δ13CDIC Values Research Articles

Stable isotope characteristics of CBM co-produced water and implications for CBM development: The example of the Shizhuangnan block in the southern Qinshui Basin, China

Coalbed methane (CBM) is a rapid growing industry in the Qinshui Basin, China. However, as a relatively new industry, few studies have examined the co-produced water from CBM in terms of either water quality or stable isotope composition. This study aims to determine the isotopic composition of coal mine roof water and CBM co-produced water, to correlate stable isotope characteristics of the co-produced water with gas and groundwater production and flow pathways, and to identify the origin of the co-produced water of the Shizhuangnan CBM block in the southern Qinshui Basin. Stable isotope analyses were conducted on 58 water samples collected from three coal mines and 29 CBM wells. Well production data including gas and water production rates, and well liquid level were also collected. Ten additional sets of literature data were also used in the analyses from this area. The stable isotope analyses show that both coal seam roof water and CBM co-produced water are predominantly meteoric origin, with a trend towards lower isotopic composition where the aquifer is shallower. It was found that co-produced water samples with δD and δ18O values above the local meteoric water line (LMWL) were associated with higher gas production, whereas those with δD and δ18O values lower than the LMWL were associated with lower gas production. In addition, the co-produced water of highly productive gas wells has positive and relatively higher δ13C-DIC (DIC, dissolved inorganic carbon) values, while water from wells with no gas production has negative δ13C-DIC values. We propose that coal seam water is characterized by “D drift” above the LMWL in the plots of δD vs. δ18O and positive δ13C-DIC values. On the contrary, sandstone and limestone aquifer water is normally characterized by “18O drift” below the LMWL and negative δ13C-DIC values. Therefore, the findings of this study will help to identify the origin of co-produced water, and guide CBM exploration and development. They can also be used in the management of CBM co-produced water.

Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

Globally, dissolved inorganic carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here, we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low-lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon, particulate organic carbon and DIC concentrations and stable isotope ratios of DIC (δ13CDIC) at two time scales: seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser was used to measure diel DIC concentration and δ13CDIC changes at a 15-min temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55 and 3.82 mg/l and δ13CDIC values ranging from −19.7 ± 0.31‰ to −17.1 ± 0.08‰. In contrast, at the seasonal scale, we observed wet season DIC variations predominantly from mixing processes and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/l) and δ13CDIC values of river water (by 5.4‰) largely result from proportional increases in subsurface inflows from the savanna plains (C4 vegetation) region relative to inflows from the rainforest (C3 vegetation) highlands. The high DIC river load during the wet season resulted in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river, there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial carbon sink in the savannas of northern Australia during wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere.

Inorganic and organic carbon dynamics in forested soils developed on contrasting geology in Slovenia—a stable isotope approach

Soil carbon dynamics were studied at four different forest stands developed on bedrocks with contrasting geology in Slovenia: one plot on magmatic granodiorite bedrock (IG), two plots on carbonate bedrock in the karstic-dinaric area (CC and CD), and one situated on Pleistocene coalluvial terraces (FGS). Throughfall (TF) and soil water were collected monthly at each location from June to November during 2005–2007. In soil water, the following parameters were determined: T, pH, total alkalinity, concentrations of Ca2+ and Mg2+, dissolved organic carbon (DOC), and Cl− as well as δ13CDIC. On the other hand, in TF, only the Cl− content was measured. Soil and plant samples were also collected at forest stands, and stable isotope measurements were performed in soil and plant organic carbon and total nitrogen and in carbonate rocks. The obtained data were used to calculate the dissolved inorganic carbon (DIC) and DOC fluxes. Statistic analyses were carried out to compare sites of different lithologies, at different spatial and temporal scales. Decomposition of soil organic matter (SOM) controlled by the climate can explain the 13C and 15 N enrichment in SOM at CC, CD, and FGS, while the soil microbial biomass makes an important contribution to the SOM at IG. The loss of DOC at a soil depth of 5 cm was estimated at 1 mol m−2 year−1 and shows no significant differences among the study sites. The DOC fluxes were mainly controlled by physical factors, most notably sorption dynamics, and microbial–DOC relationships. The pH and pCO2 of the soil solution controlled the DIC fluxes according to carbonate equilibrium reactions. An increased exchange between DIC and atmospheric air was observed for samples from non-carbonate subsoils (IG and FGS). In addition, higher δ13CDIC values up to −19.4 ‰ in the shallow soil water were recorded during the summer as a consequence of isotopic fractionation induced by molecular diffusion of soil CO2. The δ13CDIC values also suggest that half of the DIC derives from soil CO2 indicating that 2 to 5 mol m−2 year−1 of carbon is lost in the form of dissolved inorganic carbon at CC and CD after carbonate dissolution. Major difference in soil carbon dynamics between the four forest ecosystems is a result of the combined influence of bedrock geology, soil texture, and the sources of SOM. Water flux was a critical parameter in quantifying carbon depletion rates in dissolved organic and inorganic carbon forms.

The response of inorganic carbon distributions and dynamics to upwelling-favorable winds on the northern Gulf of Mexico during summer

Upwelling-favorable winds and an offshore-distributed Mississippi and Atchafalaya River plume trajectory were observed in summer 2009 in contrast to the mean conditions from 2002 to 2010 (upwelling-unfavorable winds and an alongshore river plume trajectory), a set of conditions which was also observed in summer 2007. The responses of dissolved inorganic carbon (DIC) distributions and dynamics to upwelling-favorable winds are studied by comparing the contrasting conditions between summer 2009 and summer 2007 on the northern Gulf of Mexico. Patterns of surface water partial pressure of CO2 (pCO2), DIC, δ13C in DIC, and total alkalinity (TA) determined in July 2009 and August 2007 were strongly related to river plume trajectories, and differed between the two summers.The slope of the relationship between dissolved oxygen (DO) and DIC in summer 2007 was comparable to the Redfield O/C ratio of 1.3, which was attributed to respiration of organic matter in the bottom water. The slope of the DO and DIC relationship and δ13CDIC values in bottom waters during July 2009 were clearly affected by mixing since their salinities were <35. A three end-member mixing model was used to remove mixing effects in (1) δ13CDIC, to estimate the organic source of respiration, and (2) in DIC concentrations, to calculate DIC removal and release. δ13CDIC results in both summers were consistent with an apparent release of DIC in hypoxic waters (DO less than 2mgL−1) associated with respiration of surface organic matter. The area-weighted surface DIC removal (i.e., biological production) was lower in 2009 than in 2007 on the shelf, as the plume was distributed offshore. The release of DIC in bottom waters was higher over the shelf in 2009 and was surmised to be related to stronger mixing, which was favorable for the DO supply for respiration. Overall, surface waters on the continental shelf in the region of study in July 2009 acted as a weak CO2 source to the atmosphere, but a weak CO2 sink in August 2007. We contend that the inorganic carbon distribution and concentrations on the shelf were related to regional wind forcing, through its influence on the distribution of coastal currents and plume trajectories and their subsequent impact on biogeochemical processes.

Quantitative analysis of intraspecific variations in the carbon and oxygen isotope compositions of the modern cool-temperate brachiopod Terebratulina crossei

This study unravels intraspecific variations in the carbon isotope (δ13C) and oxygen isotope (δ18O) compositions of shells of the modern cool-temperate brachiopod Terebratulina crossei collected at a water depth of 70m in Otsuchi Bay, northeastern Honshu, Japan. Brachiopod shells have been used as proxies of the δ13C values of dissolved inorganic carbon (DIC) (δ13CDIC) and seawater temperature/δ18O (δ18OSW) values to reconstruct the evolution of Phanerozoic oceans. To identify more reliable shell portions as the proxies, we conducted a rigorous time-series comparison of δ13C and δ18O values between the brachiopod shells and calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite) (δ13CEC and δ18OEC values, respectively). Samples were collected from the outer and inner surfaces of the secondary shell layer along the maximum growth axis (ontogenetic-series and inner-series samples, respectively). The ontogenetic-series δ13C values, which showed regular annual and irregular non-annual cycles, partly fell in but were mostly less than the range of the δ13CEC values. The δ13C cycles were often associated with one or two minor negative peaks. The peaks were likely resulted from an increased incorporation of respiration-derived 12C due to elevated metabolic activity during spawning. The ontogenetic-series δ18O values showed distinct seasonal variations and were mostly within the range of δ18OEC values. The amplitude of the δ18O profiles was relatively large during the younger fast-growth stage, and decreased during the senescent slow-growth stage. The inner-series δ13C and δ18O values of individual shells varied within narrow ranges. The inner-series δ13C values were close to the minimum δ13CEC values. The inner-series δ18O values were in the upper range of the δ18OEC values. Kinetic isotope fractionation effects were evident, but its degree varied among different shells. We identified the shell portions reliably recording past ocean environments. The best estimates of annual average δ13CDIC values were obtained from ontogenetic-series δ13C values near the anterior shell edge and from inner-series δ13C values; those of the annual average seawater temperature were obtained from ontogenetic-series δ18O values from the entire transect and from the fast-growth stage.

Investigation of the geographical provenance of the beer available in South Korea using multielements and isotopes

Although beer has the largest share of the global alcoholic beverage market, only limited studies of beer have been performed in terms of elemental and isotopic compositions. Here, we measured elemental and isotopic compositions—including carbon and oxygen isotopes as well as 87Sr/86Sr ratios—of beers available in South Korea to examine geographical differences. Although most of the elements analyzed in this study were not markedly different between the beer samples, there was a clear distinction in the isotopic compositions of the samples. δ13CDIC values indicated that most of the beer samples were produced from C3 plants, such as barley or wheat. δ18O values allowed the samples to be discriminated by latitude, reflecting a negative correlation between latitude and isotopic composition. Similarly, 87Sr/86Sr ratios were different between the samples due to the bedrock. Statistical analysis of the combination of elemental and isotopic compositions showed a clear difference between the beer samples according to the geographical provenance on a continental scale. This study demonstrated a powerful method for distinguishing diverse beers according to their geographical provenance, and will make an important contribution to research into discrimination of the geographical origin of diverse processed foods available commercially.

Isotopic composition of carbon in atmospheric air; use of a diffusion model at the water/atmosphere interface in Velenje Basin

CO2 concentrations (partial pressure of CO2, pCO2), and isotope compositions of carbon dioxide in air (δ13CO2), temperature (T) and relative humidity (H) have been measured in the atmosphere in the Velenje Basin. Samples were collected monthly in the calendar year 2011 from 9 locations in the area where the largest thermal power plant in Slovenia with the greatest emission of CO2 to the atmosphere (around 4M t/year) is located. Values of pCO2 ranged from 239 to 460 ppm with an average value of 294 ppm, which is below the average atmospheric CO2 pressure (360 ppm). δ13CO2 ranged from -18.0 to -6.4 ‰, with an average value of -11.7 ‰. These values are similar to those measured in Wroclaw, Poland. We performed the comparison of δ13CO2 values in atmospheric air with Wroclaw since researchers used similar approach to trace δ13CO2 around anthropogenic sources. The isotopic composition of dissolved inorganic carbon (δ13CO2) in rivers and lakes from the Velenje basin changes seasonally from -13.5 to -7.1 ‰. The values of δ13CDIC indicate the occurrence of biogeochemical processes in the surface waters, with dissolution of carbonates and degradation of organic matter being the most important. A concentration and diffusion model was used to calculate the time of equilibration between dissolved inorganic carbon in natural sources (rivers) and atmospheric CO2.

Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea

Cold hydrocarbon seepage is a frequently observed phenomenon along continental margins worldwide. However, little is known about the impact of seeping fluids on the geochemical cycle of redox-sensitive elements. Pore waters from four gravity cores (D-8, D-5, D-7, and D-F) collected from cold-seep sites of the northern South China Sea were analyzed for SO42−, Mg2+, Ca2+, Sr2+, dissolved inorganic carbon (DIC), δ13CDIC, dissolved Fe, Mn, and trace elements (e.g. Mo, U). The sulfate concentration–depth profiles, δ13CDIC values and (ΔDIC+ΔCa2++ΔMg2+)/ΔSO42− ratios suggest that organoclastic sulfate reduction (OSR) is the dominant process in D-8 core. Besides OSR, anaerobic oxidation of methane (AOM) is partially responsible for depletion of sulfate at D-5 and D-7 cores. The sulfate consumption at D-F core is predominantly caused by AOM. The depth of sulfate–methane interface (SMI) and methane diffusive flux of D-F core are calculated to be ~7m and 0.035molm−2yr−1, respectively. The relatively shallow SMI and high methane flux at D-F core suggest the activity of gas seepage in this region. The concentrations of dissolved uranium (U) were inferred to decrease significantly within the iron reduction zone. It seems that AOM has limited influence on the U geochemical cycling. In contrast, a good correlation between the consumption of sulfate and the removal of molybdenum (Mo) suggests that AOM has a significantly influence on the geochemical cycle of Mo at cold seeps. Accordingly, cold seep environments may serve as an important potential sink in the marine geochemical cycle of Mo.

Measurements of the stable carbon isotope composition of dissolved inorganic carbon in the northeastern Atlantic and Nordic Seas during summer 2012

Abstract. The stable carbon isotope composition of dissolved inorganic carbon (δ13CDIC) in seawater was measured in a batch process for 552 samples collected during two cruises in the northeastern Atlantic and Nordic Seas from June to August 2012. One cruise was part of the UK Ocean Acidification research programme, and the other was a repeat hydrographic transect of the Extended Ellett Line. In combination with measurements made of other variables on these and other cruises, these data can be used to constrain the anthropogenic component of dissolved inorganic carbon (DIC) in the interior ocean, and to help to determine the influence of biological carbon uptake on surface ocean carbonate chemistry. The measurements have been processed, quality-controlled and submitted to an in-preparation global compilation of seawater δ13CDIC data, and are available from the British Oceanographic Data Centre. The observed δ13CDIC values fall in a range from −0.58 to +2.37 ‰, relative to the Vienna Pee Dee Belemnite standard. The mean of the absolute differences between samples collected in duplicate in the same container type during both cruises and measured consecutively is 0.10 ‰, which corresponds to a 1σ uncertainty of 0.09 ‰, and which is within the range reported by other published studies of this kind. A crossover analysis was performed with nearby historical δ13CDIC data, indicating that any systematic offsets between our measurements and previously published results are negligible. Data doi:10.5285/09760a3a-c2b5-250b-e053-6c86abc037c0 (northeastern Atlantic), doi:10.5285/09511dd0-51db-0e21-e053-6c86abc09b95 (Nordic Seas).

Influence of the carbon isotopic composition of methane and the proportion of methane-derived bicarbonate on the 13C/12C ratio of dissolved inorganic carbon at the sulfate–methane transition in the Joetsu Basin area, eastern margin of the Sea of Japan

The geochemistry of interstitial waters associated with active marine hydrocarbon seepage provides insight into the source and ongoing dynamics of such systems. In this investigation of interstitial waters obtained from sediments in a gas hydrate region of the Sea of Japan, we compare carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) at the sulfate–methane transition (SMT) with δ13C of deeply sourced methane (as inferred from the deepest samples obtained beneath the SMT). Based on this comparison, we distinguish two causes for lower values of δ13CDIC at the SMT. In Group A (17 piston cores), δ13CDIC values at the SMT (−30.0 to −9.7‰ VPDB) correlate positively with δ13C of deep methane (−90.7 to −61.4‰ VPDB). Lowering of these δ13CDIC values was independent of methane flux, which controls the generation rate of methane-derived bicarbonate by anaerobic oxidation of methane, owing to the influence of a high alkalinity flux from depth. In the Group B core samples, δ13CDIC at the SMT (−41.6 to −14.2‰ VPDB) relative to δ13C of deep methane (−68.2 to −40.5‰ VPDB) is lower than that of Group A, implying a greater contribution of methane-derived HCO3−. With only two exceptions, the Group B core sites were located at areas of active gas seepage. The admixture of methane from migrating methane bubbles with deep methane in the interstitial waters of sediments has the potential to significantly increase the overall methane flux at gas seep sites. Although the admixture may reflect higher δ13C values of deep methane, the increase in methane flux results in a higher proportion of methane-derived HCO3− and lower δ13CDIC at the SMT. Recovery of two group B cores adjacent a dormant pockmark suggests that the depression with seismic blanking beneath it, similar to that of the gas seep sites, is a previous gas seepage site.

Response of dissolved inorganic carbon (DIC) and δ13CDIC to changes in climate and land cover in SW China karst catchments

Monthly hydrochemical data and δ13C of dissolved inorganic carbon (DIC) in karst water samples from September 2007 to October 2012 were obtained to reveal the controlling mechanisms on DIC geochemistry and δ13CDIC under different conditions of climate and land cover in three karst catchments: Banzhai, Dengzhanhe and Chenqi, in Guizhou Province, SW China. DIC of karst water at the Banzhai site comes mainly from carbonate dissolution under open system conditions with soil CO2 produced by root respiration and organic carbon decomposition with lowest δ13C values under its dense virgin forest coverage. Weaker carbonate bedrock dissolution due to sparse and thin soil cover results in lower δ13CDIC, pCO2, DIC and EC, and lower cation and anion concentrations. At the Chenqi site, larger soil CO2 input from a thick layer of soil results in high pCO2 and DIC, and low pH, SIc and δ13CDIC in the karst water. At the Dengzhanhe site, a lesser soil CO2 input due to stronger karst rock desertification and strong gypsum dissolution contribute to higher δ13CDIC, high EC and high cation and anion concentrations. Soil CO2 inputs, controlled by biological activity and available soil moisture, carbonate bedrock dissolution, dilution and degassing effects, vary seasonally following rainfall and temperature changes. Consequently, there are seasonal cycles in hydrochemistry and δ13CDIC of the karst water, with high pCO2 and low pH, EC, SIc, and δ13CDIC values in the warm and rainy seasons, and vice versa during the cold and dry seasons. A strongly positive shift (>3‰) in δ13CDIC occurred in the drought year, 2011, indicating that δ13CDIC in groundwater systems can be an effective indicator of environmental and/or climate changes.

Mineralogical, hydrogeochemical and isotopic characteristics of the Žveplenica sulphide karstic spring (Trebuša Valley, NW Slovenia)

Sulphide springs in Slovenia are very rare and they form a peculiar feature in a carbonate setting. The basic geological, hydrogeochemical and isotopic characteristics of the Žveplenica sulphide dolomite spring (Trebusa Valley, NW Slovenia) were investigated, along with the geochemical and mineralogical characteristics of the sediment deposited at the spring location. Geological mapping defined a small, structurally and lithologically isolated dolomitic aquifer. The major groundwater geochemical composition is HCO3− > Mg2+ > Ca2+, indicating dissolution of dolomite. The concentration of SO42− was very low. The groundwater was generally close to saturation with respect to calcite and dolomite. Geochemical modelling and other analyses indicated sulphur to originate not from gypsum and/or anhydrite, but from some other sources. We suggest the origin of sulphur be the dissolution of volcanogenic sulphidic enrichment in a highly variable, Ladinian clastic-limestone-volcanic rock association, forming the basis of early Carnian Cordevolian dolomite, from which the spring discharges. The measured δ13CDIC value of −11.9 ‰ indicates groundwater with a contribution of degraded organic matter and dissolved inorganic carbon in the aquifer. The isotopic composition of oxygen (δ18OH2O), hydrogen (δDH2O) and tritium was −8.1 ‰, δD −51.4 ‰ and 3H 3.64 TU, respectively. The δ18O and δD values indicated recharge from precipitation with H2S exsolution, while the 3H activity shows groundwater older than 40 years. Mineralogical and geochemical analysis of sediment showed a typical carbonate (mostly dolomite) composition, which was in agreement with the geochemical and isotopic composition of the groundwater of the spring, indicating a deep sourced inorganic form of sulphur such as pyrite.

Spatial and temporal variations of pCO2, dissolved inorganic carbon and stable isotopes along a temperate karstic watercourse

AbstractThis study investigated CO2 degassing and related carbon isotope fractionation effects in the Wiesent River that drains a catchment in the karst terrain of the Franconian Alb, Southern Germany. The river was investigated by physico‐chemical and stable isotope analyses of water and dissolved inorganic carbon during all seasons along 65‐km long downstream transects between source and mouth. Calculated pCO2 values at the source were 21 400 ± 2400 µatm. The pCO2 rapidly decreased in the river water and dropped to an average of 1240 ± 330 µatm near the mouth. About 90% of this decrease occurred within the first 6 km of the river. The river was supersaturated with respect to CO2 over its entire course and must have acted as a continuous year‐round CO2 source to the atmosphere. The average CO2 flux from the karst river was estimated with 450 mmol m−2 day−1 with higher fluxes up to 5680 mmol m−2 day−1 at the source. At the source, δ13CDIC values showed no seasonal variations with an average of −14.2 ± 0.2‰. This indicated that groundwater retained high pCO2 mainly from soil CO2. The contribution of soil CO2 to dissolved inorganic carbon was estimated at 65% to 72%. The downstream CO2 loss caused a positive shift in δ13CDIC values of 2‰ between source and mouth because of the preferential loss of the 12C isotope during degassing. Considering the findings of this study and the fact that carbonate lithology covers a significant part of the earth's surface, CO2 evasion from karst regions might contribute notably to the annual carbon dioxide release from global freshwater systems. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Tracing anthropogenic DIC in urban streams based on isotopic and geochemical tracers

To elucidate the type and extent of anthropogenic DIC (dissolved inorganic carbon), carbon isotopic composition of DIC and dissolved ion concentration were determined in three streams (Gap, Yudeung and Daejeon) draining populated Daejeon metropolitan area, South Korea. Seasonal variations in water chemistry and carbon isotopic composition of DIC (δ13CDIC) reflected changing contribution from two main DIC sources, i.e., soil weathering and sewages. DIC input from the soil weathering was characterized by δ13CDIC values ranging from −16.7 to −16.0 ‰ and higher during Jun–Nov. The sewage-derived DIC became dominant during Dec–May with δ13CDIC values ranging from −11.4 to −7.4 ‰. Si and Cl served as indicators of DIC inputs from soil weathering and sewages, respectively. The higher δ13CDIC values of sewages in the study area indicated that detergents (δ13CDIC of −12.0 to −6.5 ‰) carried by the urban sewages were the main anthropogenic DIC component. The extent of anthropogenic DIC relative to soil-derived DIC in stream waters increased when the precipitation was low and discharge decreased. The results of this study provide a method to quantify anthropogenic DIC, which can help manage water quality in urban stream waters.

Carbon dioxide in scree slope deposits: A pathway from atmosphere to pedogenic carbonate

A continuum of carbon, from atmospheric CO2 to secondary calcium carbonate, has been studied in a soil associated with scree slope deposits in the Jura Mountains of Switzerland. This approach is based on former studies conducted in other environments. This C continuum includes atmospheric CO2, soil organic matter (SOM), soil CO2, dissolved inorganic carbon (DIC) in soil solutions, and secondary pedogenic carbonate. Soil parameters (pCO2, temperature, pH, Cmin and Corg contents), soil solution chemistry, and isotopic compositions of soil CO2, DIC, carbonate and soil organic matter (δ13CCO2, δ13CDIC, δ13Ccar and δ13CSOM values) have been monitored at different depths (from 20 to 140cm) over one year. Results demonstrated that the carbon source in secondary carbonate (mainly needle fiber calcite) is related to the dissolved inorganic carbon, which is strongly dependent on soil respiration. The heterotrophic respiration, rather than the limestone parent material, seems to control the pedogenic carbon cycle. The correlation of δ13Corg values with Rock-Eval HI and OI indices demonstrates that, in a soil associated to scree slope deposits, the main process responsible for 13C-enrichment in SOM is related to bacterial oxidative decarboxylation. Finally, precipitation of secondary calcium carbonate is enhanced by changes in soil pCO2 associated to the convective movement of air masses induced by temperature gradients (heat pump effect) in the highly porous scree slope deposits. The exportation of soil C-leachates from systems such as the one studied in this paper could partially explain the “gap in the European carbon budget” reported by recent studies.

Stable carbon isotope values in dissolved inorganic carbon of ambient waters and shell carbonate of the freshwater pearl mussel (Hyriopsis sp.)

We investigated carbon sources and cycling in a canal connected to Lake Kasumigaura (Japan) using stable carbon isotope values (δ13C) in dissolved inorganic carbon (DIC) of ambient water and in shell carbonate of the commercially cultured freshwater pearl mussel (Hyriopsis sp., Unionidae). The δ13CDIC showed correlations with DIC concentrations, pH, monthly precipitation and pCO2. Thus, the carbon isotopic composition can be used as a proxy for environmental variables. The pCO2 ranged from 3,046 to 9,535 μatm, with a mean value of 6,035 μatm, and remained well above the atmospheric value throughout the years of study. Changes in CO2 solubility, outgassing and biological processes in the river had only minor influence on the carbon isotope composition. DIC concentration and δ13CDIC were controlled primarily by external inputs of soil and weathering-derived DIC associated with precipitation. The δ13C values in different growth zones of the Hyriopsis shells were similar, suggesting that given a constant δ13CDIC source throughout shell formation, the influence of size-related metabolic variations on shell δ13C was negligible. Comparison of δ13C values in DIC and Hyriopsis shell indicated that mussel ontogeny did not influence carbon isotope fractionation, but inter-specimen differences suggested that physiological factors exert systematic control on δ13C values in Hyriopsis. Results also suggest that if the annual mean δ13CDIC value is to be inferred for a fluvial system from shell δ13C values, multiple specimens should be sampled, and the annual mean value yields a temporally integrated measure of the stable isotopic composition of the river system. Given the carbon isotope fractionation between ambient DIC and shell is +2.0 ± 0.7 ‰ (2 se), we estimated that pCO2 averaged ~6,850 μatm during shell growth periods from 2002 to 2007. The pCO2 concentration was an order of magnitude greater than the atmospheric equilibrium value in all years, suggesting that CO2 was lost from surface water to the atmosphere or downstream to the lake. These losses must be accounted for when considering the regional redistribution of CO2. Our results extend the utility of δ13C measures in freshwater bivalves for inferring the δ13CDIC history of ambient waters. This tool provides information that is complementary to that gained from direct δ13CDIC measurement and should enable inference of long-term changes in critical variables such as pCO2.

Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes

Chemical and isotopic analyses of groundwater from piezometers located around a reclaimed landfill in Otwock (Poland) were performed in order to trace the origin of dissolved inorganic carbon (DIC) in the groundwater. Due to differences in the isotopic composition of carbon from different sources, an analysis of stable carbon isotopes in the groundwater, together with the Keeling plot approach and a two-component mixing model allow us to evaluate the relative contributions of carbon from these sources in the groundwater. In the natural (background) groundwater, DIC concentrations and the isotopic composition of DIC (δ13CDIC) comes from two sources: decomposition of organic matter and carbonate dissolution within the aquifer sediments, whereas in the leachate-contaminated groundwater, DIC concentrations and δ13CDIC values depend on the degradation of organic matter within the aquifer sediments and biodegradation of organic matter stored in the landfill. From the mixing model, about 4–54% of the DIC pool is derived from organic matter degradation and 96–46% from carbonate dissolution in natural conditions. In the leachate-contaminated groundwater, about 20–53% of the DIC is derived from organic matter degradation of natural origin and 80–47% from biodegradation of organic matter stored in the landfill. Partial pressure of CO2 (P CO2) was generally above the atmospheric, hence atmospheric CO2 as a source of carbon in DIC pool was negligible in the aquifer.P CO2 values in the aquifer in Otwock were always one to two orders of magnitude above the atmospheric P CO2, and thus CO2 escaped directly into the vadose zone.

Identification of sources and processes in a low-level radioactive waste site adjacent to landfills: groundwater hydrogeochemistry and isotopes

Multiple tracer-element and isotope approaches were applied at a 1960s-era low-level radioactive waste burial site located in the Lucas Heights area on the southwest urban fringe of Sydney, Australia. The site is situated among other municipal and industrial (solid and liquid) waste disposal sites causing potential mixing of leachates. Local rainfall contains marine-derived major ion ratios that are modified during infiltration depending on waste interactions. The local geology favours the retention of contaminants by ion-exchange processes within the clay-rich soils and the shale layer underlying the burial site. Local soils experience periodic infiltration and wetting fronts that can fully saturate the waste trenches (bathtub effect) while surrounding soils are mostly unsaturated with discontinuous perched lenses. Within the trenches, the degradation of organic matter results in localised methanogenesis, as suggested by enriched δ2H and δ13CDIC values in adjacent subsurface water. Movement of contaminants from the trenches is indicated by Na+, Br− and I− concentrations, variations in 87Sr/86Sr, enriched δ13CDIC values and evolution of δ34S of dissolved sulfate in perched water bodies above the shale. Although transport is limited by the low transmissivity of the clay-rich soils, migration and mixing processes are indicated by the variation of concentrations with distance from the trenches, disappearance of δ2H enrichments, mixing with other sources of Sr and sulfate isotope fractionations. The depth distribution of waste-derived contaminants (specifically 3H and Be) between the perched water surrounding the trenches, and the underlying shale and sandstone layers, indicates limited downward transport of contaminants. Past removal of the shale layer in an adjacent site, Harrington's Quarry, has facilitated the mixing of some municipal waste leachates (characterised by circum-neutral pH, high alkalinity, low sulfate, high 3H, high Be, enriched δ2H and δ13CDIC) into the underlying groundwater system as suggested by high TDS, Cl−/Br− ratios, Be and 3H found in deeper wells. This study demonstrates the applicability of using trace elements, stable- and radio-isotopes to document the existing geochemistry and the contaminant transport from the waste trenches. The multiple tracer approach addresses the complexities of transport at the site and differentiates various municipal, industrial and radioactive waste sources.

Effect of a controlled sub-seabed release of CO2 on the biogeochemistry of shallow marine sediments, their pore waters, and the overlying water column

The potential for leakage of CO2 from a storage reservoir into the overlying marine sediments and into the water column and the impacts on benthic ecosystems are major challenges associated with carbon capture and storage (CCS) in subseafloor reservoirs. We have conducted a field-scale controlled CO2 release experiment in shallow, unconsolidated marine sediments, and documented the changes to the chemical composition of the sediments, their pore waters and overlying water column before, during and up to 1 year after the 37-day long CO2 release. Increased levels of dissolved inorganic carbon (DIC) were detected in the pore waters close to the sediment-seawater interface in sediments sampled closest to the subsurface injection point within 5 weeks of the start of the CO2 release. Highest DIC concentrations (28.8mmolL−1, compared to background levels of 2.4mmolL−1) were observed 6 days after the injection had stopped. The high DIC pore waters have high total alkalinity, and low δ13CDIC values (−20‰, compared to a background value of −2‰), due to the dissolution of the injected CO2 (δ13C=−26.6‰). The high DIC pore waters have enhanced concentrations of metals (including Ca, Fe, Mn) and dissolved silicon, relative to non-DIC enriched pore waters, indicating that dissolution of injected CO2 promotes dissolution of carbonate and silicate minerals. However, in this experiment, the pore water metal concentrations did not exceed levels considered to be harmful to the environment. The spatial extent of the impact of the injected CO2 in the sediments and pore waters was restricted to an area within 25m of the injection point, and no impact was observed in the overlying water column. Concentrations of all pore water constituents returned to background values within 18 days after the CO2 injection was stopped.

Fate of injected CO2 in the Wilcox Group, Louisiana, Gulf Coast Basin: Chemical and isotopic tracers of microbial–brine–rock–CO2 interactions

The “2800’ sandstone” of the Olla oil field is an oil and gas-producing reservoir in a coal-bearing interval of the Paleocene–Eocene Wilcox Group in north-central Louisiana, USA. In the 1980s, this producing unit was flooded with CO2 in an enhanced oil recovery (EOR) project, leaving ∼30% of the injected CO2 in the 2800’ sandstone post-injection. This study utilizes isotopic and geochemical tracers from co-produced natural gas, oil and brine to determine the fate of the injected CO2, including the possibility of enhanced microbial conversion of CO2 to CH4 via methanogenesis. Stable carbon isotopes of CO2, CH4 and DIC, together with mol% CO2 show that 4 out of 17 wells sampled in the 2800’ sandstone are still producing injected CO2. The dominant fate of the injected CO2 appears to be dissolution in formation fluids and gas-phase trapping. There is some isotopic and geochemical evidence for enhanced microbial methanogenesis in 2 samples; however, the CO2 spread unevenly throughout the reservoir, and thus cannot explain the elevated indicators for methanogenesis observed across the entire field. Vertical migration out of the target 2800’ sandstone reservoir is also apparent in 3 samples located stratigraphically above the target sand. Reservoirs comparable to the 2800’ sandstone, located along a 90-km transect, were also sampled to investigate regional trends in gas composition, brine chemistry and microbial activity. Microbial methane, likely sourced from biodegradation of organic substrates within the formation, was found in all oil fields sampled, while indicators of methanogenesis (e.g. high alkalinity, δ13C-CO2 and δ13C-DIC values) and oxidation of propane were greatest in the Olla Field, likely due to its more ideal environmental conditions (i.e. suitable range of pH, temperature, salinity, sulfate and iron concentrations).