Glacial weakening of the AMOC and the associated increase in deep ocean carbon deposit

During the mid-Pleistocene between 1,250 and 700 kyr ago (ka), Earth’s climate oscillated between warmer interglacial periods and cooler glacial periods with reduced and expanded polar ice sheets, respectively. Paleo records indicate that the Atlantic meridional overturning circulation (AMOC)  was relatively weaker during glacial periods likely due to reduced evaporation and increased freshwater input from glaciers to the North Atlantic. In a paper published in Nature Geoscience, a team of scientists led by Lamont-Doherty Earth Observatory of Columbia University scientists analysed trace element and isotope data to show a close linkage between the AMOC and deep ocean carbon storage across the mid-Pleistocene transition (MPT; between 1950 and 900 ka). The study found that during the MPT the North Atlantic Deep Water contribution to the abyssal South Atlantic reduced by 20% and the glacial deep Atlantic carbon inventory increased by approximately 50 Gt. Consequently, the CO₂ release (or outgassing) to the atmosphere (mostly in the Southern Ocean upwelling regions) greatly reduced, and thereby enabled expanded terrestrial ice volume during the MPT. The study also showed that the Antarctic Bottom Water (AABW) formation was increased during the MPT due to ocean biogeochemical and sea-ice feedback processes, which also contributed to the increased deep ocean carbon deposit. 

Figure 4 from Farmer et al. (2019): a,c, Glacial AMOC cross-sections before (a) and after 950 ka (c). b,d, Antarctic processes within the dashed boxes in a and c, respectively. Curved arrows trace the pathways of AABW and NADW and show [PO₄³⁻]; the thickness of the arrows indicates the deep-water contribution (thicker arrows represent greater contributions). The red arrows indicate CO₂ released from the ocean and orange dots represent dust deposition. Black wavy arrows in b and d indicate brine rejection. Before 950 ka, a smaller East Antarctic Ice Sheet (EAIS) limits Antarctic sea-ice coverage, shortening the path for Antarctic surface waters before subduction. This results in a larger fraction of unutilized nutrients in AABW and greater CO₂ release. NADW dominates Atlantic MOC. After 950 ka, the EAIS expands to marine-based margins and sea-ice coverage increases. Antarctic surface waters upwell farther north, travelling further before subduction, decreasing [PO₄³⁻] in AABW and lowering CO₂ outgassing. AABW dominates deep Atlantic circulation, effectively trapping CO₂ (indicated by cross-hatching). RFZ = Romanche Fracture Zone.

Farmer, J. R., B. Hönisch, L. L. Haynes, D. Kroon, S. Jung, H. L. Ford, M. E. Raymo, M. Jaume-Seguí, D. B. Bell, S. L. Goldstein, L. D. Pena, M. Yehudai & J. Kim, 2019: Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles. Nature Geosci., https://doi.org/10.1038/s41561-019-0334-6. https://www.nature.com/articles/s41561-019-0334-6

https://phys.org/news/2019-04-carbon-lurking-deep-ocean-threw.html