δ13CDIC Values Research Articles

A rapid rise of seawater δ13C during the deglaciation of the Marinoan Snowball Earth

The Marinoan Snowball Earth glaciation (~650–635 Ma) was followed by an increase in atmospheric O2 level and the diversification of eukaryotes. These dramatic changes have been assumed to have resulted from the rapid recovery of primary productivity during deglaciation, but this hypothesis has not yet been tested. Carbonate carbon isotope (δ13Ccarb), closely related to organic matter production and burial, has the potential to provide with the most straightforward evidence for the perturbation in marine carbon cycle. However, individual δ13Ccarb record might be less well-suited to test this hypothesis than a large data compilation. Here, we analyze 1590 globally-distributed carbonate carbon isotope data from the Marinoan cap carbonate. The least square sum analysis of the shallow-water cap carbonate δ13Ccarb data indicates that the carbon isotopic composition of seawater dissolved inorganic carbon (DIC, δ13CDIC) was −3.01‰ during cap carbonate precipitation. This result sharply contrasts with the assumed synglacial seawater δ13CDIC value of −5‰, which was verified with a binary mixing model. The 2‰ increase in the seawater δ13CDIC can be attributed to intensive organic matter production and burial during the deglaciation in less than 106 years. Massive organic carbon burial in the wake of the Marinoan Snowball Earth glaciation contributed a postglacial increase of atmospheric O2 level by 3.8% to 13.4% PAL (present atmospheric level) and thus further facilitated diversification of eukaryotes.

Transport and transformation of dissolved inorganic carbon in a subtropical groundwater‑fed reservoir, south China

Most reservoirs in subtropical areas experience periodic variations in the thermal structure of their water columns, with times of strong thermal stratification being succeeded by periods of mixing, over the course of the year. Understanding of the transport and transformation of dissolved inorganic carbon over such thermal cycles in artificial reservoirs remains poor. To address this problem, this study examined the spatiotemporal behavior of dissolved inorganic carbon (DIC), the partial pressure of CO2 (pCO2), carbon isotope ratios (δ13CDIC), and CO2 emission (FCO2), from 2014 to 2018 in a subtropical, groundwater-fed reservoir in southern China. It was found that CO2 emissions during mixing periods are much higher than in thermally stratified periods (particularly during transition from stratified to mixing) as a result of upwelling and release of dissolved CO2 (CO2aq) accumulated in the hypolimnion. CO2 emission fluxes at the water-gas interface accounted for only a small proportion of the DIC in the reservoir. The relationships between of DIC and δ13CDIC displayed two distinct modes, due to spatial differences in water depths and to strong thermal stratification during warmer seasons: (1) DIC concentrations increase and δ13CDIC values decrease from epilimnion to hypolimnion, and (2) δ13CDIC values decrease with increasing DIC concentrations but δ13CDIC is progressively enriched near the bottom during periods of thermal stratification. In addition, this study found three distinct processes of DIC accumulation and consumption in the reservoir: (1) DIC accumulated in the hypolimnion during thermal stratification periods, due to carbon retention but (2) DIC was substantially consumed in the epilimnion during such periods, and (3) average DIC concentrations and pCO2 increased significantly from upstream to downstream along the reservoir, while average δ13CDIC values became lighter. These results highlight that carbon behavior in groundwater‑fed reservoirs is often controlled by a combination of biogeochemical processes and seasonal variations in thermal structure. Sampling and monitoring strategies should consider these factors in order to accurately estimate carbon budgets in reservoirs, lakes or ponds.

Seasonal variation in the total alkalinity and δ13CDIC values of cave drip water in a ventilated cave in response to the regional climatology, southwest China

The simulation and monitoring of environmental proxies in cave systems is of great significance for understanding the controlling factors of stalagmite geochemistry and for further analysing the signal transmission process and mechanism in cave systems. In this study, we collected samples from three drip points in Liangfeng Cave from 2015 to 2020, Guilin City, Southwest China. The δ13CDIC values, total alkalinity and modern calcite deposition rates at the drip points in the cave were measured during the monitoring period. Comparison of these data with atmospheric temperature and rainfall data from the study area revealed that (1) the δ13CDIC and total alkalinity values of cave drip water show a seasonal pattern involving low values that increase in summer and autumn and high values that decrease in winter and spring, which is synchronized with the changes in the regional rainfall and temperature. (2) The modern calcite deposition rate also shows characteristics of significant seasonal variation. In addition, modern calcite deposition rate of drip water (site LF-4) decreased year by year. On an interannual time scale, the recharge regime of this drip water is likely to be the main control on the modern calcite deposition rate. (3) The δ13CDIC values of the drip water in Liangfeng Cave (well ventilated) are generally higher than those of another cave (weakly ventilated) in the study area for winter and spring. The cave ventilation effect leads to a higher δ13CDIC value in the drip water during times of low cave pCO2. (4) Because Liangfeng Cave is a well-ventilated cave, the drip water could record the seasonal regional climatology variation. Observations from the well-ventilated Liangfeng Cave provide a reference point for the interpretation of regional climatology signals from speleothem proxy records.

Dissolved inorganic carbon isotopes of a typical alpine river on the Tibetan Plateau revealing carbon sources, wetland effect and river recharge

AbstractThe Nyangqu River, the largest right bank tributary of the Yarlung Zangbo River in the Qinghai–Tibet Plateau, was representative of an alpine riverine carbon cycle experiencing climate change. In this study, dissolved inorganic carbon (DIC) spatial and seasonal variations, as well as their carbon isotopic compositions (δ13CDIC) in river water and groundwater were systematically investigated to provide constraints on DIC sources, recharge and cycling. Significant changes in the δ13CDIC values (from −2.9‰ to −23.4‰) of the water samples were considered to be the result of different contributions of two dominant DIC origins: soil CO2 dissolution and carbonate weathering. Three types of rock weathering (dissolution of carbonate minerals by H2CO3 and H2SO4, and silicate dissolution by H2CO3) were found to control the DIC input into the riverine system. In DIC cycling, groundwater played a significant role in delivering DIC to the surface water, and DIC supply from tributaries to the main stream increased from the dry season to the wet season. Notably, the depleted δ13CDIC ‘peak’ around the 88.9° longitude, especially in the September groundwater samples, indicated the presence of ‘special’ DIC, which was attributed to the oxidation of methane from the Jiangsa wetland located nearby. This wetland could provide large amounts of soil organic matter available for bacterial degradation, producing 13C‐depleted methane. Our study provided insights regarding the role of wetlands in riverine carbon cycles and highlighted the contribution of groundwater to alpine riverine DIC cycles.

The temporal evolution of the carbon isotope composition of calcite in the presence of cyanobacteria

Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite (δ13CCalcite) and dissolved inorganic carbon (δ13CDIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO3 saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of 12C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75 × 10−14 to 1.71 × 10−13 mol 13C/m2/s. These rates are sufficiently fast such that the δ13CDIC value of aqueous fluids in calcite-rich rocks would be buffered by the δ13CCalcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local 12C sink such as photosynthetic microorganisms or 12C source such as decomposition of organic material.

Seawater chemistry of a modern subtropical ‘epeiric’ sea: Spatial variability and effects of organic decomposition

Direct studies of recent epeiric seawater chemistry and their geological significance are relatively few compared to the far more abundant studies from ancient epeiric marine carbonates. Acknowledging the limitations of analogue studies in the modern glaciated world, we propose that the recent Abu Dhabi offshore and lagoonal areas share important attributes with the shallow-marine coastal zones of a subtropical epeiric carbonate sea. The study area in Abu Dhabi is dwarfed by the vast epeiric seas of the geological past, but its seawater properties and related geochemistry have relevance for paleoenvironmental interpretations for similar ancient systems. This study documents the geochemical properties of seawater along the Abu Dhabi coast, and discusses their trends within a spatial context. Results show a positive correlation between salinity, temperature, elemental concentrations, and oxygen isotopic composition of seawater, indicating a significant influence of evaporation on seawater chemistry. A distal-to-proximal negative correlation between seawater Mg/Ca and Sr/Ca ratios may be related to a landward increase in abundance of high-Mg calcite foraminifera. After normalization to chloride, seawater elemental (Ca2+, Mg2+, Sr2+) concentrations and alkalinity show decreasing trends in a landward direction, consistent with seawater aging from the open marine to restricted lagoonal settings. Lower ΔAlk (alkalinity change) : ΔCa2+ (Ca2+ change) ratios in seawater compared with the theoretical value, and more negative δ13CDIC values relative to typical (sub)tropical oceans in the study area indicate an influence of factors apart from CaCO3 precipitation on properties of aged seawater. An input of alkalinity and 12C-enriched carbon by organic carbon decomposition (via sulfate reduction) during early diagenesis is proposed to account for these deviations from typical values. The input of alkalinity and 12C-enriched carbon by organic carbon decomposition may be further related to reduced seawater pCO2 and early-marine cementation. Findings discussed here suggest that organic carbon decomposition during early-marine diagenesis may influence the geochemical properties of the coastal areas of shallow epeiric seas, and thus assumedly produce significant differences in proxy data compared with those from carbonates deposited in open oceans.

Deciphering the hydroclimatic significance of dripwater δ13CDIC variations in monsoonal China based on modern cave monitoring

Stable carbon isotopic composition (δ13C) of speleothems has often been recognized as a proxy of vegetation and soil processes in many climatic regimes. In monsoonal regions, the speleothem δ13C records are thought to be able to document changes in local and/or regional precipitation as well. Due to the complexity of carbon isotopic evolution within heterogeneous karst aquifers, accurate interpretations of this proxy are challenging. It is essential to carry out detailed monitoring of cave dripwater δ13CDIC to disentangle various processes governing carbon isotopic evolution and decipher the hydroclimatic constraints. Here, we reported the results of a five-year monitoring of the δ13CDIC as well as hydrochemical compositions of dripwaters in the Maomaotou Big Cave, Guilin, South China. Great spatiotemporal variations in the dripwater δ13CDIC values were observed: (i) seepage flow-fed drips commonly had lower δ13CDIC values, with smaller temporal variation; (ii) the lowest δ13CDIC values were found at fracture-fed drips that had a large supply of soil CO2, whereas the fracture-fed drips with less, episodic CO2 recharge had much more positive δ13CDIC; (iii) the δ13CDIC exhibits remarkable season changes, particularly at fracture-fed drips where higher δ13CDIC values were observed in winter seasons; and (iv) an increasing trend in mean value of δ13CDIC at individual sites during 2015–2019 was found. Both hydrochemical and isotopic analyses revealed that the dripwater δ13CDIC is mainly controlled by spatiotemporal variations in water-CO2-rock interactions in association with hydrological processes, coupled with soil CO2 dynamics, which are closely linked to changes in local rainfall at monthly to annual timescales. Prior calcite precipitation (PCP) along the flow path above the cave could also affect the δ13CDIC of fracture-fed dripwaters in dry periods. Comparisons of the dripwater δ13CDIC records with local climate showed that the change of summer monsoonal rainfall, particularly the August-September-October precipitation, significantly influences the annual mean δ13CDIC value of dripwaters in caves of South China. Further comparison analyses of dripwater δ13CDIC records from both southern and northern China suggested that variations of dripwater δ13CDIC could reflect changes of regional monsoonal precipitation over inter-annual (and maybe decadal) timescale. This demonstrates that the δ13C of speleothems, if precipitated at isotopic equilibrium and not greatly influenced by the CO2 degassing, is likely to be a valuable proxy of monsoon rainfall variability across East Asia.

Effects of stream ecosystem metabolisms on CO2 emissions in two headwater catchments, Southeastern China

Headwater streams, as critical zones due to their proximity to terrestrial sources, emit large quantities of CO2 owing to their higher drainage density of catchments, substantial total areas, and stream lengths. Stream ecosystem metabolism (EM, i.e. photosynthesis and respiration) can significantly affect the in-stream carbon cycle. However, how stream CO2 emissions respond to ecosystem respiration (ER) is still not well known. Here we report in-situ daily observations of headwater stream gross primary production (GPP), ecosystem respiration (ER), isotopic composition of dissolved inorganic carbon (δ13CDIC) and CO2 emissions conducted in five streams in two headwater catchments of the Changjiang River in Southeastern China. The results show that δ13CDIC, pCO2, and CO2 emission rates have significant diel variations. Both the day and night δ13CDIC values were more depleted than the theoretical values in the two catchments, but the nocturnal depletion was greater. The headwater streams have high EM, with an average ER of −1.99 ± 0.82 g C m−2 d−1and GPP of 1.52 ± 0.84 g C m−2 d−1 in the two catchments. ER increases with DOC concentration but decreases with water discharge.The nocturnal pCO2 (3885 ± 2959 μatm) was significantly higher than in daytime (2525 ± 2221 μatm). The higher nocturnal CO2 emission rate (225 ± 165 mg C m−2h−1) was about 42 % higher than in daytime. DOC uptake velocity (Vf) was 0.46 ± 0.14 m d−1, and increased with lower water discharge and lower catchment-scale DOC yield. DOC loss rate (ζ) was 0.96 ± 1.25 d−1 and declined exponentially with higher water discharge and catchment-scale DOC yield. Moreover, the removal fraction of DOC via ER was about 0.32 ± 0.14, contributing to 31 ± 16 % of the CO2 flux in streams. The re-estimated annual CO2 flux was about 14.62 ± 4.99 Tg C yr−1 from the Changjiang headwater streams network. This study emphasizes the necessity of incorporating the effects of EM into the river network's CO2 emissions and the importance of the headwater streams to river carbon cycling.

Evaluating Freshwater Mussel Shell δ13C Values as a Proxy for Dissolved Inorganic Carbon δ13C Values in a Temperate River

AbstractThe stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land‐use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high‐resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo‐DIC in rivers. To test our hypotheses, we analyzed time‐series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of >30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high‐resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Role of anthropogenic sulfuric and nitric acids in carbonate weathering and associated carbon sink budget in a karst catchment (Guohua), southwestern China

Accurate estimate of carbon sink flux resulted from carbonate weathering in the karst area is of great significance for advancing the current understanding of the global carbon cycle and climate change. However, the carbon sink flux may be overestimated when the natural reaction with carbonic acid is influenced by protons of anthropogenic sulfuric and nitric acids, which can reduce the carbon sink flux during carbonate weathering. Here, we quantitatively evaluated the impact of anthropogenic sulfuric and nitric acids on carbonate weathering and carbon sink flux under base flow condition based on the stoichiometry of chemical compositions of groundwater from a typical karst catchment (Guohua), Guangxi, southwestern China. Seventy groundwater samples were analyzed for the characteristics of hydrochemistry and carbon isotopes of dissolved inorganic carbon (δ13CDIC) during the dry season in 2015. The results show that: (1) Ca2+ and Mg2+ dominate 92.4%–99.7% of the total cations, while HCO3− accounts for 83.0%–97.4% of total anions, indicating that the compositions of groundwater are primarily controlled by carbonate weathering; (2) The [Ca2++ Mg2+]/[HCO3−] equivalent ratios (1.00 to 1.41) and the corresponding δ13CDIC values (−16.8‰ to −8.1‰) of groundwater are all distributed between carbonate weathering by carbonic acid, and carbonate weathering by sulfuric and nitric acids. Meanwhile, the [Ca2++ Mg2+]/[HCO3−+ SO42−+ NO3−] ratios of groundwater are about 1, and the [Ca2++ Mg2+] equivalent concentrations have a good positive correlation with [HCO3−+ SO42−+ NO3−] and [SO42−+ NO3−], suggesting that sulfuric and nitric acids, in addition to carbonic acid, have been involved in carbonate weathering; (3) The contribution of sulfuric and nitric acids involved in carbonate weathering to (Ca2++ Mg2+) and HCO3− in groundwater varies from 0.6% to 58.0% (mean value of 20.9%) and from 0.3% to 40.9% (mean value of 12.2%), respectively. The carbonate weathering rate has increased by 20.9% while the karst carbon sink flux has decreased by 12.2%. Therefore, the carbon sink flux produced by carbonate weathering should be carefully evaluated when anthropogenic sulfuric acid and/or nitric acid are involved and the role of sulfuric and nitric acids in carbonate weathering could not be ignored in the global carbon cycle.

Extreme gradients in CO2 losses downstream of karstic springs.

Rivers are significant sources of CO2 to the atmosphere, and karstic watersheds are particularly important in this respect due to their large availability of inorganic carbon. This study examines characteristics of dissolved inorganic carbon (DIC) and excess partial pressures of CO2 (epCO2) in the source springs and headwaters of four watersheds in a Central European karstic region, via dissolved inorganic carbon concentration and stable carbon isotope measurements. Our results show the most 13C-depleted δ13CDIC values at the source springs, which become rapidly enriched downstream due to CO2 degassing. Concurrently, epCO2 values, while consistently in excess of atmospheric concentrations at the spring sources, show decreases of up to 92% within only 50m downstream distance. In conjunction with the large observed flux estimates of up to 88g C m-2day-1, these findings suggest that karstic springs are major CO2 sources to the atmosphere. Because headwater streams constitute the bulk of the surface area of most watersheds, they may provide a disproportionately large contribution to CO2 effluxes in carbonate-dominated basins, in which source springs play a particularly important role.

Stable Carbon and Sulfur Isotope Characteristics of Stream Water in a Typical Karst Small Catchment, Southwest China

Surface water samples from the Maolan National Natural Reserved Park (MNNRP) were analyzed from Sept. 2013 to June 2014, for major ion concentrations (K+, Na+, Ca2+, Mg2+, Cl−, SO42−, HCO3−), δ13C-DIC and δ34S-SO42− to quantify the sources of solutes and chemical weathering. The results show that HCO3− and SO42− are the main anions in Banzhai watershed, which account for 86.2 and 10.4% of the total anion equivalent, respectively. While Ca2+ and Mg2+ account for 76.9 and 20.5%, respectively. Considerable Mg2+ in stream water indicates that it may be affected by dolomite weathering. stream water samples present the δ13C-DIC values in the range of −16.9‰~−10.8‰ (mean value was −13.9‰), which were lower than that of the groundwater. The δ34S-SO42− values ranged from −15.2‰ to 1.7‰ (mean value was −4.4‰). There was a negative correlation between HCO3− content and δ13C value, implying the result of the interaction of temperature and precipitation intensity in different seasons. The significant positive correlation between SO42− content and δ13C-DIC indicates that H2SO4 may be involved in the weathering process of carbonate rocks in small watershed scale. The content of SO42− in a school sample site was much higher than that of other sample sites for the interference from human sources. The δ34S values show that the average δ34S-SO42− in most sites is close to the δ34S isotopic values of Guizhou coal and rain, indicating that they may be affected by local coal.

Spatiotemporal variations of sedimentary carbon and nitrogen isotopic compositions in the Yangtze Shelf Sea across the Ordovician-Silurian boundary

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted

Origin and geochemical evolution of groundwater in the Abaya Chamo basin of the Main Ethiopian Rift: application of multi-tracer approaches

The fractured volcanic aquifer of the Abaya Chamo basin in the southern Ethiopian Rift represents an important source for water supply. This study investigates the geochemical evolution of groundwater and the groundwater flow system in this volcanic aquifer system using hydrochemistry and environmental tracers. Water types of groundwater were found to transform from Ca-Mg-HCO3 (western part of Lake Abaya area) to Na-HCO3 (northwestern part), from the highland down to the Rift Valley. Silicate hydrolysis and Ca/Na ion exchange are the major geochemical processes that control groundwater chemistry along the flow path. Groundwaters are of meteoric origin. The δ18O and δD content of groundwater ranges from −4.9 to −1.1‰ and –27 to 5‰, respectively. The δ18O and δD values that lie on the summer local meteoric water line indicate that the groundwater was recharged mainly by summer rainfall. δ13CDIC values of cold groundwater range from −12 to −2.7‰, whereas δ13CDIC of thermal groundwater ranges from −8.3 to +1.6‰. The calculated δ13CCO2(g) using δ13CDIC and DIC species indicates the uptake of soil CO2 for cold groundwater and the influx of magmatic CO2 through deep-seated faults for thermal groundwater. In the western part of Lake Abaya area, the shallow and deep groundwater are hydraulically connected, and the uniform water type is consistent with a fast flow of large gradient. In contrast, in the northern part of Lake Abaya area, water underwent deep circulation and slow flow, so the water types—e.g. high F− (up to 5.6 mg/L) and Na+—varied laterally and vertically.

Different responses of ecohydrological processes in the re-vegetation area between the dip and anti-dip slope in a karst rocky desertification area in southwestern China

Rocky desertification is an economic, social, and environmental problem, which refers to the processes and human activities that transform a karst area covered by vegetation and soil into a rocky landscape. The effects of different types of soil cover on ecohydrological processes during the revegetation of areas undergoing karst rocky desertification remain unclear. Taking different measures for different ecohydrological situations will benefit re-vegetation in karst rocky desertification areas. Monthly/heavy rainfall hydrochemical data and the δ13C of dissolved inorganic carbon (DIC) in karst water samples from January 2017 to December 2019 were obtained. This revealed the controlling mechanisms on hydrochemical processes by δ13CDIC under different soil cover conditions in the dip/anti-dip slopes of a karst trough valley in Ganxi Town, Chongqing City, southwestern China. Hydrochemical variations are controlled by climate, soil, and vegetation and show clear seasonality. The δ13CDIC values (−16.88‰ to −3.49‰) in the study area were higher than the theoretical value (−14.00‰). In addition, it was observed that rain accelerated soil erosion (R2=0.7) in the karst trough valley and human activities (exogenous acid) resulted in DIC loss. The slopes, soil, and bare rock cover on the dip slope were different from that on the anti-dip slope, leading to the difference in their re-vegetation statuses. In this study, we found that different vegetation restoration schemes and treatment measures should be implemented between the dip and anti-dip slope. The protection of soil and water resources, especially soil, is the key to ecological restoration in rocky desertification areas.

Hydrogeochemical characterization of a possible carbon sink from shallow saline-alkaline groundwater in the eastern Hetao Basin of Inner Mongolia in China.

The question of how saline-alkaline groundwater can be used as a CO2 sink in arid saline-alkaline areas remains controversial. This study investigates the role of saline-alkaline groundwater as a CO2 sink using a mass balance method, Gibbs diagrams of the hydrochemistry, and carbon isotope (δ13CDIC) measurements. Twenty-eight groundwater samples of varying electrical conductivity (EC; 1.52-52.34 mS cm-1) were collected at different depths (1-2 and 5-25 m) in the Hetao Basin of Inner Mongolia, China. The results showed that groundwater ions could be primarily concentrated from water-rock interactions and evaporation, and that there are two main groundwater types: a mixed Na·Ca·Mg-Cl·SO4·HCO3 type and a Na-Cl type. The dissolved inorganic carbon (DIC) concentration in samples obtained from a depth of 1-2 m was less than that in samples from 5-25 m, and a downward migration trend of DIC in the groundwater was observed. The DIC concentration exhibited a significant positive correlation with pH (R2 = 0.61, p < 0.05) and the saturation index of carbonates (R2 = 0.93, p < 0.01). Groundwater with a higher pH contained a higher DIC concentration and could provide strong carbon sink potential. The δ13CDIC values of the groundwater samples varied from -21.22‰ to -11.02‰, indicating that DIC was derived from the dissolution equilibrium of pedogenic carbonates and atmospheric/soil CO2. The carbon sequestration of the shallow saline-alkaline groundwater in the Hetao Basin could reach 4.66 × 108 g C a-1, which represents important potential of carbon sink in the biogeochemical cycle.

Stable Carbon Isotopes of Phytoplankton as a Tool to Monitor Anthropogenic CO2 Submarine Leakages

This study aims to validate the stable carbon isotopic composition (δ13C) of phytoplankton as a tool for detecting submarine leakages of anthropogenic CO2(g), since it is characterised by δ13C values significantly lower than the natural CO2 dissolved in oceans. Three culture experiments were carried out to investigate the changes in δ13C of the diatom Thalassiosira rotula during growth in an artificially modified medium (ASW). Three different dissolved inorganic carbon (DIC) concentrations were tested to verify if carbon availability affects phytoplankton δ13C. Simultaneously, at each experiment, T. rotula was cultured under natural DIC isotopic composition (δ13CDIC) and carbonate system conditions. The available DIC pool for diatoms grown in ASW was characterised by δ13CDIC values (−44.2 ± 0.9‰) significantly lower than the typical marine range. Through photosynthetic DIC uptake, microalgae δ13C rapidly changed, reaching significantly low values (until −43.4‰). Moreover, the different DIC concentrations did not affect the diatom δ13C, exhibiting the same trend in δ13C values in the three ASW experiments. The experiments prove that phytoplankton isotopic composition quickly responds to changes in the δ13C of the medium, making this approach a promising and low-impact tool for detecting CO2(g) submarine leakages from CO2(g) deposits.

The transport mechanism of carbon isotopes based on 10 years of cave monitoring: Implications for paleoclimate reconstruction

Cave monitoring is crucial to unraveling the factors controlling carbon isotope compositions and their transportation in karst systems. Here, we report a 10-year (2010 ~ 2019) cave monitoring of carbon isotopes from the plants, soil, cave water, and speleothems in Jiguan Cave, Central China, located on the Chinese north–south divide in an area that is sensitive to the Asian monsoon. The results show that the cave ventilation and soil air CO2 controlled by the local temperature and humidity conditions are responsible for the seasonal variability in the δ13CDIC values. The variation in the δ13CDIC values and trace element ratios of the cave water are sensitive to the local hydrological conditions. Although there is the lag time from precipitation to cave water in drought year, the δ13CDIC and trace element ratios (Mg/Ca, Ba/Ca and Sr/Ca) of cave water are strongly coupled and reflect the extreme drought due to the influence of prior calcite precipitation (PCP) and water–rock interaction. The 10-year monitoring period covered two complete El Niño Southern Oscillation (ENSO) cycles. During the two strong El Niño years (2010 and 2015), the cave drip water δ13CDIC values were negative but the δ18O values were positive. This opposite trend is mainly due to their different controlling mechanisms. The rainfall amount and moisture sources are responsible for the δ13CDIC and δ18O values of the drip water, respectively. Throughout the karst cave system, the carbon isotopes in Jiguan Cave are continuously enriched when migrating from the vegetation to the soil to the cave drip water to the modern speleothems. Comparisons with the stalagmite δ13C records for the nearby Jiguan Cave suggest that stalagmite carbon isotopes may reflect the regional precipitation changes induced by the ENSO on the decadal scale. Overall, we demonstrate that the δ13C is a potentially reliable proxy that records the changes in the regional environment, especially precipitation changes, and is also a sensitive indictor that indirectly reflects ENSO.

Assessing methane cycling in the seep sediments of the mid-Okinawa Trough: Insights from pore-water geochemistry and numerical modeling

The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH4, dissolved inorganic carbon (DIC), Cl−, Br−, SO42−, Na+, Mg2+, Ca2+, Sr2+, and NH4+, and the δ13CDIC, δ13CCH4, and δDCH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13CCH4 ~ –70‰ V-PDB) and thermogenic (δ13CCH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH4 and DIC fluxes are ~30 and 20 mmol m−2yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems.