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1. Multiple carbon cycle mechanisms associated with the glaciation of Marine Isotope Stage 4

   https://doi.org/10.1038/s41467-022-33166-3  

   Menking, James A. ; Shackleton, Sarah A. ; Bauska, Thomas K. ; Buffen, Aron M. ; Brook, Edward J. ; Barker, Stephen ; Severinghaus, Jeffrey P. ; Dyonisius, Michael N. ; Petrenko, Vasilii V. ( September 2022 , Nature Communications)

   Here we use high-precision carbon isotope data (δ¹³C-CO₂) to show atmospheric CO₂during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ¹³C-CO₂during peak glaciation suggests increased ocean carbon storage. Variations in δ¹³C-CO₂in early MIS 4 suggest multiple processes were active during CO₂drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO₂remained low during MIS 4 while δ¹³C-CO₂fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO₂at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO₂during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.

    

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2. Evaluating the Glacial‐Deglacial Carbon Respiration and Ventilation Change Hypothesis as a Mechanism for Changing Atmospheric CO 2

   https://doi.org/10.1029/2020GL091296  

   Stott, Lowell D. ; Shao, Jun ; Yu, Jimin ; Harazin, Kathleen M. ( February 2021 , Geophysical Research Letters)

   The prevailing hypothesis to explain pCO₂rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O₂] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O₂] depletion was 100–140 µ mol/kg at the glacial maximum. Because respiration is coupled toδ¹³C of dissolved inorganic carbon (DIC), [O₂] depletion of 100–140 µ mol/kg from carbon respiration would lower deep waterδ¹³C_DICby ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of¹⁴C in the deep sea. We show that Pacific Deep Waterδ¹³C_DICdid not decrease relative to the surface ocean and Δ¹⁴C was only ∼50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases inδ¹³C_DIC, Δ¹⁴C, andε¹⁴C that are not recorded in observations.

    

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3. Carbon and carbon-13 in the preindustrial and glacial ocean

   https://doi.org/10.1371/journal.pclm.0000434  

   Schmittner, Andreas ; Fillman, Nathaniel J ( July 2024 , PLOS Climate)

   Carré, Matthieu (Ed.)

   Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope¹³C in the ocean are not well understood. Models typically do not decompose C and¹³C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ¹³C_DIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ¹³C_DIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ¹³C_DICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ¹³C_DIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ¹³C_DIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ¹³C_DIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ¹³C_DICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO₂. Spatial distributions of the δ¹³C_DICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors.

    

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4. Origin and Accumulation of an Anthropogenic CO 2 and 13 C Suess Effect in the Arctic Ocean

   https://doi.org/10.1029/2019GB006423  

   Ko, Young Ho ; Quay, Paul D. ( February 2020 , Global Biogeochemical Cycles)

   We determined the impact of anthropogenic CO₂(C_ant) accumulation on the δ¹³C of dissolved inorganic carbon in the Arctic Ocean (i.e., the¹³C Suess effect) based on δ¹³C measurements during a GEOTRACES cruise in 2015. The δ¹³C decrease was estimated from the amount of C_antchange derived by the transit time distribution approach and the ratio of the anthropogenic δ¹³C/dissolved inorganic carbon change (RC). A significant C_antincrease (up to 45 μmol kg⁻¹) and δ¹³C decrease (up to −0.9‰) extends to ~2,000 m in the Canada and Makarov Basin. We find distinctly different RC values for the intermediate water (300–2,000 m) and upper halocline water (<200 m) of −0.020 and −0.012‰ (μmol kg⁻¹)⁻¹, respectively, which identifies two sources of C_antaccumulation from North Atlantic and North Pacific. Furthermore, estimated RC for intermediate waters is the same as the RC observed in the Greenland Sea and the rate of anthropogenic dissolved inorganic carbon increase estimated for intermediate waters at 0.9 μmol kg⁻¹yr⁻¹is identical to the estimated rate in the Iceland Sea. These observations indicate that the high rate of C_antaccumulation and δ¹³C decrease in the Arctic Ocean is primarily a result of the input of C_ant, via ventilation of intermediate waters, from the Nordic Sea rather than local anthropogenic CO₂uptake within the Arctic Basin. We determine the preindustrial δ¹³C (δ¹³C_PI) distributions and find distinct δ¹³C_PIsignatures of the intermediate and upper halocline waters that reflect the difference in δ¹³C_PI–PO₄relationship of Atlantic and Pacific source water.

    

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5. 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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