Dissolved Inorganic Carbon Concentrations Research Articles

Carbon Pools Differ in Source and Temporal Patterns in a Tidal Marsh Creek System of the York River, VA Estuary

Estuarine ecosystems, including tidal marsh creek systems, are hotspots of carbon cycling. Despite their biogeochemical importance, the magnitude and timescales at which carbon concentrations vary in these environments remain poorly characterized. To address this, our study identified temporal patterns in carbon pools in a tidal marsh creek with the goal of identifying their primary drivers. Specifically, we measured concentrations of particulate and dissolved organic carbon (POC, DOC), dissolved inorganic carbon (DIC), water quality parameters, and organic matter source proxies (δ13C, δ15N, and colored dissolved organic matter (CDOM)). We collected samples every 2.5 h over a tidal cycle monthly (October 2013–January 2015, intermittently thereafter). We characterized organic matter composition from marsh and estuarine endmembers using samples collected at low and high tides respectively. POC concentrations were highest at high tide (4.21 ± 0.40 mg/L), whereas DIC concentrations were highest at low tide (24.82 ± 0.74 mg/L), suggesting that the marsh was a sink for POC and source of DIC. Temporally, DIC concentrations were highest in fall (23.19 ± 1.30 mg/L); DOC and POC varied little across seasons. We used linear mixed effect models to identify potential processes that explained these temporal patterns. Suspended solids and chlorophyll-a were the best predictors of POC concentrations, while CDOM was the best predictor of DOC, and percent freshwater was the best predictor of DIC (p < 0.001). Results from this study highlight the importance of measuring multiple carbon pools simultaneously, as each pool is influenced by different sources and processes and responds differently to drivers that change at tidal and seasonal timescales.

Ocean Circulation Drives the Variability of the Carbon System in the Eastern Tropical Atlantic

The carbon system in the eastern tropical Atlantic remains poorly known. The variability and drivers of the carbon system are assessed using surface dissolved inorganic carbon (DIC), alkalinity (TA) and fugacity of CO2 (fCO2) measured in the 12° N–12° S, 12° W–12° E region from 2005 to 2019. A relationship linking DIC to temperature, salinity and year has been determined, with salinity being the strongest predictor. The seasonal variations of DIC, ranging from 80 to 120 μμmol kg−1, are more important than the year-to-year variability that is less than 50 μμmol kg−1 over the 2010–2019 period. DIC and TA concentrations are lower in the northern part of the basin where surface waters are fresher and warmer. Carbon supply dominates over biological carbon uptake during the productive upwelling period from July to September. The lowest DIC and TA are located in the Congo plume. The influence of the Congo is still observed at the mooring at 6° S, 8° E as shown by large salinity and chlorophyll variations. Nevertheless, this site is a source of CO2 emissions into the atmosphere.

Deepening roots can enhance carbonate weathering by amplifying CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-rich recharge

Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century timescale. Plant roots accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports vertical downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are regulated by soil pCO2 driven by the seasonal soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating recharge (or vertical water fluxes) into the deeper carbonate zone and export reaction products at dissolution equilibrium. The numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data suggest relatively similar soil CO2 distribution over depth, which in woodlands and grasslands leads only to 1 % to ∼ 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to 200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots at all, underscoring the essential role of roots in general. Numerical experiments also indicate that weathering fronts in woodlands propagated &gt; 2 times deeper compared to grasslands after 300 years at an infiltration rate of 0.37 m/a. These differences in weathering fronts are ultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promoting recharge and CO2–carbonate contact in the deep subsurface and (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for colocated characterizations of roots, subsurface structure, and soil CO2 levels, as well as their linkage to water and water chemistry. These measurements will be essential to illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

Spatiotemporal variations in water dissolved organic carbon and dissolved inorganic carbon concentrations in Wenwusha Reservoir in subtropical estuary, Southeast China

沿海水库汇聚并埋藏着大量的碳，是全球碳循环的重要区域.水体溶解有机碳（DOC）和溶解无机碳（DIC）的生物地球化学行为是水库碳循环研究的重要组成部分，对其系统生物过程和生态环境变化具有重要的影响.为了解亚热带河口区文武砂水库表层水体DOC和DIC的时空分布特征，本研究于2018年11月、2019年3月和6月分别对库区表层水进行多空间点位采样分析.结果表明，研究期间，文武砂水库表层水体DOC和DIC浓度变化范围分别介于0.10～21.13和0.38～34.94 mg/L，其均值分别为（4.09±0.18）和（15.83±0.24） mg/L，最大值分别出现在夏季和秋季；空间分布趋势，整体表现为北库区>南库区，且由库区周边的浅水区向库区中心的深水区域呈现出递减的趋势；浮游植物光合作用强度是影响库区DOC和DIC浓度的季节变化的重要因素，而外源污染输入强度是引起库区DOC和DIC浓度空间变化的重要因素.本文结果扩展对水库碳循环的认知，对提高水库水质也提供了有益参考.;Coastal reservoirs receive and accumulate a large amount of carbon; therefore, they are important areas for global carbon cycling. The biogeochemical cycles of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) are important components of coastal reservior carbon cycle, with significant effects on biological process and environmental change. This study was conducted to analyze the spatiotemporal variation in DOC and DIC concentrations at Wenwusha Reservoir, a subtropical coastal reservoir in Southeast China, based on the high spatial resolution filed sampling in November 2018 (autumn), March 2019 (spring), and June 2019 (summer). The results showed that the DOC and DIC concentrations of the surface water in Wenwusha Reservoir during the research period changed in the ranges of 0.10-21.13 mg/L, 0.38-34.94 mg/L, with mean values of 4.09±0.18 mg/L and 15.83±0.24 mg/L, respectively. Temporally, concentrations of DOC and DIC in the reservoir showed the highest values in summer and autumn, respectively. Spatially, DOC and DIC concentrations in the northern part of the reservoir were higher than those in the southern part. Furthermore, the DOC and DIC concentrations decreased along a gradient from shallow water (<1 m) to deeper water (>3 m) in the reservoir. The seasonal variations in DOC and DIC concentrations were mainly influenced by phytoplankton photosynthesis, while the spatial changes of DOC and DIC concentrations were primarily affected by exogenous input (e.g., sewage discharge and river flow). The results improve the understanding of carbon cycling in aquatic ecosystem and also be helpful for improving water quality in reservoirs.

Hydrogeochemical characterization of a possible carbon sink from shallow saline-alkaline groundwater in the eastern Hetao Basin of Inner Mongolia in China.

The question of how saline-alkaline groundwater can be used as a CO2 sink in arid saline-alkaline areas remains controversial. This study investigates the role of saline-alkaline groundwater as a CO2 sink using a mass balance method, Gibbs diagrams of the hydrochemistry, and carbon isotope (δ13CDIC) measurements. Twenty-eight groundwater samples of varying electrical conductivity (EC; 1.52-52.34 mS cm-1) were collected at different depths (1-2 and 5-25 m) in the Hetao Basin of Inner Mongolia, China. The results showed that groundwater ions could be primarily concentrated from water-rock interactions and evaporation, and that there are two main groundwater types: a mixed Na·Ca·Mg-Cl·SO4·HCO3 type and a Na-Cl type. The dissolved inorganic carbon (DIC) concentration in samples obtained from a depth of 1-2 m was less than that in samples from 5-25 m, and a downward migration trend of DIC in the groundwater was observed. The DIC concentration exhibited a significant positive correlation with pH (R2 = 0.61, p < 0.05) and the saturation index of carbonates (R2 = 0.93, p < 0.01). Groundwater with a higher pH contained a higher DIC concentration and could provide strong carbon sink potential. The δ13CDIC values of the groundwater samples varied from -21.22‰ to -11.02‰, indicating that DIC was derived from the dissolution equilibrium of pedogenic carbonates and atmospheric/soil CO2. The carbon sequestration of the shallow saline-alkaline groundwater in the Hetao Basin could reach 4.66 × 108 g C a-1, which represents important potential of carbon sink in the biogeochemical cycle.

Cell size influences inorganic carbon acquisition in artificially selected phytoplankton.

Cell size influences the rate at which phytoplankton assimilate dissolved inorganic carbon (DIC), but it is unclear whether volume-specific carbon uptake should be greater in smaller or larger cells. On the one hand, Fick's Law predicts smaller cells to have a superior diffusive CO2 supply. On the other, larger cells may have greater scope to invest metabolic energy to upregulate active transport per unit area through CO2 -concentrating mechanisms (CCMs). Previous studies have focused on among-species comparisons, which complicates disentangling the role of cell size from other covarying traits. In this study, we investigated the DIC assimilation of the green alga Dunaliella tertiolecta after using artificial selection to evolve a 9.3-fold difference in cell volume. We compared CO2 affinity, external carbonic anhydrase (CAext ), isotopic signatures (δ13 C) and growth among size-selected lineages. Evolving cells to larger sizes led to an upregulation of CCMs that improved the DIC uptake of this species, with higher CO2 affinity, higher CAext and higher δ13 C. Larger cells also achieved faster growth and higher maximum biovolume densities. We showed that evolutionary shifts in cell size can alter the efficiency of DIC uptake systems to influence the fitness of a phytoplankton species.

The relative importance of photodegradation and biodegradation of terrestrially derived dissolved organic carbon across four lakes of differing trophic status

Abstract. Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12 %–25 % of the total carbon flux from soils and bedrock. This CO2 is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to CO2. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance – SUVA320; spectral slope ratio – Sr), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May–August 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that, in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, ∼ 30 % of the DOC was processed by sunlight, and a minimum of 1 % was photomineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs.

Concentration and Carbon Isotope Composition of DOC and DIC in the Miyun Reservoir Watershed in Summer

This study aims to complement existing research on the carbon cycle of water in reservoirs, for the effective control of nutrient input in drinking water. The content and isotopic compositions of water dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from the Miyun Reservoir watershed in Beijing were studied, and water from the Jingmi diversion canal was also studied as a baseline reference. The results showed that the content of DOC in the Miyun Reservoir watershed in summer varied from 1.07 to 5.19 mg·L-1, with an average value of 2.61 mg·L-1. DOC in tributaries was lower than that of most rivers in China and globally, while DOC in reservoirs, particularly in Miyun Reservoir, was high. The high water level might be the main reason for the high DOC in the Miyun Reservoir. In summer, the δ13CDOC in the Miyun Reservoir watershed ranged from -27.4‰ to -24.3‰, with an average value of -25.8‰. Results from the isotopic analysis showed that the water in the Miyun Reservoir watershed was less affected by human sources, with the exception of some points. In summer, DOC in tributaries and in the Miyun Reservoir was mainly derived from soil organic matter and terrestrial C3 plants. In addition, endogenous substances also contributed to DOC in the watershed. DOC concentration increased along the Chao and Bai Rivers, and DOC from soil erosion and human sources was higher in the Chao River. The δ13CDIC in the study area varied from -12.6‰ to 5.75‰, with a mean value of -9.44‰. The weathering of carbonate rocks accounted for the major river DIC, and DIC had been clearly assimilated by the phytoplankton by photosynthesis. DOC and DIC in the Miyun Reservoir were significantly higher than in the Jingmi diversion canal (P<0.01), and the δ13CDIC in the Miyun Reservoir showed a clear positive deviation. This indicated that there were differences in carbon concentration and components in the two kinds of water, which might affect the denitrification capacity of the water in the Miyun Reservoir after mixing. In general, the dissolved carbon in the water of the Miyun Reservoir watershed is less affected by human sources. The conversion of DIC to DOC may also be a potential source of DOC in the study area.

The carbonate system of the northern South China Sea: Seasonality and exchange with the western North Pacific

Using a most comprehensive, high quality, high-resolution dataset for any marginal sea up to depths >2000 db, we examined the seasonality of the carbonate system in the northern South China Sea (nSCS) and exchange with the West Philippine Sea (WPS) during 2009–2011. The carbonate system dynamics demonstrated evident spatial and seasonal variations. Winter exhibited the highest average surface dissolved inorganic carbon (DIC) concentrations (1936 ± 15 μmol kg−1), and summer had the lowest (1882 ± 12 μmol kg−1), primarily associated with more abundant freshwater inputs in summer. At 100 db depth, decreased DIC and total alkalinity (TA) values were observed within the Luzon Strait vicinity due to the influence of WPS waters. Higher DIC and TA concentrations were found within the central nSCS basin. The average Kuroshio contribution to the DIC inventory in the upper 150 db was seasonally significant, ranging between 11 and 32%, with the highest contributions during spring and winter. Below 2000 db, nSCS basin-averaged DIC was significantly higher than WPS-averaged DIC (∼23 µmol kg−1 difference) due to more organic matter decomposition in the nSCS basin. Within the basin, average deep water DIC values were highest in autumn, and averaged concentrations at >18.5°N were lower than at ≤18.5°N. Our datasets and analysis imply that (i) the significant seasonal and spatial patterns of carbonate chemistry in the nSCS are controlled by a combination of large-scale and smaller mesoscale physical processes; (ii) extrinsically from Asian monsoons via seasonal freshwater discharge and dynamic exchanges with open ocean waters; and (iii) intrinsically, through seasonal vertical mixing as well as mesoscale processes and their subsequent new productions. The seasonal and spatial variability in carbonate parameters established here serves as an essential baseline to monitor future changes to the nSCS and to compare with other marginal sea systems.

Controlling factors of riverine CO2 partial pressure and CO2 outgassing in a large karst river under base flow condition

Riverine CO2 outgassing constitutes the important vertical conduit in global carbon cycle and affects climate change. In order to investigate the factors that control riverine pCO2 and CO2 diffusion rate (FCO2), river water samples were collected in the mainstream and 42 tributaries of Xijiang River, a karst river during the dry season in 2015. The pCO2 varied from 260 to 6354μatm with a mean value of 1765μatm, indicating most of the samples were oversaturated with respect to CO2 compared to atmosphere. The pH, pCO2, and HCO3− exhibited clearly spatial distribution pattern that pH and HCO3− decreased along the flow direction while the pCO2 increased along the flow direction. Theoretical calculation indicated that the water temperature fluctuation, bio-degradation/respiration, photosynthetic activities and anthropogenic acidic inputs were not the reasonable explanations. The lithologic differences essentially predominated the riverine pH, pCO2, and HCO3−, the upper reaches of Xijiang River flow through karst carbonate terrain and thus exhibited high pH, HCO3− concentrations meanwhile low pCO2, the spatial distribution of pH, pCO2, and HCO3− became visible as the continuous addition of silicate weathering products with low pH and dissolved inorganic carbon (DIC) concentrations. The mixing calculation indicated the DIC concentrations of silicate weathering should lower than 0.5 mmol/L. The calculated FCO2 of Xijiang River ranged between −38.6 and 1579.8 mmol/m2/D, with a mean value of 574.8 mmol/m2/D and median value of 410.9 mmol/m2/D. FCO2 exhibited obviously lower values in karst terrain compared that in silicate terrains, because carbonate weathering results in high pH of karst rivers, which caused the low relative proportion of pCO2 in DIC. More estimations of CO2 outgassing rate from the karst rivers are required due to the unique characteristics and geochemical process in karst landform.

Coupled effects of hydrology and temperature on temporal dynamics of dissolved carbon in the Min River, Tibetan Plateau

Studying the temporal dynamics and fluxes of riverine dissolved carbon is crucial in understanding the regional and global carbon cycles under various climatic conditions. Here, we studied the behaviors and fluxes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) responding to various hydrologic conditions in the Min River, originated from Tibetan Plateau. The DIC concentrations decreased with increasing runoff, partially reflecting dilution effect, which may be ascribed to the shortened fluid transit time and then the reduced contact time with rocks. Nevertheless, DOC concentrations were positively correlated with runoff, which can be attributed to that a large amount of soil organic carbon flowed into the river as a result of the strong flushing effect. The negative relationship between δ13CDIC and runoff could be explained by soil CO2 influx and organic matter degradation during the high flow season. ΔDIC (the production of DIC with changing hydrologic conditions) had a strong positive correlation with water temperature due to the accelerated DIC production rates by high temperature, which always co-varied with intense precipitation in Asian monsoonal regions. The mean DIC/DOC ratio in the Min River was 15.09, and the DOC and DIC fluxes were 1.1 and 15.2 t C km−2 yr−1, respectively, for the studied year. And the DOC and DIC fluxes varied dramatically with runoff changes, suggesting that hydrologic conditions were critical factors for the variations in dissolved carbon export. This study shows that carbon dynamics of rivers draining from the Tibetan Plateau are greatly affected by short-term climatic variabilities, which has implications for understanding global carbon cycle under future climate change.

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.

Carbon biogeochemical processes in a subtropical karst river–reservoir system

Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (δ13CDIC) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L−1, 167.6 to 192.0 mg L−1, 179.1 to 181.5 mg L−1, and 182.4 to 183.8 mg L−1 while δ13CDIC decreased from −6.6‰ to −9.7‰, −7.8‰ to −10.4‰, −9.7‰ to −10.0‰ and −9.9‰ to −10.0‰ in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes’ concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, δ13CDIC had a significant positive relationship with the reservoir effect index (Ri), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by Ri.

Nutrient and dissolved inorganic carbon variability in the North Pacific

A compilation of surface water nutrient (phosphate, nitrate, and silicate) and partial pressure of CO2 (pCO2) observations from 1961 to 2016 reveals seasonal and interannual variability in the North Pacific. Nutrients and calculated dissolved inorganic carbon (DIC) reach maximum concentrations in March and minimum in August. Nutrient and DIC variability is in-phase (anti-phase) with changes in the mixed layer depth (sea surface temperature) north of 30 °N, and it is anti-phase (in-phase) with changes in Chl-a north of 40 °N (in 30 °N–40 °N). Seasonal drawdown of nutrients and DIC is larger toward the northwest and shows a local maximum in the boundary region between the subarctic and subtropics. Stoichiometric ratios of seasonal drawdown show that, compared to nitrate, silicate drawdown is large in the northwestern subarctic including the Bering and Okhotsk seas, and drawdown of carbon is larger toward the south. Net community production in mixed layer from March to July is estimated to be more than 6 gC/m2/mo in the boundary region between the subarctic and subtropics, the western subarctic, the Gulf of Alaska, and the Bering Sea. Nutrient and DIC concentrations vary with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation which cause changes in horizontal advection and vertical mixing. The DIC trend is positive in all analysis area and large in the western subtropics (> 1.0 μmol/l/yr). Averaged over the analysis area, it is increasing by 0.77 ± 0.03 μmol/l/yr (0.75 ± 0.02 μmol/kg/yr).

Differential Sensitivity of a Symbiont‐Bearing Foraminifer to Seawater Carbonate Chemistry in a Decoupled DIC‐pH Experiment

AbstractLarger benthic foraminifera (LBF) are unicellular eukaryotic calcifying organisms and an important component of tropical and subtropical modern and ancient oceanic ecosystems. They are major calcium carbonate producers and important contributors to primary production due to the photosynthetic activity of their symbiotic algae. Studies investigating the response of LBF to seawater carbonate chemistry changes are therefore essential for understanding the impact of climate changes and ocean acidification (OA) on shallow marine ecosystems. In this study, calcification, respiration, and photosynthesis of the widespread diatom‐bearing LBF Operculina ammonoides were measured in laboratory experiments that included manipulation of carbonate chemistry parameters. pH was altered while keeping dissolved inorganic carbon (DIC) constant, and DIC was altered while keeping pH constant. The results show clear vulnerability of O. ammonoides to low pH and CO32− under constant DIC conditions, and no increased photosynthesis or calcification under high DIC concentrations. Our results call into question previous hypotheses, suggesting that mechanisms such as the degree of cellular control on calcification site pH/DIC and/or enhanced symbiont photosynthesis in response to OA may render the hyaline (perforate and calcitic‐radial) LBF to be less responsive to OA than porcelaneous LBF. In addition, manipulating DIC did not affect calcification when pH was close to present seawater levels in a model encompassing the total population size range. In contrast, larger individuals (&gt;1,200 μm, &gt;1 mg) were sensitive to changes in DIC, a phenomenon we attribute to their physiological requirement to concentrate large quantities of DIC for their calcification process.

Biogeochemical Controls on 13CDIC Signatures from Circum-Neutral pH Groundwater in Cu–W–F Skarn Tailings to Acidic Downstream Surface Waters

Regular features of ground and surface waters affected by drainage from mine waste include their acidity and elevated concentrations of dissolved metals, with their attendant negative effects on drinking water quality and aquatic life. One parameter that aids in buffering these waters against acidity and sustains aquatic life is dissolved inorganic carbon (DIC). In this study, the chemical and isotopic (δ13C) composition of primary calcite and DIC (δ13CDIC) in groundwater and surface waters within and downstream, respectively, of abandoned Cu–W–F skarn tailings at Yxsjöberg, Sweden, were used to trace the biogeochemical processes controlling their respective δ13CDIC signatures. In addition, the δ13C signatures of the inorganic (carbonate) fractions of the tailings were used to verify the formation of secondary carbonates within the tailings. Lower average δ13C values of the carbonate fractions (δ13Ccarb = −2.7‰) relative to those of the primary calcite (δ13C = +0.1‰) from the orebodies from which the tailings originated pointed to the precipitation of secondary carbonates. These lower δ13Ccarb signatures were assumed to represent mixed-source C signals involving isotopically light CO2 from the atmosphere, the degradation of organic matter in the upper part of the tailings and HCO3− from calcite dissolution. The groundwater δ13CDIC values (−12.6‰ to −4.4‰) were far lower than the hypothetical range of values (−4.6‰ to +0.7‰) for primary calcite and secondary carbonate dissolution. These signatures were attributed to carbonate (calcite and secondary carbonate) dissolution and the degradation of dissolved organic carbon (DOC) from various organic sources such as peat underneath the tailings and the surrounding forests. Downstream surface water samples collected in May had low δ13CDIC values (−16‰) and high DOC (14 mg C/L) compared to the groundwater samples. These signatures represented the oxidation of the DOC from the wash out of the mires and forests during the snowmelt and spring flood. The DOC and δ13CDIC values of the surface waters from June to September ranged from 6–15 mg·C/L and −25‰ to −8.6‰, respectively. These signatures were interpreted to reflect mixed C sources, including carbonate dehydration by acidity from Fe3+ hydrolysis due to the mixing of groundwater with surface waters and the subsequent diffusive loss of CO2 (g), aquatic photosynthesis, photooxidation, DOC degradation, as well as microbial respiration. Although the 13CDIC signatures of the downstream surface waters seemed to be seasonally controlled and influenced by variable groundwater contributions, the lack of data with respect to DIC concentrations, coupled with multiple potential biogeochemical processes that could influence the DIC pool and 13CDIC values, made it difficult to identify the major regulating process of the 13CDIC signatures. Therefore, other complimentary isotopes and elemental concentrations are recommended in order to decipher the dominant biogeochemical process.

Dissolved Inorganic Carbon ([DIC], δ13С(DIC)) in Waters of the Eastern East Siberian Sea

Concentrations and carbon isotope composition of dissolved inorganic carbon (DIC) and isotope composition (δD, δ18O) of waters were studied in the eastern East Siberian Sea, along two submeridional sections extending from the Indigirka and Kolyma mouths to the edge of perennial ice. Estimated contents of river waters, sea ice meltwater, and modified waters showed that the eastern part of the shelf of the East Siberian Sea in summer, 2017 was globally freshened by riverine waters (6–57%). A slight contribution of sea ice meltwater (2–3%) was found only in the surface waters of the Kolyma section. The waters of the Indigirka section experienced intense modification, which is related to the ice formation and reaches 20% ice extraction from water. The concentration and δ13C values of DIC are controlled by mixing between sea water and river runoff. Their shift relative to the values determined by conservative mixing suggests that shelf waters contain an excess of DIC with an isotopically light carbon. Main processes leading to these shifts are the decomposition of organic matter and water modification. The rate of the latter is 3–4 times higher than that of organic matter oxidation. The isotope composition and concentrations of DIC could partially reflect the influence of Pacific seawaters coming through the Bering Strait and the Chukchi Sea.

Pore-water dissolved inorganic carbon sources and cycling in the shallow sediments of the Haima cold seeps, South China Sea

Abstract Pore-water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane-derived carbon in sediments. In this study, six sediment push cores were collected by manned submersible at the center and edge of the active Haima cold seeps, and a nearby background site. Pore-water compositions of the SO42−, Ca2+, Mg2+, Sr2+, NH4+, dissolved inorganic carbon (DIC), and δ13C-DIC at each sediments core were measured. The very shallow depths of sulfate–methane transition zone (SMTZ) suggest that high methane flux and significant anaerobic oxidation of methane (AOM) activities occur in the shallow sediments. Reaction-transport modelling results show that the relative contributions of AOM and organoclastic sulfate reduction (OSR) consuming sulfate are 84.1% and 15.9% at site HM-2-6, and 94.6% and 5.4% at site HM-3-3, respectively. Furthermore, extremely low δ13C-DIC values at the central sites HM-2-6 (−45.67‰) and HM-3-3 (−53.87‰) indicate an external methane-derived DIC. Based on the δ13C mass balance, we quantify the proportions of pore-water DIC derived from organic matter degradation and AOM as 24.7% and 65.3% at site HM-2-6, and 9.3% and 80.7% at site HM-3-3, respectively. An increase in pore-water DIC concentration leads to carbonate precipitation. Plots of Sr/Ca vs. Mg/Ca ratios reveal that the carbonate phases in sediments are dominantly high Mg-calcite in this region. Our study demonstrates that AOM can serve as a dominant biogeochemical process through incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases, significantly influencing DIC cycling within the shallow sediments of the Haima cold seep area.

Geologic Controls on Source Water Drive Baseflow Generation and Carbon Geochemistry: Evidence of Nonstationary Baseflow Sources Across Multiple Subwatersheds

AbstractThe contributions and composition of baseflow sources across an extended recession period were quantified for six subwatersheds of varying size in a structurally complex watershed in coastal California using endmember mixing analysis and related to catchment characteristics (e.g., topography, geology, land use, and soil characteristics). Both shallow subsurface and deep groundwater reservoirs were important contributors for streamflow during low flow periods, and the composition of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., low vs. high) best explained the changes in shallow subsurface water and deeper groundwater inputs through the seasonal recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA254) were used to provide additional insight into endmember characteristics and their contributions to baseflow. Stream water DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively constant DIC concentrations, while stream water DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC. Results from this study show (1) the importance of considering baseflow as a dynamic mixture of water from multiple sources, (2) the effect of geology on source composition at the subwatershed scale during low flow conditions, and (3) the impact of shifting baseflow sources on stream water dissolved carbon concentrations and the utility of using dissolved carbon concentrations to obtain additional insight into temporal variability in baseflow sources.

Effects of chemical weathering and CO2 outgassing on δ13CDIC signals in a karst watershed

As the important part of carbon cycle research, riverine dissolved inorganic carbon (DIC) has attracted continuous attentions for its close relationship with global climate change. In order to investigate the dominant factors of dissolved inorganic carbon species and its stable carbon isotopic signals in large watershed under base flow condition, 81 river water samples were collected in the Xijiang River during the dry season in 2015. The elemental ratios in river water revealed the hydro-chemistry in Xijiang River were mainly controlled by the carbonate weathering. The characteristics of the most samples were the high DIC concentrations and narrow range of δ13CDIC, which can be interpreted as the result of chemical weathering under open system based on the simulated calculation of soil respiration and mineral dissolution. The relatively lower DIC concentrations and δ13CDIC values have been observed in tributaries draining silicate terrains, this result may be controlled both by the mineral dissolution and CO2 outgassing. Most samples had the over-saturated pCO2 levels compared to the atmosphere, the respiratory quotient (RQ) demonstrated that high pCO2 values were not from the biologic respiration, instead, the groundwater influx with the high soil pCO2 increased pCO2 level in river waters. The CO2 concentration gradient between river water and atmosphere caused a violent CO2 outgassing accompanied isotope fractionation. The outgassing significantly dropped the pCO2 in river water and caused an enrichment of C13 in DIC. Based on the theoretical calculation and previous observation, the negative correlation between the δ13CDIC and pCO2 in river water was caused by the CO2 outgassing. Additionally, the isotope exchange between the DIC and atmosphere CO2 can also partially increase the riverine CO2. This study examined the impacts of soil respiration, minerals dissolution and CO2 outgassing on DIC and δ13CDIC, and found that the riverine δ13CDIC is probably not reflect the mixing of source signals such as soil CO2 and carbonate, instead, it is mainly controlled by the fractionation in minerals dissolution and CO2 outgassing.