Factors controlling frequency of turbidites in the Bengal fan during the last 248 kyr cal BP: Clues from a presently inactive channel

The seafloor of the Bay of Bengal is covered by thick sediment deposits that constitute the largest turbiditic system in the world. This system is fed primarily by the Ganges and Brahmaputra rivers, which drain the high Himalayan ranges. Sediment transfers from the delta to the deep-sea fan take place as turbidity currents in channel-levee systems. Previous studies have shown that, during high sea-level stand periods, the sediments were being mainly stored in the Ganges-Brahmaputra delta, and turbiditic transfer was occurring through active channels. Most of these channels are now inactive and sealed by hemipelagic deposits. However, the evolution of the inactive channels during the last sea-level variations has never been described in detail. Sedimentation in the currently active channel, the Active Valley, was particularly important during the last sea-level rise, which suggests a very good connection between the fluvial systems and the deep turbidite system at this time. During the MONOPOL cruise (2012), we retrieved a giant piston core (MD12-3412) near the currently inactive E4 channel. Previous studies have hypothesized that this channel is connected to the Swatch of No Ground canyon on the upper fan. The upper part of the core covers the last ~250 kyr. It reveals that, contrary to what is known about the Active Valley, the turbidite activity in E4 took place mainly during low sea-level phases (glacial stages), and stopped around 11.8 kyr cal BP. This different mode of activity suggests that (i) E4 was not abandoned but served as a secondary channel, and (ii) that the supply of turbidite material at the site of core MD12-3412 was not related to past changes in summer monsoon strength. Periods of activation of the E4 channel observed on core MD12-3412 were previously identified on the shelf area, by Hubscher and Spiess (2005), as thick Forced Regression System Tracts (FRST) after a displacement of deltaic edifices. High turbidites record on the deep basin are mainly synchronous with sea-level fall and rise conditions, but mostly during low sea-level periods. This could be explained by a residual connection between the coastal system and the E4 channel during sea level low stands.