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Tropical carbonate systems are valuable archives of paleoenvironments, as the carbonate growth is intimately affected by water depth and climatic conditions. Geochemical data from the Burdigalian interval in IODP Site U1468 in the Maldives, northern Indian Ocean, were integrated with sedimentological and paleontological data for a more detailed reconstruction of depositional history. Generally, the Sr/Ca values of slope sediments record highstand progradation in both sequence unit and whole Burdigalian interval, while the absence of higher Sr/Ca ratio close to the sequence boundary during the early Burdigalian could be related to the erosion of deeper-water sediments due to the activity of bottom current. From 20.5 to 19.1 Ma and from 17.9 to 17.2 Ma, nutrient level and productivity were moderately elevated due to the terrigenous input by the intensified South Asian Proto-Monsoon, which also helped cause more reducing conditions in the distal slope. Moreover, increased nutrient level facilitated the growths of calcareous algae and sponges, while it was not favorable for coral development. The elevated nutrient level, higher sea level, and monsoon-induced current contributed to the backstepping of the outer margin during the late Burdigalian. Our study shows an example on how a tropical carbonate platform evolved in response to the interplay of sea-level and paleoclimatic conditions. Findings are expected to be applicable to other tropical carbonate platforms. [ABSTRACT FROM AUTHOR]

Carbonate minerals are globally distributed on the modern and ancient Earth and are abundant in terrestrial and marine depositional environments. Fluid inclusions hosted by calcite retain primary signatures of the source fluid geochemistry at the time of mineral formation (i.e., pCO2) and can be used to reconstruct paleoenvironments. Confocal laser Raman spectroscopy provides a quick, nondestructive approach to measuring the constituents of fluid inclusions in carbonates and is a reliable method for qualitatively determining composition in both the aqueous and gas phases. Here, we demonstrate a method for accurately quantifying bicarbonate and carbonate ion concentrations (down to 20 mM) and pH (7–11) from calcite fluid inclusions using confocal Raman spectroscopy. Instrument calibrations for carbonate (CO32–) and bicarbonate (HCO3–) concentrations and pH were performed using stock solutions. We show that the calcite host mineral does not affect the accurate quantification of carbonate solution concentrations and that these parameters can be used to estimate the pH and pCO2 of a solution entrapped within a fluid inclusion. We apply the technique to Icelandic spar calcite and find a [CO32–] = 0.11, [HCO3–] = 0.17, pH = 10.1, and CO2 parts per million = 2217. The presence of gaseous Raman bands for CO2, CH4, and H2S suggests that the mineral precipitated in a reducing environment. [ABSTRACT FROM AUTHOR]

The results of high-precision direct determination of CO2 in the surface waters of the Black Sea are presented. These measurements made possible to obtain high precision characteristics of the content and intra-annual variations in pCO2 for the first time. The average annual value of pCO2 is 436 µatm, which is higher than the average annual value in the near-sea surface layer of the atmosphere (420 µatm). The seasonal variability has revealed decrease of pCO2 in seawater from late spring to fall. The minimum values of pCO2 are detected in January–February; the maximum ones, in July. The seasonal variations in the CO2 content in the near-sea atmosphere reveal an inverse relationship, with maximum values in February and minimum values in July. This indicates different mechanisms of the evolution of CO2 in the air and seawater. The content of CO2 in the atmosphere is determined by external sources and depends on the burning intensity of organic carbon. Variations in pCO2 in water is significantly affected by abiotic factors, such as the influence of temperature change on CO2 solubility and the state of the carbon system. In the summer–fall season, pCO2 is determined by a combination of abiotic and biotic factors, including variations in temperature and intensity of biological processes of organic matter transformation. Both the intensity and effect of these factors vary throughout the year, but the abiotic factor remains primay during the entire year. [ABSTRACT FROM AUTHOR]

The dissolution of CO2 in seawater in the form of bicarbonate ions is an attractive alternative to storage in geological formations, on the condition that the storage is stable over long periods and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO2 absorbed in seawater as bicarbonate by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity) in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists of mixing it with pre-equilibrated solutions obtained from the reaction of CO2 and Ca(OH)2, with the same pH as natural seawater. This was achieved with a pilot plant working with tons of seawater, while small-scale laboratory experiments were carried out by adding sodium bicarbonate to artificial seawater solutions. If the increase in the overall carbon concentration in the final mixture does not exceed a critical threshold (about 1000–1500 μmol/L), the resulting bicarbonate-rich solutions are found to be stable for over three months. [ABSTRACT FROM AUTHOR]

Coastal ecosystems with karstic geology have a unique characteristic where the dissolution of carbonate rocks can increase total alkalinity (TA) and dissolved inorganic carbon (DIC). This results in higher inorganic carbon budgets in coastal areas. One such ecosystem is the Terminos Lagoon, the most extensive tropical estuarine lagoon system in Mexico, located in the karstic aquifer of the Yucatan Peninsula and connected to the southern Gulf of Mexico (sGoM). We measured TA and DIC to evaluate the variability in Terminos Lagoon's of the carbonate system. We also estimated pH, partial CO2 pressure (pCO2), and aragonite saturation (ΩAr) along two transects from the main lagoon tributaries (Palizada and Candelaria rivers) to the coastal zone during the dry and rainy seasons. During the dry season, TA and DIC concentrations were significantly higher (3092 ± 452 µmol kg-1 TA, 2943 ± 522 µmol kg-1 DIC) than during the rainy season (2533 ± 228 µmol kg−1 TA, 2492 ± 259 DIC µmol kg−1). Our calculations indicate that the rainy season pCO2 (2532 ± 2371 µatm) seems higher than in the dry season (1534 ± 1192 µatm). This leads to a reduction in pH (7.9 ± 0.3 to 7.8 ± 0.3). These significant changes indicate that rain increases the flow of unsaturated river water into the lagoon. The results of this work contribute toward a dissolved inorganic carbon variability baseline in the sGoM and can be helpful to Terminos Lagoon decision-makers. [ABSTRACT FROM AUTHOR]

In this work, the seasonal variations of the carbonate system and air‐sea CO2 fluxes were investigated by two cruises in the southern Yellow Sea (SYS) and the East China Sea (ECS), which were significantly influenced by the Changjiang riverine inputs across seasons. Biological‐mediated change of dissolved inorganic carbon (DIC) was first quantitated through a three end‐member mixing model from winter to spring. Our modeling results suggested that DIC addition through biological remineralization persisted in the SYS during winter and spring, while DIC removal was evident in the Changjiang River Plume and the offshore ECS triggered by river inputs in spring. Horizontal and vertical mixing together constituted the largest contribution to the interseasonal variability of partial pressure of CO2 (pCO2) in the SYS (−69.5%), the river plume (−59.3%), and the ECS offshore (−43.8%), followed by temperature effects in the plume area (29.4%), and biological processes in the ECS offshore (24.1%) and the SYS (−7.7%), with air‐sea CO2 exchange contributing the least in both three subregions. The SYS, the river plume, and the ECS offshore all acted as atmospheric CO2 sinks in both winter and spring. Furthermore, their ability to absorb atmospheric CO2 increased from winter to spring, as reflected in a ∼1.8‐fold increase in the overall spring air‐sea CO2 flux compared to winter estimation. Plain Language Summary: The carbonate system has played an essential role in regulating the ocean carbon cycle due to its buffer capacity for ocean acidification, especially in continental shelf seas. However, complex biological and physical processes present difficulties in assessing the seasonal dynamics of the carbonate system and following air‐sea CO2 fluxes. In this study, seasonal variations of the carbonate system were identified in the southern Yellow Sea (SYS) and the East China Sea (ECS). From winter to spring, biological DIC addition happened in the SYS, while DIC removal by biological production exceeded the addition by remineralization and caused most springtime DIC consumption in the river plume and the ECS offshore. We also quantified pCO2 variations from winter to spring, revealing the importance of physical mixing in controlling the ocean carbon cycle. Our study regions acted as atmospheric CO2 sinks in winter and spring, and their sink capacity was strengthened from winter to spring. Key Points: Dissolved inorganic carbon (DIC) addition occurred in the southern Yellow Sea (SYS) during winter and spring, while springtime DIC removal occurred in the river plume and the East China Sea (ECS) offshorePhysical mixing became the primary factor in regulating the interseasonal variation of pCO2The capacity of coastal waters in absorbing atmospheric CO2 was enhanced from winter to spring [ABSTRACT FROM AUTHOR]

Ocean Negative Carbon Emission (ONCE) involves utilizing natural marine chemistry and biology, along with mariculture, to achieve carbon sink goals. Growing awareness of the interplay between aquaculture and the coastal carbonate system has drawn researchers’ attention amid ring CO2 concentrations and the negative impacts of aquaculture on the environment. In this study, twelve sites representing different maricultural types were selected, including macroalgae, shellfish, fish, and non-farming areas. The environmental factors, dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO2, were measured monthly during kelp farming periods. Nitrate is a major component of total nitrogen, and the NO3-N concentration in the macroalgal culture zone was lower than others, indicating effective nitrogen removal by macroalgae aquaculture. TA and DIC in non-farmed areas demonstrated larger variation ranges than in farming areas, probably due to the effects of precipitation on salinity. Aquaculture activities effectively maintained TA and DIC, with macroalgae cultivation playing an important role in TA stability, potentially resisting acidification. The pCO2sea-air of macroalgae culture areas in spring was slightly negative, suggesting carbon sink potential. However, further research is needed to assess the full extent of this “fourth type” of blue carbon, including accurate carbon footprint calculation and the contributions of particulate organic carbon and recalcitrant dissolved organic carbon. This study provided insight into the comprehensive contribution of different aquaculture types to the fishery environment and carbonate system, which can help guide aquaculture management and facilitate the carbon-neutral transition of aquaculture. [ABSTRACT FROM AUTHOR]

A basin-scale Mediterranean carbonate system model has been setup, building on the POSEIDON operational biogeochemical model. The spatial variability of carbonate system variables from a 13-year simulation (2010-2022) was validated against CARIMED in situ data (DIC, TA, pCO2), showing reasonable agreement in reproducing the observed patterns and preserving the dynamics in different areas, except a slight overestimation (~15 µmol/kg) of TA in the Eastern Levantine. The time-variability of model outputs (DIC, TA, pCO2, pH) was validated, against available time-series from Western (DYFAMED, Villefranche-PointB) and Eastern Mediterranean (HCB) sites, showing good agreement with the data, particularly for pCO², pH and DIC. The model failed to reproduce the observed late summer peak of TA at DYFAMED/PointB sites, which may be partly attributed to the advection of lower alkalinity Atlantic water in the area. The seasonal variability of DIC and pCO2@20°C was found to be mainly controlled by winter mixing and the subsequent increase of primary production and net CO2 biological uptake, which appeared overestimated at HCB. Along with the reference simulation, three sensitivity simulations were performed, de-activating the effect of biology, evaporation and CO2 air-sea fluxes on DIC and TA, in order to gain insight on the processes regulating the simulated carbonate system variability. The effect of biological processes on DIC was found more significant (peak during spring) in the more productive North Western Mediterranean, while evaporation had a stronger impact (peak during late summer) in the Levantine basin. CO2 air-sea flux was higher in the Western Mediterranean, particularly the Gulf of Lions and Alboran Sea, as well as in river influenced areas, such as the N. Adriatic and along the pathway of the Black Sea Water in the Aegean. A weak release of CO2 was found in the Eastern Levantine and Libyan Sea. Its basin average (+2.1 mmol/m² / day) and positive trend (+0.1 mmol/m² /day/year) indicates a gradually increasing net CO2 ocean uptake. The simulated positive trends of DIC (0.77 mmol/kg/year) and TA (0.53 mmol/kg/year) in the North Western Mediterranean were consistent with observational and modelling studies, in constrast with the Levantine basin, where no significant trends were found for TA. [ABSTRACT FROM AUTHOR]

Understanding the factors that control carbonate systems is an important goal due to the complex interactions between the hydrophysical and chemical–biological conditions in coastal basins. The results of this paper present the state of the carbonate system in Penzhina Bay and its adjacent waters—the Shelikhov Gulf—in July 2023, during spring tides with 13 m height. The area we studied included the length of the largest river in the region, the Penzhina River, from the peak of its summer flood to its boundary with the Shelikhov Gulf (the Sea of Okhotsk). This unique dynamic basin, with a length of about 800 km, was studied over 17 days. During this period, the entire water column of Penzhina Bay, down to a depth of about 60 m, and the surface water layer of the Shelikhov Gulf were undersaturated in terms of CO2, with low levels relative to those of the atmosphere. To explain this observation, the dissolved oxygen, nutrients in mineral and organic forms, humic substances, chlorophyll a, and photic zone thickness are presented for the entire basin under study, together with its hydrological data. The results of daily observations of the carbonate system at fixed anchorage stations characterize two contrasting regions of Penzhina Bay: one that was more exposed to continental runoff, which had salinity levels in the range of 8.0–21.3 psu during one tidal cycle; the second had smaller variations in salinity in the range of 31.6–32.9 psu during one tidal cycle. This study emphasizes the importance of biological processes and continental runoff on the variability of the carbonate system parameters and CO2 fluxes at a water/atmosphere boundary with extreme tidal conditions in this ecosystem that is barely affected by human activities. [ABSTRACT FROM AUTHOR]

Sustained wetland utilization has been effective in purifying urban riverine pollutants and promoting sustainable development. However, its effect on water CO2 system remains unclear in semi-arid areas. In this study, seasonal monitoring of the carbonate system was performed at two compared stations, i.e., in constructed wetland (Xiangyun station) and its upstream (Lancun station) in a semi-arid river (the Fenhe River) in China. As indicated by the result of the sustained monthly observation from May 2020 to May 2021, riverine dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) reached 30.9–46.7 mg L−1 and 524–1050 μatm in Lancun station, respectively, whereas the above-described values declined significantly in Xiangyun station with the values of 24.1–39.1 mg L−1 for DIC and 188–873 μatm for pCO2. Compared with the Lancun station where the carbonate system was primarily controlled by natural factors (e.g., carbonate weathering and temperature), significant aquatic photosynthesis and calcification precipitation due to constructed wetland triggered the decrease in DIC and pCO2 and dominated their temporal variation in Xiangyun station. Thus, the large CO2 reduction arising from constructed wetlands may create vital paths for CO2 neutralization and sustainable conservation in urban rivers in arid and semi-arid areas in the future. [ABSTRACT FROM AUTHOR]

A five-day in situ experiment was conducted in plastic enclosures to investigate the effects of different densities of the mussel Mytilus edulis, and sampling time on water variables, and phytoplankton community structure in Sanggou Bay (Shandong, China). The tested mussel densities were low (LD300: 300 g/m3), medium (MD600: 600 g/m3), high (HD900: 900 g/m3), and sampling times were at 6:00 and 17:00 h. The water variables analyzed as indicators were pH, dissolved oxygen, nutrients, carbonate system, pCO2, and chlorophyll-a (Chl-a). The results showed that density and sampling time had highly significant effects on all water parameters and phytoplankton abundance, except for dissolved oxygen (P

In the northern Gulf of Mexico shelf, the Mississippi‐Atchafalaya River System (MARS) impacts the carbonate system by delivering freshwater with a distinct seasonal pattern in both total alkalinity (Alk) and dissolved inorganic carbon (DIC), and promoting biologically driven changes in DIC through nutrient inputs. However, how and to what degree these processes modulate the interannual variability in calcium carbonate solubility have been poorly documented. Here, we use an ocean‐biogeochemical model to investigate the impact of MARS's discharge and chemistry on interannual anomalies of aragonite saturation state (ΩAr). Based on model results, we show that the enhanced mixing of riverine waters with a low buffer capacity (low Alk‐to‐DIC ratio) during high‐discharge winters promotes a significant ΩAr decline over the inner‐shelf. We also show that increased nutrient runoff and vertical stratification during high‐discharge summers promotes strong negative anomalies in bottom ΩAr, and less intense but significant positive anomalies in surface ΩAr. Therefore, increased MARS discharge promotes an increased frequency of suboptimal ΩAr levels for nearshore coastal calcifying species. Additional sensitivity experiments further show that reductions in the Alk‐to‐DIC ratio and nitrate concentration from the MARS significantly modify the simulated interannual ΩAr patterns, weakening the positive surface ΩAr anomalies during high‐discharge summers or even producing negative surface ΩAr anomalies. Our findings suggest that riverine water carbonate chemistry is a main driver of interannual variability in ΩAr over river dominated ocean margins. Plain Language Summary: The Mississippi river strongly impacts seawater chemistry and biological production over the northern Gulf of Mexico, leading to changes in ocean acidification. An increased acidity is concurrent with increased solubility in calcium carbonate (CaCO3), which has the potential to negatively impact calcifying organisms that build‐up hard structures of this mineral compound, like shellfish and corals. Near‐corrosive acidic conditions for CaCO3 have been documented in the proximity of the Mississippi delta during summer, but it is not well known how changes in the Mississippi river discharge and chemistry influence the year‐to‐year changes in CaCO3 solubility. Here we use a numerical model that realistically simulates oceanic and biogeochemical dynamics to investigate this aspect. Our model identifies seasonal distinctive mechanisms leading to increased CaCO3 solubility nearshore, mainly driven by the mixing of relatively more acidic Mississippi river waters (compared to seawater) during high‐discharge winters, and biological processes stimulated by riverine nutrients during high‐discharge summers. The increased CaCO3 solubility in high‐discharge years promotes suboptimal conditions for the growth of calcifying organisms, and the opposite is true in low‐discharge years. Our study highlights that river discharge, river acidity, and river nutrient concentrations are key variables modulating the interannual CaCO3 solubility patterns in river‐influenced coastal margins. Key Points: We simulated the Mississippi River impacts on interannual patterns of aragonite saturation (ΩAr) in the northern Gulf of MexicoHigh‐discharge conditions increase the frequency of suboptimal ΩAr levels for coastal calcifying species in all seasonsReduced ΩAr under high‐discharge is linked to low buffer capacity of riverine water in winter and enhanced bottom remineralization in summer [ABSTRACT FROM AUTHOR]

Sustained time-series measurements are crucial to understand changes in oceanic carbonate chemistry. In the North Western Mediterranean Sea, the temporal evolution of the carbonate system is here investigated based on two 10-year time-series (between January 2010 and December 2019) of monthly carbonate parameters measurements at two sampling sites in the Ligurian Sea (ANTARES and DYFAMED). At seasonal timescale, the seawater partial pressure of CO2 (pCO2) within the mixed layer is mostly driven by temperature at both sites, and biological processes as stated by the observed relationships between total inorganic carbon (CT), nitrate and temperature. This study suggests also that mixing and water masses advection could play a role in modulating the CT content. At decadal timescale, significant changes in ocean chemistry are observed with increasing trends in CT (+3.2 ± 0.9 µmol.kg-1.a-1 - ANTARES; +1.6 ± 0.8 µmol.kg-1.a-1 - DYFAMED), associated with increasing pCO2 trends and decreasing trends in pH. The magnitude of the increasing trend in CT at DYFAMED is consistent with the increase in atmospheric pCO2 and the anthropogenic carbon transport of water originating from the Atlantic Ocean, while the higher trends observed at the ANTARES site could be related to the hydrological variability induced by the variability of the Northern Current. [ABSTRACT FROM AUTHOR]

The equilibrium states and kinetics of changes in the concentrations of carbonate system components in water with different calcium hardness are analyzed and the possibility of reducing this indicator by an environmentally friendly method, aeration, is evaluated. Changes in the concentrations of dissolved carbon dioxide, calcium ions, hydrogen ions (pH), bicarbonate and carbonate ions in water depending on the partial pressure of carbon dioxide in soil and atmospheric air and the kinetics of reducing the bicarbonate-calcium hardness of water during its natural and forced aeration were traced. It is shown that water purification by a stream of atmospheric air microbubbles with an average radius of 50 µm and a flow rate of 10 l/min can reduce the hydrogen carbonate-calcium hardness from 8 mg-eq/l to 1 mg-eq/l in 100 liters of water in 20 minutes. In the paradigm of «green chemistry», this method is more rational, environmentally friendly, and economical than ion exchange or reverse osmosis, which are currently practiced to reduce hardness in decentralized water supply systems. [ABSTRACT FROM AUTHOR]

A basin-scale Mediterranean carbonate system model has been setup, building on the POSEIDON operational biogeochemical model. The spatial variability of carbonate system variables from a 13-year simulation (2010-2022) was validated against CARIMED in situ data (DIC, TA, pCO2), showing reasonable agreement in reproducing the observed patterns and preserving the dynamics in different areas, except a slight overestimation (~15 µmol/kg) of TA in the Eastern Levantine. The time-variability of model outputs (DIC, TA, pCO2, pH) was validated, against available time-series from Western (DYFAMED, Villefranche-PointB) and Eastern Mediterranean (HCB) sites, showing good agreement with the data, particularly for pCO2, pH and DIC. The model failed to reproduce the observed late summer peak of TA at DYFAMED/PointB sites, which may be partly attributed to the advection of lower alkalinity Atlantic water in the area. The seasonal variability of DIC and pCO2@20°C was found to be mainly controlled by winter mixing and the subsequent increase of primary production and net CO2 biological uptake, which appeared overestimated at HCB. Along with the reference simulation, three sensitivity simulations were performed, de-activating the effect of biology, evaporation and CO2 air-sea fluxes on DIC and TA, in order to gain insight on the processes regulating the simulated carbonate system variability. The effect of biological processes on DIC was found more significant (peak during spring) in the more productive North Western Mediterranean, while evaporation had a stronger impact (peak during late summer) in the Levantine basin. CO2 air-sea flux was higher in the Western Mediterranean, particularly the Gulf of Lions and Alboran Sea, as well as in river influenced areas, such as the N. Adriatic and along the pathway of the Black Sea Water in the Aegean. A weak release of CO2 was found in the Eastern Levantine and Libyan Sea. Its basin average (+2.1 mmol/m2/day) and positive trend (+0.1 mmol/m2/day/year) indicates a gradually increasing net CO2 ocean uptake. The simulated positive trends of DIC (0.77 μmol/kg/year) and TA (0.53 μmol/kg/year) in the North Western Mediterranean were consistent with observational and modelling studies, in constrast with the Levantine basin, where no significant trends were found for TA.

The dissolution of CO2 in seawater in the form of bicarbonate ions is an attractive alternative to storage in geological formations, on the condition that the storage is stable over long periods and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO2 absorbed in seawater as bicarbonate by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity) in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists of mixing it with pre-equilibrated solutions obtained from the reaction of CO2 and Ca(OH)2, with the same pH as natural seawater. This was achieved with a pilot plant working with tons of seawater, while small-scale laboratory experiments were carried out by adding sodium bicarbonate to artificial seawater solutions. If the increase in the overall carbon concentration in the final mixture does not exceed a critical threshold (about 1000–1500 μmol/L), the resulting bicarbonate-rich solutions are found to be stable for over three months.

Within the program "Ecosystems of the Siberian Arctic Seas," carried out by Shirshov Institute of Oceanology, Russian Academy of Sciences since 2007, studies of the water structure and spatial variability of the parameters of the carbonate system have been performed, and the intensity and direction of the carbon dioxide flux over the continental slope of the Laptev Sea and in the Vilkitsky Strait in September 2018 have been calculated. The presence of several main water masses that govern the water structure in the study area is shown. A strong spatial variability of the parameters of the carbonate system of seawater, determined by complexes of physical and chemical–biological processes, has been revealed. The intensity and direction of the carbon dioxide flux at the water–atmosphere boundary were calculated, which range from –12 to 4 mmol m–2 day–1. It was revealed that the investigated area of the outer shelf and continental slope of the Laptev Sea is an emitter of carbon dioxide into the atmosphere as of September 2018. Conversely, the area of the Vilkitsky Strait, is a CO2 sink zone. [ABSTRACT FROM AUTHOR]

Foraminiferal Mg/Ca has proven to be a powerful paleothermometer for reconstructing past sea‐surface temperature, which, among other applications, is a critical parameter for boron isotope reconstructions of past surface ocean pH and PCO2. However, recent laboratory culture studies indicate seawater pH and the total dissolved inorganic carbon content (DIC) may both exert a significant additional control on foraminiferal Mg/Ca, likely influencing paleotemperature records as a result of seawater chemistry evolution on geologic timescales. In addition, the seawater Mg/Ca composition (Mg/Casw) has been shown to reduce the sensitivity of foraminiferal Mg/Ca to temperature and possibly its sensitivity to the carbonate system as well. Here we present new Mg/Ca data from laboratory culture experiments with living planktic foraminifera— Globigerinoides ruber (p), Trilobatus sacculifer, and Orbulina universa— grown under a range of different pH and/or seawater DIC conditions and in low Mg/Casw to mimic the chemical composition of the Paleocene ocean. We also conducted targeted [Ca] experiments to help define Mg/Cacalcite–Mg/Casw relationships for each species and conducted new pH experiments with G. bulloides. We find that pH effects on foraminiferal Mg/Ca are reduced or absent at Mg/Casw = 1.5 mol/mol in all three species, and that T. sacculifer is generally insensitive to variable DIC and pH, making it the ideal species for Mg/Ca SST reconstructions back to 20 Ma. We apply our new T. sacculifer calibration to a Middle Miocene Mg/Ca record and provide recommendations for interpreting Mg/Ca records from extinct species. Plain Language Summary: Reconstructing sea surface temperatures in the geologic past is an important endeavor because it allows us to compare modern climate change to natural variability. The magnesium/calcium ratio (Mg/Ca) of fossilized foraminifera shells is known to record the seawater temperature in which the foraminifer grew. However, shell Mg/Ca can be biased by other variables, like the total amount of carbon in seawater and its pH. In addition, the Mg/Ca ratio of the ocean itself was much lower in the past. In this study, we grew living foraminifera in the laboratory with simulated "ancient" seawater chemistry to test the influence of these confounding factors on shell Mg/Ca. We find that seawater carbon chemistry (like pH) has less of an effect on Mg/Ca in "ancient" seawater chemistry. We also find that one species, Trilobatus sacculifer, is least sensitive to these variables, making it the best choice for reconstructing sea surface temperatures. Key Points: Carbonate system effects on foraminiferal Mg/Ca ratios are reduced in low seawater Mg/CaswTrilobatus sacculifer Mg/Ca is least affected by the carbonate system of the species studied, in support of previous researchCalibration models are put forward for three planktic species and recommendations are made for extinct species [ABSTRACT FROM AUTHOR]

Understanding the factors that control carbonate systems is an important goal due to the complex interactions between the hydrophysical and chemical–biological conditions in coastal basins. The results of this paper present the state of the carbonate system in Penzhina Bay and its adjacent waters—the Shelikhov Gulf—in July 2023, during spring tides with 13 m height. The area we studied included the length of the largest river in the region, the Penzhina River, from the peak of its summer flood to its boundary with the Shelikhov Gulf (the Sea of Okhotsk). This unique dynamic basin, with a length of about 800 km, was studied over 17 days. During this period, the entire water column of Penzhina Bay, down to a depth of about 60 m, and the surface water layer of the Shelikhov Gulf were undersaturated in terms of CO2, with low levels relative to those of the atmosphere. To explain this observation, the dissolved oxygen, nutrients in mineral and organic forms, humic substances, chlorophyll a, and photic zone thickness are presented for the entire basin under study, together with its hydrological data. The results of daily observations of the carbonate system at fixed anchorage stations characterize two contrasting regions of Penzhina Bay: one that was more exposed to continental runoff, which had salinity levels in the range of 8.0–21.3 psu during one tidal cycle; the second had smaller variations in salinity in the range of 31.6–32.9 psu during one tidal cycle. This study emphasizes the importance of biological processes and continental runoff on the variability of the carbonate system parameters and CO2 fluxes at a water/atmosphere boundary with extreme tidal conditions in this ecosystem that is barely affected by human activities.

Sustained time-series measurements are crucial to understand changes in oceanic carbonate chemistry. In the North Western Mediterranean Sea, the temporal evolution of the carbonate system is here investigated based on two 10-year time-series (between January 2010 and December 2019) of monthly carbonate parameters measurements at two sampling sites in the Ligurian Sea (ANTARES and DYFAMED). At seasonal timescale, the seawater partial pressure of CO2 (pCO2) within the mixed layer is mostly driven by temperature at both sites, and biological processes as stated by the observed relationships between total inorganic carbon (CT), nitrate and temperature. This study suggests also that mixing and water masses advection could play a role in modulating the CT content. At decadal timescale, significant changes in ocean chemistry are observed with increasing trends in CT (+3.2 ± 0.9 µmol.kg−1.a−1 – ANTARES; +1.6 ± 0.8 µmol.kg−1.a−1 – DYFAMED), associated with increasing pCO2 trends and decreasing trends in pH. The magnitude of the increasing trend in CT at DYFAMED is consistent with the increase in atmospheric pCO2 and the anthropogenic carbon transport of water originating from the Atlantic Ocean, while the higher trends observed at the ANTARES site could be related to the hydrological variability induced by the variability of the Northern Current.

Accurate and traceable measurements are required to understand ocean processes, to address pressing societal challenges, such as climate change and to sustainably manage marine resources. Although scientific and engineering research has resulted in advanced methods to measure Essential Ocean Variables (EOVs) there is a need for cross comparison of the techniques and traceability to recognized standards. Metrological laboratories are experienced in accredited methods and assessment of methodology. An EU INFRAIA-02-2020: Integrating Activities for Starting Communities project MINKE (Metrology for Integrated marine maNagement and Knowledge-transfer nEtwork https://minke.eu) brings European marine science and metrology Research Infrastructures together to identify synergies and create an innovative approach to Quality Assurance of oceanographic data. Quality depends both on the accuracy (that can be provided through the metrology component) and the completeness of the data sets. The collaboration between different Marine Research Infrastructures (RIs) places a fundamental role on assuring the completeness of the datasets, particularly at global scales. The MINKE project encourages enhancement through collaboration of national metrology laboratories and the oceanographic community. Metrological assessment of the accuracy and uncertainties within multidisciplinary ocean observations will provide data that are key to delivering policy information. Objectives across all the RIs are to facilitate ocean observation and build wider synergies. MINKE will investigate these synergies, then introduce metrology to the core of various EOV measurements. Currently the marine RIs cover laboratory and field operations, from the surface seafloor, coastal waters to deep sea, fixed ocean stations to ship and autonomous vehicle operations to ships of opportunity, and flux stations focusing on carbonate system variables. The nexus of these operations is the focal point for coordinated improvement of ocean observing methods. Measurement intercomparisons, traceability and uncertainty assessments should be at the core of the scientific observations. Specifically, MINKE will work with RIs and Metrology Institutes to improve the quality of dissolved oxygen, carbonate system, chlorophyll-fluorescence, ocean sound and current meter measurements, through access to metrology laboratories, Transnational Access and intercomparison studies across existing marine consortia and RIs. MINKE will also promote the development of absolute salinity observation, and improvements in marine litter measurements.

Organic acid‐base species in the dissolved organic carbon pool have been shown to make an important contribution (i.e., organic alkalinity; OrgAlk) to the total alkalinity (TA) in many coastal systems. This study documents an intensive investigation of OrgAlk characteristics in the river‐to‐coast transition zones of six southeast Chinese rivers. OrgAlk, mainly originating from river input, accounted for an important proportion of TA (0.3%–12%) in six estuaries. Carboxylic acid groups were commonly present in all estuaries. Notable differences in the TA values (1–18 μmol kg−1) determined by several established TA measurement approaches were identified in estuaries where organic acids with pKa <5.2 were abundant. The most widely used open‐cell titration method, in comparison with closed‐cell titration and single‐step titration, is the best approach to incorporate OrgAlk in titrated TA when the pKa values of organic acids were >5 in the study estuaries. Across our study sites, OrgAlk might modify H+ concentrations by 3%–69% (i.e., pH by 0.01–0.78) and aragonite saturation states by 1%–72%, indicating that OrgAlk can play a significant role in the coastal carbonate buffering system. It is essential to improve current TA measurement approaches to more accurately represent OrgAlk in the coastal system and assess impacts of OrgAlk on coastal carbonate chemistry. Plain Language Summary: Organic acid‐base species have been shown to make an important contribution (i.e., organic alkalinity; OrgAlk) to total alkalinity (TA) in the coastal systems. However, the established aquatic TA model generally does not consider OrgAlk as a significant term. In this study, we assessed the influence of OrgAlk on TA measurements and the role it plays in the carbonate buffering system in six southeastern Chinese estuaries. Some of the established TA measurement approaches could not fully incorporate OrgAlk according to the definition of TA, resulting in differences in the TA values determined by different TA measurement approaches in the study estuaries, where large quantities of organic acids with pKa <5.2 were present. OrgAlk was estimated to affect H+ concentration and aragonite saturation state considerably across our study sites, thus playing an important role in the coastal carbonate buffering system. Improving the current TA measurement approaches to better include OrgAlk in the TA model is greatly needed to avoid misinterpretation of key carbonate system indicators. This requires a comprehensive understanding of the characteristics of organic acid‐base species and their cycling across coastal systems. Key Points: Organic alkalinity can greatly affect seawater acidity and the carbonate system in six southeast Chinese estuariesOrganic acids with low‐pKa values caused significant uncertainties in different total alkalinity measurement approaches in southeastern Chinese estuariesRiver input is a major source of organic alkalinity in the study estuaries [ABSTRACT FROM AUTHOR]

The Bay of Marseille (BoM), located in the north-western Mediterranean Sea, is affected by various hydrodynamic processes (e.g., Rhône River intrusion and upwelling events) that result in a highly complex local carbonate system. In any complex environment, the use of models is advantageous since it allows to identify the different environmental 15 forcings, thereby facilitating a better understanding. By combining approaches from two biogeochemical ocean models and improving the formulation of total alkalinity, we develop a more realistic representation of the carbonate system variables at high temporal resolution which enables us study air-sea CO2 fluxes and seawater pCO2 variations more reliably. We apply this new formulation to two particular scenarios, typical for the BoM: (i) summer upwelling and (ii) Rhône River intrusion events. In both scenarios, our model was able to correctly reproduce the observed patterns of pCO2 variability. Summer upwelling events are typically associated with pCO2 decrease that mainly results from decreasing near-surface temperatures. Furthermore, Rhône River intrusion events are typically associated with pCO2 decrease, although in this case the pCO2 decrease results from a decrease in salinity and an overall increase in total alkalinity. While our model was able to correctly represent the daily range of air-sea CO2 fluxes, we were unable to correctly estimate the yearly total air-sea CO2 flux. Although the model consistent with observations, predicted the BoM to be a sink of CO2 on a yearly basis, the magnitude of this CO2 sink was underestimated which may be an indication of the limitations inherent in dimensionless models for representing air-sea CO2 fluxes. [ABSTRACT FROM AUTHOR]

Coral reef ecosystems have extremely high primary productivity and play an important role in the marine carbon cycle. However, due to the high carbon metabolism efficiency of coral communities, little is known about the carbon sink–source properties of coral reefs. In November 2022, in situ field investigations coupled with incubation experiments were conducted in typical subtropical coral reef waters, i.e., Yangmeikeng Sea Area (Area I) and Dalu Bay (Area Ⅱ), to explore the dynamics of the carbonate system and its controlling factors. The results revealed that the carbonate parameters had high variability, comprehensively forced by various physical and biochemical processes. Overall, Areas I and Ⅱ were net sinks of atmospheric CO2, with net uptake fluxes of 1.66 ± 0.40 and 0.99 ± 0.08 mmol C m−2 day−1, respectively. The aragonite saturation state (ΩA), 3.04–3.87, was within the range adequate for growth of tropical shallow-water scleractinian corals. Inorganic carbon budget results indicated that photosynthesis and microbial respiration were the main factors affecting the dynamics of carbonate systems in the whole study area. However, focusing on the reef areas, coral metabolism was also a key factor affecting the carbonate system in seawater (especially in Area I) and its contribution accounted for 28.9–153.3% of the microbial respiration. This study highlighted that metabolism of coral communities could significantly affect the seawater carbonate system, which is of great significance in the context of the current process of ocean acidification. [ABSTRACT FROM AUTHOR]

The response of coastal systems to global acidification depends strongly on river inputs, which can alter the total alkalinity (AT) and dissolved inorganic carbon (DIC) in seawater. The northern Adriatic Sea (NAd) is a shallow continental shelf region that currently receives about 15% of the total freshwater input in the Mediterranean Sea, where the role of riverine discharges on the carbonate system has been poorly studied. In particular, river discharges can alter the carbonate system in the sea, affecting both the equilibrium chemistry and biological processes. For the main rivers flowing into the NAd (the Po, Adige, Brenta, Piave, Livenza, Tagliamento, Isonzo, Timavo and Rižana), data were collected for the pH, concentrations of the total alkalinity (AT), Ca2+ and Mg2+ and the isotopic ratio of stable carbon in the dissolved inorganic carbon (δ13CDIC). The DIC fluxes were estimated using the THINCARB (THermodynamic modeling of INOrganic CARBon) model for the compilation of the AT and pH data. The results show that the total transport of the AT in the rivers was 205 Gmol yr−1 while the transport of the DIC was 213 Gmol yr−1, of which about 70% was from the Po River. About 97% of the DIC in the river waters was in the form of bicarbonates. The high Mg2+/Ca2+ ratios indicate that dolomite weathering is predominant in the Adige, Piave, and Livenza river basins, while lower ratios in the Timavo and Rižana rivers indicate a greater proportion of calcite. The mean δ13C-DIC value was estimated to be −10.0 ± 1.7 ‰, a value nowadays considered typical for the DIC flux inputs in oceanic carbon cycle modeling. The DIC flux depends on the mineral weathering and biological activity in each river basin. However, these natural processes can be modified by anthropogenic disturbances that should be better quantified. [ABSTRACT FROM AUTHOR]

The values of pH, total alkalinity, humic substance, chlorophyll a were measured in water in the Razdolnaya R. within 13 months in 2013–2014 at a station near Razdolnyi Settl. Estimates of the carbonate system of river waters showed that the major portion of the year, the river water was a source of CO2 for the atmosphere with an annual emission flux of ~25 thousands tC/year. The chemical weathering of silicate rocks in the the Razdolnaya R. leads to withdrawal of atmospheric CO2. The mean annual export of the atmospheric CO2 in the form of dissolved inorganic and organic carbon by the Razdolnaya R. into the Amur Bay (the Sea of Japan) was 47 thousands tC/year. Therefore, the the Razdolnaya R. ecosystem in the study period absorbed CO2 (>20 thousands tC/year). The period of 2003–2017 showed a tendency toward an increase in the export of alkalinity, dissolved inorganic and organic carbon by the Razdolnaya R.. Estimates of the rate of chemical weathering of rocks, composing the basin, lie within 12–24 t/(km2 year). [ABSTRACT FROM AUTHOR]

Measurements of the marine carbonate system on tropical and subtropical continental margins are poorly distributed in space and time, with many uncertainties persisting regarding the role of carbon exchanges at the ocean-atmosphere interface in these areas. To calculate sea-to-air CO2 fluxes in Marine Ecoregions along the Brazilian continental margin (4°N to 34°S), we used data from the Surface Ocean CO2 Atlas (SOCAT v2020), collected up to 400 km from the coast, at the surface (5 m), between 1991 and 2018, with the aim of investigating the role of ecoregions as potential sinks or sources of atmospheric CO2. The temperature and salinity of seawater presented variability in the north-south direction mainly because of the broad latitudinal range, reflecting typical patterns of tropical (T = 27.4°C ±1.49; S = 36.4 ±1.91) and subtropical waters (T = 22.8°C ±3.41; S = 35 ±2.91), in addition to the greater or lesser influence of river inputs in each ecoregion. The pCO2 values in the surface waters varied from 121.81 (Amazon) to 478.92 µatm (Eastern), differing significantly between ecoregions and showing an expected decadal increasing trend, both in the atmosphere and in the seawater. The calculated values of CO2 fluxes showed non-homogeneous spatio-temporal variations, from -24.37 mmol m-2 d-1 (Rio Grande) to 9.87 mmol m-2 d-1 (Southeastern). Throughout the analyzed time series, we observed that the Northeast, Amazon and Eastern ecoregions acted predominantly as sources of CO2 and the Southeastern ecoregions and, mainly, Rio Grande, acted predominantly as sinks of atmospheric CO2. [ABSTRACT FROM AUTHOR]

This study examined carbonate dynamics in the northwestern South China Sea (NWSCS), an area jointly influenced by upwelling, river plumes and submarine groundwater discharge. Data were obtained from two cruises conducted in summer 2009 and 2012. In 2009, a high salinity-low temperature water mass occurred nearshore off northeastern Hainan Island, indicative of upwelling, commonly referred to as HNEU. A river plume fueled primarily by local rivers and characterized by low salinity and high temperature was observed in the NWSCS off the mainland roughly along the 30 m isobath. In 2012, coastal upwelling off northeastern Hainan Island was not detectable at the surface, but was observed at a different location off eastern Hainan Island (HEU). River plume waters in 2012 were patchily distributed, with a low salinity zone further westerly than that in 2009 and another on the mid-shelf of the NWSCS sourced from the Pearl River which reached out ∼250 km from the mouth of the Pearl River Estuary. In 2009, elevated dissolved inorganic carbon (DIC) and total alkalinity (TA) occurred in the coastal plume, where submarine groundwater discharge contributed DIC and TA additions of 38.9±20.5 and 42.5±22.3 µmol kg−1, respectively, with a DIC/TA ratio of ∼0.92, which made a minor contribution to the variation of seawater partial pressure of CO2 (pCO2), pH and the aragonite saturation state index (Ωarag). Additionally, high surface phytoplankton production consumed DIC of 10.0±10.4 µmol kg−1 but did not significantly affect TA, which dominated pCO2 drawdown in the coastal plume water and increased the pH and Ωarag at surface. Submarine groundwater discharge was also observed in the region influenced by upwelling, but to a lesser degree than that impacted by coastal plume. Lower pH and Ωarag and higher pCO2 values than in offshore waters were observed downstream of the upwelling system, attributable largely to organic matter remineralization with a DIC addition of 23.8±8.4 µmol kg−1. In 2012, submarine groundwater discharge was not detected but high phytoplankton production dominated carbonate dynamics in the coastal plume water with a net DIC consumption of 104.2 µmol kg−1, which markedly drew down sea surface pCO2 and increased pH and Ωarag. In the Pearl River Plume, the solubility-driven CO2 sink exceeded biological CO2 uptake, resulting in an additional decrease of pH and Ωarag and increase of seawater pCO2. Taken together, this study demonstrated complex spatial and year-to-year variability, and the controls of the carbonate system under the joint modulations of upwelling, river plumes and submarine groundwater discharge. A first order estimate that considered the rise of atmospheric CO2 and seawater temperature further suggested a high risk of ocean acidification in this coastal area by the end of this century, which could be amplified under the stresses of river plumes, submarine groundwater discharge and organic matter remineralization. [ABSTRACT FROM AUTHOR]

The problem of ocean acidification caused by the increase of atmospheric carbon dioxide concentration is becoming increasingly prominent. Field observation in the northwest Pacific Ocean was carried out along the 150°E transect in November 2019. The distribution characteristics and influencing factors of the surface seawater carbonate chemistry, including dissolved inorganic carbon (DIC), total alkalinity (TA), pH, partial pressure of carbon dioxide (pCO2) and aragonite saturation state (Ωarag) were investigated. DIC and TA ranged from 1915 to 2014 µmol kg−1 and 2243 to 2291 µmol kg−1, respectively; DIC in general decreased with decreasing latitude, but TA had no clear latitudinal gradient. pCO2 values increased with the decrease of latitude and were all below the atmospheric pCO2 level, ranging from 332 to 387 µatm. pH on the total hydrogen ion concentration scale (pHT) decreased with the decrease of latitude in the range of 8.044–8.110, while Ωarag increased with the decrease of latitude in the range of 2.61–3.88, suggesting that the spatial distributions of pHT and Ωarag were out of phase. Compared with the present, the predicted values of pHT and Ωarag by the end of this century would decrease remarkedly; larger declines were found in the higher pHT and Ωarag regions, resulting in the differences along the meridional gradient becoming smaller for both pHT and Ωarag. [ABSTRACT FROM AUTHOR]

Seawater pH is a valuable parameter to describe ocean acidification. However, pH measurements are often subject to large uncertainty and the results of the pH comparison from different laboratories are not convincing. We assessed the potentiometric method for pH measurement on seawater samples with salinities from 20 to 40 and pH ranging from 7.2 to 8.6. pH glass electrodes were calibrated using both commercially available NBS buffers and the equimolal Tris (2‐amino‐2‐hydroxymethyl‐1,3‐propanediol) buffer (prepared in synthetic seawater at a salinity of 35). The results demonstrated that the uncertainty in pH measurements was within ± 0.01 in the entire salinity range and was better than ± 0.003, when the sample salinity was close to that of equimolal Tris buffer (salinity difference within ± 2.5), regardless of the sample pH. This study suggests that if the electrode calibration is performed properly, the potentiometric method can fulfill the "weather" goal (± 0.02) of the Global Ocean Acidification Observing Network in pH measurements; it might even meet the "climate" goal (± 0.003) if the difference between the salinity of the samples and the Tris buffer is small. [ABSTRACT FROM AUTHOR]

The eastern Ross Sea is a key area to understand the role of the Amundsen Sea inflow of freshwater that can influence the Ross Sea water properties and salt budget. A survey was carried out in the eastern Ross Sea during the austral summer 2019–20 to evaluate the contribution of the Amundsen Sea Water (ASW) to the salinity variability. A total of 248 seawater samples were collected f\or the analysis of total alkalinity (AT) and pH. The data collected were used together with temperature and salinity to obtain a full description of the carbonate system properties including total inorganic carbon (CT), CO2 partial pressure (pCO2), calcium carbonate saturation state of aragonite and calcite (Ω), and Revelle factor. Moreover, we estimated the anthropogenic carbon (Cant) throughout the TrOCA method to better understand the carbon cycle, also considering the effect of atmospheric CO2 uptake on ocean acidification. We used principal component analysis (PCA) to investigate the major controls on the carbonate system parameters with the aim of defining their sensitivity as chemical tracers. The changes in carbonate chemistry in surface waters were mainly due to the physical properties. AT and pH traced the entry of the ASW showing limited mixing between water masses on the shelf area. Shelf waters were enriched in Cant, which resulted lower than the estimated value for shelf waters produced in western Ross Sea.

In shallow coastal shelves like the Mid‐Atlantic Bight (MAB), ocean acidification due to increased atmospheric carbon dioxide (CO2) is compounded by highly variable coastal processes including riverine freshwater inputs, nutrient loading, biogeochemical influence, coastal currents and water mass mixing, and seasonal transitions in physical parameters. Past deconstructions of carbonate system drivers in the MAB have focused on nearshore zones or single season data, and thus lack the spatial and temporal resolution required to assess impacts to important species occupying the shelf. Deconstructing highly resolved data collected during four seasonal Slocum glider deployments in the MAB, this study uses a Taylor Series decomposition to quantify the influence of temperature, salinity, biogeochemical activity, and water mass mixing on pH and aragonite saturation state from sea surface to bottom. Results show that water mass mixing and biogeochemical activity were the most significant drivers of the carbonate system in the MAB. Nearshore water was more acidic year‐round due to riverine freshwater input, but photosynthesis reduced acidity at certain depths and times. Water mass mixing increased acidity in bottom water on the shelf, particularly in summer. Gulf Stream intrusions at the shelf break during fall acted to mitigate acidification on the shelf in habitats occupied by carbonate‐bearing organisms. The relationships quantified here can be used to improve biogeochemical forecast models and determine habitat suitability for commercially important fin and shellfish species residing in the MAB. Plain Language Summary: The coastal ocean is experiencing changes in chemistry due to human activities, including carbon dioxide emissions, nutrient runoff, and seasonal changes in temperature, salinity, and coastal currents. These drivers have been studied close to shore and/or only during single seasons, leaving a gap in our understanding of seasonal changes across the entire economically important shelf region. Here, we use high‐resolution data collected by a deep‐sea robot that measures chemistry from ocean surface to the sea floor. We determined the importance of four key influences (temperature, salinity, water mass mixing, and biological activity) on changes in coastal chemistry over the course of a year. We found that the most important driver of shelf chemistry was mixing of freshwater at the coast and warm, salty water at the edge of the shelf. Biological activity was a secondary influence, which caused smaller scale changes in chemistry. These results can help to predict how coastal chemistry might change in the future, so that we can prepare for the effects on economically important animals and industries. Key Points: Water mass mixing and biogeochemical activity are the major drivers of seasonal carbonate system dynamics in the Mid‐Atlantic BightWater mass mixing has opposing effects on carbonate chemistry in the nearshore and at the continental shelf break [ABSTRACT FROM AUTHOR]

In the period of summer flood 2016, a number of chemical characteristics associated with carbon cycle were studied in the estuaries of the Syran and Ul'ban rivers in the area of the Shantarskii Archipelago with the total water discharge of 194.7 m3/s. The zone of mixing at the salinity of <20‰ is the source of CO2 for the atmosphere with the estimated CO2 flux up to 112.7 mmol m−2 day−1. At the salinity >20‰, the thickness of the photic layer increases abruptly and photosynthesis starts to dominate, resulting in the formation of CO2 flow from the atmosphere into water with a rate of up to 30 mmol m−2 day−1. A model CO2 flow at the water/atmosphere interface is presented at a wind speed from 2 to 15 m/s for the entire mixing zone, the water in which generally absorb atmospheric CO2. A specific feature of the basin compared with the estuaries of the Uda and Usalgin rivers in the zone of the Shantarskii Archipelago is the combination of the relatively low volume of water and solid runoff with a relatively high phosphorus flux in mineral and organic forms. [ABSTRACT FROM AUTHOR]

Using Argo profiling floats, cruises and mooring data, we reconstructed the dissolved oxygen (O2) dynamics in the Gulf of Lion and the Ligurian Sea, with a focus on the intermediate waters. By applying the CANYON‐MED neural network‐based method on the large network of O2‐equipped Argo floats we derived nutrients and carbonate system variables in the Gulf of Lion and the Ligurian Sea at different depths in the water column and derived trends over the 2012–2020 period. In these waters, the O2 minimum is strongly affected by the intermittent convection process, and the two areas show dissimilar responses to the mixing events. In the absence of deep convection events, the O2‐depleted layer tends to spread vertically and intensify even more so in the Ligurian than in the Gulf of Lion. In both areas, over the 2012–2020 period, nutrients increase overall in deep layers, with a concomitant impact on nutrient molar ratios tending toward an increase in P‐limitation. Acidification estimates derived in different layers of the water column show an overall increase in dissolved inorganic carbon and a concurrent pH decrease. These trends were strongly affected by convection events slowing down the overall acidification trend. Plain Language Summary: Using multiple observation platforms such as Argo profiling floats (autonomous, free‐floating instruments equipped with sensors and profiling regularly in the first 2,000 m of the water column), cruises on oceanographic vessels, and moorings (fixed position stations), we reconstructed the dissolved oxygen (O2) dynamics in the northwestern Mediterranean Sea. The intermediate waters are characterized by an O2 minimum strongly affected by the intermittent convection process (due to winter forcings and density changes, mixing of water masses occurs over the water column). We also derived nutrients and carbonate system variables (related to acidification) from a neural network methodology developed for the Mediterranean Sea, CANYON‐MED, applied to Argo floats equipped with O2 sensors in different layers of the water column. Over the 2012–2020 period, the derived trends show an overall increase of nutrients in deep layers, with an impact on nutrient limitations. Acidification estimates show an overall acidification. These trends were strongly affected by convection events. Key Points: In the absence of deep convection events, the O2‐depleted layer spreads vertically and intensifies more in the Ligurian than Gulf of LionNutrients increase in deep and to a lesser extent in intermediate waters with a decoupling between nitrate and phosphate trendsDissolved inorganic carbon increases in intermediate and deep waters with a concurrent pH decrease over the period of study, 2012–2020 [ABSTRACT FROM AUTHOR]

The wind‐driven part of the South Atlantic Ocean is primarily ventilated through central and intermediate water formation. Through the water mass formation processes, anthropogenic carbon (Cant) is introduced into the ocean's interior which in turn makes the South Atlantic region vulnerable to ocean acidification. Cant and the accompanying acidification effects have been estimated for individual sections in the region since the 1980s but a comprehensive synthesis for the entire basin is still lacking. Here, we quantified the Cant accumulation rates and examined the changes in the carbonate system properties for the South Atlantic using a modified extended multiple linear regression method applied to five hydrographic sections and data from the GLODAPv2.2021 product. From 1989 to 2019, a mean Cant column inventory change of 0.94 ± 0.39 mol C m−2 yr−1 was found. Cant accumulation rates of 0.89 ± 0.33 μmol kg−1 yr−1 and 0.30 ± 0.29 μmol kg−1 yr−1 were observed in central and intermediate waters, accompanied by acidification rates of −0.0020 ± 0.0007 pH units yr−1 and −0.0009 ± 0.0009 pH units yr−1, respectively. Furthermore, increased remineralization was observed in intermediate waters, amplifying the acidification of this water mass, especially at the African coast along 25°S. This increase in remineralization is likely related to circulation changes and increased biological activity nearshore. Assuming no changes in the observed trends, South Atlantic intermediate waters will become unsaturated with respect to aragonite in ∼30 years, while the central water of the eastern margins will become unsaturated in ∼10 years. Key Points: The western margin and southern limit of the South Atlantic Ocean are more impacted by anthropogenic carbon (Cant) uptakeA Cant column inventory change of 0.94 ± 0.39 mol C m−2 yr−1 was found in the region, higher than the global mean storage rateAragonite unsaturation is expected in the next two decades along the eastern margin if the anthropogenic changes maintain the present trend [ABSTRACT FROM AUTHOR]

The interplay of global climate change and anthropogenic activities has significantly affected the carbon cycle in coastal ocean environments. Consequently, further investigation into the carbonate system, carbon source and sink processes, and acidification mechanisms is essential. This study examined the surface carbonate system offshore of Qingdao, utilizing data from nine spring cruises in 2011-2019. The inter-annual variations in sea surface temperature revealed a gradual upward trend, whereas chlorophyll a (Chl-a) and apparent oxygen utilization (AOU) exhibited alternating trends of increase and decrease. Over the period from 2011 to 2019, pH levels ranged from 8.00 to 8.19, partial pressure of CO 2 (pCO 2 ) values varied between 187.8 and 364.1 μatm, and calcium carbonate saturation (Ω arag ) ranged from 3.22 to 4.79. From 2011 to 2017, both pH and Ω arag showed a decreasing trend, attributed to the increasing influence of respiration. In contrast, an upward trend was observed for both parameters during 2018 and 2019, indicating a decline in respiration's contribution. Biological processes, especially the increase in respiration caused by the nutrients and organic matter released through human activities, were pivotal in determining spatial variations in pCO 2 and Ω arag offshore, while the impact of mixing processes and temperature was comparatively negligible. Furthermore, the Ulva prolifera bloom event in July 2015 significantly affected the coastal waters of Qingdao's carbonate system when compared to the no-bloom period in March 2015. Overall, the study area functioned as a sink for atmospheric CO 2 in the spring. The fluctuations in air-sea CO 2 fluxes corresponded with changes in ΔpCO 2 and wind speeds, with surface seawater air-sea CO 2 fluxes ranging from -17.7 to 12.2 mmol m -2 d -1 between 2011 and 2019, yielding an average flux of -9.4 ± 5.3 mmol m -2 d -1 ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

The Antarctic continental shelf is known as a critical anthropogenic CO2 (Cant) sink due to its cold waters, high primary productivity, and unique circulation, which allow it to sequester large amounts of organic and inorganic carbon into the deep ocean. However, climate change is currently causing significant alteration to the Antarctic marine carbon cycle, with unknown consequences on the Cant uptake capacity, making model-based estimates of future ocean acidification of polar regions highly uncertain. Here, we investigated the marine carbonate system in the Ross Sea in order to assess the current anthropogenic carbon content and how physical–biological processes can control the Cant sequestration along the shelf-slope continuum. The Winter Water mass generated from convective events was characterized by high Cant level (28 µmol kg−1) as a consequence of the mixed layer break-up during the cold season, whereas old and less-ventilated Circumpolar Deep Water entering the Ross Sea revealed a very scarce contribution of anthropogenic carbon (7 µmol kg−1). The Cant concentration was also different between polynya areas and the shelf break, as a result of their specific hydrographic characteristics and biological processes: surface waters of the Ross Sea and Terra Nova Bay polynyas served as strong CO2 sink (up to −185 mmol m−2), due to the remarkable net community production, estimated from the summertime surface-dissolved inorganic carbon deficit. However, a large amount of the generated particulate organic carbon was promptly consumed by intense microbial activity, giving back carbon dioxide into the intermediate and deep layers of the continental shelf zone. Further Cant also derived from High-Salinity Shelf Water produced during winter sea ice formation (25 µmol kg−1), fueling dense shelf waters with additional input of Cant, which was finally stored into the abyssal sink through continental slope outflow (19 µmol kg−1). Our results suggest that summer biological activity over the Ross Sea shelf is pivotal for the shunt of anthropogenic CO2 between the organic and inorganic carbon pools, enhancing the ocean acidification of the upper mesopelagic zone and the long-term Cant sequestration into the deep ocean.

This paper is based on a comparison of the carbon parameters at the western and eastern borders of the tropical Atlantic using data collected from 55 cruises. Oceanic and coastal data, mainly total alkalinity (TA), total dissolved inorganic carbon (CT), sea surface salinity (SSS) and sea surface temperature (SST), were compiled from different sources. These data were subdivided into three subsets: oceanic data, coastal data and adjacent to the Brazilian (western) and African coastal areas (eastern) data. Significant differences between the TA data (2099.4 ± 286.4 µmol kg−1) at the western and eastern edges (2198 ± 141.9 µmol kg−1) were observed. Differences in the CT values between the western edge (1779.6 ± 236.4 µmol kg−1) and eastern edge (1892.2 ± 94.2 µmol kg−1) were also noted. This pattern was due to the different variabilities in the carbon parameters between the eastern and western border coastal areas and to the biogeochemistry that drives these parameters. In the western coastal area, the physical features of the continental carbon and oceanic waters mixing with the freshwater that flows from the Amazon and Orinoco Rivers to the South American coast are different than the physical features of the water that flows from the Congo, Volta and Niger Rivers in the eastern region. Applying the TA empirical relationship to TA with values of SSS < 35 in the western and eastern regions leads to a higher root mean square error (rmse) in the eastern and western regions. Therefore, most of the existing TA empirical relationships are most useful at the regional scale due to the difference in the water properties of each region. The relationships of TA and CT determined in the western and eastern regions do not reproduce in situ data well, especially at the adjacent edges. This difference is explained by the difference between the African and Brazilian coasts in terms of their carbon parameter characteristics and processes responsible for their variation. Based on the mixing model, it has been shown that the primary productivity in the eastern region is higher than that in the western region. This is one of the reasons why the carbon parameters are higher in the eastern region. For each region studied, an equation for TA is introduced in this study. [ABSTRACT FROM AUTHOR]

Lava flows from the 2021 volcanic eruption in La Palma, Canary Islands, reached the shoreline during three events on September 28th, November 10th, and November 22nd, strongly affecting the seawater properties as they interacted with the seawater. The evolution of surface and water column physicochemical properties (temperature, salinity, carbonate system variables, and dissolved oxygen) was characterized during 13 visits to the frontal zone of the newly formed deltas. A large volume of hot (50°C measured on November 12th) and high salinity seawater promoted pH values in the frontal zone of 7.0 with important decreases in alkalinity, ΔAT, and total dissolved inorganic carbon, ΔCT, that reached 566 and 272 µmol kg−1, respectively. The addition of acids generated during the lava–seawater interaction (44.4 mmol), together with those used in the titration of carbonate alkalinity (796 ± 72 mmol) plus acids used in metal dissolution (21 mmol), was estimated with an average added proton concentration of 0.31 µmol kg−1 of seawater in the affected 2.7 · 106 m3 for November 12th. During this event, the decrease in pH and the increase in temperature increased the partial pressure of CO2, outgassing 2 tons of CO2, 40 times the daily emitted CO2 for this area under non-eruptive conditions. One month after the eruption, the studied physicochemical properties of the seawater close to the new deltas returned to the usual non-eruptive normal values, which include the presence of low salinity, low pH, and high CO2 gas diffusive emissions through submarine groundwater discharges observed between the formed deltas. The new mineral-rich deltas and the increase in solubility due to the low pH conditions contributed to the recovery of the affected area.

The biogeochemical dynamics of fjords in the southeastern Pacific Ocean are strongly influenced by hydrological and oceanographic processes occurring at a seasonal scale. In this study, we describe the role of hydrographic forcing on the seasonal variability of the carbonate system of the Sub-Antarctic glacial fjord, Seno Ballena, in the Strait of Magellan (53°S). Biogeochemical variables were measured in 2018 during three seasonal hydrographic cruises (fall, winter and spring) and from a high-frequency pCO2-pH mooring for 10 months at 10 ± 1 m depth in the fjord. The hydrographic data showed that freshwater input from the glacier influenced the adjacent surface layer of the fjord and forced the development of undersaturated CO2 (< 400 μatm) and low aragonite saturation state (ΩAr < 1) water. During spring, the surface water had relatively low pCO2 (mean = 365, range: 167 - 471 μatm), high pH (mean = 8.1 on the total proton concentration scale, range: 8.0 - 8.3), and high ΩAr (mean = 1.6, range: 1.3 - 4.0). Concurrent measurements of phytoplankton biomass and nutrient conditions during spring indicated that the periods of lower pCO2 values corresponded to higher phytoplankton photosynthesis rates, resulting from autochthonous nutrient input and vertical mixing. In contrast, higher values of pCO2 (range: 365 – 433 μatm) and relatively lower values of pHT (range: 8.0 – 8.1) and ΩAr (range: 0.9 – 2.0) were recorded in cold surface waters during winter and fall. The naturally low freshwater carbonate ion concentrations diluted the carbonate ion concentrations in seawater and decreased the calcium carbonate saturation of the fjord. In spring, at 10 m depth, higher primary productivity caused a relative increase in ΩAr and pHT. Assuming global climate change will bring further glacier retreat and ocean acidification, this study represents important advances in our understanding of glacier meltwater processes on CO2 dynamics in glacier–fjord systems.

The carbonate chemistry in river-dominated marginal seas is highly heterogeneous, and there is ongoing debate regarding the definition of atmospheric CO 2 source or sink. On this basis, we investigated the carbonate chemistry and air-sea CO 2 fluxes in a hotspot estuarine area: the Changjiang Estuary during winter and summer. The spatial characteristics of the carbonate system were influenced by water mixing of three end-members in winter, including the Changjiang freshwater with low total alkalinity (TA) concentration, the less saline Yellow Sea Surface Water with high TA, and the saline East China Sea (ECS) offshore water with moderate TA. While in summer with increased river discharge, the carbonate system was regulated by simplified two end-member mixing between the Changjiang freshwater and the ECS offshore water. By performing the end-member mixing model on DIC variations in the river plume region, significant biological addition of DIC was found in winter with an estimation of −120 ± 113 μmol kg−1 caused by wintertime organic matter remineralization from terrestrial source. While this biological addition of DIC shifted to DIC removal due to biological production in summer supported by the increased nutrient loading from Changjiang River. The p CO 2 dynamics in the river plume and the ECS offshore were both subjected to physical mixing of freshwater and seawater, whether in winter and summer. In the inner estuary without horizontal mixing, the p CO 2 dynamics were mainly influenced by biological uptake in winter and temperature in summer. The inner estuary, the river plume, and the ECS offshore were sources of atmospheric CO 2 , with their contributions varying seasonally. The Changjiang runoff enhanced the inner estuary's role as a CO 2 source in summer, while intensive biological uptake reduced the river plume's contribution. [Display omitted] • Mixing of end-members drove the variation of carbonate system among seasons. • Winter biological DIC addition and summer DIC removal occurred in the rive plume. • The important role of freshwater inputs in CO 2 emissions from the Changjiang estuary. [ABSTRACT FROM AUTHOR]

Ocean acidification poses a substantial threat to global marine ecosystems, estuaries are more vulnerable to ocean acidification compared to open oceans due to their weaker buffering capacity. This study examined the carbonate parameters off the Yangtze River estuary (YRE) during summer 2019 and investigated seasonal variations in total alkalinity (TA) and dissolved inorganic carbon (DIC) transport in the lower Yangtze River in 2019. Monthly DIC (1566–2164 μmol/kg) and TA (1471–2128 μmol/kg) in the Yangtze River were negatively correlated with water discharge. Buffer factor (β DIC) was calculated and used to evaluate the buffering capacity, which ranged from 65 to 256 μmol/kg and increased seaward along the YRE. Conservative mixing models indicated that the estuary had a minimum buffer zone (MBZ) at salinity of 2–9 during the high discharge periods. And the salinity of the MBZ was positively correlated with the riverine DIC:TA ratio. The construction of the Three Gorges Dam has resulted in a decrease in the Yangtze River's DIC:TA ratio, leading to the migration of the estuarine MBZ towards lower salinity regions. The effect of anthropogenic CO 2 invasion on estuarine buffering capacity was opposite to that of dam construction, leading to the migration of the estuarine MBZ towards higher salinity regions. Biological influences on the buffering capacity in the YRE were also quite considerable. Net autotrophy slightly enhanced the buffering capacity of the estuarine surface water, while net heterotrophy significantly weakened the buffering capacity of the estuarine bottom water. Eutrophication could intensify the biological influences on the buffering capacity. Globally, mid-latitude estuaries, such as the YRE, generally exhibit the strongest buffering capacity, while estuaries in Arctic regions tend to have the weakest buffering capacity. • Estuarine minimum buffer salinity is positively correlated with river DIC:TA ratio. • The dam construction makes the minimum buffer zone move to lower salinity. • Anthropogenic CO 2 invasion leads to a decline in the estuarine buffering capacity. • Eutrophication exacerbates the decline in the estuarine bottom buffering capacity. [ABSTRACT FROM AUTHOR]

Coral reef ecosystems have extremely high primary productivity and play an important role in the marine carbon cycle. However, due to the high carbon metabolism efficiency of coral communities, little is known about the carbon sink–source properties of coral reefs. In November 2022, in situ field investigations coupled with incubation experiments were conducted in typical subtropical coral reef waters, i.e., Yangmeikeng Sea Area (Area I) and Dalu Bay (Area Ⅱ), to explore the dynamics of the carbonate system and its controlling factors. The results revealed that the carbonate parameters had high variability, comprehensively forced by various physical and biochemical processes. Overall, Areas I and Ⅱ were net sinks of atmospheric CO2, with net uptake fluxes of 1.66 ± 0.40 and 0.99 ± 0.08 mmol C m−2 day−1, respectively. The aragonite saturation state (ΩA), 3.04–3.87, was within the range adequate for growth of tropical shallow-water scleractinian corals. Inorganic carbon budget results indicated that photosynthesis and microbial respiration were the main factors affecting the dynamics of carbonate systems in the whole study area. However, focusing on the reef areas, coral metabolism was also a key factor affecting the carbonate system in seawater (especially in Area I) and its contribution accounted for 28.9–153.3% of the microbial respiration. This study highlighted that metabolism of coral communities could significantly affect the seawater carbonate system, which is of great significance in the context of the current process of ocean acidification.

Seawater pH is a direct proxy of ocean acidification, and monitoring the global pH distribution and long-term series changes is critical to understanding the changes and responses of the marine ecology and environment under climate change. Owing to the lack of sufficient global-scale pH data and the complex relationship between seawater pH and related environmental variables, generating time-series products of satellite-derived global sea surface pH poses a great challenge. In this study, we solved the problem of the lack of sufficient data for pH algorithm development by using the massive underway sea surface carbon dioxide partial pressure (pCO2) dataset to structure a large data volume of near in situ pH based on carbonate calculation between underway pCO2 and calculated total alkalinity from sea surface salinity and relevant parameters. The remote sensing inversion model of pH was then constructed through this massive pH training dataset and machine learning methods. After several tests of machine learning methods and groups of input parameters, we chose the random forest model with longitude, latitude, sea surface temperature (SST), chlorophyll a (Chla), and Mixed layer depth (MLD) as model inputs with the best performance of correlation coefficient (R2 = 0.96) and root mean squared error (RMSE = 0.008) in the training set and R2 = 0.83 (RMSE = 0.017) in the testing set. The sensitivity analysis of the error variation induced by the uncertainty of SST and Chla (SST ≤ ±0.5 °C and Chla ≤ ±20%; RMSESST ≤ 0.011 and RMSEChla ≤ 0.009) indicated that our sea surface pH model had good robustness. Monthly average global sea surface pH products from 2004 to 2019 with a spatial resolution of 0.25° × 0.25° were produced based on the satellite-derived SST and Chla products and modeled MLD dataset. The pH model and products were validated using another independent station-measured pH dataset from the Global Ocean Data Analysis Project (GLODAP), showing good performance. With the time-series pH products, refined interannual variability and seasonal variability were presented, and trends of pH decline were found globally. Our study provides a new method of directly using remote sensing to invert pH instead of indirect calculation based on the construction of massive underway calculated pH data, which would be made useful by comparing it with satellite-derived pCO2 products to understand the carbonate system change and the ocean ecological environments responding to the global change. [ABSTRACT FROM AUTHOR]