Distribution and Origin of Disseminated Organic Carbon in Glacial Drift, Southwestern Michigan, USA

Measurements of disseminated organic carbon (OC) content were conducted on samples from 4 complete rotasonic cores of glacial drift in southwestern Michigan. The cores range in depth from 177-267 ft and terminate in Mississippian Coldwater Shale bedrock. Grain size distribution was determined every 3-4 feet and 85 samples were analyzed for organic carbon content. The method included removal of inorganic carbon by repeated centrifuging with HCL, combustion with cupric oxide to oxidize the OC, and extraction and measurement of CO₂ in a vacuum extraction line.

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.