Revisit of ballast hypothesis in the Bay of Bengal

Abstract

The ballast hypothesis involving rapid sinking of organic carbon in association with riverine mineral particles is proposed in the Bay of Bengal (BoB) in 1991. The ballast hypothesis was used intensively to explain several biogeochemical processes, such as low primary production, weak oxygen minimum zone (OMZ), low bacterial respiration rates and lack of denitrification in the BoB. In contrast, the recent measurements indicated that high primary production, intense OMZ with occurrence of denitrification in the sinking particles but not in the water column. Hence the ballast hypothesis is re-visited using the recent experiments conducted on sinking particle flux using sediment traps, 234Thorium based particulate organic carbon export, particle back scatter, water column biogeochemistry, stable isotopic composition of carbon and nitrogen of sinking particles and surface sediment of shelf region. The isotopic data suggests the sinking organic matter is mainly contributed by in-situ production supported by dissolved organic nitrogen (DON). The amount of organic matter decomposed within the water column was higher in the north and decreased towards southern BoB and it is consistent with the spatial pattern of rate of sinking particle flux suggesting against ballast hypothesis of removal of organic matter to the sediment with weaker modifications in the water column. The higher organic carbon trapped in the middle and deep than shallow traps was observed and it is attributed to cross-shelf transport of sedimentary organic carbon as evidenced from the back-scatter of particles and isotopic composition carbon and nitrogen of shelf sediments. Variations in the river discharge did not show impact on the magnitude of sinking carbon fluxes indicating that river discharge is not a drive force for higher sinking carbon fluxes to deep BoB than hitherto hypothesized as ballast effect.