δ13C Signatures Research Articles

Nutrient supplementation changes chemical composition of soil organic matter density fractions in desert steppe soil in northern China

The influence of reactive nitrogen (N) and phosphorus (P) supplementation on ecosystem dynamics of soil organic matter (SOM) shows extensive variations. The underlying mechanisms of regulation of the chemical composition and dynamics of different SOM density fractions (DFs) in aggregates in a desert steppe by N and P supplementation are unclear. Here, the chemical composition and organic carbon (C) and N dynamics of various SOM DFs under N and P addition were assessed. Surface soil samples taken at 0–10 cm depth were obtained from 4-year (2017–2021) experiments under four conditions, namely N treatment, P treatment, combined N and P treatment (NP), and no nutrient supplementation (CK), in a desert steppe located in northern China. Nutrient supplementation significantly increased (P < 0.05) SOM content by 8.89–35.4%; however, the total N content remained constant. N addition mainly increased SOM content in the macroaggregates (>0.25 mm), while P enhanced SOM in microaggregates (0.25–0.053 mm) and silt-clay (<0.053 mm). The fine particulate organic matter (fPOM) proportion was increased significantly (P < 0.05) by 18.4% and 16.4% in macro- and microaggregates under N addition, respectively; P addition mainly enhanced mineral-associated SOM (mSOM) proportion in aggregates. N supplementation remarkably enhanced (P < 0.05) fPOM’s organic C content by 27.3% and 32.0%, and N content by 40.9% and 145.7% in macro- and microaggregates, respectively. Single N and P addition promoted the enrichment of labile and resistant organic components in light fraction (LF), respectively. N promoted the enrichment of stable organic components in the mSOM of microaggregates. Nutrient inputs drive mSOM formation through the interaction of carboxyl-C and mineral surface. Based on a stronger δ13C and δ15N signal, N addition enhanced SOM decomposition in aggregates. Nutrient supplementation promoted SOM accumulation through the formation of more stable old C in POM and silt-clay. Our findings suggest that SOM accumulation in desert steppe probably increases following nutrient addition, but the mechanism is different through which N or P enrichment affects soil dynamics and chemical composition of SOM DFs in desert steppe.

Assessing chemostratigraphic and biostratigraphic correlation of Ediacaran phosphorites (Bocaina, Khesen and Doushantuo Formations): Diachronic, local signals of the Neoproterozoic phosphogenic-taphonomic event

The Neoproterozoic phosphogenic event records biotic and biogeochemical changes that seem to indicate intrinsic relationships between phosphogenesis and phosphatic fossil preservation. However, chemo-litho and biostratigraphic correlations conducted on fossiliferous, phosphatic successions of the Doushantuo Formation, South China, evidence a highly heterogeneous record, featuring diachronic depositional ages, variable taxonomic diversity and inconsistent, “noisy” δ13C records. Similar detailed chemo-litho and biostratigraphic correlations targeted in high-grade phosphorite strata have not been performed along Ediacaran units elsewhere. Considering the significance and scarcity of Ediacaran high-grade phosphorite strata, establishing a comprehensive correlation framework within and between those units is fundamental to understanding Ediacaran biogeochemical and biotic evolution. This work aims to integrate new δ13C isotope stratigraphy, and existing Doushantuo-Pertatataka acritarch assemblages and geochronological records from Bocaina (Brazil), Khesen (Mongolia) and Doushantuo (China) Formations. Phosphate rich strata in those units feature inconsistent δ13C signatures characterized by negative values showing large bed-to-bed variations, possibly reflecting local modification of original marine signals by phosphogenesis, suggesting caution against the use in global/regional chemostratigraphic correlation. The record of phosphatized acritarch assemblages shows contrastive taxonomic diversities, and a small number of common taxa including Megasphaera. Whereas the presence of biozonal Doushantuo Formation taxa on Khsesen Fm, resonates with growing views of strong environmental resilience and large temporal ranges of acanthomorph acritarchs throughout the Ediacaran-early Cambrian, the scarcity of similar taxa in the Bocaina Fm may question the reliability of Doushantuo Fm acritarch biozones in other paleogeographic or paleoenvironmental scenarios. Alternatively, the low taxonomic diversity recorded in Bocaina and Khesen Fms may also reflect the effects of the late Ediacaran decline in acritarch diversity. Further, combined evidence of selective paleoenvironmental phosphatization, local geochemical signals, narrow time spans and heterogeneous taxonomic diversity found in phosphatized DPK acanthomorph assemblages indicate the local nature of Ediacaran phosphatic fossil preservation events. Finally, evidence for diachronic preservation between high-grade phosphatic strata and phosphatized microfossils show non-causal relationships between Ediacaran phosphogenesis and phosphatic fossil preservation, highlighting the punctuated, intrinsic local nature of those events.

An overview of Miocene to Pleistocene methane-seep faunas from Taiwan: Paleoecology and paleobiogeographic implications

The invertebrate macrofauna of four methane-seep deposits of Miocene to Pleistocene age from Taiwan is taxonomically evaluated. The Neogene faunas, with one example each from the early and late Miocene and the Pliocene, consist exclusively of one or two large, infaunal lucinid bivalve genera per site, namely Meganodontia and Lucinoma. The former includes the large, charismatic Pliocene ‘Loripes’ goliath, which is here identified as belonging to the widespread, Neogene to Recent genus Meganodontia. The Pleistocene seep fauna is more diverse, includes extant species like the mytilid Gigantidas horikoshii and Meganodontia aff. acetabulum, unidentified species belonging to Lucinoma and the vesicomyid Isorropodon, and specimens resembling the enigmatic bivalve Sisonia frijellanae, described so far only from the late Miocene of the Philippines; the gastropods could not be identified below family level. The two Miocene seep faunules formed on the shelf of the Eurasian plate, the Pliocene faunule in an upper offshore setting in the Tainan foreland basin, whereas the Pleistocene site with its more diverse fauna formed closer to the shelf edge of the Tainan foreland basin. We suggest that water depth is the most likely driver of this difference in species diversity. The very negative δ13C signature of the seep carbonates of all fossil sites indicates the seepage of mostly biogenic rather than thermogenic methane, analogous to the extant seep sites around Taiwan. Biogeographically, the Miocene seep faunas show, with Meganodontia, links to seep faunas from lower to mid-latitudes worldwide (Caribbean Sea, Mediterranean Sea, Philippines, and New Zealand). From the Pliocene onward, biogeographic ties are more restricted to areas in the central Indo-West Pacific Ocean.

Phenotypic plasticity and carryover effects in an ecologically important bivalve in response to changing environments

Phenotypic plasticity can improve an organism’s fitness when exposed to novel environmental conditions or stress associated with climate change. Our study analyzed spatiotemporal differences in phenotypic plasticity and offspring performance in Olympia oysters Ostrea lurida. This species is an ecosystem engineer and is of great interest for commercial and restoration aquaculture. We used a multidisciplinary approach to examine acute and long-term physiological differences in O. lurida in response to in situ oceanographic conditions in a dynamic inland sea. We outplanted oysters to different areas in Puget Sound, Washington, affixing cages to anchor lines of oceanographic monitoring buoys. This allowed us to couple high-resolution oceanographic data with organism’s phenotypic response. To assess spatiotemporal differences in oyster physiological performance, we collected oysters after six-months and one year of acclimatization at four field sites. During each collection period we evaluated changes in shell properties, diet, metabolism, and reproduction. Adult growth, δ13C and δ15N isotopic signatures, and gametogenesis were affected by both seasonal and environmental conditions. In the winter, oysters from all sites had higher respiration rates when exposed to acute thermal stress, and lower respiration response to acute pH stress. Lipid content, sex ratio and shell strength were unchanged across locations. Offspring growth rates between sites at experimental temperature 20°C closely reflected parental growth rate patterns. Offspring survival was not correlated with growth rates suggesting different energetic trade-offs in oyster offspring. The metabolic response (respiration) of larvae reached its highest point at 20°C but sharply decreased at 25°C. This indicates that larvae are more sensitive to temperature stress, as adults did not exhibit a reduction in metabolic response at 25°C. By deploying genetically similar oysters into distinct environments and employing a wide range of physiological methodologies to examine performance and fitness, our results indicate that Olympia oysters exhibit a high degree of phenotypic plasticity and show evidence of parental carryover.

Dietary reconstruction and influencing factors of oysters cultured in a typical estuarine bay of South China

Blue carbon can be transferred from primary producers to primary consumers through oyster filter feeding. Understanding the carbon (food) sources is crucial for understanding coastal food webs and management of oyster aquaculture ecosystems. In the present study, Bayesian stable isotope mixing models (MixSIAR) were used to identify and quantify food composition within a cultured oyster ecosystem at six sites along a salinity gradient in the estuarine bay of Zhenhai (ZHB). The carbon stable isotope (δ13C) signatures of oyster Crassostrea hongkongensis ranged from −22.06 to −27.27‰, with significant spatial differences (p < 0.001), indicating variations in food resources along the salinity gradient. The primary food source for cultured oysters was suspended particle organic matter across ZHB (55.7–90.6%), with sedimentary organic matter contributing more in the mid-estuary (28.9–44.3%). Partial least squares regression (PLSR) was used to explore the effects of habitat environment and human disturbance on spatial variation in oyster diets. The results indicated that dissolved oxygen, sediment particle size, elevation, and mangrove-related landscape indices were the main factors influencing oyster diet in ZHB. The findings provide insights into the dietary composition of cultured oysters and the underlying factors in a typical shallow bay in southern China, with particular emphasis on the role of terrestrial landscape features. In addition, the findings validate PLSR as a reasonable tool for predicting the food composition of consumers in estuarine bays.

Changes of soil carbon along a topo-climatic gradient in rangelands of Iran: insights from 14C mean residence time and δ13C

Context Soils can be the largest terrestrial carbon source and a potential sink of atmospheric CO2. Soil organic carbon (SOC) dynamics can be unravelled by 14C-derived mean residence times (MRT). Aims We aimed to understand SOC dynamics in surface and subsurface soils along a topo-climatic gradient in the rangelands of Khuzestan Province, Iran. Methods Study sites were selected under two contrasting regional climates in Izeh (MAT + 19.2°C, MAP 623 mm) and Ramhormoz (MAT + 27.5°C, MAP 200 mm). Soil physicochemical properties, SOC forms, and 14C MRT and δ13C signatures were determined in the control profiles. Key results The average MRT up to 1 m depth in Izeh and Ramhormoz were 2980 and 6582 years before present, respectively. On average, a loss of 300 Mg C ha−1 in SOC stocks and a rise of 430 years in SOC MRT up to 1 m can be expected per 1°C increase in MAT, 50 mm reduction in MAP, and 100 m decrease in elevation, highlighting the potential significance of MAT in SOC dynamics. Using optimistic and pessimistic carbon emission scenarios, carbon emissions in the upland areas were projected to be between 50 and 100 Mg C ha−1 over 80 years. Conclusions While the most influential element on SOC stock and its relative age was likely the temperature, other factors like erosion and deposition processes can cause enhanced SOC dislocation along the topo-climatic gradient. Implications Soil carbon pools stabilised for centuries to millennia are susceptible to alterations due to climate and land cover change.

Quantification of allochthonous and autochthonous organic carbon in large and shallow Lake Wuliangsu based on distribution patterns and δ13C signatures of n-alkanes

Identification and quantification of allochthonous and autochthonous organic carbon (OC) in large and shallow lakes are crucial for knowledge of OC burial and recycling behaviors, as well as sustainable management practices. However, quantitative methods to estimate the contributions from various OC sources are still limited. In this study, we analyzed distribution patterns and δ13C signatures of mid- and long-chain n-alkanes from aquatic plants, riverine sediment, and surface sediments from a typically large and shallow Lake Wuliangsu in the Hetao Irrigation District, China. The results indicate that n-alkanes among submerged macrophytes, emergent plants, and riverine sediment show unique distribution patterns and δ13C signatures, supporting the practicability of identification and quantification of OC sources by end-member mixing models of n-alkanes’ distribution patterns and 13C values. Our results also suggest that introducing δ13C values into the end-member mixing models could effectively reduce the uncertainty, as n-alkane distribution patterns might be modified by different species, habitat environments, and potential OC degradation. In the case of Lake Wuliangsu, the model results demonstrate that the riverine sourced OC from the main channel has decreased during southward transport, indicating that Lake Wuliangsu serves as an important trap and sink for OC transported from the upper reaches of the Yellow River. The model results also show a predominant contribution of the autochthonous OC to sediments in Lake Wuliangsu, with open-water areas dominated by submerged macrophyte-sourced OC and the other areas by emergent plant-sourced OC. Higher productivities of submerged macrophytes are mainly modulated by increases in water depth, water transparency, and nutrient concentrations, while higher productivities of emergent plants are mainly associated with increasing nutrient concentrations. To improve the ecological conditions and OC burial of Lake Wuliangsu, it is necessary to manage the quality of the inflowing water from the agricultural farmland, as well as the preservation of the water body from current paludification.

Sources and transport of CO2 in the karst system of Jiguan Cave, Funiu Mountains, China

Conveyance and modification of carbon-isotope signals within the karst system remain difficult to constrain, due to the complexity of interactions between multiple components, including precipitation, bedrock, soil, atmosphere, and biota. Cave monitoring is thus critical to understanding both their transport in the karst system and dependence on local hydroclimatic conditions. Jiguan Cave, located in Funiu Mountain in central China, is representative of karst tourist caves with relatively thin epikarst zone. We conducted a comprehensive monitoring program of cave climate from 2018 to 2021 and measured δ13C during 2021 in monthly and heavy-rainfall samples of soil CO2, cave CO2, cave water (drip water and underground river), and underground river outlet. Our results demonstrate synchronous variations between CO2 concentration and δ13CCO2 in both soil and cave air on seasonal time scales. Cave pCO2 and carbon-isotope composition further exhibited a high sensitivity to human respiration with fluctuations of ~2000–3000 ppm within 4 days during the cave closure period in July 2021 without tourists. 13C-depleted isotopic signal of cave air in summer is the mixture of human respiration and soil CO2 which varies as a function of regional hydrological conditions of the summer monsoon during the rainy season with high temperatures and humidity. However, respired CO2 from the overlying soil was expected to be the only principal source of the cave CO2 when the anthropogenic CO2 source was removed. The high seasonal amplitude of cave air δ13CCO2 reflects ventilation dynamics, which leads to a prominent contribution from the external atmosphere during winter. Intriguingly, although the δ13C signal reflects complex vertical processes in the vertical karst profile, a heavy summer rainfall event was related to anomalously high δ13C values of cave water that can be utilized to interpret rainfall intensity and regional hydroclimate.

Network Analysis Reveals Species-Specific Organization of Microbial Communities in Four Co-Occurring Elasmobranch Species along the Georgia Coast

Comparing co-occurring species may provide insights into how aspects of ecology may play a role in influencing their microbial communities. During the 2019 commercial shrimp trawl season off coastal Georgia, swabs of skin, gills, cloaca, and gut were taken for three species of batoids (Butterfly Ray, Bluntnose Stingray, and Atlantic Stingray) and one shark species (Atlantic Sharpnose) for high-throughput sequencing of the V4 region of the bacterial 16S rRNA gene. White muscle was analyzed for stable isotopes (δ13C and δ15N) to evaluate potential niche overlap in these four sympatric mesopredators. Significant differences were found in both δ13C and δ15N signatures across species, suggesting a degree of resource partitioning. When examined within tissue type, the host species had a weak effect on β-diversity for cloaca and skin, with no differences found for gill and gut samples. However, network analysis metrics demonstrated a stronger species-specific effect and distinct microbial community relationships were apparent between the shark and batoids, with the former having tighter networks for both internally- and externally-influenced tissues (gut/cloaca and skin/gills, respectively). Despite overlapping habitat use, species’ microbiomes differed in their organizational structuring that paralleled differences in stable isotope results, suggesting a mediating role of species-specific ecology on bacterial microbiomes.

Biodegradation of polyethylene terephthalate by Tenebrio molitor: Insights for polymer chain size, gut metabolome and host genes

Polyethylene terephthalate (PET or polyester) is a commonly used plastic and also contributes to the majority of plastic wastes. Mealworms (Tenebrio molitor larvae) are capable of biodegrading major plastic polymers but their degrading ability for PET has not been characterized based on polymer chain size molecular size, gut microbiome, metabolome and transcriptome. We verified biodegradation of commercial PET by T. molitor larvae in a previous report. Here, we reported that biodegradation of commercial PET (Mw 29.43 kDa) was further confirmed by using the δ13C signature as an indication of bioreaction, which was increased from − 27.50‰ to − 26.05‰. Under antibiotic suppression of gut microbes, the PET was still depolymerized, indicating that the host digestive enzymes could degrade PET independently. Biodegradation of high purity PET with low, medium, and high molecular weights (MW), i.e., Mw values of 1.10, 27.10, and 63.50 kDa with crystallinity 53.66%, 33.43%, and 4.25%, respectively, showed a mass reduction of > 95%, 86%, and 74% via broad depolymerization. Microbiome analyses indicated that PET diets shifted gut microbiota to three distinct structures, depending on the low, medium, and high MW. Metagenome sequencing, transcriptomic, and metabolic analyses indicated symbiotic biodegradation of PET by the host and gut microbiota. After PET was fed, the host’s genes encoding degradation enzymes were upregulated, including genes encoding oxidizing, hydrolyzing, and non-specific CYP450 enzymes. Gut bacterial genes for biodegrading intermediates and nitrogen fixation also upregulated. The multiple-functional metabolic pathways for PET biodegradation ensured rapid biodegradation resulting in a half-life of PET less than 4 h with less negative impact by PET MW and crystallinity.

Climate monitoring in the Caumont cave and quarry system (northern France) reveal near oxygen isotopic equilibrium conditions for carbonate deposition

The study of modern cave deposits forming under near isotopic equilibrium conditions can potentially help disentangle the processes influencing the oxygen isotope system and suitability of stalagmites as archives of past hydrological or thermal changes. We used cave monitoring to evaluate the impact of kinetic isotope fractionation and assess the conditions under which modern cave carbonates form in the Caumont cave and quarry system, located in Normandy, northwest France. Over 20 months, we collected climatological data, dripwater, and modern carbonate samples at 2–4-week intervals at three different stations inside the Caumont cave and quarry system. We find highly stable (10.4 ± 0.3°C – 11.3 ± 0.1°C) temperature in the deeper sections of the Caumont cave and quarry system. The temporal dynamics of δ18Odrip indicates that the drip water composition in Caumont reflects the original (though subdued) signal of precipitation, rather than the impact the seasonal to interannual cave air temperature has on isotopic fractionation. The monitoring reveals that δ13C of modern carbonate is influenced by prior carbonate precipitation that occurs during the summer season when evapotranspiration can minimize effective infiltration. Comparison of δ18O from dripwater and modern calcite, precipitated on glass plates and collected every two to four weeks, reveals that modern calcite forms near oxygen isotope equilibrium. A Hendy test on modern carbonate deposited on a stalagmite-shaped glass flask over 20 months confirms this finding because neither does δ13C increase with distance from the apex, nor are δ13C and δ18O positively correlated. We conclude that the δ13C signal in speleothems reflect summer (and longer-term) prior carbonate precipitation in response to effective infiltration dynamics, and that the δ18O signal likely reflects annual to multi-annual changes in the composition of precipitation above the cave.

The effect of forest structure on drought stress in beech forests (Fagus sylvatica L.)

The unprecedented drought between 2018 and 2020 had a significant impact on European beech (Fagus sylvatica L.) forests in Central Europe. The role of different forest structures in mitigating drought stress remains controversial. This contentious debate prompted our study, in which we aimed to quantify the effect of forest structure on drought stress in beech forests in two ecoregions of northern Bavaria, Germany. Using a mobile laser scanner, we surveyed 240 plots in drought-stressed forests. We analyzed the responses of beech trees to the drought period through radial growth, wood-derived δ13C signal, and crown defoliation. Results revealed significant responses of beech forests in both regions to the drought event, including increased crown defoliation, reduced tree growth, and altered δ13C signatures compared to pre-drought conditions. Our results show a relationship between crown closure and crown defoliation in beech, suggesting an increased vulnerability of beech to drought in more open canopies. However, the potential for silvicultural intervention to mitigate drought stress, as measured by BAI and δ13C signal, appears limited. Neighboring trees and forest structure had little influence on average drought resistance. The δ13C signal showed minimal responsiveness to variations in canopy openness, as well as to distinctions between single and multi-layered forests. However, increased structural complexity within stands tended to increase resistance due to the compensatory effects of understory trees. Future forest management strategies could focus on promoting structural diversity, selecting resilient individuals but also actively enrich the forests with more drought-adapted species to increase the adaptive capacity of beech-dominated forests in the face of changing climate conditions.

Focused recharge enhances soil carbon leakage and carbonate weathering in groundwater: Implications for subsurface carbon sinks

Soil carbon leakage into groundwater is a crucial and natural carbon sink process in the subsurface carbon cycling. However, the primary driving force for soil carbon downward leakage and the following fate of the carbon in groundwater remains elusive. Here, we investigated multi-seasonal hydrodynamics, hydrogeochemistry and environmental isotopes (δ13C, δ2H and δ18O) in groundwater wells in a humid region. Frequent water-table rise (1 ∼ 3 m) and isotopic signature fluctuation after the concentrated rainfall supported evidence for the occurrence of focused groundwater recharge. We found that focused recharge during the shorter and wetter season induced the increase of groundwater DOC (306.5 ± 231 μmol/L) and CO2 (2154 ± 726.6 μmol/L), the dilution of CH4 (1.75 ± 1.2 μmol/L) and DIC (4486 ± 790.8 μmol/L) as well as depleted δ13C signatures (−17‰ ∼ −18‰). These variations jointly elucidated accelerated soil carbon downward leakage and hydrodynamic mixing driven by focused recharge. Saturation index calculation and PHREEQC simulations confirmed that massive groundwater CO2 from soil carbon leakage during focused recharge exerted positive perturbations on carbonate weathering, and ultimately stored in a more stable form by transforming into HCO3−. Carbon isotope analysis further identified carbonate weathering accounted for approximately one third of DIC dynamics compared to biogenic sources. This study highlights that focused recharge has a disproportional contribution to flushing soil-derived carbon into groundwater and activating carbonate weathering in the groundwater, with important implications for temporal variability of non-negligible subsurface carbon sinks in the global carbon budget.

Source apportionment of methane emissions from the Upper Silesian Coal Basin using isotopic signatures

Abstract. Anthropogenic emissions are the primary source of the increase in atmospheric methane (CH4) levels. However, estimates of anthropogenic CH4 emissions still show large uncertainties at global and regional scales. Differences in CH4 isotopic source signatures δ13C and δ2H can help to constrain different source contributions (e.g., fossil, waste, agriculture). The Upper Silesian Coal Basin (USCB) represents one of the largest European CH4 emission regions, with more than 500 Gg CH4 yr−1 released from more than 50 coal mine ventilation shafts, landfills, and wastewater treatment plants. During the CoMet (Carbon Dioxide and Methane Mission) campaign in June 2018 methane observations were conducted from a variety of platforms including aircraft and cars to quantify these emissions. Besides the continuous sampling of atmospheric methane concentration, numerous air samples were taken from inside and around the ventilation shafts (1–2 km distance) and aboard the High Altitude and Long Range Research Aircraft (HALO) and DLR Cessna Caravan aircraft, and they were analyzed in the laboratory for the isotopic composition of CH4. The airborne samples downwind of the USCB contained methane from the entire region and thus enabled determining the mean signature of the USCB accurately. This mean isotopic signature of methane emissions was -50.9±0.7 ‰ for δ13C and -226±9 ‰ for δ2H. This is in the range of previous USCB studies based on samples taken within the mines for δ13C but more depleted in δ2H than reported before. Signatures of methane enhancements sampled upwind of the mines and in the free troposphere clearly showed the influence of biogenic sources. We determined the source signatures of individual coal mine ventilation shafts using ground-based samples. These signatures displayed a considerable range between different mines and also varied for individual shafts from day to day. Different layers of the USCB coal contain thermogenic methane, isotopically similar to natural gas, and methane formed through biogenic carbonate reduction. The signatures vary depending on what layer of coal is mined at the time of sampling. Mean shaft signatures range from −60 ‰ to −42 ‰ for δ13C and from −200 ‰ to −160 ‰ for δ2H. A gradient in the signatures of subregions of the USCB is reflected both in the aircraft data and in the ground samples, with emissions from the southwest being most depleted in δ2H and emissions from the south being most depleted in δ13C, which is probably associated with the structural and lithostratigraphic history of the USCB and generation and migration processes of methane in the coal. The average signature of -49.8±5.7 ‰ in δ13C and -184±32 ‰ in δ2H from the ventilation shafts clearly differs from the USCB regional signature in δ2H. This makes a source attribution using δ2H signatures possible, which would not be possible with only the δ13C isotopic signatures. We assume that the USCB plume mainly contains fossil coal mine methane and biogenic methane from waste treatment, because the USCB is a highly industrialized region with few other possible methane sources. Assuming a biogenic methane signature between and −320 ‰ and −280 ‰ for δ2H, the biogenic methane emissions from the USCB account for 15 %–50 % of total emissions. The uncertainty range shows the need of comprehensive and extensive sampling from all possible source sectors for source apportionment. The share of anthropogenic–biogenic emissions of 0.4 %–14 % from this densely populated industrial region is underestimated in commonly used emission inventories. Generally, this study demonstrates the importance of δ2H-CH4 observations for methane source apportionment in regions with a mix of thermogenic and biogenic sources and, especially in our case, where the δ13C signature of the coal mine gas has a large variability.

A carbon, nitrogen, and multi-isotope study of basalt glasses near 14°N on the Mid-Atlantic Ridge. Part A: Degassing processes

14°N on the Mid-Atlantic ridge (MAR) is one of only a few locations worldwide where volatile-saturated, geochemically enriched mid-ocean ridge basalts (E-MORBs) have been recovered. These basaltic glasses are so gas-rich that CO2-filled bubbles may “pop” when brought to the surface, due to the pressure and temperature change. Although these “popping rocks” have long been regarded as representative samples of un-degassed magmas sourced from the upper mantle, uncertainties regarding both their generation mechanism(s) and the potential effects of gas loss/accumulation processes have hampered unambiguous quantification of the upper mantle volatile element (water, carbon, nitrogen, noble gas) inventory. Fortunately, the extent and consequences of gas loss/accumulation processes can be tested by studying characteristic changes in volatile elements compositions, including 4He/40Ar* (where 40Ar* is 40Ar corrected for atmospheric contamination). To document the mechanism of popping rock generation and potential effects of degassing and gas accumulation processes on MORB volatile systematics, we present a comprehensive volatile characterization (carbon, nitrogen and noble gas systematics) of popping rocks and associated MORBs (n = 19) recently sampled at 14°N on the MAR, including 2 normal MORBs (N-MORB) from an oceanic core complex (OCC) and 17 E-MORBs. In line with previous studies, we find that PR exhibit the lowest 4He/40Ar* (1.08 ± 0.04) among all MORB samples, lower than the conventional mantle production ratio of 3 ± 1. Such low 4He/40Ar* could either (i) derive from accumulation of first-generated bubbles originating from open-system degassing of underlying magmas, or (ii) represent the actual upper mantle production ratio. We summarize the arguments in favor of each of these two scenarios (including the required accumulation times for radiogenic noble gases accumulation, the K/U and 232Th/238U of the upper mantle, popping rock vesicle size distributions, and physical considerations for vesicle growth and upwelling through a basaltic magma), and discuss their implications for the volatile composition of un-degassed magmas from the upper mantle. We find homogenous N isotope compositions (average δ15N of −4.49 ± 1.40 ‰ at 14°N) but variable δ13C (from −11 ‰ to −3.4 ‰), potentially compatible with the expectations for residual dissolved gas after Rayleigh fractionation. However, explaining the light δ13C signatures of PR via this process appears incompatible with any of the two scenarios proposed for explaining their low 4He/40Ar*, which would predict (i) 13C-enrichements and (ii) no fractionation relative to an initial composition at δ13C ∼ -5‰, respectively. After correction for solubility-controlled degassing fractionation and potential gas accumulation processes using 4He/40Ar* systematics, we find relatively homogeneous C/3He ((2.65 ± 0.51) × 109) but variable C/N (from 125 up to 4578), whose potential origins are discussed as part of a companion paper (Part B: Source effects).

Stratigraphic distributions of biomarkers and carbon isotopes in coals constrain the Permo-Carboniferous climatic changes and floral turnovers in the north China block

The intricate relationship between climate and vegetation dynamics, particularly during the late Paleozoic ice age marked by multiple icehouse-greenhouse transitions, is a compelling aspect in Earth's historical narrative. In this study, we decipher the interactions between the Permo-Carboniferous climate fluctuations and floral turnovers within the North China Craton, based on the stratigraphic distributions of biomarkers and carbon isotopes from cored coals in Huainan coalfield, North China. Our findings reveal that the changes in n-alkane, terpane, sterane, and polycyclic aromatic hydrocarbon (PAH) distributions result from a confluence of maturation, depositional environment, and organofacies effects. Molecular maturity-sensitive ratios align with the maturity stage determined by Rock-Eval pyrolysis and petrography, categorizing the coals as high volatile bituminous A/B with a composite organic matter derived from terrestrial plants and marine algae. Notably, the 18α-22,29,30-trisnorneohopane (Ts) to 17α-trisnorhopane ratio (Ts/(Ts + Tm)), as well as C31–C34 22S/(22S + 22R) hopanes and C29 20S/(20S + 20R) steranes ratios exhibit moderate correlations with %Ro, indicative of epimerization reactions induced by maturation processes. The stratigraphic variations in pristane to phytane ratios (Pr/Ph), Pr/n-C17, Ph/n-C18, C30 αβ/(αβ+βα) hopanes, and organofacies sensitive PAH indices collectively capture a paleoenvironmental shift from reducing to oxic conditions attributed to marine regression. The systematic changes in PAH concentrations and their maturity indices reflect condensation and dealkylation reactions during the maturation process. The δ13C signature manifests a negative excursion in coals of the Artinskian Shanxi Formation and a subsequent positive excursion in the Capitanian Upper Shihezi Formation coals, respectively, reflecting changes in the δ13C values of contemporaneous atmospheric CO2 in response to regional aridity fluctuations.

Enhanced Bioaccumulation and Transfer of Monomethylmercury through Periphytic Biofilms in Benthic Food Webs of a River Affected by Run-of-River Dams.

Run-of-river (ROR) power plants impound limited terrestrial areas compared to traditional hydropower plants with large reservoirs and are assumed to have reduced impacts on mercury cycling. We conducted a study on periphyton and benthic communities from different habitats of the St. Maurice River (Québec, Canada) affected by two ROR power plants and their effect on the bioaccumulation and biomagnification of monomethylmercury (MMHg). Proportion of total mercury as MMHg reached maximum values about 2.9 times higher in flooded sites compared to unflooded sites. Impoundment by ROR would therefore provide favorable environments for the growth of periphyton, which can produce and accumulate MMHg. Periphyton MMHg concentrations significantly explained concentrations in some benthic macroinvertebrates, reflecting a local transfer. Through the analysis of δ13C and δ15N signatures, we found that flooding, creating scattered lenthic habitats, led to modifications in trophic structures by the introduction of new organic matter sources. The computed trophic magnification slopes did not show significant differences in the transfer efficiency of MMHg between sectors, while intercepts of flooded sectors were higher. Increases in MMHg concentrations in flooded areas are likely due to the impoundment, combined with watershed disturbances, and the creation of small habitats favorable to periphyton should be included in future predictive models.

Divergence in soil particulate and mineral-associated organic carbon reshapes carbon stabilization along an elevational gradient

Variations in climates and vegetation types along elevations in mountain ecosystems strongly reshape soil organic matter (SOM) stabilization. However, soil carbon (C) stabilization pathways from litter to SOM and potential mechanisms along elevations in the subalpine area remain unclear. Here, we investigated the C stabilization pathway from litter to particulate and mineral-associated organic matter (POM and MAOM) and associated drivers along elevational gradients in the Hengduan Mountains, China, based on δ13C signatures. Our results showed C stabilization pathways from litter to POM to MAOM, irrespective of elevation. Unexpectedly, the δ13C values of SOM and its fractions (POM and MAOM) were not significantly related to litter δ13C across elevations although litter was the main source of SOM, suggesting that other potential drivers could weaken the coordination of δ13C among litter and SOM fractions. We observed that the△13CLitter→POM (△13C between litter and POM) was more dependent on the direct effect of soil texture (i.e., sand, silt and clay). However, the △13CPOM→MAOM (△13C between POM and MAOM) was more related to microbial attributes (i.e., microbial community and enzyme activities).These results emphasizing the different stabilization procedures between POM and MAOM. Particularly, soil temperature dominantly determined △13CLitter→POM, but soil pH was the primary factor regulating the △13CPOM→MAOM. Overall, our results suggested that POM and MAOM formation occurred with different regulation processes, the POM formation was primarily via more physical pathways and MAOM formation mainly via more microbial pathways, thus providing novel insights into the better understanding of SOM formation and stabilization mechanisms in mountain ecosystems.

δ13C and δ15N zooplankton isoscapes as trace of water masses and mesoscale activity in the Pacific Tropical-Subtropical Convergence off Mexico during June 2010

This research analyzes for the first time in the Pacific Tropical-Subtropical Convergence off Mexico during June 2010 the relationships among the δ13C and δ15N signal distribution in the bulk zooplankton, water masses, and mesoscale structures. The environmental analysis showed that the California Current Water (CCW) and Transitional Water (TrW) converge where the 22 °C isotherm and 34.6 g kg−1 isohaline were observed (22°N). Two cyclonic eddies were detected: one in CCW at 24°N about 70 km in diameter, the other one located in TrW at 21°N with 150 km in diameter. A cluster analysis defined three zooplankton isoscapes with significant differences. Isoscape 1, immerses in CCW – the lowest δ13C (−22.37 ± 0.89 ‰) and δ15N (9.89 ± 1.32 ‰) – showed temperature (19.86 ± 1.97 °C), salinity (34.08 ± 0.37 g kg−1) values, the deepest thermocline (77.93 ± 25.51 m) and oxygen minimum zone (164.78 ± 44.96 m). The CCW was relatively oligotrophic without important mesoscale features in the sampled time within the sampled area. Isoscape 2, a cyclonic eddy in CCW – relatively enriched zooplankton δ13C values (−16.78 ± 1.95 ‰) due to the entry of enriched coastal biota– reflected in high chlorophyll a and subsequently assimilated into zooplankton tissues. Isoscape 3, a cyclonic eddy immerses in TrW – the highest δ15N (13.14 ± 1.60 ‰), salinity (34.76 ± 0.19 g kg−1) and temperature (19.90 ± 2.27 °C) values and shallowest thermocline (32.27 ± 18.63 m), and oxygen minimum zone (66.82 ± 37.68 m) in depth. Likely, this 15N-enriched recycled nitrate was pumped from the deoxygenated subsurface to the surface, reflected in the zooplankton bulks. The results showed that the isotopic signals in bulk zooplankton may be a good water mass and mesoscale activity tracer.

Methane Source Attribution in the UK Using Multi‐Year Records of CH4 and δ13C

AbstractIsotopic measurements of atmospheric methane are valuable for the verification of bottom‐up atmospheric emissions inventories. The balance of sources in emissions inventories must be consistent with the δ13C‐CH4 isotopic record in the air. Long‐term records of both methane mole fraction and δ13C from five sites across the UK are presented, showing post‐2007 growth in CH4 and negative trend in δ13C, consistent with global background sites. Miller‐Tans analyses of atmospheric measurements identified that the δ13C signature of the methane source mix varied between −50.1 and −56.1‰, with less depleted δ13C signatures at sites receiving air from urban areas, consistent with an increased proportion of thermogenic sources. Isotopic signatures calculated for all sites are more enriched than those expected from the bottom‐up emissions inventory, suggesting that inventories for the UK either underestimate contributions of thermogenic/pyrogenic emissions or overestimate biogenic sources.