HR: 09:50h
AN: PP41A-06    [Abstracts]
TI: Dynamic Proxies of Ocean Circulation in the North Atlantic During the Younger Dryas
AU: * Praetorius, S
EM: spraetorius@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology & Geophysics deptartment,MS#23, Woods Hole, MA 02543, United States
AU: McManus, J
EM: jmcmanus@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology & Geophysics deptartment,MS#23, Woods Hole, MA 02543, United States
AB: The North Atlantic Deep Water (NADW) is a major component of the Atlantic's meridional overturning circulation, which is strongly linked to climate through the sea-to-air heat transfer by water transported from low to high latitudes. Changes in this circulation system have been implicated in the abrupt climate reversal of the Younger Dryas. Previous studies using nutrient proxies such as δ¹³C and Cd/Ca show a nutrient enrichment in the North Atlantic during the Younger Dryas, reflecting a reduction in the volume of nutrient-depleted NADW. Although valuable, water mass tracers cannot constrain the rate of overturning; a crucial factor in the overall heat flux of deep water formation. Dynamic proxies such as ²³¹Pa/²³⁰Th disequilibria and the grain size of deep sea sediments provide tools to measure changes in the vigor of ocean circulation. ²³¹Pa/²³⁰Th ratios act as a proxy for the export rate of subsurface waters from the North Atlantic. Changes in the non-cohesive sortable silt (SS) size fraction (10-63μm) of terrigenous sediments reflect variations in the current strength as a result of the relative entrainment capacity of flow velocity. Here we compare the grain size record from site 984, along the Rekjanes Ridge, with ²³¹Pa/²³⁰Th data from core GGC5 on the Bermuda Rise. Site 984 is well situated to monitor both the modern deep water overflows and the intermediate depth waters of the glacial period, whereas core GGC5 offers a more basin-wide measure of circulation export. These records indicate similarly robust overturning circulation during the last glacial maximum and Holocene. In contrast, the deglacial period reveals significant reductions in the circulation. The Younger Dryas exhibits the most dramatic decrease in grain size throughout the 20,000 year record, and the ²³¹Pa/²³⁰Th data indicate a reduction in export rate that is rivaled only by the first Heinrich iceberg discharge event. The reduction in current strength during the Younger Dryas is concurrent with an increase in δ¹⁸O and polar fauna at both sites, indicating colder surface temperatures. The dynamic circulation proxies demonstrate that decreases in the rate of circulation occurred in association with deglacial melting, supporting the hypothesis that freshwater influx can have a dampening effect on the rate of overturning. This confirms the close connection between ocean circulation and such rapid climate oscillations as the Younger Dryas.
DE: 4558 Sediment transport (1862)
DE: 4860 Radioactivity and radioisotopes
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4962 Thermohaline
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Joint Assembly