Natural Radionuclides (234Th, 7Be and 210Pb) as Indicators of Sediment Dynamics in Jamaica Bay, New York.

摘要

This study has been motivated by the issue of marsh loss in Jamaica Bay, New York. A deficit in sediment supply has been implicated as a factor in the dramatic marsh loss in the bay, and we have used particle-reactive natural radionuclides as tracers for the transport and deposition of particles in the bay. The short-lived radionuclides 7Be (half-life = 53 d) and 234Th (half-life = 24.1 d) serve as tracers of particle dynamics on short-term (seasonal) time scales, while the longer lived 210Pb (half-life = 22.3 y) traces the fate of particles on decadal time scales. In Jamaica Bay, the well-characterized supply of 7Be and 210Pb from the atmosphere is augmented by inputs of these radionuclides from combined sewer overflow (CSO) events that add storm water to the bay. As well, the supply of 234Th from decay of dissolved 238U in situ is augmented by particles with excess 234Th transported into the bay from the New York Bight. Inventories of excess 234Th in bay sediments show significant temporal variation, as evidenced in four sampling campaigns of the bay carried out in 2004-2006. 234Th is deposited in sediments of the western bay during times of low wave height outside the bay. However, particles and associated 234Th are transported to the northeastern portion of the bay (e.g. Grassy Bay) following storms. We have used a mass balance of 234Th in the bay to estimate an annual input of sediment of 5.6 OE 1010 to 6.7 x 1011 g from the New York Bight into the bay. This estimate is approx. 4-45 times greater than those based on sediment balances and extrapolations based on measurement of sediment transport across Rockaway Inlet over a tidal cycle.