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

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 (δ¹³C_org), organic carbon to total nitrogen ratio (C_org/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, δ¹³C_org shows minimal fluctuations, indicating a consistent supply of organic matter from both angiosperms and gymnosperms. However, a significant negative carbon isotopic excursion (nCIE; δ¹³C_org 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 C_org/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.