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

摘要

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 NWHimalaya. 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 ~1 Ma and 12.2 Ma, were analyzed for δ~(13)C and δ~(18)O values. The δ~(13)C values vary from ?24.77‰ to ?1.1‰ and δ~(18)O values vary from ?15.34‰ to ?7.81‰. Pedogenic and nonpedogenic carbonates ranging in age between ~1 Ma and 6 Ma have largely similar δ~(13)C values and the range of δ~(13)C values indicate the dominance of C4 type of vegetation. However, unlike pedogenic carbonates which showed the dominance of C3 type of vegetation pre- 7 Ma on the basis of δ~(13)C –depleted isotopic values (Singh et al., 2011), δ~(13)C values are largely enriched in the corresponding aged non-pedogenic carbonates revealing no information on specific type of vegetation. Likewise, paleoprecipitational reconstructions from δ~(18)O values in pedogenic carbonates showed a progressive increase in aridity from ~12Ma to recent excluding short termincreases in rainfall/monsoon intensity at around 10Ma, 5Ma, and 1.8Ma (Singh et al., 2012). On the contrary, such reconstructions are not possible fromthe δ~(18)O values of non-pedogenic carbonates and indeed the δ~(18)Ovaluesof non-pedogenic carbonates are largely depleted to as much as 6‰from the corresponding pedogenic carbonates. Such differences in δ~(13)C and δ~(18)O 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-6 Ma and pre-6 Ma non-pedogenic carbonates were largely formed by shallow and deep groundwater conditions respectively. The result of these investigative studies therefore, suggests that the stable δ~(13)C andδ~(18)O 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.