Heather W. Hill

Phasing of deglacial warming and Laurentide Ice Sheet meltwater in the Gulf of Mexico

Evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation. However, existing studies show that deglacial warming was asyn- chronous across the tropical band, complicating the identification of causal mechanisms. The Orca Basin in the northern Gulf of Mexico is ideally located to record subtropical Atlantic sea-surface temperature (SST) warming in relation to meltwater input from the Laurentide Ice Sheet. Paired d 18 O and Mg/Ca data on the planktonic foraminifer Globigerinoides ruber from core EN32-PC6 are used to separate deglacial changes in SST and d 18 O of sea- water. SST as calculated from Mg/Ca data increased by .3 8C from ca. 17.2 to 15.5 ka in association with Heinrich event 1 and was not in phase with Greenland air temperature. Subtracting tem- perature effects from d 18 O values in G. ruber reveals two excur- sions representing Laurentide meltwater input to the Gulf of Mex- ico, one of .1.5‰ from ca. 16.1 to 15.6 ka and a second major spike of .2.5‰ from ca. 15.2 to 13.0 ka that encompassed melt- water pulse 1A and peaked ca. 13.8 ka during the Bolling-Allerod. Conversion to salinity through the use of a Laurentide meltwater end member of 225‰ indicates that near-surface salinity de- creased by 2‰-4‰ during these spikes. These results suggest that Gulf of Mexico SST warming preceded peak Laurentide Ice Sheet decay and the Bolling-Allerod interval by .2 k.y. and that heat was retained in the subtropical Atlantic during Heinrich event 1, consistent with modulation of deglacial climate by thermohaline circulation.

Laurentide Ice Sheet Meltwater and the Atlantic Meridional Overturning Circulation During the Last Glacial Cycle: A View From the Gulf of Mexico

0GM001016-RA-Flower.3 Pro of Meltwater input from the Laurentide Ice Sheet (LIS) has often been invoked as a cause of proximal sea surface temperature (SST) and salinity change in the North Atlantic and of regional to global climate change via its influence on the Atlantic meridional overturning circulation (AMOC). Here we review the evidence for meltwater inflow to the Gulf of Mexico and its reduction relative to the onset of the Younger Dryas, compare inferred meltwater inflow during marine isotope stage 3 (MIS 3), and thereby assess the role of LIS meltwater routing as a trigger of abrupt climate change. We present published and new Mg/Ca and δO data on the planktic foraminifer Globigerinoides ruber from four northern Gulf of Mexico sediment cores that provide detailed records of SST and δO of seawater (δOsw) for most of the last glacial cycle (48–8 ka). These results generally support models that suggest meltwater rerouting away from the Gulf of Mexico and directly to the North Atlantic may have caused Younger Dryas cooling via AMOC reduction. Alternatively, southern meltwater input may simply have been reduced during the Younger Dryas. Indeed, Dansgaard-Oeschger cooling events must have had a different cause because southern meltwater input during MIS 3 does not match their number or timing. Furthermore, the relationships between Gulf of Mexico meltwater input, Heinrich events, Antarctic warm events, and AMOC variability suggest bipolar warming and enhanced seasonality during meltwater episodes. We formulate a “meltwater capacitor” hypothesis for understanding enhanced seasonality during abrupt climate change in the North Atlantic region.