Quantifying the ocean's role in glacial CO 2 reductions

• Published in: Climate of the Past
• Main Authors: R. Ohgaito, A. Oka, A. Abe-Ouchi, M. O. Chikamoto, A. Timmermann
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2012
• Subjects: Environmental pollution; TD172-193.5; Environmental protection; TD169-171.8; Environmental sciences; GE1-350; Antarctic; Southern Ocean; Antarc*; NADW; North Atlantic Deep Water; North Atlantic; Sea ice
• Online Access: https://doi.org/10.5194/cp-8-545-2012; https://doaj.org/article/dbb5925406164e148d81ba64bb29d3cb

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 p CO 2 . 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 p CO 2 changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m −3 a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (δ 13 C) decreases by 0.4‰ 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 p CO 2 . 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 p CO 2 oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the air-sea gas exchange of CO 2 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 p CO 2 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 2 and the difficulty of ...