Dissolved Inorganic Carbon Pool Research Articles

Source partitioning of dissolved inorganic carbon addition to Pacific Winter Water in the western Arctic Ocean

AbstractPacific Winter Water (PWW) with high dissolved inorganic carbon (DIC) is the key water mass in which subsurface acidification occurs in the western Arctic Ocean. To investigate and partition carbon sources added to PWW across the Chukchi shelf to the adjacent Canada Basin, we investigated the distributions of DIC and its stable isotope (δ13C‐DIC) with other hydrographic and biogeochemical parameters during the Mirai cruise in the late summer of 2021. Using a four‐end‐member mixing model, we deciphered the water masses and separated DIC changes induced by biological processes from those induced by conservative mixing. We demonstrated that DIC dynamics in PWW were mainly controlled by the biological decomposition of organic carbon (OC). A mass balance model analysis of DIC and δ13C‐DIC suggested that the apparent δ13C signature of the respired organic carbon (δ13COCx) within PWW was −22.2‰ ± 1.1‰ on the shelf and −25.6‰ ± 1.6‰ in the basin. Therefore, we concluded that marine‐origin OC was the dominant carbon source that was decomposed in the Chukchi shelf bottom water, while respired terrestrial‐origin carbon made a major contribution to DIC pool in the basin PWW. We proposed that terrestrial OC from the Chukchi and Beaufort coastal seas could be an important carbon source, which was associated with winter water formation on the shelves and influenced by Beaufort Gyre state shift and circulation changes. This unconventional finding has important ramifications for the prediction of the future state of ocean acidification in the western Arctic Ocean.

Impacts of wastewater treatment plant effluents on dissolved inorganic carbon in highly urbanized karst rivers, Southwest China

Wastewater treatment plants (WWTPs) are an important manifestation of the impact of urbanization on aquatic ecosystems, and the influences of effluents discharge from WWTPs on receiving river carbon biogeochemical cycle processes has been a research hotspot. However, our knowledge about potential impacts of WWTP effluents on dissolved inorganic carbon (DIC) cycle in karst rivers is rather limited. This study investigates the water chemistry and carbon isotope composition (δ13CDIC) in the urbanized reach of a river that drains the world’s largest continuous karst region in Southwest China, in an attempt to determine the effects of WWTP effluents on karst river DIC cycle and source discrimination. The results reveal: (1) Partial pressure of dissolved carbon dioxide (pCO2) levels in the river increased noticeably during the dry and normal seasons after receiving WWTP effluents; (2) There existed an ionic ratio overlap between end-members of silicate rock weathering and WWTP effluents which requires caution in identifying end-members of rock weathering in urbanized karst rivers; (3) DIC in the highly urbanized karst rivers was affected by complex geochemical processes and these processes varied in different seasons; (4) The contribution of WWTP effluents to the riverine DIC pool was 21±13 %, 58±21 % and 66±33 % in the wet, dry, and normal seasons, respectively. These findings highlight the interference of WWTP effluents with DIC carbon cycling in the receiving karst rivers. Since DIC from WWTPs do not relate with consuming atmospheric or soil CO2, it is important to exclude their contribution from the watershed’s rock weathering carbon sink budget. Based on these findings, we insist that the role and effect of WWTP effluents should be carefully considered when examining DIC carbon source and cycling in rivers that drain urbanized watersheds with abundant wastewater treatment plants.

Unexpected contributions by carbonates and organic matter in a silicate-dominated tropical catchment: An isotope approach

The understanding of global carbon has rarely extended to small-scale tropical river basins. To address these uncertainties, this study aims to investigate the importance of rock weathering and organic matter turnover in the carbon cycle in a terrain dominated by crystalline silicate rocks. The geochemical composition of the dissolved and particulate carbon phases (DIC, DOC and POC) and their stable carbon isotopes were studied in the Deduru Oya River in Sri Lanka. Dissolved inorganic carbon (DIC) was the most dominant carbon phase and its contribution to the total carbon pool varied between 67 and 89 %. Furthermore, the δ13CDIC values in the river varied between −1.1 and −16.5 ‰. The lithological characteristics and molar ratios between Ca2+, Mg2+ and HCO3− indicated rock weathering mainly by CO2 and carbonic acid. The δ13CDIC values for groundwater input were −15.9 ‰, while for carbonate weathering, mainly due to fertiliser input, they reached a value of −12.7 ‰. This input was fed into an isotope mass balance to determine the relative contributions. However, the isotope mass balance was only plausible after correcting for the effects on δ13CDIC caused by degassing and photosynthesis. Our study demonstrated that carbonate weathering and organic matter turnover are essential components of the river carbon cycle even in a silicate dominated catchment. They can represent up to 60 % of the DIC pool. Combined with the higher organic matter turnover and high pCO2 in the river water, it can be suggested that the Deduru Oya River acts as a net source of CO2 in the atmosphere. Our study shows that CO2 degassing and in-stream photosynthesis in tropical river systems need to be considered along with chemical weathering to account for carbon transport and turnover in tropical rivers.

Dual clumped (Δ47-Δ48) isotope data for amorphous carbonates and transformation products reveal a novel mechanism for disequilibrium clumped isotope effects

Amorphous precursors to minerals have been observed in laboratory materials and in nature, including across diverse phyla. These metastable phases allow for the incorporation of cations at higher concentrations than classical crystallization pathways, thus, their chemistry and behavior have implications in an array of disciplines. Currently, little is known about the isotopic composition of the anion in amorphous carbonates and how isotopic values evolve during transformation into a mineral. Here, we examined the evolution of isotopic values in amorphous carbonates and mineral transformation products to identify the potential origins of disequilibrium isotopic compositions in carbonate minerals that form from an amorphous precursor. We measured dual carbonate clumped isotopes (13C18O16O - Δ47; 12C18O18O - Δ48), bulk stable isotope ratios (δ13C, δ18O), and chemical and structural data throughout the transformation of amorphous calcium magnesium carbonate (ACMC) into high Mg-calcite (HMC) over 1 year, with crystallization occurring in solutions from 10 to 60 °C. The Δ47, Δ48, and δ18O values evolved significantly during transformation, indicating dissolution of ACMC and reprecipitation of HMC. After crystallization, the Δ47 and Δ48 values achieved a disequilibrium steady state, while δ18O values continued to evolve. For the fully crystallized HMC samples, the low temperature samples formed at 10 °C had the greatest extent of oxygen isotope disequilibrium (measured value – equilibrium value = −39‰); conversely; the greatest clumped isotope disequilibrium was observed in high temperature samples formed at 40 and 60 °C (0.068‰ for Δ47, 0.072‰ for Δ48). These results are consistent with a new potential mechanism of disequilibrium clumped isotope values in carbonate minerals. Specifically, the dissolution of ACMC during transformation causes disequilibrium oxygen and clumped isotope values in the dissolved inorganic carbon (DIC) pool. The extent of isotopic disequilibrium in DIC during transformation is temperature dependent, and is recorded by the forming mineral. Isotopic results may also reflect mixing effects, as scanning electron microscopy (SEM) showed ACMC and HMC existing simultaneously during transformation, indicating that ACMC likely progressively dissolves and reprecipitates as the DIC pool isotopically evolves. This may result in heterogeneous isotopic values in HMC. In total, these data suggest a highly dynamic localized environment could exist in biomineralizing organisms and abiotic systems that utilize amorphous precursors to form carbonate minerals, potentially resulting in isotopic values that are not representative of formation temperature.

Recent Trends and Variability in the Oceanic Storage of Dissolved Inorganic Carbon

AbstractSeveral methods have been developed to quantify the oceanic accumulation of anthropogenic carbon dioxide (CO2) in response to rising atmospheric CO2. Yet, we still lack a corresponding estimate of the changes in the total oceanic dissolved inorganic carbon (DIC). In addition to the increase in anthropogenic CO2, changes in DIC also include alterations of natural CO2. Once integrated globally, changes in DIC reflect the net oceanic sink for atmospheric CO2, complementary to estimates of the air‐sea CO2 exchange based on surface measurements. Here, we extend the MOBO‐DIC machine learning approach by Keppler et al. (2020a, https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.nodc%3A0221526) to estimate global monthly fields of DIC at 1° resolution over the top 1,500 m from 2004 through 2019. We find that over these 16 years and extrapolated to cover the whole global ocean down to 4,000 m, the oceanic DIC pool increased close to linearly at an average rate of 3.2 ± 0.7 Pg C yr−1. This trend is statistically indistinguishable from current estimates of the oceanic uptake of anthropogenic CO2 over the same period. Thus, our study implies no detectable net loss or gain of natural CO2 by the ocean, albeit the large uncertainties could be masking it. Our reconstructions suggest substantial internal redistributions of natural oceanic CO2, with a shift from the midlatitudes to the tropics and from the surface to below ∼200 m. Such redistributions correspond with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. The interannual variability of DIC is strongest in the tropical Western Pacific, consistent with the El Nio Southern Oscillation.

Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean.

Taking the world's largest green tide caused by the macroalga Ulva prolifera in the South Yellow Sea as a natural case, it is studied here if macroalgae can perform inorganic carbon sequestration in the ocean. Massive macroalgae released large amounts of organic carbon, most of which were transformed by microorganisms into dissolved inorganic carbon (DIC). Nearshore field investigations showed that, along with seawater deoxygenation and acidification, both DIC and total alkalinity (TAlk) increased significantly (both >50%) in the areas covered by dense U. prolifera at the late-bloom stage. Offshore mapping cruises revealed that DIC and TAlk were relatively higher at the late-bloom stage than at the before-bloom stage. Laboratory cultivation of U. prolifera at the late-bloom stage further manifested a significant enhancement effect on DIC and TAlk in seawater. Sulfate reduction and/or denitrification likely dominated the production of TAlk. Notably, half of the generated DIC and almost all the TAlk could persist in seawater under varying conditions, from hypoxia to normoxia and from air-water CO2 disequilibrium to re-equilibrium. The enhancement of TAlk allowed more DIC to remain in the seawater rather than escape into the atmosphere, thus having the long-term legacy effect of increasing DIC pool in the ocean.

Mineralization of autochthonous particulate organic carbon is a fast channel of organic matter turnover in Germany's largest drinking water reservoir

Abstract. Turnover of organic matter (OM) is an essential ecological function in inland water bodies and relevant for water quality. This is especially important for the potential of dissolved organic carbon (DOC) removal as well as for emissions of CO2. In this study, we investigated various phases of OM including DOC, autochthonous particulate organic carbon (auto-POC), allochthonous particulate organic carbon (allo-POC), and sedimentary matter (SED) in a temperate drinking water reservoir (Rappbode Reservoir, Germany) by means of dissolved inorganic carbon (DIC) concentrations and carbon stable isotope ratios. In order to best outline carbon turnover, we focused on the metalimnion and the hypolimnion of the reservoir, where respiration is expected to be dominant and hardly disturbed by atmospheric exchange or photosynthesis. DIC concentrations ranged between 0.30 and 0.53 mmol L−1, while δ13CDIC values ranged between −15.1 ‰ and −7.2 ‰ versus the VPDB (Vienna PeeDee Belemnite) standard. Values of δ13CDOC and δ13Cauto-POC ranged between −28.8 ‰ and −27.6 ‰ and between −35.2 ‰ and −26.8 ‰, respectively. Isotope compositions of sedimentary material and allochthonous POC were inferred from the literature and from measurements from previous studies with δ13CSED=-31.1 ‰ and δ13Callo-POC ranging from −31.8 ‰ to −28.6 ‰. Comparison of DIC concentration gains and stable isotope mass balances showed that auto-POC from primary producers was the main contributor to increases in the DIC pool. Calculated OM turnover rates (0.01 to 1.3 µmol L−1 d−1) were within the range for oligotrophic water bodies. Some higher values in the metalimnion are likely due to increased availability of settling auto-POC from the photic zone. Samples from a metalimnetic oxygen minimum (MOM) also showed dominance of respiration over photosynthesis. Our work shows that respiration in temperate lentic water bodies largely depends on auto-POC production as a major carbon source. Such dependencies can influence the vulnerabilities of these aqueous systems.

Radiocarbon and Stable Carbon Isotope Constraints on the Propagation of Vent CO 2 to Fluid in the Acidic Kueishantao Shallow Water Hydrothermal System

We report radiocarbon and stable carbon isotope measurements from vent gas (CO2(g)), dissolved inorganic carbon (DIC), and particulate organic carbon (POC), from two vents of the Kueishantao (KST) shallow-water hydrothermal system, offshore northeast Taiwan. The purpose of this research is to investigate how magmatic-sourced carbon enters various carbon pools in the hydrothermal system. We utilize a precipitation method to eliminate sulfur compounds from CO2 samples to facilitate Accelerator Mass Spectrometry (AMS) analysis, and evaluate radiocarbon background levels during processing to characterize hydrothermal-sourced CO2 that contains negligible radiocarbon. The result shows that both CO2(g) and DIC in the fluids below two vent orifices fall within a narrow range of fraction of modern carbon (F14C) from 0.013 to 0.136 and δ13C from −8.3‰ to −5.1‰. The F14C values correspond to approximately 90% magmatic-sourced carbon in CO2(g) and DIC. A combination of equilibrium and kinetic isotopic fractionation can adequately explain relatively high CO2(g) δ13C to DIC, beneath vent orifices. Above the vent orifices, DIC becomes enriched in both 14C and 13C as a result of physical mixing with ambient seawater. POC F14C values confirm a significant magmatic carbon contribution into the POC pool within the KST system, with rapid hydrothermal circulation. Our results identify the physiochemical processes responsible for magmatic carbon migration from CO2(g) into the DIC pool, and demonstrate how a dual carbon isotope approach can serve as an effective tool in understanding carbon flow in high temperature-low pH hydrothermal systems.

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

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

Sulfate concentrations affect sulfate reduction pathways and methane consumption in coastal wetlands

Coastal wetlands are an important source of methane emissions, and understanding the mechanisms that control methane emissions from coastal wetlands is of great significance to global warming. Anaerobic oxidation of methane driven by sulfate is an important process to prevent methane emissions from coastal wetlands. The effects of environmental changes on this process and the function of the sulfate-methane transition zone (SMTZ) are poorly understood. In this study, spatiotemporal variations in pore-water geochemistry (concentrations of SO42-, CH4 and DIC as well as δ13C-DIC and δ13C-CH4) in the Beidagang wetland, Tianjin, China, were investigated to unravel factors controlling the role of anaerobic oxidation of methane in coastal wetlands. Results show that the geochemical profile of pore-water is characterized by significant spatial and temporal variability, which may be related to changes in sulfate concentration, temperature and dissolved oxygen. The carbon isotope fractionation factors (εC) during methane oxidation range from 8.9‰ to 12.5‰, indicating that the sulfate-driven anaerobic oxidation of methane (S-AOM) dominates the methane oxidation in the Beidagang coastal wetland in both winter and summer, in both high and low salinity wetlands, and in both open water and littoral areas. However, sulfate concentration has a strong influence on the sulfate reduction pathways and methane consumption. The consumption of methane and sulfate by S-AOM is more significant in coastal wetlands with high sulfate concentrations, with S-AOM consuming nearly all of the upward-diffusing methane (96%) and downward-diffusing sulfate (96%). In addition, the dissolved inorganic carbon (DIC) produced in the pore-water mainly comes from methanogenesis, accounting for more than 80% of the total DIC pool, but in the areas with high sulfate concentrations in water column, the contribution of S-AOM to the DIC pool is greater, although only a small fraction of the total DIC pool (9%). The depth and width of the SMTZ show a clear spatial and temporal pattern, with active methanogenesis activity and upward high methane flux shoaling the SMTZ and increasing the risk of high methane emissions from coastal wetlands with low sulfate concentrations. Our findings highlight the importance of sulfate-driven anaerobic oxidation of methane in coastal wetlands and the effect of sulfate concentration on it. It contributes to our understanding of the mechanism of methane production and emissions from the coastal wetland system, particularly in light of the increased demand for coastal wetland restoration under global warming.

Oxygen isotopes of calcite precipitated at high ionic strength: CaCO3-DIC fractionation and carbonic anhydrase inhibition

Background electrolytes affect calcite precipitation and dissolution rates and hence have the potential to impact kinetic isotope fractionation between calcite and the dissolved inorganic carbon (DIC) species. We grew inorganic calcite from NaCl-CaCl2 solutions (T = 25 °C, pH = 8.3, and crystal growth rate = 10−6.3 ± 0.3 mol m−2 s−1) to test for potential ionic strength effects on the oxygen isotope fractionation between calcite and an isotopically equilibrated inorganic carbon pool (αc/EIC). Calcite was grown in the presence of the enzyme carbonic anhydrase from bovine erythrocytes (bCA) to promote isotopic equilibration of the DIC pool.No evidence of an ionic strength effect on αc/EIC was found for NaCl concentrations up to 0.35 M (1000lnαc/w = 28.0 ± 0.1; n = 7). For experiments conducted with [NaCl] > 0.35 M, the DIC pool could not be maintained in isotopic equilibrium with water due to the inhibitory effect of NaCl on bCA. The use of other types of CA may be required to maintain isotopic equilibration of DIC in solution with ionic strength close to or above that of seawater.The oxygen isotope results were modeled successfully with an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each DIC species and CaCO3. The experimental and modeling results suggest that the efficacy of bCA decreases exponentially with increasing [NaCl]. We quantify the salt effect on the quantity kcat/KM, which relates the catalyzed rate constant for CO2 hydration to the concentration of bCA. The salt effect can be implemented in models of biomineralization with potential to extend our knowledge of oxygen isotope vital effects in marine organisms.

Carbon isotope excursions during the late Miocene recorded by lipids of marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea

Abstract Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly, the δ13C signature of their diagnostic biomarker crenarchaeol was suggested as a potential δ13CDIC proxy in marine basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed basins are sensitive to carbon-cycle perturbations, because they tend to develop thermohaline stratification. Water column stratification typified the semi-enclosed basins of the Mediterranean Sea during the late Miocene (Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the crisis affected the carbon cycle, we studied sediments of the Piedmont Basin (northwestern Italy), the northernmost Mediterranean subbasin. A potential bias of our δ13CDIC reconstructions from the input of soil Thaumarchaeota is discarded, since high and increasing branched and isoprenoid tetraether (BIT) index values do not correspond to low and decreasing δ13C values for thaumarchaeal lipids, which would be expected in case of high input from soil Thaumarchaeota. Before the onset of the crisis, the permanently stratified distal part of the basin hosted a water mass below the chemocline with a δ13CDIC value of approximately −3.5‰, while the well-mixed proximal part had a δ13CDIC value of approximately −0.8‰. The advent of the crisis was marked by 13C enrichment of the DIC pool, with positive δ13CDIC excursions up to +5‰ in the upper water column. Export of 12C to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.

Evaluation of the moderate DI13C isotope enrichment method for measuring photochemical mineralization of marine dissolved organic carbon

AbstractThe moderate DI13C isotope enrichment (MoDIE) method by Powers et al. (2017) is a promising method to precisely measure the photochemical mineralization of dissolved organic carbon (DOC) in water samples without dramatically altering a sample's pH or organic carbon pool. Here, we evaluated the analytical uncertainties of the MoDIE method and used Monte Carlo simulations to optimize the experimental design for the most precise measurements of dissolved inorganic carbon (DIC) that is produced photochemically (DIChν). Analytically, we recommend calculating yields of DIChv with an exact expression of conservation of mass that intrinsically reduces error and uncertainty. Methodologically, the overall uncertainty and detection limit of the MoDIE method can be significantly reduced by partially stripping away the original DIC pool, enriching the residual DIC with more DI13C, and increasing the yields of DIChv via longer irradiation. Instrumentally, more precise measurements of enriched δ13C values before and after irradiation are needed to further improve the precision of DIChν concentration determinations. Higher precision DIChv measurements via the optimized MoDIE method can improve our understanding of the photochemical mineralization of DOC and thus the budget of marine DOC. The optimizations and detection limits reported here will become more refined as measurements and associated uncertainties from future MoDIE experiments become available.

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.

Oxygenated conditions in the aftermath of the Lomagundi-Jatuli Event: The carbon isotope and rare earth element signatures of the Paleoproterozoic Zaonega Formation, Russia

The c. 2.0 Ga Zaonega Formation of the Onega Basin (NW Russia) has been central in efforts to understand what led to the initial rise (Great Oxidation Event, GOE) and postulated fall in free atmospheric oxygen and associated high-amplitude carbon cycle excursions, the Lomagundi-Jatuli Event (LJE) and subsequent Shunga Event during Paleoproterozoic time. The Formation accumulated shortly after the LJE and encompasses both the recovery in the carbon cycle and hypothesised contraction of the oceanic oxidant pool. However, interpreting the correct environmental context recorded by geochemical signatures in the Zaonega rocks is difficult due to a complex depositional and diagenetic history. In order to robustly constrain that history, we undertook a multiproxy study (mineralogy, petrography, carbon isotope and rare earth element composition) of carbonate beds in the upper part of the Zaonega Formation recovered in the 102-m composite section of the OnZap drill-cores. Our findings differentiate primary environmental signatures from secondary overprinting and show that: (i) the best-preserved carbonate beds define an upwards increasing δ13Ccarb trend from c. −5.4‰ to near 0‰; and that (ii) large intra-bed δ13Ccarb variations reflect varying contributions of methanotrophic dissolved inorganic carbon (DIC) to the basinal DIC pool. Rare earth element and yttrium (REYSN) patterns confirm a marine origin of the carbonate beds whereas a consistent positive EuSN anomaly suggests a strong high temperature hydrothermal input during accumulation of the Zaonega Formation. Importantly, the presence of a negative CeSN anomaly in the REYSN pattern indicates an oxygenated atmosphere–ocean system shortly after the LJE and indicates that models invoking a fall in oxygen at that time require reassessment.

Changes in dissolved inorganic carbon in river water due to urbanization revealed by hydrochemistry and carbon isotope in the Pearl River Delta, China.

Under natural conditions, the dissolved inorganic carbon (DIC) in river water is dominantly derived from carbonate or silicate dissolution by carbonic acid. However, sulfuric and nitric acids produced by human activities provide additional acidity for chemical weathering, which would affect the DIC flux and change its isotopic composition. To identify the natural and anthropogenic impacts on DIC, the major ion concentrations and stable carbon isotopes of the DIC (δ13C-DIC) of river waters were measured in the Pearl River Delta (PRD) region, which is one of the most developed and populated areas in China. The mass balance calculations for DIC-apportionment showed that carbonate dissolution by carbonic acid was the dominant origin of DIC in the Beijiang (BJ) River (67%) and Xijiang (XJ) River (78%) and silicate dissolution by carbonic acid was the dominant origin of DIC in the Guangzhou (GZ) Channel (37%) and Dongjiang (DJ) River (50%), which was related to the lithology of the catchment. The contribution of carbonate dissolution by sulfuric and nitric acids, which represented the contribution of human activities to the total DIC concentrations in river water, showed high proportions in the GZ Channel and DJ River, with averages of 42% and 34%, respectively, which were associated with a high degree of urbanization. Evidence of hydrochemical parameters and δ13C-DIC signatures indicated that human activities had impacts on the DIC pool. Carbonate dissolution by sulfuric and nitric acids caused by human activities changed DIC apportionments rather than the DIC flux, and this part of DIC would ultimately become a source of CO2 to the atmosphere on the geological timescale and affects the CO2 budget. An increase in nutrient concentration due to increased sewage discharge in the urbanized area could promote phytoplankton photosynthesis, which could change the DIC pool and increase the δ13C-DIC value. This study quantitatively highlights the influence of human activities on DIC apportionment in river water, suggesting that anthropogenic impacts should be seriously considered when evaluating the evolution of DIC.

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

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

Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave “McMaster Cave”

Understanding carbon and oxygen isotope systematics in cave environments is a prerequisite for the interpretation of stable isotopes in speleothem-based paleoclimate records. Here we present a series of experimental data collected under laboratory conditions with controlled temperature, relative humidity, drip water chemistry, flow rate and cave pCO2, simulating the growth of speleothems in natural cave settings. Drip water with high pCO2 and low calcium concentration (Ca2+ = 2 mmol/L) flowed along a three-step glass path, similar to a stalactite-stalagmite-pool route in natural caves, forming a thin water film that allowed CO2 degassing and CaCO3 precipitation as a result of the pCO2 gradient between the drip water and ambient cave atmosphere. The experiments were conducted at 15, 25 and 32 °C and flow rates of 700, 270 and 125 ml/d.The growth rate of calcite on the stalagmite-like settings increases linearly with increasing flow rate and/or temperature. The δ13CCc and δ18OCc of calcite formed on the stalagmite-like settings increases with decreasing flow rate (corresponding to increasing exposure time of water) at a given temperature, indicating non-equilibrium isotope effects between calcite, water and dissolved inorganic carbon (DIC). Nevertheless, these non-equilibrium isotope effects still display regular temperature dependence under a constant flow rate. This suggests that non-equilibrium isotope effects in natural stalagmites might be used to provide useful qualitative paleoclimate information (such as differentiating wet/dry and warm/cold climate conditions). The non-equilibrium carbon and oxygen isotope effects in the stalagmite-like settings were most likely caused by rapid CO2 degassing and CaCO3 precipitation that rapidly consume the available DIC pool in the thin water film. Furthermore, CO2 exchange between DIC and cave atmosphere quickly amplified the observed non-equilibrium carbon isotope effects in the precipitated calcite.In the pool-like settings, calcite was buffered by oxygen isotope exchange between DIC species and water, and slowly precipitated at or near to oxygen isotopic equilibrium with the temperature dependence of 1000ln18αCc-H2O = 18.33 (103/T) – 33.31 regardless of flow rate. This fractionation relation agrees with that determined by Kim and O’Neil (1997) when a newly recommended value for the acid fractionation factor for calcite is used (i.e., Kim et al., 2015). Carbon isotope fractionation between calcite and bicarbonate was temperature-independent between 15 and 32 °C and the average magnitude was 1000ln13αCc–HCO3− = 1.7 ± 0.7‰. Observed variability of 1000ln18αCc-H2O in modern calcite speleothems from natural cave settings lies in the range predicted by this study, between the predicted maximum non-equilibrium deviation at the stalagmite-like settings and the equilibrium 1000ln18αCc-H2O achieved in the pool-like settings.

Isotope exchange rates in dissolved inorganic carbon between 40 °C and 90 °C

Carbonate minerals are normally closely related to the stable and clumped isotope composition of the parent solution and are often assumed to record the equilibrium conditions in these proxies. Variations in e.g., temperature, pH or salinity lead to changes in the dissolved inorganic carbon (DIC) composition and to temporary deviations from expected isotopic equilibrium values. The exchange rate, at which the new equilibrium of stable and clumped isotopes is reached, has only been assessed experimentally for clumped isotopes between 5 °C and 25 °C and for δ18O up to 40 °C.In this study the δ18O and Δ47 evolution in the DIC pool of 0.1 molar sodium bicarbonate solutions at pH 8 and various temperatures (40 °C, 55 °C, 70 °C, 90 °C) was examined. At time intervals corresponding to the evolution of the DIC, a quantitative and instantaneous precipitation of the DIC was accomplished by adding strontium chloride and sodium hydroxide solutions. As the δ13C of the SrCO3 precipitates was constant and no significant non-first order trend was observed in Δ47, δ18O and Δ47 values could be simply fitted with an exponential approach to isotopic equilibrium. The temperature dependence of the rate constants was evaluated with the Arrhenius equation. The resulting rate constant based on the combined data points from δ18O and Δ47 follows askT[min-1]=exp-1.00±0.15·104∗1T+(29±4)The inferred exchange rate constants for Δ47 are of the same order of magnitude as those of the oxygen isotopes and show a similar temperature dependence in the investigated temperature range.Our experimental results suggest that the isotopic equilibration of DIC in a solution with pH < 10 and elevated temperatures is rapid (at 70 °C: 4.5 min, at 90 °C: 36 s for 99% equilibrium) and that even in the case of short-term disturbances an equilibrated DIC composition is very likely for solutions in the diagenetic temperature range above 100 °C. In contrast, it is important to consider the generally long equilibration times at low temperatures (e.g., 2–6 days at pH 8.0–8.5 and 0 °C) and, in particular, at higher pH (e.g., >100 days at pH 11 and 0 °C).

Kinetic clumped isotope fractionation in the DIC-H2O-CO2 system: Patterns, controls, and implications

The carbonate clumped isotope thermometer is a powerful tool in paleoclimate research, because it constrains carbonate formation temperature based on the extent of 13C and 18O clumping within the carbonate lattice (Δ47) and thus does not require knowledge of the isotopic composition of the water from which carbonates precipitate. However, there is growing evidence that kinetic processes in the precipitating solution, particularly the slow inter-conversion between CO2 and HCO3−, can cause deviations of the clumped isotope composition of dissolved inorganic carbon (DIC) from their expected equilibrium values, leading to disequilibrium clumped isotope composition in the carbonate precipitates. Such disequilibrium effects impede the application of the carbonate clumped isotope thermometer to important climate archives (e.g., speleothems and corals), and hinder the realization of the full potential of this novel thermometer. Here, I systematically examine the patterns and controls of kinetic clumped isotope fractionations (i.e., Δ47, Δ48 and Δ49) in the DIC-H2O-CO2 system through first-principles theoretical calculations and numerical modeling (IsoDIC), and explore their implications for the carbonate clumped isotope thermometry.I show that, in contrast to the large carbon and oxygen isotope effects, intrinsic clumped isotope fractionations associated with CO2 hydration and hydroxylation and their reverse reactions (i.e., HCO3− dehydration and dehydroxylation) will lead to only relatively small disequilibrium Δ47 effects in the HCO3− that are directly produced or consumed by these reactions. Instead, the disequilibrium Δ47 effects observed in most natural and laboratory-synthesized carbonates arise in large part from the mixing or removal of DIC pools with similar Δ47 compositions but distinct bulk carbon and oxygen isotope compositions. Further, my model simulations show characteristic evolutions of the clumped isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H2O isotope exchange, CO2 degassing, and CO2 absorption, and predict correlated enrichments in δ13C, δ18O and Δ48 but depletions in Δ47 and Δ49 of DIC during the early stage of CO2 degassing and vice versa during CO2 absorption, yielding typical disequilibrium Δ47/δ18O, Δ48/δ18O and Δ47/Δ48 slopes of about −0.03, 0.03, and −1.0 for CO2 degassing and about −0.02, 0.04, and −0.6 for CO2 absorption. While both the physicochemical condition (e.g., T, pH, [DIC], pCO2) and the isotopic composition (e.g., δ13C, δ18O, Δ47, Δ48, Δ49) of the aqueous solution and air CO2 can affect the magnitudes of these disequilibrium isotope effects, the correlations among these disequilibrium effects are relatively insensitive to changes in most environmental parameters except the isotopic composition of the aqueous solution and air CO2. The quantitative agreements between my model results and the existing observations from laboratory experiments and natural carbonates suggest that the model captures the key processes governing clumped isotope fractionations in the DIC-H2O-CO2-CaCO3 system, and support the development of novel approaches, e.g., coupled Δ47-Δ48 measurements, to discern the kinetic processes involved in carbonate formation and to correct for disequilibrium clumped isotope effects in carbonate archives and derive accurate estimates of their formation temperatures.