Paper No. 33-2
Presentation Time: 1:55 PM
EVIDENCE OF EXTENSIVE PERMAFROST BETWEEN DEGLACIATION AND 15,000 YEARS AGO IN WESTERN NEW YORK, USA
The chronology of the last deglaciation of the Laurentide Ice Sheet in western New York (WNY) is poorly constrained. To fill this spatial and temporal gap, we collected sediment cores from six bogs in the Kent (terminal) and Lake Escarpment (first major recessional) moraine systems in WNY to constrain moraine abandonment via radiocarbon dating of macrofossils from basal sediments. Unlike prior work, we collected mineral-rich sediments lower than Livingstone corer penetration using a percussion Geoprobe system. Generally, the cores capture a basal silty diamicton followed by gray and tan lacustrine silts. The lacustrine silt increases in organic matter before a transition to peat. The lowest ages from the gray lacustrine units from all six bogs range from ~15 – 14 cal kyr BP. Other radiocarbon ages from the same unit occasionally come back older than the lowest age, spanning from 15 – 19.5 cal kyr BP. The oldest age (19.5 cal kyr BP) appears to be from an organic-rich rip-up clast within the gray silt and suggests sediment re-working. Our working hypothesis is that the mixed ages are the result of persistent permafrost conditions that inhibited kettle basin formation until ~15 cal kyr BP. Between at least 19.5 and 15 cal kyr BP, large ice blocks likely survived in the moraines until 15 – 14 cal kyr BP. Once these ice blocks began to melt, organic material from thin, frozen organic deposits (dating back to at least 19.5 cal kyr BP) could have been re-worked into the kettle basins. According to our hypothesis, the ice sheet began retreating from its terminal position before 19.5 cal kyr BP, but permafrost conditions and later basin formation prohibited a typical post-glacial deposition sequence. As such, the oldest macrofossils found within minerogenic sections of sediment cores from moraines in WNY are the best minimum constraints on moraine abandonment, not necessarily the standard ‘bog-bottom’ ages. This model potentially reconciles recent work which proposed that ages from 13 – 14 cal kyr BP were the result of a re-occupation of WNY by the Laurentide Ice Sheet, which instead could be the result of delayed deposition from permafrost. The Laurentide Ice Sheet’s proximal location over Lake Ontario until ~15 cal kyr BP and the Bølling/Allerød warming may have contributed to the extended cold conditions and eventual thawing of the landscape.