Stable Strontium Isotopic Compositions of River Water, Groundwater and Sediments From the Ganges–Brahmaputra–Meghna River System in Bangladesh

Abstract

The Sr isotopic composition of rivers and groundwaters in the Bengal Plain is a major contributor to the global oceanic Sr inventory. The stable strontium isotope ratios (δ88Sr) provide a new tool to identify chemical weathering reactions in terrestrial water. In this study, we investigated the spatiotemporal variations of δ88Sr in samples of river water, bedload sediment, and groundwater collected from the Ganges–Brahmaputra–Meghna drainage basin in Bangladesh, which is known to strongly influence the 87Sr/86Sr ratio in seawater. The average δ88Sr values of waters of the Ganges, Brahmaputra, and Meghna rivers were 0.269, 0.316, and 0.278‰, respectively. Our data showed little difference between seasons of high and low discharge. The δ88Sr values measured in sequential leaching fractions of sediments varied from –0.258 to 0.516‰ and were highest in the silicate fraction, followed in turn by the carbonate fraction and the exchangeable fraction. Both 87Sr/86Sr and δ88Sr of these waters are primarily controlled by the inputs of Sr in weathering products from the Bengal Plain and Sr from the Himalayan rivers (Ganges and Brahmaputra). Values of δ88Sr and Sr/Ca were higher in the Brahmaputra River than in the Ganges River, a difference we attribute to greater input from silicate weathering. The variations of δ88Sr and 87Sr/86Sr were greater in groundwater than in river waters. Mineral sorting effects and dissolution kinetics can account for the large scatter in 87Sr/86Sr and δ88Sr values. The depth profile of δ88Sr showed wide variation at shallow depths and convergence to a narrow range of about 0.31‰ at depths greater than 70 m, which reflects more complete equilibration of chemical interactions between groundwater and ambient sediments owing to the longer residence time of deeper groundwater. We found that δ88Sr values in the groundwater of Bangladesh were almost identical to those of river water from the lower Meghna River downstream of its confluence with the Ganges–Brahmaputra river system, thus confirming that the δ88Sr composition of the groundwater discharge to the Bay of Bengal is very similar to that of the river discharge.

The Influence of Weathering, Water Sources, and Hydrological Cycles on Lithium Isotopic Compositions in River Water and Groundwater of the Ganges–Brahmaputra–Meghna River System in Bangladesh

“…The similar magnitude of δ 7 Li fluctuation (3.7‰) have been reported in the outlet glacier of the Greenland Ice Sheet over a sub-weekly time series during the melt season (Hindshaw et al, 2019c). (Manaka et al, 2017;Manaka et al, 2019;Yoshimura et al, 2021). A red open symbol with a line represents the Ganges River's monthly water samples at Pakshi from October 2012 to August 2013.…”

“…Moreover, we conducted a monthly water sampling of the Ganges River at Pakshi from October 2012 to August 2013. Carbonate and major ion concentrations in these samples have been previously reported (Manaka et al, 2017(Manaka et al, , 2019Yoshimura et al, 2021).…”

“…Compared to the river water, the chemical compositions of groundwater, predominantly reflecting floodplain weathering in the Bengal Plain, vary widely, especially in the shallow groundwater. Groundwater tends to show high Na and K contents, suggesting that silicate weathering influences water chemistry more strongly than carbonate weathering (Yoshimura et al, 2021).…”

“…The downstream evolution of δ 7 Li in the Ganges River is described by Rayleigh fractionation with a fractionation factor of 0.996-0.980 (Bickle et al, 2018). Incongruent dissolution and sorting effects of silicate materials results in the high Sr/Ca ratios of the silicate fractions of bedload (Yoshimura et al, 2021). (Bagard et al, 2015;Manaka et al, 2017;Pogge von Strandmann et al, 2017).…”

“…The floodplain sediment load consists of incompletely weathered material that is delivered from high mountain areas. Continued weathering of Himalayan-derived sediment during its transport through the low-altitude plain makes a significant Yoshimura et al, 2021) showing geomorphologic units (Paul et al, 2010) as well as sampling locations for groundwater (blue triangles) and river water of the Ganges (red circle), Brahmaputra (yellow circle), upper Meghna (blue diamond), and lower Meghna (green hexagon) Rivers. Blue and red numbers represent δ 7 Li compositions of groundwater and river water, respectively.…”