Temporal variations in lead concentrations and isotopic composition in the Southern California Bight

Abstract

Lead concentrations in surface waters of the Southern California Bight appear to have decreased threefold (from > 170 to <60 pM) since they were initially measured by Clair Patterson and his associates in the 1970s. The decrease parallels a threefold decline in anthropogenic inputs of industrial lead to the bight over the past two decades. Moreover, mass balance calculations indicate that the primary source of lead to the bight now is upwelling. This is evidenced by the isotopic compositions of surface waters in the bight, which are most characteristic of Asian industrial lead aerosols ( 0.4793 ≤ 206Pb 208Pb ≤ 0.4833 ) deposited in oceanic waters of the North Pacific. While the decrease in surface water lead concentrations in the bight reflects the reduction in industrial lead emissions from the United States, the isotopic compositions of surface waters in the southern reach of the bight reflect a concurrent increase in industrial lead emissions from Mexico ( 0.4852 ≤ 206Pb 208Pb ≤ 0.4877 ). The isotopic composition ( 208Pb 207Pb ≈ 2.427 ) of elevated lead concentrations of surface waters in San Diego Bay indicate that lead is being remobilized from contaminated sediments within that bay.