Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

• Main Authors: Kobayashi, Hidetaka, Oka, Akira, Obase, Takashi, Abe-Ouchi, Ayako
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: article; Verlagsveröffentlichung; Pacific; Southern Ocean; ice core
• Online Access: https://doi.org/10.5194/egusphere-2023-2526; https://noa.gwlb.de/receive/cop_mods_00069832; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00068203/egusphere-2023-2526.pdf; https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2526/egusphere-2023-2526.pdf

We conducted a transient numerical experiment on the ocean carbon cycle during the last deglaciation. We used a three-dimensional ocean field from a transient climate model MIROC4m simulation of the last deglaciation as input to an ocean biogeochemical model, which allowed us to evaluate the effects of the gradual warming and the abrupt climate changes associated with the Atlantic Meridional Overturning Circulation during the last deglaciation. During Heinrich Stadial 1 (HS1), the atmospheric partial pressure of carbon dioxide (pCO2) increased as a result of rising sea surface temperature. Subsequently, during the Bølling–Allerød period, characterized by a rapid strengthening of the Atlantic Meridional Overturning Circulation (AMOC), atmospheric pCO2 showed a decreasing trend. Our decomposition analysis indicates that the declining atmospheric pCO2 in response to the enhanced AMOC during the BA period were primarily driven by an increase in ocean surface alkalinity, although this effect was partially offset by changes in sea surface temperature. Meantime, we found that our model generally underestimated atmospheric pCO2 changes compared to the ice core data. To understand this, we conducted an analysis of ocean circulation and water masses using radiocarbon and stable carbon isotope signatures (Δ14C and δ13C). We found that the overall changes in the deep water Δ14C in response to the AMOC change are quantitatively consistent with the sediment core data. However, the model underestimates the increased ventilation in the deep ocean and the reduced efficiency of biological carbon export in the Southern Ocean during mid-HS1 compared to estimates derived from sediment core data. In addition, the model underestimates the active ventilation in the North Pacific Intermediate Water during mid-HS1, as suggested by sediment core data. These underestimations in the activation of the deep ocean circulation and the limitation of biological productivity could be the primary reasons why our model exhibits smaller atmospheric ...