Sedimentary processes in the inner New York Bight: evidence from excess ²¹⁰Pb and ^239,240Pu

Abstract

Particle-reactive radionuclides were determined in sediments from the inner New York Bight to trace transport and storage of fine-grained sediments and associated reactive materials. Seven sediment cores 20-50 cm in length were analyzed for water content, loss on ignition (LOI) and excess ²¹⁰Pb; three of these were also analyzed for ^239,240Pu. Excepting some depth horizons in a core from a dredge-spoil dumpsite, every sample analyzed contained excess ²¹⁰Pb. Variations in the concentration of excess ²¹⁰Pb with depth in the sediment at all stations correlated strongly with LOI, which apparently traces that fraction of the sediment which is active in removing reactive elements from the water column. In the cores analyzed for ^239,240Pu, every sample contained finite Pu, and Pu concentrations correlated strongly with excess ²¹⁰Pb. The radionuclide distributions may be simply viewed as products of steady-state sediment accumulation or of mixing. Geochemically reasonable accumulation rates are very high (0.5-2.6 g/cm² y) and could probably only be sustained by offshore transport of dumped materials. At the other extreme the relationships between excess ²¹⁰Pb and LOI are compatible with rapid mixing of a ²¹⁰Pb carrier phase (traced by LOI) into the pre-existing substrate with little or no actual accumulation. Other non-steady-state processes, such as sediment gravity flow, could also explain the observed distributions. Measured sediment inventories (dpm/cm²) of excess ²¹⁰Pb and Pu at these stations are greatly in excess of those supportable by direct atmospheric deposition: lateral supply is required. Incorporation of sedimentary fines into the sand substrate could make the inner New York Bight an important repository of reactive materials.