Palaeocene-Eocene organic sedimentary archives of Bikaner-Nagaur Basin, Rajasthan, India: An integrated revelation from biogeochemical and elemental proxies

The Palaeocene-Eocene organic sedimentary archives comprising lignites, carbonaceous shales, and shales from the Palana Formation of the Bikaner-Nagaur Basin, Rajasthan, India, are investigated in this study using elemental and biogeochemical proxies. The objectives of this investigation are to elucidate the sources of organic matter and their responses to palaeoredox and palaeoclimatic conditions, as well as to reconstruct the palaeoenvironment of the organic matter deposition. Elemental distributions, Rock Eval pyrolysis, stable carbon isotope, n-alkane and its ratios, acyclic isoprenoids and terpenoid biomarkers are employed as the tools to carry out this research. The hydrogen index shows that the samples contain type III, admixed type II-III and type II kerogen. The presence of admixed type II-III kerogen in the samples may indicate mixing of terrestrial plant-derived organic matter with autochthonous organic matter in a coastal depositional setting. Deposition in a coastal setting may also raise the possibility of organic matter supply from coastal vegetation. The large HI values (>300 mg HC/g TOC) may result from selective preservation of hydrogen-rich biomolecules derived from terrestrial and coastal vegetation. The stable carbon isotopic composition of bulk organic matter (−30.66 to −25.51‰) and the organic carbon to organic nitrogen ratio (26.62–131.12) may suggest that the organic matter was sourced primarily from C3 plants. The integration of these two parameters may also indicate that mangroves supplied significant amount of organic matter to the coastal peatland. The n-alkane distributions and carbon preference indices show the derivation of organic matter from waxy terrestrial plants, aquatic submerged vegetation, and microbial communities. The palaeohydrological proxies, i.e., proxy-aqueous ratio (0.24–0.86), proxy-wax ratio (0.33–0.84), and average chain length (26.32–28.30) may illustrate intermittent changes in organic matter sources (mangrove-dominated emergent vegetation and waxy plants to aquatic submergent plants and vice-versa) and their relations with shifts in palaeohydrological conditions of mire and associated palaeoclimatic oscillations between wet (high rainfall) and dry spells (low rainfall). Additionally, terpenoid distributions reveal organic matter input from angiosperms (including mangroves) (olean-12-ene, de-A-olean-13(18)-ene, de-A-lupane, and other de-A-triterpenoids), conifers (C18 diterpane, abietane, and ent-beyerane) and microbial sources (hopanoids). Besides, regarding the palaeodepositional environment, acyclic isoprenoid distributions (pristane and phytane), relative hydrocarbon potential, and the newly introduced palaeoredox factor may put forward mostly suboxic to oxic redox conditions of organic matter deposition. The total organic carbon and sulfur contents may further imply deposition of organic matter in topogenous-transitional mire conditions under a marginal marine/coastal environment. In complementary, the ranges of production index (0.01–0.19), Tmax (388–422 °C) and the presence of 17β(H),21β(H)-hopanes (ββ hopanes), 17α(H),21β(H)-homohopane, hop-17(21)-ene, ββ-hopanes, norhopanes and angiosperm-derived unsaturated triterpenoids may suggest thermal immaturity of organic matter in the studied samples.