Lead-210 as a tracer of atmospheric input of heavy metals in the northern Venice Lagoon

Abstract

A chronology for the accumulation of a salt marsh core taken in the Venice Lagoon has been obtained from the activity-depth profile of excess 210 Pb , a naturally occurring radionuclide supplied to the marsh from the atmosphere. Over the past century, the accretion rate of the marsh increased from 0.07–0.08 cm yr −1 to a maximum (0.2 cm yr −1 ) in ∼1970, followed by a decrease to ∼0.18 cm yr −1 . The increase and recent constancy of accretion of the marsh follow the historical pattern of sea level change due to natural and human-induced (groundwater extraction) causes. The 210 Pb chronology is used to determine depositional histories of anthropogenic trace metals, defined as metal concentration in excess of the background value determined at depth in the core. Several patterns are evident: fluxes of excess Ag and Cd increase to the present, Ni and Zn show increasing fluxes to ∼1970 with factor-of-two decreases in recently deposited material. Lead shows high and variable fluxes between 1940 and 1984, with recent decreases. Copper fluxes display a broad peak around 1960. Recent (∼1990) metal fluxes recorded in the marsh core agree within factors of 1.5 to 4 with directly measured (1993–1994) atmospheric fluxes in wet and dry deposition at a site in Venice. The data show that Cu and Pb inputs to the marsh can be accounted for by atmospheric deposition and significant fractions (27–42%) of Cd, Ni and Zn can be supplied by this pathway. Excess 210 Pb inventories in both marsh and subtidal sediments from the Venice Lagoon are consistent with atmospheric supply of this radionuclide. If we assume that atmospheric supply of trace metals is the dominant source of heavy metal contaminants to the marsh, integrated deposition of excess metals in the marsh core, normalized to that of excess 210 Pb , can be used to determine upper limits on the atmospheric contribution of trace metals to lagoon sediments. The results demonstrate that point source inputs of Cu, Ni, Pb and Zn are evident in sediments near the mainland, especially near Porto Marghera, but atmospheric inputs of Pb (and possibly Zn) tend to dominate in the northern and eastern portions of the lagoon.