Polonium-210 and Lead-210 as tracers of particle export and attenuation on the first EXPORTS cruise at Station PAPA

Montserrat Roca Martí, Universitat Autònoma de Barcelona (UAB), Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Cerdanyola del Vallès, Spain, Margaret L Estapa, Skidmore College, Saratoga Springs, NY, United States, Pere Masque, School of Science, Centre for Marine Ecosystems Research, Edith Cowan University, Joondalup, Western Australia, Australia, Claudia R Benitez-Nelson, University of South Carolina, School of the Earth, Ocean, and Environment, Columbia, United States and Ken Buesseler, Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, United States
The NASA supported EXPORTS (EXport Processes in the Ocean from RemoTe Sensing) Program focuses on linking remotely sensed properties to the mechanisms that influence the export of surface ocean primary production to depth. Here, we present results from the naturally-occurring radionuclide pair Polonium-210/Lead-210 as part of the first EXPORTS cruise in the NE Pacific at Station PAPA (August-September 2018). 210Po, with a half-life of 138 days, traces particle cycling processes on seasonal timescales. Its disequilibrium from 210Pb was used to quantify sinking particle fluxes and their attenuation below the euphotic zone and results compared to other methods of particle export that integrate shorter time scales: Thorium-234 (weeks) and sediment traps (days). A remarkable consistency between 210Po and 234Th profiles sampled over a period of three weeks suggests relatively small variability in particle fluxes at Station PAPA in the spring and summer. We measured net removal of both 210Po and 234Th on sinking particles from 0 to 75 - 125 m depth. Particulate organic carbon (POC) and other elemental (biogenic silica, nitrogen, inorganic carbon and phosphorus) fluxes were estimated using the POC (or other element) to 210Po ratios on size-fractionated particles collected using in-situ pumps and sinking particles sampled with surface-tethered and neutrally-buoyant traps. Overall, we show low POC export fluxes with a maximum of < 8 mmol C m-2 d-1 at 50 - 100 m in the euphotic zone. POC fluxes were rapidly attenuated below, leaving < 1 mmol C m-2 d-1 reaching 500 m. Station-to-station differences in 210Po and 234Th profiles, their derived elemental fluxes, and sediment traps are further explored to examine specific influences on particle export, such as changes in the biological environment and particle composition.