Award: OCE-1736319

GEOTRACES is an international program which aims to map the concentrations and fluxes of key trace elements and their isotopes (TEIs), and to identify the processes controlling TEI distributions in the oceans. Interest in TEIs stems from their functions as nutrients or pollutants in ocean ecosystems, as well as their role in the global carbon cycle. Many processes in the ocean cannot be directly observed and as such, tracers are used to provide important constraints on their rates and pathways. In this study, we used the naturally occurring radioactive isotope Be-7 as a tracer to provide estimates of processes affecting the distribution and fate of trace elements and carbon along a transect of the Pacific Ocean from Alaska to Tahiti, French Polynesia in 2018. Following its formation in the upper atmosphere, Be-7 rapidly attaches to aerosol particles. Rain washes Be-7 from the atmosphere and deposits it on the ocean surface where mixing processes control its vertical distribution. Some important trace elements follow a similar path. For example, lead and aluminum from the continents are transported on aerosols and deposited via precipitation in remote parts of the ocean. Since Be-7 is radioactive and has a known half-life (53.3 days), it can be used to estimate atmospheric deposition rates of other trace elements. Using Be-7 measurements from the 2018 cruise together with data from other ocean basins, we derived a global relationship between rainfall rate and aerosol deposition velocities. Pending further testing, the relationship will allow for estimating fluxes of bioactive trace elements to remote areas using satellite rainfall products. These estimates are critical for evaluating limitations on phytoplankton growth, the strength of the biological carbon pump, and constraining ocean biogeochemical models. In the water column, Be-7 concentrations are constant within the well-mixed surface layer. Below this, where vertical mixing is slower, concentrations tail off and approach zero. The shape of the profile below the mixed layer, which is a result of radioactive decay and downward mixing, can be used to estimate the downward fluxes of nutrients, carbon and trace elements (e.g. sharper tailing in the Be-7 profile suggests slower mixing rates and thus lower fluxes). Another process that affects the Be-7 profile is upwelling, which brings nutrient rich, Be-7 ?dead? water to the surface. We used vertical mixing and upwelling rates estimated from Be-7 with profiles of oxygen and nutrients to estimate rates of carbon export from the surface layer. Along the cruise transect, the highest export rates were found in the subpolar zone, and the lowest rates were found within the subtropical North Pacific. Export to the deep ocean effectively removes carbon from the upper ocean and thus the atmosphere. Our carbon export estimates can be utilized in broader models of the atmosphere-ocean carbon system, and thus contribute to a better understanding of the role of the North Pacific in the global carbon cycle. Last Modified: 12/19/2022 Submitted by: Mark P Stephens