Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: Weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic 87Sr/ 86Sr

Abstract

The concentrations of Rb and Sr and 87Sr/ 86Sr isotopic ratios have been measured in the dissolved load of the Ganga-Brahmaputra (G-B) river system. The Ganga was sampled extensively from its source at Gangotri (in the Higher Himalaya) to Patna (on the alluvial plains). The Brahmaputra was sampled in its stretch in Assam, in India. The average Sr concentration in the Ganga (at Patna) is 1.2 μmol/l and that in the Brahmaputra (at Goalpara) is 0.73 μmol/l; the mean 87Sr/ 86Sr ratios are 0.7239 and 0.7192, respectively. The 87Sr/ 86Sr in the Ganga source waters (the Alaknanda, the Bhagirathi and their tributaries) range between 0.7300 and 0.7986, considerably higher than the global average runoff value of 0.7119. The high 87Sr/ 86Sr in the Ganga source waters result from the intense weathering of Precambrian granites and gneisses enriched in radiogenic Sr. The Sr isotope systematics of the Ganga waters is dominated by silicate weathering, whereas carbonate weathering plays a significant role in their major ion chemistry. The G-B system transports about 910 million moles of dissolved Sr annually to the Bay of Bengal, with an average 87Sr/ 86Sr of 0.7213. The Sr transported by the G-B system is about 2.7% of the global dissolved Sr flux to the oceans via rivers. Model calculations reveal that the G-B system has contributed significantly to the Sr isotope evolution of seawater during the past ∼ 20 Ma. The flux of Rb through the G-B system is ∼ 24 million moles per year. This is about 5% of the global river supply of Rb to the oceans, nearly twice the contribution of water via the G-B system to the oceans. Our study suggests that the marine geochemistries of Sr and Rb (and possibly U) may have been influenced considerably by the Himalayan orogeny.