Enormous Ganges-Brahmaputra sediment discharge during strengthened early Holocene monsoon

Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence d evelopment. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing clim atic regimes. In this study we present the first quantified estimate of sed iment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11 000 yr B.P., when rising sea level flooded the Bengal basin, t hereby trapping most of the river's discharge on the inner margin. Chronost ratigraphic data from these deltaic deposits are used to calculate the rate s of sediment storage on the margin, which provide a minimum estimate of th e river's past sediment load. Results reveal that similar to5 x 10(12) m(3) of sediment was stored in the Bengal basin from ca, 11 000 to 7000 yr B.P. , which corresponds to a mean load of 2.3 x 10(9) t/yr. In comparison, mode rn sediment load of the Ganges-Brahmaputra is similar to1 x 109 t/yr, ranki ng it first among the world's rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a strong er than present southwest monsoon in South Asia. Similar patterns of high m onsoon-related sediment discharge have been noted throughout the tropics an d subtropics, suggesting a widespread fluviosedimentary response, the poten tial magnitude of which is show-cased by the Ganges-Brahmaputra system.