POC export fluxes in the Western Indian Ocean based on 234Th as tracer

Abstract

Thorium-234 (234Th; half life = 24.1 days) has been used as an excellent tracer of element scavenging, particle settling, and the export of particulate organic carbon (POC) in the ocean. We measured the vertical and horizontal distributions of particulate and total phases of 234Th, POC, and other oceanographic parameters during July 5-22, 2017 and April 6-25, 2018 in the Western Indian Ocean along the transects (60°E and 67°E). In the study region, Seychelles-Chagos Thermocline Ridge (SCTR) was observed depending on the latitude (5°S-15°S). Interestingly, 234Th activities in the SCTR were relatively lower than those observed in the other regions. POC fluxes (7.62 ± 6.33 mmol C m-2 d-1) based on 234Th fluxes in the SCTR also were higher in the other regions. These high POC fluxes were similar to the observed in the North Pacific and Arabian Ocean where the primary production is much higher than in the Western Indian Ocean. We propose that the higher POC fluxes in the SCTR should be due to the upwelling of subsurface water with nutrients.