Intensification and variability of ocean thermohaline circulation through the last deglaciation

Abstract

Neodymium isotope ratios in the authigenic ferromanganese oxide component in a southeastern Atlantic core reveal a history of the global overturning circulation intensity through the last deglaciation. From a minimum during the Last Glacial Maximum (LGM), North Atlantic Deep Water (NADW) began to strengthen between 18 and 17 kyr cal. BP, approximately 2000–3000 years before the Bølling warming. It exhibits a gradually increasing baseline intensity that plateaus in the early Holocene, tracks increasing Northern Hemisphere insolation and parallels atmospheric CO 2 concentration. Millennial-scale fluctuations are superimposed on this baseline NADW increase, corresponding to events in Northern Hemisphere climate records. The millennial excursions show sharp increases associated with the pre-Bølling retreat of continental glaciers and the Bølling warming itself, a decrease with Allerød cooling, and a recovery through the Younger Dryas and early Holocene. The data confirm a close link between deep-ocean circulation and North Atlantic climate changes. There is no clear linkage with meltwater pulses, indicating that rapid meltwater additions did not trigger observable mode changes in NADW production. However, rapid changes in North Atlantic sea ice cover show a strong relationship with the millennial perturbations of NADW flux to the deep Southern Ocean. The data indicate that the baseline intensity of NADW reaching the Cape Basin is paced by Northern Hemisphere insolation and that North Atlantic sea ice coverage acts as a major amplifying link between the oceans and the atmosphere.