Paper No. 3-2
Presentation Time: 8:20 AM
INTRA DANSGAARD–OESCHGER CYCLES IN ICE MARGIN, LOESS, DUNE, AND LACUSTRINE DEPOSITS IN THE CENTRAL USA AND THEIR FORCING MECHANISM
Dansgaard–Oeschger (D-O) cycles, inferred from d18O variations in Greenland ice, are large temperature oscillations representing cold stadial and warm interstadial conditions at high latitudes during the last glaciation. Heinrich events, inferred from ice-rafted deposits in the North Atlantic, represent extensive growth of Northern Hemisphere (NH) ice sheets and sea ice cover, and have been tied to the coldest D-O stadials. The main triggers for these climate cycles are thought to have been iceberg surges and associated freshwater fluxes into the North Atlantic during the Heinrich Stadials (HS), which weakened or shut down the Atlantic meridional overturning circulation (AMOC). Alternatively, the massive iceberg discharges could have been caused by changes in oceanic or atmospheric circulation, such as through the expansion of Antarctic Bottom Water and intensified NH Westerlies. Here, I show that, according to loess-paleosol, dune-wetland, and marl-lacustrine records near the retreating Lake Michigan Lobe (LML) of the Laurentide Ice Sheet in Illinois, climate oscillation occurred more rapidly than D-O cycles. These intra D-O/Heinrich scale climate changes can be observed between alternating loess and paleosol, dune and wetland, and marl and lacustrine units. Accelerator mass spectrometry 14C dates of short-lived fossil plants from proglacial and ice-walled lacustrine deposits show that the LML advanced to its first, second, and third outermost ice margins only during certain subphases of the coldest HS. These intra D-O/Heinrich scale climate changes correlate reasonably well with d18O variations in Greenland ice and Hulu cave speleothems, which suggests large-scale atmospheric changes played a role over the NH during the last deglaciation. I conclude that westerly winds, steered by strong or muted polar jet streams over the NH ice sheets with or without AMOC changes, modulated the intra D-O/Heinrich cycles.