Extreme precipitation amplified the cumulative effects of DOM availability on organic-sourced DIC in the Yangtze River.

Optimizing Mytilus edulis and Gracilaria lemaneiformis ratios for maintaining water environment stability in IMTA systems.

Application of acid digestion-based total inorganic carbon measurement for carbonated cement-based materials

This study investigated the mineral carbonation and neutralization behavior of red mud (RM) using CO<sub>2</sub> microbubbles (CO<sub>2</sub> MBs) under ambient temperature and pressure conditions, and further assessed the feasibility of utilizing carbonated RM as a cement substitute. Batch experiments were conducted at various solid-to-liquid ratios (S/L=0.001–1.0), monitoring pH, electrical conductivity (EC), and aqueous carbonic acid (H<sub>2</sub>CO<sub>3</sub>(<sub>aq</sub>)) concentrations. In the RM–CO<sub>2</sub> MBs system, pH initially dropped sharply and then recovered to the buffering zone (pH 7–8.5), while EC exhibited a rapid rise followed by gradual decline, indicating sequential ion release and carbonate precipitation. The H<sub>2</sub>CO<sub>3</sub>(<sub>aq</sub>) concentration decreased over time due to both carbonation consumption and pH-induced speciation shift. In continuous experiments (reactor dimensions: D=14.6cm, H=34 cm, S/L=0.025), both powdered (RM-P) and sludge-type (RM-S) samples achieved neutralization (pH=7) within 4 minutes, accompanied by a characteristic EC decrease–rebound pattern. The total inorganic carbon (TIC)-based CO<sub>2</sub> uptake of RM-S reached 8.87g-CO<sub>2</sub>/kg-RM, corresponding to approximately 84% of the theoretical maximum carbonation potential (TMCP). Mortar specimens incorporating carbonated RM as a partial cement replacement (0–15 wt%) exhibited decreasing compressive strength with increasing substitution ratio, yet 5 wt% replacement maintained adequate strength for non-structural construction materials. These results demonstrate that CO<sub>2</sub> MBs enable rapid (≤2 min), high-efficiency carbonation and neutralization of RM under ambient conditions without pressurized systems. The proposed process provides a low-energy, environmentally friendly pathway for simultaneous CO<sub>2</sub> sequestration and red mud valorization, contributing to sustainable carbon-neutral technology.

Abstract. Marine dissolved inorganic carbon (DIC) is by far the largest pool of carbon in the Earth surface system that exchanges with the atmosphere on human-relevant timescales. Measurements of DIC are therefore necessary to study the changing marine carbon cycle. The most accurate routine DIC measurement method is coulometry. In this method, the signal detected by a coulometer for each measurement must be corrected for background noise, which is termed the blank. The current best practice recommendation is to measure the blank once per analysis session and use this constant value to correct all measurements. However, calculating the blank for each measurement separately shows that the blank sometimes changes during analysis sessions. Correcting measurements to a constant blank when the blank is actually changing leads to an apparent drift in DIC results and therefore lower accuracy. Here, we propose an alternative method for coulometer blank corrections in which the blank is calculated on a per-measurement basis. The per-measurement blank values are then fitted to a smoothing function to determine a set of fitted blank values with which the measurements are corrected. We test the three different approaches (constant, per-measurement and fitted) by applying them to 263 measurements of a laboratory internal standard conducted during 89 analysis sessions over ∼ 7 years. Switching from the constant blank to either the per-measurement or fitted blank improves the precision from 1.85 to 1.31 µmol kg−1. This improvement is statistically significant and important relative to the climate-quality uncertainty target for DIC measurements of ±2 µmol kg−1. Using the fitted blank rather than per-measurement blank eliminates a number of outliers, notably reducing the total range and kurtosis of the residuals. A free and open source Python package (koolstof) has been made available to perform fitted blank corrections for some common coulometer data types. We recommend that in future coulometric DIC analyses, per-measurement blanks should be routinely calculated as part of the quality control process and the fitted blank method applied either as standard or when a changing blank is observed.

In light of the rapidly increasing CO2 emissions, extreme climate change now described as “global boiling”, and tightening regulatory standards, direct air capture (DAC) has attracted significant attention, even surpassing that given to low-carbon or carbon-neutral power generation technologies. Among various DAC methods, the electrochemical pH-swing process is considered particularly promising because of its potential energy efficiency. Our research group has developed a bench-scale electrochemical pH-swing CO2 capture system that employs electrodes composed of proton intercalation materials in an alkaline electrolyte.[1-3] This method requires periodic electrolyte switching to facilitate recurring charge/discharge cycles and to achieve the desired pH swing. A three-way valve alternates connection between liquid channels to switch two electrolyte streams. However, during valve switching inter-stream mixing inadvertently moderates both pH and dissolved inorganic carbon (DIC) levels, thereby affecting the attainment of the desired pH swing, the corresponding swing in potassium concentration, and the amount of CO2 transferred.This theoretical study quantitively investigates the changes in ion composition resulting from inter-stream mixing, which is a process elucidated in electrochemical desalination using Na+ intercalation.[ 4,5] It compares these changes with those of an ideal cycle and proposes strategies to mitigate and harness the mixing effects to enhance overall cycle performance. The ion composition for each state is determined thermodynamically based on K+ and DIC concentrations as state variables. These variables are interrelated through Henry’s law, CO2 speciation, water dissociation, and electroneutrality. We model the valve-switching process as a sequence of four steps (Fig.1a). The change in ion concentration, arising from either (de)alkalizing steps or inter-stream mixing, is determined using a mass balance between the two electrolyte streams. By combining the concentration changes due to both effects, K+ and DIC concentrations for each half-cycle of the electrochemical swing are obtained.Application of this model to our DAC system revealed that larger mixing volumes require more electrochemical cycles to achieve the desired K+-swing and result in lower DIC retention at the endpoint (Figs.1b,c). Figure 1d shows that increased mixing volume leads to reduced DIC retention, even though the desired K+-swing is attained. This implies that greater energy input is required and that CO2 transfer is diminished. Moreover, the results indicate that DIC retention is more sensitive to inter-stream than K+-swing attainment. Figure 1e depicts this aspect, as K+ charge efficiency consistently exceeds DIC retention efficiency at the same ratio of dimensionless electrode capacity (Q*) to dimensionless mixing volume (V* mix). One approach to mitigate the adverse effects resulting from inter-stream mixing is to minimize the mixing volume or enhance the electrode’s specific capacity. Alternatively, the mixing effect can be intentionally exploited (Fig.1g). By fully mixing the two streams before the (de)alkalizing steps, the duration of the electrochemical process is reduced, thereby lowering energy consumption. Although this pre-mixed cycle achieves roughly half the ideal DIC retention shown in Fig. 1f, a significant reduction in energy consumption compensates for the loss in DIC retention. This strategy enhances overall energy efficiency.Reference:[1] K.C. Smith and A. Shrivastava, "Method and system for electrochemical-based carbon capture and sequestration/valorization," Patent Cooperation Treaty Application WO 2023/096735 A1 (2023).[2] K.C. Smith, J. Lee, P.G. Rozzi, T.S. Arthur, and C.A. Roberts, "Electrochemical Device for Carbon Dioxide Direct Air Capture using Solid Proton Intercalation Electrodes to Swing the pH of an Alkaline Electrolyte," US Patent Application 63/745,219 (2025).[3] P.G. Rozzi, J. Lee, V.Q. Do, T.S. Arthur, C.A. Roberts, and K.C. Smith, "Toward Low-Energy Direct-Air CO2 Capture by Reversible Proton-Intercalation Mediated Alkalization," in review (2025).[4] E.R. Reale, L. Regenwetter, A. Agrawal, B. Dardón, N. Dicola, S. Sanagala, K.C. Smith, “Low porosity, high areal-capacity Prussian blue analogue electrodes enhance salt removal and thermodynamic efficiency in symmetric Faradaic deionization with automated fluid control,” Water Res X 13 (2021) 100116. https://doi.org/10.1016/J.WROA.2021.100116.[5] V.Q. Do, E.R. Reale, I.C. Loud, P.G. Rozzi, H. Tan, D.A. Willis, K.C. Smith, “Embedded, micro-interdigitated flow fields in high areal-loading intercalation electrodes towards seawater desalination and beyond †,” Energy Environ. Sci 16 (2023) 3025. https://doi.org/10.1039/d3ee01302b. Figure 1

Abstract. Ocean alkalinity enhancement (OAE) is a carbon dioxide (CO2) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO2. Increasing seawater alkalinity leads to an increase in the saturation state (Ω) with respect to several minerals, which may trigger mineral precipitation, consuming the added alkalinity and thus decreasing the overall efficiency of OAE. To explore mineral formation due to alkalinity addition, we present results from shipboard experiments in which an aqueous solution of NaOH was added to unfiltered seawater collected from the surface ocean in the Sargasso Sea. Alkalinity addition ranged from 500 to 2000 µmol kg−1, and the carbonate chemistry was monitored through time by measuring total alkalinity (TA) and dissolved inorganic carbon (DIC), which were used to calculate Ω. The amount of precipitate and its mineralogy were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant precipitate phase is aragonite with trace amounts of calcite and magnesium hydroxide (MgOH2, i.e., brucite). Aragonite crystallite size increases and its micro-strain decreases with time, consistent with Ostwald ripening. The precipitation rate (r) in our experiments and those of other OAE-related calcium carbonate precipitation studies correlate with the aragonite saturation state (ΩA), and the resulting fit of log10(r) = n × log10 (ΩA−1) + log10 (k) yields a reaction order n=2.15 ± 0.50 and a rate constant k=0.20 ± 0.10 µmol h−1. The reaction order is comparable to that derived from previous studies, but the rate constant is 1 order of magnitude lower, which we attribute to the fact that our experiments are unseeded compared with previous studies that used aragonite seeds which act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlated with the initial Ω. Mineral precipitation occurred in a runaway manner, decreasing TA to values below those of seawater prior to alkalinity addition. This study demonstrates that the highest risk of mineral precipitation is immediately following alkalinity addition and before dilution and CO2 uptake by seawater, both of which lower Ω. Aragonite precipitation will decrease OAE efficiency because aragonite is typically supersaturated in surface ocean waters. Thus, once formed, aragonite essentially permanently removes the precipitated alkalinity from the CO2 uptake process. Runaway mineral precipitation also means that mineral precipitation following OAE may not only decrease OAE efficiency at sequestering CO2 but could also render this approach counterproductive. As such, mineral precipitation should be avoided by keeping Ω below the threshold of precipitation and quantifying its consequences for OAE efficiency if it occurs. Lastly, in order to be able to quantitatively determine the impact of mineral precipitation during OAE, a mechanistic understanding of precipitation in the context of OAE must be developed.

Recent strategies to mitigate the most drastic consequences of climate change increasingly depend upon direct air capture (DAC) of CO2 to offset CO2 emissions from hard-to-abate sectors and counteract legacy CO2 emissions. To achieve economical DAC, electrochemical pH-swing methods have been studied as an effective alternative to amine-based methods based on their room-temperature regeneration and reduced complexity. In this work, we experimentally demonstrate for the first time CO2 capture at 400 ppm and release using a pH swing driven by solid proton-intercalation electrodes in an aqueous alkaline electrolyte.[1-3] The bench-scale experimental apparatus used in this work utilized manganese dioxide (MnO2) to (de)intercalate protons. Both electrolytic manganese dioxide (EMD) and α-phase MnO2 with pre-intercalated potassium (α-K0.05MnO2) were screened as candidate materials. Ultimately, α-K0.05MnO2 was chosen as our active material due to its higher specific capacity and lower capacity fade when (de)intercalating protons (Fig. 1c). This process of (de)alkalization shifted the pH and thus dissolved inorganic carbon (DIC) solubility in two separate electrolyte streams in a flow cell architecture. High-pH electrolyte was chosen for its higher CO2 solubility and solubility gradient compared to low-pH electrolyte (Fig. 1a). Valve switching enabled the pH swing to be deepened over several galvanostatic cycles. (De)alkalization was performed separately from CO2 transfer, and these two sequential processes constituted an asynchronous CO2 capture cycle (Fig. 1b). A bench-scale apparatus was constructed (Fig. 1d) to perform (de)alkalization and CO2 transfer and autonomously switch between the two processes.Before system modifications, or the “DIC retained” case, we found that our system yielded a CO2 transfer extent of 7.15 mmol g-1, a specific energy consumption (SEC) of 2.77 GJ tonne-1 CO2, and a productivity of 0.0341 kg m-2 day-1 (Fig. 1e) for a single capture cycle. The observed extent of CO2 transfer for each cycle indicated that significant inter-stream mixing of electrolytes was likely occurring during valve switching events, which has been observed in similar electrochemical separations work.[4-5] This theoretically led to reduced CO2 transfer compared to the theoretical maximum, thereby limiting the best attainable SEC and productivity.To reduce energy consumption, the electrolyte streams were intentionally mixed together immediately prior to (de)alkalization. This pre-mixing strategy reduced the number of electrochemical cycles required to (de)alkalize the electrolyte streams, thus lowering energy consumption without a major reduction of CO2 transfer or productivity. This method yielded a CO2 transfer of 5.85 mmol g-1, a specific energy consumption of 1.68 GJ tonne-1 CO2 and a productivity of 0.0319 kg m-2 day-1 (Fig. 1f) when releasing CO2 at 400 ppm. This SEC extrapolated to roughly 2.60 GJ tonne-1 CO2 when including compression to a CO2 partial pressure of 1 bar, which is low compared to other CO2 capture studies using feed gases at atmospheric concentrations.

This study presents a novel integrated system for electrochemical carbon capture (ECC) coupled with simultaneous energy storage capabilities, inspired by vanadium redox flow battery (VRFB) chemistry [1]. The proposed process employs established vanadium (VO2 +/ VO2+) and ferricyanide (Fe(CN)6 3−/ Fe(CN)6 4−) redox couples to induce proton-driven modulation of solution pH, facilitating CO₂ capture and release via electrochemical pH-swing. The key advantage of this process is its potential dual functionality: it could capture CO2 and store energy when renewable electricity is abundant and regenerate the absorbent while releasing the stored energy to grid during periods of limited renewable generation. The initial phase of the study focused on the fundamental electrochemical principles and operational viability of the process through bench-scale experimentation. Cyclic voltammetry (CV) was performed to isolate desired proton-coupled electron transfer (PCET) reactions of vanadium while preventing parasitic or undesired redox reactions, particularly those involving lower oxidation states of vanadium. Using polarization curve analysis, an optimal practical potential of 0.5 V was determined to overcome kinetic and ohmic losses.Electrochemical impedance spectroscopy (EIS) was used to study the redox kinetics, highlighting significant performance improvements achieved through plasma treatment of graphite electrode surfaces. This treatment decreased the contact angle, increased hydrophilicity, and improved wettability, thereby enhancing electrochemical surface activity. Consequently, a 43% reduction in charge transfer resistance was achieved, enhancing current density and reaction kinetics. Additionally, comprehensive mass transfer studies established an optimized electrolyte composition consisting of a balanced 1:1 ratio of redox-active species to background electrolyte due to the trade-off between electrical conductivity and Faradaic efficiency. Bench-scale experiments demonstrated reversible system operation, achieving an energy consumption of approximately 54 kJ/mol CO2—comparable or superior to existing electrochemical capture methods reported in the literature. Additionally, the reversibility of the developed process was studied by its operation over multiple charge and discharge cycles.A detailed thermodynamic modeling effort was also carried out to support experimental observations and serve as the backbone of techno-economic analysis (TEA). The process modeling included four stages: 1-CO2 absorption using potassium carbonate (H2CO3) in the absorber, 2-electrochemical acidification during the charge cycle, 3-CO2 outgassing in the flash tank, and 4-electrochemical absorbent regeneration during the discharge cycle. Equilibrium speciation analysis of dissolved inorganic carbon (DIC) species, including bicarbonate (HCO3 −), carbonate (CO3 2−), and carbonic acid (H2CO3), was studied because of proton concentration modulation on the carbon capture process at different states of charge (SOC). The model revealed a substantial increase in CO2 partial pressure during proton-driven acidification, confirming the strong driving force for gas-phase CO2 desorption upon pH reduction. Furthermore, modeling outcomes guided the optimization of electrolyte concentrations and identified critical operational parameters influencing overall system performance, such as SOC, cell potential and redox speciation control within electrolytes.Complementing the experimental proof-of-concept studies, an extensive techno-economic analysis (TEA) was performed to evaluate the economic feasibility and potential commercial scalability of the technology. The TEA utilized detailed cost and performance metrics from commercially available VRFB literature, addressing a common limitation in ECC research due to the low technology readiness levels (TRL) and associated scarcity of accurate scaled-up cost data. This approach allowed for a more realistic TEA and reducing uncertainties typically associated with early-stage electrochemical systems. The economic analysis identified two primary capital expenditure drivers: (i) the electrode active surface area (ECSA)-to-geometric area (GA) ratio, critical for enabling three-dimensional scaling of electrochemical modules using porous electrodes, and (ii) membrane costs, particularly for proton exchange membranes such as Nafion. Meanwhile, operating expenses were mainly influenced by cell potential and Faradaic efficiency. A systematic sensitivity analysis conducted within the TEA framework evaluated the individual and combined impacts of these parameters, specifically examining the ECSA-to-GA ratio, membrane prices, current density, cell potential, and Faradaic efficiency. Additionally, the study introduced a novel "cost-target analysis," designed to assess the synergistic effects of simultaneous improvements across multiple performance metrics rather than evaluating them separately. This multi-variable approach identified operational regimes capable of achieving targeted capture costs of $100 and $75/tonneCO2 through incremental improvements in key parameters. Specifically, the analysis showed that reducing Nafion membrane prices to below $200/m2—or alternatively, adopting cost-effective membranes such as sulfonated poly(ether ether ketone) (SPEEK)—is essential for meeting these economic targets. However, the performance and long-term durability of SPEEK membranes must be improved to ensure viability at scale. Additionally, increasing the ECSA-to-GA ratio above 10 was identified as another critical requirement.

The pressing need for scalable direct air carbon capture (DAC) technologies motivates the development of novel electrochemical systems that overcome the high energy demands of traditional approaches. Supercapacitive swing adsorption (SSA) is an energy-efficient electrochemical CO₂ capture method, but with low CO₂ adsorption capacity. The mechanism of SSA is hypothesized to occur via pH swings, whereby hydrolysis of CO₂ in an initially neutral pH aqueous electrolyte leads to proton adsorption at the carbon electrode. In this study, we hypothesize that we can increase the CO₂ adsorption capacity of SSA via an asymmetric design that integrates a proton insertion transition metal oxide electrode. These electrodes have higher specific capacities for proton adsorption than activated carbon, which could increase the pH swing effect, improve capture efficiency, and reduce energy requirements. We designed and evaluated three electrochemical flow cell configurations: symmetrical SSA using two activated carbon electrodes, an asymmetrical SSA system combining a proton insertion electrode with an activated carbon counter electrode, and a full proton rocking-chair battery-type SSA. We characterized the CO₂ adsorption/desorption performance, electrochemical efficiency, and long-term operational stability of each system. Our results show that proton insertion electrodes enable a pronounced pH increase at negative potentials due to proton uptake. This leads to increased dissolved inorganic carbon (DIC) within the system through dissociation of carbon dioxide into bicarbonate ions. Utilizing proton insertion electrodes resulted in an increase in CO₂ capture capacity and a reduction in energy consumption to approximately 100 kJ/mol CO₂, near the threshold for commercial DAC. Moreover, the system demonstrated stable performance over extended cycling, highlighting its potential for real-world implementation. These findings establish that proton-coupled pH swing mechanisms can be leveraged to improve the viability of electrochemical CO₂ capture without forfeiting the longevity of previous SSA systems.

Abstract Travertine is widely distributed in northwestern (NW) Iran and Turkey and serves as a valuable sample for paleoenvironmental reconstructions in semi-arid areas. Previous studies have analyzed the chemical compositions, carbon and oxygen isotopes of the travertines in NW Iran for paleoenvironmental reconstructions, but little dating has been done because travertine 14 C dating faces the problem of identifying the initial 14 C concentration of each sample. The objective of this study is to determine the formation age of travertine in NW Iran using radiocarbon ( 14 C) and δ 13 C from a travertine mound and its related spring water. Travertine samples were collected from the base to the top of a cone-shaped travertine mound, Zendan-e Soleyman, in the Takab region of NW Iran. The 14 C concentrations of the travertine samples ranged from 0.67 to 3.72 pMC, with values fluctuating considerably and higher 14 C being observed at higher elevations. The δ 13 C values were lower at higher elevations (+10.1 to +7.4‰) with fluctuations. The values suggest that the travertines were formed through the decarbonation of limestone and rapid degassing. The dissolved inorganic carbon (DIC) of nearby spring water samples had 14 C concentrations of about 10.4 pMC, about 89.6% dead carbon fraction (DCF), and δ 13 C value of +1.3‰. These values indicate that one of the of CO 2 sources in the travertine-deposited spring water was of hydrothermal origin. Considering the DCF of the spring water DIC, the formation of the travertine mound began about 20 kyr BP, and the growth of the mound ended about 7 kyr BP.

Massive carbon inputs from fish farming reduce carbon sequestration capacity in a macroalgae mariculture area.

Abstract The chemical nature of river water significantly influences the coastal carbonate system, contributing to coastal acidification and creating suboptimal conditions for marine calcifiers. While several regional efforts have assessed observationally based riverine concentrations and fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC), these values in global ocean biogeochemical models have generally been simplified, often set to zero or balanced against global sediment calcium carbonate burial. To enhance our understanding of rivers' role in the coastal carbonate system, we applied multiple linear regression (MLR) to develop global empirical relationships for estimating river TA and DIC from watershed properties. We find that river TA values are primarily controlled by forest, carbonate rock coverage, and annual mean precipitation, explaining 74% of the spatial variability in TA. The variability explained improves to 77% with the inclusion of permafrost and glacial coverage, especially in high latitude and altitude regions. Additionally, nearly 30% of the spatial variability in the river DIC‐to‐TA ratio can be explained by terrestrial gross primary production and carbonate rock coverage. Applying these MLR‐derived TA and DIC concentrations to a 1/4° resolution global ocean model reduces the high bias in model estimates of global coastal uptake by 69% (equivalent to 0.11 Pg C yr −1 less uptake) compared to the case with zero river TA and DIC. This study elucidates key drivers of the river carbonate system and underscores the importance of accurately representing riverine inputs to improve predictions of global coastal carbon dynamics and ecosystem responses to environmental changes.

Long-term trends and anthropogenic forcing of surface ocean carbon storage and acidification.

Abstract Coastal peatlands are important carbon, nutrient, and trace metal stores that have been extensively drained and degraded. Rewetting may restore ecological and hydrological functions but could also enhance the export of solutes through the submarine groundwater discharge (SGD) pathway to adjacent marine environments. We directly measured SGD and its associated solute concentrations in front of a coastal peatland using seepage meters across two shore-perpendicular transects in summer and fall 2021. In addition to SGD, groundwater and porewater samples were analyzed for dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), nutrients, trace metals, and stable water and carbon isotopes to characterize the discharging groundwater. Results indicated a mean SGD rate of 1.3 ± 1.7 cm d⁻ 1 (max. 3.9 cm d⁻ 1 ) during the summer campaign while recharge conditions prevailed in fall, due to intense landwards winds. Notably, salinity of SGD exceeded that of ambient seawater, and isotope analyses revealed that most SGD originated from the recirculation of brackish seawater. Solute concentrations were significantly higher in SGD than in seawater, but lower than values encountered in porewater and groundwater. The flushed permeable coastal sediments thus serve as a source for carbon, nutrients and trace metals. Summer solute fluxes were substantial (mmol m −2 d −1 ) – DOC: 10; DIC: 44; TDN: 2.8; NH₄⁺: 1.5, PO₄ 3 ⁻: 0.13; Fe: 0.23; and Mn: 0.17. However, estimates of solute fluxes might be conservative as land-derived groundwater influence was minor during the sampling campaigns. Our findings suggest that SGD in front of coastal peatlands is significant and could influence local and regional biogeochemical budgets.

Is it possible to valorize bicarbonates from reclaimed wastewater by CO2 electroreduction into formic acid? Investigation under low bicarbonate concentration and low-conductivity solutions.

Lakes function as receivers, regulators, reactors, and reservoirs in the global carbon cycle. However, long-term and national-scale monitoring data on lake carbon parameters are not available. Alternatively, long-term archived Landsat data have potential for enhancing the spatial and temporal retrievals of lake carbon parameters. Using Landsat reflectance during 1984-2023 and in-situ measurements at 5,503 stations, this study developed several two-step Random Forest algorithms, systematically incorporating findings from previous studies, to remotely retrieve concentrations and storage of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) across 24,366 lakes in China. This integrated 40-year dataset provides the grid-based distributions (1.0°) of different carbon components, overcoming limitations of prior single-component studies. The accuracies of the developed algorithms were validated by comparing the remotely derived results with publicly available reference data. The remotely retrieved dataset provides grid-based spatial and temporal distributions of lake carbon concentrations and stocks during 1984-2023, offering a valuable resource for lake water environment management, lake carbon stock estimation, and global carbon balance assessment.

Abstract The occurrence of marine heatwaves, sustained periods of anomalously elevated temperatures in the marine environment, appear to be increasing with time which is a symptom of anthropogenic climate change. These extreme temperatures impact large oceanic areas, and they can have catastrophic impacts on marine ecosystems. However, the impact on the marine carbonate system, air-sea carbon dioxide (CO2) exchange and the resulting ocean carbon sink before and after the heatwave appears unclear. Here we investigate the impact on the ocean carbonate system of five documented marine heatwaves in Western Australia (2011), the North Pacific ‘blob’ (2015), the Southern Ocean (2016), South Pacific (2016) and Equatorial Indian Ocean (2016). The changes in the carbonate system and the air-sea CO2 exchange 12 months before, during and 12 months after each heatwave were evaluated using a well characterised and spatially complete surface observation-based carbonate system dataset. The signature of each heatwave was clearly identified within the data, but the carbonate-system specific signature was regionally different. All the heatwaves studied showed a significant reduction in dissolved inorganic carbon (DIC), where negative anomalies increased in magnitude before the heatwave peak, followed by a decrease afterwards. The carbonate chemistry changes mean that fugacity of CO2 (fCO2 (sw)) and pH anomalies in all the heatwaves were controlled by the variations in DIC and temperature, with anomalies occurring both before and after the heatwaves, but the specifics varied across regions. The effect of these heatwaves on the air-sea exchange of CO2 varied during the heatwave lifetime, which was driven by a combination of the carbonate system state and meteorological condition. These results identify that the impact of the heatwaves on the carbonate system and ocean uptake cannot be easily generalised, and it is important to assess the heatwave impacts before, during and after the severe heatwave conditions.



Abstract. Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (mCDR) approach that relies on the addition of liquid or solid alkalinity into seawater to take up and neutralize carbon dioxide (CO2) from the atmosphere. Documenting the effectiveness of OAE for carbon removal requires research and development of measurement, reporting, and verification (MRV) frameworks. Specifically, direct observations of carbon uptake via OAE will be critical to constrain the total carbon dioxide removal (CDR) and to validate the model-based MRV approaches currently in use. In September 2023, we conducted a ship-based rhodamine water tracer (RT) release in United States federal waters south of Martha's Vineyard, MA, followed by a 36 h tracking and monitoring campaign. We collected RT fluorescence data and a suite of physical and chemical parameters at the sea surface and through the upper water column using the ship's underway system, a conductivity–temperature–depth (CTD) rosette, and Lagrangian drifters. We developed an OAE analytical framework that explicitly references the OAE intervention and the resulting CDR to the baseline ocean state using these in situ observations. We evaluated the effectiveness of defining a “dynamic” baseline, in which the carbonate chemistry was continuously constrained spatially and temporally using the shipboard data outside of the tracer patch. This approach reduced the influence of baseline variability by 25 % for CO2 fugacity (fCO2) and 60 % for TA. We then constructed a hypothetical alkalinity release experiment using RT as a proxy for OAE. With appropriate sampling, and with suitable ocean conditions, OAE signals were predicted to be detectable in total alkalinity (TA > 10 µmol kg−1), pH (> 0.01), and CO2 fugacity (fCO2 > 10 µatm). Over 36 h, an ensuing additional CO2 uptake was driven by this persistent gradient in surface fCO2. The calculated CDR signal was detectable as a 4 µatm surface fCO2 increase, a pH decrease of 0.004 units, and a dissolved inorganic carbon (DIC) increase of 1.8 µmol kg−1, translating to 10 % of the total potential CDR. This signal, and the CDR itself, would continue to grow as long as an fCO2 gradient persisted at the sea surface. Climatological results from a regional physical circulation model supported these findings and indicated that models and in-water measurements can be used in concert to develop a comprehensive MRV framework for OAE-based mCDR.

Ocean carbon removal represents a promising pathway for mitigating residual anthropogenic carbon dioxide (CO2), yet existing methods are constrained by high energy demands and potential ecological risks. Here, inspired by the natural calcification process of corals, we present a bio-inspired capacitive decarbonization (CDC) reactor that sequesters dissolved inorganic carbon (DIC) from seawater as CaCO3 using only seawater-derived Ca2+ and renewable electricity. The CDC system integrates a Ca2+-selective electrode with a weak electric field to regulate ion transport and disrupt the hydration shell of Ca2+, enhancing its reaction with CO3 2-. To address the limited concentration of CO3 2- relative to Ca2+ in seawater, we introduce an asymmetric electrosorption strategy to preferentially enrich CO3 2- at the electrode interface, achieving a DIC conversion rate of up to 34% with an ultralow intrinsic electrochemical energy input of 2.5kJ mol-1 CO2 for the CDC reactor. The reactor exhibits stable continuous operation for over 100h without fouling, enabled by spatially decoupled CaCO3 precipitation. To mitigate the reduction in seawater alkalinity, we introduce a mineral-assisted re-alkalinization step that effectively restores pH and supports continued CO2 absorption. A global integrated analysis model shows the CDC technology could remove up to 11-438 million tonnes of CO2 by 2050-2100. This work demonstrates a scalable and low-energy solution for durable ocean carbon removal.