰), The Z values, higher than 120, reflect the marine sediments of the formation. Four facies were identified, (Lime mudstone Microfacies, Sandy Lime mudstone Microfacies, Peloidal Bioclastic Packstone-Wackestone Microfacies, Bioclastic Wackestone Microfacies) and based on them, the formation environment was determined (restricted marine, and shallow open marine environments).

Revisiting Elevated δ13C Values of Sediment on Modern Carbonate Platforms

AbstractThe measured carbon isotopic compositions of carbonate sediments (δ13Ccarb) on modern platforms are commonly 13C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ13Ccarb values of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ13CDIC values on a modern carbonate platform. These data demonstrate that δ13Ccarb values on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ13C value of atmospheric CO2 and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the 13C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs.