Atmospheric Deposition of Metals to Coastal Waters (Long Island Sound, New York U.S.A.): Evidence from Saltmarsh Deposits

Abstract

Deposition chronologies of six high saltmarsh cores collected in New York City and Long Island, New York, determined using excess210Pb, show that accretion rates range from 0·1 to 0·4cm year−1. On average the marshes are accreting within about a factor of two of the rise in mean sea level for the area. Excess210Pb inventories in the marshes average 31±14dpm cm−2, comparable to values expected from atmospheric deposition of210Pb. Chronologies of fluxes of137Cs and excess Pb, Cu, Zn and Cd (defined as values in excess of the pre-anthropogenic background at depth in a core) indicate significant migration of137Cs and Cd in the marsh cores. Lead shows maximum fluxes between 1970 and 1980, consistent with the historical record of consumption of leaded gasoline in the U.S.A. Copper shows recent decreases in flux at four of the six sites and Zn shows variable patterns. Fluxes of Pb, Cu and Zn are generally less than or equal to directly measured atmospheric fluxes of these metals, indicating the importance of atmospheric supply of the metals to the marsh sites. Assuming that the atmospheric pathway is dominant for both metal and210Pb supply to the marshes, the regional pattern of atmospheric metal fluxes and the relative importance of the atmosphere in contributing metals to adjacent Long Island Sound can be determined. The marsh data indicate the atmospheric fluxes of Pb, Cu and Zn are greatest in New York City and decrease eastward along the north shore of Long Island. Comparison of the ratios of the inventories of excess metals to excess210Pb in the marshes and in the subtidal sediments of adjacent Long Island Sound suggests that atmospheric supply can contribute large fractions of Pb (at most 70–90% of integrated input). The atmospheric contributions of Cu and Zn are <50%, indicating the dominance of non-atmospheric sources for these metals. For all the metals, lower contributions of atmospheric input are evident in western Long Island Sound, suggesting that non-atmospheric sources of metals (point and non-point) are of greater importance in this portion of the estuary.