Disequilibrium between 226Ra and supported 210Pb in a sediment core from a shallow Florida lake

Abstract

210Pb 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 226Ra and total 210Pb activities in a core from Lake Rowell, Florida. 226Ra activity was high and strongly variable throughout the core, even exceeding total 210Pb 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 226Ra and supported 210Pb. High and variable 226Ra 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 226Ra and supported 210Pb makes it difficult to distinguish between supported and unsupported 210Pb activity throughout the Lake Rowell core and would require special assumptions and nonconventional dating models to establish age‐depth relationships.