A stable Strontium isotopic (δ88/86Sr) study of seawater from the Bay of Bengal, coastal groundwater from the Bengal Basin, and the Ganges, Brahmaputra, Mahanadi, and Godavari rivers in India

Abstract

Strontium (Sr) is an alkaline earth element that has four naturally occurring stable isotopes, 84Sr, 86Sr, 87Sr and 88Sr. During measurements of the radiogenic Sr isotope ratio (87Sr/86Sr), which is a widely used tracer in Earth sciences, the instrumental mass dependent fractionation is corrected by assuming the 88Sr/86Sr ratio as 8.375209. However, with the recent advancements in mass spectrometry, it is now possible to resolve variations in the 88Sr/86Sr ratio (expressed as δ88/86Sr) in natural samples. In this study, a measurement protocol for stable Sr isotope ratio measurements was developed using a double spike thermal ionization mass spectrometry (DS-TIMS) technique and δ88/86Sr variability in depth-bound seawater, groundwater, and large rivers are investigated. The analytical technique for high precision TIMS measurements of δ88/86Sr was developed using a new Sr double spike (87Sr-84Sr); the single spikes used to prepare this double spike were less pure compared to the spikes reported in literature and hence the DS of this study is less expensive and differs in isotopic composition from DS used in previous studies. The robustness of the method was validated by measurements of multiple standards of different matrices (carbonate, silicate, seawater, and pure Sr) and the long-term external reproducibility of δ88/86Sr was less than ± 0.035 ‰ (2SD) which is comparable to, if not better than previous studies. The δ88/86Sr values are not affected by the nature of the loading filament (tantalum versus zone refined rhenium) or amount of Sr loaded on the filament; further, the δ88/86Sr values are invariant over a wide range of spike/sample ratios, consistent with modelling calculations for error propagations. The δ88/86Sr values, relative to the NIST SRM987 standard, are reported for an Alfa Aesar ICPMS Sr standard (0.101 ± 0.033 ‰), NASS-6 seawater (0.387 ± 0.034 ‰), JCp-1 coral carbonate (0.196 ± 0.006 ‰), JCt-1 clam shell (0.252 ± 0.004 ‰), BCR-2 basalt (0.264 ± 0.003 ‰), and NIST SRM987 (0 ± 0.023 ‰). The Bay of Bengal, which has crucial control on marine Sr budget, receives large influx of continental Sr through the Himalayan rivers in form of surface flow with additional contribution from groundwater discharge. The present study reports restricted variability in the δ88/86Sr values (0.373-0.411 ‰) of seawater samples collected from multiple depths (0-1500 m) in the Bay of Bengal with an average δ88/86Sr = 0.388 ± 0.025 ‰ (2SD), which overlaps with previous estimates of global seawater. The chemistry of coastal seawater can be significantly influenced by groundwater and interactions between Bengal Basin groundwater and the adjacent Bay of Bengal seawater are well documented. This thesis presents δ88/86Sr values, 87Sr/86Sr, as well as stable Ca isotope (δ44/40Ca) data for a set of groundwater samples collected from multiple depths (14-333 mbgl) from coastal aquifers in and around the Sundarbans delta, India. Significant variabilities were observed in 87Sr/86Sr, salinity, and cation concentrations across depths which indicate seawater incursion at shallower depths (14-42 mbgl) while deep aquifer samples (~333 mbgl) retained freshwater signature. A substantial variability in δ88/86Sr values (~0.542 ‰) was observed in these groundwater samples with shallow aquifer samples showing high δ88/86Sr (up to 0.666 ‰), which is higher than modern seawater (~0.388 ‰). These variations along with modelling results suggest appreciable amounts of precipitation of secondary carbonates in the saline groundwaters along with indications of seawater-freshwater mixing. The δ44/40Ca values of the same samples show similar trends as the δ88/86Sr values and are consistent with removal of solute Ca as aragonites or calcites (up to 45%). Rivers are major transport pathways of weathering-derived continental Sr to the oceans and play a major role in modulating the overall Sr budget among modern reservoirs. The δ88/86Sr values in seasonally and spatially resolved river water samples from the lower Ganges (Hooghly River) varied from -0.167 to 0.418 ‰ while those in seasonally distributed river water samples from the Brahmaputra, collected from Guwahati and Jorhat, range between 0.136-0.304 ‰. Spatially and seasonally resolved river water from the Mahanadi Basin display δ88/86Sr values between 0.263-0.638 ‰; few samples from coastal regions overlapped with Bay of Bengal δ88/86Sr values underscoring seawater intrusion. The Mahanadi Basin groundwater samples display δ88/86Sr values between 0.276-0.423 ‰, which overlap with compositions of the Mahanadi River water, consistent with riverwater-groundwater interactions. Seasonally resolved river water samples from the Godavari River, collected from Rajahmundry, showed δ88/86Sr variability between 0.174-0.640 ‰. Overall, the significant variability in δ88/86Sr values of these large rivers broadly overlap with global river water data and reflect varying sources, congruent versus incongruent chemical weathering, with implications for role of water mass mixing and secondary minerals, such as carbonates and clays in river waters.