Thermal Impact of Deccan Volcanism on Organic Sediments: A Key Factor in Late Cretaceous Climate Instability

 

The Chicxulub impact in Mexico and Deccan volcanism in India are both associated with the end-Cretaceous mass extinction, but the precise timing of the impact, volcanic eruptions, and environmental changes remains debated, hindering a comprehensive understanding of their respective roles. Geochronological data from Malwa Plateau basalts, located on the northern margin of the Deccan Large Igneous Province (LIP), show that the first pulse of Deccan volcanism coincided with a ∼200 kyr warming event in the Late Maastrichtian, recorded globally in contemporaneous stratigraphic sections. This warming, estimated at 2.5–8°C, has been inferred from δ18O studies on benthic foraminifera, pedogenic carbonate, bivalve shells, and leaf morphology. The timing of this excursion corresponds to the early decline in oceanic 187Os/188Os ratios and rising mercury (Hg) concentrations. The first phase of Deccan volcanism erupted through organic-rich Permian sediments in the Narmada-Tapti rift basin. Direct CO2 emissions from basalt are unlikely to account for the scale of warming observed, unless the eruption rates were extremely high, which conflicts with evidence suggesting lower eruption rates and a longer eruption duration. It is likely that thermal contact metamorphism of the sedimentary rocks was a significant source of CO2 that contributed to the Late Maastrichtian warming event. This study aims to investigate the fate of carbon (C), mercury (Hg), tellurium (Te), and sulfur (S) during the contact metamorphism associated with the first pulse of Deccan volcanism and to assess the role of this process in the global cycles of C, Hg, and S.Our data were derived from measurements of contact aureoles around dikes and sills intruding into organic-rich Permian coal deposits in the Narmada-Tapti rift basin. We focused on total organic carbon (TOC), Hg, Te, and S concentrations. While sediments farther from the intrusions show high TOC (>20%) and notable levels of Hg, Te, and S, samples from the aureoles (5-10 m thick) exhibit a near-total loss of these elements. Vitrinite reflectance values >5% indicate temperatures above 300°C in the aureoles. We present strong evidence for thermal alteration of coals and shales due to dike-sediment interactions, leading to the production of CH4 and CO2 gases. Furthermore, all analyzed sections show significant depletion of TOC, with a distinct zone of negligible Hg levels near the contact areas. Scaling these findings with data from Mittal et al. (2021) and Kubo Hutchison et al. (2023), assuming 50% TOC loss in the contact aureoles, a median dike width of 10 m, a median dike length of 1000 m, and a coal density of 1500 kg/m³, we estimate that approximately 900 Gt of carbon may have been emitted—comparable to the total extrusive outgassing from the Deccan Traps. Our preliminary results suggest that large igneous province (LIP)-scale sill and dike emplacement in organic-rich sedimentary rocks can significantly perturb global C, S, Te, and Hg cycles. Deccan volcanism likely contributed to climate instability during the Late Cretaceous and may have amplified the environmental impacts of the Chicxulub impact.