Changing sedimentary environment during the Late Quaternary: Sedimentological and isotopic evidence from the distal Bengal Fan

The sediments recovered from two gravity cores of the lower and distal Bengal Fan were investigated for sedimentological properties and Sr-Nd isotopes. Each core exhibits two distinct units, the lower unit 2 and upper unit 1 sediments. The unit 2 sediments are predominantly olive black/grey in colour with abundant finer silt-size fractions, low organic carbon and CaCO3, quartz and mica in the coarse fraction, dominant illite and chlorite in the <2 μm fraction and uniform rock-magnetic properties. Biogenic constituents are extremely rare or restricted to the lower part of unit 2. The unit 1 sediments, on the other hand, are moderate brown/yellowish brown in colour with intermittent thin dark-coloured sediment layers. Higher clay, organic carbon, CaCO3, and biogenic constituents in the coarse fraction, and enriched smectite and kaolinite in the <2 μm fraction are typical. Magnetic susceptibility values are higher and correlate well with acid-insoluble residue content. Higher Rb, Sr, Sm and Nd concentrations, 87Sr/86Sr ratios and more radiogenic εNd values are characteristic for unit 2 sediments compared to unit 1 in both the cores. The unit 2 sediments represent Pleistocene hemiturbidites, older than 13 14C kyr BP with their source from the northern Bay of Bengal (NBOB), derived from the Himalayas and transported by the Ganges-Brahmaputra (G-B) River system. Unit 1 sediments are calcareous pelagic sediments, which started depositing ∼12 14C kyr BP, with its clastic sediments derived from the Himalayas and SE Indian/Sri Lankan margins. The change in lithofacies from unit 2 to unit 1 suggests that the sediment deposition by turbidity current activity ceased in the distal Bengal Fan at ∼12 14C kyr BP, perhaps because of the rapid rise in sea-level during the melt water pulse 1A and Holocene.