Bengal arsenic, an archive of Himalaya orogeny and paleohydrology
Résumé
Holocene groundwater in many districts of the West Bengal and parts of Bangladesh are enriched in arsenic enhancing poisoning effect on humans. One of the main problems to depict the source of arsenic is that this element is very mobile and can be easily removed and recombined from the source during alteration processes, transport and mobilization in sediments. The Ganga-Brahmaputra river system mainly contributed to the buildup of the Bengal fan, which is considered one of the largest modern deltas of the world, then the possible source of the As has probably to be search within the Himalayan belt. We propose that the Indus-Tsangpo suture zone dominated by arc-related rocks and more particularly by large volume of serpentinites enriched in arsenic could be one of the primary source of arsenic. The fact that, the present day arsenic concentration in the main Himalayan river, and particularly the Siang-Brahmaputra river system is not so high as expected can be explained by strong aridic conditions present day prevailing in the Indus-Suture zone and do not favored the weathering of serpentinites into As rich-smectite and Fe-hydroxydes. For the Ganga basin, the original source of arsenic has to be search in the weathering of arc related rocks in the Indus-Tsangpo suture zone followed by its intermediate storage into the sediments of the Siwalik foreland basin, playing the role of arsenic reservoir from Miocene to Pleistocene. Intense tectonic activity in the front of the Himalayan belt associated with high rainfall conditions during the Holocene allowed the arsenic to be remobilized and transported toward the Bay of Bengal.