PATTERNS OF FINE-GRAINED SEDIMENT ACCUMULATION IN THE DELAWARE ESTUARY: NEW INSIGHT FROM PB-210 AND CS-137 INVENTORIES

Despite the ecological, industrial, and commercial importance of the Delaware Estuary, little is known about the patterns and rates of modern fine-grained sediment accumulation within the system. This study utilizes a suite of sedimentological data and radioisotope inventories (Pb-210 and Cs-137) as tracers of fine-grained sediment accumulation, non-deposition, and erosion in order to identify and quantify sediment sinks in the Estuary. Preliminary results based on cores from 33 sites in the estuarine channel provide new insight into patterns of modern depositional processes. Excess Pb-210 (22.3-yr half life) inventories from the channel range from non-detectable to 60 dpm/cm² and have an overall mean of 11+/-14 dpm/cm². Roughly 90% of these excess Pb-210 inventories fall below the theoretical value supported by a steady-state atmospheric flux (32 dpm/cm²). Additionally, excess Pb-210 inventories are considerably lower than the 27-78 dpm/cm² range determined for adjacent tidal marshes where long-term sediment accumulation has been relatively continuous. Cesium-137 (a stratigraphic marker appearing in 1954) inventories range from non-detectable to 6 dpm/cm² and have an overall mean of 0.5+/-1.2 dpm/cm², relatively low compared to marsh inventories (2-22 dpm/cm²). The low excess Pb-210 and Cs-137 inventories suggest that the estuarine channel is largely non-depositional or erosional, with only localized areas of fine-grained sediment accumulation. These results are consistent with sidescan sonar observations in the channel, which reveal highly heterogeneous sediment distribution. The radioisotope inventories imply that fine-grained sediments derived from fluvial or marine sources are preferentially deposited elsewhere in the Estuary, perhaps on channel-flanking shoals and (or) within adjoining tidal marshes. Future work will employ high-resolution seismic records to extrapolate the coring results throughout the channel environment and to develop a fine-grained sediment budget.