Preliminary Evidence of Transport‐Limited Chemical Weathering and Element Immobility in the Ganges Tidal Delta Plain of Bangladesh

Previous studies have documented a weathering‐limited regime in the upper reaches of the Ganges River Basin. Chemical weathering and element mobility at six sites in the lower reaches of the Ganges‐Brahmaputra tidal floodplain of southwest Bangladesh were investigated by comparing compositions of rice paddy soils, precursor tidal channel sediments, surface waters, and extract solutions, which represent the soluble fraction of solids. Little spatial variation in water and solid compositions is observed in each season, indicating similar processes are acting to transport elements across this region. Roughly one to several decades after deposition, rice paddy soils are not significantly different in mineralogy or composition from precursor tidal channel sediments, and both are similar to the composition of average upper continental crust. Soil and sediments contain mostly stable, cation depleted minerals, and coexisting waters are saturated in those minerals. There is no detectable change in composition of tidal channel water between upstream and downstream sites. Together, these observations indicate the dominance of weathered material and weak chemical weathering in the tidal floodplain, consistent with a transport‐limited regime. Multiple lines of evidence indicate a lack of exchange equilibrium between surface waters and coexisting solids, which may be a common feature in tidal river deltas where transport‐limited regimes likely dominate. Plain Language Summary Previous studies of the upper reaches of the Ganges River near the Himalaya have shown that an abundant supply of easily weathered minerals produced by erosion results in rapid chemical weathering. We measured the chemical compositions of water from rivers and rice paddies, and of associated river sediments and the soils that form from them, in the lower reaches of the Ganges River tidal delta plain in SW Bangladesh. We found that soils were similar in composition to river sediments and that river water compositions were constant across our study area. These observations suggest that little material is transferred from solids to water. This is either because the soluble minerals were dissolved in the upper reaches and are not present in sediment in the lower reaches or the water has already dissolved as much of the minerals as possible. Key Points In the Ganges tidal delta plain in Bangladesh, chemical weathering rates are low and transport‐limited Tidal channel sediments and rice paddy soils are s