Sr, C and O isotopes in carbonate nodules from the Ganga Plain: evidence for recent abrupt rise in dissolved ⁸⁷Sr/⁸⁶Sr ratios of the Ganga

Abstract

The evolution of ⁸⁷Sr/⁸⁶Sr ratio of the Ganga water during the past ˜ 100 ka has been reconstructed in this study. The ⁸⁷Sr/⁸⁶Sr, δ¹³C and δ¹⁸O values have been measured in carbonate nodules from different depths of two sediment cores raised from the Indian Institute of Technology Kanpur (IITK) campus (50 m long) and Jagdishpur (JP, 25 m long) respectively. The ⁸⁷Sr/⁸⁶Sr ratios of the carbonate nodules range from 0.7142 to 0.7189 in the IITK and 0.7142 to 0.7367 in the JP cores. The nodules in general, display significantly lower ⁸⁷Sr/⁸⁶Sr ratio compared to contemporary Ganga river water at Kanpur, however, values of ⁸⁷Sr/⁸⁶Sr ratio are observed in the nodules of the JP core near the surface are consistent with that of the present day Ganga water at Kanpur as well as groundwater samples from adjacent areas indicate recent abrupt increase in ⁸⁷Sr/⁸⁶Sr ratio of the Ganga. These findings are also consistent with the concomitant increase in ⁸⁷Sr/⁸⁶Sr ratio of silicates in contemporary sediments of the Ganga at Kanpur compared to that of past ˜ 100 ka. The sudden rise of ⁸⁷Sr/⁸⁶Sr ratio of the Ganga is probably due to increase in the relative proportion of Sr from the Lesser Himalaya containing silicates and carbonates with higher ⁸⁷Sr/⁸⁶Sr ratio. The cause for the recent increase in Sr contribution from the Lesser Himalaya is not clear; this could be due to enhanced agricultural activities and deforestation and or climatic variability during recent times resulting in more erosion in the Lesser Himalaya. The δ¹³C and δ¹⁸O, measured in the nodules from the IITK core, vary from - 6.8 to + 1.6%. and - 8.3 to - 5.4%. respectively. The co-variation of δ¹³C and δ¹⁸O suggest the impact of paleoclimate/paleovegetation. Petrography and chemical composition of carbonate nodules indicate little or no diagenetic alteration. Research highlights:› ⁸⁷Sr/⁸⁶Sr of the Ganga water was low during the last 100 ka except the recent past. › Recent increase in ⁸⁷Sr/⁸⁶Sr could be due to recent climatic/anthropogenic impact. › These recent activities could have resulted more weathering in the Lesser Himalaya. › Role of Himalayan rivers towards marine ⁸⁷Sr/⁸⁶Sr budget needs to be reevaluated. › This requires better constraint on the temporal variation in ⁸⁷Sr/⁸⁶Sr of the Ganga.