Expanding our Use of Thorium-234 as a Particle Flux Tracer in the EXPORTS Program

Ken Buesseler1, Claudia R Benitez-Nelson2, Samantha Jade Clevenger1, Jessica Drysdale3, Montserrat Roca Martí4, Steven M Pike5, Laure Resplandy6, Blaire Umhau7 and Abigale Wyatt8, (1)Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, United States, (2)University of South Carolina, School of the Earth, Ocean, and Environment, Columbia, United States, (3)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (4)Universitat Autònoma de Barcelona (UAB), Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Cerdanyola del Vallès, Spain, (5)Woods Hole Oceanographic Institute, Woods Hole, United States, (6)Princeton University, Department of Geosciences, Princeton, NJ, United States, (7)Sea Education Association, Woods Hole, MA, United States, (8)Princeton University, Geosciences, Princeton, NJ, United States
The EXPORTS (Exports Processes in the Ocean from RemoTe Sensing) Program focuses on linking remotely sensed properties to the physical, chemical, and biological regime that controls the magnitude and composition of material that is transferred from surface waters to depth. To this end, we used the naturally occurring radionuclide thorium-234 (half-life = 24.1 d), as a tracer of sinking particle fluxes. The disequilibrium between 234Th and its soluble parent, uranium-238, provides quantitative information on particle export and remineralization at scales similar to the physical and biological processes that influence particulate organic carbon (POC) and associated elemental fluxes. We present final results from close to 1000 measurements of total and particulate 234Th in the water column as part of the first NASA supported EXPORTS cruise in 2018 the NE Pacific at Ocean Station PAPA. Our high resolution vertical sampling (13 -18 depths, 0-500m), and large number of stations (almost 60 profiles in 28 days) allowed us to map the spatial and temporal evolution of particle formation, remineralization, and sinking flux in a Lagrangian fashion that will be linked to other EXPORTS measurements. Remarkable consistency across water column total 234Th profiles suggests that variability in the processes that lead to particle flux are relatively small during this time of year. The POC/234Th ratios in particles decrease systematically with depth and are applied to calculate POC fluxes. Overall, the maximum POC flux is estimated to be 7.5 mmol C/m2/d at 50m, with POC flux decreasing by more than 50% between 60-110 m. These results support earlier studies showing that the NE Pacific has a low efficiency for the biological carbon pump.