The Transatlantic Equatorial Cruise II (TRATLEQ-II) conducted in April – May 2022 was focused on upwelling conditions in the tropical Atlantic, its physical forcing, its importance for biological production, and plankton communities, associated chemical cycles, and the downward particulate organic carbon (POC) export. To achieve this goal, precise observations of POC export are needed. Here, we used the naturally occurring radiotracer ²³⁴Th-²³⁸U pair to estimate the magnitude of POC export and dynamics in the euphotic zone. The disequilibrium between this isotope pair allows estimating the POC export from the upper water column integrated over monthly time scales. The profiles of ²³⁴Th in the upper 500 m of the water column at 33 stations were obtained along the equator between 2°E and 44°W. Th-234 deficit was observed in the upper water column indicating the export of POC. Integration of ²³⁴Th deficit relative to ²³⁸U, observed in the productive surface layer yields ²³⁴Th flux, from which POC flux can be estimated by using the POC/²³⁴Th ratio on sinking particles. Particulate samples were collected at 10 stations, evenly distributed every 5° longitude between using in-situ pumps deployed at 6 depths per station to determine the POC/²³⁴Th ratio. The particulate organic carbon export from the euphotic zone and remineralization along the Equator determined using this approach will be presented and compared to estimates based on Underwater Vision Profiler data, also obtained during the cruise.

Primary Presenter: Wokil Bam, Woods Hole Oceanographic Institution (wokil.bam@whoi.edu)

Authors:

Beat Gasser, International Atomic Energy Agency, 4 Quai Antoine 1er, 98000 Monaco, Monaco   (B.Gasser@iaea.org)

Pere Masque, International Atomic Energy Agency, 4 Quai Antoine 1er, 98000 Monaco, Monaco   (P.Masque-Barri@iaea.org)

Ken Buesseler, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution  (kbuesseler@whoi.edu)

Rainer Kiko, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany  (rkiko@geomar.de)