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 calculate rates of sediment accumulation, and determine the timing of recent changes in lake-watershed ecosystems. We used low-background gamma counting to meaSUre Ra-226 and total Pb-210 activities in a core from Lake Rowell, Florida. Ra-226 activity was high and strongly variable throughout the core, even exceeding total Pb-201 activity in recently deposited sediments. We traced one source of Ra-rich sediments to the only inflow, Alligator Creek, where stream-bottom deposits display disequilibrium 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 unsupported Pb-210 activity throughout the Lake Rowell core and would require special assumptions and nonconventional dating models to establish age-depth relationships.