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 flux es. Samples were collected from January to August 1995 on four cruises duri ng the Northeast Monsoon, the Spring Intermonsoon and the mid- and late-Sou thwest Monsoon periods. In this study, Th-234 activity distributions are us ed to quantify the Th-234 flux On Sinking particles, and the measured ratio of particulate organic carbon (POC) to particulate Th-234 is used to conve rt from Th-234 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 a re observed. The single largest feature is a basin-wide export maximum asso ciated with the late-SW Monsoon cruise when POC export rates are 17-28% of the observed primary production rates along the southern sampling line. Dur ing all other cruises, this export ratio is <2-10%, with an increase near s hore where POC fluxes are generally elevated. Also, during the Spring Inter monsoon, 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 diatom s. The timing of the late-SW Monsoon flux peak agrees with the observed flu x 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 al so corresponds to measured decreases in the stocks of total organic C in th e upper 150 m (Hansell and Peltzer, 1998) and a sharp decline in surface wa ter Al and Fe (Measures and Vink, 1998). These 100 m flux results, plus a s eries of POC flux profiles, allow for a more complete understanding of the magnitude and timing of sub-euphotic zone export in the Arabian Sea. (C) 19 98 Elsevier Science Ltd. Ail rights reserved.