Palaeovegetational reconstruction in Late Miocene: A case study based on early diagenetic carbonate cement from the Indian Siwalik

摘要

Carbon isotope ratio of early diagenetic carbonate cement (EDCC) from sandstones was measured from Mohand Rao (age: 9 to 4.5 Ma) and Haripur Khol (6 to 1.8 Ma) sections of Siwalik Group of India to reconstruct palaeovegetation. The delta(13)C of cement from Mohand Rao section varies from -10.5 parts per thousand to -0.2 parts per thousand with progressive increase in values from 9 to 7.3 Ma indicating gradual change of existing C-3 type vegetation to C-4 type vegetation. Post 7.3 Ma, the delta(13)C value is anchored around zero per mil indicating mixed C-3-C-4 environment with C-4 dominating the ecosystem. In Haripur Khol section, the delta(13)C value of EDCC from sandstone indicates presence of both C-3 and C-4 type of plants with dominance of C4 in the ecosystem, which corroborates the results of our previous study based on carbon isotope ratio of soil carbonates. The oxygen isotope ratio of EDCC from sandstones does not show any systematic variation with time. The delta(18)O values of EDCC from Mohand Rao section ranges from - 8.9 parts per thousand to - 13.6 parts per thousand and in Haripur Khol section, from - 9.9 parts per thousand to - 13.6 parts per thousand. At a given stratigraphic level, the average delta(18)O value of EDCC of sandstones is lower (up to 4 parts per thousand) compared to the average delta(18)O of soil carbonate from the same or adjacent level (using our earlier published data from Haripur Khol section). The depletion in O-18 of EDCC may be due to contribution of contemporary river water infiltrating the groundwater system in the post-monsoon period. Carbon and oxygen isotope ratio of soil carbonate nodules and carbon isotope ratio of associated organic matter from the same nodules were also measured from the Mohand Rao section in a few cases (n = 9). From 9 to 8 Ma, the carbon isotope ratio of soil carbonates varies from -10.8 parts per thousand to -7.8 parts per thousand indicating dominance of C-3 type vegetation in the flood plain. In contrast, from 5.4 to 4.8 Ma, the delta(13)C ranges from 0.1 parts per thousand to -4.3 parts per thousand indicating that the vegetation consisted of mixed C-3-C-4 plants with C4 dominating the ecosystem. The carbon isotope ratio of the organic matter from the same soil carbonate nodules ranges from -25.2 parts per thousand to -24.4 parts per thousand (from 9 to 8 Ma) and -17.4 parts per thousand to -24.6 parts per thousand (from 5.4 to 4.8 Ma) corroborating the above results. The average delta(18)O value of soil carbonate nodules for the time period 9 to 8 Ma is -8.8 parts per thousand; for 5.4 to 4.8 Ma, the value is - 7.9 parts per thousand. These average delta(18)O values are comparable with previously published oxygen isotope results of soil carbonates from Haripur Khol and Kangra valley. (c) 2005 Elsevier B.V. All rights reserved.