Response of Atmospheric pCO₂ to Glacial Changes in the Southern Ocean Amplified by Carbonate Compensation

Abstract

Atmospheric carbon dioxide concentration (pCO2) varies by about 100ppm during glacial‐interglacial cycles. Previous studies suggest that the strongly stratified Southern Ocean at the Last Glacial Maximum increases the oceanic storage of carbon, but the glacial reduction of atmospheric pCO2 simulated by ocean general circulation models (OGCMs) does not reach 100ppm. One candidate for the underestimation is that carbonate compensation is not explicitly incorporated in the previous OGCM simulations. Therefore, we quantitatively evaluate the impact of carbonate compensation on the glacial atmospheric pCO2 by using an OGCM coupled with an ocean sediment model. As suggested by previous box model studies, our OGCM simulations show that the enhanced Southern Ocean stratification amplifies the decrease in atmospheric pCO2 due to carbonate compensation. Considering the enhanced stratification in the Southern Ocean, we obtain a 26‐ppm drawdown of atmospheric pCO2 by carbonate compensation, and the full reduction from our pre‐industrial simulation reaches 73ppm. Both the increase in ventilation ages in the deep Atlantic and Southern Oceans and the growth of export production in the subantarctic region reduce the bottom‐water carbonate ion and promote deposited carbonate dissolution. Consequently, a greater imbalance between the river inflow and burial loss of carbonate rises ocean alkalinity, lowering atmospheric pCO2. We suggest that the reproducibility of the Southern Ocean process is essential for controlling the magnitude of atmospheric pCO2 decline due to carbonate compensation.

Development of the MIROC-ES2L Earth system model and the evaluation of biogeochemical processes and feedbacks

“…Detritus contains nitrate, phosphorus, iron, and carbon, most of which is remineralized while sinking downward. The detritus that reaches the ocean floor is removed from the system; however, a fraction of OM in the sediment is assumed to return to the bottom layer of the water column at a constant rate in each location (Kobayashi and Oka, 2018).…”

Deep‐Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age

“…Dissolution of CaCO 3 raised the ocean's alkalinity (8.1% increase in salinity-normalized [S N ] TALK, Table 4), thereby contributing to the ice age drawdown of atmospheric CO 2 (Archer et al, 2000;Archer & Maier-Reimer, 1994;Sigman & Boyle, 2000). The additional CO 2 drawdown attributable to this carbonate compensation effect has been calculated to fall in the range of 15-30% depending on ocean circulation and other boundary conditions Kobayashi & Oka, 2018;Omta et al, 2018). Furthermore, the greater S N TALK during the LGP should be taken into account when past changes in the partial pressure of CO 2 and other carbon system parameters are calculated by combining information from boron isotope proxies for pH with an assumed value of S N TALK (e.g., Honisch et al, 2009;Martinez-Boti et al, 2015;Palmer & Pearson, 2003;Rae et al, 2018).…”

Abrupt Bølling‐Allerød Warming Simulated under Gradual Forcing of the Last Deglaciation

“…The coefficient of the horizontal isopycnal layer thickness diffusivity of the ocean model was changed to 7.0 × 10 6 cm 2 /s from 3.0 × 10 6 cm 2 /s, and the present model produces weak AMOC under the LGM because of enhanced dense Antarctic Bottom Water (AABW) formation (Kawamura et al, 2017). The MIROC has been used to investigate the climate of the LGM with radiative forcing and climate feedback (Yoshimori et al, 2009), the effect of ice sheets on the climate and the AMOC Kawamura et al, 2017;Sherriff-Tadano et al, 2018), ocean biogeochemical cycles (Kobayashi et al, 2015;Kobayashi & Oka, 2018;Yamamoto et al, 2019), and mass balance of the Antarctic ice shelves (Kusahara et al, 2015;Obase et al, 2017).…”