Dissolved Inorganic Carbon Concentrations Research Articles

Submarine groundwater discharge drives biogeochemistry in two Hawaiian reefs

AbstractGroundwater inputs are typically overlooked as drivers of environmental change in coastal reef studies. To assess the impact of groundwater discharge on reef biogeochemistry, we examined two fringing reef environments, located in Maunalua Bay on the south shore of O‘ahu, Hawai‘i, that receive large inputs of submarine groundwater discharge. We supplemented 25‐ and 30‐d time series measurements of salinity, water temperature, pH, dissolved oxygen, and 222Rn with high‐resolution 24‐h nutrient, dissolved inorganic carbon (DIC), total alkalinity (TA), and δ13C–DIC measurements to evaluate both groundwater‐induced and biologically‐driven changes in coastal carbonate chemistry across salinity gradients. Submarine groundwater discharge at these two locations was characterized by low pHT (7.36–7.62), and variable DIC (1734–3046 μM) and TA (1716–2958 μM) content relative to ambient seawater. Groundwater‐driven variability in coastal carbonate system parameters was generally on the same order of magnitude as biologically‐driven variability in carbonate system parameters at our study locations. Further, our data revealed a shift in reef metabolism from net dissolution to net calcification across this groundwater‐driven physicochemical gradient. At sites with high levels of groundwater exposure, net community production and calcification rates were reduced. Our findings shed light on the importance of considering groundwater inputs when examining coastal carbonate chemistry.

Modeling uranium(VI) adsorption onto montmorillonite under varying carbonate concentrations: A surface complexation model accounting for the spillover effect on surface potential

The prediction of U(VI) adsorption onto montmorillonite clay is confounded by the complexities of: (1) the montmorillonite structure in terms of adsorption sites on basal and edge surfaces, and the complex interactions between the electrical double layers at these surfaces, and (2) U(VI) solution speciation, which can include cationic, anionic and neutral species. Previous U(VI)-montmorillonite adsorption and modeling studies have typically expanded classical surface complexation modeling approaches, initially developed for simple oxides, to include both cation exchange and surface complexation reactions. However, previous models have not taken into account the unique characteristics of electrostatic surface potentials that occur at montmorillonite edge sites, where the electrostatic surface potential of basal plane cation exchange sites influences the surface potential of neighboring edge sites (‘spillover’ effect).A series of U(VI) – Na-montmorillonite batch adsorption experiments was conducted as a function of pH, with variable U(VI), Ca, and dissolved carbonate concentrations. Based on the experimental data, a new type of surface complexation model (SCM) was developed for montmorillonite, that specifically accounts for the spillover effect using the edge surface speciation model by Tournassat et al. (2016a). The SCM allows for a prediction of U(VI) adsorption under varying chemical conditions with a minimum number of fitting parameters, not only for our own experimental results, but also for a number of published data sets. The model agreed well with many of these datasets without introducing a second site type or including the formation of ternary U(VI)-carbonato surface complexes. The model predictions were greatly impacted by utilizing analytical measurements of dissolved inorganic carbon (DIC) concentrations in individual sample solutions rather than assuming solution equilibration with a specific partial pressure of CO2, even when the gas phase was laboratory air. Because of strong aqueous U(VI)-carbonate solution complexes, the measurement of DIC concentrations was even important for systems set up in the ‘absence’ of CO2, due to low levels of CO2 contamination during the experiment.

Freshwater-Saltwater Mixing Effects on Dissolved Carbon and CO2 Outgassing of a Coastal River Entering the Northern Gulf of Mexico

The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO2 outgassing at six locations along an 88-km-long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river’s mouth. We found that throughout the sampling period, all six sites exhibited CO2 supersaturation with respect to the atmospheric CO2 pressure during most of the sampling trips. The average CO2 outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m−2 year−1, respectively, with a mean of 140 mol m−2 year−1 for the entire river reach. In the short freshwater river reach before a saltwater barrier, 0.079 × 108 kg carbon was emitted to the atmosphere during the study year. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 × 108 kg) was emitted to the atmosphere during the same period. For the entire study period, the river’s freshwater discharged 0.25 × 109 mol dissolved inorganic carbon (DIC) and 1.77 × 109 mol dissolved organic carbon (DOC) into the mixing zone. DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 to 0.56 mM). These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world’s estuarine systems.

Space and time variability of the Southern Ocean carbon budget

AbstractThe upper ocean dissolved inorganic carbon (DIC) concentration is regulated by advective and diffusive transport divergence, biological processes, freshwater, and air‐sea CO2 fluxes. The relative importance of these mechanisms in the Southern Ocean is uncertain, as year‐round observations in this area have been limited. We use a novel physical‐biogeochemical state estimate of the Southern Ocean to construct a closed DIC budget of the top 650 m and investigate the spatial and temporal variability of the different components of the carbon system. The dominant mechanisms of variability in upper ocean DIC depend on location and time and space scales considered. Advective transport is the most influential mechanism and governs the local DIC budget across the 10 day–5 year timescales analyzed. Diffusive effects are nearly negligible. The large‐scale transport structure is primarily set by upwelling and downwelling, though both the lateral ageostrophic and geostrophic transports are significant. In the Antarctic Circumpolar Current, the carbon budget components are also influenced by the presence of topography and biological hot spots. In the subtropics, evaporation and air‐sea CO2 flux primarily balances the sink due to biological production and advective transport. Finally, in the subpolar region sea ice processes, which change the seawater volume and thus the DIC concentration, compensate the large impact of the advective transport and modulate the timing of biological activity and air‐sea CO2 flux.

Eddy‐induced carbon transport across the Antarctic Circumpolar Current

AbstractThe implications of a mesoscale eddy for relevant properties of the Southern Ocean carbon cycle are examined with in situ observations. We explored carbon properties inside a large (~190 km diameter) cyclonic eddy that detached from the Subantarctic Front (SAF) south of Tasmania in March 2016. Based on remote sensing, the eddy was present for ~2 months in the Subantarctic Zone (SAZ), an important region of oceanic carbon dioxide (CO2) uptake throughout the annual cycle and carbon subduction (i.e., where mode and intermediate waters form), before it was reabsorbed into the SAF. The eddy was sampled during the middle of its life, 1 month after it spawned. Comparatively, the eddy was ~3°C colder, 0.5 practical salinity unit fresher, and less biologically productive than surrounding SAZ waters. The eddy was also richer in dissolved inorganic carbon (DIC) and had lower saturation states of aragonite and calcite than the surrounding SAZ waters. As a consequence, it was a strong source of CO2 to the atmosphere (with fluxes up to +25 mmol C m−2 d−1). Compared to the SAF waters, from which it originated, DIC concentration in the eddy was ~20 μmol kg−1 lower, indicating lateral mixing, small‐scale recirculation, or eddy stirring with lower‐DIC SAZ waters by the time the eddy was observed. As they are commonly spawned from the Antarctic Circumpolar Current, and as 50% of them decay in the SAZ (the rest being reabsorbed by the SAF‐N), these types of eddies may represent a significant south‐north transport pathway for carbon across the ACC and may alter the carbon properties of SAZ waters.

Assessing dissolved carbon transport and transformation along an estuarine river with stable isotope analyses

Estuaries play an important role in the dynamics of dissolved carbon from rivers to coastal oceans. However, our knowledge of dissolved carbon transport and transformation in mixing zones of the world's coastal rivers is still limited. This study aims to determine how dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations and stable isotopes (δ13CDIC and δ13CDOC) change along an 88-km long estuarine river, the Calcasieu River in Louisiana, southern USA, with salinity ranging from 0.02 to 21.92. The study is expected to elucidate which processes most likely control carbon dynamics in a freshwater-saltwater mixing system, and to evaluate the net metabolism of this estuary. Between May 2015 and February 2016, water samples were collected and in-situ measurements on ambient water conditions were performed during five field trips at six sites from upstream to downstream of the Calcasieu River, which enters the Northern Gulf of Mexico (NGOM). The DIC concentration and δ13CDIC increased rapidly with increasing salinity in the mixing zone. The average DIC concentration and δ13CDIC at the site closest to the NGOM (site 6) were 1.31 mM and −6.34‰, respectively, much higher than those at the site furthest upstream (site 1, 0.42 mM and −20.83‰). The DIC concentrations appeared to be largely influenced by conservative mixing, while high water temperature may have played a role in deviating DIC concentration from the conservative line due likely to increased respiration and decomposition. The δ13CDIC values were close to those suggested by the conservative mixing model for May, June and November, but lower than those for July and February, suggesting that an estuarine river can fluctuate from a balanced to a heterotrophic system (i.e., production/respiration (P/R) < 1) seasonally. Unlike the DIC longitudinal trend, the DOC concentrations in the river estuary decreased from upstream to downstream, but to a much smaller degree. The DOC concentrations consistently showed a deviation from those suggested by the conservative mixing model, which may have been a consequence of in-stream photosynthesis. This river estuary consistently showed depleted δ13CDOC values (i.e., from −30.56‰ to −25.92‰), suggesting that the DOC source in the mixing zone was highly terrestrially derived. However, in this relatively small isotopic range, δ13CDOC alone has limitations in differentiating carbon produced by aquatic photosynthesis from carbon produced by terrestrial photosynthesis in a river-ocean continuum.

Impacts of Thermal Stratification on the Hydrochemistry and Dissolved Inorganic Carbon in a Typical Karst Reservoir in Summer

Thermal stratification leads to significant stratification characteristics of hydrochemistry and aquatic organisms in reservoirs, and thus affects the biogeochemical cycle in the reservoir. This study aims to understand physico-chemical properties and dissolved inorganic carbon change processes and its factors in a karst groundwater-fed reservoir, Dalongdong Reservoir, located in Shanglin County, Guangxi Zhuang Autonomous Region, China. The eight sampling points were placed along the direction of the water flow on June 19-21, 2015. The results show that: ① The reservoir exhibited obvious thermal stratification in the summer. There were significant differences in physical and chemical parameters, such as pH and conductivity (Spc) between the epilimnion and thermocline; ② The dissolved oxygen (DO) and chlorophyll a (Chl-a) content from the surface to the bottom did not show a single decreasing trend, but the maximum value occurred 2.5 m or 5 m below the surface; ③ From the surface to the bottom, dissolved inorganic carbon (DIC) concentrations showed an increasing trend with the average DIC concentration of 2.03 mmol·L-1 in the epilimnion and the average DIC concentration of 4.18 mmol·L-1 at the bottom of the thermocline. The value of stable carbon isotope (δ13CDIC) was more positive in the epilimnion than in the thermocline, where δ13CDIC gradually became partially negative with water depth. Possible reasons of these results include: ① The significant differences in temperature, distribution of aquatic organisms, and strength and direction of metabolisms in different water layers due to thermal stratification; ② The DIC variations in the epilimnion were mainly affected by the carbonate precipitation process and phytoplankton photosynthesis, thereby affecting the DIC stable isotope fractionation. DIC was mainly controlled by biological respiration and the organic matter decomposition process in the thermocline.

Remote effects of mixed layer development on ocean acidification in the subsurface layers of the North Pacific

Using the outputs of projections under the highest emission scenario of the representative concentration pathways performed by Earth system models (ESMs), we evaluate the ocean acidification rates of subsurface layers of the western North Pacific, where the strongest sink of atmospheric CO2 is found in the mid-latitudes. The low potential vorticity water mass called the North Pacific Subtropical Mode Water (STMW) shows large dissolved inorganic carbon (DIC) concentration increase, and is advected southwestward, so that, in the sea to the south of Japan, DIC concentration increases and ocean acidification occurs faster than in adjacent regions. In the STMW of the Izu-Ogasawara region, the ocean acidification occurs with a pH decrease of ~0.004 year−1 , a much higher rate than the previously estimated global average (0.0023 year−1), so that the pH decreases by 0.3–0.4 during the twenty-first century and the saturation state of calcite (ΩCa) decreases from ~4.8 down to ~2.4. We find that the ESMs with a deeper mixed layer in the Kuroshio Extension region show a larger increase in DIC concentration within the Izu-Ogasawara region and within the Ryukyu Islands region. Comparing model results with the mixed layer depth obtained from the Argo dataset, we estimate that DIC concentration at a depth of ~200 m increases by 1.4–1.6 μmol kg−1 year−1 in the Izu-Ogasawara region and by 1.1–1.4 μmol kg−1 year−1 in the Ryukyu Islands region toward the end of this century.

Sources of dissolved inorganic carbon in rivers from the Changbaishan area, an active volcanic zone in North Eastern China

Major elements and carbon isotopes of dissolved inorganic carbon (DIC) have been measured in the waters of Changbaishan mountain, a volcanic area in northeastern China, between June and September 2016 to decipher the origin of the CO2 involved in chemical weathering reactions. Spatial variations of major elements ratios measured in water samples can be explained by a change of the chemical composition of the volcanic rocks between the volcanic cone (trachytes) and the basaltic shield as evidenced by the variations in the composition of these rocks. Hence, DIC results from the neutralization of CO2 by silicate rocks. DIC concentrations vary from 0.3 to 2.5 mmol/L and carbon isotopic compositions of DIC measured in rivers vary from −14.2‰ to 3.5‰. At a first order, the DIC transported by rivers is derived from the chemical weathering’s consumption of CO2 with a magmatic origin, enriched in 13C (−5%) and biogenic soil CO2 with lower isotopic compositions. The highest δ13C values likely result from C isotopes fractionation during CO2 degassing in rivers. A mass balance based on carbon isotopes suggest that the contribution of magmatic CO2 varied from less than 20% to more than 70%. Uncertainties in this calculation associated with CO2 degassing in rivers are difficult to quantify, and the consequence of CO2 degassing would be an overestimation of the contribution of DIC derived from the neutralization of magmatic CO2 by silicate rocks.

Carbon isotope exchange between gaseous CO2 and thin solution films: Artificial cave experiments and a complete diffusion-reaction model

Speleothem stable carbon isotope (δ13C) records provide important paleoclimate and paleo-environmental information. However, the interpretation of these records in terms of past climate or environmental change remains challenging because of various processes affecting the δ13C signals. A process that has only been sparsely discussed so far is carbon isotope exchange between the gaseous CO2 of the cave atmosphere and the dissolved inorganic carbon (DIC) contained in the thin solution film on the speleothem, which may be particularly important for strongly ventilated caves.Here we present a novel, complete reaction diffusion model describing carbon isotope exchange between gaseous CO2 and the DIC in thin solution films. The model considers all parameters affecting carbon isotope exchange, such as diffusion into, out of and within the film, the chemical reactions occurring within the film as well as the dependence of diffusion and the reaction rates on isotopic mass and temperature. To verify the model, we conducted laboratory experiments under completely controlled, cave-analogue conditions at three different temperatures (10, 20, 30°C). We exposed thin (≈0.1mm) films of a NaHCO3 solution with four different concentrations (1, 2, 5 and 10mmol/l, respectively) to a nitrogen atmosphere containing a specific amount of CO2 (1000 and 3000ppmV). The experimentally observed temporal evolution of the pH and δ13C values of the DIC is in good agreement with the model predictions. The carbon isotope exchange times in our experiments range from ca. 200 to ca. 16,000s and strongly depend on temperature, film thickness, atmospheric pCO2 and the concentration of DIC. For low pCO2 (between 500 and 1000ppmV, as for strongly ventilated caves), our time constants are substantially lower than those derived in a previous study, suggesting a potentially stronger influence of carbon isotope exchange on speleothem δ13C values. However, this process should only have an influence in case of very long drip intervals and slow precipitation rates.

Riverine carbon fluxes to the South China Sea

AbstractThe high precipitation around Southeast Asia results in abundant freshwater outflow, which transports terrestrial dissolved and particulate material to the world's largest marginal sea, namely, the South China Sea (SCS). To estimate the riverine carbon flux to the SCS, carbonate data from 42 rivers were collected. These results, combined with literature data for 13 rivers, indicate that the concentrations of dissolved inorganic carbon (DIC) are positively correlated with the cation exchange capacity and the bulk density, except for the Low Discharge category. The highest DIC concentration and flux are in the Low and Medium Discharge categories. Negative correlations exist between dissolved organic carbon (DOC) concentrations and the base saturation in the Low and High Discharge categories and between DOC and bulk density in the Low and Medium Discharge categories. However, the correlation between the DOC concentration and the organic carbon content in the soil is only significant in the Medium Discharge category. Once the negative exponential relationships between the particulate inorganic carbon (PIC) or the particulate organic carbon (POC) and the total suspended matter are determined, these can be used to estimate the PIC and POC carbon fluxes. Of the annual riverine carbon flux, 83.0 ± 8.1 Tg C, 40.14 ± 6.11 Tg is DIC, 25.03 ± 4.54 Tg is DOC, 1.83 ± 0.35 Tg is PIC, and 16.03 ± 2.87 Tg is POC. The total discharge amounts to 6.2–10.3% of the global riverine discharge for a sea covering only 1% of the world's ocean surface area and for watersheds covering only 2.2% of the global land mass.

A new version of the Hydrochemical Dystrophy Index to evaluate dystrophy in lakes

Dystrophic freshwaters referred to as humic, brown, or black waters are typical for boreal or some mountainous regions where fens and coniferous forests form a significant part of basins. I evaluated the usefulness of the Hydrochemical Dystrophy Index (HDI) which has been developed for dystrophic lakes in Poland, on the basis of a rich database for lakes in Sweden and data provided by other researchers for lakes from Finland and Russia. I propose a new version of HDI for synthetic and quantitative description of habitat conditions in lakes and for use in limnological monitoring of protected areas. The state of dystrophy was evaluated using data for surface water pH, electric conductivity (EC), DIC (dissolved inorganic carbon) and DOC (dissolved organic carbon) concentrations. HDI values between 50 and 65 indicate semidystrophic conditions and with advanced a real dystrophy, they reach HDI values from 65 up to 100. Long term data shows that lake dystrophy is fairly steady with seasonal fluctuations. I show that the level of lake dystrophy is not correlated to latitude, but rather to small lake areas, regional geochemical spots, and favorable local hydrological conditions. A summary description of the main parameters of various types of humic lakes is presented.

An exploiting of cost-effective direct current conductivity detector in gas diffusion flow injection system

In this work, a homemade direct current (DC) conductivity detector as an alternative cost-effective detection device has been fabricated and investigated to use in flow analysis system. Under the selected appropriate conditions of flow system, the electrolysis of a carrier stream at the conductivity detector was negligible and provides well-defined signal. The cost-effective DC conductivity detector was demonstrated to couple with gas diffusion flow injection system for determination of dissolved inorganic carbon (DIC) in water. The method is based on the conversion of DIC (dissolved CO2, HCO3- and CO32-) presented in the injected sample to carbon dioxide in an acidic donor stream and then CO2 gas diffuses through a hydrophobic porous membrane to an acceptor stream. As a result, the change of conductivity signal was observed corresponding to DIC concentration. A linear calibration range of DIC in 1.0–10mmolL−1, with limit of detection of 70µmolL−1, repeatability of <3% RSD and 15 injections h−1 sample throughput can be obtained. This method was applied for DIC determination in natural water.

MoDIE: Moderate dissolved inorganic carbon (DI13C) isotope enrichment for improved evaluation of DIC photochemical production in natural waters

Photochemical reactions in natural waters are a sink for dissolved organic carbon (DOC) and a source of dissolved inorganic carbon (DIC). Although considered significant to the carbon budget of the oceans, DIC photoproduction rates remain poorly constrained due to the analytical challenge involved in accurately measuring very low production rates (likely sub-μM h− 1 for blue ocean waters) relative to high background DIC concentrations (~ 2 mM). In an attempt to overcome this analytical limitation, almost all previous DIC photoproduction studies in marine systems have relied on the stripping of background DIC prior to irradiation, with unknown consequences for the integrity of the DOC pool and its photoreactivity, and none have resulted in a satisfactory determination of photoproduced DIC in open ocean waters. Here, we use small additions of NaH13CO3 (< 10% of background DIC concentrations) to achieve moderate DI13C isotope enrichment (MoDIE) of river, estuarine, and seawater samples. We then quantify the shift in δ13C of DIC in irradiated samples via liquid chromatography - isotope ratio mass spectrometry (LC-IRMS) and use these shifts to calculate photoproduced DIC. MoDIE provides a new method to more precisely (± 56 to ± 340 nM DIC) and more rapidly quantify DIC photoproduction compared to previous methods. MoDIE was evaluated by determining initial DIC photoproduction rates, along with associated broadband photochemical efficiency data (ranging from 144 to 280 μmol DIC per mol photons absorbed), in riverine to offshore waters, and produced the earliest time point and most precise measurement of DIC photoproduction in unmodified, low-CDOM, blue water (ag(325) = 0.30 m− 1) reported to date. In addition to measuring small DIC changes from photochemistry, the use of MoDIE could provide a major advance for other ocean carbon cycling studies, including examination of oceanic respiration, carbonate mineral precipitation/dissolution, and assessment of ocean acidification.

Transport, anoxia and end-product accumulation control carbon dioxide and methane production and release in peat soils

Anaerobic respiration and methanogenesis have been found to slow-down in water saturated peat soils with accumulation of metabolic end-products, i.e. dissolved inorganic carbon (DIC) and methane (CH4), due to a lack of solute and gas transport. So far it is not well understood how solute and gas transport may control this effect. We conducted a column experiment with homogenized ombrotrophic peat over a period of 300 days at 20 °C. We specifically evaluated the effects of diffusive flux as control, downward advective water flux, intensified ebullition by conduit gas transport and diffusive oxygen supply on controlling anaerobic decomposition rates and carbon (C) turnover. To simulate advective flux, water and solutes were recirculated downward through the column after stripping of dissolved gases. We analyzed DIC and CH4 concentrations, production rates and fluxes, gas filled porosity, oxygen profiles (O2) and microbial C biomass over time. DIC residence time thereby served as proxy to characterize transport. A slowdown of anaerobic respiration and methanogenesis evolved with the accumulation of the end-products DIC and CH4 and set in after 150 days. This slow-down was accompanied by a decrease in the distribution of microbial biomass C with depths. Anaerobic DIC and CH4 production rates were fastest close to the water table and sharply slowed with depth. Accumulation of DIC and CH4 in the homogeneous peat material throughout the column decreased decomposition constants from about 10−5 near the surface to 10−9 year−1 deeper in the profile. Advective water transport extended the zone of active methanogenesis compared to a diffusive system; experimental enhancement of ebullition had little or no effect as well as strictly anoxic conditions. DIC residence time was negatively correlated to anaerobic respiration suggesting this parameter to be a predictor of anaerobic peat decomposition in peatlands. Overall, this study suggests that burial of peat and accumulation of metabolic end-products effectively slows decomposition and that this effect needs to be considered to explain peat accumulation and the response of peat mineralization rates to changes in environmental conditions.

Technical note: Coupling infrared gas analysis and cavity ring down spectroscopy for autonomous, high-temporal-resolution measurements of DIC and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C–DIC

Abstract. A new approach to autonomously determine concentrations of dissolved inorganic carbon (DIC) and its carbon stable isotope ratio (δ13C–DIC) at high temporal resolution is presented. The simple method requires no customised design. Instead it uses two commercially available instruments currently used in aquatic carbon research. An inorganic carbon analyser utilising non-dispersive infrared detection (NDIR) is coupled to a Cavity Ring-down Spectrometer (CRDS) to determine DIC and δ13C–DIC based on the liberated CO2 from acidified aliquots of water. Using a small sample volume of 2 mL, the precision and accuracy of the new method was comparable to standard isotope ratio mass spectrometry (IRMS) methods. The system achieved a sampling resolution of 16 min, with a DIC precision of ±1.5 to 2 µmol kg−1 and δ13C–DIC precision of ±0.14 ‰ for concentrations spanning 1000 to 3600 µmol kg−1. Accuracy of 0.1 ± 0.06 ‰ for δ13C–DIC based on DIC concentrations ranging from 2000 to 2230 µmol kg−1 was achieved during a laboratory-based algal bloom experiment. The high precision data that can be autonomously obtained by the system should enable complex carbonate system questions to be explored in aquatic sciences using high-temporal-resolution observations.

Continuous CO2 escape from the hypersaline Dead Sea caused by aragonite precipitation

Chemical precipitation of CaCO3 occurs in diverse marine and lacustrine environments. In the hypersaline Ca-chloride lakes that have been occupying the Dead Sea basin since the late Pleistocene, CaCO3 precipitated, mostly as aragonite. The aragonite sediments precipitated mainly during periods of high lake level stands as a result of mixing of bicarbonate-rich freshwater runoff with Dead Sea brine, that is Ca-rich and have high Mg/Ca ratio. During periods of arid conditions with limited freshwater inflow, water level declined, salinity increased and gypsum and halite became the dominant evaporitic minerals to precipitate.The present study investigates the carbon cycle of the Dead Sea under the current limited water and bicarbonate supply to the brine, representing periods of extremely arid conditions. The decrease of inflows to the Dead Sea in recent years stems mainly from diversion of freshwater from the drainage basin and results in dramatic water level decline and massive halite precipitation. During 2013–2014, bi-monthly depth profiles of total alkalinity, dissolved inorganic carbon (DIC) and its isotopic composition (δ13C) were conducted in the Dead Sea, from surface down to the bottom of the lake (290m). Mass balance calculations conducted for the period 1993–2013 show that while inventories of conservative ions such as Mg2+ remained constant, the net DIC inventory of the lake decreased by ∼10%. DIC supply to the lake during this period, however, amounted to ∼10% of lake’s inventory indicating that during 20years, the lake lost ∼20% of its 1993s inventory. Compilation of historical data with our data shows that during the past two decades the lake's low DIC (∼1mmolkg−1) and very high PCO2 (1800ppmV) remained relatively constant, suggesting that a quasi-steady-state situation prevails. In spite of the surprisingly stable DIC and CO2 concentrations, during this 20year period δ13CDIC increased significantly, from 1.4‰ to 2.7‰. An isotopic mass balance calculation together with the high PCO2 of the brine show that during that period the lake lost about 3.7·1010 moles of DIC, of which ∼60% by CO2 degassing and ∼40% by aragonite precipitation. The deviation from the common 1:1 CO2:CaCO3 ratio in most aqueous systems is facilitated by the dominance of the borate alkalinity of the Dead Sea. Nucleation and crystal growth experiments suggest that throughout this time the Dead Sea remained supersaturated with respect to aragonite, a situation facilitated by combination of slow nucleation rates and absence of growth surfaces. The very high PCO2 of the Dead Sea is maintained by the low CO2 piston velocity of the brine, calculated to be only ∼4.5myr−1, more than an order of magnitude slower than seawater value.

Stable carbon isotopic composition of submerged plants living in karst water and its eco-environmental importance

The stable carbon isotopic composition of submerged plants (δ13CP) can be controlled by physiological and environmental factors. Herein, we took advantage of a short, natural karst river with an annual mean bicarbonate (HCO3−) value of 3.8mmolL−1 to study the stable carbon isotopic composition of submerged plants along the river and the influence of environmental conditions on the δ13CP values. The δ13CP values of Ottelia acuminata, Potamogeton wrightii, Vallisneria natans, and Hydrilla verticillata from upstream to downstream show a gradient and ranged from −34.8‰ to −27.8‰, −36.6‰ to −23.7‰, −35.1‰ to −25.3‰, and −38.6‰ to −26.3‰, respectively and even more depleted values for the first two species at the uppermost site. Diurnal variation of water chemistry and concentration of the dissolved inorganic carbon (DIC) and the stable carbon isotopic composition of DIC (δ13CD) indicate that the macrophytes and other primary producers in the river have a very high net photosynthetic rate. The gradient of δ13CP values was consistent with CO2 being a declining source of inorganic carbon for photosynthesis in the downstream transect. The results demonstrate that the high DIC concentration with lower negative δ13C value, particularly in karst water environment has a significant role in controlling the stable carbon isotopic composition of submerged plants living in it.

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

ABSTRACTThe aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ13CDIC) with the most negative value of –18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of −28.5 ‰ from DOC and −23.4, −31.8 and −30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ13CDIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.

Temporal and Spatial Variations of Dissolved Inorganic Carbon and Its Stable Isotopic Composition in the Surface Stream of Karst Groundwater Recharge

Stable carbon isotope of dissolved inorganic carbon (δ13CDIC), which is mainly constituted by HCO3- in karst water, is widely used to trace the different sources and influential factors of dissolved inorganic carbon (DIC). In order to understand the distribution of DIC and δ13CDIC in subtropical karst area, this paper researched the water chemistry and δ13CDIC in a karst surface stream in detail, which is fed by Guancun subterranean stream in Liuzhou City, Guangxi Province, in the southwest of China. The results showed that the contents of DIC in subterranean stream outlet (G1 site) ranged from 4.60 to 4.90 mmol·L-1 with an average of 4.73 mmol·L-1 in dry season, and from 2.80 to 4.70 mmol·L-1 with an average of 4.23 mmol·L-1 in rainy season. The contents of DIC in 1.35 km downstream site (G2 site) ranged from 4.30 to 4.90 mmol·L-1 with an average of 4.56 mmol·L-1 in dry season, and from 3.00 to 4.70 mmol·L-1 with an average of 4.20 mmol·L-1 in rainy season. The δ13CDIC of subterranean stream outlet (G1 site) varied from -12.8‰ to -11.53‰ with an average of -12.22‰ in dry season, and from -13.12‰ to -11.01‰ with an average of -12.28‰ in rainy season. The δ13CDIC of stream downstream site (G2 site) ranged from -11.71‰ to -9.55‰ with an average of -10.73‰ in dry season, and ranged from -12.18‰ to -9.85‰ with an average of -11.10‰ in rainy season. The contents of DIC of G1 site were higher than those of G2 site. The DIC contents in dry season in both G1 and G2 site were higher than those in rainy season. The values of δ13CDICof G1 and G2 site in dry season were more positive than those in rainy season. The δ13CDICvalue of G1 site was more negative than that of G2 site. The main sources of DIC in underground river and surface stream were soil CO2and carbonate dissolution. However, the differences of DIC and δ13CDICbetween G1 and G2 site showed that CO2degassing and photosynthesis of aquatic plants had significant influence on water DIC and δ13CDIC value. This study is helpful to understand the dynamic change and distribution of DIC and δ13CDIC in karst surface stream.