13 Nov 2023
 | 13 Nov 2023
Status: this preprint is open for discussion and under review for Climate of the Past (CP).

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

Hidetaka Kobayashi, Akira Oka, Takashi Obase, and Ayako Abe-Ouchi

Abstract. 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 pCO2 changes than ice core data.

Our decomposition analysis, which estimates the quantitative contribution to the oceanic pCO2, suggests that changes in alkalinity have played a central role in driving variations and trends in atmospheric pCO2 as the deep ocean circulation changes. This finding may provide valuable insights into the model-dependent response of the ocean carbon cycle to changes in the AMOC, as several previous studies have emphasized the importance of the AMOC in influencing changes in atmospheric pCO2, but the magnitude and direction of these changes have varied widely between studies.

Hidetaka Kobayashi et al.

Status: open (until 08 Jan 2024)

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Hidetaka Kobayashi et al.

Model code and software

Data for journal articles Takashi Obase and Ayako Abe-Ouchi

Hidetaka Kobayashi et al.


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Short summary
The transient response of the ocean carbon cycle during the last deglaciation was investigated. During the early deglaciation, ocean temperature rise increased atmospheric pCO2, whereas a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) decreased atmospheric pCO2. We emphasize the critical influence of ocean alkalinity on pCO2 trends. Our carbon isotope modeling confirms the AMOC response and emphasizes the need to simulate prompt ventilation activation.