CP Climate of the Past CP Clim. Past 1814-9332 Copernicus Publications Göttingen, Germany 10.5194/cp-8-545-2012 Quantifying the ocean's role in glacial CO<sub>2</sub> reductions Chikamoto M. O. 1 Abe-Ouchi A. 1 2 Oka A. 2 Ohgaito R. 1 Timmermann A. 3 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Kanagawa, Japan Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba, Japan International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, USA 16 03 2012 8 2 545 563 Copyright: © 2012 M. O. Chikamoto et al. 2012 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://cp.copernicus.org/articles/8/545/2012/cp-8-545-2012.html The full text article is available as a PDF file from https://cp.copernicus.org/articles/8/545/2012/cp-8-545-2012.pdf

A series of Last Glacial Maximum (LGM) marine carbon cycle sensitivity experiments is conducted to test the effect of different physical processes, as simulated by two atmosphere-ocean general circulation model (AOGCM) experiments, on atmospheric <i>p</i>CO<sub>2</sub>. One AOGCM solution exhibits an increase in North Atlantic Deep Water (NADW) formation under glacial conditions, whereas the other mimics an increase in Antarctic Bottom Water (AABW) associated with a weaker NADW. None of these sensitivity experiments reproduces the observed magnitude of glacial/interglacial <i>p</i>CO<sub>2</sub> changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m<sup>−3</sup> a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (&delta;<sup>13</sup>C) decreases by 0.4&permil; at intermediate depths in the South Atlantic in accordance with sedimentary evidence. The shift of deep and bottom water formation sites from the North Atlantic to the Southern Ocean increases the total preformed nutrient inventory, so that the lowered efficiency of Southern Ocean nutrient utilization in turn increases atmospheric <i>p</i>CO<sub>2</sub>. This change eventually offsets the effect of an increased abyssal carbon pool due to stronger AABW formation. The effects of interhemispheric glacial sea-ice changes on atmospheric <i>p</i>CO<sub>2</sub> oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the air-sea gas exchange of CO<sub>2</sub> in agreement with previous theoretical considerations, glacial advances of sea-ice in the Northern Hemisphere lead to a weakening of the oceanic carbon uptake through the physical pump. Due to enhanced gas solubility associated with lower sea surface temperature, both glacial experiments generate a reduction of atmospheric <i>p</i>CO<sub>2</sub> by about 20–23 ppmv. The sensitivity experiments presented here demonstrate the presence of compensating effects of different physical processes in the ocean on glacial CO<sub>2</sub> and the difficulty of finding a simple explanation of the glacial CO<sub>2</sub> problem by invoking ocean dynamical changes.

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