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Different from rivers in humid areas, the variability of riverine CO2 system in arid areas is heavily impacted by anthropogenic disturbance with the increasing urbanization and water withdrawals. In this study, the water chemistry and the controls of carbonate system in an urbanized river (the Fenhe River) on the semi-arid Loess Plateau were analyzed. The water chemistry of the river water showed that the high dissolved inorganic carbon (DIC) concentration (about 37 mg L−1) in the upstream with a karst land type was mainly sourced from carbonate weathering involved by H2CO3 and H2SO4, resulting in an oversaturated partial pressure of CO2 (pCO2) (about 800 µatm). In comparison, damming resulted in the widespread appearance of non-free flowing river segments, and aquatic photosynthesis dominated the DIC and pCO2 spatiality demonstrated by the enriched stable isotope of DIC (δ13CDIC). Especially in the mid-downstream flowing through major cities in warm and low-runoff August, some river segments even acted as an atmospheric CO2 sink. The noteworthy is wastewater input leading to a sudden increase in DIC (> 55 mg L−1) and pCO2 (> 4500 µatm) in the downstream of Taiyuan City, and in cold November the increased DIC even extended to the outlet of the river. Our results highlight the effects of aquatic production induced by damming and urban sewage input on riverine CO2 system in semi-arid areas, and reducing sewage discharge may mitigate CO2 emission from the rivers. [ABSTRACT FROM AUTHOR]

Using hydrochemical and isotopic compositions of springs and wells, we trace carbon from critical zone carbon dioxide (CO2) into groundwater of the semiarid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Dissolved inorganic carbon (DIC) concentrations, pH and stable isotope tracers of carbon for DIC (δ13CDIC), are used to show that most groundwater evolves under open system conditions, moving carbon into the groundwater and acting as a carbon sink. However, one sample (-10.94‰ δ13CDIC, 6,350 14C years before present (yrs. BP)) may have evolved under closed system conditions with a higher partial pressure of critical zone CO2 than present-day soils. By characterizing the carbon cycle, we show that (1) carbon evolution is primarily under open-system conditions, (2) shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs. BP) than deeper groundwater samples (1,469 to 6,350 14C yrs. BP), and (3) the older portion of the groundwater may be even older than the calculated 14C ages, as indicated by the mixing of age tracers in intermediate wells. Our global conception of the deep critical zone should include carbon cycling of critical zone CO2 in old groundwater. Characterizing the deep critical zone in a semiarid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling. [ABSTRACT FROM AUTHOR]

Four species of shellfish, blue mussel (Mytilus galloprovincialis), Pacific abalone (Haliotis discus hannai), zhikong scallops (Chlamys farreri), and Pacific oyster (Crassostrea gigas), were exposed to decoupled carbonate system variables to investigate the impacts of different seawater carbonate parameters on the CO2 excretion process of mariculture shellfish. Six experimental groups with two levels of seawater pH (pH 8.1 and pH 7.7) and three levels of total alkalinity (TA = 1000, 2300, and 3600 μmol/kg, respectively) were established, while pH 8.1 and TA = 2300 μmol/kg was taken as control. Results showed that the CO2 excretion rates of these tested shellfish were significantly affected by the change in carbonate chemistry (P < 0.05). At the same TA level, animals incubated in the acidified group (pH 7.7) had a lower CO2 excretion rate than those in the control group (pH 8.1). In comparison, at the same pH level, the CO2 excretion rate increased when seawater TA level was elevated. No significant correlation between the CO2 excretion rate and seawater pCO2 levels (P > 0.05) was found; however, a significant correlation (P < 0.05) between CO2 excretion rate and TA-DIC (the difference between total alkalinity and dissolved inorganic carbon) was observed. Blue mussel has a significantly higher CO2 excretion rate than the other three species in the CO2 excretions per unit mass of soft parts, with no significant difference observed among these three species. However, in terms of CO2 excretion rate per unit mass of gills, abalone has the highest CO2 excretion rate, while significant differences were found between each species. Our studies indicate that the CO2 buffering capacity impacts the CO2 excretion rate of four shellfish species largely independent of pCO2. Since CO2 excretion is related to acid–base balancing, the results imply that the effects of other carbonate parameters, particularly the CO2 buffering capacity, should be studied to fully understand the mechanism of how acidification affects shellfish. Besides, the species difference in gill to soft parts proportion may contribute to the species difference in responding to ocean acidification. [ABSTRACT FROM AUTHOR]

A three-dimensional (3-D) physical-biogeochemical-carbon cycle coupled model is used to investigate the interannual variability of the air-sea carbon dioxide (CO2) flux (FCO2) in the Northern Yellow Sea (NYS) from 2009 to 2018. The verification of the model indicate that the simulation results for multiple variables exhibit consistency and fit well with the observed data. The study show that although the multi-year average FCO2 in the NYS is close to the source-sink balance, there are obvious interannual differences between different years. In particular, a relatively strong source of atmospheric CO2 (1.0 mmol m-2 d-1) is exhibited in 2014, while a relatively strong sink of atmospheric CO2 (-0.7 mmol m-2 d-1) emerges in 2016. Mechanism analysis indicates that the abnormally high temperature is the main controlling factor for the relatively high CO2 efflux rate in the NYS in 2014, while the abnormally low dissolved inorganic carbon (DIC) concentration is the main factor contributing to the relatively high CO2 influx rate in 2016. Further analysis reveals that the primary reason for the low DIC concentration since the onset of winter in 2016 is the high net decrease rate of DIC in the NYS in 2015, influenced by net community production in the summer and advection processes during the autumn. The abnormally high primary production during the summer of 2015 results in the excessive reduction of DIC concentration through biological processes. In addition, due to the strong northeasterly wind event in November 2015, low-concentration-DIC water from the Yellow Sea (YS) extends into the Bohai Sea (BS). This further leads to higher DIC flux from the NYS into the BS in the upper mixed layer and increases the inflow of low-concentration-DIC water from the Southern Yellow Sea (SYS) into the NYS. These ultimately result in the abnormal reduction of DIC concentration in the upper mixed layer of the NYS during the autumn of 2015. This study enriches our understanding of interannual variability of FCO2 in the NYS, which will not only help to further reveal the variations of FCO2 under human activities and climate change, but also provide useful information for guiding the comprehensive assessment of the carbon budget. [ABSTRACT FROM AUTHOR]

The Western Tropical Atlantic Ocean (WTAO) is crucial for understanding CO2 dynamics due to inputs from major rivers (Amazon and Orinoco), substantial rainfall from the Intertropical Convergence Zone (ITCZ), and CO2-rich waters from equatorial upwelling. This study, spanning 1998 to 2018, utilized sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W to reconstruct the surface marine carbonate system. Empirical models derived total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS, with subsequent estimation of pH and fCO2 from TA, DIC, SSS, and SST data. Linear trend analysis showed statistically significant temporal trends: DIC and fCO2 increased at a rate of 0.7 μmol kg-1 year-1 and 1.539 μatm year-1, respectively, and pH decreased at a rate of -0.001 pH units year-1, although DIC did not show any trend after data was de-seasoned. Rainfall analysis revealed distinct dry (July to December) and wet (January to June) seasons, aligning with lower and higher freshwater influence on the ocean surface, respectively. TA, DIC, and pH correlated positively with SSS, exhibiting higher values during the dry season and lower values during the wet season. Conversely, fCO2 correlated positively with SST, showcasing higher values during the wet season and lower values during the dry season. This emphasizes the influential roles of SSS and SST variability in CO2 solubility within the region. Finally, we have analysed the difference between TA and DIC (TA-DIC) as an indicator for ocean acidification and found a decreasing trend of -0.93 ± 0.02 mmol kg-1 year-1, reinforcing the reduction in the surface ocean buffering capacity in this area. All trends found for the region agree with data from other stations in the tropical and subtropical Atlantic Ocean. In conclusion, the use of empirical models proposed in this study has proven to help filling the gaps in marine carbonate system data in the Western Tropical Atlantic. [ABSTRACT FROM AUTHOR]

Land use in karst areas affects soil properties, impacting carbon sinks. Accurate estimation of carbon sink flux in karst areas through zoning and classification is crucial for understanding global carbon cycling and climate change. The peak cluster depression is the largest continuous karst landform region in southern China, with the depressions primarily covered by farmland and influenced by agricultural activities. This study focused on the Guancun Underground River Basin, a typical peak cluster depression basin, where sampling and analysis were conducted during the agricultural period of 2021–2022. Using hydrochemical analysis and isotopic methods, the results indicated that: (1) The primary hydrochemical type in the Guancun Underground River Basin is HCO3-Ca, with hydrochemical composition mainly controlled by carbonate rock weathering. (2) The primary sources of Cl−, SO42−, and NO3− are agricultural activities, with agriculture contributing 0.68 mmol/L to dissolved inorganic carbon (DIC), accounting for about 13.86%, as confirmed by ion concentration analysis and isotope verification. (3) The size of the depression area is proportional to the contribution of agricultural activities to DIC, while also being influenced by dilution effects. A comparison was made regarding the contribution of other land use types to DIC. The impact of land use on DIC in karst processes should not be overlooked, and zoning and classification assessments of carbon sink flux under different influencing factors contribute to carbon peaking and carbon neutrality goals. [ABSTRACT FROM AUTHOR]

North Atlantic subtropical mode water (NASTMW) serves as a major conduit for dissolved carbon to penetrate into the ocean interior by its wintertime outcropping events. Prior research on NASTMW has concentrated on its physical formation and destruction, as well as Lagrangian pathways and timescales of water into and out of NASTMW. In this study, we examine how dissolved inorganic carbon (DIC) concentrations are modified along Lagrangian pathways of NASTMW on subannual timescales. We introduce Lagrangian parcels into a physical‐biogeochemical model and release these parcels annually over two decades. For different pathways into, out of, and within NASTMW, we calculate changes in DIC concentrations along the path (ΔDIC), distinguishing contributions from vertical mixing and biogeochemical processes. The strongest ΔDIC is during subduction of water parcels (+101 μmol L−1 in 1 year), followed by transport out of NASTMW due to increases in density in water parcels (+10 μmol L−1). While the mean ΔDIC for parcels that persist within NASTMW in 1 year is relatively small at +6 μmol L−1, this masks underlying dynamics: individual parcels undergo interspersed DIC depletion and enrichment, spanning several timescales and magnitudes. Most DIC enrichment and depletion regimes span timescales of weeks, related to phytoplankton blooms. However, mixing and biogeochemical processes often oppose one another at short timescales, so the largest net DIC changes occur at timescales of more than 30 days. Our new Lagrangian approach complements bulk Eulerian approaches, which average out this underlying complexity, and is relevant to other biogeochemical studies, for example, on marine carbon dioxide removal. Plain Language Summary: Mode waters are relatively thick water masses with homogeneous properties, such as temperature and salinity. The North Atlantic subtropical mode water (NASTMW), found in the Sargasso Sea, is one such water mass. Lying underneath the ocean surface, it comes into contact with the atmosphere during winter, when the surface layer is vigorously mixed due to strong winds, causing the mixed layer to connect with NASTMW. This way, NASTMW can buffer atmospheric temperature and carbon anomalies during the summer, when there is no surface connection. It is also a conduit for carbon to penetrate beneath the ocean's upper mixed layer, with the potential to sequester it. We study NASTMW from the viewpoint of a water parcel that moves with the currents and see how carbon concentrations in the water parcels change along different NASTMW pathways. For each pathway, the carbon concentration changes due to an interplay of vertical mixing and biogeochemical processes, for example, related to plankton growth and decay. These processes can unfold over different timescales and may counteract or enhance themselves or one another. The largest change in carbon concentration is found when a parcel moves from the upper ocean mixed layer into NASTMW, mostly due to vertical mixing. Key Points: Carbon transformations along pathways of North Atlantic Subtropical Mode Water are split into mixing and biogeochemical contributionsAlong paths into, within, and out of this mode water, mixing and biogeochemistry alter carbon in water parcels over a range of timescalesEnrichment is highest during mixed layer subduction, which few parcels undergo annually; persistence in mode water is the dominant pathway [ABSTRACT FROM AUTHOR]

In the oligotrophic Southeastern Mediterranean Sea (SEMS), it has been shown that dissolved inorganic nutrient (DIN) from fresh submarine groundwater discharge (FSGD) enhance primary production in coastal waters. In this study pH, Total Alkalinity (TA) and DIN of seawater and fresh water in a sea‐cave in the northern part of the Israeli Mediterranean coast and a nearby contact spring, respectively, were measured during October 2018–March 2020. The results show gradients of measured salinity, TA, pH and DIN along the cave axis year‐round, suggesting that they are influenced by FSGD. The seawater near the back of the cave was supersaturated with respect to atmospheric CO2 nearly year‐round and there is a strong positive divergence from its regional open‐water thermal dependence, which suggests that FSGD is also a source of atmospheric CO2 in this region. Comparison of TA, salinity and pCO2 from the back of the sea cave to their corresponding values from an abrasion platform monitoring site, ca. 3 km south of the cave, suggests that FSGD is occurring along the entire shoreline in this region. Thus, despite the increased productivity due to FSGD mediated nutrient enrichment of adjacent coastal waters of the oligotrophic SEMS, they are still a source of atmospheric CO2 nearly year‐round. Finally, the apparent trends of seawater acidification (ΔpH/Δt = −0.006 yrs−1) and pCO2 increase (+8 ppmV yr−1) observed at the nearby monitoring site since 2013 are explained by increased groundwater recharge and resulting FSGD total alkalinity compared to dissolved inorganic carbon inputs (ΔTA/ΔDIC = 1:1.2). Plain Language Summary: Submarine groundwater discharge is an important source of dissolved inorganic nutrients that support organic matter production in the Mediterranean Sea. Furthermore, the contribution of fresh groundwater discharge to the alkalinity budget of the Mediterranean Sea compared to riverine inputs is also significant. Approximately half of the groundwater discharged into the Mediterranean originates from karst lithologies. In this study we followed the annual variations in seawater chemical properties in the Rosh‐Hanikra sea‐cave, located along the northern Mediterranean coast of Israel. The results suggest that fresh groundwater discharge supports the observed gradients of measured seawater chemistry along the cave axis year‐round with a stronger influence during the wintertime compared to summertime and that it helps maintain coastal waters as a source of atmospheric CO2 year‐round. Furthermore, the data suggest that submarine groundwater discharge, which is enriched in dissolved inorganic carbon increases the rate of ocean acidification associated with increasing atmospheric CO2 in the coastal waters of this region. Key Points: Fresh submarine groundwater discharge (FSGD) is apparent in seawater properties of the Rosh‐Hanikra Mediterranean Sea sea‐cave year‐roundFSGD into coastal waters in the Rosh‐Hanikra region causes them to be a source of atmospheric CO2 year‐roundFSGD augments seawater acidification along the northern Mediterranean coast of Israel despite nutrient enrichment and increased productivity [ABSTRACT FROM AUTHOR]

Study region: Agricultural Irrigation and Drainage system - Ulansuhai Lake Continuum, located in the Hetao Irrigation District, Inner Mongolia, China. Study focus: In this study, carbon isotope tracing technology was applied to analyze the sources and migration-transformation characteristics of Dissolved Organic Carbon (DOC) and Dissolved Inorganic Carbon (DIC) in the continuous system of the agricultural irrigation and drainage system - Ulansuhai Lake in the Hetao Irrigation Area of Inner Mongolia during typical irrigation periods. New hydrological insights for the region: During typical irrigation periods, DOC and DIC increase simultaneously with the increase in salinity, and dissolved carbon dioxide in the irrigation and drainage channels escapes. The main sources of DOC in farmland drainage water are terrestrial C3 plants (94.00 %) and plankton (6.00 %). The primary sources of DOC in the lake are farmland drainage (98.19 %) and macroaquatic plants (1.81 %). The main sources of DIC in farmland drainage water are irrigation water from the Yellow River (64.73 %) and the equilibrium dissolution process between carbonates and soil CO₂ (35.27 %); while the primary sources of DIC in the lake are farmland drainage (93.10 %) and air-water exchange (6.90 %). Crop types in the irrigated area, soil salinization, rock weathering, metabolic processes, and the hydrological connectivity induced by irrigation from the Yellow River jointly control the sources, migration, and transformation of carbon within the continuum.

The Eastern Indian Ocean (EIO) is an ideal region to explore the variability and controlling mechanisms of the seawater carbonate system and their potential influence on global climate change due to the distinctive environmental features, while studies in the EIO is far from sufficient. The spatiotemporal distributions of pH, dissolved inorganic carbon (DIC), alkalinity (Alk), and partial pressure of carbon dioxide (pCO2) were investigated in the EIO during autumn 2020 and spring 2021. The respective quantitative contributions of different controlling processes to DIC were further delineated. Significant seasonal variations were observed in the study area. Overall, the surface pH was lower and DIC, Alk, and pCO2 were higher during spring 2021 than during autumn 2020. The pH generally decreased from east to west during autumn 2020, whereas it decreased from north to south during spring 2021. The low values of DIC and Alk that were detected in the Bay of Bengal in these two seasons were mainly attributed to the influence of river inputs. Coastal upwelling during monsoon periods led to higher pCO2 and DIC values near Sumatra and Sri Lanka during spring 2021. The relationships of carbonate system parameters with different types of nutrients and different sized chlorophyll-a in the two seasons indicated the shifts of nutrients utilized by the phytoplankton, and phytoplankton species dominated the carbonate system variabilities. In vertical profiles, carbonate system parameters showed strong correlations with other physical and biogeochemical parameters, and these correlations were more robust during spring 2021 than during autumn 2020. The average sea-air flux of CO2 was 10.00 mmolm-2 d-1 during autumn 2020 andwas 16.00 mmolm-2 d-1 during spring 2021, which revealed that the EIO served as a CO2 source during the study period. In addition, the separation of different controlling processes of DIC indicated stronger mixing processes, less CaCO3 precipitation, more intensive sea-air exchange, and weaker photosynthesis during spring 2021 than during autumn 2020. [ABSTRACT FROM AUTHOR]

Lake carbon cycle in lake ecosystems is critical for regional carbon management. The application of carbon isotope techniques to terrestrial and aquatic ecosystems can accurately elucidate carbon flow and carbon cycling. Lake ecosystems on the Qinghai–Tibetan Plateau are fragile and sensitive to climate and environment changes, and the carbon cycle impact on the carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) in these systems has not been well studied, limiting the ability to devise effective management strategies. This study explored the relationship among the δ13C position of the DIC (δ13CDIC) in Genggahai Lake, the lake environment, and the climate of the watershed based on the observed physicochemical parameters of water in areas with different types of submerged macrophyte communities, combined with concomitant temperature and precipitation changes. Overall, the Genggahai Basin δ13CDIC exhibited a large value range; the average δ13CDIC for inflowing spring water was the most negative, followed by the Shazhuyu River, and then lake water. Variations in the photosynthetic intensity of different aquatic plants yielded significantly changing δ13CDIC-L values in areas with varied aquatic plant communities. Hydrochemical observations revealed that δ13CDIC-I and aquatic plant photosynthesis primarily affected the differences in the δ13CDIC-L values of Genggahai Lake, thereby identifying them as the key carbon cycle components in the lake. This improves the understanding of the carbon cycle mechanism of the Qinghai–Tibetan Plateau Lake ecosystem, which is beneficial to improving sustainable lake development strategies. [ABSTRACT FROM AUTHOR]

Estimation of residence time of groundwater, particularly in regions with inadequate surface waters are very important for formulating sustainable groundwater management policies. We developed a technique for extracting dissolved inorganic carbon (DIC) quantitatively from water for measuring its 14C contents and presented the analytical details here. We also measured stable carbon isotope ratio (δ13C) in soil CO2 and groundwater DIC to correct the groundwater 14C ages. In addition, 14C in soil CO2 were measured for making necessary correction in the initial activity of the recharging water. The corrected 14C contents in the groundwater samples were used to estimate their residence times employing Lumped Parameter Models (LPM), a set of mathematical models to account for the processes that take place during transport from the recharge to the sampling spots. We present a case study focused on the calculation of radiocarbon ages and residence times for a groundwater sample collected from the campus of Physical Research Laboratory in Ahmedabad, Gujarat, India. The study also includes estimations of groundwater residence times using previously measured 14C ages of groundwater samples from Gujarat, India. Various factors controlling the groundwater ages in the LPM and their applicability are discussed. [ABSTRACT FROM AUTHOR]

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m-2d-1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m-2d-1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets. [ABSTRACT FROM AUTHOR]

The sum of ions of the basic Lake Ladoga water mass has varied quite significantly from 55.6 to 71.6 mg/L over the past 60 years. In the present paper, separate periods of the mineralization growth are identified. It is shown that the change in the mineralization of Lake Ladoga has been always accompanied by a significant change in the ratio of basic ion concentrations. Any significant trends in the total mineralization of Lake Ladoga over the analyzed period have not been identified, but periods of the growth and decrease in the number of ions have been revealed. Until 1998, the periods of increased mineralization, as a rule, were accompanied by an increase in concentrations of and , most likely coming from the catchment area due to industrial and household pollution. During 2009–2019, the mineralization was growing mainly due to an increase in the absolute and relative concentrations of . At the same time, the concentration of dissolved CO2 in the main water mass of the lake also was increasing (and, consequently, pH was decreasing). Thus, the changes in the bicarbonate ion concentration in the water of Lake Ladoga may be largely determined by an increase in its inflow from the catchment area, whose possible reason is the enhancement of the chemical weathering of carbonate rocks due to the increasing concentrations of dissolved in the water. [ABSTRACT FROM AUTHOR]

The watershed carbon cycle is an important part of the global carbon cycle, playing an important role in seeking global carbon sinks that have been missed. Dissolved inorganic carbon (DIC) is one of the important indicators for studying the carbon cycle of water bodies in the basin. As an important component of terrestrial ecosystems, karst water bodies containing high DIC concentrations play an important role in the global carbon cycle. The Lujiang river basin, a typical development area of karst graben basin, is an applicable place for us to study the carbon migration and transformation of water bodies. In order to study the hydrochemical types and sources of DIC in the rivers of the Lujiang river basin, sampling data of hydrochemistry and carbon isotope from 14 surface water points, 8 underground river points, and 10 spring points during the rainy season were analyzed. It is believed that exploring the proportional contribution of DIC sources in different water bodies may help to understand the sources and cycles of groundwater DIC under the influence of human activities, providing scientific references for water resource management. Firstly, this study determines the hydrochemical types of different water bodies through the Piper trilinear map and Schukalev classification. The results show that regardless of the type of water body in the study area, the hydrochemical type in the drainage basin is mainly HCO3-Ca, belonging to the "type of carbonate rock weathering". Due to the long interaction time between water and rock, the rock weathering in groundwater is more obvious than in surface water. Secondly, the analyses of ion sources in water bodies by Gibbs plot indicate the consistency in the hydrochemical composition, changes, and origin mechanisms between groundwater and surface water. The controlled mechanism belongs to the "leaching type of rock weathering". Thirdly, due to the widespread distribution of carbonate rocks in the study area, an analysis has been conducted on carbonate rock dissolution in order to distinguish the relative contribution of limestone and dolomite dissolution to the chemical ion compositions of water bodies in the watershed. It is found that there are additional sources of Mg2+ and Ca2+ in the surface water of the Lujiang river basin, while other acids participate in the chemical weathering reaction in the water bodies of the underground river points and spring points. Furthermore, it is determined that SO42- and NO3- are involved in the rock weathering in the basin, which has caused a large disturbance to the hydrochemical characteristics of water bodies in the basin. The Lujiang river basin is mixed with carbonate and silicate rocks. Therefore, the main sources of HCO3- in water are classified into three types: carbonate rocks weathered and dissolved by carbonic acid, carbonate rocks weathered by sulfuric acid/nitric acid, and silicate rocks weathered by carbonic acid. Based on the quantitative analysis by ion ratio method, carbonate rocks weathered by carbonic acid contribute average 68.8% to HCO3-, carbonate rocks weathered by sulfuric acid/nitric acid 27.2%, and silicate rock weathered by carbonic acid 3.9%. During the weathering process of water bodies in the Lujiang river basin, carbonate rocks are mainly weathered by carbonic acid. Finally, the verification by carbon isotope method has been compared with the measured values to show that the theoretical average values of underground river water and spring water δ13CDIC&lowbar;Th are relatively close, while the values of surface water are positive too, indicating that the source of DIC in surface water is not only controlled by rock weathering, but also by biogeochemical processes within river water bodies. In comparison, the areas of surface rivers are more significantly affected by human activities with severe urbanization and agricultural activities. [ABSTRACT FROM AUTHOR]

Using hydrochemical and isotopic compositions of springs and wells, we trace carbon from critical zone carbon dioxide (CO2) into groundwater of the semi-arid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Dissolved inorganic carbon (DIC) concentrations, pH and stable isotope tracers of carbon for DIC (δ13CDIC), are used to show that most groundwater evolves under open system conditions, moving carbon into the groundwater and acting as a carbon sink. However, one sample (−10.94‰ δ13CDIC, 6,350 14C years before present (yrs. BP)) may have evolved under closed system conditions with a higher partial pressure of critical zone CO2 than present-day soils. By characterizing the carbon cycle, we show that (1) carbon evolution is primarily under open-system conditions, (2) shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs. BP) than deeper groundwater samples (1,469 to 6,350 14C yrs. BP), and (3) the older portion of the groundwater may be even older than the calculated 14C ages, as indicated by the mixing of age tracers in intermediate wells. Our global conception of the deep critical zone should include carbon cycling of critical zone CO2 in old groundwater. Characterizing the deep critical zone in a semi-arid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling.

A three-dimensional (3-D) physical-biogeochemical-carbon cycle coupled model is used to investigate the interannual variability of the air-sea carbon dioxide (CO2) flux (FCO2) in the Northern Yellow Sea (NYS) from 2009 to 2018. The verification of the model indicate that the simulation results for multiple variables exhibit consistency and fit well with the observed data. The study show that although the multi-year average FCO2 in the NYS is close to the source-sink balance, there are obvious interannual differences between different years. In particular, a relatively strong source of atmospheric CO2 (1.0 mmol m–2 d–1) is exhibited in 2014, while a relatively strong sink of atmospheric CO2 (–0.7 mmol m–2 d–1) emerges in 2016. Mechanism analysis indicates that the abnormally high temperature is the main controlling factor for the relatively high CO2 efflux rate in the NYS in 2014, while the abnormally low dissolved inorganic carbon (DIC) concentration is the main factor contributing to the relatively high CO2 influx rate in 2016. Further analysis reveals that the primary reason for the low DIC concentration since the onset of winter in 2016 is the high net decrease rate of DIC in the NYS in 2015, influenced by net community production in the summer and advection processes during the autumn. The abnormally high primary production during the summer of 2015 results in the excessive reduction of DIC concentration through biological processes. In addition, due to the strong northeasterly wind event in November 2015, low-concentration-DIC water from the Yellow Sea (YS) extends into the Bohai Sea (BS). This further leads to higher DIC flux from the NYS into the BS in the upper mixed layer and increases the inflow of low-concentration-DIC water from the Southern Yellow Sea (SYS) into the NYS. These ultimately result in the abnormal reduction of DIC concentration in the upper mixed layer of the NYS during the autumn of 2015. This study enriches our understanding of interannual variability of FCO2 in the NYS, which will not only help to further reveal the variations of FCO2 under human activities and climate change, but also provide useful information for guiding the comprehensive assessment of the carbon budget.

The Western Tropical Atlantic Ocean (WTAO) is crucial for understanding CO2 dynamics due to inputs from major rivers (Amazon and Orinoco), substantial rainfall from the Intertropical Convergence Zone (ITCZ), and CO2-rich waters from equatorial upwelling. This study, spanning 1998 to 2018, utilized sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W to reconstruct the surface marine carbonate system. Empirical models derived total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS, with subsequent estimation of pH and fCO2 from TA, DIC, SSS, and SST data. Linear trend analysis showed statistically significant temporal trends: DIC and fCO2 increased at a rate of 0.7 µmol kg-1 year-1 and 1.539 µatm year-1, respectively, and pH decreased at a rate of -0.001 pH units year-1, although DIC did not show any trend after data was de-seasoned. Rainfall analysis revealed distinct dry (July to December) and wet (January to June) seasons, aligning with lower and higher freshwater influence on the ocean surface, respectively. TA, DIC, and pH correlated positively with SSS, exhibiting higher values during the dry season and lower values during the wet season. Conversely, fCO2 correlated positively with SST, showcasing higher values during the wet season and lower values during the dry season. This emphasizes the influential roles of SSS and SST variability in CO2 solubility within the region. Finally, we have analysed the difference between TA and DIC (TA-DIC) as an indicator for ocean acidification and found a decreasing trend of -0.93 ± 0.02 μmol kg-1 year-1, reinforcing the reduction in the surface ocean buffering capacity in this area. All trends found for the region agree with data from other stations in the tropical and subtropical Atlantic Ocean. In conclusion, the use of empirical models proposed in this study has proven to help filling the gaps in marine carbonate system data in the Western Tropical Atlantic.

Determination of the isotopic composition and concentration of dissolved inorganic carbon (DIC) in sea water requires not only high measurement accuracy, but also the development of unified approaches to data standardization and material collection. In this work, we studied the effect of sampling methods ("into containers" and "into vials with acid") with and without the addition of a toxin) on the results of determining the values of δ13С(DIC) and the concentration of DIC in sea water. The analytical protocol is described in detail, based on many years of experience in the sample collection, measurement and standardization of data obtained for a large number of water samples of the Arctic seas. According to the above protocol, the values of δ13С(DIC) and [DIC] can be determined with an error of less than 0.05‰ (1σ) and 4.5 rel % respectively. It has been shown that sampling "in a vial with acid" with their storage for 4 months is accompanied by significant contamination by atmospheric carbon dioxide with an underestimation of δ13С(DIC) values by an average of 0.3–0.8‰ and an overestimation of [DIC] values by an average of two times. The absence of a toxin that stops biological activity does not lead to significant shifts in the DIC concentration, but it strongly affects the δ13С(DIC) values, which become underestimated by an average of 1‰. Using sampling "in a container" and toxin addition, along with following other recommendations, the samples retain the isotope and concentration parameters of DIC throughout the year. [ABSTRACT FROM AUTHOR]

In coastal regions and marginal bodies of water, the increase in partial pressure of carbon dioxide (pCO2) in many instances is greater than that of the open ocean due to terrestrial (river, estuarine, and wetland) influences, decreasing buffering capacity and/or increasing water temperatures. Coastal oceans receive freshwater from rivers and groundwater as well as terrestrial-derived organic matter, both of which have a direct influence on coastal carbonate chemistry. The objective of this research is to determine if coastal marshes in Georgia, USA, may be "hot-spots" for acidification due to enhanced inorganic carbon sources and if there is terrestrial influence on offshore acidification in the South Atlantic Bight (SAB). The results of this study show that dissolved inorganic carbon (DIC) and total alkalinity (TA) are elevated in the marshes compared to predictions from conservative mixing of the freshwater and oceanic end-members, with accompanying pH around 7.2 to 7.6 within the marshes and aragonite saturation states (ΩAr) <1. In the marshes, there is a strong relationship between the terrestrial/estuarine-derived organic and inorganic carbon and acidification. Comparisons of pH, TA, and DIC to terrestrial organic material markers, however, show that there is little influence of terrestrial-derived organic matter on shelf acidification during this period in 2014. In addition, ΩAr increases rapidly offshore, especially in drier months (July). River stream flow during 2014 was anomalously low compared to climatological means; therefore, offshore influences from terrestrial carbon could also be decreased. The SAB shelf may not be strongly influenced by terrestrial inputs to acidification during drier than normal periods; conversely, shelf waters that are well-buffered against acidification may not play a significant role in mitigating acidification within the Georgia marshes. [ABSTRACT FROM AUTHOR]

Following the principle of oxygenic photosynthesis, electron transport in the thylakoid membranes (i.e., light reaction) generates ATP and NADPH from light energy, which is subsequently utilized for CO2 fixation in the Calvin-Benson-Bassham cycle (i.e., dark reaction). However, light and dark reactions could discord when an alternative electron flow occurs with a rate comparable to the linear electron flow. Here, we quantitatively monitored O2 and total dissolved inorganic carbon (DIC) during photosynthesis in the pennate diatom Phaeodactylum tricornutum, and found that evolved O2 was larger than the consumption of DIC, which was consistent with 14CO2 measurements in literature. In our measurements, the stoichiometry of O2 evolution to DIC consumption was always around 1.5 during photosynthesis at different DIC concentrations. The same stoichiometry was observed in the cells grown under different CO2 concentrations and nitrogen sources except for the nitrogen-starved cells showing O2 evolution 2.5 times larger than DIC consumption. An inhibitor to nitrogen assimilation did not affect the extra O2 evolution. Further, the same physiological phenomenon was observed in the centric diatom Thalassiosira pseudonana. Based on the present dataset, we propose that the marine diatoms possess the metabolic pathway(s) functioning as the O2-independent electron sink under steady state photosynthesis that reaches nearly half of electron flux of the Calvin-Benson-Bassham cycle. [ABSTRACT FROM AUTHOR]

Caulerpa is a widely distributed genus of chlorophytes (green macroalgae) which are important for their dietary, social and coastal ecosystem value. Ocean acidification (OA) threatens the future of marine ecosystems, favouring macroalgal species that could benefit from increased seawater carbon dioxide (CO2) concentrations. Most macroalgae species possess CO2 concentrating mechanisms (CCMs) that allow active uptake of bicarbonate (HCO3−). Those species without CCMs are restricted to using CO2, which is currently the least abundant species of dissolved inorganic carbon (DIC) in seawater. Thus, macroalgae without CCMs are predicted to be likely benefit from OA. Caulerpa is one of the rare few genera that have species both with and without CCMs. The two most common Caulerpa species in New Zealand are C. geminata (possesses a CCM) and C. brownii (non-CCM). We investigated the responses of growth, photo-physiology and DIC utilisation of C. geminata and C. brownii to four mean seawater pH treatments (8.03, 7.93, 7.83 and 7.63) that correspond to changes in pH driven by increases in pCO2 simulating future OA. There was a tendency for the mean growth rates for C. brownii (non-CCM) to increase under lower pH, and the growth rates of C. geminata (CCM) to decline with lower pH, although this was not statistically significant. However, this is likely because variability in growth rates also increased as seawater pH declined. There were few other differences in physiology of both species with pH, although there was tendency for greater preference for CO2 over HCO3− uptake in the CCM species with declining seawater pH. This study demonstrates that DIC-use alone does not predict macroalgal responses to OA. [ABSTRACT FROM AUTHOR]

Saline lakes across the globe have experienced severe reduction in their surface area as a result of climate change and human-induced perturbations like water diversion and extraction. The changing lake volume is predicted to have large-scale implication on the in-lake biogeochemistry. This study explores the carbon (C) and nitrogen (N) cycling in a desiccating hypersaline lake (Sambhar Lake, India) along with adjacently located brine reservoir and salt pans by measuring concentrations and stable isotopic ratios of different C and N pools during winter and monsoon. Incubation experiments to estimate the net nitrification and mineralization rates in lake sediments were also performed. The Lake witnessed a large decrease in surface area and showed a clear signature of desiccation on lake biogeochemistry. Both particulate and dissolved fractions of C and N in the lake increased as the lake desiccated from monsoon to winter. Low N isotopic composition (δ15N) of particulate organic matter during winter suggested the presence of N2 fixers in this nutrient-rich saline environment. Taken together, significant difference in C and N concentrations and isotopic compositions were observed across the lake, brine reservoir, and salt pans, suggesting considerable modulation of in-lake processes due to human interventions. [ABSTRACT FROM AUTHOR]

The subsurface layer of the Canada Basin contains large reservoirs of dissolved inorganic nitrogen and carbon (DIN and DIC, respectively). Under rapid change of Arctic climate system, these reservoirs may not only serve as potential material bases for primary production, but also for carbon source and sink. However, the long‐term trends in reservoirs and the interaction between reservoirs and the upper ocean are not fully understood. This study used two data sets to evaluate the long‐term trends in the DIN and DIC reservoirs in the subsurface layer of the Canada Basin during 1990–2015. The results indicate an increase of ∼35% in the subsurface DIN reservoir over 25 years, whereas the DIN concentration remained fairly constant. The discrepancy between the DIN reservoir and DIN concentration was primarily owing to the expansion of the subsurface layer, which increased the DIN stock but maintained a relatively stable DIN concentration. Additionally, the ΔDIC/ΔDIN ratio in the subsurface layer increased by ∼33% over 25 years, primarily due to denitrification on the pathway of Pacific inflow, such as Chukchi shelf. In the Canada Basin, the enhanced mixing between the subsurface and surface layers may promote primary production and render the subsurface layer a carbon source. Consequently, when studying the biogeochemical cycles in the changing Arctic Ocean, the long‐term interactions between the subsurface and surface layers in the Canada Basin should be further considered. Plain Language Summary: Nutrients in the subsurface layer of the Canada Basin are potential material bases for primary production. Rapid climate change and enhanced mixing between the subsurface and surface layers impact the biogeochemical cycles in the Canada Basin. Two data sets were collected to determine the dissolved inorganic nitrogen and carbon (DIN and DIC, respectively) concentrations, DIN reservoir, and ΔDIC/ΔDIN ratio in the subsurface layer from 1990 to 2015. The results revealed that the subsurface DIN concentration remained constant, whereas the DIN and DIC reservoirs and ΔDIC/ΔDIN ratio dramatically increased. The increased Pacific inflow and expansion of subsurface layer may have led to an increase in the subsurface DIN and DIC stocks in the Canada Basin. Simultaneously, denitrification on the Chukchi shelf may have slowed the increase in the DIN reservoir, leading to an increase in the ΔDIC/ΔDIN ratio. Increased regional events such as upwellings, eddies, and storms can enhance the mixing of the upper layers (subsurface and surface layers) and thus may promote primary production. However, the increased ΔDIC/ΔDIN ratio implies that the mixing of the upper layers also supplies DIC upwardly, which might negatively impact the air‐sea CO2 flux in the Canada Basin and reduce carbon uptake from the atmosphere. Key Points: The subsurface dissolved inorganic nitrogen (DIN) reservoir in the Canada Basin increased by ∼35% during 1990–2015The subsurface DIN concentration in the Canada Basin remained fairly constant during 1990–2015The ratio of subsurface delta dissolved inorganic carbon (DIC) to delta DIN (ΔDIC/ΔDIN) increased by ∼33% during 1990–2015 [ABSTRACT FROM AUTHOR]

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m−2d−1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m−2d−1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets.

Land use in karst areas affects soil properties, impacting carbon sinks. Accurate estimation of carbon sink flux in karst areas through zoning and classification is crucial for understanding global carbon cycling and climate change. The peak cluster depression is the largest continuous karst landform region in southern China, with the depressions primarily covered by farmland and influenced by agricultural activities. This study focused on the Guancun Underground River Basin, a typical peak cluster depression basin, where sampling and analysis were conducted during the agricultural period of 2021–2022. Using hydrochemical analysis and isotopic methods, the results indicated that: (1) The primary hydrochemical type in the Guancun Underground River Basin is HCO3-Ca, with hydrochemical composition mainly controlled by carbonate rock weathering. (2) The primary sources of Cl−, SO42−, and NO3− are agricultural activities, with agriculture contributing 0.68 mmol/L to dissolved inorganic carbon (DIC), accounting for about 13.86%, as confirmed by ion concentration analysis and isotope verification. (3) The size of the depression area is proportional to the contribution of agricultural activities to DIC, while also being influenced by dilution effects. A comparison was made regarding the contribution of other land use types to DIC. The impact of land use on DIC in karst processes should not be overlooked, and zoning and classification assessments of carbon sink flux under different influencing factors contribute to carbon peaking and carbon neutrality goals.

The production of metabolites in microalgae is influenced by extreme cultivation conditions. Bicarbonate is an inorganic carbon source for phototrophic microalgae culturing. In this study, the effect of dissolved inorganic carbon (DIC) (0.4–13 g L−1) on the accumulation of metabolites in Dunaliella tertiolecta biomass is presented. The highest levels of primary metabolites [lipid (239.6 ± 24.3 mg g−1), protein (336.2 ± 47.5 mg g−1)], secondary metabolites [total phenolic (12.8 ± 2.0 mg g−1), total flavonoid (14.4 ± 2.3 mg g−1), total ascorbate (4.7 ± 1.1 mg g−1)], and pigments [chlorophyll (27.2 ± 3.1 mg g−1), carotenoid (2.0 ± 0.1 mg g−1)] were observed when cells were grown with 5.7 g L−1 of DIC (NaHCO3). The highest biomass concentrations (1.5 ± 0.1 g L−1) were obtained for cells grown in a mixture of DIC (4.3 + 1.1 g L−1, NaHCO3 + Na2CO3). This study recommends the optimal levels of bicarbonate carbon of 5.7 g L−1 for maximising the generation of metabolites in the biomass. It also demonstrates that exogenous excessive DIC in the growth medium would be an effective stressor to produce high-value metabolites in Dunaliella or alkali-halophilic strains. [ABSTRACT FROM AUTHOR]

Dissolved inorganic carbon (DIC) represents an important but poorly constrained form of lateral carbon flux to the oceans. With high precipitation rates, large glaciers, and dense temperate rainforest, Southeast Alaska plays a critical role in the transport of carbon to the Gulf of Alaska (GOA). Previous estimates of DIC flux across the Southeast Alaska Drainage Basin (SEAKDB) are poorly constrained in space and time. Our goal was to incorporate recent measurements of DIC concentrations with previous measurements from the U.S. Geological Survey in order to model the spatial and temporal patterns of riverine DIC transport from SEAK to the GOA. We aggregated DIC concentration measurements from 1957 to 2020 and associated measurements of mean daily discharge. We then constructed load estimation models to generate concentration predictions across 24 watersheds. By spatially matching measurements of DIC with SEAKDB watersheds, we extrapolated concentration predictions across 2,455 watersheds encompassing approximately 190,000 km2. Models were aggregated according to two factors, the presence of karst and the discharge regime. Finally, monthly flux predictions were generated for each watershed using predicted concentrations and runoff estimates from the Distributed Climate Water Balance Model. Mean annual DIC flux from the SEAKDB was 2.36 Tg C with an average yield of 12.52 g C m−2. Both karst presence and flow regimes modified DIC flux and speciation across coastal marine areas. The high resolution of DIC flux estimates will provide useful inputs for describing seasonal C dynamics, and further refines our understanding of C budgets in the Pacific temperate rainforest and the surrounding marine environment. Plain Language Summary: Understanding how carbon moves through ecosystems is critical in a changing climate. Dissolved carbon in aquatic environments plays a critical role in driving large‐scale processes such as ocean acidification, which represents a threat to many marine ecosystems. Despite the importance of understanding and accounting for carbon as it moves through the environment, the transfer of dissolved inorganic carbon (DIC) (such as carbon dioxide) from the terrestrial environment to the marine environment is often overlooked. Streams and rivers transfer carbon from land to ocean and represent a significant source of carbon to the marine environment, especially in areas that have large amounts of freshwater discharge such as Southeast Alaska. In this study, we created a model which generates predictions for how much DIC is entering the marine environment of Southeast Alaska. For each of 2,455 watersheds identified in this region we calculated monthly flux estimates which we grouped into large marine zones. Our overall flux estimate agrees well with previous estimates, but here our model provides more highly resolved spatial and temporal flux values which reveals seasonal and geographic patterns of DIC transfer from rivers to the marine environment. Key Points: Lateral flux of dissolved inorganic carbon (DIC) is not well resolved in time and space in Southeast AlaskaWe present robust models for DIC flux on a watershed basis that are spatially and temporally explicitCalculated DIC flux from Southeast Alaska watersheds is heavily influenced by streamflow regime and karst presence [ABSTRACT FROM AUTHOR]

The eutrophication of lakes is a global environmental problem. Regulating nitrogen (N) and phosphorus (P) on phytoplankton is considered to be the most important basis of lake eutrophication management. Therefore, the effects of dissolved inorganic carbon (DIC) on phytoplankton and its role in mitigating lake eutrophication have often been overlooked. In this study, the relationships between phytoplankton and DIC concentrations, carbon isotopic composition, nutrients (N and P), and hydrochemistry in the Erhai Lake (a karst lake) were investigated. The results showed that when the dissolved carbon dioxide (CO 2(aq)) concentrations in the water were higher than 15 µmol/L, the productivity of phytoplankton was controlled by the concentrations of TP and TN, especially by that of TP. When the N and P were sufficient and the CO 2(aq) concentrations were lower than 15 µmol/L, the phytoplankton productivity was controlled by the concentrations of TP and DIC, especially by that of DIC. Additionally, DIC significantly affected the composition of the phytoplankton community in the lake (p <0.05). When the CO 2(aq) concentrations were higher than 15 µmol/L, the relative abundance of Bacillariophyta and Chlorophyta was much higher than those of harmful Cyanophyta. Thus, high concentrations of CO 2(aq) can inhibit harmful Cyanophyta blooms. During lake eutrophication, when controlling N and P, an appropriate increase in CO 2(aq) concentrations by land-use changes or pumping of industrial CO 2 into water may reduce the proportion of harmful Cyanophyta and promote the growth of Chlorophyta and Bacillariophyta , which may provide effectively assist in mitigating water quality deterioration in surface waters. [ABSTRACT FROM AUTHOR]

Rivers act as a source of CO2 to the atmosphere and some of the implied inorganic carbon comes from the aquifer-river connectivity through groundwater discharges to surface water. This study aims to quantify groundwater discharge entering the stream and to estimate this external input to the riverine inorganic carbon cycle, as both dissolved inorganic carbon (DIC) and CO2 in the Matane River (Qc, Canada). Two approaches based on radon (222Rn) mass balance models, DIC, total alkalinity (TA), pH and PCO2 measurements were developed to quantify groundwater discharges and associated DIC and CO2 fluxes at a high- (< 1 km) and low- (> 1 km) resolution scales. Groundwater discharges were heterogeneous along the riverbed with mean linear inputs varying from 3.1 to 51.9 m3·day−1·m−1 depending on the scale. The associated fluxes of DIC ranged between 7.4 and 132.9 mol·day−1·m−1 and corresponded to only less than 13% of the total DIC transported by the river. Regarding CO2, however, the contribution of groundwater to CO2 emission fluxes reached 81% to 287% of the river's internal CO2 production. Exceeding groundwater-derived CO2 flux compared to the total CO2 flux from the river probably highlights the instantaneous degassing of CO2 as soon as groundwater discharges to the surface water. These results shed light on the key role of groundwater in the riverine inorganic carbon cycle in a subarctic region, and specifically in the CO2 evasion to the atmosphere. Such quantifications are particularly important in northern systems where important changes in hydroclimatic conditions and terrestrial carbon storage are undergoing and are expected to continue to undergo. [ABSTRACT FROM AUTHOR]

The boom in dam construction and continuous river‐lake exchange has had a profound impact on the transmission and transformation of riverine dissolved inorganic carbon (DIC). An in‐depth understanding of the change mechanisms of DIC concentrations and sources driven by dam operation and lake recharge is crucial for regulating greenhouse gas emissions and evaluating the impact of DIC on the global carbon cycle. This study investigated dam‐ and lakes‐driven DIC via the concentration and δ13C of DIC, combined with anions, cations, δD and δ18O in the main stream of the Yangtze River. DIC showed a decreasing trend from upper reach (2,262.31 ± 113.69 μmol kg−1) to lower reach (1,771.61 ± 89.36 μmol kg−1). Carbonate dissolution proportion (from 36.45% to 28.44%) and atmospheric CO2 proportion (from 37.51% to 22.94%) of DIC decreased from upper reach to lower reach, whereas soil CO2 proportion of DIC (from 26.01% to 48.63%) increased. The control of dam operation on DIC biogeochemical process was revealed from different time scales. From the perspective of short‐term seasonal changes (from 2020 to 2021), the mineralization of organic matter in the dry season strengthened CO2 degassing and calcite precipitation, reducing the DIC and increasing the proportion of soil CO2. Meanwhile, longer periods of runoff retention provided sufficient time for water–rock reactions in the wet season and increased the DIC and carbonate dissolution source in the reservoir area. On a long‐term scale (from 2009 to 2021), a decrease in pH driven by sediment mineralization contributed to an annual increase in DIC in the reservoir. The flow of lakes mixed into the mainstream was revealed by the enrichment of δ18O, and river‐communicating recharge decreased the DIC and carbonate dissolution source. We show that dam operation and lake inflow change DIC concentrations and sources and therefore need to be considered in the transmission and transformation processes of DIC in the river‐ocean continuum. [ABSTRACT FROM AUTHOR]

Quantifying the flux of dissolved inorganic carbon (DIC) from the Chukchi shelf to the deep ocean (i.e., the continental shelf pump) is of great significance for understanding the carbon cycle and ocean acidification in the changing Arctic Ocean. Using a coupled ocean‐sea ice‐biogeochemical model, this study shows that the Pacific inflow drives the highly efficient continental shelf pump in the Chukchi Sea, with a multi‐year average of 828 Tg C‐DIC exported to deep basins during 1998–2015. The Pacific annual inflow carries 385 Tg C‐DIC to the Chukchi Sea in summertime (May–September) and 436 Tg C in wintertime (October–April). Downstream, DIC is mainly exported via Barrow Canyon (BC) (266 Tg C, 64%) in summer and along the shelf under the 100 m isobath (217 Tg C, 53%) in winter. This variability corresponds to the seasonal variation of local winds. The stronger winter northeasterly winds hinder the outflow through BC. In both seasons, interannual variability exists in the proportion of DIC exports through different pathways. The fraction of DIC export via BC has a significant correlation (r > 0.8) with the along‐canyon wind speed. This is further related to the sea level pressure (SLP) gradient between the Arctic and North Pacific. A weaker SLP gradient leads to weaker local northeasterly wind, and thus favors a higher proportion of DIC export via BC and lower DIC export through other pathways. This process plays a key role in the redistribution of shelf‐sourced DIC in deep basins of the Arctic Ocean. Plain Language Summary: The Chukchi Sea exports acidified water to the western Arctic Ocean Basin, where there is undergoing unprecedented ocean acidification in recent decades. Quantifying the dissolved inorganic carbon (DIC) flux from the Chukchi shelf to deep basins is of great significance for understanding the carbon cycle and ocean acidification in the changing Arctic Ocean. This study reveals that the Pacific inflow determines the annual DIC flux from the Chukchi Sea to deep basins by analyzing a hindcast simulation of a three‐dimensional ocean–sea ice–biogeochemical model. DIC is mainly exported via Barrow Canyon (BC) in summer and through the 100 m isobath pathway in winter. The year‐to‐year variations of proportions of DIC export through different pathways are regulated by the along‐canyon wind speeds in both summer and winter. The local wind is associated with the sea level pressure (SLP) gradient between the Arctic and North Pacific, in which the weaker SLP gradient leads to weaker northeasterly wind and subsequently favors a higher proportion of DIC export via BC. Thus, the change in atmospheric conditions will adjust the redistribution of DIC in the western Arctic Ocean Basin by affecting the ocean circulation structure. Key Points: Pacific inflow drives the continental shelf pump for dissolved inorganic carbon (DIC) (828 Tg C yr−1) exported from the Chukchi Sea to deep basinsIn summer, DIC is mainly exported via Barrow Canyon (BC) (266 Tg C) while through 100 m isobath pathway (217 Tg C) in winterInterannually, local winds regulate the proportion of DIC exports between BC and other pathways [ABSTRACT FROM AUTHOR]

Karst ecological stresses are harmful to plant growth, especially high bicarbonate concentrations, drought, high pH, etc. In this study, the effects of 0, 2.0, 7.0 and 12.0 mmol L−1 sodium bicarbonate concentrations on the biomass, electrophysiological properties, intracellular water metabolism, nutrient transport, photosynthesis and chlorophyll fluorescence of Coix lacryma-jobi L. were investigated. The results show that 2.0 mmol L−1 sodium bicarbonate effectively improved the biomass formation of Coix lacryma-jobi L., notably increased its intrinsic capacitance (IC) and decreased its intrinsic resistance (IR), intrinsic impedance (IZ), intrinsic capacitive reactance (IXc) and intrinsic inductive reactance (IXL) as well as reliably enhanced its intracellular water metabolism, nutrient transport and photosynthetic capacities. However, 7.0 and 12.0 mmol L−1 sodium bicarbonate concentrations exhibited marked inhibitory effects on the plants' photosynthetic rate, stomatal conductance, transpiration rate and dry weight, whereas they did not significantly change the intracellular water metabolism or the nutrient transport capacity of Coix lacryma-jobi L. This study highlights that appropriate bicarbonate levels could enhance the intracellular water metabolism, nutrient transport, photosynthesis and growth of Coix lacryma-jobi L., which can be rapidly monitored by the plant's electrophysiological properties. Importantly, plant electrophysiological measurement is significantly superior to photosynthesis measurement. In the future, plant electrophysiological measurement can be used as a means to quickly and effectively evaluate the physiological response of plants to the external environment. [ABSTRACT FROM AUTHOR]

In deep karst reservoirs, the internal environment is complex, and thermal stratification isnot the only factor controlling the vertical distribution of the DIC concentration. Previous studies have not fully understood the migration and transformation of DIC in a deep-water reservoir. In this study, a deep-water reservoir in southwest China was chosen, and the spatial and temporal characteristics of the DIC concentration, pCO2, δ13CDIC value, and SIc were investigated. It was found that the Pingzhai Reservoir is a double temperature leapfrog reservoir. The DIC concentration, pCO2, Sic, and δ13CDICvalues showed annual cycle variation. During the thermal stratification phase, the DIC concentration, pCO2, Sic, and δ13CDICvalues were significantly different between the surface layer and the lower layer. However, during the mixing and mixed phases, the differences were not significant. The vertical divergence of the DIC in the Pingzhai Reservoir was influenced by the subtemperate layer, human activities, and sources. The formation of the subtemperate layer was due to the submerged flow formed when river water enteredthe reservoir, which provides a channel for DIC from the river to enter the lower layer of the reservoir. Human activities increased the solubility of carbonate rocks in the reservoir, and the source of DIC was one of the factors contributing to the concentration stratification of DIC in the reservoir. [ABSTRACT FROM AUTHOR]

Volcanic activities are known to release a substantial amount of deep carbon to the Earth's surface, thereby influencing the global carbon cycle on geological timescales. It is crucial to clarify the contribution of deep carbon and its changes under hydrological variations in rivers draining volcanic areas. This study reported the temporal variations of chemical compositions and isotopic compositions (δ13C and Δ14C) of dissolved inorganic carbon (DIC) from the Erdaobai River, which drains the Changbaishan volcanic area. The results showed that hydrothermal fluids had a considerable impact on the chemical and carbon isotopic compositions in this study area. The chemical mass balance model indicated that hydrothermal fluids composed 4.6 ± 0.7% of the water budget for the Erdaobai River. Additionally, the isotopic mixing model of Δ14CDIC showed that hydrothermal DIC accounted for 83.6 ± 1.9% and 40.8 ± 3.7% of the DIC budget for the upstream and downstream, respectively, and the proportion of hydrothermal contribution also varied in a narrow range in response to hydrological changes. However, the DIC and dissolved organic carbon fluxes showed significant seasonal variations, verifying the controlling role of hydrological processes. This study highlighted the impact of hydrological processes and deep carbon release on the carbon budget in the volcanic catchment, which is of great significance for understanding atmospheric CO2 balance and climate change. [ABSTRACT FROM AUTHOR]

The northwest Atlantic Ocean is an important sink for carbon dioxide produced by anthropogenic activities. However the strong seasonal variability in the surface waters paired with the sparse and summer biased observations of ocean carbon makes it difficult to capture a full picture of its temporal variations throughout the water column. We aim to improve the estimation of temporal trends of dissolved inorganic carbon (DIC) due to anthropogenic sources using a new statistical approach: a time series generalization of the extended multiple linear regression (eMLR) method. Anthropogenic increase of northwest Atlantic DIC in the surface waters is hard to quantify due to the strong, natural seasonal variations of DIC. We address this by separating DIC into its seasonal, natural and anthropogenic components. Ocean carbon data is often collected in the summer, creating a summer bias, however using monthly averaged data made our results less susceptible to the strong summer bias in the available data. Variations in waters below 1000m have usually been analyzed on decadal time scales, but our monthly analysis showed the anthropogenic carbon component had a sudden change in 2000 from stationary to an increasing trend at the same rate as the waters above. All depths layers had similar rates of anthropogenic increase of ∼0.57µmol kg−1 year−1, and our uncertainty levels are smaller than with eMLR results. Integration throughout the water column (0–3,500 m) gives an anthropogenic carbon storage rate of 1.37 ± 0.57 mol m−2 year−1, which is consistent with other published estimates. Plain Language Summary: We need to measure the ocean sink for the CO2 emitted by industrialized societies, and it is particularly important for the northwest Atlantic Ocean. The rate of carbon increase is often overshadowed by natural and seasonal variability. We introduce new statistical approaches to better estimate the rate of anthropogenic carbon that has accumulated due to human activities. Ocean carbon data is often collected in the summer, creating a summer bias, however using monthly averaged data made our results less susceptible to the strong summer bias in the available data. From 1993 to 2015 in the northwest Atlantic Ocean, anthropogenic carbon increased at ∼0.57 μmol kg−1 year−1 within all depth‐layers. Integration of results throughout the water column (0–3,500 m) gives an anthropogenic carbon storage rate of 1.37 ± 0.57 mol m−2 year−1. Key Points: A time series generalization of the extended multiple linear regression (eMLR) method is developed to produce monthly estimates with uncertainties of anthropogenic ocean carbonThe rate of anthropogenic carbon increase in northwest Atlantic is roughly the same for all depth layers, at 0.57 µmol/kg/yearOur method produces estimates of anthropogenic carbon increase that are comparable to those from eMLR, but with smaller uncertainties [ABSTRACT FROM AUTHOR]

This study combines measurements of dissolved inorganic carbon (DIC), total alkalinity (TA), pH, earth observation (EO), and ocean model products with deep learning to provide a good step forward in detecting changes in the ocean carbonate system parameters at a high spatial and temporal resolution in the North Atlantic region (Long. −61.00° to −50.04° W; Lat. 24.99° to 34.96° N). The in situ reference dataset that was used for this study provided discrete underway measurements of DIC, TA, and pH collected by M/V Equinox in the North Atlantic Ocean. A unique list of co-temporal and co-located global daily environmental drivers derived from independent sources (using satellite remote sensing, model reanalyses, empirical algorithms, and depth soundings) were collected for this study at the highest possible spatial resolution (0.04° × 0.04°). The resulting ANN-estimated DIC, TA, and pH obtained by deep learning shows a high correspondence when verified against observations. This study demonstrates how a select number of geophysical information derived from EO and model reanalysis data can be used to estimate and understand the spatiotemporal variability of the oceanic carbonate system at a high spatiotemporal resolution. Further methodological improvements are being suggested. [ABSTRACT FROM AUTHOR]

The present study reviewed the carbon-biogeochemistry-related observations concerning CO2 and CH4 dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO2 and CH4 [pCO2(water) and pCH4(water)] and air–water CO2 and CH4 fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO2 emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in pCO2(water) and pCH4(water), enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated pCO2(water) and pCH4(water) and their fluxes. Higher chlorophyll-a concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH4 in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the pCO2(water) and water-to-air CO2 fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future. [ABSTRACT FROM AUTHOR]

The Southern Ocean plays a critical role in the uptake, transport, and storage of carbon by the global oceans. It is the ocean's largest sink of CO2, yet it is also among the regions with the lowest storage of anthropogenic carbon. This behavior results from a unique combination of high winds driving the upwelling of deep waters and the subduction and northward transport of surface carbon. Here we isolate the direct effect of increasing anthropogenic CO2 in the atmosphere from the indirect effect of climate variability and climate change on the reorganization of carbon in the Southern Ocean interior using a combination of modeling and observations. We show that the effect of climate variability and climate change on the storage of carbon in the Southern Ocean is nearly as large as the effect of anthropogenic CO2 during the period 1998–2018 compared with the climatology around the year 1995. We identify a distinct climate fingerprint in dissolved inorganic carbon (DIC), with elevated DIC concentration in the ocean at 300–600 m that reinforces the anthropogenic CO2 signal, and reduced DIC concentration in the ocean around 2,000 m that offsets the anthropogenic CO2 signal. The fingerprint is strongest at lower latitudes (30°–55°S). This fingerprint could serve to monitor the highly uncertain evolution of carbon within this critical ocean basin, and better identify its drivers. Key Points: The effect of decadal climate variability on dissolved inorganic carbon (DIC) in the Southern Ocean is nearly as large as that of atmospheric CO2Climatic drivers cause a distinct fingerprint on the change in DIC concentration in the Southern Ocean interiorThis fingerprint could serve to detect future trends in Southern Ocean carbon storage [ABSTRACT FROM AUTHOR]

In a future ocean, dissolved organic carbon (DOC) release by seaweed has been considered a pathway for organic carbon that is not incorporated into growth under carbon dioxide (CO2) enrichment/ocean acidification (OA). To understand the influence of OA on seaweed DOC release, a 21- day experiment compared the physiological responses of three seaweed species, two which operate CO2 concentrating mechanisms (CCMs), Ecklonia radiata (C. Agardh) J. Agardh and Lenormandia marginata (Hooker F. and Harvey) and one that only uses CO2 (non-CCM), Plocamium cirrhosum (Turner) M.J. Wynne. These two groups (CCM and non-CCM) are predicted to respond differently to OA dependent on their affinities for Ci (defined as CO2 + bicarbonate, HCO3 −). Future ocean CO2 treatment did not drive changes to seaweed physiology—growth, Ci uptake, DOC production, photosynthesis, respiration, pigments, % tissue carbon, nitrogen, and C:N ratios—for any species, regardless of Ci uptake method. Our results further showed that Ci uptake method did not influence DOC release rates under OA. Our results show no benefit of elevated CO2 concentrations on the physiologies of the three species under OA and suggest that in a future ocean, photosynthetic CO2 fixation rates of these seaweeds will not increase with Ci concentration. [ABSTRACT FROM AUTHOR]