DISEQUILIBRIUM BETWEEN 226RA AND SUPPORTED PB-210 IN A SEDIMENT CORE FROM A SHALLOW FLORIDA LAKE

Pb-201 dating can be used to assign ages in lake sediment cores calcul ate rates of sediment accumulation, and determine the timing of recent changes in lake-watershed ecosystems. We used low-background gamma co unting to meaSUre Ra-226 and total Pb-210 activities in a core from La ke Rowell, Florida. Ra-226 activity was high and strongly variable thr oughout the core, even exceeding total Pb-201 activity in recently dep osited sediments. We traced one source of Ra-rich sediments to the onl y inflow, Alligator Creek, where stream-bottom deposits display disequ ilibrium between Pa-226 and supported Pb-210. High and variable Pa-226 activity in the Lake Rowell profile argues for direct estimates of in situ Ra in lake sediment cores from disturbed watersheds that have Ra -bearing bedrock. Isotopic disequilibrium between Ra-226 and supported Pb-210 makes it difficult to distinguish between supported and unsupp orted Pb-210 activity throughout the Lake Rowell core and would requir e special assumptions and nonconventional dating models to establish a ge-depth relationships.