POST-CHIPPEWA OXYGEN-ISOTOPE RECORD FROM COWLES BOG, SOUTHERN LAKE MICHIGAN BASIN

Previous studies of sediments and molluscs recovered from vibracores a t Cowles Bog, a fen located in the Indiana Dunes National Lakeshore, a long the south shore of Lake Michigan, reveal long and short term wate r level fluctuations during the last 6000 years. Low water events are indicated by zones of organic detritus, in which occasionally, iron ox ide and calcium carbonate nodules, as well as selenite crystals have b een precipitated. Oxygen isotope data from aragonitic shells of the ga stropod 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 wate r level fluctuations based on changes in lithology and molluscan fauna l abundance and composition. Below 366 cm the molluscan record is eith er absent or represented by shell fragments. The condition of shells i n this interval suggests that the molluscs may have been exposed to su baerial weathering and reworking of older Holocene lake sediments, pos sibly during the low water Chippewa phase in the Lake Michigan basin ( 10 000 YBP to 6000 YBP). Above 366 cm the core is characterized by a w ell preserved molluscan fauna. Relatively light isotopic values for th e interval between 366 cm to 300 cm correlate with the transition from nonfossiliferous sands, peat and diamict to silty marl and calcareous sand, with a molluscan fauna dominated by taxa associated with perman ent water bodies. The event producing these alterations, the Nipissing Transgression, marks a change from subaerial to 'permanent' lacustrin e conditions that were not characterized by high net evaporation. Evid ence for another series of environmental changes occurs between 284 cm and 198 cm. This evidence includes the: (1) appearance of aquatic mol luscs at 280 cm that are associated with water bodies subject to signi ficant seasonal water level changes; (2) intermittent accumulations of iron oxide nodules, calcium carbonate nodules, and organic layers int erbedded with crudely horizontal layers of fine, calcareous, sand, sug gesting periodic water level oscillations; (3) onset of major excursio ns in the oxygen isotopic values between 260 cm to 198 cm. Relatively high delta(18)O (PDB) values, possibly indicating evaporative enrichme nt of the water, correlate with a prominent shell debris layer at a de pth of about 235 cm. Taken together, this evidence suggests that the c ore site was in the process of becoming isolated from Lake Michigan. T his isolation occurred during a series of low water events during the later part of the Nipissing Transgression.