Neoproterozoic-Early Cambrian biota and ancient niche: A synthesis from molecular markers and palynomorphs from Bikaner-Nagaur Basin, western India

Abstract

The time frame of present study is Neoproterozoic-Early Cambrian, an enigmatic and consequential time in the history of Earth. The present study elucidates the terminal Proterozoic biota and palaeoenvironment by characterizing the soluble and insoluble fractions of organic matter extracted from sediment core samples recovered from Marwar Supergroup, Bikaner-Nagaur Basin, western India. The normal alkanes range from C15 to C33 wherein C19 to C21n-alkanes predominate and carbon preference index ranges from 1.12 to 1.19. Pristane/phytane ratio varies between 0.9 and 1.2. The kerogen matter occurs as homogenous laminae and discrete organic remains which are mostly derived from microbial mat. Triterpenoids include cheilanthanes and hopanes which are confirming evidences of microbial input. Algal presence has also been confirmed from palynological data. Green algae evolved to be a major biota during the Neoproterozoic-Early Cambrian time and this is validated by the appreciable abundance of C29 steranes in the bitumen fraction. The amorphous pieces of kerogen residues lack any visibly distinct internal features. However, a very few simple ornamented acritarchs like Micrhystridium sp., Lophosphaeridium sp. and Leiosphaeridia sp. are observed in the organic residues. Micro-FTIR spectroscopy of the amorphous organic particles reveals the presence of long chain aliphatic components and unusually high CC aromatic ring stretching band. The organic remains were deposited in restricted marine water and elevated saline condition as indicated by several salinity biomarkers. The present study clearly demonstrates a microbial mat dominated niche with significant eukaryotic green algae that had been capable of surviving in the high salinity of the terminal Proterozoic oceans.