EVIDENCE FOR PERMAFROST CONDITIONS IN NEW YORK BETWEEN THE LGM AND THE B-A WARMUP

Ideas for deglacial conditions in New York between the LGM and the B/A warmup range from a continuous ice cover to a complete ice retreat into the Ontario basin. Two new data streams from central and southern New York suggest other possible responses. The first is a thick pro-glacial lacustrine sequence south of Cayuga Lake that underlies Port Bruce deposits and overlies till of probable Nissouri age. Plant macros recovered from near the middle of this inorganic exposure indicate a tundra environment, with an age of 18.2 cal yr BP, confirming an Erie Interstade age. The very cold environment at this time corroborates the local evidence for a lack of Erie ice retreat north of Ithaca and indicates local permafrost conditions during this interval. The second data set is the compilation of the AMS macrofossil dates from inorganic basal clays in lakes and bogs over a large latitudinal transect from the NY-NJ terminal moraine to near the north end of Cayuga Lake. The ages over this 350 km transect show no discernable gradient, but vary mostly between 14.2 and 15.1 cal yr BP. Three of the ages at the terminal moraine are older than 16 cal kyr, and include a basal tundra assemblage (Dryas, Betula, Salix). Younger southern sites sometimes indicate tundra but include conifer needles as well, suggesting park-tundra conditions. Understanding this inorganic deposition is important, linking it to a modern analog. Kettle bog bases near Ithaca tend to be slightly younger, have no Dryas and are Picea dominated, suggesting boreal conditions. One way to explain this age distribution and vegetational response is that most of the region continued to have very cold and permafrost conditions until about 14.7 ka when the Bølling-Allerød warming began. During this interval buried ice masses could not melt, but during the subsequent warming the ice blocks melted at rates dependent on their size and burial conditions, which could account for the scatter in ages. Alternatively, isolated ice regions were still present surrounding southern sites until the Bolling. These models fit our age distribution better than postulating a large-scale ice retreat to the Ontario basin and very rapid re-advance to the Valley Heads moraine location.