Middle Holocene Hydrologic Change and Hypolimnion Formation in Lake Erie

Abstract

Data from a nearshore sediment core and a deep-water sediment core from the central basin of Lake Erie reveal shifts in sediment properties and stable isotope composition of shell carbonate between ca. 4,600 and 3,500 14C yrs BP. Radiocarbon dates are corrected for the hardwater effect by subtracting 670 years based on a modern calibration for the central basin. Silt content increased in the deep water core at 4,600 and again, slightly, at 3,900 14C yrs BP. Sand size increased in the nearshore core at 3,500 14C yrs BP. δ 18O of shell carbonate increased and δ 13C decreased in both cores between about 4,200 and 3,500 14C years BP. Magnetic susceptibility and percent calcite decreased sharply and percent organic carbon increased slightly in the deep water core beginning at 4,000 14C yrs BP. Most of the changes in sediment properties and stable isotope composition of shell carbonate occurred between 4,200 and 3,900 14C yrs BP. They coincide in time with the Nipissing II highstand of Lake Nipissing, in the Huron and Michigan basins, and with evidence for higher-than-present lake levels in Lake Erie. The changes in proxy data are interpreted as evidence for an influx of surface water as drainage from the Upper Great Lakes was rerouted through Lake Erie. There is little evidence in the sediment proxy record for changes in Lake Erie during the earlier Nipissing I highstand or the middle Holocene transition in regional climate. A 9,000-year composite stable isotope record for the central basin shows that the sediment cores document a transformation in drainage that established the modern hydrologic system for the lake. High lake level induced a seasonal hypolimnion, setting the stage for the low pH, oxygen-depleted bottom waters of today.