Early deglacial Atlantic overturning decline and its role in atmospheric CO 2 rise inferred from carbon isotopes (δ 13 C)

• Published in: Climate of the Past
• Main Authors: A. Schmittner, D. C. Lund
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2015
• Subjects: Environmental pollution; TD172-193.5; Environmental protection; TD169-171.8; Environmental sciences; GE1-350; Pacific; Indian; ice core; North Atlantic
• Online Access: https://doi.org/10.5194/cp-11-135-2015; https://doaj.org/article/7af261af795642edb9f32d3e3e89eef8

The reason for the initial rise in atmospheric CO 2 during the last deglaciation remains unknown. Most recent hypotheses invoke Southern Hemisphere processes such as shifts in midlatitude westerly winds. Coeval changes in the Atlantic meridional overturning circulation (AMOC) are poorly quantified, and their relation to the CO 2 increase is not understood. Here we compare simulations from a global, coupled climate–biogeochemistry model that includes a detailed representation of stable carbon isotopes (δ 13 C) with a synthesis of high-resolution δ 13 C reconstructions from deep-sea sediments and ice core data. In response to a prolonged AMOC shutdown initialized from a preindustrial state, modeled δ 13 C of dissolved inorganic carbon (δ 13 C DIC ) decreases in most of the surface ocean and the subsurface Atlantic, with largest amplitudes (more than 1.5‰) in the intermediate-depth North Atlantic. It increases in the intermediate and abyssal South Atlantic, as well as in the subsurface Southern, Indian, and Pacific oceans. The modeled pattern is similar and highly correlated with the available foraminiferal δ 13 C reconstructions spanning from the late Last Glacial Maximum (LGM, ~19.5–18.5 ka BP) to the late Heinrich stadial event 1 (HS1, ~16.5–15.5 ka BP), but the model overestimates δ 13 C DIC reductions in the North Atlantic. Possible reasons for the model–sediment-data differences are discussed. Changes in remineralized δ 13 C DIC dominate the total δ 13 C DIC variations in the model but preformed contributions are not negligible. Simulated changes in atmospheric CO 2 and its isotopic composition (δ 13 C CO 2 ) agree well with ice core data. Modeled effects of AMOC-induced wind changes on the carbon and isotope cycles are small, suggesting that Southern Hemisphere westerly wind effects may have been less important for the global carbon cycle response during HS1 than previously thought. Our results indicate that during the early deglaciation the AMOC decreased for several thousand years. We propose that the ...