TREE-RING INVESTIGATION OF THE YOUNGER DRYAS IN THE U.S. UPPER MIDWEST

The warming from Late Glacial to Early Holocene was interrupted by an abrupt, cold excursion known as the Younger Dryas (YD) event. There is abundant evidence of this event in Europe, but there is less evidence and clarity of its effects in N. America, even though events in central N. America (melting of the continental ice sheet) may well have been responsible for triggering the YD. In this project, the environmental changes in mid-North America is being investigated over several millenia (ca. 10,000 to 14,000 ¹⁴C-yr BP) during the Late Glacial-Early Holocene transition, including the YD interval. More than 10 sites have been identified from which wood of this interval has been preserved by sedimentation, a few of which contain dead trees in their growth position (stumps). All sites will be sampled (or re-sampled) over the next 2 years, but a large number of pieces of wood have been examined from 3 sites: two from archival collections collected at least a decade ago, and a third site collected in August 2001. Like many of the sites of this age interval in the Midwest, two of the sites consist of spruce trees (the other is hardwood). Tentative tree-ring chronologies have been built from these specimens including from the Wisconsin Two Creeks forest (11,640 ¹⁴C-yr BP) and the Michigan Gribben Basin forest (10,000 ¹⁴C-yr BP). The chronology from the Illinois Lincoln Quarry site is much less certain because trees were not in growth position and a wide range of ¹⁴C ages were measured. This is the first step in systematically constructing the first high-resolution portrait of the Late Glacial/Early Holocene transition in mid-N. America using chronologies of tree-ring widths, ring anomalies/events (such as frost rings), stable isotopes and radiocarbon activity.