| 英文摘要: | The Paleocene-Eocene Thermal Maximum (PETM) is one of the most pronounced and widely documented global warming events in the geological history that occurred at the beginning of the Eocene (similar to 56 Ma) causing substantial changes in biota and geochemistry. It is marked worldwide by a negative isotopic excursion of delta C-13. The biotic changes associated with the PETM in marine realm include bloom of the dinoflagellate Apectodinium, turnover of larger foraminiferids, diversification of planktic foraminiferids, and carbonate dissolution of calcareous test shells, etc. while the geochemical changes include decrease in carbonates and increase in abundance of silicates and phyllosilicates, etc.
Conspicuously, there has been no attempt to identify the PETM in the Himalaya even though some Himalayan sections are known to have Paleocene-Eocene transition beds. To fill this lacuna, we investigated the basal part of the late Paleocene-middle Eocene Subathu Group exposed at the village Kurla near Subathu in Himachal Pradesh (NW sub-Himalaya) for biotic, mineralogical, and geochemical signatures of this abrupt warming event. The significant results of this study include carbon isotope excursion (CIE) of 3.4 parts per thousand, occurrence of index di-noflagellate genus Apectodinium, and carbonate dissolution of the larger benthic foraminiferids. The mineralogical changes noted across the Paleocene - Eocene transition include increase in quartz and phyllosilicates and decrease in carbonates. The geochemical changes include (i) increase in SiO2, Al2O3, K2O, and Fe2O3, (ii) decrease in CaCO3, (iii) decreasing trend of Si/Al, Fe/Al, and Mg/Al ratios, (iv) increasing trend of K/Al, Ti/Al, and Zr/Al ratios, (v) changes in trace element abundance, (vi) maximum chemical index of alteration (CIA) of 85-89%, and (vii) increase in abundance of rare earth elements. The aforementioned dataset is clearly very close to the PETM and even reflects its probable onset and peak phases, however, it is insufficient to definitively identify the PETM in the studied section. Nevertheless, since this study is the first from a Himalayan section and brings out considerable new information, all results and interpretations are presented and discussed here.
|