Time Scales of Sediment Focusing in Large Lakes as Revealed by Measurement of Fallout Cs-137

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The sediments of large lakes provide a sink for many of the trace contaminants that have been mobilized in the environment. Several models have been developed to describe the resultant concentration profiles in sediment cores—in terms of input fluxes, sedimentation rates, and mixing depths. Implicit in these models are the assumptions that sedimenting particles are disconnected from prior input events and that transport is only by vertical advection, even though the measured integrated fluxes indicated that there had been considerable sediment focusing. Recent measurements of Cs-137 and Pb-210 in sediment cores from Green Bay in Lake Michigan and least-squares best fits for sedimentation rates and mixing depths from these data suggest that these simple models are incorrect and do not adequately reflect the resuspension, horizontal transport, and redeposition of bottom sediments. Furthermore, such processes are not confined to shallow water; a comparison of total Cs-137 content, sedimentation rates, and mixing depths measured in a series of cores taken from Lake Michigan at the same locations ten years apart indicate that while the overall integrated Cs-137 content of the sediments, corrected for radioactive decay, has remained constant, there has been an increase within and a decrease at the periphery and outside of the depositional zones. Over the same period there was, as would be predicted from the simple models, an increase in the thickness of the measured mixing depth.KeywordsSedimentation RateSediment CoreGreat LakeLarge LakeRadioactive DecayThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.