North Atlantic Meridional Overturning Circulation (AMOC) and Abrupt Climate Change through the Last Glaciation

Abstract:

The climate of the glacial North Atlantic was punctuated by catastrophic discharges of icebergs (Heinrich events), as well as by more mysterious, abrupt warming events associated with Dansgaard-Oeschger oscillations. These events are suspected to be related to changes in AMOC and its influence on heat transport and the regional and global heat budget. Investigation of these rapid oscillations is often limited by the resolution of sediment records. High accumulation rates at our study site (33.69°N, 57.58°W, 4583m water depth) on the Bermuda Rise allow improved resolution by one to two orders of magnitude.

Cores CDH19 (38.81m) and CDH13 (36.70m), were recovered during KNR191, the initial deployment of the RV Knorr's long coring system developed at the Woods Hole Oceanographic Institution with support from the NSF. These cores contain high quality sediment sections that allow high resolution studies extending through the last glacial cycle at a key location for monitoring past oceanographic and climatic variability. Here we present detailed multi-proxy data from Bermuda Rise sediments reflecting deep ocean chemistry and dynamics of the last glaciation, and combine them with published data to produce a continuous, high resolution record spanning the last 70,000 years.

CaCO3 burial fluxes, foraminifera stable isotopes, and sedimentary uranium-series disequilibria (including seawater-derived ²³¹Pa /²³⁰Th), display coherent, complementary variability throughout the last glaciation. Glacial values in each proxy are consistent with reduced ventilation and overturning compared to the Holocene, with intervals that indicate substantial millennial reductions in each, and others when they briefly approach Holocene levels. In multiple instances, particularly spanning interstadials eight through twelve (IS8-IS12) our results are consistent with an abrupt, subcentennial acceleration in the export of excess ²³¹Pa from the North Atlantic during stadial-interstadial transitions, accompanied by increases in CaCO3 preservation and greater ventilation by low-nutrient deep waters, followed by a more gradual decline in those indicators as glacial conditions were reestablished in the basin. This pattern is consistent with a strong link between AMOC and millennial climate change throughout the last glaciation.