Award: OCE-1736996

The goals of the project are: (1) to provide long-term intercalibration of Pb concentrations and Pb isotopes for US GEOTRACES (2) to obtain Pb isotope data from the southern section of the US GEOTRACES GP15 PMT section. The data will be used to evaluate the penetration of anthropogenic Pb into the Pacific Ocean and processes that affect it, and to evaluate mechanisms that control Cr stable isotope distributions in the marine environment. The latter also extends to support for geological interpretations of Cr isotopes in sediments. We have analyzed 450 samples from US GEOTRACES GP15 Alaska to Tahiti ocean section for Pb isotopes (204,206,207,208). This is probably the most detailed ocean section for Pb isotopes. We also obtained ~60 analyses for Pb concentrations from three stations. We also obtained Cr isotope data from a station near Tahiti. We are tracing the sources of anthropogenic (mainly) and natural Pb to the ocean, how they are transported within the ocean, and how they are evolving with time. At the time of sampling (2018) The uppermost waters north of 10.5?S are dominated by Pb emitted by China, which has a higher 208/206 ratio at a given 206/207 ratio. Much of the Chinese Pb is emitted from coal combustion. In the deep South Pacific, the Pb either comes dominantly from natural sources or South American sources. In the deep North Pacific, Pb has evolved on a decadal-century scale as the dominant sources were initially US-Australian but now are US-Chinese, transported by sinking particles that are reversibly exchanged. Under high productivity "particle veils" at the equator and ~42?N, the shift from natural-U.S. Pb to Chinese Pb in the upper ocean is sufficiently strong that lateral ocean mixing is overcome by the local flux from the surface. In the mid-depths (300-700m) of the South Pacific, there is a strong signature of Australian-type Pb showing as a strong minimum in 206/207 (and maximum in 208/207) in the northward transport of Sub-Antarctic mode water formed in the South Pacific. This Pb may either come from atmospheric transport of Pb from Australian sources or from the Indian Ocean transported to the South Pacific by the Antarctic Circumpolar Current. We have seen a snapshot of Pb evolution that a significant stage of the Pb evolution documenting significant U.S., Chinese, and Australian-type Pb sources. The Pb isotope distribution is sufficiently distinct in the deep waters under the high-flux particle veils that we can see a Pb isotope anomaly that overcomes the homogenizing influence of lateral ocean eddy mixing, quantitatively constrained. Last Modified: 08/16/2022 Submitted by: Edward A Boyle