The estuarine sediment record preserves a history of inputs of sediment to the estuary as well as of certain chemical species that strongly associate with particles. Reading the sediment record thus provides the opportunity to reconstruct events in the development of the estuary, including sediment transport events and inputs of contaminants to the system. Deciphering the sediment record is far from straightforward, however, and requires sorting out the effects of processes that perturb sediments after deposition. In the Hudson, these processes can include the effects of organisms living on or in the sediments and physical effects caused by river flow, as well as human-induced perturbations caused by dredging and shipping. One of the most powerful tools for determining rates of sediment accumulation in estuarine sediments is the use of natural and anthropogenic radioactive chemical species that are added to the estuary and become rapidly associated with sediments through aprocess referred to as 'scavenging'. Owing to their known rates of radioactive decay or varying rate of input to the estuary, these radionuclides serve as chronometers to estimate the time since they were present at the sediment-water interface and consequently the rate of sediment accumulation. Once the history of sediment deposition is determined at a given location, it can be applied to the estimation of inputs of particle-associated contaminants such as trace metals or Polychlorinated Biphenyls (PCBs). The purpose of this chapter is to review the primary methods used to determine sediment chronologies in the Hudson River and to describe the results.
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