LATE QUATERNARY OSTRACODES AS INDICATORS OF SALINITY CONDITIONS AND PALEOHYDROLOGY IN A MID-CONTINENT WETLAND, CHEYENNE BOTTOMS, KANSAS

An understanding of prehistoric hydrologic conditions of mid-continent lakes is needed to characterize changes in fresh-water recharge to wetlands resulting from anthropogenic activities. Salinity-dependent, freshwater ostracode species from a 13.8 m sediment core are used here as a tool for interpreting the paleohydrology of one such wetland. Ostracode microfossils and sediment core analysis enabled the reconstruction of local water salinities and flow regimes. The core was divided into 66 samples based on visual description and hand texturing. Fossil assemblages and soil morphologic features indicate a transition from wetland to drier conditions at a depth of 6.2 m. Ostracodes were found from the bottom of the core upward to 6.2 m, and indicate a range of salinity conditions. Throughout this section, mixosaline ostrocodes, particularly Fabaeformiscandona rawsoni and Limnocythere ceriotuberosa, are abundant. Between 8.76 m and 8.63 m the mixosaline species occur along with abundant Cyprideis salebrosa and Limnocythere staplini. This suggests a transition upward from mixosaline to eusaline/hypersaline conditions. Between 7.8 m and 6.2 m the abundance of oligosaline species Candona caudata and Limnocythere itasca indicate fresher-water conditions. Here, also, the presence of Cypridopsis vidua suggest an environment of sub-aquatic vegetation with shallow groundwater influx and/or additional freshwater recharge. Cretaceous foraminifera reworked from nearby sandstones and limestones are present throughout the ostracode-bearing sediments. However, they are more abundant in the freshwater zone, providing possible evidence for fluvial transport of sediments and fresh surface water into the basin. The lack of ostracodes in the upper part of the core implies the absence of standing water and is in agreement with interpretations of a Late Pleistocene - Holocene unconformity and Holocene climate change made by others.