Abstract:
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, and we have used particle-reactive natural radionuclides as tracers for the transport and deposition of particles in the bay. The short-lived radionuclides <super>7</super>Be (half-life = 53.3 days) and <super>234</super>Th (half-life = 24.1 days) serve as tracers of particle dynamics on short-term (seasonal) time scales, while the longer lived <super>210</super>Pb (half-life = 22.3 y) traces the fate of particles on decadal time scales. In Jamaica Bay, the well-characterized supply of <super>7</super>Be and <super>210</super>Pb from the atmosphere comprised 77-92% and 23-48% of the supply of these radionuclides, respectively and is augmented by inputs of these radionuclides from CSO events that add storm water to the bay. Mean <super>234</super>Th<sub>xs</sub> inventories in the subtidal sediments in 2004. 2006 ranged from 3.4 to 5.6 dpm cm<super>-2</super>. The supply of <super>234</super>Th from decay of dissolved <super>238</super>U in situ is ~ 0.9 dpm cm<super>-2</super> and is augmented by particles with excess <super>234</super>Th transported into the bay from the New York Bight. Inventories of excess <super>234</super>Th in bay sediments show significant temporal variation, as evidenced in four sampling campaigns of the bay carried out in 2004-2006. The upper 5 cm of subtidal sediments were sampled to insure that the entire <super>234</super>Th<sub>xs</sub> inventory was obtained and could be counted quickly, but additional samples in fine-particle dominated sediments indicated that 79. 100% of the <super>234</super>Th<sub>xs</sub> inventory was confined to a highly active veneer of sediment. Results indicate that <super>234</super>Th is deposited in sediments of the western bay during times of low wave height outside the bay and winds out of the south. However, particles and associated <super>234</super>Th in the highly active surficial sediments are transported to the northeastern portion of the bay (e.g. Grassy Bay) following storms and winds out of the southwest. We have used a mass balance of <super>234</super>Th in the bay to estimate an annual input of sediment of 4.3 to 35.8 x 1010 g y<super>-1</super> from the New York Bight into the bay. A mass balance of <super>210</super>Pb for Jamaica Bay indicated that a sediment import of 4.2 to 15.8 1010 g y<super>-1</super> would be needed to balance the bay's <super>210</super>Pb budget. Both these radionuclide-based sediment import estimates are upper limits. We have also used down-core distributions of excess <super>210</super>Pb activity to estimate average long-term rates of sediment deposition in the muddy sediments of the bay to be 0.47 ñ 0.27 g cm<super>-2</super> y<super>-1</super> (7.6 ñ 1.4 10<super>10</super> g y<super>-1<super>). A sediment import of 5.8. 8.8 10<super>10</super> g y<super>-1</super> would be required to balance the sediment budget of the bay. This annual import estimate is higher than a previous estimate made from the attempt at a sediment budget for the bay (1.5. 2.9 10<super>10</super> g y<super>-1</super>). However, estimates from the mass balances of the radionuclides and the sediment budgets all indicate that there is an import of sediment from into Jamaica Bay. Measurements of <super>7</super>Be and <super>234</super>Th in marsh peat complement the distribution of these radionuclides in subtidal sediments. While <super>7</super>Be was observed at all sites sampled (ranging from 0.7 to 3.2 dpm cm<super>-2</super>), as a consequence of its direct supply to the marsh from the atmosphere. In contrast, the input of <super>234</super>Th depends on the supply of particles from the subtidal to the marsh surface. Inventories of <super>234</super>Th<sub>xs</sub> on the marshes ranged from 0. 9.8 dpm cm<super>-2</super> with higher mean inventories observed on the marshes in the western bay in both September-2004 and May-2005 (6.0 ñ 1.3 and 3.4 ñ 1.3 dpm cm<super>-2</super>, respectively) than in the eastern bay (4.1 ñ 1.2 and 1.8 ñ 1.7 dpm cm<super>-2</super>, respectively). Elevated inventories of <super>234</super>Th 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. Sediment accumulation on the marshes was estimated using the <super>234</super>Th<sub>xs</sub> inventories measured on the marsh islands and the activity of <super>234</super>Th<sub>xs</sub> in the upper 5 mm of the fine-fraction sediments near the marsh islands. Sediment accumulation rates derived from this method ranged from 0 to 2.5 g cm<super>-2</super> y<super>-1</super>, while mass accumulation rates on the marshes, derived from <super>210</super>Pb geochronologies in a previous study ranged from 0.05. 0.1 g cm<super>-2</super> y<super>-1</super>. The difference in these two methods may reflect sampling bias to the marsh edges during this study, where deposition is likely higher, and the short-term patterns of deposition, reflected with the <super>234</super>Th<sub>xs</sub> inventories.