δ13CDIC Values Research Articles

A comparison of hydro-geochemistry and stable isotope composition of travertine-depositing springs, Garab in NE Iran and Pamukkale in SW Turkey

In this study, the hydro-geochemistry and stable isotope compositions (δ18O, δ2H, and δ13C) of travertine-depositing springs were investigated in two regions of Garab and Pamukkale, located in NE-Iran and SW-Turkey, respectively. The physical, chemical and isotopic dataset of water and travertine samples were obtained in situ measurements, laboratory analysis and from the literature. According to the high EC values (~ 2400 and ~ 10,500 μS/cm), the average δ13C-DIC values of water samples (10.4 and 7.2‰ VPDB), and its δ13C–CO2 values (1.5 and − 1.8‰ VPDB), it seems that the Garab and Pamukkale spring water were supplying from deep thermal groundwater with thermogenic origins and with contribution of carbonate dissolution through the rock-water interactions process. The more concentrations of Na+, K+, and Cl− in Garab water are related to subsequent admixture processes, which is originated from dissolving overloaded impure dissolve materials during upwelling water toward the ground level. The more enriched δ13C and δ18O values of Garab travertine samples (10.4 and − 7.1‰ VPDB, respectively) than that of Pamukkale travertine (7.2 and − 10.4‰ VPDB, respectively) is due to more CO2 degassing. The isotopic compositions of precipitation in both Garab (δ2H = 7.2∗δ18O + 11.2‰) and Pamukkale (δ2H = 8∗δ18O + 16‰) areas are characterized by greater d-excess compared to GMWL but smaller than of Mediterranean area. Although the isotopic compositions of both Garab and Pamukkale springs show the meteoric origin; however, the deviation from meteoric water lines is probably evident to oxygen isotope exchange with the deep host bedrock.

Rapid groundwater recharge dynamics determined from hydrogeochemical and isotope data in a small permafrost watershed near Umiujaq (Nunavik, Canada)

Hydrogeochemical data are used to better understand recharge dynamics and to support a hydrogeological conceptual model in a 2-km2 watershed in a discontinuous permafrost zone in Nunavik, Canada. The watershed contains an upper (surficial) and lower aquifer within Quaternary deposits, above and below a marine silt layer containing ice-rich permafrost mounds. The analysis is based on water samples from precipitation, groundwater monitoring wells, ground ice in permafrost mounds, thermokarst lakes and a perennial stream. Groundwater geochemistry in both aquifers reflects young, poorly evolved waters, with mainly Ca-HCO3 water types and low mineralisation ranging from 11 to 158 mg/L total dissolved solids (TDS), implying short pathways and rapid travel times of a year or less. While relatively low, TDS signatures in groundwater and surface water show increasing values downgradient. Groundwater isotope values (δ18OH2O and δ2HH2O) are often strongly influenced by snowmelt, while those of thermokarst lakes show evidence of evaporation. Recharge along the cuesta contributes to a transverse component of groundwater flow within the valley with higher TDS and δ13CDIC values influenced by open-system weathering. Even where permafrost-free, the marine silt unit has a strong confining effect and plays a more important role on recharge dynamics than the discontinuous permafrost. Nevertheless, the vulnerability of these types of hydrogeological aquifer systems is expected to increase due to rapid recharge dynamics associated with the gradual loss of the confining effect of permafrost. This hydrogeochemical data set will be useful as a baseline to document impacts of permafrost degradation on the hydrogeological system.

Using environmental isotopes to investigate the groundwater recharge mechanisms and dynamics in the North-eastern Basin, Palestine

ABSTRACTThis study aims to differentiate the potential recharge areas and flow mechanisms in the North-eastern Basin, Palestine. The results differentiate the recharge into three main groups. The first is related to springs and some of the deep wells close to the Anabta Anticline, through the Upper Aquifer (Turonian) formation, with depleted δ18O and δ2H. The second is through the Upper Cenomanian formation surrounding the Rujeib Monocline in the southeast, where the lineament of the Faria Fault plays an important role, with relatively enriched δ13CDIC values of about −4‰ (VPDB). The third is the Jenin Sub-series, which shows higher δ13CDIC values, with enriched δ18O and δ2H and more saline content. The deep wells from the Nablus area in the south of the basin indicate low δ13CDIC values due to their proximity to freshwater infiltrating faults. The deep wells located to the northwest of the basin have δ13CDIC values from −8 to −9‰ (VPDB), with enriched δ18O signatures, indicating slow recharge through thick soil.

Understanding the inorganic carbon transport and carbon dioxide evasion in groundwater with multiple sulfate sources during different seasons using isotope records

The geochemical cycle of carbon under anthropogenic activities controls the transport of carbon between surface sediments, hydrosphere and atmosphere, especially for atmospheric and marine carbon dioxide. Currently, the anthropogenically-induced carbon cycle in groundwater is an important part of the cycle in inland waters and cannot be underestimated as a geochemical process affecting the global carbon cycle. Therefore, there is a need to conduct deep research using isotopes in which the isotopic mixing and fractionation response to the carbon cycle can be assessed. The results showed that the δ34S-SO42− and δ18O-SO42− values in groundwater inherited the isotopic characteristics of four sulfate sources and shifted from acid mine drainage (AMD) derived from pyrite oxidation to atmospheric sulfates and agricultural fertilizers as well as domestic sewage along the groundwater flow path. In particular, the evolution of sulfate sources was accompanied by changes in the δ13C-DIC values in different seasons. During the dry season, the Rayleigh function described a possibility of enriched δ13C-DIC values in an AMD-affected aquifer, where a dropping water table or decreasing water content in unsaturated zones provided favorable acid buffering conditions. Bicarbonate neutralized protons to CO2 that was in turn spilled over the water surface. During the wet season, the depleted δ13C-DIC values in the AMD-affected aquifer reflected another possibility of the carbon cycle in which a rising water table caused a similar closed system to some extent and slowed down the replenishment of bicarbonate so that δ13C-DIC almost recorded the δ13C of soil CO2. In spite of the same AMD source, their influences on the carbon cycle in the dry and wet seasons were CO2 evasion and deficient CO2 uptake, respectively. By contrast, regardless of the season, the carbon cycle gradually shifted towards a net consumption of CO2 in the aquifer with atmospheric and agricultural sulfates as well as domestic sewage, and δ13C-DIC inherited the characteristic of HCO3−, indicating that these sources in anthropogenically-induced carbon processes hardly involved in isotopic fractionation of δ13C-DIC.

Testing the preservation potential of early diagenetic dolomites as geochemical archives

AbstractEarly marine diagenetic dolomite is a rather thermodynamically‐stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post‐depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U‐Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike‐slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric‐preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto‐Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high‐grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late‐stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically‐complex dolomite archives must be considered with caution.

Application of stable isotopes and dissolved ions for monitoring landfill leachate contamination.

We evaluated groundwater contamination by landfill leachate at a municipal landfill and characterized isotopic and hydrogeochemical evidence of the degradation and natural attenuation of buried organic matter at the study site. Dissolved ion content was generally much higher in the leachate than in the surrounding groundwater. The leachate was characterized by highly elevated bicarbonate and ammonium levels and a lack of nitrate and sulfate, indicating generation under anoxic conditions. Leachate δD and δ13CDIC values were much higher than those of the surrounding groundwater; some groundwater samples near the landfill showed a significant contamination by the leachate plume. Hydrochemical characteristics of the groundwater suggest that aquifer geology in the study area plays a key role in controlling the natural attenuation of leachate plumes in this oxygen-limited environment.

Temporal and spatial trends in marine carbon isotopes in the Arctic Ocean and implications for food web studies.

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ13C) signatures in dissolved inorganic carbon (δ13C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ13C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ13C values in consumers can change as a result of environmentally driven variation in the δ13C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ13C baseline, with spatial gradients in the δ13C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ13C‐DIC values (−0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO2) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ13C‐POC values and δ13C in arctic marine mammals reflects the anthropogenic CO2 signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ13C‐POC values in the Arctic Ocean and partially explain the δ13C values in marine mammals with consideration of time‐varying integration of δ13C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.

Geochemical Characteristics and Ecological Significance of Carbon Isotopes in Groundwater Under the Influence of Different Land Use Types in Karst Areas

Based on the seasonal characteristics of groundwater hydrochemistry and the carbon isotopes (δ13C) of dissolved inorganic carbon (DIC) in the Hongjiadu Basin, Guizhou Province, this paper discusses the natural processes and anthropogenic factors affecting the characteristics of δ13CDIC in karst groundwaters under different land use types. The results show that the main sources of DIC in groundwater are carbonate weathering and soil CO2. In winter, the δ13CDIC values for groundwater ranged from -14.8‰ to -4.1‰ with an average of -10.1‰ and, in summer, ranged from -14.5‰ to -6.3‰ with an average of -10.2‰. Sulfuric acid from sulfide oxidation in coal-bearing strata and acid rain is involved in carbonate weathering, resulting in the enrichment of groundwater with heavy carbon isotopes. Due to the soil CO2 effect, the δ13CDIC values of woodland groundwater experiencing less disturbance from human activities are lower in summer than in winter. The degradation of organic matter input from residential areas is a significant contributor of DIC to groundwater. The average values of δ13CDIC in winter and summer were -11.9‰ and -11.6‰, respectively, and the seasonal difference was relatively small in residential areas. During different seasons and for different types of land use, human activities could lead to differences in groundwater δ13CDIC values and hydrochemistry. Therefore, δ13CDIC can reflect the impact of human activities on karst aquifers, which has important ecological significance.

Application of Geochemical and Isotopic Tracers for Characterization of SAGD Waters in the Alberta Oil-Sand Region

Summary Mineral precipitation (scale) can significantly hinder production in petroleum reservoirs. This includes steam-assisted-gravity-drainage (SAGD) operations used for bitumen recovery in the Athabasca oil-sand region (AOSR) of northeastern Alberta, Canada. We explored whether select geochemical and isotope tracers (δ2H, δ18O, δ11B, δ34S, δ13C, 87Sr/86Sr) in SAGD-water sources can help to improve the understanding of the dynamics of reservoir fluids and their mixing in SAGD operations that might contribute toward scale precipitation. Pore water, bottom-formation water, steam condensate, and returned emulsions (produced bitumen and water) were sampled from an SAGD reservoir in northeastern Alberta and analyzed for geochemical and isotopic parameters. The results obtained indicate distinct Na and Cl concentrations and δ18O and δ2H values for these fluid sources. Significant differences in δ13CDIC, δ11B, and δ34S values and 87Sr/86Sr ratios were observed between bottom-formation water, steam condensate, and returned-water samples and hence constitute excellent tracers for bottomwater (BW) influx.

Hydrochemistry and Dissolved Inorganic Carbon (DIC) Cycling in a Tropical Agricultural River, Mun River Basin, Northeast Thailand.

Dissolved inorganic carbon isotope composition (δ13CDIC), together with major ion concentrations were measured in the Mun River and its tributaries in March 2018 to constrain the origins and cycling of dissolved inorganic carbon. In the surface water samples, the DIC content ranged from 185 to 5897 μmol/L (average of 1376 μmol/L), and the δ13CDIC of surface water ranged from −19.6‰ to −2.7‰. In spite of the high variability in DIC concentrations and partial pressure of carbon dioxide (pCO2), the δ13CDIC values of the groundwater were relatively consistent, with a mean value of −16.9 ± 1.4‰ (n = 9). Spatial changes occurred in the direction and magnitude of CO2 flux through water-air interface (FCO2). In the dry season, fluxes varied from −6 to 1826 mmol/(m2·d) with an average of 240 mmol/(m2·d). In addition to the dominant control on hydrochemistry and dissolved inorganic carbon isotope composition by the rock weathering, the impacts from anthropogenic activities were also observed in the Mun River, especially higher DIC concentration of waste water from urban activities. These human disturbances may affect the accurate estimate contributions of carbon dioxide from tropical rivers to the atmospheric carbon budgets.

Factors controlling the carbon isotope composition of dissolved inorganic carbon and methane in marine porewater: An evaluation by reaction-transport modelling

Abstract Carbon isotope compositions of dissolved inorganic carbon (DIC) and methane (CH4) in porewater of marine sediments at seafloor temperatures show very large variation covering a δ13C range from −100‰ to +35‰. These extreme values are the result of isotope fractionation during microbial carbon metabolism, but the combined effect of all factors controlling the isotope distributions is still not completely understood. We used a model approach to evaluate the effects of reaction and transport on carbon isotope distributions in modern sediment porewater under steady state. Simulated δ13CDIC profiles typically show negative values in the sulphate reduction zone and more positive values in the methanogenic zone. With increasing depth in the methanogenic zone, δ13C values approach a distribution where the offset of δ13CDIC from δ13C of total organic carbon (TOC) to more positive values is similar to the offset of δ13CCH4 to more negative values (δ13CDIC and δ13CCH4 approach a symmetric distribution relative to δ13CTOC). The model never exceeds this symmetry of the DIC-CH4 couple towards more positive values under steady-state conditions in a purely diffusive system. Our model shows that to reach an offset in δ13C between DIC and CH4 in the order of 70‰, as frequently observed in methanogenic zones, a larger fractionation than reported from culture experiments with acetoclastic or autotrophic methanogens would be required. In fact, the observed isotope offset in natural systems would be consistent with the known inorganic equilibrium fractionation factor at in-situ temperature, which may suggest isotope exchange via a microbial pathway, during methanogenesis. Furthermore, the model reproduces strongly negative δ13CCH4 values at the sulphate methane-transition (SMT) as result of a reverse flux of carbon from DIC to CH4 during AOM. Such a reverse AOM has no influence on the δ13CDIC at the SMT as methane is almost completely consumed. Only at high sedimentation rate combined with low porosity, δ13CDIC values significantly more negative than δ13CTOC occur at the SMT.

Current travertines precipitation related to artificial CO2 leakages from a natural reservoir (Gañuelas-Mazarrón Tertiary Basin, SE Spain)

In the framework of a natural CO2 reservoir with CO2 leakages as an analogue of a failed CO2 deep geological storage, the current precipitation of travertines and the associated upwelling of CO2-rich saline groundwater were analysed. This natural analogue is located in the Gañuelas-Mazarrón Tertiary Basin (SE Spain). The study comprises of the chemistry of both groundwater and travertines, including stable isotopes, mineralogy and petrography of the travertines, all this performed after a review of the geology of the basin. In this sense, the basin gathers the main features of a safe natural CO2 reservoir in a deep saline aquifer sealed by a thick marl formation. The aquifer was artificially perturbed by the drilling of wells, inducing the travertines precipitation at these water discharge points. Groundwater is saline, slightly acid, oversaturated in aragonite and calcite and with significant concentrations of heavy elements, some of them toxic. From an isotopic viewpoint, the relative constant δ13C-DIC values suggest that carbon is mainly inorganic in origin with minor organic and mantle contributions. Travertines are basically composed of aragonite or calcite, their precipitation being controlled by a sudden CO2 degassing and minor biological activity. Their δ13C signatures indicate that carbon mainly has an inorganic origin, although some contribution of organic carbon must be considered as well. Furthermore, these carbonate deposits did not precipitate in isotopic equilibrium, as determined by δ18O values. Finally, it is suggested that the appearance of travertines along with their carbon isotopic signatures represent efficient tools for detecting CO2 leakages from any CO2 storage site.

Factors affecting diurnal dissolved inorganic carbon and its isotopic composition in a small stream on a volcanic island in South Korea

Water samples were collected hourly at a small stream on Jeju Island, South Korea in both wet and dry seasons to identify the factors affecting dissolved inorganic carbon (DIC) and its isotopic composition (δ13CDIC). In the wet season, dissolved oxygen (DO) displayed a diurnal pattern with lower values in samples collected between sunset and sunrise compared to samples at other times. Likewise, a diurnal pattern was observed for pH, DIC and δ13CDIC. An overall negative correlation between DIC and DO indicated that photosynthesis and respiration in the stream are the main factors controlling DIC concentration and behavior. Notably, samples at night displayed two trends in plots of DO and DIC, with different δ13CDIC values. This was due to CO2 outgassing from the stream to the ambient atmosphere. Partial pressure of CO2 (pCO2), calcite saturation index (SIcalcite), and equilibrium isotope fractionation between HCO3− and CO2 gas were related to the δ13CDIC value, verifying that photosynthesis and respiration determined DIC behavior. In addition to microbial activity, CO2 outgassing contributed to DIC in the stream after respiration and before photosynthesis. In comparison, δ13CDIC values in the dry season were similar to those in the wet season but showed no clear relationship with pCO2 and SIcalcite. Unlike the wet season, this indicated that in the dry season, photosynthesis and respiration are not the dominant processes and that CO2 exchange between the atmosphere and water is more important in determining DIC.

Spatiotemporal variations of dissolved inorganic carbon and controlling factors in a small karstic catchment, Southwestern China

AbstractDissolved inorganic carbon (DIC) is the most important carbon component in karst aquatic system where fluid is highly transmissive, but has rarely been examined in the subtropical karst critical zone (K‐CZ). In this study, concentrations of dissolved solutes and isotopic compositions of DIC (δ13CDIC) at 11 sites of a 73.4 km2karstic catchment in Southwestern China were analysed monthly in order to uncover the spatiotemporal variations of both DIC and its dominant sources, and to identify relevant controlling factors. Both DIC concentrations and δ13CDICwere highly variable, ranging from 2.52 to 5.85 mmol l−1and from −15.7 to −4.5‰, respectively. DIC in underground water (UGW) was higher in concentration and more depleted in13C compared to surface water (SFS). DIC concentrations showed an inconsistent seasonal trend with other solutes, with higher values in the wet season at some sites. δ13CDICvalues were lower in the wet season than in the dry season. The results of mixing model IsoSource revealed spatiotemporal patterns of DIC sources. During the dry season, carbonate weathering was the primary contributor to DIC in UGW (excluding in the middle reaches). However, during the wet season, soil CO2was the dominant source of DIC in both UGW and SFS, and it was higher than in the dry season. Overall, there are significant spatiotemporal disparities and highly transmissive characteristics of both DIC and its sources in the K‐CZ, which are controlled by multiple factors. This study also highlights that rainfall may play a crucial role in accelerating carbon dynamics in the K‐CZ. High‐frequency sampling campaigns in high‐flow periods and deep analyses are needed in future work to elucidate the related processes and mechanisms. © 2019 John Wiley & Sons, Ltd.

CO2-dependent carbon isotope fractionation in Archaea, Part II: The marine water column

Abstract Stable carbon isotope ratios of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids have been proposed as a proxy to infer past changes in the carbon isotope composition (δ13C) of dissolved inorganic carbon (DIC). The premise for paleo-δ13CDIC reconstructions from GDGTs is based on observations of relatively constant δ13CGDGT values in recent depositional environments. Marine Thaumarchaeota, thought to be the dominant source of GDGTs to marine sediments, fix inorganic carbon using the 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway, which is specific to HCO3– as the substrate. Bicarbonate-dependent autotrophy has been the basis for predicting that the stable carbon isotopic composition of GDGTs (δ13CGDGT) should vary in parallel with water column δ13CDIC values, because HCO3– is by far the dominant fraction of DIC in modern seawater. However, this relationship has never been systematically tested. Here we examine the carbon isotopic composition of GDGTs from four water column profiles in the Southwest and Equatorial Atlantic Ocean. Values of δ13CGDGT increase with depth in the water column, in contrast to the characteristic decrease in δ13CDIC values. These divergent trends imply a decrease in the observed total biosynthetic isotope effect (eAr) with depth, i.e., the offset between δ13CDIC and δ13CGDGT is not constant. Instead, we find that values of eAr specifically correlate with oceanographic variables associated with extent of organic remineralization, decreasing as CO2 concentration increases. This observed relationship is consistent in both magnitude and direction with the results of an isotope flux-balance model for Thaumarchaeota that suggests eAr should be sensitive to growth rate (μ) and CO2 availability under conditions of atmospheric pCO2

Geochemical and isotopic characteristics of geothermal discharges in the Emet Basin, Western Anatolia, Turkey

The geochemical and isotopic compositions of low-temperature thermal waters in the N–S-oriented Emet Basin, located in Western Anatolia, Turkey, were comprehensively investigated. Three distinct physicochemical processes affecting the chemical composition of the thermal waters were distinguished at this site: dissolution of CaCO3, gradual transformation of Ca–HCO3-type waters to Ca–SO4-type waters by the addition of SO4, and mixing of deep Na–HCO3-type waters with Ca–SO4-type waters. The first two processes are associated with the dissolution of carbonates and evaporites, respectively, and predominantly control the chemistry of thermal waters in the basin. The third process is observed locally and is manifested as CO2-rich thermal waters with relatively higher total ionic concentration.The majority of the thermal waters lie close to the local meteoric water line with a deuterium excess of +17‰. The single CO2-rich thermal water in the basin, located in the Yeniceköy area, shows a slight enrichment in 18O and a shift from the local meteoric water line. The δ34SSO4 and δ18OSO4 values of the dissolved sulfate range from 14.49‰ to 22.46‰ (vs. V-CDT) and 11.9‰–17.8‰ (vs. V-SMOW), respectively, which indicates that the source of sulfate in the thermal waters is evaporites such as gypsum and anhydrite in the basin. The δ13CDIC values of the thermal waters vary between −0.63‰ and −10.36‰ (vs. V-PDB). By using a binary mixing model, the δ13C value of the deep carbon source associated with the formation of CO2-rich thermal waters was calculated as 0‰, which is a typical value of marine limestones. Thermal decomposition of marine limestones and dissolution of carbonates dominate the carbon budgets of thermal waters, whereas the organic carbon contribution is generally less than 20% and the mantle-derived carbon contribution was found to be insignificant.Air-corrected 3He/4He ratios of the thermal waters ranging from 0.34 RA to 1.57 RA indicate that radiogenic (crustal) helium is dominant in the region, although up to 19.4% of the total helium is of mantle origin. The 3He/heat ratios vary from 0.4 × 10−15 cm3 STP/J to 13.2 × 10−15 cm3 STP/J, which are very close to the ratios in the continental environment. This indicates that the dominant heat source in the region is radiogenic.

Carbon, nitrogen and sulfur isotopic features and the associated geochemical processes in a coastal aquifer system of the Pearl River Delta, China

The coastal zone is not only a terrestrial-marine buffer system but also the main site of the organic matter decomposition process, which complicates the identification of the sources of bioactive elements (C, N and S) and the factors that control C-N-S cycling. Here, a multi-isotope approach is developed to distinguish C, N and S sources and the associated geochemical processes in a coastal aquifer system of the Pearl River Delta (PRD), China. The PRD is characterized as a typical multilayer groundwater system with a reducing environment in the confined aquifers and an oxidizing environment in the unconfined aquifer. For the groundwater samples from the unconfined aquifer, the δ15N values showed that the ammonium and nitrate originated from manure and sewage, the δ13C-DIC indicated that dissolved inorganic carbon (DIC) was mainly derived from soil CO2 and weathering of carbonate rocks, and the δ34S and δ18O signals of sulfate suggested that SO42− originated from the dissolution of continental evaporate. The oxidizing environment and the positive relationship between δ15N-NO3− and δ18O-NO3− suggested that nitrification and denitrification were the key factors influencing the transformation of nitrogen in the unconfined aquifer. No obvious correlation between C, N and S content and isotopic composition suggested C-N-S was loosely coupled in this aquifer.For the groundwater samples in the confined aquifers, the δD-H2O and δ18O-H2O signals suggested that partially confined groundwater samples were affected by seawater intrusion. In the non-seawater intrusion region (NSI zone), the C, N and S content combined with C, N and S isotopic composition indicated that ammonium was derived from the mineralization of organic matter, sulfate mainly originated from rock weathering, and DIC mainly originated from carbonate mineral dissolution and the CO2 produced by organic matter mineralization. Extremely high CH4 concentrations suggested the existence of methanogenesis, while the molar ratio of CH4/(CH4 + (1 + α) × CO2) indicated that both acetate fermentation and CO2 reduction were ubiquitous. CO2 reduction had a stronger isotopic fractionation effect on the δ13C-DIC values, thus, the relatively high δ13C-DIC values were observed. The electron acceptor was the key factor for organic matter mineralization, and CO2 acted as the main electron acceptor in the NSI Zone. Accordingly, acetate fermentation and CO2 reduction became the key factors driving ammonium release in the NSI zone. In the seawater intrusion region (SI zone), ammonium was derived from the mineralization of organic matter, sulfate was derived from the mixing of fresh water and seawater, and DIC was derived from carbonate mineral dissolution, seawater and the CO2 produced by organic matter mineralization. The δ34S-SO42− showed a positive correlation with δ18O-SO42− and a negative correlation with SO42−, suggesting the existence of bacterial sulfate reduction (BSR). High CH4 concentrations also suggested the existence of methanogenesis. The CH4/(CH4 + (1 + α) × CO2) ratios indicated that CO2 reduction existed in more than half of the confined groundwater samples in the SI zone. C-N-S was highly coupled in this region because both BSR and methanogenesis can increase the DIC content, while BSR enriches lighter carbon isotopes and methanogenesis enriches heavier carbon isotopes; thus, a wide range of δ13C-DIC values was found in SI zone. Sulfate or both sulfate and CO2 were the electron acceptors. Accordingly, acetate fermentation, sulfur reduction and CO2 reduction worked together to promote ammonium release for the groundwater samples in the SI Zone. However, the exhaustion of sulfate will make CO2 reduction the key factor for organic matter mineralization in the future.

Effects of depositional environment and organic matter degradation on the enrichment and mobilization of iodine in the groundwater of the North China Plain

Groundwater iodine has direct importance for human dietary iodine intake in areas where drinking water is of groundwater origin. However, little is known about enrichment and mobilization mechanisms of groundwater iodine in the North China Plain (NCP). Geochemistry, inorganic/organic carbon isotope and biomarker of groundwater and sediment samples were studied to reveal the effects of depositional environment and organic matter (OM) degradation on the generation of high iodine groundwater (>100 μg/L) in NCP. Results showed that groundwater iodine had a range of 7.2–800 μg/L and was increasing with increase in HCO3 concentration and decrease in groundwater δ13CDIC value, indicating the potential effects of microbial activity on the elevation of groundwater iodine. Sediments iodine ranged from 0.03 to 2.54 μg/g and higher contents occurred under the oxidizing depositional environment (higher Pr/Ph ratios). Biomarker analysis indicated that the marine iodine-rich OM is considered as the main source of groundwater iodine, which is prone to be released into groundwater by the microbial degradation under the reducing conditions. The hypothesis was evidenced by the 13Corg, 13CDIC and 3-D excitation emission matrices of groundwater. These results suggest that carbon-related biogeochemical cycling and redox condition are important in the enrichment and mobilization of iodine in groundwater system.

Groundwater quality of a public supply aquifer in proximity to oil development, Fruitvale oil field, Bakersfield, California

Due to concerns over the effects of oil production activities on groundwater quality in California, chemical, isotopic, dissolved gas and age-dating tracers were analyzed in samples collected from public-supply wells and produced-water sites in the Fruitvale oil field (FVOF). A combination of newly collected and historical data was used to determine whether oil formation fluids have mixed with groundwater used for public supply and what the potential pathways for the migration of oil formation fluids into groundwater may be. Stable isotopes of water (δ2H and δ18O) and age dating (3H, 3Hetrit, SF6 and 14C) tracers in groundwater samples were consistent with the Kern River being the main source of recharge to aquifers. The distribution of major ion concentrations and pH with distance from the Kern River indicate that natural processes were the primary controls on groundwater salinity. Two of 14 groundwater samples had δ13C-DIC values (−2.4 to +1.9 per mil) consistent with mixtures of <1 to about 9 percent oil-field water. Concentrations of TDS in groundwater samples were generally much lower (129–1,200 milligrams per liter (mg/l), median 216 mg/l) than produced water samples (586–24,930 mg/l, median 2,717 mg/l), suggesting that any mixing of oil-field water with groundwater has not significantly affected groundwater salinity. Trace concentrations of thermogenic methane were detected in three groundwater samples that did not have dissolved inorganic or isotopic indicators consistent with mixing of oil-field water, suggesting that stray gases may have migrated from the subsurface via preferential pathways such as leaky well bores into groundwater aquifers. Low concentrations of petroleum hydrocarbons were detected in samples that also contained anthropogenic VOCs and components of post- and pre-1950s recharge, indicating that petroleum hydrocarbons could have come from subsurface and/or surface sources. Overall, the results of this study indicated that groundwater currently used for public supply in the FVOF was of good quality with little, if any, effects from oil production activities. This may be due in part to the relatively rapid flushing of the aquifer system by recharge from the Kern River.

The Effect of a Tropical Climate on Available Nutrient Resources to Springs in Ophiolite-Hosted, Deep Biosphere Ecosystems in the Philippines.

Springs hosted in ophiolites are often affected by serpentinization processes. The characteristically low DIC and high CH4 and H2 gas concentrations of serpentinizing ecosystems have led to interest in hydrogen based metabolisms in these subsurface biomes. However, a true subsurface signature can be difficult to identify in surface expressions such as serpentinizing springs. Here, we explore carbon and nitrogen resources in serpentinization impacted springs in the tropical climate of the Zambales and Palawan ophiolites in the Philippines, with a focus on surface vs. subsurface processes and exogenous vs. endogenous nutrient input. Isotopic signatures in spring fluids, biomass, and carbonates were examined to identify sources and sinks of carbon and nitrogen, carbonate geochemistry, and the effect of seasonal precipitation. Seasonality affected biomass production in both low flow and high flow spring systems. Changes in meteorological precipitation affected δ13CDIC and δ13CDOC values of the spring fluids, which reflected seasonal gain/loss of atmospheric influence and changes in exogenous DOC input. The primary carbon source in high flow systems was variable, with DOC contributing to biomass in many springs, and a mix of DIC and carbonates contributing to biomass in select locations. However, primary carbon resources in low flow systems may depend more on endogenous than exogenous carbon, even in high precipitation seasons. Isotopic evidence for nitrogen fixation was identified, with seasonal influence only seen in low flow systems. Carbonate formation was found to occur as a mixture of recrystallization/recycling of older carbonates and rapid mineral precipitation (depending on the system), with highly δ13C and δ18O depleted carbonates occurring in many locations. Subsurface signatures (e.g., low DOC influence on Cbiomass) were most apparent in the driest seasons and lowest flow systems, indicating locations where metabolic processes divorced from surface influences (including hydrogen based metabolisms) are most likely to be occurring.