Upper Ocean Carbon Export Flux Estimation in the East Indian Ocean Using 234Th

Abstract

The strength of the Biological Carbon Pump (BCP) to sequester atmospheric CO₂ in the East Indian Ocean is unclear due to lack of studies. Here, we estimated Particulate Organic Carbon (POC) export flux by using ²³⁴Th as a flux proxy in the upper Indian Ocean (0–300 m depth), including the East Indian Ocean. In seawater, the soluble parent radionuclide, ²³⁸U (t_1/2 = 4.47 × 10⁹ yr) decays to produce a particle-reactive daughter, ²³⁴Th (t_1/2 = 24.1 d), which surface adsorbs onto particles, and sinks from the euphotic zone to the sea bottom. Disequilibrium between ²³⁸U and ²³⁴Th in seawater and POC/²³⁴Th ratio in sinking particles is used to estimate POC export flux. In this study, euphotic depth integrated ²³⁴Th deficit fluxes and the estimated POC export flux varied from negligible to 2,025 ± 87 dpm m⁻² d⁻¹ and negligible to 6.6 ± 0.6 mmol C m⁻² d⁻¹, respectively. The BCP efficiency varied from negligible (in coastal Arabian Sea) to 14% (near equator), except for the Andaman Sea (0%–80%). Temporal decoupling of primary productivity and POC export flux in the Andaman Sea resulted in a high export ratio. Compilation of spring intermonsoon ²³⁴Th based POC export flux and export efficiency from JGOFS and GEOTRACES showed high export flux and efficiency in the open Arabian Sea and in the Equatorial Indian Ocean but low POC export flux and efficiency in the Bay of Bengal, Andaman Sea, East Indian Ocean, and South Indian Ocean. Although low in magnitude, the Equatorial Indian Ocean sequesters atmospheric CO₂ like the equatorial- Atlantic and Pacific Oceans.