Summary: Recent studies of the Greenland ice cores have offered many insights into Holocene climatic dynamics at decadal to century
timescales. Despite the abundance of continental records of Holocene climate, few have sufficient chronological control and
sampling resolution to compare with the Greenland findings. Butannually laminated sediments (varves) from lakes can provide
high-resolution continental palaeoclimate data with secure chronologies. Here we present analyses of varved sediments from
Deep Lake in Minnesota, USA. Trends in the stable oxygen-isotope composition of the sedimentary carbonate indicate a pronounced
climate cooling from 8.9 to 8.3 kyr before present, probably characterized by increased outbreaks of polar air, decreased
precipitation temperatures, and a higher fraction of the annual precipitation falling as snow. The abrupt onset of this climate
reversal, over several decades, was probably caused by a reorganization of atmospheric circulation and cooling of the Arctic
airmass in summer that resulted from the final collapse of the Laurentide ice near Hudson Bay and the discharge of icebergs
from the Quebec and Keewatin centres into the Tyrell Sea. The timing and duration of this climate reversal suggest that it
is distinct from the prominent widespread cold snap that occurred 8,200 years ago in Greenland and other regions. No shifts
in the oxygen-isotope composition of sediment carbonate occurred at 8.2 kyr before present at Deep Lake, but varve thickness
increased dramatically, probably as a result of increased deposition of aeolian dust. Taken together, our data suggest that
two separate regional-scale climate reversals occurred between 9,000 and 8,000 years ago, and that they were driven by different
mechanisms.
More Info on Paleolimnology |
