Evidence of Gymnosperm-Dominant Forests in the Early Paleocene of Western India Based on Lipid Biomarkers

Summary The geoterpenoid composition of the Paleocene sediments from the Barsingsar lignite mine of Bikaner-Nagaur Basin (Rajasthan) have been studied with the help of gas chromatography-mass spectrometry (GC-MS) to understand their botanical origin. The terpenoids include sesquiterpenoids, diterpenoids and few triterpenoids. The sesquiterpenoids and terpenoids indicate higher plant-derived. The diterpenoid primarly comprising of abietane, phyllocladane, kaurane, pimarane/isopimarane and podocarpane-class compounds show affinities to Southern Hemisphere conifer families, Podocarpaceae and Araucariaceae. The tetracyclic diterpenoids include 18-norkaurane, ent-beyerane, ent-16s(H)-kaurane and 16a(H)-phyllocladane. The tricyclic diterpenoid, rosane and its derivative are identified. Exceptional void of phenolic abietanes and labdane derivatives rules out the probability of Pinaceae being the source. Low abundance in pentacyclic terpenoids like oleananes and their degraded forms suggest a negligible angiosperm contribution . The average value of pr/phy in the realm is 1.8 indicating dysaerobic depositional setting. The ar-AGI values for the samples range from 0.01 to 1. The t-AGI 1 and t-AGI 2 values range from 0.009 to 1.23 and 0.002 to 0.34 respectively indicating low angiosperm input. This work provided evidence of the occurrence of a gymnosperm dominated tropical rainforest as refugia in the midst of emerging angiosperms and a probable clue to biotic exchange out-of-India during the Paleocene.