Dissolved Inorganic Carbon Values Research Articles

Distributions and controlling processes of the carbonate system in the Eastern Indian Ocean during autumn and spring

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 mmol m−2 d−1 during autumn 2020 and was 16.00 mmol m−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.

Simple bulk pigment analysis suggests microphytobenthos contributions to food webs may be underestimated due to isotopic contamination by remineralized wetland carbon

Stable isotope studies have revealed the importance of microphytobenthos (MPB) in coastal food webs. Microalgae typically have δ13C values between depleted C3 wetland/terrestrial macrophytes, and enriched C4 macrophytes and seagrasses. However, the challenges of obtaining clean samples of microalgae from sediments means they are often represented by limited sampling in many food web studies; consequently, we have a limited understanding of spatial and temporal variation in their δ13C values. We tested a simple technique to measure the δ13C of bulk pigments extracted from surficial sediments to represent MPB and applied it to quantify fine scale spatial variation in MPB δ13C around salt marshes. The bulk extraction method is logistically simple, and drives substantial but relatively consistent fractionation in δ13C of −3.5 ± 0.13 ‰ (mean ± 1 S.E., range = 2.3–4.4 ‰, n = 18 paired comparisons) compared to whole cell values. The consistency in fractionation suggests that spatial and temporal δ13C patterns measured in field samples should reflect real variation in source values, and that measured values could be corrected and incorporated into isotope mixing models. In 88 MPB samples among four marsh sites over two summers, MPB δ13C in marsh creeks was lower by an average of 4.4 ± 0.72 ‰ and up to 8.4 ‰ compared to sites along the outer marsh-open water fringe 10's of m away. Few food web studies incorporate this magnitude of variation in their MPB source estimates into mixing models. Over three weekly samplings at one marsh creek site, low tide dissolved inorganic carbon (DIC) δ13C was similarly lower by 4.8 ± 0.36 ‰ and up to 6.2 ‰ compared to high tide and adjacent open water DIC values. The significant small-scale variability in MPB δ13C appears to be driven by remineralized marsh carbon which depletes the DIC δ13C in the marsh creeks, a phenomenon that has long been recognized but is rarely considered in food web studies. Mixing models that assume a narrower range in MPB source values will erroneously attribute isotopically variable MPB contributions to end-member production sources thereby clouding our understanding of energy flows through coastal seascapes.

Coupling of Carbon and Oxygen in the Pearl River Plume in Summer: Upwelling, Hypoxia, Reoxygenation and Enhanced Acidification

AbstractAcidification and hypoxia are universal environmental issues in coastal seas, especially in large river estuaries such as the Pearl River estuary. In July and August of 2015, two legs of a field survey were conducted in the Pearl River plume. Leg 1 was sampled during the influence of upwelling favorable winds, while Leg 2 was during downwelling favorable winds. During both legs, instead of the typically observed dissolved inorganic carbon (DIC) consumption and dissolved oxygen (DO) over‐saturation, upwelling‐induced high DIC (>2,000 μmol kg−1), low pH (7.7–7.8) and low DO (140–150 μmol kg−1) values were observed in surface waters at the estuary mouth and the area off Hong Kong. In the bottom waters, hypoxia, acidification (pH 7.6–7.8) and DIC accumulation (DIC addition of ∼100–180 μmol kg−1) were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. The stoichiometry of oxygen depletion to DIC addition was 0.89 for bottom water, suggesting remineralization was dominated by marine sourced organic matter. However, a comparison of data from the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2 (0.73 ± 0.03 for Leg 1 vs. 0.80 ± 0.05 for Leg 2), although N/P ratios were the same (13.54 ± 1.93 for Leg 1 vs. 13.51 ± 2.04 for Leg 2). This phenomenon was attributed mainly to enhanced ventilation (re‐oxygenation) under the influence of the downwelling favorable winds during Leg 2. Although ventilation relieves some hypoxia, it might enhance acidification in bottom waters after a short‐term ventilation event. The enhanced acidification after short‐term ventilation is worthy of further study considering that most hypoxia and acidification are found in shallow coastal seas.

Distributions and Influencing Factors of Carbonate Species in Bohai Bay and Relevant Rivers Flowing into the Bay in Summer 2020

The distributions of pH, total alkalinity (Alk), dissolved inorganic carbon (DIC) and the partial pressure of carbon dioxide (pCO2) were investigated in Bohai Bay in northeastern China during summer 2020. The CO2 air–sea fluxes were also estimated for that summer. The results of the survey showed that the distributions of Alk and DIC in surface and bottom seawater were generally consistent, with the maximum values of DIC and Alk appearing at the same station, station BHB33 at the surface and station BHB35 at the bottom, which were in the middle of the bay. The minimum values of pH and the maximums value of pCO2 were both located at station HH2 at the surface and station HH3 at the bottom, which were near the mouth of the Haihe River. Bohai Bay was estimated as a source of CO2 in summer 2020. Carbonate species presented significant correlations with biochemical parameters, such as chlorophyll α (Chl-α), dissolved oxygen (DO) and nutrients. In addition, terrestrial inputs and human activities, together with the poor hydrodynamics of Bohai Bay are also the main factors influencing the carbonate species in Bohai Bay.

Interpretation of abnormally negative carbon isotope excursions in Eifelian–Givetian (Middle Devonian) sediments of South China and implications for paleo-environmental variation

Many negative carbon isotope excursions (NCIEs) have been employed to interpret global biogeochemical events in Earth history, but they are also influenced by local depositional settings and diagenetic alteration, thus making it difficult to link them to global events. Like the NCIEs in Middle Devonian sediments of North America, similar NCIEs were also observed in the age-equivalent sediments of South China, and therefore it is necessary to further address the logical significance of these intercontinentally comparable NCIEs. Here, we present major and trace elements as well as δ15N, δ13Ccarb, and δ13Corg records across the Middle Devonian Eifelian–Givetian transition in the intra-platform basin of South China. The changes in stable isotopes (δ13Ccarb, δ13Corg, δ18Ocarb, and δ15N) and their relationships may jointly indicate limited influence of diagenetic processes. Our abnormally negative δ13Ccarb values (−7 to −5‰), even lighter than the mantle input end-member value (∼ −5‰), cannot be easily explained by dramatic changes in the δ13C values of global dissolved inorganic carbon (DIC) reservoir. In a restricted platform basin, enhanced nutrient input may stimulate high paleo-productivity, resulting in the depletion of DIC concentrations in surface waters. CO2 is then replenished via invasion from the atmosphere, and intense evaporative conditions may promote the large isotopic fractionation of CO2 invasion owing to hydration reactions of aqueous CO2. Our NCIEs are most likely related to the local 13C-depleted values of platform DIC owing to CO2 invasion associated with large isotopic fractionation due to enhanced evaporation and primary productivity. The “productivity-anoxia feedback” mechanism can explain the coupled changes in our terrestrial clastics input, productivity, and redox proxies. By combining redox proxies and C and N isotopic records, we further propose a model for local carbon and nitrogen cycling in a restricted and evaporative intra-platform basin. Our study highlights local depositional controls on NCIEs, and these local factors should be considered before NCIEs are applied to understand the global carbon cycle perturbations during the Middle Devonian.

Carbon isotope fractionation and its tracer significance to carbon source during precipitation of calcium carbonate in the presence of Bacillus cereus LV-1

Microbial-induced carbonate precipitation (MICP) has the potential to immobilize carbon durably. The carbon source of carbonate minerals is the crucial issue for understanding the fixation mechanism of CO2 by MICP. However, the carbon source and its temporal changes in microbial-induced carbonate minerals remain little explored. In this study, calcium carbonate (CaCO3) biomineralization experiments have been carried out using Bacillus cereus in the medium without additional dissolved inorganic carbon (DIC). X-ray diffraction (XRD), attenuated total reflection-infrared spectroscopy (ATR-IR) and scanning electron microscopy (SEM) indicated that the precipitate produced mainly consisted of rhombohedral and irregular calcite. The δ13C values of DIC, dissolved organic carbon (DOC), and calcite were the main parameters studied. Carbon isotope fractionation was characterized by carbon isotope offset (Δ13Ccalcite-DIC or Δ13Ccalcite-fluid). The Δ13Ccalcite-DIC values ranged from +8.2‰ to +21.5‰, indicating that strain LV-1 induced the accumulation of 13C in calcite. The Δ13Ccalcite-fluid values indicated that the calcite was up to +15.0‰ (on average) 13C-enriched relative to the fluid. Calculated chemical mass balance data showed that the proportion of CO2(g) derived from DOC on days 16 and 20 was negative, but it became positive after day 20. Meanwhile, the δ13CCO2(g) value calculated by isotope mass balance was −8.5‰, near to δ13C for air (−8.0‰) on day 16, and then shifted to −16.4‰, similar to δ13C for tryptone (−17.2‰) after day 20. These results suggested that the amount of CO2 arising from organic matter through bacterial respiration and the action of enzymes might determine whether the carbon in calcite was derived from organic carbon or atmospheric CO2. Our findings corroborate the potential utility of MICP for immobilization of CO2 to reduce net soil CO2 emissions and to mitigate the greenhouse effect. Microbial-driven carbon isotope fractionation can also cause great carbon isotope shifts in calcite. Thus, carbon isotope value might be used as a marker to identify whether carbonate minerals/rocks are of microbial origin.

Influence of the advection of water masses in the Ballenas Channel on the CO[formula omitted] system in Bahía de los Angeles (Mexico)

The surface waters of the Ballenas Channel (BC) are characterized by thermohaline and biogeochemical conditions that depend on the water masses present. Since these surface waters are transported into Bahía de los Ángeles (BLA), it is relevant to assess their timing, intensity and biogeochemical implications on a seasonal scale. For this purpose, the seasonal and spatial variability of temperature, salinity, dissolved inorganic carbon (DIC) and total alkalinity (TA), were quantified over 19 discrete monthly monitoring campaigns that covered 12 stations in BLA from June 2017 to October 2019. In addition, vertical CTD profiles of thermohaline, apparent oxygen utilization (AOU), and nitrate were obtained for the purposes of identifying the water masses present and determining how they change by season. It was found that the dynamics of DIC, AOU, nitrate, and temperature in BLA respond to the presence of Subtropical Subsurface Water (SSW), evident by the enrichment of the bay with high DIC and nitrate concentrations during winter, but during summer occurs the opposite, with the presence of the Gulf of California Water (GCW). In addition, zonation analysis within BLA indicated three regions consistently found along the year, attributed to different physical and biogeochemical processes. Finally, we also interpreted the data in the context of the interannual variability due to ENSO events by using a relationship between observed temperature–DIC data. A 20-year calculated DIC (DICcalc) time series for an area near BLA indicated that during summer periods under the influence of La Niña events, the DICcal in front to BLA could be 10-12μmol kg −1 higher than El Niño events. These high DIC values, likely promoted by an increased volume of SSW in the BC, is accompanied by a higher nutrients concentration, suggesting that the interannual variability must interact with biological processes within BLA and other adjacent embayment’s in this region.

Effects of Shellfish and Macro-Algae IMTA in North China on the Environment, Inorganic Carbon System, Organic Carbon System, and Sea–Air CO2 Fluxes

Shellfish and macro-algae integrated multi-trophic aquaculture (IMTA) contribute greatly to the sustainability of aquaculture. However, the effects of large-scale shellfish and macro-algae aquaculture on the functions of the ocean carbon sink are not clear. To clarify these effects, we studied the spatial and temporal changes of inorganic and organic carbon systems in seawater under different aquaculture modes (monoculture or polyculture of shellfish and macro-algae) in Sanggou Bay, together with the variation of other environmental factors. The results show that the summertime dissolved oxygen (DO) concentration in the shellfish culture zone was significantly lower than other zones (p < 0.05), with a minimum value of 7.07 ± 0.25 mg/L. The variation of pH and total alkalinity (TA) were large across different culture modes, and the seawater in the shellfish culture zone had the lowest pH and TA than the other zones. Seasonal environment and aquaculture modes significantly affected the variation of dissolved inorganic carbon (DIC), CO2 partial pressure (pCO2), dissolved organic carbon (DOC), and particulate organic carbon (POC) concentrations. The highest values of DIC, pCO2, and POC appeared in summer, and the lowest appeared in winter. For DOC concentration, the lowest value appeared in autumn. Spatially, DIC and pCO2 were highest in the shellfish culture zone and lowest in the macro-algae culture zone, DOC was highest in the macro-algae culture zone and lowest in the shellfish culture zone, and POC was lower in the shellfish culture zone and macro-algae culture zone and higher in the remaining zones. The results of sea–air CO2 fluxes showed that except for the shellfish culture zone during summertime, which released CO2 to the atmosphere, all culture zones were the sinks of atmospheric CO2 during the culture period, with the whole bay being a strong CO2 sink during autumn and winter. In summary, large-scale shellfish–macro-algae IMTA plays an important role in the local carbon cycle and contributes to mitigating ocean acidification and hypoxia.

Understanding upper water mass dynamics in the Gulf of Mexico by linking physical and biogeochemical features

In the Gulf of Mexico (GoM), the upper 300 m of the water column contains a mixture of water types derived from water masses from the North Atlantic and the Caribbean Sea, namely Caribbean Surface Water (CSW), Subtropical Underwater (SUW), Gulf Common Water (GCW), and Tropical Atlantic Central Water (TACW). These are mainly altered by mesoscale processes and local evaporation, which modulate biogeochemical cycles. In this study, we improve our understanding of water mass dynamics by including biogeochemical data when evaluating the T-S relationship to define water-mass boundaries, particularly when the observed thermohaline characteristics overlap. The variables considered were apparent oxygen utilization (AOU), nitrate, and dissolved inorganic carbon (DIC). The data were obtained from eight cruises carried out in the central and southern regions of the GoM and an additional cruise that covered the entire coastal-ocean region. The new proposed boundaries were instrumental in clarifying the dynamics of surface waters. Of note, GCW on the western side of the GoM is not formed from the mixing of CSW and SUW but by the mixing of remnant CSW with TACW. In winter, a remnant of CSW mixed with GCW, and the biogeochemical composition of surface waters was affected, as observed from an increase in nitrate and DIC concentrations and positive AOU values. CSW was mainly detected at the surface during summer with negative AOU values, low DIC values, and almost undetectable nitrate concentrations. The presence or absence of CSW modulated the depth of the nitracline and likely influenced primary productivity.

Geochemical and Microbial Signatures of Siboglinid Tubeworm Habitats at an Active Mud Volcano in the Canadian Beaufort Sea

During the ARA08C expedition in 2017, sediment push cores were collected at an active mud volcano (420 m water depth) in the Canadian Beaufort Sea from two visually discriminative siboglinid tubeworm (ST) habitats that were colonized densely and less densely (ST1 and ST2, respectively). In this study, we investigated the biogeochemical and microbial community characteristics at ST1 by analyzing the geochemical properties, microbial lipids, and nucleic acid signatures, and comparing them with the data previously reported from ST2. The two ST sites showed distinct differences in vertical geochemical gradients [methane, sulfate, dissolved inorganic carbon (DIC), total organic carbon, and total sulfur], with a higher methane flux recorded at ST1 (0.05 mmol cm–2 y–1) than at ST2 (0.01 mmol cm–2 y–1). Notably, the δ13C values of DIC were more depleted at ST1 than at ST2, resulting in a higher proportion of DIC derived from the anaerobic oxidation of methane (AOM) at ST1 than at ST2. Moreover, both the ST1 and ST2 sites revealed the dominance of AOM-related lipid biomarkers (especially sn-2-hydroxyarchaeol), showing highly 13C-depleted values. The 16S rRNA analyses showed the presence of AOM-related archaea, predominantly anaerobic methanotrophic archaea (ANME)-3 at ST1 and ST2. Our results suggest that AOM-related byproducts (sulfide and DIC) potentially derived from ANME-3 were more abundant at ST1 than at ST2. This variation was attributed to the intensity and persistence of ascending methane. Therefore, our study suggests that AOM-derived byproducts are possibly an essential energy source for tubeworms during chemosynthetic metabolism, shaping different colony types on the seafloor.

Global trends of ocean CO2 sink and ocean acidification: an observation-based reconstruction of surface ocean inorganic carbon variables

Ocean acidification is likely to impact marine ecosystems and human societies adversely and is a carbon cycle issue of great concern. Projecting the degree of ocean acidification and the carbon-climate feedback will require understanding the current status, variability, and trends of ocean inorganic carbon system variables and the ocean carbon sink. With this goal in mind, we reconstructed total alkalinity (TA), dissolved inorganic carbon (DIC), CO2 partial pressure (pCO2sea), sea–air CO2 flux, pH, and aragonite saturation state (Ωarg) for the global ocean based on measurements of pCO2sea and TA. We used a multiple linear regression approach to derive relationships to explain TA and DIC and obtained monthly 1° × 1° gridded values of TA and DIC for the period 1993–2018. These data were converted to pCO2sea, pH, and Ωarg, and monthly sea-air CO2 fluxes were obtained in combination with atmospheric CO2. Mean annual sea–air CO2 flux and its rate of change were estimated to be − 2.0 ± 0.5 PgC year−1 and − 0.3 (PgC year−1) decade−1, respectively. Our analysis revealed that oceanic CO2 uptake decreased during the 1990s and has been increasing since 2000. Our estimate of the globally averaged rate of pH change, − 0.0181 ± 0.0001 decade−1, was consistent with that expected from the trend of atmospheric CO2 growth. However, rates of decline of pH were relatively slow in the Southern Ocean (− 0.0165 ± 0.0001·decade−1) and in the western equatorial Pacific (− 0.0148 ± 0.0002·decade−1). Our estimate of the globally averaged rate of pH change can be used to verify Indicator 14.3.1 of Sustainable Development Goals.

Tracing source and transformation of carbon in an epikarst spring-pond system by dual carbon isotopes (13C[sbnd]14C): Evidence of dissolved CO2 uptake as a carbon sink

δ13C and D14C measurements on dissolved inorganic carbon (DIC), particulate organic carbon (POC) and aquatic plants from a karst spring and two spring-fed ponds in Laqiao, Maolan Township, Libo County, southeastern Guizhou of China in January, July and October of 2013 have been carried out to understand the roles of aquatic photosynthesis through DIC uptake in surface karst waters. The mean D14C and δ13C values of DIC for the spring, midstream pond (MP) and downstream pond (DP) are −26 ± 36‰ and −13 ± 2‰, 6 ± 56‰ and −12 ± 3‰, and 0 ± 64‰ and −9 ± 2‰, respectively. The carbon source for the DIC is mainly from biogenic CO2 rather than the dissolution of limestone rock as the D14C and δ13C of limestone are about −1000‰ and 2‰, respectively. The enrichment trend of D14CDIC and δ13CDIC from the spring to the DP indicates CO2 exchange between atmospheric CO2 and DIC, because D14C and δ13C values of atmospheric CO2 are ca. 50‰ and −8‰, respectively. The average D14CPOC values in the spring, MP and DP were −325‰, −123‰ and −158‰, respectively, which are all lower than these of the DIC in each reservoir. The lower D14C values of the POC may be caused by older soil carbon from surface runoff and dust fall. More aquatic algae were formed through photosynthesis in the stream ponds, especially in summer, shown by strongly increased D14CPOC and evidence of growth in EDS/SEM analyses. Furthermore, the D14C values of the submerged aquatic plants range from −153‰ to −26‰, reflecting that the aquatic plants used DIC for photosynthesis. The D14C value of an emergent plant which uses atmospheric CO2 during photosynthesis is 52.5 ± 0.3‰, equivalent to the atmospheric D14C. Seasonal variations of D14CDIC and δ13CDIC are influenced by soil CO2 input, primary productivity in the ponds, and CO2 exchange; hydrochemical condition show lower D14C values but higher δ13C values in cold/dry season, and vice versa in summer rainy season. A simple mass balance calculation indicates ~90% of carbon for the spring DIC is from biogenic CO2, with higher contribution in summer due to higher productivity. Although this simple calculation may overestimate the biogenic CO2, it indicates that organic decomposition is a major carbon source for DIC in the karst hydrological system. The results of the present study have implications for 14C dating on aquatic plant remains, regional and perhaps global carbon budgets, and the different behaviors of 13C and 14C in karst systems.

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.

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.

Novel reverse radioisotope labelling experiment reveals carbon assimilation of marine calcifiers under ocean acidification conditions

Abstract Ocean acidification by anthropogenic carbon dioxide emissions is projected to depress metabolic and physiological activity in marine calcifiers. To evaluate the sensitivity of marine organisms against ocean acidification, the assimilation of nutrients into carbonate shells and soft tissues must be examined. We designed a novel experimental protocol, reverse radioisotope labelling, to trace partitioning of nutrients within a single bivalve species under ocean acidification conditions. Injecting CO2 gas, free from radiocarbon, can provide a large contrast between carbon dissolved in the water and the one assimilated from atmosphere. By culturing modern aquifer organisms in acidified seawater, we were able to determine differences in the relative contributions of the end members, dissolved inorganic carbon (DIC) in seawater and metabolic CO2, to shell carbonate and soft tissues. Under all pCO2 conditions (463, 653, 872, 1,137 and 1,337 μatm), radiocarbon (Δ14C) values of the bivalve Scapharca broughtonii shell were significantly correlated with seawater DIC values; therefore, shell carbonate was derived principally from seawater DIC. The Δ14C results together with stable carbon isotope (δ13C) data suggest that in S. broughtonii shell δ13C may reflect the kinetics of isotopic equilibration as well as end‐member contributions; thus, care must be taken when analysing end‐member contributions by a previous method using δ13C. The insensitivity of S. broughtonii to perturbations in pCO2 up to at least 1,337 µatm indicates that this species can withstand ocean acidification. Usage of radioisotope to dope for tracer experiments requires strict rules to conduct any operations. Yet, reverse radioisotope labelling proposing in this study has a large advantage and is a powerful tool to understanding physiology of aquifer organisms that can be applicable to various organisms and culture experiments, such as temperature, salinity and acidification experiments, to improve understanding of the proportions of nutrients taken in by marine organisms under changing environments.

Autotrophic metabolism considered to extend the applicability of the carbon balances model for assessing biodegradation in petroleum-hydrocarbon-contaminated aquifers with abnormally low dissolved inorganic carbon

The carbon balance model has been used as a general method to assess biodegradation in petroleum hydrocarbon-contaminated (PHC) aquifers for two decades. The model assumes that heterotrophic metabolism is the dominant process for degrading hydrocarbon to CO2. Another important metabolic process that autotroph consumption of CO2 is usually considered to be negligible. However, a phenomenon of abnormally low dissolved inorganic carbon (DIC) was found in the North China Plain. According to the biodegradation mechanism, the existing model encounters the problem and only the consideration of autotrophic metabolism can explain the phenomenon. Therefore, in this paper, the carbon balance model was extended to four specific processes “heterotrophic metabolism (H)→ heterotrophic metabolism with precipitation (HP) → autotrophic metabolism (A) → autotrophic metabolism with precipitated (AP)”. Using the model, measurements of biodegradation at three time points over two years were 2.02, 1.46 and 1.36 (×105 mol.a−1) respectively, close to another common method of terminal electron acceptors (TEAs) method (1.93, 0.71, and 1.34 (×105 mol.a−1)). Moreover, supported by the δ 13C of DIC values and, metagenomics data, this developed model was evaluated to be reliable. Given that PHC sites are widespread, the developed model can give more accurate carbon emission estimation for climatic change evaluation and is significant for the environmental communication for energy utilization.

Dynamics of the Carbonate System in the Western Indonesian Seas During the Southeast Monsoon

AbstractWe present a unique water column data set of dissolved inorganic carbon (DIC) and total alkalinity (TAlk) from a cruise to the western Indonesian Seas during the southeast monsoon, covering the Karimata Strait, western Java Sea, and Sunda Strait. Salinity‐normalized TAlk (NTAlk) in the surface water ranged 2,297–2,348 μmol kg−1, very close to typical values observed in the tropical ocean. In the Karimata Strait, the Kapuas River plume was observed, featuring low salinity, DIC, and TAlk. In the western Java Sea, where waters were well mixed, we observed relatively homogeneous distributions of salinity, DIC, and TAlk. In the Sunda Strait, waters intruding from the Java Sea occupied the upper layer, and below was the Indian Ocean water with lower values of salinity, DIC, and TAlk. In its deep portion, depth profiles of normalized DIC and NTAlk were very similar to those observed in the Indian Ocean. Physical processes and air‐sea gas exchange exerted predominant controls on the carbonate system in the Karimata Strait and western Java Sea. While both processes play large roles in the Sunda Strait, a net DIC removal of 31 ± 23 μmol kg−1 in the surface mixed layer were revealed. The drawdown of DIC is consistent with an overall supersaturation of dissolved oxygen (102–107%), suggesting significant organic carbon production. In the subsurface‐intermediate waters of the Sunda Strait mainly influenced by the advection of Indian Ocean water, a net DIC consumption of 54 ± 45 μmol kg−1 was distinct, likely stimulated by the nutrients supplied from the Indian Ocean.

Spatiotemporal changes of ocean carbon species in the western North Pacific using parameterization technique

We constructed parameterizations for the estimation of dissolved inorganic carbon (DIC) and pH in the western North Pacific, including Japanese coastal regions. Parameterizations, determined as a function of potential temperature (θ) and dissolved oxygen (DO), provided strong correlations with direct measurements for DIC [the coefficient of determination (R2) = 0.99; the root mean square error (RMSE) = 8.49 µmol kg−1] and pH (R2 = 0.98, RMSE = 0.030). Predicted DIC and pH values were consistent with previous independent hydrographic observations, generally within 3 µmol kg−1 for DIC and 0.010 for pH. By applying these parameterizations to climatological seasonal θ and DO data and time-series θ and DO data from autonomous profiling floats (Argo) in the western North Pacific, large spatiotemporal variations in DIC and pH were demonstrated. Results indicated that the rate of increase in anthropogenic DIC above a depth of 400 m along 30° N in the western North Pacific was + 0.86 ± 0.11 µmol kg−1 year−1 between 1994 and 2004. This decreased by + 0.58 ± 0.05 µmol kg−1 year−1 after 2004 partly due to increasing anthropogenic nitrogen input, suggesting that the decrease has induced the slowdown of ocean acidification in the western North Pacific. The declining trend in anthropogenically induced ocean pH was found to be 0.0021 ± 0.0016 during 1994–2013.

Quantifying carbon isotope disequilibrium during in-cave evolution of drip water along discreet flow paths

Paleoclimate reconstructions that use speleothem proxy data have increased our understanding of terrestrial climate change, but gaps remain in our understanding of in-cave processes that influence speleothem chemistry. The δ13C values of speleothem calcite are typically influenced by kinetic isotope effects that operate during CO2 degassing and calcite precipitation. Therefore the identification and quantification of these isotopic effects is important in interpreting speleothem stable isotope records. Here we studied the change in water chemistry and δ13C values of dissolved inorganic carbon (DIC) along discreet flow paths at multiple drip sites in Inner Space Cavern, central Texas. We quantified the extent to which the water remains in C isotopic equilibrium during flow along speleothem surfaces as CO2 degasses and calcite precipitates.Two locations in the study cave that have long in-cave flow paths were examined to determine the geochemical evolution and its driving processes along these paths. At each location cave water was sampled at two points 1–2 meters apart along each flow path. Among the key spatial changes observed is a <∼1‰ to ∼4‰ increase in δ13C values of DIC along the flow paths. The magnitude of the increase in δ13C values is controlled by the extent of DIC loss to CO2 degassing. The extent of DIC loss and CO2 degassing is controlled by the pCO2 gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ13C value of the DIC reservoir can be accounted for by a Rayleigh distillation model with equilibrium C-isotope fractionation factors for (CO2(g)-HCO3−(aq)) and (CaCO3-HCO3−(aq)). As the depletion of the DIC reservoir exceeds 15% the DIC δ13C values become progressively higher such that the (HCO3−(aq)-CO2(g)) fractionation values needed to explain the observations change from equilibrium values of ∼8‰ to non-equilibrium values of up to ∼25‰. This variance in magnitude of C-isotope fractionation during CO2 degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss. Such kinetic effects have significant implications for speleothem C-isotope proxy interpretations, as these kinetic isotope effects are of a similar magnitude as those used to infer past changes in drought and vegetation.

Methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the Mid-Okinawa Trough

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).