High gradients of lead isotopic composition in north-east Pacific upwelling filaments

Abstract

SUBSTANTIAL differences in stable lead isotopic composition in coastal waters of the north-east Pacific have been inferred from a single vertical profile taken near western Canada1. Here we present measurements of lead isotopes in two upwelling filaments off central California (39° N, 124° W) which confirm that such marked differences exist. The distinct, filamentary structures appear in association with surface jets which transport cold upwelled water hundreds of kilometres offshore2. Surface-water ratios of stable lead isotopes (204Pb,206Pb, 207Pb, 208Pb) within the core of the first filament (for example,206Pb/207Pb = 1.17) contrasted with those of surrounding coastal waters (206Pb/207Pb = 1.19) and those of industrial lead aerosols from the United States (206Pb/207Pb = 1.22). Contrasts in the isotopic composition indicated that the lead within the core was primarily derived from aeolian inputs of Asian industrial lead (206Pb/207Pb = 1.16) to remote waters, which were then upwelled near the North American coastline. High gradients of lead isotopic composition in coastal waters were confirmed by surface and sub-surface measurements surrounding a second upwelling filament. These gradients illustrate the complexity of trace-element cycles in coastal waters, and the offshore fluxes within the ephemeral filaments demonstrate significant influences of coastal dynamics on composition within the ocean gyre.