RECONSTRUCTING LATE HOLOCENE PALEOENVIRONMENTS BASED ON FAUNA AND ISOTOPES FROM TWO INLAND LAKES ON MICHIGAN'S WEST COAST

The reconstruction of past climates may be determined by studying marl horizons deposited at times of known climate variability. Several lacustrine cores from Silver and Stony Lakes, Oceana County, MI were obtained by vibracoring for the purpose of determining if there was climate variability recorded during the Little Ice Age, the hypsithermal episode, the 8.2ka event and the Younger Dryas cold period. Lab analysis consisted of: 1) radiocarbon dating of wood in cores to determine marl deposition rates, 2) sediment type, molluscan and ostracod identification, and 3) isotopic analysis using a Finnigan Mat 252 isotope ratio mass spectrometer. Molluscan fauna and ostracods provide good insight into the paleoecology at the time of formation, specifically proxies for water temperature, depth, turbulence, salinity and other environmental parameters. Isotopic analysis provided an accurate record of key parameters such as, water temperature and salinity, which is critical when reconstructing past climates. Initial ƒÔ 18O and ƒÔ 13C values from an early Holocene marl at Silver Lake record a shift from heavier isotopes to lighter isotopes 9500 cal kyr B.P. Preliminary fauna results, from Silver Lake, showed the addition of gastropod Valvata sincera, which lives in cold water, at depths when the 8.2 ka event is thought to have occurred. Results also include gastropod Stagnicola sp. was only present in older marl horizons from Silver Lake, dated at 9490 cal yr B.P. (8500 14C yr B.P.) and bivalve Pisidium sp., typical of low turbulence, was more diverse. These preliminary fauna results indicate that Silver Lake levels were lower and possibly experienced dry periods throughout the year, compared to the larger lake of today