Post-Chippewa oxygen isotope record from Cowles Bog, southern Lake Michigan basin

Previous studies of sediments and molluscs recovered from vibracores at Cowles Bog, a fen located in the Indiana Dunes National Lakeshore, along the south shore of Lake Michigan, reveal long and short term water level fluctuations during the last 6000 years. Low water events are indicated by zones of organic detritus, in which occasionally, iron oxide and calcium carbonate nodules, as well as selenite crystals have been precipitated. Oxygen isotope data from aragonitic shells of the gastropod Amnicola limosa (Say) collected from a sediment core provide a record of Middle to Late Holocene environmental changes for the fen. These data are in good agreement with previous interpretations of water level fluctuations based on changes in lithology and molluscan faunal abundance and composition. Below 366 cm the molluscan record is either absent or represented by shell fragments. The condition of shells in this interval suggests that the molluscs may have been exposed to subaerial weathering and reworking of older Holocene lake sediments, possibly during the low water Chippewa phase in the Lake Michigan basin (10000 YBP to 6000 YBP). Above 366 cm the core is characterized by a well preserved molluscan fauna. Relatively light isotopic values for the interval between 366 cm to 300 cm correlate with the transition from non-fossiliferous sands, peat and diamict to silty marl and calcareous sand, with a molluscan fauna dominated by taxa associated with permanent water bodies. The event producing these alterations, the Nipissing Transgression, marks a change from subaerial to ‘permanent’ lacustrine conditions that were not characterized by high net evaporation. Evidence for another series of environmental changes occurs between 284 cm and 198 cm. This evidence includes the: (1) appearance of aquatic molluscs at 280 cm that are associated with water bodies subject to significant seasonal water level changes; (2) intermittent accumulations of iron oxide nodules, calcium carbonate nodules, and organic layers interbedded with crudely horizontal layers of fine, calcareous, sand, suggesting periodic water level oscillations; (3) onset of major excursions in the oxygen isotopic values between 260 cm to 198 cm. Relatively high δ18O (PDB) values, possibly indicating evaporative enrichment of the water, correlate with a prominent shell debris layer at a depth of about 235 cm. Taken together, this evidence suggests that the core site was in the process of becoming isolated from Lake Michigan. This isolation occurred during a series of low water events during the later part of the Nipissing Transgression.