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.