Sediment ages and flux variations from depth profiles of 210Pb: lake and marine examples

Abstract

Sediment accumulation rates are redetermined for several freshwater and marine settings to demonstrate the application of a new methodology for interpreting sediment core profiles of radioisotope activity versus depth. Depositional histories were reconstructed using the sediment isotope tomography (SIT) methodology, which abandons the stringent requirement of current methods that sediment accumulation rates and isotopic fluxes may not both vary with time. Using inverse numerical analysis techniques in conjunction with a predictive model, the SIT method is able to separate the effects of sediment accumulation from flux variations to determine age-to-depth relationships. Cumulative probability procedures provide a framework for interpreting the sensitivity and accuracy of the model-determined age-to-depth relationships. Restrictions applied to previous methods on interpreting sediment profiles affected by mixing must also be adhered to when using the SIT method. Previously published data from two Estonian lakes, Kurtna and Nômmejärv, from Rockwell Reservoir in Northeastern Ohio and from Cape Lookout Bight, NC, are examined to demonstrate the broad applicability of the methodology. With the exception of one location, the resultant age-to-depth relationships determined by the model agree with earlier interpretations. In the Estonian Lake Nômmejärv, previous use of the Constant Flux model overestimates sediment ages for two known time horizons, giving ages of 43 yr at 22 cm and 111 yr at 45 cm. The age-to-depth relationship determined by the SIT method reproduces satisfactorily the time horizons of 16±1 yr at 22 cm and 35±2 yr at 45 cm, in agreement with the onset of industrial activities in the region. These examples illustrate the suitability of this new methodology for application in a variety of complex depositional environments.