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We present the carbon isotope (14C and 13C), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) concentration measurements in the South China Sea (SCS) to reveal the different sources and cycling time scales of the two major carbon pools in the SCS. The DIC concentrations ranged from 1,776 to 2,328 μmol kg−1, and they were lower at the surface and increased with depth. Conversely, the DOC concentrations ranged from 38 to 95 μM, and they were higher on the surface and decreased rapidly in the upper 500‐m water depth. The DIC Δ14C and DOC Δ14C values varied from −227‰ to 68‰ and −557‰ to −258‰, respectively, and both decreased with depth until 1,500 m and then remained relatively constant. DOC Δ14C values were −330‰ lower than DIC Δ14C, indicating that DOC has cycled for much longer than DIC in the SCS. The lower Δ14C‐DIC and Δ14C‐DOC values at depths shallower than 700 m were mainly influenced by intensified vertical mixing, which upwelled the deep water with low Δ14C‐DIC and Δ14C‐DOC values for thorough mixture with the upper layer water. Conversely, the small difference in the Δ14C signature in deep water (>1,500 m) between the SCS and the North Pacific confirmed the rapid water exchange through the Luzon Strait and rapid water mixing in the SCS basin, which plays an important role in controlling carbon cycling in the deep SCS. Plain Language Summary: Dissolved inorganic carbon (DIC) is the largest carbon pool in the ocean and is closely linked to dissolved organic carbon (DOC), which is the largest exchangeable organic carbon pool in the ocean. Both DIC and DOC play important roles in the global carbon cycle, but their sources, distribution, and cycling time are different and controlled by different processes in the ocean. Here we report radiocarbon and stable carbon isotope measurements of DIC and DOC collected in the South China Sea (SCS) to reveal the sources and cycling time scales of the two major carbon pools in the SCS. The Δ14C values and 14C ages indicate that DOC has cycled for much longer than DIC in the SCS. The rapid water exchange and mixing between the SCS and the Kuroshio Current in the Northwestern Pacific play important roles in controlling the distributions and cycling of DIC and DOC in the SCS. Key Points: The 14C levels in DOC and DIC of the South China Sea are equal to the levels in the western and central North PacificThe South China Sea is largely influenced by the Kuroshio Current extrusionThe distribution and cycling of DIC and DOC in the deep South China Sea are largely controlled by hydrodynamic processes [ABSTRACT FROM AUTHOR]

Dissolved inorganic carbon (DIC) transport by rivers is an important control on the pH and carbonate chemistry of the coastal ocean. Here, we combine DIC and total alkalinity (TAlk) concentrations from four tropical rivers of the Great Barrier Reef region in Australia with daily river discharge to quantify annual river loads and export rates. DIC in the four rivers ranged from 284 to 2,639 μmol kg−1 and TAlk ranged from 220 to 2,612 μmol kg−1. DIC:TAlk ratios were mostly greater than one suggesting elevated exports of free [CO2*]. This was pronounced in the Johnstone and Herbert rivers of the tropical wet north. The largest annual loads were transported in the two large river catchments of the southern Great Barrier Reef region, the Fitzroy and Burdekin rivers. The carbon stable isotopic composition of DIC suggests that carbonate weathering was the dominant source of DIC in the southern rivers, and silicate weathering was likely a source of DIC in the northern Wet Tropics rivers. Annual loads and export rates were strongly driven by precipitation and discharge patterns, the occurrence of tropical cyclones, and associated flooding events, as well as distinct seasonal dry and wet periods. As such, short‐lived hydrological events and long‐term (seasonal and inter‐annual) variation of DIC and TAlk that are pronounced in rivers of the tropical and subtropical wet and dry climate zone should be accounted for when assessing inorganic carbon loads to the coastal ocean and the potential to buffer against or accelerate ocean acidification. [ABSTRACT FROM AUTHOR]

The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape‐level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial‐to‐aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%–77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra‐high‐resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO2 concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land‐use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular‐plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere. [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope thermometry has emerged as a useful tool for the reconstruction of precipitation temperatures for minerals from surface and subsurface environments. Few studies have examined the impact of variable solution chemistry and mineral chemistry on clumping. Therefore, we used theoretical calculations to predict the impact of the presence of cations in a solution on the equilibrium clumped, oxygen (18O/16O and 17O/16O, θ and κ), and carbon isotopic (13C/12C) compositions of dissolved inorganic carbon species (DIC), and the impact of cation type and stoichiometry on carbonate mineral composition within the calcite-dolomite-magnesite solid solution. We examined the impact of the presence of Mg2+, K+, Na+, and Ca2+ on equilibrium Δ 63 values of carbonate ion groups in DIC species and the composite DIC pool using the B3LYP/6-311++G(2d, 2p) model. We developed new cluster models for dolomite, magnesite, and high-magnesium calcite (Ca 0.83 Mg 0.17)CO 3 and (Ca 0.67 Mg 0.33)CO 3) and examined the effect of extraneous cations substituted into the crystal lattices of the carbonate minerals. We find the presence of added cations in the DIC pool increases equilibrium Δ 63 values slightly, depending upon their solution concentrations. For the cations examined, the identity of the added cation does not seem to be significant. Cumulative effects may account for small-scale differences (<0.02‰) but are unlikely to be substantial unless cation/anion association and ion pairing are important in the solution that carbonates are growing from. We also find that at a given temperature, stoichiometric dolomite and magnesite Δ 63 values are similar to calcite and less than aragonite. Surprisingly, substitution of an extraneous cation into a carbonate mineral (e.g., as observed in high-magnesium calcite) generally lowered Δ 63 by a slight amount, which suggests that high and low-magnesian calcites may have resolvable differences in equilibrium clumped isotope signatures, potentially providing an explanation for observations of different Δ 47 values in slow-growing minerals. These results imply the cationic composition of the solution may be a minor factor that should be considered for crystals that rapidly grow from solution, and that cationic substitution into carbonate mineral lattices will influence equilibrium compositions of carbonate minerals. Solution chemistry effects on mineral composition are likely to be observed at high growth rates. In contrast, equilibrium isotope compositions arising from cationic substitution in minerals, such as differences between high-Mg calcite and low-Mg calcite, or high-Mg calcite and dolomite, would likely be observed at slow growth rates. [ABSTRACT FROM AUTHOR]

Combined pressures from inland agricultural intensification and coastal development are dramatically altering estuaries' structure and function. Despite the established global significance of estuarine carbon (C) cycling, the impact of growing anthropogenic stress on coastal C inputs and exports is unclear. To address this gap, we evaluated the magnitude and drivers of estuary C fluxes in eight sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of nutrient enrichment. We measured changes in the concentration and isotopic composition (δ13C) of the major C pools (organic and inorganic) and gaseous product of C turnover (CO 2) over wet and dry seasons. Over both sampling periods estuaries classified Moderate and High emitted far more CO 2 (37 ± 10 mmol m−2 d−1) than those classified Low (6.3 ± 4 mmol m−2 d−1). However, estuaries with both high nutrients and high turbidity produced less CO 2 , and thus exported more DIC, than expected from hydrodynamics (freshwater flushing time). Differences in estuary phytoplankton biomass (Chl a concentrations) corresponded with differences in the biological CO 2 production (respiration) rates estimated from δ13C-DIC variations, although respiration rates were higher than predicted based on hydrodynamics (surface area/discharge) in high nutrient, low turbidity systems. Together these findings demonstrate that land-use intensification can alter both the source and the production of estuary CO 2 , and suggest that the direction of this shift can depend on ancillary factors like turbidity as well as nutrient enrichment. Evidence that human alterations to coastal ecosystems can shift the balance between DIC downstream export and CO 2 emissions outside of the range predicted by hydrodynamic factors like residence time, surface area, and discharge has implications for global C models. [ABSTRACT FROM AUTHOR]

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

Laboratory studies that test the responses of coastal organisms to ocean acidification (OA) typically use constant pH regimes which do not reflect coastal systems, such as seaweed beds, where pH fluctuates on diel cycles. Seaweeds that use CO2 as their sole inorganic carbon source (non-carbon dioxide concentrating mechanism species) are predicted to benefit from OA as concentrations of dissolved CO2 increase, yet this prediction has rarely been tested, and no studies have tested the effect of pH fluctuations on non-CCM seaweeds. We conducted a laboratory experiment in which two ecologically dominant non-CCM red seaweeds (Callophyllis lambertii and Plocamium dilatatum) were exposed to four pH treatments: two static, pHT 8.0 and 7.7 and two fluctuating, pHT 8.0 ± 0.3 and 7.7 ± 0.3. Fluctuating pH reduced growth and net photosynthesis in C. lambertii , while P. dilatatum was unaffected. OA did not benefit P. dilatatum , while C. lambertii displayed elevated net photosynthetic rates. We provide evidence that carbon uptake strategy alone cannot be used as a predictor of seaweed responses to OA and highlight the importance of species-specific sensitivity to [H+]. We also emphasize the importance of including realistic pH fluctuations in experimental studies on coastal organisms. [ABSTRACT FROM AUTHOR]

We present radiocarbon measurements of dissolved inorganic carbon (DIC) in depth profiles from reoccupations of our central North Pacific Ocean and Sargasso Sea (SS) sites. From 1985 to 1999, an increase in Δ14C values of 6–17‰ was measured between 1150 and 2400 m depth in the North Central Pacific. Natural changes in deep circulation are likely responsible for variability of Δ14C signatures in the deep ocean, though we cannot rule out the presence of bomb 14C at this depth range. Bomb 14C had increased in the deep SS from 1989 to 2000; this is the result of southward transport of North Atlantic Deep Water (NADW) to this site.

Large-scale intra-continental strike-slip faults provide the pathways for groundwater circulation and the release of CO 2 -rich fluids through thermal springs, which is a significant type of deep CO 2 emissions into the atmosphere and hydrosphere. India-Asia collision created numerous strike-slip faults and thermal springs in the Tibetan Plateau and adjacent regions, but their CO 2 origins and fluxes are still not well understood. Here, we present a quantitative study on CO 2 emissions from thermal springs in the Karakoram fault system (KKFS) and adjacent regions, western Tibetan Plateau, with the aim of revealing the origins of CO 2 -rich fluids and estimating the dissolved and gaseous CO 2 fluxes. Hydrogen and oxygen isotopic data show that the geothermal waters were recharged by local precipitation (e.g., rainfall and snow-melt water) and experienced variable degrees of water-rock interaction, giving rise to the dominant Na–HCO 3 type and minor Na–Cl and Ca–HCO 3 type waters discharging from thermal springs. Spatially, the thermal springs within or close to the KKFS (Group 1 and Group 2) have larger circulation depths of 1.3–5.1 km, while those away from the KKFS (Group 3) exhibit shallow (0.6−0.8 km) groundwater circulation. Mass balance calculations reveal high deeply-sourced carbon inputs (60.4−94.3%) to dissolved inorganic carbon (DIC) of the Group 1 and Group 2 samples, while the Group 3 samples have higher shallow carbon contributions (50.4−60.9%) to DIC via dissolution of carbonate and organic carbon. This is consistent with higher Δ14C DIC values (−788.6‰ to −692.1‰) in Group 3 samples and the lack of radioactive 14C in Group 1 and Group 2 samples (Δ14C DIC = −999.9‰ to −985.9‰). Based on chemical compositions and fluxes of spring water and bubbling gases, the total CO 2 output from thermal springs (including dissolved and gaseous CO 2) in the KKFS is estimated to be ∼2332 t a−1, in which the endogenic carbon such as metamorphic CO 2 accounts for ∼1863 t a−1. Our results provide the first estimate of deep CO 2 flux for the KKFS and would contribute to a better understanding of active CO 2 emissions facilitated by strike-slip faulting processes in continental collision zone. • We studied CO 2 origins and fluxes of thermal springs in Karakoram fault system. •Water-rock interaction is more evident for thermal springs of the Karakoram fault. •Deep and shallow carbon sources are discriminated for dissolved inorganic carbon. •Thermal springs of the Karakoram fault release ∼2332 t a−1 CO 2 to Earth's surface. [ABSTRACT FROM AUTHOR]

Based on observations in China's east coastal oceans, we conducted a preliminary assessment of 16 sets of carbonic acid dissociation constants (K 1 * and K 2 *) by comparing spectrophotometrically measured pH values at 25 °C with those calculated from total alkalinity and dissolved inorganic carbon. We obtained that K 1 * and K 2 * often performed differently within different salinity ranges, and that the constants of Millero et al. (2002) (M02) demonstrated the best performance for the salinity range of 24–35. In contrast, the often recommended constants of Mehrbach et al. (1973) refit by Dickson and Millero (1987) (DM87-M) and Lucker et al. (2000) (L00) would underestimate pH at salinities of 24–30. This was mainly associated with the higher product of K 1 * and K 2 * by DM87-M and L00 than by M02 at this salinity range. Also, we found almost no differences between pH values calculated with DM87-M and L00. • Compared to pH measured at 25 °C, M02 exhibited the best internal consistency. • Recommended constants of DM87-M and L00 underestimated pH at low salinities. • There were almost no differences between pH values calculated with DM87-M and L00. • PH can be divided into two components associated with K 1 * × K 2 * and [CO 2 *]/[CO 3 2−]. [ABSTRACT FROM AUTHOR]

Climate variability drives significant changes in the physical state of the North Pacific, and there may be important impacts of this variability on the upper ocean carbon balance across the basin. We address this issue by considering the response of seven biogeochemical ocean models to climate variability in the North Pacific. The models' upper ocean pCO2 and air-sea CO2 flux respond similarly to climate variability on seasonal to decadal timescales. Modeled seasonal cycles of pCO2 and its temperature- and non-temperature-driven components at three contrasting oceanographic sites capture the basic features found in observations (Takahashi et al., 2002, 2006; Keeling et al., 2004; Brix et al., 2004). However, particularly in the Western Subarctic Gyre, the models have difficulty representing the temporal structure of the total pCO2 seasonal cycle because it results from the difference of these two large and opposing components. In all but one model, the air-sea CO2 flux interannual variability (1σ) in the North Pacific is smaller (ranges across models from 0.03 to 0.11 PgC/yr) than in the Tropical Pacific (ranges across models from 0.08 to 0.19 PgC/yr), and the time series of the first or second EOF of the air-sea CO2 flux has a significant correlation with the Pacific Decadal Oscillation (PDO). Though air-sea CO2 flux anomalies are correlated with the PDO, their magnitudes are small (up to ±0.025 PgC/yr (1σ)). Flux anomalies are damped because anomalies in the key drivers of pCO2 (temperature, dissolved inorganic carbon (DIC), and alkalinity) are all of similar magnitude and have strongly opposing effects that damp total pCO2 anomalies.

As a result of anthropogenic activities, it has been predicted that the ocean will be challenged with rising temperature, increased stratification, ocean acidification, stronger more frequent tropical storms, and oxygen depletion. In the tropical Pacific off central Mexico all these phenomena are already occurring naturally, providing a laboratory from which to explore ocean biogeochemical dynamics that are predicted under future anthropogenic forcing conditions. Here, seasonally anomalous surface tropical waters were detected as a result of the developing “Godzilla El Niño 2015–2016.” The incursion of this oxygenated water modified the local structure of an intense and shallow oxygen minimum zone (OMZ), partially eroding and intensifying the oxycline while having an associated impact on the carbon maximum zone. The core of the OMZ (

We report on an initial long-term study of dissolved inorganic and organic carbon (DIC) from Sabino Creek, located in Sabino Canyon, Pima County, Arizona. The purpose of this study was to monitor changes in dissolved radiocarbon (14C) with time and to understand the processes contributing to these variations. Our results span the period 2009–2016 and show a mixing trend between dissolved inorganic and organic carbon modern end-members with an older component. This study provides preliminary information for more detailed research on recycling of organic components in this stream system. [ABSTRACT FROM AUTHOR]

Dissolved inorganic carbon (DIC) is the most important carbon component in karst aquatic system where fluid is highly transmissive, but has rarely been examined in the subtropical karst critical zone (K‐CZ). In this study, concentrations of dissolved solutes and isotopic compositions of DIC (δ13CDIC) at 11 sites of a 73.4 km2 karstic catchment in Southwestern China were analysed monthly in order to uncover the spatiotemporal variations of both DIC and its dominant sources, and to identify relevant controlling factors. Both DIC concentrations and δ13CDIC were highly variable, ranging from 2.52 to 5.85 mmol l−1 and from −15.7 to −4.5‰, respectively. DIC in underground water (UGW) was higher in concentration and more depleted in 13C compared to surface water (SFS). DIC concentrations showed an inconsistent seasonal trend with other solutes, with higher values in the wet season at some sites. δ13CDIC values were lower in the wet season than in the dry season. The results of mixing model IsoSource revealed spatiotemporal patterns of DIC sources. During the dry season, carbonate weathering was the primary contributor to DIC in UGW (excluding in the middle reaches). However, during the wet season, soil CO2 was the dominant source of DIC in both UGW and SFS, and it was higher than in the dry season. Overall, there are significant spatiotemporal disparities and highly transmissive characteristics of both DIC and its sources in the K‐CZ, which are controlled by multiple factors. This study also highlights that rainfall may play a crucial role in accelerating carbon dynamics in the K‐CZ. High‐frequency sampling campaigns in high‐flow periods and deep analyses are needed in future work to elucidate the related processes and mechanisms. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In order to better understand the response of coral calcification to changes in seawater chemistry, fragments of the coral Pocillopora damicornis were grown in seawater with varying [Ca2+]. Using a combined trace element (B/Ca) and stable isotope (δ 11 B, δ 13 C) approach, this paper explores the effect of seawater calcium concentrations ([Ca2+] SW) on coral calcification mechanisms and describes the manipulation of the extracellular calcifying fluid (ECF) pH (pH ECF), saturation state (Ω ECF) and dissolved inorganic carbon (DIC) speciation of the ECF. In these experiments, increases in [Ca2+] SW did not significantly influence calcification rates but caused the skeletal B/Ca ratio to significantly increase. To explain these results we propose a mechanism by which [CO 3 2 − ] ECF is influenced by [Ca2+] SW. This mechanism suggests that the pH ECF will be lowest in our highest [Ca2+] treatment and using this relationship we can place constraints on the pH ECF. The difference in pH ECF in our treatments is supported in part by a slight, but insignificant decrease in skeletal δ 11 B and δ 13 C values. We propose a novel dual-proxy approach to estimate pH ECF , where calcification occurs at a pH ECF > 9. This work emphasizes the importance of multi-element proxy approaches in understanding pH up-regulation and CO 3 2 − concentrations during calcification of corals. • Corals were grown in seawater with varying [Ca] to understand calcification response. • Using a multi-proxy approach, we propose a mechanism to describe calcification. • Growth rates are not influenced by seawater [Ca]. • Experiments suggest that seawater [Ca] modify Ca pumping in the calcifying space. • We propose a novel method to estimate calcifying fluid pH with B and C isotopes. [ABSTRACT FROM AUTHOR]

Deep convection in the Labrador Sea supplies large amounts of anthropogenic carbon to the ocean's interior. We use measurements of all four measurable CO 2 system parameters made along AR7W (across Labrador Sea) between 2013 and 2015 to assess the internal consistency of the carbonate system, including, as appropriate, conversion to in situ temperature (T) and pressure (P). The best agreement between measured and calculated values was obtained through combination of T,P-dependent (pH or p CO 2) and non-dependent (TA or DIC) parameters. Use of the dissociation constants of Mehrbach et al. (1973) as refit by Dickson and Millero (1987) and Lueker et al. (2000) yielded the best internal consistency irrespective of the input parameters used. A Monte Carlo simulation demonstrated that the propagated uncertainty (i.e. combined standard uncertainty) of calculated parameters of the carbonate system is (a) always larger than the analytical precision of the measurements themselves; (b) strongly dependent on the choice of input parameters and uncertainties; (c) less dependent on choice of the specific set of constants. For calculation of other parameters of the carbonate system from TA and DIC measurements made throughout the Labrador Sea time-series, the estimated combined standard uncertainty of calculated p CO 2 and pH based on the Monte Carlo simulation is ~13 μatm and ~0.012 pH units respectively, with accuracy relative to laboratory-based measurement estimated to be between −3 and − 13 μatm and 0.002 and 0.007 pH units. Internal consistency especially at in situ temperature and pressure conditions is important for rapidly developing sensor-based monitoring programs in the region, including measurement of pH and/or p CO 2 from gliders, profiling floats and moorings. We highlight uncertainty associated with the large pressure effect on pH and p CO 2 , and recommend a study of carbonate system internal consistency under deep ocean conditions that addresses pressure effects on calculations. • Best calculations from combination of T,P-dependent and non-dependent parameters • The dissociation constants of M73 and L yielded the best internal consistency • Monte Carlo simulation shows combined uncertainty to be more dependent on input parameters, less on dissociation constants • Internal consistency study for deep ocean conditions is required [ABSTRACT FROM AUTHOR]

• Using δ13C DIC and water chemistry to refine the sources and multiple processes in the waters. • A new model to understand migration and transformation of DIC in a karst river-reservoir system. • HRT is a key factor for carbon transportation and transformation in the reservoir. In order to advance our understanding of the driving factors controlling carbon dynamics and evolution of water quality in river-reservoir systems for the generation of hydropower, we conducted a study in a karst deep-water reservoir (Wujiangdu Reservoir), southwest China. Water samples were collected from the inflow/outflow of the Wujiangdu Reservoir, four vertical columns along the reservoir, and from three tributaries to the reservoir in January, April, July, and October 2017. The dissolved inorganic carbon (DIC) concentrations and carbon isotope composition (δ13C DIC) varied greatly (1.99–3.45 mmol/L and −10.7‰ to −6.0‰, respectively) and were controlled by multiple processes including CO 2 outgassing, primary production, and organic matter degradation. In the four vertical profiles, the difference between the values of samples with those at 15 m of Δ[DIC] and Δ[δ13C DIC ], Δ[DIC] and ΔCa2+ in the same water column had positive correlations, and the variation in dissolved O 2 , partial pressure of CO 2 , and flux of CO 2 suggested that primary production dominated above the epilimnion and degradation of OM dominated below the epilimnion during the warm season. Continuous CO 2 outgassing was found from riverine water to surface water of the reservoir before the dam based on carbon isotopic compositions and water chemistry. These processes would cause isotopic fractionation between the residual DIC and CO 2 (aqueous), the degree of which varied among different seasons (July > April > October > January). Carbon transport and biogeochemical processes were highly controlled by the hydraulic retention time (HRT) in the river-reservoir system. It was estimated that approximately 71.5% of the annual DIC flux was produced in the water column below 15 m depth during the study year, suggesting that the processes of DIC generation and consumption occurred at the same time. The results highlight that carbon behavior in the impounded rivers is influenced by multiple processes. The carbon transformation processes should be taken into account for improving the estimation accuracy of the carbon budget calculations and the management of water quality for river-reservoir systems. [ABSTRACT FROM AUTHOR]

Liu, J.; Han, G.; Liu, X.; Yang, K.; Li, X., and Liu, M., 2019. Examining the distribution and variation of dissolved carbon species and seasonal carbon exports within the Jiulongjiang River Basin (Southeast China). Journal of Coastal Research, 35(4), 784–793. Coconut Creek (Florida), ISSN 0749-0208. The water chemistry, including major ions and dissolved carbon of Jiulongjiang River in SE China, were determined to examine the distributions and variations of dissolved carbon species. Dissolved inorganic carbon (DIC) concentrations in the river water were within the range of 7.50 to 49.04 mg/L (averaged 22.12 mg/L) in the wet season and from 8.83 to 84.91 mg/L (averaged 41.17 mg/L) in the dry season. Dissolved organic carbon (DOC) varied from 0.54 to 2.89 mg/L (averaged 1.04 mg/L) in the wet season and from 1.34 to 3.56 mg/L (averaged 2.34 mg/L) in the dry season. The concentrations of DIC were comparable among the three tributaries, whereas the DOC was significantly higher in the Xixi River and Nanxi River than the Beixi River. The calculations employing the mass balance method indicated that weathering process and anthropogenic inputs played an important role in the dissolved carbon production in the Jiulongjiang watershed. The DOC exhibited significant correlations with the rural area whereas the DIC did not show any significant correlations with land-use types, indicating that the DOC was closely related to agricultural activities. The carbon export fluxes from the Jiulongjiang River were calculated based on the discharge data. The DIC-C export reached to 1.2 × 105 t/y, whereas the DOC-C export was nearly 3.2 × 104 t/y. In addition, seasonal variation of carbon exports had been observed; because of the large water discharge in the wet season, more DIC and DOC were transported from the Jiulongjiang River in the wet season than in the dry season. The source analysis demonstrated natural processes account for the majority of carbon exports. [ABSTRACT FROM AUTHOR]

Understanding sequestration of carbon by coastal ecosystems is central to addressing the role they play in climate change mitigation. To quantify this process, accurate measurements of CO 2 fluctuation, coupled with variations in residence time of coastal water-bodies are required. Nearshore ecosystems, including coastal lagoons, may provide an effective sink for atmospheric carbon dioxide, particularly those containing productive biota such as seagrass. However, the rate and pattern of carbon sequestration in seagrass meadows across a range of environmental settings is still poorly constrained. In this study, we utilize a robust physical tidal model, along with biogeochemical dissolved inorganic carbon (DIC) assessment, to estimate water residence time and net sequestration of atmospheric CO 2 in an intertidal lagoon containing a seagrass (Zostera noltii) meadow. Total alkalinity and pH measurements taken from advected water mass exchanged with the open ocean at inlet boundaries are used to calculate DIC and p CO 2. A predictive model of hydrodynamics provides good approximation of mean water residence time to within 6 h (±3 s.d). Results indicate that during the daytime study period the lagoon is a sink for carbon, having a mean net ecosystem productivity (NEP) of 3.0 ± 0.4 mmol C m−2 hr−1. An equivalent diel NEP range of between 15.23 and −9.24 mmol C m−2 d−1 is calculated based on reported shallow water pelagic respiration rates. Moreover, approximately 4% of DIC availability occurs from atmospheric CO 2 transfer to lagoon water. However, a negative diel rate of −82 ± 81 mmol C m−2 d−1 is found, assuming overnight respiration ascertained from converted Zostera noltii O 2 utilization. We hypothesize that analogous regional nearshore ecosystems provide baseline study sites suitable to elucidate the carbon capture potential of planned, nearby tidal range energy schemes. • Tidal range energy schemes require additional benefits to boost feasibility. • Carbon offsetting could provide an additional income source. • Man-made nearshore lagoons containing seagrass may enhance carbon sequestration. • Results show existing regional lagoons may help elucidate this potential. • High-resolution, multi-disciplinary studies are required to constrain uncertainty. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on 226Ra and 228Ra, the total SGD flux into the KRE surface layer was estimated to be (12.8–16.2) × 105 m3/day. A conservative estimation of the fresh groundwater flux was (5.0–8.3) × 105 m3/day, which accounts for 10–17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD‐derived dissolved inorganic nitrogen (DIN) and silicates contributed 58–90% and 24–64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD‐derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD‐derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol · m2 · day) was much higher than those in most estuaries worldwide, suggesting that the DIC‐enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD‐derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise. Plain Language Summary: Coastal karst aquifers are very vulnerable and sensitive to climate and environmental changes. In this study, we evaluated the impacts of submarine groundwater discharge in a typical oligotrophic highly stratified karstic estuary. Using multiple parameters and mathematical models, we found that the net SGD‐derived nutrient flux could affect the productivity of this coastal ecosystem, notably contributing to forming the conditions that lead to the occurrence of phytoplankton blooms. In addition, nutrient‐enriched SGDs from the karst aquifers are likely to be important but easily ignored sources for the global carbon cycle. Taken together, our study revealed the impacts of submarine groundwater from the karst aquifers on the coastal seas, which shall become increasingly evident and substantial with further development of global climate change, such as the rise of sea level. Key Points: 226Ra and 228Ra were used to estimate SGD in an oligotrophic stratified karstic estuaryNet SGD‐derived nutrients have potential to impact the ecosystem in the Krka River Estuary by high DIN flux and DIN:DIP ratioSGD from DIC‐enriched coastal karst aquifers make a significant contribution to the carbon cycle of the coastal seas worldwide [ABSTRACT FROM AUTHOR]

Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (AT). The brine sinks, potentially exporting DIC and AT to deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 year pCO2 mooring time series where a ~35‐μatm increase in pCO2 was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resulting pCO2 is sensitive to the ratio of AT and DIC retained in the ice and the mixed layer depth, which controls dilution of the ice‐derived AT and DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing the pCO2 at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC and pCO2 in the surface of the Arctic Ocean as seasonal ice production and wind‐driven mixing increase with diminishing ice cover. Plain Language Summary: When sea ice forms, the dissolved solutes are squeezed out of the ice matrix, forming a dense brine that can sink into deeper water. This process has a potentially important role in the inorganic carbon cycle because dissolved inorganic carbon can sink with the brine, exporting it from the surface ocean and increasing the sea surface CO2 partial pressure (pCO2). We deployed a pCO2 sensor at 30‐m depth in the Canada Basin, the largest of the Arctic Ocean subbasins, that documented an increase in pCO2 during the fall to winter ice formation period. This increase correlated with an increase in salinity over the same time period. A simple model calculation supports that the increase in pCO2 and inorganic carbon is primarily due to ice formation. This finding has important implications for the future inorganic carbon cycle as seasonal ice formation increases due to loss of perennial ice cover in the Arctic. Key Points: We deployed a pCO2 sensor in the northwest Canada Basin of the Arctic Ocean; the time series shows a steady increase from fall to winterThe pCO2 increase is attributed to sea ice formation that excludes inorganic carbon; the inorganic carbon concentrated in the mixed layerIce exclusion of inorganic carbon could become an increasingly important part of the Arctic Ocean carbon cycle with diminishing ice cover [ABSTRACT FROM AUTHOR]

We report marine dissolved organic carbon (DOC) concentrations, and DOC ∆14C and δ13C values in seawater collected from the central Pacific. Surface ∆14C values are low in equatorial and polar regions where upwelling occurs and high in subtropical regions dominated by downwelling. A core feature of these data is that 14C aging of DOC (682 ± 86 14C years) and dissolved inorganic carbon (643 ± 40 14C years) in Antarctic Bottom Water between 54.0°S and 53.5°N are similar. These estimates of aging are minimum values due to mixing with deep waters. We also observe minimum ∆14C values (−550‰ to −570‰) between the depths of 2,000 and 3,500 m in the North Pacific, though the source of the low values cannot be determined at this time. Plain Language Summary: Most of the organic carbon in ocean water is in the dissolved form, like the broth in chicken soup. Even though it is believed that dissolved organic carbon (DOC) is formed during photosynthesis in the surface ocean using modern carbon, its radiocarbon age is surprisingly thousands of years old. We present the first transect of radiocarbon in DOC for the Pacific Ocean. We find that the radiocarbon age of DOC in the bottom waters decreases similarly to that found in the more abundant dissolved inorganic carbon. We conclude that DOC ages in the bottom water as it flows northward toward Alaska. Key Points: 14C aging of dissolved organic and inorganic carbon in the bottom waters of the Pacific are equivalentSurface DOC ∆14C values are low in equatorial and polar regions where upwelling occurs and high in the subtropics where downwelling dominatesMinimum DOC ∆14C values (‐550 to ‐570 per mil) are observed in Pacific Deep Water north of the equator [ABSTRACT FROM AUTHOR]

With rapid development of global wastewater treatment plants (WWTPs), acidification and enhanced CO2 release in receiving waters caused by high-CO2 treated wastewater input have raised concerns. Insights into the variations in dissolved inorganic carbon (DIC) species in treated wastewater contribute to understanding the mechanisms of the acidification process. Here, we investigated three large-scale municipal WWTPs that discharged into the coast of Qingdao, China, for variations in effluent DIC species and their control mechanisms. The results showed that the effluent DIC concentrations, with a range of 2554–5718 μmol/L, significantly exceeded the concentration in seawater and mainly increased from winter to spring and decreased from summer to autumn. The effluent DIC and its δ13CDIC showed a good negative correlation. The ratios of effluent DIC to total alkalinity (DIC/TAlk) ranged from 1.00 to 1.24, and the proportions of CO2 in DIC ranged from 0.9 to 19.7%; both sets of values significantly exceeded those in seawater. The proportions of CO32− in DIC were only ~ 0.4%. These features determined that the CO2 concentrations in effluents fluctuated from 3 to 80 times the concentration in seawater, whereas the CO32− concentrations were less than 1/15 of those in seawater. Organic matter degradation and nitrogen removal processes made important contributions to the high effluent CO2 concentrations. The increase in solubility induced by decreased temperature may be the main cause for the higher effluent CO2 concentrations during winter as well as spring months with low effluent temperatures. Correspondingly, the effluent pH values were significantly lower than the seawater pH values and showed a good negative logarithmic correlation with the DIC/TAlk values, reflecting the control of DIC species on the pH values in treated wastewater. Variations in DIC species in treated wastewater can cause changes in the affected region and the degree of the induced acidification in receiving waters. [ABSTRACT FROM AUTHOR]

Carbon (C) in riverine ecosystems play an important role in regulating terrestrial carbon reservoir to the oceans and atmosphere, thus, understanding C dynamics in riverine ecosystem are essential for understanding global carbon cycle. Here, we investigated the seasonal and spatial variations in concentrations of dissolved inorganic carbon (DIC), particulate organic carbon (POC), and their carbon isotopic signatures (i.e., δ13C-DIC, δ13C-POC) in the tributaries and main channel of the upper Han River, China. Dissolved organic carbon (DOC), total suspended solid (TSS), and other physicochemical parameters including water temperature, pH, dissolved oxygen (DO), and electrical conductivity (EC) were also measured. We found that POC concentration was significantly higher in wet season and in main channel, and DIC concentration did not show significant spatiotemporal variability. DIC and POC were positively related to DOC, and they were also positively correlated with water temperature and TSS, respectively. δ13C-DIC and δ13C-POC had significant seasonal and spatial variations, and δ13C-DIC was significantly higher in dry season and δ13C-POC was significantly higher in the tributaries and in dry season. δ13C-DIC was negatively related to the TSS concentration, while δ13C-POC was negatively correlated with water temperature, DIC, and DOC concentrations. Our results suggest that water temperature and TSS are the primary factors in regulating C concentrations, and their stable isotopic signatures are primarily affected by aquatic photosynthesis and riverine respiration in the upper Han River, China. [ABSTRACT FROM AUTHOR]

Abstract Dissolved inorganic carbon (DIC) is an important source of carbon in aquatic ecosystems, especially under conditions of increased frequency of cyanobacterial bloom. However, the importance of bacteria in direct or indirect utilization of DIC has been widely overlooked in eutrophic freshwater. To identify the functional bacteria that can actively utilize DIC in eutrophic freshwater during cyanobacterial bloom, stable-isotope probing (SIP) experiments were conducted on eutrophic river water with or without inoculation with cyanobacteria (Microcystis aeruginosa). Our 16S rRNA sequencing results revealed the significance of Betaproteobacteria, with similar relative abundance as Alphaproteobacteria, in the active assimilation of H13CO 3 − into their DNA directly or indirectly, which include autotrophic genera Betaproteobacterial ammonia-oxidizing bacteria. Other bacterial groups containing autotrophic members, e.g. Planctomycetes and Nitrospira , also presented higher abundance among free-living bacteria in water without cyanobacteria. Microcystis aggregates showed a preference for some specific bacterial members that may utilize H13CO 3 − metabolized by Microcystis as organic matter, e.g. Bacteroidetes (Cytophagales, Sphingobacteriales), and microcystin-degrading bacteria Betaproteobacteria (Paucibacter /Burkholderiaceae). This study provides some valuable information regarding the functional bacteria that can actively utilize DIC in eutrophic freshwater. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Metabolic processes of the submerged aquatic community (photosynthesis and respiration) play important roles in regulating diel cycles of dissolved inorganic carbon (DIC) and sequestering carbon in a karst stream. However, little is known of whether diel DIC cycling occurs during rainfall in a karst groundwater-fed stream, even though this question is critical for the accurate estimation of what may be a major terrestrial carbon sink. Here, we measured diel variations of water chemical composition in a small karst groundwater-fed stream in southwest China during a rainfall event to assess the influences of rainfall and rising discharge on DIC diel cycling and the potential carbon sink produced by in-stream metabolism. Our results show that water chemical composition at the source spring (CK site) is relatively stable due to chemostatic behavior during rising discharge after a rainfall period. This site lacked submerged aquatic vegetation and, thus, had no diel variations in water chemistry. However, diel cycles of all hydrochemical parameters occurred at a site 1.3 km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+, and pCO2. These results indicated that diel cycling of DIC due to in-stream metabolism of submerged aquatic community was still occurring during elevated discharge from rainfall. We estimate the carbon sink through the in-stream metabolism of the submerged aquatic community to be 5.6 kg C/day during the studied rainfall event. These results imply that submerged aquatic communities in a karst stream can significantly stabilize carbon originating from the carbonate rock weathering processes in karst areas. [ABSTRACT FROM AUTHOR]

Radiocarbon (14C) generated by the thermonuclear tests in the late 1950s to early 1960s has been used as a tracer to study atmospheric and oceanic circulations, carbon exchange between different reservoirs, and fossil fuel emissions. Here we report the first measurements of 14C in atmospheric CO2 of maritime air collected over the South China Sea (SCS) during July 2014. We also present 14C of the dissolved inorganic carbon (DIC) in the sea surface water in the same region. Most of the Δ14C values of the atmospheric CO2 vary in the range of 15.6±1.6‰– 22.0±1.6‰, indicating that the central SCS maritime air is well-mixed and consistent with the clean background air in the Northern Hemisphere. The 14C values of the DIC (DI14C) in the surface seawater vary between 28.3±2.5‰ and 40.6±2.7‰, mainly due to the lateral mixing between the SCS and western Pacific. The average surface seawater DI14C is 15.4 ± 5.1‰ higher than that of the maritime air 14CO2. The reversal of the sea–air Δ14C gradient occurred at ∼2000, marking the start of the upper ocean transferring bomb 14C back to the atmosphere in the SCS. [ABSTRACT FROM AUTHOR]

Abstract The high-relief catchment of the Tavignanu River (Corsica Island, France) with an elevation range from sea level to 2622 m above sea level was investigated for its riverine carbon budget and stable carbon isotopes. Major riverine dissolved inorganic carbon (DIC or TCO 2) sources depended on seasons and sub-catchment lithology. In winter δ 13C DIC values of −2 to −7‰ (VPDB) indicated influences of atmospheric CO 2. δ 13C DIC values decreased gradually to values between −9 and −12‰ in July, which indicates elevated soil CO 2 contribution. An observed downstream increase in the total amount of carbon species correlated with inputs from carbonate bearing tributaries and evaporation effects in summer. Main channel partial pressure of CO 2 (p CO 2) was seasonally highly variable in the upper silicate catchment and the lower coastal plain, where summer values exceed up to six times atmospheric levels. During winter, the central section of the Tavignanu River was found to be undersaturated with respect to atmospheric CO 2 levels. The median values for main channel p CO 2 were below atmospheric levels in winter and spring and above in summer and autumn. The annual carbon flux across the air-water boundary (F CO 2) along the Tavignanu River was calculated with (0.77 ± 0.24) Gg C yr−1, which is about seven times higher than the riverine TCO 2 transport to the sea of about 0.11 Gg C yr−1. While large sections of the river experienced year-round atmospheric CO 2 uptake or equilibrium, the river as a whole was a small but continuous net source of carbon to the atmosphere. This underlines the important, but so far not well-constrained, contributions of smaller streams and rivers to the terrestrial carbon flux and the need of incorporating them into future global carbon cycle models. Graphical abstract Unlabelled Image Highlights • Carbon isotope values of river water indicate atmospheric CO 2 influence in winter. • In summer, soil CO 2 was the main carbon contributor to the stream. • Half of the main stream length is undersaturated with respect to CO 2 year-round. • Annual main stream air-water carbon flux is 7 times higher than riverine DIC export. [ABSTRACT FROM AUTHOR]

Abstract Mangroves sequester large amounts of carbon in soils but limited information is available on carbon losses from tropical mangrove systems. Here we quantify carbon outwelling, CO 2 emissions and pore-water exchange rates from two nearby (∼2 km apart) tropical mangrove creeks located in different geomorphic settings on the island of Palau, Micronesia during the monsoon storm season. On average, POC and p CO 2 were >100% higher and DOC, DIC and TA were 62%, 25%, 16% higher, respectively, from Creek 1 (located within a semi-enclosed bay) than from Creek 2 (located along the coast adjacent to fringing reefs). Both creeks were net exporters of DIC, DOC, POC, TA and emitters of CO 2. However, outwelling rates of POC, DIC and DOC and CO 2 emissions were 27-fold, 8-fold, 4-fold and 3-fold higher at Creek 1. DIC outwelling (37%) and CO 2 emissions (39%) were the major terms contributing to total carbon losses at Creek 1, whilst CO 2 emission (61%) was the major contributor at Creek 2. Monsoon storms appeared to explain the organic carbon dynamics whilst tidal pumping appears to drive the inorganic carbon dynamics at both creeks. Our data demonstrates the considerable heterogeneity of mangroves creeks that are in close proximity and subject to similar weather conditions but in differing geomorphological settings. [ABSTRACT FROM AUTHOR]

Abstract Paleoenvironmental reconstructions based on the clumped isotopes (Δ 47) and traditional isotope (δ13C or δ18O) techniques are often problematic for carbonates that have undergone diagenetic alteration. One of the most common types of diagenesis is the transition between polymorphs, such as the replacement of aragonite by calcite. The isotope fractionation during such transitions in aqueous solutions remains unclear. We conducted a series of aragonite-to-calcite transition experiments in aqueous solutions of varying salinity and experiment durations at 25 °C and 90 °C to examine the variations of δ13C, δ18O, and Δ 47 values for carbonates at varying degrees of the transition. The results confirm the retarding effect of Mg2+ and the catalytic effect of Na+ and Ca2+ on the transition, which are consistent with previous findings. Compared with the results from the transitions at 25 °C, the 90 °C experiments show a greater transition to calcite and a more depleted oxygen isotope composition. However, both clumped and carbon isotope values show no significant variation in the various experiments, suggesting that they are unaffected by mineralogical transition. Based on published equilibrium calibrations for δ18O and Δ 47 , the results demonstrate that kinetic effects in isotope systems are controlled primarily by the rate of polymorph transition, but there is a significant kinetic difference between the clumped isotope bond (13C 18O) reordering and the 18O exchange within the dissolved inorganic carbon pool. This kinetic difference results in partial equilibration in δ18O with no significant reordering in Δ 47 toward their equilibriums, which can be accounted for by the difference of equilibration rates between the oxygen isotopes bound to 12C and those bound to 13C. This study provides a clear observation of the response of isotope systems to carbonate polymorph transitions and sheds light on their reliability as paleoenvironmental proxies. [ABSTRACT FROM AUTHOR]

Abstract Seafloor hydrothermal systems and cold seeps contribute considerable amounts of dissolved carbon to ocean water. To investigate the compositional characteristics and potential sources of the dissolved carbon in the Okinawa Trough (OT), East China Sea (ECS), we measured the concentrations and distributions of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and their 13C stable isotopes in a full-depth water column in this area. Our results indicate that in the upper layer (water depth < 200 m), the dissolved carbon is mainly controlled by biological processes. In contrast, in the deeper layer, the concentration and isotopes are clearly influenced by seafloor hydrothermal and cold seep fluids in the OT. Based on our mass balance model estimation, cold seeps and hydrothermal fluids contribute up to 14.3% and 19.6%, respectively, of the seawater DIC. Our study shows that both of these extreme seafloor systems contribute considerable proportions of the local carbon budget. Moreover, they may also have an important impact on the extreme seafloor ecosystems therein. [ABSTRACT FROM AUTHOR]

In order to determine the influence of damming on the spatial distribution of dissolved inorganic carbon and dissolved inorganic nitrogen in river，we stratified sampling in the karst area deeply water reservoir (Longtan Reservoir) in September 2016 (rainfall season) to access to the main physical and chemical parameters of water. We studied the spatial distribution pattern of D/C，DIN and C/N in water by classical statistics and geo statistics methods. The results show that the temperature shows obvious stratification and results stratification of total dissolved solids，pH，Alkalinity，D/C，D/N and NH4+- N at Longtan Reservoir in summer. Apart from water temperature and pH，the main physical and chemical parameters have large variation in space and decrease from the surface to the bottom，in the order of DIC>C/N> D/N. The horizontal variation of D/C，D/N and C/N in the reservoir area has obvious spatial structure with the dam body is larger than the tail. The spatial heterogeneity produced by the experimental random error is small and the three have strongly spatial auto correlation. However， the natural structural factors play a significantly role in spatial heterogeneity caused on D/C，D/N and C/N. [ABSTRACT FROM AUTHOR]

Abstract Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H 2 O-CO 2 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H 2 O isotope exchange, CO 2 degassing and CO 2 absorption. We show that under thermodynamic equilibrium dissolved HCO 3 – and CO 3 2– both exhibit similar Δ ′ 17 O as calcite and aragonite (within 10 per meg at 25 °C), with temperature dependences around 0.61 per meg/°C. However, kinetic isotope fractionations associated with CO 2 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, e.g., temperature, pH and initial composition. Particularly, we predict correlated enrichments in Δ ′ 17 O and δ 18 O of DIC during CO 2 degassing but depletions during CO 2 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO 2 , yielding values of 8.8 and 12.0 per meg/‰ at pH = 8 and 9, respectively, at 25 °C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (e.g., speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates. [ABSTRACT FROM AUTHOR]

Past research in seasonally ice‐covered Arctic seas has suggested that ice algae play a role in reducing dissolved inorganic carbon (DIC) during spring, preconditioning surface waters to low dissolved CO2 (pCO2sw), and uptake of atmospheric CO2 during the ice‐free season. The potential role of under‐ice phytoplankton blooms on DIC and pCO2sw has not often been considered. In this study we examined the inorganic carbon system beneath landfast sea ice starting midway through a bottom ice algae bloom and concluding in the early stages of an under‐ice phytoplankton bloom. During most of the ice algae bloom we observed a slight increase in DIC/pCO2sw in surface waters, as opposed to the expected reduction. Biomass calculations confirm that the role of ice algae on DIC/pCO2sw in the study region were minor and that this null result may be widely applicable. During snow melt, we observed an under‐ice phytoplankton bloom (to 10 mg/m3 Chl a) that did reduce DIC and pCO2sw. We conclude that under‐ice phytoplankton blooms are an important biological mechanism that may predispose some Arctic seas to act as a CO2 sink at the time of ice breakup. We also found that pCO2sw was undersaturated at the study location even at the beginning of our sampling period, consistent with several other studies that have measured under‐ice pCO2sw in late winter or early spring. Finally, we present the first measurements of carbonate saturation states for this region, which may be useful for assessing the vulnerability of a local soft‐shelled clam fishery to ocean acidification. Key Points: An ice algae bloom beneath landfast sea ice did not impact DIC/pCO2, and is not expected to contribute to CO2 uptake from the atmosphereUnder ice phytoplankton blooms did significantly reduce DIC/pCO2, likely making the region a stronger sink for atmospheric CO2 at break‐upArctic seas do not often experience supersaturation of pCO2 during the Arctic winter [ABSTRACT FROM AUTHOR]

The origin and the chemical and isotopic evolution of dissolved inorganic carbon (DIC) in groundwater of the Okavango Delta in semi-arid Botswana were investigated using DIC and major ion concentrations and stable oxygen, hydrogen and carbon isotopes (δD, δ18O and δ13CDIC). The δD and δ18O indicated that groundwater was recharged by evaporated river water and unevaporated rain. The river water and shallow (<10 m) groundwater are Ca-Na-HCO3 type and the deep (≥10 m) groundwater is Na-K-HCO3 to HCO3-Cl-SO4 to Cl-SO4-HCO3. Compared to river water, the mean DIC concentrations were 2 times higher in shallow groundwater, 7 times higher in deep groundwater and 24 times higher in island groundwater. The δ13CDIC indicate that DIC production in groundwater is from organic matter oxidation and in island groundwater from organic matter oxidation and dissolution of sodium carbonate salts. The ionic and isotopic evolution of the groundwater relative to evaporated river water indicates two independent pools of DIC. [ABSTRACT FROM AUTHOR]

Highlights • A geochemical method to calculate carbonate weathering from acid rain. • SO 4 , NO 3 , Ca, NH 4 and H are the main anions and cations (>15%) in acid rainfall. • The average proportion of carbonate weathering by acid rain is 36%. • The proportion represents an upper limit for carbonate weathering by acid rain. Abstract Carbonate weathering by CO 2 is considered to play an important role in the global carbon cycle. In acid rain-affected areas, the carbon cycle can be impacted by acid rain weathering. When previous studies have considered all possible weathering processes including H 2 SO 4 in rainfall, sulfide oxidation and agricultural activities, this study proposed a geochemical method to specifically calculate the extent of carbonate weathering by acid rain using hydrogeochemical data from karstic spring and rainfall. This method has considered the H concentration and nitrification of NH 4 in rain during recharge, and provided the upper limit of the weathering amount. Seasonal sampling for spring (wet and dry season) was conducted from a typical carbonate area in Jiaoshi, Chongqing, SW China, with annual rain pH values ranging from 3.94 to 4.70. The major hydrogeochemistry types of spring are HCO 3 –Ca and HCO 3 –Ca·Mg. The average proportion of carbonate weathering by acid rain is estimated to be 36 ± 15%. The quantitative geochemical method provided in this study is important for understanding the hydrogeochemical processes in acid rain areas and for calculating CO 2 sources & sinks of carbonate weathering. [ABSTRACT FROM AUTHOR]

Abstract Rivers and lakes have been traditionally studied as separate entities for carbon transport. However, there is a gap in our knowledge of the connectivity of dissolved carbon in a river-lake continuum. In this study, we analyzed dissolved carbon along the Little River-Catahoula Lake in subtropical Louisiana, United States to assess carbon biogeochemistry in such a river-lake continuum. Monthly in-situ measurements and water sample collections were made at four locations during April 2015 to February 2016 to determine riverine carbon transport into and out of the lake. Results show that much of the dissolved inorganic carbon (DIC) in the river-lake continuum originated from 13C depleted sources with an average δ13C DIC of −18.5‰. Significant decreases in DIC were found after the river passed through the lake (from 482 to 399 μmol L−1), which was most prevalent when the lake was not affected by backwater flow from the downstream river. CO 2 outgassing could be mainly responsible for the sink behavior of the lake for DIC. Dissolved organic carbon (DOC) in the studied watershed were mostly terrigenous with low δ13C DOC averaged at −29.2‰. Significant, consistent decreases in DOC concentrations were found from the river to the lake inflow and then to the lake outflow. During the majority of the year, the lake reduced DOC concentrations from the river inflow water, but switched to functioning as a source of DOC during warmer, dryer conditions in September and October due to increased water residence time. Therefore, the lake functioned both as a sink and as a source for DOC. Graphical abstract Unlabelled Image Highlights • In 9 out of 11 months the lake was a sink for DOC sustained by terrigenous sources. • The lake changed to a DOC source during months with extended water residence time. • DIC in the river-lake continuum originated mainly from 13C depleted sources. • CO 2 outgassing made the lake a sink for the down-network DIC transport. • Backwater containing high-level DIC could offset the sink function of the lake. [ABSTRACT FROM AUTHOR]

Abstract Pairs of marine carbonate system variables are often used to calculate others, but those results are seldom reported with estimates of uncertainties. Although the procedure to propagate these uncertainties is well known, it has not been offered in public packages that compute marine carbonate chemistry, fundamental tools that are relied on by the community. To remedy this shortcoming, four of these packages were expanded to calculate sensitivities of computed variables with respect to each input variable and to use those sensitivities along with user-specified estimates of input uncertainties (standard uncertainties) to propagate uncertainties of calculated variables (combined standard uncertainties). Sensitivities from these packages agree with one another and with analytical solutions to within 0.01%; similar agreement among packages was found for the combined standard uncertainties. One package was used to quantify how propagated uncertainties vary among computed variables, seawater conditions, and the chosen pair of carbonate system variables that is used as input. The relative contributions to propagated uncertainties from the standard uncertainties of the input pair of measurements and various other input data (equilibrium constants etc) were explored with a new type of diagram. These error-space diagrams illustrate that further improvement beyond today's state-of-the-art measurement uncertainties for the input pair would generally be ineffective at reducing the combined standard uncertainties because the contribution from the constants is larger. Likewise, using much more uncertain measurements of the input pair does not always substantially worsen combined standard uncertainty. The constants that contribute most to combined standard uncertainties are generally K 1 and K 2 , as expected. Yet more of the propagated uncertainty in the computed saturation states of aragonite and calcite comes from their solubility products. Thus percent relative combined standard uncertainties for the saturation states are larger than for the carbonate ion concentration. Routine propagation of these uncertainties should become standard practice. Highlights • Uncertainty propagation added to four public software packages that make CO 2 system calculations • New type of diagram helps to assess how propagated uncertainty changes with different input uncertainties • Uncertainties from the constants often dominate propagated uncertainty, so that measurement uncertainty plays little role • Relative uncertainties are larger for CaCO 3 saturation states than for CO 3 2- because of uncertainties in solubility products [ABSTRACT FROM AUTHOR]

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106-0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1, and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.” [ABSTRACT FROM AUTHOR]