Sarin, M.M.¹, R. Rengarajan¹ and V. Ramaswamy²

¹Physical Research Laboratory, Ahmedabad 380 009, India and ²National Institute of Oceanography, Goa 403 004, India, E-mail: sarin@prl.ernet.in

Particulate organic carbon export in the Arabian Sea determined from the upper ocean ²³⁴Th/²³⁸U disequilibrium

Arabian Sea is known to be a region of strong seasonal reversals in surface circulation driven by monsoon winds that lead to extreme variability in biogeochemical processes. During the Indian JGOFS programme, we have used ²³⁴Th/²³⁸U disequilibrium in the water column to obtain information on particle and C_org export fluxes from the upper 100 m. Time-series measurements were also carried out at 21°N, 64°E during the late NE monsoon. The deficiency of ²³⁴Th in the upper 100 m (relative to equilibrium with ²³⁸U), at open ocean sites along the 64°E transect, varied as (4.7 to 7.2) x10⁴ dpm m^-2 over the annual cycle. The ²³⁴Th export fluxes during the late NE monsoon, intermonsoon and SW monsoon ranged as 1250 – 2170, 1345 - 1605 and 1620 – 2045 dpm m^-2 d^-1 respectively. The particulate C_org/²³⁴Th ratio in the shallow traps at 100 - 140 m averaged 20.3 and 43.3 µmol dpm^-1 during the intermonsoon and late NE monsoon. ²³⁴Th and C_org export in the northern Arabian Sea was relatively high, sustained by summer upwelling and convective mixing in the winter. At the 14-day time-series station, characterized by extensive salp swarm, ²³⁴Th deficiency centered around 8.1 x 10⁴ dpm m^-2; ²³⁴Th export and C_org/²³⁴Th averaged 2305 dpm m^-2 d^-1 and 10µmol dpm^-1 respectively. We, thus, estimate C_org export flux of 23 mmol m^-2 d^-1 from upper 100 m; representing ~30% of the primary production (70-82 mmol C m^-2 d^-1) and equals that exported during the entire NE monsoon. Such extreme variabilities and associated processes are the key issues to understand the upper ocean carbon export.