Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234

Thorium-234 is used in the Arabian Sea as a tracer of sinking particle fluxes. Samples were collected from January to August 1995 on four cruises during the Northeast Monsoon, the Spring Intermonsoon and the mid- and late-Southwest Monsoon periods. In this study, 234Th activity distributions are used to quantify the 234Th flux on sinking particles, and the measured ratio of particulate organic carbon (POC) to particulate 234Th is used to convert from 234Th to POC export at 100 m. The calculated POC fluxes range from <1 to >25 mmols C m-2 d-1, and strong seasonal and spatial gradients are observed. The single largest feature is a basinwide export maximum associated with the late-SW Monsoon cruise when POC export rates are 17–28% of the observed primary production rates along the southern sampling line. During all other cruises, this export ratio is <2–10%, with an increase near shore where POC fluxes are generally elevated. Also, during the Spring Intermonsoon, a POC export maximum is observed along the northern sampling line. Both this Spring export feature and late-SW Monsoon flux maximum appear to be associated with a phytoplankton community structure dominated by diatoms. The timing of the late-SW Monsoon flux peak agrees with the observed flux maximum in the deep moored time-series sediment traps (Honjo et al., 1998). This dramatic increase in export between the mid- and late-SW Monsoon also corresponds to measured decreases in the stocks of total organic C in the upper 150 m (Hansell and Peltzer, 1998) and a sharp decline in surface water Al and Fe (Measures and Vink, 1998). These 100 m flux results, plus a series of POC flux profiles, allow for a more complete understanding of the magnitude and timing of sub-euphotic zone export in the Arabian Sea.