Dissolved Inorganic Carbon Values Research Articles

Multiple isotope (O, S and C) approach elucidates the enrichment of arsenic in the groundwater from the Datong Basin, northern China

Hydrochemical data, the δ34S and δ18O values in dissolved sulfate SO42- and the δ13C signature in dissolved inorganic carbon (DIC) have been analyzed and used to deduce the predominant biogeochemical/geochemical processes controlling the mobilization of arsenic within aquifers of the Datong Basin. High dissolved Fe(II), HS− and NH4+ concentrations and low NO3-, SO42- and ORP values were observed in the high-arsenic groundwater. The coupled occurrence of high dissolved Fe(II), HS−, NH4+ and HCO3- concentrations in groundwater indicates that microbially mediated Fe(III) and SO42- reduction and organic matter oxidation has occurred. Wide ranges of δ34S and δ18O values were measured in dissolved SO42- (ranging from 7.0‰ to 36.8‰ and from 3.2‰ to 13.0‰ for δ34S and δ18O, respectively) and depleted δ13C values of DIC (varying from −6.9‰ to −22.0‰) were detected. The wide ranges of the δ34SSO4 and δ18OSO4 values and the correlation between the low δ13CDIC values and the high δ34SSO4 values indicate that the microbial reduction of SO42- and organic matter biodegradation have occurred or are occurring. The correlation between δ34SSO4 and δ18OSO4 values and Fe concentrations indicates that Fe(III) oxides/hydroxides reduction and iron sulfide formation may be the main processes controlling the enrichment of arsenic in groundwater. Furthermore, the negative correlations between the δ13CDIC values and the δ34SSO4 values and Fe concentrations demonstrate that arsenic mobility are influenced by processes: (1) the reduction of arsenic-bearing crystalline Fe(III) oxide/hydroxides and SO42- coupled to the oxidation of organic matter in aquifer sediments or unsaturated zone and (2) the reduction of amorphous Fe(III) oxyhydroxide without the significant reduction of sulfate.

Calcification and ocean acidification: new insights from the coccolithophore Emiliania huxleyi

Calcification and ocean acidification: new insights from the coccolithophore <i>Emiliania huxleyi</i>

Sequential injection methodology for carbon speciation in bathing waters

A sequential injection method (SIA) for carbon speciation in inland bathing waters was developed comprising, in a single manifold, the determination of dissolved inorganic carbon (DIC), free dissolved carbon dioxide (CO2), total carbon (TC), dissolved organic carbon and alkalinity. The determination of DIC, CO2 and TC was based on colour change of bromothymol blue (660nm) after CO2 diffusion through a hydrophobic membrane placed in a gas diffusion unit (GDU). For the DIC determination, an in-line acidification prior to the GDU was performed and, for the TC determination, an in-line UV photo-oxidation of the sample prior to GDU ensured the conversion of all carbon forms into CO2. Dissolved organic carbon (DOC) was determined by subtracting the obtained DIC value from the TC obtained value. The determination of alkalinity was based on the spectrophotometric measurement of bromocresol green colour change (611nm) after reaction with acetic acid. The developed SIA method enabled the determination of DIC (0.24–3.5mgCL−1), CO2 (1.0–10mgCL−1), TC (0.50–4.0mgCL−1) and alkalinity (1.2–4.7mgCL−1 and 4.7–19mgCL−1) with limits of detection of: 9.5μgCL−1, 20μgCL−1, 0.21mgCL−1, 0.32mgCL−1, respectively. The SIA system was effectively applied to inland bathing waters and the results showed good agreement with reference procedures.

Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this “missing carbon” is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore‐water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and δ13C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon (222Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and δ13C values displayed consistent tidal variations, and mirrored the trend in 222Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar δ13C value in summer and winter (∼ −30‰). The exported δ13C‐DIC showed no difference between summer and winter and had a δ13C value slightly more enriched (∼ −22.5‰) than the exported DOC. The imported POC had differing values in summer (∼ −16‰) and winter (∼ −22‰), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222Rn and carbon model showed that 93–99% of the DIC and 89–92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m−2 d−1 and was an order of magnitude higher than DOC export, and similar to global estimates of the mangrove missing carbon (i.e., ∼ 1.9–2.7 g C m−2 d−1).

Comparison of Particulate Organic and Dissolved Inorganic Radiocarbon Signatures in the Surface Northeast Pacific Ocean

It has long been assumed that radiocarbon (Δ14C) content of dissolved inorganic carbon (DIC) is equal to that of particulate organic carbon (POC) in surface seawater; however, little research has been conducted to explicitly test this assumption. Here, we report Δ14C measurements of surface POC samples and compare them with contemporaneous DIC Δ14C measurements from the northeast Pacific Ocean (Hwang et al. 2004; Druffel et al. 2010). Samples were collected from surface waters at Station M off California between 1995 and 2004. The POC Δ14C values decreased 3.2% per year from 1995 to 2004, similar to the decline observed in the DIC Δ14C values during the same period. Overall, our results show no statistical difference between POC and DIC Δ14C—consistent with the assumption that DIC and POC Δ14C values can generally be considered equivalent. However, significant variability was observed for POC Δ14C values during several fall/summer events, where POC Δ14C signatures were lower than DIC Δ14C values. An evaluation of 2 sample pretreatments also suggests that non-homogenized POC samples deviated less from average POC Δ14C values and more closely matched the DIC Δ14C average for the time series. The presence of seasonal POC/DIC Δ14C disagreements, combined with sample processing effects, suggest that infrequent contributions of allochthonous, older carbon may have originated from deeper in the water column, especially during periods when upwelling in this area was prominent.

Carbon Cycling and Organic Radiocarbon Reservoir Effect in a Meromictic Crater Lake (Lac Pavin, Puy-de-Dôme, France)

Lac Pavin is a meromictic maar lake for which the interpretation of sediment radiocarbon dates is complicated by the existence of a largely undefined reservoir effect resulting from degradation of carbon stored in the bottom layer of the water column. A data set of the contemporary 14C distribution of dissolved and particulate organic pools in the water column is presented to address this issue. Dissolved inorganic carbon (DIC) and organic carbon (DOC), plankton, suspended particulate organic carbon (POCsusp), sinking POC (POCsink), and bottom sediment organic carbon (SOC) were analyzed. Present-day Δ14C values of DIC were measured ranging from −750′ in the monimolimnion to atmospheric values in lake surface waters and in spring inlet waters. A range of Δ14C values between −200 and −300′ was observed for superficial POCsusp, POCsink, SOC, and DOC. This relatively uniform 14C offset of the exported organic production from the surface waters to the bottom represents a contemporary reservoir effect of ∼2500 yr. Laminated buried sediment samples and terrestrial vegetal macro-remains were used to evaluate temporal reservoir effect variations since the formation of the crater lake (7 ka cal BP). Buried sediment layers presented a similar offset or showed larger differences between Δ14C values of bulk sediment and terrestrial plant remains (–400 to −500′). Furthermore, an almost 0-yr reservoir effect was inferred from the sediment layers deposited just above the volcanic bedrock at the early flooding of the crater, and increasing slightly within the first centuries of the lake's history. A second objective was to tentatively model a defined scenario of the cycling of carbon in the lake capable of predicting a modern reservoir effect. Alternative scenarios were then tested for which a larger contribution of deeper DIC would provide a model compatible with larger past reservoir effects. It is concluded that using Δ14C SOC variation in laminated lake sediments as a proxy of paleolimnological conditions may be valuable provided that more data on the dynamics of the 14C composition of plankton and more detailed sampling of laminated sediment layers are available.

Variability of Air-Sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Fluxes and Dissolved Inorganic Carbon Distribution in the Atlantic Basin: A Coupled Model Analysis

The biogeochemical dynamics of carbon in the ocean is a subject of fundamental interest to environmental studies. In this context, we have implemented a ten year run of the Brazilian Earth System Coupled Ocean-Atmosphere Model (BESM-OA2.3) integrated with TOPAZ biogeochemical model for the Atlantic basin. The modeled ΔpCO2 for the tropical Atlantic shows very clearly a high dominance of positive fluxes, that is, the CO2 fluxes are sea-to-air throughout the tropical region and for both winter and summer periods. In the mid-latitudes regions negatives fluxes (air-to-sea) were observed for both seasons. An exception to this pattern is an extensive negative tongue on the latitude 10N. The occurrence of this negative ΔpCO2 tongue region in the Tropical Atlantic is highly correlated to negative Evaporation-Precipitation values during this season. In the northern hemisphere (NH) summer the negative values of ΔpCO2 in the tropical Atlantic region are concentrated in the adjacent zone of the Amazon river mouth due to the North Equatorial Counter Current intensification. This process favors the formation of a carbon sink in the adjacent region of the Amazon river mouth. Model results show lowest values of dissolved inorganic carbon (DIC) in a surface layer (100 - 150 m). Highest DIC values are observed in deeper layers and concentrated in an equatorial band. The chlorophyll bloom in equatorial zones was well represented by the model. These blooms are the result of equatorial upwelling that brings the high concentration tongues of DIC present in the equatorial band towards the euphotic zone. This is the first published paper about the BESM-OA2.3 integrated with TOPAZ. The presented results suggest that this modeling system is able to reproduce the main regional carbon dynamics features of the mid-latitude/tropical Atlantic.

Degradation of terrestrial organic carbon, primary production and out-gassing of CO2 in the Laptev and East Siberian Seas as inferred from δ13C values of DIC

The cycling of carbon on the Arctic shelves, including outgassing of CO2 to the atmosphere, is not clearly understood. Degradation of terrestrial organic carbon (OCter) has recently been shown to be pronounced over the East Siberian Arctic Shelf (ESAS), i.e. the Laptev and East Siberian Seas, producing dissolved inorganic carbon (DIC). To further explore the processes affecting DIC, an extensive suite of shelf water samples were collected during the summer of 2008, and assessed for the stable carbon isotopic composition of DIC (δ13CDIC). The δ13CDIC values varied between −7.2‰ to +1.6‰ and strongly deviated from the compositions expected from only mixing between river water and seawater. Model calculations suggest that the major processes causing these deviations from conservative mixing were addition of (DIC) by degradation of OCter, removal of DIC during primary production, and outgassing of CO2. All waters below the halocline in the ESAS had δ13CDIC values that appear to reflect mixing of river water and seawater combined with additions of on average 70±20μM of DIC, originating from degradation of OCter in the coastal water column. This is of the same magnitude as the recently reported deficits of DOCter and POCter for the same waters. The surface waters in the East Siberian Sea had higher δ13CDIC values and lower DIC concentrations than expected from conservative mixing, consistent with additions of DIC from degradation of OCter and outgassing of CO2. The outgassing of CO2 was equal to loss of 123±50μM DIC. Depleted δ13CPOC values of −29‰ to −32‰ in the mid to outer shelf regions are consistent with POC from phytoplankton production. The low δ13CPOC values are likely due to low δ13CDIC of precursor DIC, which is due to degradation of OCter, rather than reflecting terrestrial input compositions. Overall, the δ13CDIC values confirm recent suggestions of substantial degradation of OCter over the ESAS, and further show that a large part of the CO2 produced from degradation has been outgassed to the atmosphere.

Constraining carbonate chemistry at a potential ocean acidification event (the Triassic–Jurassic boundary) using the presence of corals and coral reefs in the fossil record

Ocean acidification associated with emplacement of the Central Atlantic Magmatic Province (CAMP) has been hypothesized as a kill mechanism for the end-Triassic mass extinction, but few direct proxies for ancient ocean acidity are available. This paper describes a new proxy that uses the presence of fossil corals and coral reefs to determine aragonite saturation state (ΩArag). Modern scleractinian corals struggle to biomineralize a skeleton below an ΩArag of 2 and modern shallow water coral reefs are typically only found in areas with source water of ΩArag>3; so when corals or coral reefs are preserved in the fossil record, these ocean saturation states can be inferred. Atmospheric pCO2 reconstructions are combined with the coral ΩArag limitations to calculate the total dissolved inorganic carbon (DIC) in the Late Triassic ocean, which is a measure of the buffering capacity or ocean sensitivity to acidification. Once DIC is known, the severity of an acidification due to a carbon dioxide injection can be determined, for example the Triassic–Jurassic (T–J) event. Our results suggest that if Late Triassic DIC values were low to moderate (2000–3000μmol/kg), the T–J pCO2 increases would have depressed saturation state to the point where coral biomineralization would have been extremely challenging (ΩArag<2), resulting in the observed coral and coral reef gap in the fossil record. While the average pCO2 elevations recorded in stomatal and pedogenic proxies are not sufficient to cause complete carbonate undersaturation, modeled scenarios for CAMP-related T–J pCO2 increases (within error of pedogenic pCO2 proxies) suggest that aragonite undersaturation is plausible and in extreme cases calcite undersaturation is achievable. Thus, a short but extreme acidification event could occur and would satisfactorily explain the significant extinction of calcareous organisms, the Early Hettangian coral gap, and the T–J carbonate crisis.

Analyses of pre-injection reservoir data for stable carbon isotope trend predictions in CO2 monitoring: preparing for CO2 injection

Baseline monitoring at the proposed enhanced gas recovery site in Altmark (Germany) was carried out in combination with theoretical and laboratory investigations to describe and predict the principles of expected stable carbon isotope and dissolved inorganic carbon (DIC) trends during CO2 injection in reservoirs. This provides fundamental data for site-specific characterisation for monitoring purposes. Baseline δ13C values at the Altmark site ranged between −1.8 and −11.5 ‰ and DIC values were about 2 mmol L−1. These baseline values form the basis for a theoretical study on the influences of the ambient reservoir conditions on the state of geochemical and isotope equilibrium of the reservoir fluids. Transferring this theoretical study to the Altmark site enables predictions on geochemical trends during potential injection. Assuming that CO2 would be injected at the Altmark site to pCO2 = 100 bar and with a δ13C of −30 ‰, at isotopic and geochemical equilibrium, δ13CDIC values would approach this end-member, and DIC concentrations of 1,000 mmol L−1 would be expected. Laboratory experiments were conducted at low pCO2 levels (4–35 bars) to mimic the approach of a CO2 plume at a monitoring well. These results support field investigations from other sites: that δ13CDIC is a sensitive tool for monitoring CO2 migration in the subsurface and simultaneously allows quantification of geochemical trapping of CO2.

Hydrogeochemical processes in clastic sedimentary rocks, South Korea: A natural analogue study of the role of dedolomitization in geologic carbon storage

Abstract For long-term mineral trapping of sequestered CO 2 in deep saline aquifers, base cations such as Ca 2 + and Mg 2 + are essential. As a natural analogue study of geologic carbon storage in deep aquifers hosted in sedimentary formations, we examined the hydrochemistry of sulfate-rich (up to 1140 mg/l SO 4 2 − ) and moderately high P CO 2 (10 − 1.1 to 10 − 2.4 atm) groundwater in a Cretaceous non-marine sedimentary basin in South Korea with the objective to elucidate water–rock interactions controlling the concentrations and behavior of base cations. Principal component analysis of the acquired hydrochemical data indicated that dissolution of carbonates (calcite and dolomite) and evaporite minerals (halite and gypsum) controls the chemical composition of groundwater, resulting in substantial increases of the concentration s of Ca 2 + , Mg 2 + , Na + , Cl − , SO 4 2 − and total dissolved solids (TDS) in deep groundwater. Na + versus Cl − and Ca 2 + + Mg 2 + versus HCO 3 − + SO 4 2 − plots provided evidence for dissolution of halite and gypsum. Progressively increasing δ 34 S values of dissolved SO 4 2 − (from 15.1 to 19.2‰) with increasing sulfate concentrations indicated gypsum dissolution. Ion–ion plots (esp., Mg 2 + /Ca 2 + , Ca 2 + /SO 4 2 − and Mg 2 + /SO 4 2 − ) and saturation indices of calcite, dolomite and gypsum suggest that the groundwater chemistry (esp., the concentrations of Ca 2 + and Mg 2 + ) is controlled by dedolomitization driven by gypsum dissolution. Groundwater in the study area does not reach complete equilibrium with respect to calcite and dolomite because of gypsum dissolution, which controls the Mg 2 + /Ca 2 + ratios of groundwater. Continued calcite precipitation triggered by an excess Ca supply from gypsum dissolution reduces the concentrations of dissolved inorganic carbon (DIC) in groundwater. The increase of δ 13 C DIC values from − 11.1 to − 6.5‰ concomitantly with increasing sulfate concentration was explained via geochemical modeling by dedolomitization under the rate constant ratio of about 0.038 between dolomite and calcite. The model results agree well with the observed Mg 2 + /Ca 2 + ratios and further suggest a potential increase of the void volume in the aquifer through dedolomitization by about 0.72 cm 3 /l. Based on this analogue, we suggest that dedolomitization in concert with dissolution of gypsum may constitute an important process releasing base cations for mineral trapping of injected CO 2 in non-marine clastic sedimentary strata containing carbonates and gypsum in South Korea and elsewhere.

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

Abstract. The scientific motivation for this study is to understand the processes in the ocean interior controlling carbon transfer across 30° S. To address this, we have developed a unified framework for understanding the interplay between physical drivers such as buoyancy fluxes and ocean mixing, and carbon-specific processes such as biology, gas exchange and carbon mixing. Given the importance of density in determining the ocean interior structure and circulation, the framework is one that is organized by density and water masses, and it makes combined use of Eulerian and Lagrangian diagnostics. This is achieved through application to a global ice-ocean circulation model and an ocean biogeochemistry model, with both components being part of the widely-used IPSL coupled ocean/atmosphere/carbon cycle model. Our main new result is the dominance of the overturning circulation (identified by water masses) in setting the vertical distribution of carbon transport from the Southern Ocean towards the global ocean. A net contrast emerges between the role of Subantarctic Mode Water (SAMW), associated with large northward transport and ingassing, and Antarctic Intermediate Water (AAIW), associated with a much smaller export and outgassing. The differences in their export rate reflects differences in their water mass formation processes. For SAMW, two-thirds of the surface waters are provided as a result of the densification of thermocline water (TW), and upon densification this water carries with it a substantial diapycnal flux of dissolved inorganic carbon (DIC). For AAIW, principal formatin processes include buoyancy forcing and mixing, with these serving to lighten CDW. An additional important formation pathway of AAIW is through the effect of interior processing (mixing, including cabelling) that serve to densify SAMW. A quantitative evaluation of the contribution of mixing, biology and gas exchange to the DIC evolution per water mass reveals that mixing and, secondarily, gas exchange, effectively nearly balance biology on annual scales (while the latter process can be dominant at seasonal scale). The distribution of DIC in the northward flowing water at 30° S is thus primarily set by the DIC values of the water masses that are involved in the formation processes.

Importance of water mass formation regions for the air-sea CO2flux estimate in the Southern Ocean

[1] CARIOCA drifters and ship data from several cruises in the Subantarctic Zone (SAZ) of the Pacific Ocean, approximately 40°S–55°S, have been used in order to investigate surface CO2 partial pressure (pCO2) and dissolved inorganic carbon (DIC) patterns. The highest DIC values were determined in regions of deep water formation, characterized by deep mixed layer depths (MLD) as estimated from Argo float profiles. As a result, these areas act as sources of CO2 to the atmosphere. Using an empirical linear relationship between DIC, sea surface temperature (SST), and MLD, we then combine DIC with AT based on salinity and compute pCO2. Finally, we derive monthly fields of air-sea CO2 flux in the SAZ. Our fit predicts the existence of a realistic seasonal cycle, close to equilibrium with the atmosphere in winter and a sink when biological activity takes place. It also reproduces the impact that deep water formation regions close to the Subantarctic Front (SAF) and in the eastern part of the SAZ have on the uptake capacity of the area. These areas, undersampled in previous studies, have high pCO2, and as a result, our estimates (0.05 ± 0.03 PgC yr−1) indicate that the Pacific SAZ acts as a weaker sink of CO2 than suggested by previous studies which neglect these source regions.

Influence of diel biogeochemical cycles on carbonate equilibrium in a karst river

Variations in temperature, photosynthesis, and respiration force diel variations in pH and dissolved CO 2 concentrations of surface streams, possibly controlling carbonate equilibrium between river water and carbonate stream beds. Diel cycles of water chemistry and δ 13C of dissolved inorganic carbon (DIC) were measured to assess how biogeochemical processes affect dissolution and precipitation of calcite and thus channel development in Ichetucknee River, a large spring-fed river (discharge > 6 m 3/s) flowing over carbonate karst terrain in north central Florida (USA). Samples were collected at a 4-h sampling interval during two one-week periods and at a 1-h interval during a single 24-h period. Simultaneously, temperature, pH, dissolved oxygen (DO) and NO 3 − concentrations were measured using in situ sensors at 15-min or 1-h intervals. Ca 2+, DIC and NO 3 − concentrations decreased during the day and increased at night causing diel changes of in-stream specific conductivity. These changes were inversely related to diel changes in pH, PCO 2 and DO concentrations. This work shows that photosynthesis and respiration of subaquatic vegetation are the dominant processes influencing in-stream diel variation. During the day, a simultaneous increase of δ 13C DIC and decrease in DIC indicates that photosynthesis was the primary control on DIC concentrations. Calcite saturation indices ranged from 0 to 0.5, with the highest value in daylight as a result of CO 2 consumption causing carbonate precipitation. The water remained saturated with respect to calcite at night and δ 13C DIC values decreased, indicating that CO 2 production from ecosystem respiration was the dominant process affecting DIC concentrations but was insufficient to induce significant carbonate dissolution. At night outgassing maintained in-stream DIC concentrations lower than the supersaturated DIC springs source but a drop in δ 13C DIC indicates that ecosystem respiration had a dominant influence over outgassing. Although CO 2 outgassing occurs, it is shown to be a minor component of the DIC mass balance while carbonate precipitation represents 88% of DIC loss. These results indicate that in-stream biological processes influence carbonate mineral diagenesis in large clear-water rivers.

Δ14C and Δ13C of Seawater DIC as Tracers of Coastal Upwelling: A 5-Year Time Series from Southern California

Marine radiocarbon (14C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Here, we report a time series of carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from the Newport Beach pier in Orange County, within the Southern California Bight, from 2005 to 2010. Surface seawater was collected bimonthly and analyzed for Δ14C, δ13C, and salinity. Results from May 2005 to November 2010 show no long-term changes in δ13C DIC values and no consistent variability that can be attributed to upwelling. Δ14C DIC values have lowered from ∼34‰ to about ∼16‰, an 18‰ decrease from the beginning of this project in 2005, and is consistent with the overall 14C depletion from the atmospheric thermonuclear bomb pulse at the end of the 1950s. Δ14C DIC values, paired with salinity, do appear to be suitable indicators of upwelling strength with periods of upwelling characterized by more saline and lower DIC Δ14C values. However, a similar signal was not observed during the strong upwelling event of 2010. These results were obtained in the Southern California Bight where upwelling is fairly weak and there is a complex occanographic circulation in comparison with the remaining western USA coastline. It is therefore likely that the link between DIC Δ14C, salinity, and upwelling would be even stronger at other sites. These data represent the longest time series of Δ14C data from a coastal Southern California site performed to date.

Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves

Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of 13C and 12C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ 13C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO 2, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO 2 concentrations and tested against the carbon isotope data. Air advection forces winter pCO 2 to drop in the cave air to ca. 500 ppm from a summer peak of ca. 1500 ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4 days. The process of cave ventilation forces degassing of CO 2 from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. 13C-enrichment of dripwater whose δ 13C DIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ 13C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO 2 evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic 13C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO 2 between cave water and cave air. This implies that speleothem δ 13C values may not be simply interpreted either in terms of hydrology or soil processes.

Mixing of magmatic CO 2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

To understand deep groundwater flow systems and their interaction with CO 2 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 (δ 13C DIC) and DIC concentrations of the sampled waters show that magma-derived CO 2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ 13C DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ 18O) 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 δ 13C DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO 2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO 2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.

Geochemistry of dissolved inorganic carbon and carbonate weathering in a small typical karstic catchment of Southwest China: Isotopic and chemical constraints

The sources of dissolved inorganic carbon (DIC) and rock weathering processes were studied in the Houzhai catchment, a typical karstic catchment in the Changjiang River Basin, Southwest China. The carbon isotopic compositions ( δ 13C DIC) of DIC in the samples collected from the catchment vary from − 13.5‰ to − 6.9‰, with a mean value of − 9.8‰. The DIC in the catchment is thereby considered to be mainly derived from soil CO 2 and weathering of carbonate rocks. The DIC concentrations and the δ 13C DIC values show pronounced seasonal variations, with the lowest values being observed during the high flow season (from May to October). The logPCO 2 values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite (SIc) and δ 13C DIC values. These observations indicate that CO 2 derived from organic matter oxidation plays an important role in the dissolution of carbonate for this typical karstic environment. Based on a chemical mass balance, the weathering rate of carbonate rocks in the Houzhai catchment was estimated to be approximately 133 t/km 2/year or 584 × 10 3 mol/km 2/year in terms of CO 2 consumption rate, which is higher than the reported values for other rivers in Southwest China.

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ13C values of DIC become heavier with increasing subbottom depth, and vary between −9.5 and +21.4‰ PDB. Enrichment of DIC in 13C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO2-reduction) were revealed by determining the offset of δ13C between dissolved CH4 and CO2, and also by using δ13C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.

Diurnal dynamic of inorganic carbon and oxygen dissolved in a Nile tilapia (Oreochromis niloticus Linnaeus, 1758) fish pond, São Paulo, Brasil

Natural waters may play the role of sinks or carbon dioxide (CO2) emitters, depending on the physicochemical characteristics of the system (diffusion and reaction of this gas into water) as well as on the pH, and the primary production of micro-organisms as a result of the consumption of such compounds. Evidence suggests that the CO2 concentrations in ponds are mainly governed by the aquatic metabolism, i.e. by the balance between respiration and photosynthesis; AIM: The purpose of this study was to describe aspects of the metabolism of tilapia cultivation based on the dynamic and balance of the oxygen concentrations (DO) and forms of dissolved inorganic carbon (DIC): carbon dioxide (CO2) and bicarbonate (HCO3-). Other variables analyzed are: total phosphorus, water transparency, total alkalinity, water temperature, pH, underwater radiation and quantitative analysis of phytoplankton community; METHODS: Sampling was collected infor 5 consecutive days from 6:00 AM to 8:00 PM (December/2006) every 2 hours; RESULTS: During the test it was observed periodicity in the fluctuations of the DIC concentrations, being CO2 and HCO3- the predominant fractions. The values of DIC were strongly influenced by the fraction of CO2 and it was observed a predominance of the fraction HCO3- in the afternoon. CO2 concentrations ranged from 0.48 µM through 138.94 µM, reaching a daily average of 18.04 µM. The flow of CO2 in the interface atmosphere/water showed variations during the day. In the afternoon (from 12:00 PM until 6:00 PM) the variation pointed to the flow atmosphere/fish pond; however, the balance was 577 µmol.m-2.h-1 in the flow fish pond/atmosphere; CONCLUSIONS: The observed dynamics indicated that under the conditions of this study, the metabolism of aquatic organisms was the main driving force of this system, a fact corroborated by the intense process of euthrophication in the pond.