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1. Multi‐Century Changes in the Ocean Carbon Cycle Controlled by the Tropical Oceans and the Southern Ocean

   https://doi.org/10.1029/2021GB007090  

   Chikamoto, Megumi O.; DiNezio, Pedro (November 2021, Global Biogeochemical Cycles)

   Abstract The oceanic absorption of anthropogenic carbon dioxide (CO₂) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO₂peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi‐century changes in uptake are dominated by changes in sea‐air CO₂fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon‐rich thermocline waters, suppressing natural outgassing of CO₂. In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO₂in the next centuries. The slowdown in CO₂uptake in the subsequent centuries is caused by the decrease in CO₂solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO₂. In sum, predicting multi‐century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC. 

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2. The Deep Ocean's Carbon Exhaust

   https://doi.org/10.1029/2021GB007156  

   Chen, Haidi; Haumann, F. Alexander; Talley, Lynne D.; Johnson, Kenneth S.; Sarmiento, Jorge L. (July 2022, Global Biogeochemical Cycles)

   Abstract The deep ocean releases large amounts of old, pre‐industrial carbon dioxide (CO₂) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO₂release is relevant to the global climate because its changes could alter atmospheric CO₂levels on long time scales, and also affects the present‐day potential of the Southern Ocean to take up anthropogenic CO₂. Here, year‐round profiling float measurements show that this CO₂release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea‐ice edge. This band of high CO₂subsurface water coincides with the outcropping of the 27.8 kg m⁻³isoneutral density surface that characterizes Indo‐Pacific Deep Water (IPDW). It has a potential partial pressure of CO₂exceeding current atmospheric CO₂levels (∆PCO₂) by 175 ± 32 μatm. Ship‐based measurements reveal that IPDW exhibits a distinct ∆PCO₂maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO₂decreases. Most of this vertical ∆PCO₂decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO₂outgassing from the high‐carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO₂fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles. 

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3. Ocean Oxygen, Preformed Nutrients, and the Cause of the Lower Carbon Dioxide Concentration in the Atmosphere of the Last Glacial Maximum

   https://doi.org/10.1029/2023PA004775  

   Sigman, Daniel M.; Hain, Mathis P. (January 2024, Paleoceanography and Paleoclimatology)

   Abstract All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O₂) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO₂) concentrations of the ice ages. Vollmer et al. (2022,https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed¹³C/¹²C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO₂storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange. 

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4. Proxy‐Based Preformed Phosphate Estimates Point to Increased Biological Pump Efficiency as Primary Cause of Last Glacial Maximum CO ₂ Drawdown

   https://doi.org/10.1029/2021PA004339  

   Vollmer, T. D.; Ito, T.; Lynch‐Stieglitz, J. (November 2022, Paleoceanography and Paleoclimatology)

   Abstract Upwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO₂levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO₂leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO₂can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO₄(P_pre,AOU) and preformed() of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO₂drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO₂drawdown (87 ± 40 ppmv). Preformedshows similar results. The reconstructed LGM P_pre,AOUand oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean. 

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5. Source‐Labeled Anthropogenic Carbon Reveals a Large Shift of Preindustrial Carbon From the Ocean to the Atmosphere

   https://doi.org/10.1029/2022GB007405  

   Holzer, Mark; DeVries, Tim (October 2022, Global Biogeochemical Cycles)

   Abstract Two centuries of anthropogenic CO₂emissions have increased the CO₂concentration of the atmosphere and the dissolved inorganic carbon (DIC) concentration of the ocean compared to preindustrial times. These anthropogenic carbon perturbations are often equated to the amount of anthropogenically emitted carbon in the atmosphere or ocean, which ignores the possibility of a shift of natural carbon between the oceanic and atmospheric carbon reservoirs. Here we use a data‐assimilated ocean circulation model and numerical tracers akin to ideal isotopes to label carbon when it is emitted by anthropogenic sources. We find that emitted carbon accounts for only about 45% of the atmospheric CO₂increase since preindustrial times, the remaining 55% being natural CO₂that outgassed from the ocean in response to anthropogenically emitted carbon invading the ocean. This outgassing is driven by the order‐10 seawater carbonate buffer factor which causes increased leakage of natural CO₂as DIC concentrations increase. By 2020, the ocean had outgassed ∼159 Pg of natural carbon, which is counteracted by the ocean absorbing ∼347 Pg of emitted carbon, about 1.8 times more than the net increase in oceanic carbon storage of ∼188 PgC. These results do not challenge existing estimates of anthropogenically driven changes in atmospheric or oceanic carbon inventories, but they shed new light on the composition of these changes and the fate of anthropogenically emitted carbon in the Earth system. 

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