BRACKETING THE CHRONOLOGY OF ICE SHEET ADVANCE/RETREAT USING RADIOCARBON ANALYSIS OF KETTLE LAKE CORES, UPPER PENINSULA OF MICHIGAN

Cores taken from ten kettle lakes in central Upper Peninsula of Michigan were analyzed using AMS radiocarbon dating to determine the chronology of the last ice advance in the region. The kettle lakes span an area both inside and outside the last ice-contact margin, thought to be the Marquette Readvance, located on the northern shore of the Peninsula (Blewett and Reick, 1987). Using kettle lake cores, plus additional chronology from others (Hack, 1965, Saarnisto, 1974, Futyma, 1981, Lowell et al., 1999 and Fisher and Whitman, 1999), ice advance and retreat in the area can be bracketed. Kettle lake sediments, representing the minimum age of deglaciation, yielded AMS radiocarbon dates of 9000-9500 14C BP. Previous studies include dates from material buried in glacial outwash and till, yielding an age of ~10000 14C BP (Lowell et al., 1999 and Hack, 1965). This brackets the timing of ice advance and retreat to ~500 years. The clustering of radiocarbon dates around 9500 14C BP in this study and others (Futyma, 1981, Fisher and Whitman, 1999, Hack, 1965) across the northern shore of the Upper Peninsula also suggest a synchronous deglaciation in the region.