Abstract The low-latitude flow of water masses from the Pacific to the Indian Ocean, the Indonesian Throughflow (ITF), is a choke point of the surface ocean return flow of the ocean conveyor belt. Even though the significance of the ITF for the modern global ocean circulation and climate has long been established, little is known about the hemispheric origin of the water masses contributing to its overall transport in the past. Here, we take advantage of the distinctly different isotopic composition of subsurface nitrate in the Northern and Southern Hemisphere source waters to document the admixture of these waters in the ITF through time. Our record of bulk sedimentary δ15N from the Banda Sea, at the heart of the ITF, shows that Southern Hemisphere-sourced subsurface waters contributed significantly to the total ITF transport during the last 800,000 years. Because Southern Ocean processes ultimately set the biogeochemical source signature of the Southern Hemisphere endmember, the Banda Sea record implies an important conduit by which high southern latitude climate and ocean variability is transmitted into the global ocean.

Proxy records of seawater radiocarbon (14C/C) provide strong constraints on how changes in ocean ventilation contributed to the increase in atmospheric CO2 during the termination of the last ice age (approximate to 18,000-to-12,000 years ago). One outstanding problem, however, is the existence of anomalously low deglacial benthic foraminiferal 14C/C in the intermediate-depth Eastern Tropical North Pacific (ETNP) near the Gulf of California (GoC). This deglacial ETNP 14C/C anomaly is hypothesized to reflect either (a) an artifact of the proxy record, (b) the advection of low 14C/C seawater, or (c) the input of 14C/C-depleted geologic carbon related to local seafloor volcanism. To test these hypotheses, we first use new sediment-trap and seaweed 14C/C to establish a new baseline understanding of ETNP seawater 14C/C, which suggest that anomalously low 14C/C is upwelled in the modern GoC. We then apply new geochemical experiments to test and ultimately validate the utility of the benthic foraminiferal 14C/C as a proxy for seawater 14C/C. Finally, we present a compilation of published and new glacial-interglacial benthic foraminiferal 14C/C records, specifically developed to map the spatial and temporal variability of the intermediate-depth water mass containing the deglacial ETNP 14C/C anomaly. These results clearly show that the ETNP deglacial 14C/C anomaly develops near the GoC mouth, concomitant with local hydrothermal systems. Considering these results and those of our companion paper (Green et al., 2026, https://doi.org/10.1029/2025pa005217), we argue that the input of pH-neutral geologic carbon from hydrothermal vents near and within the GoC could explain the anomalous intermediate-depth 14C/C values both during the deglaciation and today.

Upwelling generates a nutrient-rich "cold tongue" in the eastern equatorial Pacific Ocean (EEP), with impacts on global climate, oceanic biological productivity, and the carbon cycle. The cold tongue was reduced during the Pliocene Epoch, a feature attributed to weaker upwelling and an associated deepening of the surface mixed layer in the EEP. Here, we report nitrogen-isotope evidence that modern-like upwelling occurred in the EEP during the Pliocene and has persisted over the past 5 million years. We explain the reduced Pliocene cold tongue as an expression of the reduced temperature difference between surface and subsurface waters in the tropical Pacific. The attendant reduction in the vertical density gradient may have maintained EEP upwelling despite the expected slackening of the trade winds under Pliocene warmth.

Despite their importance for long-term climate regulation, the rates and mechanisms of seafloor carbonate dissolution are poorly understood, especially with respect to calcite saturation and the role of sedimentary metabolic CO2 production. Here, we present results from an in situ porewater sampler deployed at the Cocos Ridge in the eastern equatorial Pacific, where we examine seafloor carbonate dissolution in locations with bottom water Omega(calcite) ranging from 1.0 to 0.84 (1600-3200 m). With cm-scale resolution from the sediment-water interface to 35 cm, we present porewater profiles of total alkalinity, pH, dissolved inorganic carbon (DIC), delta C-13 of DIC, Omega(calcite), [Mn], [Ca], and [Sr], as well as solid phase porosity, % CaCO3, and % organic C. These profiles provide evidence that deep-sea sedimentary carbonate dissolution occurs via sediment-side control, wherein dissolution is dominated by sedimentary processes rather than strictly bottom water saturation state. We estimate dissolution fluxes using three independent approaches: alkalinity fluxes, delta C-13 of DIC combined with DIC fluxes, and [Ca] fluxes. We report seafloor dissolution fluxes with uncertainties < 38 %: 40 +/- 15, 98 +/- 20, 100 +/- 32, and 89 +/- 27 mu mol CaCO3/m(2)/day at sites 3200, 2900, 2700, and 1600 m deep, respectively. The magnitude of dissolution fluxes is a function of bottom water saturation state (Omega(calcite)), bottom water dissolved oxygen, and sedimentary CaCO3 content, but not correlated with any of these parameters independently. We observe dissolution occurring at all stations, including where bottom water is saturated with respect to calcite, and present evidence that this occurs through respiration-driven dissolution within the sediment. At all sites, porewater Omega(calcite) decreases below bottom water values before increasing toward saturation deeper in the sediment. Using the delta C-13 of DIC, we partition the DIC fluxes across the sediment-water interface and find 21-48 % of DIC is sourced from CaCO3 dissolution, with the remainder sourced from organic matter respiration. We present a sedimentary mass balance, assembled with dissolution rates and mass accumulation rates obtained through Delta C-14 of foraminiferal calcite, and calculate CaCO3 burial efficiencies between 2 and 67 %, inversely correlating with water depth. Our results also provide evidence that net chemical erosion of 5,000--10,000 year old carbonate is occurring at the deepest site. Aerobic organic C respiration coupled with sedimentary CaCO3 dissolution, as documented here, will provide more alkalinity to bottom waters than from undersaturation-driven dissolution alone. This process can neutralize anthropogenic CO2 at the seafloor in a larger range of saturation states than previously estimated.

AbstractThe ocean carbon reservoir controls atmospheric carbon dioxide (CO2) on millennial timescales. Radiocarbon (14C) anomalies in eastern North Pacific sediments suggest a significant release of geologic 14C‐free carbon at the end of the last ice age but without evidence of ocean acidification. Using inverse carbon cycle modeling optimized with reconstructed atmospheric CO2 and 14C/C, we develop first‐order constraints on geologic carbon and alkalinity release over the last 17.5 thousand years. We construct scenarios allowing the release of 850–2,400 Pg C, with a maximum release rate of 1.3 Pg C yr−1, all of which require an approximate equimolar alkalinity release. These neutralized carbon addition scenarios have minimal impacts on the simulated marine carbon cycle and atmospheric CO2, thereby demonstrating safe and effective ocean carbon storage. This deglacial phenomenon could serve as a natural analog to the successful implementation of gigaton‐scale ocean alkalinity enhancement, a promising marine carbon dioxide removal method.

Ice sheets may influence the global carbon cycle by releasing chemical weathering products and carbon from basal environments. However, limited data describing subglacial biogeochemical cycles beneath Antarctic and Greenland ice leaves open fundamental questions regarding the feedbacks between climate, ice sheets, and the carbon cycle. Most notably: does subglacial chemical weathering beneath ice sheets act as a source or sink of atmospheric CO2? Here, we present constraints on biogeochemical reactions beneath the Antarctic ice sheet using geochemical and geochronologic measurements of carbonate precipitates, which formed from basal waters in ten locations along the ice sheet margins and Transantarctic Mountains (TAM), since the late Miocene. Precipitate carbon and oxygen isotopic compositions reveal a consistent geographic pattern in subglacial carbon cycling and chemical weathering, where parent waters in catchments draining through the TAM source CO2 through microbial metabolism of organic matter that drives silicate weathering, while waters beneath the ice sheet periphery acquire carbon mainly through the dissolution of carbonate minerals. Proxies for parent water pH (P/Ca) and solute source (87Sr/86Sr) show that bedrock composition and reactivity are the main factors determining intensity of silicate weathering in TAM waters. While the dominance of subglacial silicate versus carbonate weathering is determined by the presence of carbonate minerals and/or basal water residence time, which can fluctuate across global climate cycles. Because carbonate and silicate dissolution have quantitively different effects on the carbon cycle, the balance between them dictates whether Antarctica acts as a positive or negative climate-carbon cycle feedback.

The tropical Pacific is one of the largest ocean regions on Earth where the trace element iron limits new primary production and therefore the efficiency of carbon export to the deep sea. Although there is a long history of marine biogeochemical research in the tropical Pacific, recent advancements using GEOTRACES key parameters such as iron and nitrate isotopes (nitrate δ15N and δ18O) make this a good time to review the current understanding of tropical Pacific nitrate dynamics—how both regional subsurface nitrate characteristics and surface ocean nitrate utilization change with time. While this article provides a comprehensive overview of the biological, chemical, and physical processes shaping equatorial Pacific subsurface-to-surface nutrients, it principally explores the findings from the first nitrate isotope time series in iron-limited high nutrient, low chlorophyll waters. Results indicate that the preferential recycling of bioavailable iron within the euphotic zone is required to explain even the lowest observed nitrate utilization in the eastern equatorial Pacific (EEP). Furthermore, because seasonal-to-interannual nitrate utilization variability in the EEP cannot be driven by changes in iron supply, this work argues that iron recycling (and therefore bioavailable iron) is modulated by upwelling rate changes, creating a predicted and recently observed spectrum of iron limitation in the iron-limited EEP surface waters. In other words, upper ocean physics overwhelmingly dominates seasonal-to-interannual nitrate utilization in the iron-​limited EEP. This new understanding of nitrate utilization in iron-limited waters helps to explain long-term changes in past equatorial Pacific nitrate utilization obtained via sedimentary proxy records and potentially complicates the efficacy of future iron fertilization of the equatorial Pacific.

Abstract In this work, we utilize a transect of core top, mid- to late Holocene, sediments from the Eastern Siberian Sea to the central Arctic Ocean, spanning gradients in upper-ocean water column properties, to examine regional planktic foraminiferal species abundances and geochemistry. We present species- and morphotype-specific foraminiferal assemblages at these sites and stable isotope analyses of neogloboquadrinids. We find little variation in planktic species populations, and only small variations in N. pachyderma morphotype distributions, between sites. Spatial averages of N. pachyderma morphotype and N. incompta δ18O values show no significant differences, suggesting a similar calcification depth for all morphotypes of N. pachyderma and N. incompta across our sites, which we estimate to be between ∼ 50–150 m. Values of δ18O of a group of unencrusted specimens delineate a shallower calcification habitat. Neogloboquadrina pachyderma-2 Mg/Ca values yield temperatures outside the range of observations using available calibration equations, pointing toward the need for more Arctic-specific Mg/Ca-temperature calibrations.

We present the mass and carbonate annual fluxes, collected by two sediment traps at 1000 m depth, located in the western and southern deep-water region of the Gulf of Mexico, and the total mass and carbonate accumulation rates from 48 sediment cores retrieved from continental slopes and the abyssal plain of the southern Gulf of Mexico (sGM). We also presented the conventional and calibrated radiocarbon age analyzed in planktic foraminifera (> 250 μm size fraction) in sediment cores from the southern Gulf of Mexico, collected in the XIXIMI-7 cruise (May 2019).

In the intermediate depth Eastern Tropical Pacific Ocean, multiple deglacial radiocarbon (14C) records show anomalously low 14C/C values that appear to be best explained by the addition of 14C-free geologic carbon. We use inverse carbon cycle modeling and data assimilation of reconstructed atmospheric CO2 and ∆14C to develop an upper bound constraint on this speculated deglacial geologic carbon release. Our analysis suggests two primary opportunities where large bicarbonate pulses (up to 1.3 PgC yr-1) could occur with little effect on atmospheric CO2 and without upsetting 14C mass-balance constraints. Including the release of 14C-free permafrost carbon and regrowth of the terrestrial biosphere, we obtain a set of permissible scenarios for ocean geologic carbon release that ranges from 900-2400 PgC. Based on these results, we conclude that geologic carbon release is a plausible interpretation for the spatio-temporal cluster of anomalous 14C data near the East Pacific Rise.

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (delta N-15, delta O-18, delta(15-18) = delta N-15-delta O-18) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate delta N-15 is lower (4.9-5.8 parts per thousand) than the sub-thermocline source, Subantarctic Mode Water (6.9 parts per thousand); we attribute this difference to local N-2 fixation. Using a one-box model to simulate the newly fixed nitrate flux, we estimate a local N-2 fixation rate of 7-25 Tg N.a(-1), with the upper limit likely biased high. In the mixed layer, nitrate delta N-15 and delta O-18 rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N-2 fixation can be used to approximate carbon export. Thermocline and mixed-layer nitrate delta(15-18) is low, due to both N-2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low-delta(15-18) nitrate in Agulhas rings indicates leakage of low-delta N-15 nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Gas exchange between the atmosphere and ocean interior profoundly impacts global climate and biogeochemistry. However, our understanding of the relevant physical processes remains limited by a scarcity of direct observations. Dissolved noble gases in the deep ocean are powerful tracers of physical air-sea interaction due to their chemical and biological inertness, yet their isotope ratios have remained underexplored. Here, we present high-precision noble gas isotope and elemental ratios from the deep North Atlantic (~32°N, 64°W) to evaluate gas exchange parameterizations using an ocean circulation model. The unprecedented precision of these data reveal deep-ocean undersaturation of heavy noble gases and isotopes resulting from cooling-driven air-to-sea gas transport associated with deep convection in the northern high latitudes. Our data also imply an underappreciated and large role for bubble-mediated gas exchange in the global air-sea transfer of sparingly soluble gases, including O2, N2, and SF6. Using noble gases to validate the physical representation of air-sea gas exchange in a model also provides a unique opportunity to distinguish physical from biogeochemical signals. As a case study, we compare dissolved N2/Ar measurements in the deep North Atlantic to physics-only model predictions, revealing excess N2 from benthic denitrification in older deep waters (below 2.9 km). These data indicate that the rate of fixed N removal in the deep Northeastern Atlantic is at least three times higher than the global deep-ocean mean, suggesting tight coupling with organic carbon export and raising potential future implications for the marine N cycle.

In eastern boundary current systems, strong coastal upwelling brings deep, nutrient‐rich waters to the surface ocean, supporting a productive food web. The nitrate load in water masses that supply the region can be impacted by a variety of climate‐related processes that subsequently modulate primary productivity. In this study, two coastal upwelling regimes along central and southern California were sampled seasonally for nitrogen and oxygen stable isotopes of nitrate (i.e., nitrate isotopes) over several years (2010–2016) on 14 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises. Seasonal, interannual, and spatial variations in euphotic zone nitrate isotopes were largely driven by the extent of nitrate utilization, sometimes linked to iron limitation of diatom productivity. Pronounced isotopic enrichment developed with the El Niño conditions in late 2015 and early 2016 which likely resulted from increased nitrate utilization linked to reduced nitrate supply to the euphotic zone. Differential enrichment of nitrogen and oxygen isotopes was observed in the surface ocean, suggesting that phytoplankton increased their reliance on locally nitrified (recycled) nitrate during warmer and more stratified periods. Overall, nitrate isotopes effectively differentiated important euphotic zone processes such as nitrate assimilation and nitrification, while archiving the influence of disparate controls such as iron limitation and climatic events through their effects on nitrate utilization and isotopic fractionation.

Using new and published marine fossil radiocarbon (14C/C) measurements, a tracer uniquely sensitive to circulation and air-sea gas exchange, we establish several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. We find the most 14C-depleted water in glacial Pacific bottom depths, rather than the mid-depths as they are today, which is best explained by a slowdown in glacial deep-sea overturning in addition to a "flipped" glacial Pacific overturning configuration. These observations cannot be produced by changes in air-sea gas exchange alone, and they underscore the major role for changes in the overturning circulation for glacial deep-sea carbon storage in the vast Pacific abyss and the concomitant drawdown of atmospheric CO2.

Processes underlying changes in the oceanic carbon storage during the Last Glacial Maximum and the subsequent deglaciation are not fully understood. Here, we present a new high‐resolution radiocarbon reconstruction (expressed as δ14R) at the depth of the modern Lower Circumpolar Deep Water from the Pacific Sector of the Southern Ocean. Our record shows δ14R increases during Heinrich Stadial 1 and the Younger Dryas that agree with the deep‐to‐shallow transfer of old carbon in the Southern Ocean during these two periods. Our record also shows, for the first time, a clear ∼80‰ decline in δ14R during the Antarctic Cold Reversal (ACR), indicating the development of poorly ventilated conditions in the deep Southwest Pacific. These conditions are consistent with the increased Southern Ocean sea‐ice and associated stratification between Upper and Lower Circumpolar Deep Waters. This enhanced stratification in the deep South Pacific possibly facilitated greater carbon storage in the ocean interior during the ACR, effectively limiting oceanic CO2 release and contributing to the atmospheric CO2 plateau as observed in ice cores at that time.