Climatic reconstruction of the Late Palaeocene using sedimentary archives from the Bikaner-Nagaur Basin, Rajasthan, India

Abstract

The present study examines Late Palaeocene climatic changes recorded in the shale-lignite‑carbonaceous shale sequence of the Bikaner-Nagaur Basin, Rajasthan, India. Various analytical methods such as organic petrography, stable isotopes of organic carbon (δ13Corg), organic carbon to total nitrogen ratio (Corg/TN), Rock-Eval parameters, and Fourier transform infrared spectroscopy are employed to investigate the palaeoclimatic shifts. From the bottom shale to the overlying lignite sequence, δ13Corg shows minimal fluctuations, indicating a consistent supply of organic matter from both angiosperms and gymnosperms. However, a significant negative carbon isotopic excursion (nCIE; δ13Corg ranging from −26.8 to −29.9 ‰) is observed from the lignite to the overlying carbonaceous shales. This suggests a sudden increase in isotopically lighter CO2 or its higher partial pressure in the atmosphere during the Late Palaeocene. This nCIE is accompanied by a sharp decrease in the Corg/TN values and a notable increase in ash yield for the carbonaceous shales, suggesting peatland flooding by sediment-laden surface runoffs triggered by intense precipitation. The increased rainfall also raised groundwater levels, stabilizing hydrological balance within the mire. This facilitated the selective preservation of hydrogen-rich alginite under anoxic conditions, as indicated by geochemical proxy (relative hydrocarbon potential = 4.52, on average), contributing to elevated hydrogen index values (443 mg HC/g TOC on average) in the carbonaceous shale samples. The increased groundwater level is linked with the observed nCIEs in the carbonaceous shale samples, suggesting an abrupt climatic transition characterized by 13C-depleted atmospheric CO2 and intensified rainfall under warm-humid conditions during the deposition of these carbonaceous shales. These findings point towards potential influences of a Late Palaeocene global-scale hyperthermal event.