Do stable isotopes in carbonate cement of Mio-Pleistocene Himalayan sediments record paleoecological and paleoclimatic changes?

Abstract

Non-pedogenic carbonates, such as carbonate cement and nodules in the sandstones, are quite common in the terrestrial geological record. Unlike pedogenic carbonates, their stable isotope ratios lack investigations for paleoclimatic reconstructions. The present investigation therefore, explores the possibility of use of stable isotope studies of non-pedogenic carbonates from the Mio-Pleistocene Siwalik Group of sediments exposed in the Ramnagar sub-basin of the NW Himalaya. Petrographic studies reveal the dominance of micrite fabric in carbonate nodules both of pedogenic and non-pedogenic samples irrespective of specific stratigraphic unit. However, calcite as cement in the sandstones shows the dominance of micrite fabric in the younger in age sediments. Seventy-two non-pedogenic carbonate samples from the carbonate nodules and cement in the Siwalik sandstones, ranging in age between ~1Ma and 12.2Ma, were analyzed for δ13C and δ18O values. The δ13C values vary from −24.77‰ to −1.1‰ and δ18O values vary from −15.34‰ to −7.81‰. Pedogenic and non-pedogenic carbonates ranging in age between ~1Ma and 6Ma have largely similar δ13C values and the range of δ13C values indicate the dominance of C4 type of vegetation. However, unlike pedogenic carbonates which showed the dominance of C3 type of vegetation pre- 7Ma on the basis of δ13C –depleted isotopic values (Singh et al., 2011), δ13C values are largely enriched in the corresponding aged non-pedogenic carbonates revealing no information on specific type of vegetation. Likewise, paleoprecipitational reconstructions from δ18O values in pedogenic carbonates showed a progressive increase in aridity from ~12Ma to recent excluding short term increases in rainfall/monsoon intensity at around 10Ma, 5Ma, and 1.8Ma (Singh et al., 2012). On the contrary, such reconstructions are not possible from the δ18O values of non-pedogenic carbonates and indeed the δ18O values of non-pedogenic carbonates are largely depleted to as much as 6‰ from the corresponding pedogenic carbonates. Such differences in δ13C and δ18O values of non-pedogenic carbonates from pedogenic carbonates are primarily due to the dependence of the former on groundwater conditions responsible for precipitating carbonate. Further, a comparison of isotopic values between non-pedogenic and pedogenic carbonates can be interpreted that post-6Ma and pre-6Ma non-pedogenic carbonates were largely formed by shallow and deep groundwater conditions respectively.The result of these investigative studies therefore, suggests that the stable δ13C and δ18O values of non-pedogenic carbonates, unlike the pedogenic carbonates and irrespective of nature of calcite fabric, showed their little importance in paleoclimatic and paleoecological reconstructions.