Inorganic and organic deposition and preservation in the Ganges-Brahmaputra (G-B) delta: trends through the Holocene and with recent anthropogenic perturbations

Elucidating sediment distribution patterns in deltas is needed for understanding delta development and sustainability, and unraveling the impacts of anthropogenic modification (e.g., dams, dikes, canals, land used change) as these could exacerbate subsidence and flooding to coastal communities and impact global carbon budgets. We present an overview of organic and inorganic deposition and preservation in the G-B delta revealed from sediment boreholes, and address recent anthropogenic perturbations. Where the G-B rivers discharge into the Bengal Basin, fluvial fan deltas have formed from the aggradation of laterally mobile braided channels, resulting in the wide-scale deposition and preservation of bed- and suspended-load sand. Below the backwater transition, channel mobility is limited, sedimentation is predominantly via overbank flooding: fine-grained mud and organic-rich successions are concentrated within broad interdistributary basins. Borehole analyses reveal these mud and peat deposits are 5-30 m thick and superimposed above massive fine-grained sand, indicative of transgression of the fluvial fan with sea-level rise throughout the Holocene. Further, anthropogenic activity in recent decades has perturbed these processes: river embankments preclude sediment delivery, and in the historical Golpalganj-Khulna peat basins, >300 km²has been converted to rice paddy or aquacultural ponds, potentially remineralizing significant volumes (327 x 10^8 m3) of peat.

In the tidal deltaplain, the stratigraphy similarly comprises 5–20 m of fine-grained silts, yet these are tidally deposited and bioturbated, and overlie river-mouth sands. Although peat and wood preservation occurs (Allison et al. 2003, Khan & Islam 2008), in general, little peat is found here. Preliminary data from shallow piezometers reveal seasonal drying of the shallow subsurface likely produces oxidizing conditions within the tidal deltaplain, remineralizing organics. Anthropogenic modification is also rampant in this part of the delta, and recent work by Auerbach et al (2015) documented that elevation loss due to embankment construction severely increases the risk of coastal communities to storm surge or even regular tidal flooding.