A SOURCE-TO-SINK APPROACH TO UNDERSTANDING THE ROLES OF MASS EXTRACTION AND SEDIMENT LOAD DISPERSAL IN THE CONSTRUCTION OF STRATIGRAPHY IN THE HOLOCENE BENGAL BASIN

Sediment eroded from the Himalayas is transported by the Ganges and Brahmaputra rivers to the Bengal Basin, where >7.5x10³ km³ of sediment have been sequestered in the delta over the Holocene. The stratigraphic sequence of the Ganges-Brahmaputra delta (GB) can be used to interpret signals of changing mass inputs, river behavior, and accommodation over the last 12 kyr. We seek to use this record to test ideas related to sediment dispersal, mass extraction, and stratigraphic construction in one of the largest source-to-sink transport systems in the world. Average Holocene storage rates on the GB are ~1x10⁹ t/yr (roughly equal to modern mass inputs), with rates nearly 1.5x greater between ~8-12 ka. Grain size distributions for >5,000 samples help characterize the Holocene sediment package and how mass has been distributed to build the GB. We examine the GB holistically and through 5-m depth bins for 9 physiographic provinces to quantify the mass and caliber of sediment stored in delta depocenters. Nearly 75% of sediments stored in the subaerial GB are sand-sized, but the distribution of grain sizes is distinct between river valley and interfluve settings. The transport system may be self-organized within these settings and have a characteristic grain size distribution, despite differences in the river systems (i.e., Ganges vs. Brahmaputra) and significant spatiotemporal variations in sediment supply. There are also unique grain size distributions between the geomorphically distinct upper fan delta and lower tidal delta. The observed differences in grain size between the fan and tidal delta likely reflect downstream fining due to sediment mass extraction and preferential deposition of sediments based on size. An exception to this downstream fining is reflected by a spike in coarse sand (500-1000 μm) content in early Holocene sediments of the lower Jamuna (Brahmaputra), Ganges, and Meghna valleys. There, the average fraction of coarse sand in stored sediments increases from ~3% to ~10% in the Jamuna, from ~5% to ~16% in the Ganges, and from ~4% to ~13% in the Meghna, suggesting a basin-wide response and potentially, an overall coarser sediment load during this time. Insights from this study can inform modeling and prediction efforts with improved understanding of mass inputs and storage trends for the GB over the last 12 kyr.