Stable Carbon Isotope Biogeochemistry and Anthropogenic Impacts on Karst Ground Water, Zunyi, Southwest China

Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO₃–Ca type, HCO₃ · SO₄–Ca type, or HCO₃ · SO₄–Ca · Mg type, according to mass balance considerations. It is found that the average δ¹³C_DIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ¹³C_DIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO₂ makes a larger contribution to the DIC in summer than in winter. The range of δ¹³C_DIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L⁻¹ and 0.5 mg C L⁻¹, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ¹³C_DIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.     

Stable isotope geochemical characteristics of dissolved inorganic carbon in the Jiulong River Estuary,Fujian Province,China

The isotopic composition of dissolved inorganic carbon （DIC） in estuarine environments has been studied for its significant role in determining the isotopic composition of inorganic/organic matter and its applications to the study of various natural processes. In this paper, based on the stable isotope geochemical characteristics of dis- solved inorganic carbon in the Jiulong River Estuary, the following conclusions are drawn： （1） δ13CDIC values are mainly controlled by the mixing ratio of fresh water and sea water; （2）δ13Cphytoplankton values are linearly related to the δ13CDIC values; （3） δ13CpoM values for the Jiulong River Estuary are affected by anthropogenic pollution signifi- cantly; and （4） the comprehensive analysis of δ13Cphytoplankton, δ13CpoM and δ13CDIc shows that along with increasing salinity, the proportion of POM derived from the degradation of phytoplanktons gradually increases.     

Metabolic effects on stable carbon isotopic composition of freshwater bivalve shell Corbicula fluminea

The stable isotopic composition of the bivalve shell has been widely used to reconstruct the pa-laeo-climate and palaeo-environment. The climatic and environmental significance of carbon isotopic composition of the bivalve shell is still in dispute, and incorporation of metabolic carbon can obscure carbon isotope records of dis-solved inorganic carbon. This study deals with freshwater bivalve, Corbicula fluminea aragonite shell. The results indicated that the δ13C values of bivalve shells deposited out of equilibrium with the host water and showed an onto-genic decrease, indicating that there are metabolic effects and more metabolic carbon is incorporated into larger shells. The proportion of metabolic carbon of shells varies between 19.8% and 26.8%. However, δ13CS can still be used as qualitative indicators of δ13CDIC and environmental processes that occurred during shell growth.     

Stable Carbon Isotope Variations in Cave Percolation Waters and their Implications in Four Caves of Guizhou, China

Monitoring and sampling of main plants,soil CO2,soil water,bedrock,spring water,drip water and its corresponding speleothem were performed at four cave systems of Guizhou,Southwest China,from April 2003 to May 2004,in order to understand stable carbon isotope ratios variations of dissolved inorganic Carbon(DIC) in cave percolation waters(δ13CDIC) and their implications for paleoclimate.Stable carbon isotopic compositions and ions(Ca,Mg,Sr,SO4,Cl etc.) were measured for all samples.The results indicate that there are significant differences among the δ13CDIC values from inter-cave,even inter-drip of intra-cave in the four caves.The δ13CDIC values from the Liangfeng Cave(LFC) is lightest among the four caves,where vegetation type overlying the cave is primary forest dominated by tall trees with lighter average δ13C value(–29.9‰).And there are remarkable differences in δ13CDIC values of different drip waters in the Qixing Cave(QXC) and Jiangjun Cave(JJC),up to 6.9‰ and 7.8‰,respectively.Further analyses show that the δ13CDIC values in cave drip waters are not only controlled by vegetation biomass overlying the cave,but also hydro-geochemical processes.Therefore,accurate interpreting of δ13C recorded in speleothems cannot be guaranteed if these effects of the above mentioned factors are not taken into consideration.     

The Response of Carbon Geochemistry to Hydrological Events within an Urban River,Southwestern China

Natural and anthropogenic impacts on dissolved inorganic carbon (DIC) within an urban river, Nanming River in southwestern China, were investigated using hydrochemistry and carbon isotopic compositions of dissolved inorganic carbon (δ¹³C_DIC). Because of the anthropogenic inputs, generally, the TDS values and major ionic compositions showed an increasing trend along the mainstream. The TDS values and most of the dissolved solutes compositions showed a dilution effect during storms, but the dilution effect did not strictly follow the theoretical dilution curve. Lighter δ¹³C_DIC values in the river after a rainstorm reflected the influx of rain water with biological CO₂ during the rain event. Meanwhile, the negative relationship between δ¹³C_DIC values and dissolved inorganic carbon concentrations in the mainstream at different sampling campaigns suggested significant degradation of organic matter in the riverine channels. The variabilities of DIC in an urban river were mainly impacted by biological activities and infiltration of soil carbon dioxide. This study demonstrated that hydrological events and anthropogenic inputs are the main controls on the variations of dissolved solutes compositions and the DIC dynamics for an urban river.     

Identical carbon isotope trends of carbonate and organic carbon and their environmental significance from the Changhsingian (end-Permian), Meishan,South China

High resolution carbon isotope analyses of carbonate and organic carbon from Meishan, South China showed that the variation of δ13Ccarb is marked by three large positive excursions during the Changhsingian (end-Permian). Carbon isotope stratigraphy during this stage shows three cyclic intervals in δ13Ccarb, with two cycles corresponding to the lower (Paleofusulinid minima Zone) and one corresponding to the upper Changhsingian (P. sinensis Zone). The large positive δ13Ccarb excursions indicate episodes of enhanced burial of isotopically light or-ganic carbon, presumably in response to deep-water anoxia episodically extending into shallow water with the rise of sea level. The organic carbon during the Changhsingian is distinguished into two groups, and the δ13Corg of each group parallels (separately) the more detailed profile of δ13Ccarb, strongly showing that the values of fractionation Δ13Ccab-org remain relatively constant, with only two intervals with anomaly. The enhanced fractionation Δ13Ccab-org with large negative δ13Corg excursions apparently indicates significant inputs from sulfide-oxidizing bacteria and green sulfur bacteria, notably at bed 24 just predating mass extinction. Our evidence appears to support that the ex-tended euxinic water is possible for the main pulse of mass extinction at the end-Permian.     

Mixing of magmatic CO2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

To understand deep groundwater flow systems and their interaction with CO₂ emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ¹³C_DIC) and DIC concentrations of the sampled waters show that magma-derived CO₂ mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ¹³C_DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ¹⁸O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ¹³C_DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO₂ is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO₂ mixes into deeper groundwater flowing nearer the magma conduit or chamber.     

Geochemical characteristics and possible origin of natural gas in the Taibei Depression,Turpan-Hami Basin,China

292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ13C1. Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-ane and propane are defined as d13C1: -45.5‰ to -33.5‰, d13C2: -30.2‰ to -10.5‰, and d13C3: 27.6‰ to -11.2‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of Δ(d13C1-d13C2) and d13C2 and that of Δ(d13C1-d13C2) and d13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.     

Damming effects on dissolved inorganic carbon in different kinds of reservoirs in Jialing River,Southwest China

To assess the effects of river damming on dissolved inorganic carbon in the Jialing River, a total of 40 water samples, including inflow, outflow, and stratified water in four cascade reservoirs (Tingzikou, Xinzheng, Dongxiguan, Caojie) were collected in January and July, 2016. The major cations, anions, and δ¹³C_DIC values were analyzed. It was found that the dissolved compositions are dominated by carbonate weathering, while sulfuric acids may play a relatively important role during carbonate weathering and increasing DIC concentration. Different reservoirs had variable characteristics of water physiochemical stratification. The DIC concentrations of reservoir water were lower in summer than those in winter due to the dilute effects and intensive aquatic photosynthesis, as well as imported tributaries. The δ¹³C_DIC values in Tingzikou Reservoir were higher during summer than those in winter, which indicated that intensive photosynthesis increased the δ¹³C_DIC values in residual water, but a similar trend was not obvious in other reservoirs. Except for in Xinzheng Reservoir, the δ¹³C_DIC values in inflow and outflow reservoir water were lower than those in the surface water of stratified sampling in summer. For stratified sampling, it could be found that, in summer, the Tingzikou Reservoir δ¹³C_DIC values significantly decreased with water depth due to the anaerobic breakdown of organic matter. The significant correlation (p < 0.01 or 0.05) between the DIC concentrations, the δ¹³C_DIC values and anthropogenic species (Na⁺+K⁺, Cl^–, \({\text{SO}}_{4}^{2 - }\)and \({\text{NO}}_{3}^{ - }\)) showed that the isotope composition of DIC can be a useful tracer of contaminants. In total, Tingzikou Reservoir showed lacustrine features, Xinzheng Reservoir and Dongxiguan Reservoir had “transitional” features, and Caojie Reservoir had a total of “fluvial” features. Generally, cascade reservoirs in the Jialing River exhibited natural river features rather than typical lake features due to characteristics of reservoir water in physiochemical stratification, spatiotemporal variations of DIC concentrations and isotopic compositions. It is evident that the dissolved inorganic carbon dynamics of natural rivers had been partly remolded by dam building.     

Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia

Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l⁻¹ and in concentrations of Ca²⁺, Mg²⁺, Na⁺ and Si(OH)₄ compared to the shallow type.The δ²H and δ¹⁸O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ¹³C_DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO₂ from local soil horizons, whereas δ¹³C_DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO₂, δ¹³C_DIC = +1‰ of the deep groundwater with highest PCO₂ = 10^−0.9 atm yields δ¹³C_CO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO₂. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO₂ from soil and magmatic origin. The uptake of magmatic CO₂ results in elevated cations and Si(OH)₄ concentrations. Weathering of local basalts is documented by ⁸⁷Sr/⁸⁶Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO₂ from the solution leading to secondary calcite formation.     

Stable carbon isotope biogeochemistry of a shallow sand aquifer contaminated with fuel hydrocarbons

Ground-water chemistry and the stable C isotope composition (δ¹³C_DIC) of dissolved inorganic C (DIC) were measured in a sand aquifer contaminated with JP–4 fuel hydrocarbons. Results show that ground water in the upgradient zone was characterized by DIC content of 14–20 mg C/L and δ¹³C_DIC values of −11.3‰ to −13.0‰. The contaminant source zone was characterized by an increase in DIC content (12.5 mg C/L to 54 mg C/L), Ca, and alkalinity, with a significant depletion of ¹³C in δ¹³C_DIC (−11.9‰ to −19.2‰). The source zone of the contaminant plume was also characterized by elevated levels of aromatic hydrocarbons (0 μg/L to 1490 μg/L) and microbial metabolites (aromatic acids, 0 μg/L to 2277 μg/L), non-detectable dissolved O₂, NO₃ and SO₄. Phospholipid ester-linked fatty acid analyses suggest the presence of viable SO₄-reducing bacteria in ground water at the time of sampling. The ground-water chemistry and stable C isotope composition of ground-water DIC are interpreted using a chemical reaction model involving rainwater recharge, contributions of CO₂ from soil gas and biodegradation of hydrocarbons, and carbonate dissolution. The major-ion chemistry and δ¹³C_DIC were reconciled, and the model predictions were in good agreement with field measurements. It was concluded that stable C isotope measurements, combined with other biogeochemical measures can be a useful tool to monitor the dominant terminal electron-accepting processes in contaminated aquifers and to identify mineralogical, hydrological, and microbiological factors that affect δ¹³C of dissolved inorganic C.     

Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks——A thermal simulation experiment

1 IntroductionThe compositional characteristics of natural gasesand their carbon isotopic composition are controlledmainly by the type of organic matter and the degree ofits thermal evolution in the source rocks (Song Yan,1995; Xu Yongchang et al., 1994; …     

Early cambrian carbon isotope stratigraphy in the tarim basin and a correlation with the yangtze platform

Carbon isotope chemostratigraphy has been used worldwide for stratigraphic correlation.In this study,δ13Ccarb values are estimated for the Early Cambrian Sugaitebulake section in the Tarim Basin,Xinjiang Autonomous Region,China.As a result,one positive and two negative carbon isotope excursions in the studied section were iden-tified.The δ13Ccarb values reached the maximum negative excursion(N1:-12.39‰) at the basal of the Yuertusi For-mation,and then increased to P1.After P1,δ13Ccarb values sharply decreased to about-7.06‰(N2) in the studied section.The pattern of δ13Ccarb in the Early Cambrian is comparable to the synchronous records of other sections,such as the Laolin section,the Xiaotan section and the Anjiahe section of the Yangtze Platform.It is concluded that the Early Cambrian Yuertusi Formation from the Tarim Basin is within the Nemakit-Daldynian stage,and the lower strata of the Yuertusi Formation may belong to the Zhujiaqing Formation(Meishucun Formation) of the Yangtze Platform.The Ediacaran/Cambrian boundary of the Tarim Block should be located in cherts and phosphorites suc-cessions at the basal of the Yuertusi Formation.The δ13Ccarb negative excursion N1 is just across the PC/C boundary,and may be related to certain biomass extinction due to anoxic sedimentary environment,transgression and/or the oceanic overturn.The second δ13Ccarb negative excursion N2 may account for the sea-level falling in the Early Cam-brian.     

Seasonal water-column dynamics of dissolved inorganic carbon stable isotopic compositions (δCDIC) in small hardwater lakes in Minnesota and Montana

The carbon stable isotopic value of dissolved inorganic carbon (δ¹³C_DIC) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. δ¹³C_DIC behavior in three of these lakes departed from the conventional model wherein epilimnetic waters are seasonally enriched relative to all hypolimnetic waters, and in general δ¹³C_DIC values in the water column were not readily correlated to parameters such as lake stratification, algal productivity, hydraulic residence time, or water chemistry. Additionally, the processes implicated in generating the δ¹³C_DIC values of individual lakes differ between lakes with similar δ¹³C_DIC compositions. Each lake thus initially appears idiosyncratic, but when the effects of carbonate mineral equilibria, microbial activity, and lake residence time are viewed in terms of the magnitude of distinct DIC pools and fluxes in stratified lakes, generalizations can be made that allow lakes to be grouped by δ¹³C_DIC behavior. We recognize three modes in the relationship between δ¹³C_DIC values and DIC concentration ([DIC]) of individual lakes: (A) δ¹³C_DIC values decreasing with increasing [DIC]; (B) δ¹³C_DIC values increasing with increasing [DIC]; (C) δ¹³C_DIC values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding δ¹³C_DIC dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary δ¹³C components in the lacustrine paleorecord.     

Carbon Geochemistry of Gas Hydrate‐associated Sediments in the Southwestern Taiwan Basin

Marine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate-bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973-3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon(DIC) and high total organic carbon(TOC)(mean = 0.46%) originated from terrigenous organic matter(mean δ~(13)C_(TOC) value of-23.6‰) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon(IC), and the depleted δ~(13)C_(IC), coupled with the increase of TOC and the decrease of δ~(13)C_(TOC) values owing to the methanotroph ‘Methanosarcinales/ANME' in 430–840 cm. Environmental factors and archaeal communities in core 973-3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate-associated sediments.     

Carbon and sulfur isotopic compositions of Early Cambrian black shales,NW Hunan,China: Implications for the Paleoceanographic sedimentary environment

In order to better understand the paleoceanographic sedimentary environment of the Lower Cambrian black shales extensively distributed in South China, outcropped along the present southern margin of the Yangtze Platform with a width of ca. 200-400 km and a length of more than 1500 km, we present new paired δ13C data on carbonates (δ13Ccarb) and associated organic carbon (δ13Corg) and δ34Spy data on sedimentary pyrite in black shales from three sections (Ganziping, Shancha and Xiaohekou) located in NW Hunan, China. In these sections, a total of 82 Lower Cambrian black shale samples have δ13Ccarb values ranging from -4.0‰ to 1.7‰ with an average value of -2.1‰, and δ13Corg values between -34.9‰ and -28.8‰, averaging -31.9‰. The ?34Spy values of 16 separated sedi-mentary pyrite samples from the black shales vary between +10.2‰ and +28.7‰ with an average value of +19.5‰, presenting a small isotope fractionation between seawater sulfate and sedimentary sulfide. The model calculation based on credible data from the paired analyses for δ13Ccarb and δ13Corg of 11 black shale samples shows a high CO2 concentration in the Early Cambrian atmosphere, about 20 times higher than pre-industrial revolution values, consis-tent with previous global predictions. The small sulfur isotope fractionation between seawater sulfate and sedimen-tary sulfide in black shales, only 15.5‰ on average, implies a low sulfate level in the Early Cambrian seawater around 1 mmol. In combination with a high degree of pyritization (DOP) in the black shales, it is suggested that sul-fidic deep-ocean water could have lingered up to the earliest Cambrian in this area. The black shale deposition is envisaged in a stratified marine basin, with a surface euphotic and oxygenated water layer and sulfidic deeper water, controlled by a continental margin rift.     

Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.     

Invasion of anthropogenic CO<Subscript>2</Subscript> recorded in planktonic foraminifera from the northern Gulf of Aqaba

The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in ¹³C_DIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The ¹³C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63 over the last two centuries. This decrease seems to track the decrease of ¹³C_DIC in surface waters, which is mainly caused by the increase of anthropogenic input of ¹³C-depleted CO₂ into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (¹³C_org) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal ¹³C. Furthermore, direct measurements of seawater show that ¹³C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 during the period 1979–2000. The average annual decrease is 0.021, which is similar to that observed globally. The ¹³C values of planktonic foraminifera combined with organic matter ¹³C from marine sediments are good indicators for reconstructing past changes in atmospheric CO₂ concentrations from the northern Gulf of Aqaba.     

Tracing sources of carbon in urban groundwater using δ<Superscript>13</Superscript>C<Subscript>TDIC</Subscript> ratios

Total dissolved inorganic carbon (TDIC) and its stable isotope ratio δ¹³C_TDIC are used to trace the evolution of the carbon system of groundwater in three UK Permo-Triassic sandstone aquifers. Samples were collected from multilevel piezometers, open boreholes and sewer sampling points in the British Midlands (Nottingham, Birmingham and Doncaster) to evaluate both local and regional variations in δ¹³C_TDIC. δ¹³C samples of matrix and pore water have also been analysed in each aquifer to further constrain the interpretations. Combining δ¹³C_TDIC ratios with measurements of TDIC and pH clearly distinguishes the principal processes underlying the geochemical evolution of groundwater in Triassic sandstone aquifers, where processes can be both natural (e.g. carbonate dissolution) and anthropogenic (sewer-derived recharge). The paper shows that δ¹³C_TDIC resolves ambiguities that arise from the interpretation of TDIC and pH measurements in isolation. Field measurements demonstrate that, under natural conditions, the carbonate system evolves similarly in each aquifer. An open-system evolution during recharge largely saturates the groundwater with carbonate depending upon its availability in the sandstone matrix. The contribution of sewer exfiltration to urban recharge is readily distinguished by lower pH and higher TDIC values without significant changes in δ¹³C_TDIC.