NATURAL RADIONUCLIDES
(²³⁴Th, ⁷Be AND ²¹⁰ Pb) AS INDICATORS OF SEDIMENT DYNAMICS IN JAMAICA BAY, NEW YORK

Natural Resource Technical Report
NPS/NERO/NRTR—2010/324

Alisha A. Renfro¹, J. Kirk Cochran¹, David J. Hirshberg ¹, Henry J. Bokuniewcz¹, and Steven L. Goodbred, Jr.<sup>2

1</sup>School of Marine and Atmospheric Sciences
Stony Brook University
Stony Brook, NY 11794-5000

² Present Address:
Department of Earth and Environmental Sciences
Vanderbilt University
Nashville, TN 37240

May 2010

U.S. Department of the Interior
National Park Service
Natural Resource Program Center
Fort Collins, Colorado
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Executive Summary

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 × 1010 to 6.7 x 1011 g from the New York Bight into the bay. This estimate is ~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. The difference may be due to the fact that the methods are integrating over different time scales and data characterizing the 234Thxs transported into the bay with particles are limited. However, all estimates agree that there is an import of sediment from New York Bight into Jamaica Bay. We have also used down-core distributions of excess 210Pb to estimate average long-term rates of sediment deposition in the muddy sediments of the bay to be 0.47 ± 0.27 g cm-2 y-1. Both of these radionuclide-based estimates are upper limits.

Measurements of 7Be and 234Th in marsh peat complement the distribution of these radionuclides in subtidal sediments. While 7Be is observed at all sites sampled, as a consequence of its direct supply to the marsh from the atmosphere, the input of 234Th depends on the supply of particles from the subtidal to the marsh surface. Elevated inventories of 234Th are typically observed near marsh edges, although the pattern is complicated by the proximity of interior sites to tidal creeks that serve as conduits for sediment supply to the marsh. Independently measured profiles of 210Pb in several Jamaica Bay marshes (JoCo, Big Egg and East High; Kolker, 2005) show that these marshes are accreting at rates that keep pace with sea level rise. Thus, a lack of sediment supply to the marsh surface does not appear to be the cause of marsh loss in Jamaica Bay. Other factors may be responsible for the inability of marshes to increase their elevation in concert with sea level rise. Such factors include eutrophication of the surrounding waters and build-up of phytotoxins (e.g. hydrogen sulfide) in the marsh peat pore water.

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