The role of the Ganges-Brahmaputra mixing zone in supplying barium and ²²⁶Ra to the Bay of Bengal

The Ganges-Brahmaputra river system is ranked fourth among world rivers as a source of freshwater to the oceans and is believed to supply more sediment to the ocean than any other; 1.5 × 10¹² kg/yr (Milliman and Meade, 1983). Barium and ²²⁶Ra are typically enriched in waters where sediment-laden rivers enter the ocean. As such, the Ganges-Brahmaputra is likely to produce globally significant barium and ²²⁶Ra fluxes to the ocean. Water samples for barium and ²²⁶Ra were collected within four major channels of the Ganges-Brahmaputra mixing zone during a period of low sediment and freshwater discharge. The data suggest that, in addition to suspended sediments supplied directly from rivers, river sediments deposited during high discharge in mangroves and on islands are desorbing barium and ²²⁶Ra to seawater. The release of barium and ²²⁶Ra from these sediment deposits is out-of-phase with the direct supply of sediments from the rivers. Estimates of the annual fluxes of barium and ²²⁶Ra from the Ganges-Brahmaputra mixing zone were also derived. The fluxes of barium and ²²⁶Ra are 5.3 × 10⁸ mol barium/yr and 9.5 × 10¹⁴ dpm radium/yr. The first silicate and phosphate mixing profiles for this system are reported.