Conifer-mixed tropical rainforest in the Indian Paleogene: New evidences from terpenoid signatures

Abstract

The Paleocene sediments from Barsingsar lignite mine section of Bikaner-Nagaur Basin in Rajasthan, India were subjected to gas chromatography–mass spectrometry (GC–MS) analysis to resolve its botanical history in relation to its terpenoid composition. The terpenoid suite was characterised by sesquiterpenoids, diterpenoids, and sporadic triterpenoids. The presence of sesquiterpenoids along with drimane, norcadalenes, cadalenes and their related compounds hint to higher plant input. High abundance of diterpenoids and their temporal universality imply a significant gymnosperm input. The diterpenoid-class of terpenoids, primarly comprises of conifer-derived abietane, phyllocladane, kaurane, pimarane/isopimarane and podocarpane-class compounds. It further point to Southern Hemisphere conifer families, Podocarpaceae and Araucariaceae. Exceptional void of phenolic abietane and labdane derivatives rule out the probability of Pinaceae as the source of these terpenes. A very low abundance of pentacyclic terpenoids like oleanane-derivatives suggest low angiosperm-derived terpenoid contribution. The average value of pristane/phytane in the realm, 1.7 indicated oxic to suboxic depositional setting. The values of ar-AGI (aromatic Angiosperm/Gymnosperm Index), t-AGI 1 and t-AGI 2 spans over 0.01 to 1, 0.009 to 1.23, and 0.002 to 0.34 respectively for the section. The research provided evidence of the occurrence of a tropical rainforest where conifers were a significant element or remained a refugia in the midst of emerging angiosperms as life stabilized in the Paleogene after the devastating Cretaceous-Paleogene (K/Pg) boundary.