Quantifying mass inputs, sediment dispersal, and long-term sediment storage in the Holocene Bengal Basin

Abstract:

A high rate of sediment delivery (>1x10⁹ t/yr) and relatively efficient sediment trapping has built a robust sedimentary archive of Holocene change for the Bengal Basin. Sediment eroded from the Himalayas is transported to the delta by the Ganges and Brahmaputra rivers, where it is stored at a volume >7.5x10³ km³. We use grain size and bulk geochemical data for >5,000 samples from >400 boreholes collected across the basin to explore how changes in sediment and water supply and accommodation are reflected in sediment storage patterns. We partition the basin into 9 physiographic provinces and then at 5-m depth bins to quantify the mass and size fraction of stored sediment ranging from clay (0.49 μm) to coarse sand (1000 μm). Nearly ¼ of sediments are stored in the mud-rich subaqueous delta, which has been active since the mid-Holocene and serves as an offshore sink for sediments prior to export to the Bengal Fan. In the subaerial delta, while there is enhanced mud preservation in the surface stratigraphy, nearly 75% of the total mass stored is sand. The distribution of grain size fractions is not uniform across the delta but appears to be discretely distributed between river valley and interfluve settings. This suggests that the transport system may be self-organized within different settings and have a characteristic grain-size distribution, despite significant spatiotemporal variations in sediment supply. The average fraction of coarse sand in stored sediments of the entire Brahmaputra (Jamuna) valley remains ~10% throughout the Holocene. However, the coarse-sand fraction decreases downstream to just 1-2% of the total stored load. An exception to this downstream fining is reflected by a spike in coarse sand content in early Holocene sediments of the lower Jamuna valley, where the average fraction of coarse sand increases from ~3% to ~10% with up to 1.4x10¹⁰ t of coarse sand stored within a single 5-m depth interval across ~9.5x10³ km². Similar spikes in coarse sand also appear in deposits from the lower Ganges and Meghna rivers and suggest a basin-wide response. Mass inputs to the delta during this time exceed the average long-term rate by ~1.5x, and these preliminary results may demonstrate that the caliber of sediment reaching the delta between 12-8 ka was overall coarser than during the last 8 kyr.