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Ocean deoxygenation is an important consequence of climate change that poses an imminent threat to marine life and global food security. However, our understanding of the complex interactions between changes in circulation, solubility, and respiration that drive global‐scale deoxygenation is incomplete. Here, we consider idealized biogeochemical steady states in equilibrium with perpetually slower and warmer oceans constructed from climate‐model simulations of the 2090s that we hold constant in time. In contrast to simulations of the end‐of‐century transient state, our idealized states are intensely deoxygenated in the abyss, consistent with perpetually reduced ventilation and throttled Antarctic Bottom Water formation. We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper‐ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming‐driven declines in solubility account for less than 10% of the total oxygen loss. Although idealized, our analysis suggests that long‐term changes in the marine oxygen cycle could be driven dominantly by changes in circulation rather than by thermodynamics or biology. Plain Language Summary: Climate change is driving oxygen out of the ocean, threatening marine life and global food security. However, the precise contributions of the chemical, physical, and biological processes that control oxygen levels are not well known because of their complex interactions. To better understand these interactions, we consider idealized simulations of a global oxygen cycle that is fully equilibrated with a perpetually warmer and slower ocean constructed from climate‐model simulations of the 2090s but held constant in time for our analyses. Compared to typical predictions, these idealized states exhibit intense deep‐ocean deoxygenation, for which we precisely quantify the contributions from changes in solubility, respiration, and ocean circulation. We find that deep‐ocean deoxygenation is driven by the slower circulation allowing respiration to act for 2–3 times longer thereby overcoming lower respiration rates. The surface origin of oxygen shifts away from cold high‐latitude waters toward warmer waters, in which atmospheric oxygen is less soluble, further reducing oxygen levels. Warming‐driven decreases in solubility alone only account for a mere 10% of the total oxygen loss. The upper ocean remains well oxygenated because changes in respiration and circulation compensate almost perfectly. Our results highlight the central importance of circulation in controlling oxygen in the ocean. Key Points: Key drivers of deoxygenation are quantified for oxygen cycles idealized by being in equilibrium with perpetually slower oceansWidespread intense abyssal ocean deoxygenation is driven predominantly by slower circulations allowing respiration to act over longer timesMost of the reduction in preformed oxygen is driven by changes in ventilation patterns and not by warming‐driven reduced solubility [ABSTRACT FROM AUTHOR]

Accurate predictive modeling of the ocean's global carbon and oxygen cycles is challenging because of uncertainties in both biogeochemistry and ocean circulation. Advances over the last decade have made parameter optimization feasible, allowing models to better match observed biogeochemical fields. However, does fitting a biogeochemical model to observed tracers using a circulation with known biases robustly capture the inner workings of the biological pump? Here we embed a mechanistic model of the ocean's coupled nutrient, carbon, and oxygen cycles into two circulations for the current climate. To assess the effects of biases, one circulation (ACCESS-M) is derived from a climate model and the other from data assimilation of observations (OCIM2). We find that parameter optimization compensates for circulation biases at the expense of altering how the biological pump operates. Tracer observations constrain pump strength and regenerated inventories for both circulations, but ACCESS-M export production optimizes to twice that of OCIM2 to compensate for ACCESS-M having lower sequestration efficiencies driven by less efficient particle transfer and shorter residence times. Idealized simulations forcing complete Southern Ocean nutrient utilization show that the response of the optimized system is sensitive to the embedding circulation. In ACCESS-M, Southern Ocean nutrient and dissolved inorganic carbon (DIC) trapping is partially short circuited by unrealistically deep mixed layers. For both circulations, intense Southern Ocean production deoxygenates Southern-Ocean-sourced deep waters, muting the imprint of circulation biases on oxygen. Our findings highlight that the biological pump's plumbing needs careful assessment to predict the biogeochemical response to ecological changes, even when optimally matching observations. [ABSTRACT FROM AUTHOR]

Southern Ocean acidification via anthropogenic C[O.sub.2] uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate ion (C[O.sup.2-.sub.3]) to levels where calcium carbonate (both aragonite and calcite) shells begin to dissolve. Natural seasonal variations in carbonate ion concentrations could either hasten or dampen the future onset of this undersaturation of calcium carbonate. We present a large-scale Southern Ocean observational analysis that examines the seasonal magnitude and variability of C[O.sup.2-.sub.3] and pH. Our analysis shows an intense wintertime minimum in C[O.sup.2-.sub.3] south of the Antarctic Polar Front and when combined with anthropogenic C[O.sub.2] uptake is likely to induce aragonite undersaturation when atmospheric C[O.sub.2] levels reach [approximately equal to] 450 ppm. Under the IPCC IS92a scenario, Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038. Some prominent calcifying plankton, in particular the Pteropod species Limacina helicina, have important veliger larval development during winter and will have to experience detrimental carbonate conditions much earlier than previously thought, with possible deleterious flow-on impacts for the wider Southern Ocean marine ecosystem. Our results highlight the critical importance of understanding seasonal carbon dynamics within all calcifying marine ecosystems such as continental shelves and coral reefs, because natural variability may potentially hasten the onset of future ocean acidification. carbon cycle | climate change

We estimated the oceanic inventory of anthropogenic carbon dioxide (C[O.sub.2]) from 1980 to 1999 using a technique based on the global chlorofluorocarbon data set. Our analysis suggests that the ocean stored 14.8 petagrams of anthropogenic carbon from mid-1980 to mid-1989 and 17.9 petagrams of carbon from mid-1990 to mid-1999, indicating an oceanwide net uptake of 1.6 and 2.0 ± 0.4 petagrams of carbon per year, respectively. Our results provide an upper limit on the solubility-driven anthropogenic C[O.sub.2] flux into the ocean, and they suggest that most ocean genera[ circulation models are overestimating oceanic anthropogenic C[O.sub.2) uptake over the past two decades., Despite improvements in our understanding of the partitioning of anthropogenic C[O.sub.2] between the atmosphere, ocean, and terrestrial biosphere, substantial uncertainties and insufficient direct observational constraints continue. Recent decadal-scale changes in [...]

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal climate variability. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air‐sea CO2 fluxes as the biological carbon pump removes carbon from the surface ocean. We aim to understand how the physical drivers of sea surface temperature and wind speed influence interannual and decadal variability of the equatorial Pacific carbon cycle. In the equatorial Pacific, there are three biogeochemical regimes: the upwelling cold tongue east of 140°W and south of the equator (3°N–15°S); the eastern Pacific warm pool north of the equator (3°–15°N); and the 28.5°C western Pacific warm pool, west of 140°W. We find that between 2000 and 2020, air‐sea CO2 flux and ΔpCO2 increased in the cold tongue (45 mmolC m−2 yr−2, 1.5 μatm yr−1, respectively) but decreased elsewhere, while new production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO and this sink was strongest at 165°E during central Pacific "Modoki" El Niño events. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget. The mean basin‐wide (150°E−90°W and 15°N–15°S) new production for 2000–2020 was 1.2 ± 0.1 PgC yr−1 and air‐sea CO2 flux was 0.5 ± 0.1 PgC yr−1. New production decreased at −7.7 ± 1.6 TgC yr−2, compared to the CO2 flux trend of −1.7 ± 1.4 TgC yr−2. Plain Language Summary: The equatorial Pacific is the largest source of carbon dioxide that outgasses from the ocean into the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal changes in the climate. Winds in the eastern Pacific drive upwelling which supplies nutrients and carbon to the surface. Some of this carbon is outgassed to the atmosphere, some is consumed by phytoplankton and can either move through the food web or sink into the ocean interior. We are interested in how much carbon is removed from the surface equatorial Pacific through outgassing of carbon and biological consumption. We find that changes in wind speeds, surface temperatures, freshening surface water and changing patterns of ENSO have influenced the equatorial Pacific carbon budget. For example, between 2000 and 2020, CO2 release to the atmosphere increased in the upwelling zone but decreased elsewhere, while biological consumption decreased everywhere. The western Pacific occasionally absorbs carbon from the atmosphere during central Pacific "Modoki" El Niño events, a particular type of El Niño that is becoming more frequent. Key Points: Decadal shifts in upwelling have increased CO2 flux in the cold tongue but not biological production, which is likely nutrient limitedIncreasing frequency of central Pacific El Niño events may amplify decadal CO2 flux trends and drive sink conditions in the westIncreasing surface temperatures contribute to stratification and decreasing new production despite increasing dissolved inorganic carbon [ABSTRACT FROM AUTHOR]