Arvind Singh

Ocean alkalinity enhancement (OAE) is a proposed carbon dioxide removal strategy. Although several methods have been proposed for OAE, a systematic assessment of the mineral‐based alkalinity generation potential, along with the associated carbonate chemistry and biogeochemical responses to alkalinity additions, is not fully constrained. Previous studies indicate a high alkalinity generation potential of periclase and limited dissolution of olivine. We conducted a 9‐day mineral‐based mesocosm experiment to evaluate the potential of periclase and olivine for OAE across a broader range of targeted alkalinity additions: periclase (P1: +500, P2: +1,000, P3: +1,500, and P4: +2,000 μmol ) and olivine (O1: +250, O2: +500, O3: +750, and O4: +1,000 μmol ). Olivine additions did not result in measurable changes in alkalinity or pH, likely due to high dissolved organic matter, high pH, and limited mixing over time. In contrast, for periclase treatments (P1–P4), alkalinity increased by ∼200, ∼450, ∼447, and ∼463 μmol , while pH increased by 0.31, 0.42, 0.60, and 0.59, respectively. Dissolved inorganic carbon (DIC) declined within the first 24 hr in P2, P3, and P4, with alkalinity decreasing subsequently after day one. This decline in DIC, together with the isotopic signature of DIC (), revealed that secondary precipitation dominated at alkalinity additions of 1,000 μmol for periclase. This suggests that the threshold for effective alkalinity enhancement lies between 500 and 1,000 μmol . We further conclude that temporal changes observed in the mesocosms were primarily driven by nutrient availability and biogeochemical processes rather than the alkalinity addition.

Ocean alkalinity enhancement, a marine carbon dioxide removal method, involves the addition of alkaline substances to seawater, thereby shifting the carbonate chemistry speciation toward carbonate ions. Ongoing research seeks to evaluate the efficacy and durability associated with these substances. Here, we investigate the alkalinity generation potential and response of carbonate chemistry speciations for three natural minerals—olivine, kaolinite, and dolomite, and two anthropogenic minerals—periclase and hydrated lime. Overall, no significant changes in alkalinity (), pH and dissolved inorganic carbon (DIC) were observed for the natural minerals, suggesting that these minerals have a lower efficiency in increasing . In contrast, the dissolution of anthropogenic minerals increased up to 18, representing more than 80 of the total added , and elevated pH up to 0.6 units. Additionally, we used the temporal changes in the DIC concentration and its isotopic composition (C) within the mesocosms to trace secondary carbonate precipitation, remineralization, photosynthesis, ingassing and outgassing. No clear alkalinity‐loss or secondary‐precipitation signal was detected over 9 days under the tested mesocosm conditions. As the loss of alkalinity via secondary precipitation diminishes the efficiency of the alkalinity enhancement, identifying the range of alkalinity levels that do not induce secondary precipitation is valuable for evaluating the effectiveness of this method.

The ocean nitrogen cycle is influenced by climatic conditions, which regulate atmospheric greenhouse gases and generate feedback effects. These feedbacks occur through the release of nitrous oxide and alterations in the fixed‐nitrogen inventory, which in turn regulates carbon sequestration. The Arabian Sea hosts an intense oxygen minimum zone, making it a critical region for understanding marine biogeochemical cycling. Understanding past variability in nitrogen cycling offers crucial insights into the relationship between ocean circulation and productivity, which is essential for predicting future biogeochemical changes. Here, we present reconstructions of spatiotemporal variation in water column denitrification and fixation, the influence of oceanic ventilation, and the role of monsoonal variability on nitrogen cycling in the Arabian Sea. Based on our sedimentary nitrogen isotope ( ¹⁵ N) records, we found N ₂ fixation and denitrification to be concomitantly high during the Last Glacial Maximum (LGM; ∼23–19 ka) in the northern and southeastern Arabian Sea. In contrast, both processes weakened during the deglaciation. The dominance of denitrification, with the decrease in dissolved oxygen as a result of the reduced influx of Antarctic Intermediate Water, appears to have begun in the early Holocene. On millennial timescales, denitrification intensity is mainly controlled by subsurface ventilation by water masses from the Southern Ocean. Despite temporal variations, both processes remained more intense in the northern Arabian Sea. Our findings underscore the sensitivity of the nitrogen cycle in the Arabian Sea to climate change, with continued warming causing denitrification to intensify, providing positive feedback to the climate.

Phytoplankton productivity, abundance, and growth in marine ecosystems are influenced by nutrient availability and elemental ratios. It is now well established that the classical Redfield ratio (106C:16N:1P) is not universal; instead, nutrient ratios vary with environmental conditions and follow strong latitudinal patterns. However, these observations are biased toward specific regions and seasons. Here, we analyze a large data set on carbon:nitrogen:phosphorus (C:N:P) ratios in particulate organic matter, as well as in dissolved inorganic and organic pools of the northern Indian Ocean. On average, the particulate organic C:P (∼130 39) and N:P (∼19 4) ratios in the surface ocean were higher than the Redfield ratio but consistently lower than those observed in the Pacific and Atlantic oceans. In contrast, the particulate organic C:N ratios (∼7 1) across tropical oceans reflect the Redfield value. We found seasonal variations, with higher particulate organic C:P, N:P, and C:N values during summer and winter compared to spring and autumn. High particulate organic C:P, N:P, and C:N ratios in the mixed layer during nutrient‐enrichment periods, along with increased inorganic N:P ratios, indicate that phytoplankton acclimate to ambient N:P ratios. Differential remineralization of nitrogen‐ and phosphorus‐rich biomolecules relative to carbon‐rich compounds regulates these ratios at greater depths. We found that inorganic nutrient ratios and temperature play key roles in shaping planktonic composition in the northern Indian Ocean. Generalized Additive Model reveals relationships between particulate C:P and N:P ratios with temperature and nutrients, indicating that future warming may lower these ratios, with implications for the biological pump under climate change scenarios.

Marine Carbon Dioxide Removal (mCDR) approaches are increasingly considered for climate change mitigation as a supplement to rapid emissions reduction. However, it is still unclear if mCDR approaches could be effective, safe, and accountable. A critical requirement for mCDR to work, potentially, is robust monitoring, reporting, and verification (MRV) of greenhouse gas(es) removed through mCDR activities. MRV frameworks and protocols are currently developed in a scattered manner by individual stakeholders, so that the principles they are built upon may or may not appeal to the broader international community that is interested in the ocean commons. International agreement and consolidation on MRV for mCDR seem crucial to legitimize and validate mCDR, considering that the ocean is a globally interconnected fluid and that activities by some may affect many others. Here, we undertake a step toward consolidation of MRV by consulting the international scientific community. We established a global network of scientists organized into 6 “continental” nodes, each of which addressed the same set of MRV-related questions and whose thoughts were equally weighted in the synthesis. Our consultation shows that while there are many converging views on MRV (e.g., the importance of modeling for MRV), there are also differences in the regional MRV priorities (e.g., the importance of regional vs global models). The areas of consensus and divergence identified herein may be instrumental in the design of more widely accepted MRV frameworks, informed equally by scientists from 6 continents.

On behalf of our ocean community of authors and readers, the editors at JGR‐Oceans would like to sincerely thank all those who gave their valuable time to review manuscripts for the journal in 2025. Thank YOU!

The mechanisms by which ongoing climate change influences ocean Oxygen Minimum Zones (OMZs) are insufficiently understood, making it essential to examine their long‐term variations under substantial climatic forcing. Here, we present the first planktic foraminifera iodine‐to‐calcium records in two Arabian Sea sediment cores: one located within the core of the modern OMZ in the north and other on its southeastern flank, enabling us to infer the lateral extent of the OMZ across the last glacial cycle. Our results indicate persistently stronger deoxygenation in the northern Arabian Sea throughout the last glacial cycle. The OMZ expanded southward during the Last Glacial Maximum—likely driven by reduced ventilation and increased productivity from enhanced winter mixing of nutrients. During deglaciation, it retreated northward as southern‐sourced intermediate waters intruded and productivity declined. We suggest that changes in ventilation and productivity, rather than water temperature, exerted the dominant control on Arabian Sea OMZ variability.

Picophytoplankton are drivers of biogeochemical cycling and serve as key indicators of seafood productivity, ocean health, and climate change. The Arabian Sea and the Bay of Bengal—two basins of the northern Indian Ocean that support the livelihoods of over a billion people—share similar climatic conditions but differ in their hydrographic and ecological characteristics. However, the environmental factors governing picophytoplankton abundance and their implications on carbon biomass in these regions remain poorly understood. This study investigated the abundance, carbon biomass, and environmental relationships of three groups of picophytoplankton: Prochlorococcus , Synechococcus , and picoeukaryotes. We observed Synechococcus dominating the surface waters (47% in the Arabian Sea and 63% in the Bay of Bengal), while Prochlorococcus showed the highest abundance and variability in the water column between basins. Picoeukaryotes contributed more to the total picophytoplankton carbon biomass in both basins. In particular, picophytoplankton carbon biomass contribution to particulate organic carbon were not substantially distinct between the two basins. The estimated picophytoplankton carbon stock was 1.05 Tg C in the Arabian Sea and 0.5 Tg C in the Bay of Bengal. Furthermore, we determined key environmental factors—temperature, salinity, light, dissolved inorganic phosphorus (DIP), and dissolved inorganic silicate (DSi)—that shape picophytoplankton dynamics in the region. Temperature and salinity primarily influenced picophytoplankton distribution in the Arabian Sea, while nutrients were critical in the Bay of Bengal. Overall, we underscore the role of physicochemical dynamics in carbon cycling by presenting the contribution of picophytoplankton carbon biomass to the region's particulate organic carbon pool.

Ocean alkalinity enhancement (OAE) is emerging as a promising yet largely untested marine carbon dioxide CO₂ removal approach. It involves the addition of alkaline substances such as powdered minerals and aqueous hydroxide solutions to seawater, shifting the carbonate chemistry speciation towards carbonate ions so as to store more CO₂. Contemporaneous studies are being carried out to evaluate the efficacy, durability, and risks associated with these substances. Given the heterogeneity in a natural ecosystem, each substance will have distinct implications on carbonate chemistry as well as biogeochemistry of a given ecosystem. Our study contributes to ongoing research by examining the response of the ocean's carbonate chemistry to the addition of various alkalinity feedstocks — including both naturally occurring and anthropogenically (industrially) produced minerals, along the southeastern coastal Arabian Sea. We tested the alkalinity (A_T) generation potential and traced the associated changes in the carbonate chemistry speciations for three naturally occurring minerals: (i) olivine ((MgFe)₂SiO₄), (ii) kaolinite (Al₂Si₂O₅(OH)₄), and (iii) dolomite (CaMg(CO₃)₂), and for two anthropogenically produced minerals: (i) periclase (MgO) and (ii) hydrated lime (Ca(OH)₂) of two different mineral compositions, using 300 L mesocosms. Overall, no significant changes in A_T, pH and dissolved inorganic carbon (DIC) were observed for the naturally occurring minerals, suggesting the lower efficiency of these minerals to increase A_T,. In contrast, the dissolution of periclase and hydrated lime increased A_T (up to 16 %, which corresponds to 80 % of the total added A_T) and pH by up to 0.6 units. We further demonstrate that the temporal changes in the carbon isotopic composition (δ¹³C) of DIC as well as the changes in the DIC concentration occurring within the mesocosms can serve as an effective and reliable proxy for tracing secondary carbonate precipitation. As loss of alkalinity via secondary precipitation diminishes the overall efficiency of the OAE approach, accurate determination of the threshold at which secondary precipitation is triggered is critical for maximizing the effectiveness of this method. We determine these thresholds and provide an assessment of various alkalinity feedstocks that could work best for OAE.