Award: OCE-1737023

In this project we successfully analyzed over 400 seawater samples (about half of which analyzed at USM) and over 300 filtered particle samples (about a quarter of which analyzed at USM) for the naturally-occuring radionuclides Thorium-232, Thorium-230 and Protactinium-231 from the Pacific Meridional Transect, an expansive set of stations occupied between Alaska and Tahiti, also known as GEOTRACES section GP15. The research cruise was highly successful in sample collection and samples were subsequently distributed to the participating labs for onshore analysis. As data collection was being completed, we participated in several virtual and in-person workshops comparing data collected by other labs for different trace elements and isotopes, as well as a few ancillary workshops and field work opportunities. Overall the results for Th-232, Th-230 and Pa-231 reveal insights into Asian dust supply to the North Pacific Ocean, carbon flux to the deep sea, and geographical variations in trace element removal by particle scavenging (both from biogenic and hydrothermal particles). Previous work on the nature of trace element removal (or scavenging) in the North Pacific was based on scattered stations across this basin, whereas in this project we have a consistent dataset covering the major biomes of the North and tropical Pacific, including the subarctic, northern subtropical gyre, equatorial zone and southern subtropical gyre. With this transect we can also clearly see how hydrothermal plumes from the East Pacific Rise overprint the background scavenging distribution. These two effects are key to developing proxy interpretations of Th-230 as a tracer of particle flux, both today and in the geologic past as preserved in marine sediments. With regard to trace element supply, the Th-232 distributions demonstrate the long-term (decadal) average input of Asian dust to the North Pacific. The majority of the subarctic was elevated in Th-232, with relatively gradual decreases throughout the subtropical North Pacific, to quite low fluxes in the subtropical South Pacific. By extrapolating the Th-232 supply to Fe supply, this new data supports expected regions for the presence of diazotrophy or organisms that can perform nitrogen-fixation. The high Fe requirement of nitrogen fixation is supported more in the northern subtropical, low nitrate, Pacific with Asian dust, rather than the Fe-poor region of the southern subtropical, low nitrate, region. The Th-232 data is useful in that it represents decadal average inputs. The thorium-based dust estimates can provide good targets for atmospheric-ocean models testing dust-related hypotheses. In particular in the aerosol community, there is increasing awareness of aerosol sources other than mineral dust being important for iron delivery to the ocean, including biomass and/or fossil fuel burning. Thorium is likely to be particularly useful here as thorium is present in mineral dust but not present to a significant extent in biomass or fossil fuels, thus providing a dust-specific tracer of iron delivery. It may be possible to help de-convolve iron sources this way. The PI has benefitted in his teaching activities from the results of this project as well as the network on colleagues in the GEOTRACES programs, including the development of lecture material for a graduate course in Marine Chemistry and an undergraduate course in Marine Pollution. Through teaching and other outreach events including yearly participation in the Regional Ocean Sciences Bowl in Mississippi and local festivals where USM has a presence, this project benefits society in terms of environmental awareness. Our work can very broadly be described as looking for where nutrients (trace metals, namely) are added or removed from the ocean. Casual interactions with members of the public over time help to increase this environmental awareness, that the ocean has biological needs and that it may not always have these needs met. Beyond this public impact, this project led to the professional development of several participants ranging from undergraduates to professors, further supporting scientific careers within our society. Last Modified: 04/26/2023 Submitted by: Christopher T Hayes