AGE OF ORGANIC CARBON IN LATE WISCONSIN TILL, SOUTHWESTERN MICHIGAN

Dating of late Wisconsin glacial events is typically based on radiocarbon dates of wood incorporated in glacial sediment. In this pilot study, an attempt was made to date diamicton units of the Saginaw (?) Lobe in southwestern Michigan, using disseminated organic carbon from the till matrix. The organic carbon was combusted and collected in a vacuum extraction line as CO₂ after removal of inorganic carbon. Five dates were obtained from a detailed study of the organic carbon content (85 samples) of four rotasonic cores of the complete glacial section, ranging from 167 to 277 ft in thickness. An additional 6 dates were obtained from one pit exposure and 2 archived cores of Lake Michigan lobe tills. Based on the regional stratigraphic framework and the lack of paleosols in the cores, the sections were considered before dating to be late Wisconsin in age.

Organic carbon in the till matrix is derived from incorporation of soils and other organic material into the till matrix during glacial advance. Organic carbon in soil profiles is partitioned into several fractions which can differ in age by a thousand years or more. In addition to carbon from the soil profiles present at the time of advance, the tills could also contain organic carbon from 3 potential other sources, all of which would bias the dates toward older ages: (1) mid-Wisconsin wood incorporated into late-Wisconsin sediment, (2) the organic rich Coldwater Shale, which directly underlies the glacial sediments, and (3) coal derived from advance of the Saginaw Lobe over Tertiary units. Despite these potential sources of contamination, all dates are consistent with either the LGM or younger re-advances of the ice (15,800-25,200 ¹⁴C yr BP).