Lead and lead isotopes in the U.S. GEOTRACES East Pacific zonal transect (GEOTRACES GP16)

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Most of the lead (Pb) in today's ocean is from human activities, although natural sources contribute small amounts of Pb that are usually masked by massive anthropogenic emissions. In the U.S. GEOTRACES East Pacific Section (GP16), anthropogenic Pb is observed throughout the water column, and as seen elsewhere in the Pacific, Pb concentrations are highest in the upper hundreds of meters of the ocean and generally decrease with depth. Thermocline Pb concentrations are higher in the eastern portion of the section near the South American coast, decreasing with distance from the South American continent, as previously reported for redox sensitive trace elements (e.g., Mn). High precision Pb isotope ratios show that there has been a change over the past few decades in the source of Pb in the westernmost upper waters, shifting from an American-Australian type Pb intermixture to a Chinese-American-type Pb intermixture. Although it is known from the study of hydrothermal solutions and sediments that there is a large primary Pb flux from high temperature ridge crest hydrothermal springs, most of this Pb precipitates near the hydrothermal vents and very little escapes into the open water column. In fact, hydrothermal ferromanganese oxyhydroxides precipitated from the hot spring emissions strip anthropogenic Pb out of the water column, resulting in the lowest Pb concentrations ever observed in the ocean (a few tenths of a picomole/kg) in the high 3He-plume to the west of the East Pacific Rise.Deeper waters have similarly low Pb concentrations, probably as a result of the sinking veil of hydrothermal-source ferromanganese oxyhydroxides. Core top sediments from this section and previous work show clear evidence of less-radiogenic Mid Ocean Ridge Basalt (MORB) Pb distinct from the crustal and anthropogenic Pb seen in non-ridge crest sediments, specifically, low values of 206Pb/207Pb (1.189), 208Pb/207Pb (2.449), and 206Pb/204Pb (18.42), compared to ~1.195, ~2.475, and ~ 18.75 respectively for the non- hydrothermal sediments. Near bottom dissolved (<0.2 μm filter) water column Pb (with concentrations <2 pmol kg−1) at the ridge crest and in the near-downstream neutrally-buoyant hydrothermal plume show 206Pb/207Pb - 208Pb/206Pb trends that are lower than observed throughout the rest of the water column and fall within the mixing end- member isotope ratios of less radiogenic MORB and more radiogenic anthropogenic Pb sources. Thus there is a very small leakage (~1%) of primary high temperature hydrothermal Pb into the water column. But at the westernmost station (36), which shows a diluted continuation of the 3He, manganese (Mn), and iron (Fe) plume from 2200 to 2700 m, the >2200 m Pb isotope ratios deeper than 2000 m return to the non-hydrothermal ratios with no remaining detectable MORB.