Particle reworking in sediments from the New York Bight apex: Evidence from ²³⁴Th/²³⁸U disequilibrium

Two diver-collected cores of mud-rich sediment from the New York Bight apex have been analyzed for ²³⁸U and ²³²Th decay series nuclides with emphasis on ²³⁴Th/²³⁸U disequilibrium. Excess ²³⁴Th is present in both cores and shows exponential decrease in the top 4 cm. Biogenic reworking by a deposit-feeding community characterized by a Nucula proxima-Nephtys incisa assemblage apparently controls the form of the ²³⁴Th profiles. Mixing coefficients of 0.3-0.6 x 10^-6 cm²s^-1 are calculated. No decrease of ²¹⁰Pb with depth (0-11 cm) is found nor is there any vertical structure in profiles of ²²⁸Th. Episode deposition followed by periods of stability cause the observed homogeneity of the longer lived nuclides as well as laminated horizons in the sediment. ²³⁴U/²³⁸U isotope ratios reflect addition of either sea water or sewage source uranium to the collection area.