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1. The Subtropical Ocean's Biological Carbon Pump Determined From O 2 and DIC/DI 13 C Tracers

   https://doi.org/10.1029/2018GL081239  

   Yang, Bo ; Emerson, Steven R. ; Quay, Paul D. ( May 2019 , Geophysical Research Letters)

   The ocean's biological organic carbon pump regulates thepCO₂of the atmosphere and helps maintain the oxygen distributions in the ocean. Global models of this flux are poorly verified with observations. We used upper‐ocean budgets of O₂and the¹³C/¹²C of dissolved inorganic carbon (DIC) to estimate the biological pump in the subtropical gyres. These two tracers yield, within errors, similar result (~2.0 mol C·m⁻²·year⁻¹) at three Northern Hemisphere subtropical locations. Values for three Southern Hemisphere subtropical regions are lower and more variable determined by the O₂mass balance than by the DI¹³C method (−0.5 to 0.8 mol C·m⁻²·year⁻¹and 0.9 to 1.3 mol C·m⁻²·year⁻¹, respectively). Both methods suggest that the subtropical ocean is, on the whole, autotropic. The gas exchange residence times of O₂and dissolved inorganic carbon result in different spatial and temporal averaging creating complementary tracers for biological pump model verification.

    

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2. Impact of the Elemental Composition of Exported Organic Matter on the Observed Dissolved Nutrient and Trace Element Distributions in the Upper Layer of the Ocean

   https://doi.org/10.1029/2020GB006902  

   Quay, Paul ( October 2021 , Global Biogeochemical Cycles)

   Systematic regional variations in the ratio of nutrient depth gradients of dissolved inorganic carbon (ΔDIC):nitrate (ΔNO₃):phosphate (ΔPO₄) in the upper layer (300 m) of the Pacific Ocean are observed. Regional variations in the ΔDIC/ΔNO₃/ΔPO₄are primarily the result of three processes, that is, the C/N/P of organic matter (OM) being exported and subsequently degraded, nitrogen fixation, and air‐sea CO₂gas exchange. The link between the observed dissolved ΔDIC/ΔNO₃/ΔPO₄and the C/N/P of exported OM is established using surface layer dissolved DIC, NO₃, and PO₄budgets. These budgets, in turn, provide a means to indirectly estimate the C/N/P of OM being exported from the surface layer of the ocean. The indirectly estimated C/N/P of exported OM reach maxima in the subtropical gyres at 177/22/1, that is, significantly greater than the Redfield ratio and a minimum in the equatorial ocean at 109/16/1 with both results agreeing with available observed particle C/N/P and ocean biogeochemical models. The budget approach was applied to a bioactive trace element (TE) using the measured dissolved Cadmium (Cd) to PO₄gradients to estimate the Cd/P of exported OM in the Pacific Ocean. Combining the budget method with the availability of high‐quality dissolved nutrient and TE data collected during the GOSHIP and GEOTRACES programs could potentially provide estimates of the C/N/P/TE of exported OM on global ocean scales which would significantly improve our understanding of the link between the ocean's biological pump and dissolved nutrient distributions in the upper ocean.

    

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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)

   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. Important Contribution of Bacterial Carbon and Nitrogen to Sinking Particle Export

   https://doi.org/10.1029/2022GL102485  

   Shen, Yuan ; Guilderson, Thomas P. ; Chavez, Francisco P. ; McCarthy, Matthew D. ( June 2023 , Geophysical Research Letters)

   Photosynthesis in the surface ocean converts atmospheric CO₂into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Majord‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms.

    

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5. Carbon Export Buffering and CO 2 Drawdown by Flexible Phytoplankton C:N:P Under Glacial Conditions

   https://doi.org/10.1029/2019PA003823  

   Matsumoto, Katsumi ; Rickaby, Rosalind ; Tanioka, Tatsuro ( July 2020 , Paleoceanography and Paleoclimatology)

   Modern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO₂. These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO₂over glacial‐interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.

    

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