Distribution and Origin of Disseminated Organic Carbon in Glacial Drift, Southwestern Michigan, USA
The stratigraphic sequence consists of 2 fine-grained diamicton units with interbedded glaciofluvial and sandy glaciolacustrine sediments. The section is interpreted to record a late-Wisconsin advance and re-advance of the Saginaw Lobe of the Laurentide Ice Sheet. Organic carbon is enriched in the diamicton samples (generally 2000-12,000 mg/kg) and much less abundant in the sorted sediments (generally less than 2000 mg/kg).
The disseminated OC has three possible sources: (1) mid-Wisconsin (Marine Isotope Stage 3) OC from soils and vegetation incorporated into the late-Wisconsin (MIS-2) advances because the area was ice-free in the mid-Wisconsin and macroscopic fragments of wood and other plant material of mid-Wisconsin age (typically >30 kyr 14C BP) are commonly found in late-Wisconsin sediment; (2) late-Wisconsin OC corresponding to sources present at the time of the advance(s); and (3) OC from the Coldwater Shale bedrock. The extracted CO2 from 9 diamicton samples (4 from these cores and 5 from other late-Wisconsin diamictons in the region) was submitted for radiocarbon dating. The dates range from 15,800 to 23,850 yr 14C BP, thereby supporting source number 2.