Hemipelagic and turbiditic deposits constrain lower Bengal Fan depositional history through Pleistocene climate, monsoon, and sea level transitions

Abstract

The Bengal Fan contains the most complete record of Himalayan climate and tectonics. As the largest fan system of the world it is fed by the Ganges-Brahmaputra river systems. International Ocean Discovery Program (IODP) Expedition 354 cored seven sites along an east-west core transect at 8°N in the lower Bengal Fan. We studied the Pleistocene sections of these sites to provide constraints on the depositional history of the last 1.25 Ma. Low-resolution bio- and magnetostratigraphic constraints identified two regionally-extensive hemipelagic units and intercalated turbidite deposits that provide a chronostratigraphic framework for the 320 km transect. Using a combination of lithostratigraphy and sediment physical properties that vary on orbital timescales, we tuned high-resolution data sets to the LR04 benthic δ18O isotope stack to obtain new constraints on the timing of depositional changes.Above the underlying Unit 1, which consists of Early to Middle Pleistocene turbiditic deposits, we divide four more units for the last 1.25 Ma: Unit 2, a Middle Pleistocene hemipelagic layer deposited at all sites from Marine Isotopic Stages (MIS) 37 to ∼17 (∼1.24–0.68 Ma) during the entire time of the Mid-Pleistocene Transition, when the Bengal Fan depocenter must have been distal to our transect; Unit 3, mostly turbidites with some intercalated hemipelgic sediments deposited from ∼MIS 16–∼13 (∼0.68–0.48 Ma), with sandy lithologies only found at the easternmost site on the west flank of the Ninetyeast Ridge; Unit 4, massive turbiditic sediments that started to dominate deposition with the Mid-Brunhes Transition (∼MIS 12) until MIS 7/8 (∼0.48–0.25 Ma), when the channel-levee system was focused east of the 85°E basement ridge at 8°N; and Unit 5, a Late Pleistocene hemipelagic layer at the top that spans MIS 7/8–1 (∼0.25 Ma to present) while the turbidite deposition was focused west of the 85°E basement ridge at 8°N. We find evidence that, on these timescales, deposition across the lower Bengal Fan changed with the evolution of Pleistocene climate, monsoon and sea-level history and hence responded to external controls on sediment accumulation and fan architecture rather than only fan-internal, autocyclic mechanisms.