Riverine Dissolved Inorganic Carbon Research Articles

Spatiotemporal variability of dissolved carbon and sources of dissolved inorganic carbon influenced by freeze–thaw and subsurface flow in an alpine headwater catchment of the Qinghai-Tibetan Plateau

The temporal freeze–thaw (FT) and the spatial rock distribution have great impacts on riverine dissolved carbon transport and dissolved inorganic carbon (DIC) sources. However, the effects of the subsurface flow (SSF) process during the FT process and spatial riverine transport on dissolved carbon transport and DIC sources are still unclear due to the lack of available spatiotemporal measurements. Herein, we designed and implemented a complete scheme of water sampling including stream water (from upper headwater to the middle and downward) and SSF (in the soil, saprolite, and weathered bedrock layers) in an alpine headwater catchment. The characteristics of the dissolved organic and inorganic carbon (DOC and DIC), and δ13C-DIC were analyzed, and the spatiotemporal variability of the DIC source was explored. Our findings reveal that the impact of the FT process on the dissolved carbon dynamics and DIC sources is revealed in the dissolved carbon transport process controlled by the SSF process. Specifically, the deepening of the active layer during the FT process enhances the movement of SSF into the deeper layer, thereby activating DIC in this deep layer. Concurrently, DOC from the shallow layer rapidly infiltrates deeper layers, leading to minimal differences in DOC concentrations across layers. This results in an enriched behavior of DOC (b = 0.21) and a depleted behavior of DIC (b = −0.27) during the FT process. In terms of influence on DIC sources, the deepening active layer promotes silicate weathering in the silicate-rich deep layers and strengthens organic carbon mineralization. On a spatial scale, the rock distribution, particularly the Si/Ca ratio, predominantly controls the chemical weathering process and, consequently, determines the DIC sources. In contrast, the influence of organic carbon mineralization on DIC sources becomes less significant. In this study, the impact mode of SSF on dissolved carbon transport and transformation is elucidated, and the spatial influence of rock distribution on riverine DIC sources is emphasized.

Contribution of carbonate-derived dissolved inorganic carbon into autochthonous particulate organic carbon in two small temperate Korean rivers (Geum and Seomjin)

In this study, we estimated the contributions of carbonate mineral weathering to dissolved inorganic carbon (DIC) and carbonate-derived DIC to autochthonous particulate organic carbon (POC) in two temperate Korean rivers. We combined stoichiometric and stable carbon isotopic approaches to calculate the contribution of autochthonous POC, considering diverse riverine DIC sources. We collected surface water samples from May 2016 to May 2018 and analyzed the major ion composition of rivers along with the concentrations and stable carbon isotopes of DIC. Our estimates showed that the relative abundances of carbonate mineral weathering (0.41 ± 0.11 in the Geum River and 0.43 ± 0.07 in the Seomjin River) were only slightly lower than those of silicate mineral weathering (0.59 ± 0.1 in the Geum River and 0.57 ± 0.07 in the Seomjin River). The resulting percentage contributions of DIC derived from the carbonate mineral weathering to riverine autochthonous POC, if we consider the additional DIC sources of atmospheric and soil-derived CO2, were 10 ± 3 % in the Geum River and 2 ± 1 % in the Seomjin River. The calculated annual fluxes of carbonate-derived DIC for 2016–2018 were 23.2 ± 0.3 Gg C yr−1 in the Geum River and 1.1 ± 0.4 Gg C yr−1 in the Seomjin River. Moreover, the calculated annual fluxes of carbonate-derived POC were 3.6 ± 0.5 Gg C yr−1 in the Geum River and 0.1 ± 0.7 Gg C yr−1 in the Seomjin River. Accordingly, our study provides the first insight into the contribution of carbonate-derived DIC to riverine autochthonous POC in small temperate Korean river systems, dominated by silicate rocks.

Porewater dissolved inorganic carbon released due to artificial sediment scouring in the Yellow River

The water-sediment regulation scheme (WSRS) in the Yellow River has been adjusted annually to maintain the optimal capacity of the Xiaolangdi Reservoir via the scouring of deposits downstream before the rainy season. The high-intensity sediment scouring in a short time has given rise to elevated dissolved inorganic carbon (DIC) exportation; however, the source of the DIC is still unclear. Here, we hypothesized that sediment scouring released porewater DIC into the river water, increasing the riverine DIC concentration and enhancing CO2 outgassing during the adjustment of the WSRS. To test this hypothesis, we used the isotopic composition of the DIC (δ13CDIC) to constrain the sources and transformations of the DIC. River water samples were collected below the Xiaolangdi Reservoir dam before, during, and after the adjustment of the WSRS. In these three stages, the pH of the river water decreased from 8.2 to 8.0 and then increased to 8.4; and the DIC concentrations increased from 185.30 to 216.37 mg L−1, and then decreased to 199.93 mg L−1. The δ13CDIC values decreased from −7.01 to −11.53‰ and then increased to −6.82‰; and the partial pressure of the CO2 increased from 5.21 × 10−3 to 35.30 × 10−3 atm, and then decreased to 3.40 × 10−3 atm. The increase in the DIC concentrations with negative δ13CDIC values during the adjustment of the WSRS resulted from the release of porewater DIC due to the sediment scouring, which was derived from the anaerobic oxidation of organic matter in the permeable sediment environment. More broadly, the sedimentary organic carbon-derived DIC released into the ocean would counteract CO2 drawdown via the burial of organic carbon in the geological timescales.

Long‐term changes in dissolved inorganic carbon across boreal streams caused by altered hydrology

AbstractA major challenge for predicting future landscape carbon (C) balances is to understand how environmental changes affect the transfer of C from soils to surface waters. Here, we evaluated 14 yr (2006–2019) of stream dissolved inorganic carbon (DIC) concentration and export data for 14 nested boreal catchments that are subject to climatic changes, and compared long‐term patterns in DIC with patterns in dissolved organic carbon (DOC). Few streams displayed significant concentration or export trends in DIC at annual time scales. However, most streams showed decreasing DIC concentrations during spring flood over this 14‐yr period, and about half showed declines during summer. Although annual runoff has generally not changed during this period, an intra‐annual redistribution in runoff, with increases in spring flood discharge, explained much of the seasonal changes in DIC concentration. We observed negative DIC–discharge relationships in most streams, suggesting source limitation of DIC, whereas DOC mostly showed chemostatic behavior. The different trends and patterns observed for DIC vs. DOC underpin intra‐annual changes in the composition of the total C pool (i.e., the DIC/DOC ratio) and reflect fundamental differences in how these C forms are produced, stored in riparian soils, and mobilized by hydrological events. Collectively, our results highlight the sensitivity of riverine DIC to the intra‐annual distribution of runoff, but also important heterogeneity across the network that suggests local processes can also modify the mobilization of DIC in boreal landscapes.

Anthropogenic land use and urbanization alter the dynamics and increase the export of dissolved carbon in an urbanized river system

Greenhouse gas emissions from urban rivers play a crucial role in global carbon (C) cycling, this is tightly linked to dissolved C in rivers but research gaps remain. The effects of urbanization and anthropogenic land-use change on riverine dissolved carbon dynamics were investigated in a temperate river, the River Kelvin in UK. The river was constantly a source of methane (CH4) and carbon dioxide (CO2) to the atmosphere (excess concentration of CH4 ranged from 13 to 4441 nM, and excess concentration of CO2 ranged from 2.6 to 230.6 μM), and dissolved C concentrations show significant spatiotemporal variations (p < 0.05), reflecting a variety of proximal sources and controls. For example, the concentration variation of dissolved CH4 and dissolved CO2 were heavily controlled by the proximity of coal mine infrastructure in the tributary near the river head (~ 2 km) but were more likely controlled by adjacent landfills in the midstream section of the rivers main channel. Concentration and isotopic evidence revealed an important anthropogenic control on the riverine export of CO2 and dissolved organic carbon (DOC). However, dissolved inorganic carbon (DIC) input via groundwater at the catchment scale primarily controlled the dynamics of riverine DIC. Furthermore, the positive relationship between the isotopic composition of DIC and CO2 (r = 0.79, p < 0.01) indicates the DIC pool was at times also significantly influenced by soil respiratory CO2. Both DIC and DOC showed a weak but significant correlation with the proportion of urban/suburban land use, suggesting increased dissolved C export resulting from urbanization. This research elucidates a series of potentially key effects anthropogenic activities and land-use practices can have on riverine C dynamics and highlights the need for future consideration of the direct effects urbanization has on riverine C dynamics.

Dissolved inorganic carbon isotopes of a typical alpine river on the Tibetan Plateau revealing carbon sources, wetland effect and river recharge

AbstractThe Nyangqu River, the largest right bank tributary of the Yarlung Zangbo River in the Qinghai–Tibet Plateau, was representative of an alpine riverine carbon cycle experiencing climate change. In this study, dissolved inorganic carbon (DIC) spatial and seasonal variations, as well as their carbon isotopic compositions (δ13CDIC) in river water and groundwater were systematically investigated to provide constraints on DIC sources, recharge and cycling. Significant changes in the δ13CDIC values (from −2.9‰ to −23.4‰) of the water samples were considered to be the result of different contributions of two dominant DIC origins: soil CO2 dissolution and carbonate weathering. Three types of rock weathering (dissolution of carbonate minerals by H2CO3 and H2SO4, and silicate dissolution by H2CO3) were found to control the DIC input into the riverine system. In DIC cycling, groundwater played a significant role in delivering DIC to the surface water, and DIC supply from tributaries to the main stream increased from the dry season to the wet season. Notably, the depleted δ13CDIC ‘peak’ around the 88.9° longitude, especially in the September groundwater samples, indicated the presence of ‘special’ DIC, which was attributed to the oxidation of methane from the Jiangsa wetland located nearby. This wetland could provide large amounts of soil organic matter available for bacterial degradation, producing 13C‐depleted methane. Our study provided insights regarding the role of wetlands in riverine carbon cycles and highlighted the contribution of groundwater to alpine riverine DIC cycles.

Carbon dioxide, methane, and dissolved carbon dynamics in an urbanized river system

AbstractEstimates of greenhouse gas evasion from rivers have been refined over the past decades to constrain their role in global carbon cycle processes. However, despite 55% of the human population living in urban areas, urban rivers have had limited attention. We monitored carbon dynamics in an urbanized river (River Kelvin, 331 km2, UK) to explore the drivers of dissolved carbon lateral and vertical export. Over a 2‐year sampling period, riverine methane (CH4) and carbon dioxide (CO2) concentrations were consistently oversaturated with respect to atmospheric equilibria, leading to continual degassing to the atmosphere. Carbon stable isotopic compositions (δ13C) indicated that terrestrially derived carbon comprised most of the riverine CH4 and dissolved CO2 (CO2*) load while dissolved inorganic carbon (DIC) from groundwater was the main form of riverine DIC. The dynamics of CH4, CO2*, and DIC in the river were primarily hydrology‐controlled, that is, [CH4] and [CO2*] both increased with elevated discharge, total [DIC] decreased with elevated discharge while the proportion of biologically derived DIC increased with increasing discharge. The concentration of dissolved organic carbon (DOC) showed a weak relationship with river hydrology in summer and autumn and was likely influenced by the combined sewer overflows. Carbon emission to the atmosphere is estimated to be 3.10 ± 0.61 kg C·m−2·yr−1 normalized to water surface area, with more than 99% emitted as CO2. Annual carbon loss to the coastal estuary is approximately 4.69 ± 0.70 Gg C yr−1, with annual DIC export approximately double that of DOC. Per unit area, the River Kelvin was a smaller carbon source to the atmosphere than natural rivers/streams but shows elevated fluxes of DIC and DOC under comparable conditions. This research illustrates the role urban systems may have on riverine carbon dynamics and demonstrates the potential tight link between urbanization and riverine carbon export.

Source and fate of dissolved inorganic carbon in Jiulong River, southeastern China

In this study, surface water samples were collected over several seasons from the Jiulong River mainstream (North and West Rivers) and estuary from 2013 to 2014. The concentration of dissolved inorganic carbon (DIC), stable carbon isotope composition (δ13CDIC), in-situ temperature, pH, and conductivity of the surface water were measured to study the source and fate of DIC in the Jiulong River. The results showed that the DIC concentrations decreased from upstream to downstream in the North River but increased in the West River, which was determined mainly by the lithology of the drainage basin. In spring, the river is fed by deep groundwater resources that are older and contain more DIC originating from carbonate weathering compared to surface water, resulting in the high concentration and δ13CDIC value of DIC. Whereas during the summer and autumn, the high runoff caused by frequent precipitation dilutes the DIC concentration of river water and the contribution of soil CO2 is more important, resulting in lower DIC concentration and δ13CDIC value. δ13CDIC values indicate that the DIC in the Jiulong River was derived mainly from the chemical weathering of bedrock, and CO2 outgassing was one of the main fates of the riverine DIC. Results of mixing ratio calculation in freshwater and seawater suggest that additional processes existed with the mixing process, which were identified as organic carbon degradation and respiration in summer and autumn and offshore water vertical mixing in winter and spring, but in general the spatial variation of DIC in the estuary was determined mainly by mixing of freshwater and seawater. This study complements our understanding of the sources and fates of DIC in small river systems on the continental margin of the East China Sea.

Dissolved Inorganic Carbon Pump in Methane-Charged Shallow Marine Sediments: State of the Art and New Model Perspectives

Methane transport from subsurface reservoirs to shallow marine sediment is characterized by unique biogeochemical interactions significant for ocean chemistry. Sulfate-Methane Transition Zone (SMTZ) is an important diagenetic front in the sediment column that quantitatively consumes the diffusive methane fluxes from deep methanogenic sources toward shallow marine sediments via sulfate-driven anaerobic oxidation of methane (AOM). Recent global compilation from diffusion-controlled marine settings suggests methane from below and sulfate from above fluxing into the SMTZ at an estimated rate of 3.8 Tmol yr-1 and 5.3 Tmol yr-1 respectively, and wider estimate for methane flux ranges from 1—19 Tmol yr-1. AOM converts the methane carbon to dissolved inorganic carbon (DIC) at the SMTZ. Organoclastic sulfate reduction (OSR) and deep-DIC fluxes from methanogenic zones contribute additional DIC to the shallow sediments. Here, we provide a quantification of 8.7 Tmol yr-1 DIC entering the methane-charged shallow sediments due to AOM, OSR, and the deep-DIC flux (range 6.4–10.2 Tmol yr-1). Of this total DIC pool, an estimated 6.5 Tmol yr-1 flows toward the water column (range: 3.2–9.2 Tmol yr-1), and 1.7 Tmol yr-1 enters the authigenic carbonate phases (range: 0.6–3.6 Tmol yr-1). This summary highlights that carbonate authigenesis in settings dominated by diffusive methane fluxes is a significant component of marine carbon burial, comparable to ~15% of carbonate accumulation on continental shelves and in the abyssal ocean, respectively. Further, the DIC outflux through the SMTZ is comparable to ~20% of global riverine DIC flux to oceans. This DIC outflux will contribute alkalinity or CO2 in different proportions to the water column, depending on the rates of authigenic carbonate precipitation and sulfide oxidation and will significantly impact ocean chemistry and potentially atmospheric CO2. Settings with substantial carbonate precipitation and sulfide oxidation at present are contributing CO2 and thus to ocean acidification. Our synthesis emphasizes the importance of SMTZ as not only a methane sink but also an important diagenetic front for global DIC cycling. We further underscore the need to incorporate a DIC pump in methane-charged shallow marine sediments to models for coastal and geologic carbon cycling.

Seasonal Shift From Biogenic to Geogenic Fluvial Carbon Caused by Changing Water Sources in the Wet‐Dry Tropics

AbstractThe riverine export of carbon is expected to be driven by changes in connectivity between source areas and streams. Yet we lack a thorough understanding of the relative contributions of different water sources to the dissolved carbon flux, and of the way these contributions vary with seasonal changes in flow connectivity. Here we assess the temporal variations in water and associated dissolved inorganic carbon (DIC) sources and fluxes in a wet‐dry tropical river of northern Australia over two years. We use linear mixing models integrated into a Bayesian framework to determine the relative contributions of rainfall, seasonal wetlands, shallow groundwater, and a deep carbonate aquifer to riverine DIC fluxes, which we relate to the age of water sources. Our results suggest extreme shifts in water and DIC sources between the wet and dry seasons. Under wet conditions, most DIC was of biogenic origin and transported by relatively young water sources originating from shallow groundwater and wetlands. As rainfall ceased, the wetlands either dried out or became disconnected from the stream network. From this stage, DIC switched to a geogenic origin, nearly entirely conveyed via older water sources from the carbonate formation. Our findings demonstrate the importance of changing patterns of connectivity when evaluating riverine DIC export from catchments. This work also illustrates the need to systematically partition DIC fluxes between biogenic and geogenic sources, if we are to quantify how the riverine export of carbon affects net carbon soil storage.

Spatiotemporal Variability and Sources of DIC in Permafrost Catchments of the Yangtze River Source Region: Insights From Stable Carbon Isotope and Water Chemistry

AbstractRiverine dissolved inorganic carbon (DIC) exports play a central role in the regional and global carbon cycles. Here, we investigated the spatiotemporal variability and sources of DIC in eight catchments in the Yangtze River source region (YRSR) with variable permafrost coverage and seasonally thawed active layers. The YRSR catchments are DIC‐rich (averagely 25 mg C L−1) and export 3.51 g m−2 yr−1 of DIC. The seasonal changes of temperature, active layer, flow path, and discharge can alter DIC and stable carbon isotope of DIC (δ13C‐DIC). The most depleted δ13C‐DIC values were found in the thawed period, suggesting the soil‐respired CO2 during the active layer thaw period can promote bicarbonate production via H2CO3 weathering. Spatially, δ13C‐DIC values increased downstream, likely due to CO2 outgassing and changed permafrost coverage and runoff. We found that evaporite dissolution and silicate weathering in the seasonally thawed active layer contributed 44.2% and 30.9% of stream HCO3‐, respectively, while groundwater and rainwater contributed 16.7% and 7.3% of HCO3‐, respectively. Pure carbonate rock weathering played a negligible role in DIC production. These results were compatible with δ13C‐DIC source approximation results. Silicate weathering increased from initial thaw to thawed period, reflecting the active layer thaw and subsequent hydrology change impacts. Silicate weathering consumed 1.25 × 1010 mol of CO2 annually, while evaporite dissolution may produce CO2 and neutralize this CO2 sink. This study provides new understanding of the riverine DIC export processes of the YRSR. As permafrost degrades, the quantity, sources, and sinks of riverine DIC may also change spatiotemporally.

Perspectives on provenance and alteration of suspended and sedimentary organic matter in the subtropical Pearl River system, South China

Large river systems accumulate, process, and transport huge quantities of organic matter (OM) from their catchments, part of which is exported to the ocean. Although this suite of processes comprises an important component of the global carbon cycle, integrated studies examining the nature and extent of OM processing on a basin-wide scale remain rare. Here, we provide an overview of provenance and composition of OM in suspended and deposited sediments within the Pearl River watershed in South China. We present new data on the organic carbon (OC) and total nitrogen (TN) contents, stable carbon and radiocarbon isotopic compositions of OC (δ 13 C and F m ), as well as grain size distribution and mineral-specific surface area of Pearl River sediments. These results are combined with published data on suspended particulate matter (SPM) and soil profiles in the Pearl River watershed in order to determine the provenance and transformation of OM in this large subtropical fluvial system. We find that the low 14 C contents, expressed as fraction modern ( F m ) values, in suspended ( F m : 0.58–0.87) and sedimentary OM ( F m : 0.38–0.82) are attributed to contributions from 14 C-depleted soils, bedrock, as well as riverine primary productivity ( R pp ) that utilizes 14 C-depleted sources of dissolved inorganic carbon (DIC). For SPM, soil OM ( F m : 0.87 ± 0.13) is inferred to be the dominant fraction during the wet season, whereas the contributions of R pp ( F m : 0.86 ± 0.04) and petrogenic OC (devoid of 14 C) are enhanced during the dry season. This manifests itself in differences in OC soil , OC Rpp , and OC petro contributions in SPM between wet and dry seasons (1.05 ± 0.18 vs. 1.20 ± 0.50%, 0.16 ± 0.03 vs. 0.32 ± 0.15% and 0.21 ± 0.07 vs. 0.38 ± 0.19%). During erosion and transport, the most labile OM in the top soil is rapidly degraded, as indicated by a stronger contribution from soil CO 2 into riverine DIC during the wet season and flood event (39 ± 1% and 45 ± 3%) compared to the dry season (31 ± 2%). River sediments are primarily accumulated during the wet season when suspended sediment fluxes are high. Refractory deep soil OM ( F m : 0.74 ± 0.07) dominate in these sediments, whereas moderately labile soil OM components are further degraded during settling and storage. This sedimentary OM is predominantly composed of aged soil (92 ± 4%), with a minor contribution from bedrock (7 ± 4%) and negligible input from R pp (0.2 ± 0%). The longitudinal changes in the composition of suspended and sedimentary OM are mainly controlled by input of R pp and ongoing degradation processes in the river system. The riverine particulate OC flux to the Pearl River estuary and ocean thus contains a mixture of soil, petrogenic and R pp OM, all exhibiting relatively low F m values. The riverine OM transformation and dynamics are important for the short-term carbon cycle, whereas the remaining signature and fate of the extensively processed, refractory OM has implications for the long-term carbon cycle.

Inorganic carbon distribution and CO2 fluxes in a large European estuary (Tagus, Portugal)

Ten field cruises were carried out in Tagus estuary from 1999 to 2007 to study the dynamics of the inorganic carbon system. Dissolved inorganic carbon (DIC) and total alkalinity (TA) increased with salinity. DIC and TA were generally conservative in the estuarine mixing zone (salinity > 10), while a complex distribution pattern was observed at the upper estuary. DIC values peaked 1786.9 ± 155.8 µmol kg−1 at that segment. Estimated annual mean fluxes of DIC were 0.27 Tg C yr−1 from the river to the estuary, and 0.37 Tg C yr−1 from here to the coastal area. The Tagus estuary was always CO2 supersaturated, with partial pressure of CO2 (pCO2) reaching 9160 µatm in the upper estuary. An average emission of 0.11 Tg C yr−1 was estimated from the estuary to the atmosphere, corresponding to 23% of exported DIC. Only 8% of the riverine DIC was ventilated. The non-conservative behaviour of CO2 parameters in the estuary segment under freshwater influence was attributed to alternations in the relevance of riverine/terrestrial runoff, photosynthesis, aerobic respiration, organic matter mineralization and CaCO3 precipitation/dissolution.

The carbon flux of global rivers: A re-evaluation of amount and spatial patterns

Global rivers connect three large carbon reservoirs in the world: soil, atmosphere, and ocean. The amount and spatial pattern of riverine carbon flux are essential for the global carbon budget but are still not well understood. Therefore, three linear regression models for riverine DOC (dissolved organic carbon), POC (particulate organic carbon), and DIC (dissolved inorganic carbon) fluxes were established with related generating and transfer factors based on an updated global database. The three models then were applied to simulate the spatial distribution of riverine DOC, POC, and DIC fluxes and to estimate the total global riverine carbon flux. The major conclusions of this study are as follows: the correlation analysis showed that riverine DOC flux is significantly related to discharge (r2=0.93, n=109) and soil organic carbon amount (r2=0.60), POC flux increases with discharge (r2=0.55, n=98) and amount of soil erosion (r2=0.48), and DIC flux is strongly linked to CO2 consumption by rock weathering (r2=0.66, n=111) and discharge (r2=0.63). In addition, Asia exports more DOC and POC than other continents and North America exports more DIC. The Atlantic Ocean accepts the major portion of riverine DOC, POC, and DIC fluxes of all the oceans. The highest riverine DOC flux occurs in the 0–30°S zone, and the highest riverine POC and DIC fluxes appear in the 30–60°N zone. Furthermore, re-estimation revealed that global rivers export approximately 1.06PgC to oceans every year, including 0.24Pg DOC, 0.24Pg POC, 0.41Pg DIC, and 0.17Pg PIC.

Is basalt weathering a major mechanism for atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; consumption?

In the science of global change, a main focus of researchers who investigate the global carbon cycle is determining the fate of missing carbon sinks. Because atmospheric CO2 that is consumed by carbonate chemical weathering is thought to return to the atmosphere through the precipitation of carbonate, it is widely accepted that it is silicate weathering, rather than carbonate weathering, that constitutes the major mechanism of atmospheric CO2 consumption. In particular, the chemical weathering of basalt, which is a type of silicate rock, is considered to be an important “carbon sink” due to the CO2 drawdown that occurs during the basalt-carbonic acid reaction. However, the high CO2 consumption rates of basalt chemical weathering may also derive from the following four aspects, as identified in the extant literature. First, study areas with high carbon fluxes (CF) are usually ocean islands, volcanic arcs or situated in tropical regions, where precipitation is high, which results in large runoff depth. This may be one of the primary reasons for the high CF, since CF is equal to the product of the runoff depth and the concentration of bicarbonate. In addition, because of the presence of chemostatic behaviors of the concentration of bicarbonate, CF is mainly determined by runoff depth. In other words, high runoff depth will directly result in high CF. Furthermore, the interface between water and minerals will be enlarged, while the saturation state of water will be decreased, by the increase of runoff depth. Therefore, more minerals will take part in the dissolution, and the dissolving capacity of water will be increased. However, this kind of high CF results from high runoff depth, rather than basaltic properties. Second, the high concentration of dissolved inorganic carbon (DIC) may result from the chemical weathering of trace amounts of carbonate dispersed in silicate rock, rather than silicate minerals. This should be regarded as the contribution of carbonate chemical weathering, rather than silicate chemical weathering. Third, the reactions between exogenous acid and trace amounts of carbonate will contribute to DIC fluxes, during which no atmospheric CO2 is consumed. Fourth, the CO2 that participates in the rock chemical weathering in basalt watersheds may be from a deep-source, rather than the atmosphere or soil. If this is the case, the deep-source CO2 consumed by rock chemical weathering will be released into the atmosphere with the precipitation of carbonate in the oceans. In other words, the riverine DIC may become a carbon source instead of a carbon sink. Therefore, further research is needed to conclusively determine whether basalt chemical weathering constitutes a major consumption mechanism for atmospheric CO2.

Sensitivity of chemical weathering and dissolved carbon dynamics to hydrological conditions in a typical karst river

To better understand the mechanisms that hydrological conditions control chemical weathering and carbon dynamics in the large rivers, we investigated hydrochemistry and carbon isotopic compositions of dissolved inorganic carbon (DIC) based on high-frequency sampling in the Wujiang River draining the carbonate area in southwestern China. Concentrations of major dissolved solute do not strictly follow the dilution process with increasing discharge, and biogeochemical processes lead to variability in the concentration-discharge relationships. Temporal variations of dissolved solutes are closely related to weathering characteristics and hydrological conditions in the rainy seasons. The concentrations of dissolved carbon and the carbon isotopic compositions vary with discharge changes, suggesting that hydrological conditions and biogeochemical processes control dissolved carbon dynamics. Biological CO2 discharge and intense carbonate weathering by soil CO2 should be responsible for the carbon variability under various hydrological conditions during the high-flow season. The concentration of DICbio (DIC from biological sources) derived from a mixing model increases with increasing discharge, indicating that DICbio influx is the main driver of the chemostatic behaviors of riverine DIC in this typical karst river. The study highlights the sensitivity of chemical weathering and carbon dynamics to hydrological conditions in the riverine system.

Biotic and abiotic controls on CO2 partial pressure and CO2 emission in the Tigris River, Turkey

Rivers act not only as vectors from the terrestrial biosphere to the ocean but also as significant sources of CO2 to the atmosphere, which plays a key role in the global carbon cycle. In this study, dissolved inorganic carbon (DIC) and carbon dioxide partial pressure (pCO2) in the Tigris River were investigated. The alkalinity concentrations of the Tigris River ranged from 1718 to 4995μeq/L with an average of 3064μeq/L (median: 2877μeq/L), 2.3 times the global median value of 1256μeq/L. Higher alkalinity values occurred in autumn and winter months that were due to carbonate rock weathering. pCO2 values varied from 95.6 to 16,001.8μatm with a total average of 1277±1883μatm. >80% of samples had pCO2 above atmospheric equilibrium (390μatm). The highest levels of pCO2 occurred at stations Diyarbakır, Bismil and Cizre, which are categorized as polluted stations. The lowest levels of pCO2 generally occurred in the wet season (October–April) due to dilution effect of rainfall. The estimated water-to-air CO2 degassing flux from the Tigris River was around 39mol/m2/y. Thus, 0.15Tg C/y as CO2 released from the river to the atmosphere, about 0.6 fold the riverine DIC flux (0.25Tg C/y). Correlative analyses demonstrated that population density and air temperature increased pCO2, while slope degree decreased pCO2. pCO2 also had positive correlations with COD, NH4-N, PO4-P, SO4, Cl, phytoplankton abundance and number of phytoplankton taxa, while negative correlations with DO saturation and pH.

Factors controlling the isotope composition of dissolved inorganic carbon in a karst-dominated wetland catchment, Guizhou Province, Southwest China

Wetlands contain a large proportion of the world’s carbon, of which dissolved inorganic carbon (DIC) is one of the important fractions in wetland carbon cycle. We collected groundwater, river water, and wetland water from the Caohai catchment to investigate the isotope composition of DIC and inorganic carbon cycling. The main sources of groundwater DIC were carbonate dissolution and soil CO2. Groundwater was the main source of riverine DIC, but the δ13CDIC value in river water showed a more positive excursion than groundwater because of phytoplankton photosynthesis and CO2 evasion. Seasonal variations in DIC concentrations and the δ13CDIC indicate the influence of local rainfall events in the high-flow season and the longer residence time of water in the low-flow season. There was marked seasonal variation in the pCO2 and δ13CDIC in wetland water. During the high-flow season, the pCO2 in the emergent plant zone was higher than that of the atmosphere because of respiration of roots and sediments. Because of photosynthesis, the pCO2 in the submerged plant zone was lower than that of the atmosphere. Furthermore, in this zone, the δ13CDIC ranged from −15.13 to −6.68 ‰ (mean value of −10.79 ‰), and may have been influenced by chemically enhanced fractionation and photosynthesis. In the low-flow season, the pCO2 was mostly higher than atmospheric pCO2 because of organic matter decomposition in sediments. Additionally, during this season, the δ13CDIC ranged from −6.23 to −0.06 ‰ (mean value of −2.52 ‰), and may have been influenced by CO2 evasion, CO2 reduction to methane, and phytoplankton photosynthesis.

Influences of riverine and upwelling waters on the coastal carbonate system off Central Chile and their ocean acidification implications

AbstractA combined data set, combining data from field campaigns and oceanographic cruises, was used to ascertain the influence of both river discharges and upwelling processes, covering spatial and temporal variation in dissolved inorganic carbon (DIC) and aragonite saturation state. This work was conducted in one of the most productive river‐influenced upwelling areas in the South Pacific coasts (36°S). Additionally, further work was also conducted to ascertain the contribution of different DIC sources, influencing the dynamics of DIC along the land‐ocean range. Six sampling campaigns were conducted across seven stations at the Biobío River basin, covering approximately 200 km. Three research cruises were undertaken simultaneously, covering the adjacent continental shelf, including 12 sampling stations for hydrographic measurements. Additionally, six stations were also sampled for chemical analyses, covering summer, winter, and spring conditions over 2010 and 2011. Our results evidenced that seaward extent of the river plume was more evident during the winter field campaign, when highest riverine DIC fluxes were observed. The carbonate system along the river‐ocean continuum was very heterogeneous varying over spatial and temporal scales. High DIC and pCO2 were observed in river areas with larger anthropogenic effects. CO2 supersaturation at the river plume was observed during all campaigns due to the influence of low pH river waters in winter/spring and high‐pCO2 upwelling waters in summer. δ13CDIC evidenced that main DIC sources along the river and river plume corresponded to the respiration of terrestrial organic matter. We have linked this natural process to the carbonate saturation on the adjacent river‐influenced coastal area, suggesting that Ωaragonite undersaturation in surface/subsurface waters is largely modulated by the influence of both river discharge and coastal upwelling events in this productive coastal area. Conditions of low Ωaragonite might impact negatively physiological traits for marine organisms, such as bivalves, gastropods, and crustaceans. Therefore, local populations from river‐influenced sites could be inherently more tolerant to ocean acidification than organisms living in regions with lower Ωaragonite variability.

Controls on the sources and cycling of dissolved inorganic carbon in the Changjiang and Huanghe River estuaries, China: 14C and 13C studies

AbstractThe Changjiang (Yangtze River) and Huanghe (Yellow River) are the two largest rivers in China, and they transport large amounts of terrestrial carbon to the coastal waters of the East China Sea and the Bohai Sea. The sources and cycling of riverine carbon in these two large river estuaries, however, have not been well studied. In this article, we present the results of dual isotope (Δ14C and δ13C) measurements of dissolved inorganic carbon (DIC) collected in the low reaches of the Changjiang and Huanghe and their estuaries during two cruises in 2014. Our results indicate that both the Changjiang and Huanghe carry very high concentrations of DIC ranging from 1384 μmol kg−1 to 1732 μmol kg−1 and 2711 μmol kg−1 to 4120 μmol kg−1, respectively, and DIC levels varied with flow rates during high and low discharge periods. The cycling of DIC exhibited conservative behavior in both the Changjiang and Huanghe estuaries, suggesting DIC levels were controlled mainly by physical mixing processes.Δ14C‐DIC values indicate that the Changjiang and Huanghe transport aged DIC (1060–1380 yr old). Both Δ14C‐DIC and δ13C‐DIC values also showed conservative mixing in the two estuaries. Using a dual carbon isotopic model, we calculated that atmospheric CO2 consumed mainly by silicate weathering was a major source, contributing 65.2 ± 9.0% and 73.4 ± 3.0% of DIC in the Changjiang and Huanghe, and 96.9–97.7% (by air‐sea exchange) of DIC in the coastal waters of the East China Sea (ECS) and Bohai Sea, respectively. Our results indicate that carbonate dissolution was an important (12.3–17.4%) but not major process controlling the high DIC levels in both rivers, as suggested previously. Compared with the large Amazon River, respiration of riverine organic matter (OM) played a less important role, contributing only 15.4–17.2% of DIC in the two Asian rivers. Flux calculations indicate that the Changjiang and Huanghe discharged 1.46 × 1013 g and 6.28 × 1011 g DIC into the ECS and Bohai Sea in 2014, which were 9 and 17 times higher than the DOC fluxes in the two rivers. These large fluxes of riverine DIC, especially of aged DIC, could have significant impacts on primary production and carbon cycling in the ECS and Bohai Sea.