AGE AND PALEOENVIRONMENT OF DECCAN VOLCANISM AND THE K-T MASS EXTINCTION

Deccan volcanism has long been proposed as the key to global climate change, mainly as a result of CO2 and SO2 gases released to the atmosphere, and by implications a decisive contributor to the end-Cretaceous mass extinction. But a clear link between volcanism, climate and extinctions has remained elusive mainly due to large (1%) uncertainties in radiometric dating that preclude assessment of the environmental impact of volcanism. We have now succeeded to obtain high-resolution age control from the intertrappean sediments between the two largest Deccan Trap flows that extended 1000 km across India and erupted in C29r and C29n. Our results are from four outcrops in the Krishna-Godavari Basin and based on planktic foraminifera and nannofossils. We report two main age results: (1) the end of the most massive phase of Deccan volcanism coincided precisely with the K-T mass extinction. And (2) the last Deccan volcanic phase occurred in the early Danian zone P1b (lower C29n) just prior to the long delayed post-KT marine recovery. Volcanism may have played a decisive role in both the K-T mass extinction and delayed biotic recovery. Comprehensive paleontologic, sedimentologic and mineralogic studies of the intertrappean sediments indicate deposition occurred during the early Danian zone P1a in varied environments ranging from shallow estuarine, restricted and open coastal to marginal and open marine settings punctuated by periods of subaerial deposition marked by paleosoils. Mineralogic studies indicate paleoclimates that varied from semi-arid to seasonal and humid, punctuated by tempestites. Marine life indicates highly stressful conditions through zone P1a, as expressed by low diversity, dwarfism and deformed benthic foraminifera. These stresses are partly due to local conditions, though similar high-stress assemblages are well known from the early Danian globally with recovery in C29n apparently after the last phase of Deccan volcanism. These results demonstrate the importance of volcanism in the K-T mass extinction and post-K-T recovery. The combined effects of Deccan volcanism and a large K-T impact (i.e., Ir anomaly) remain to be evaluated. The Chicxulub impact, commonly assumed to be the K-T impact, predates the mass extinction by 300,000 years and may have coincided with an earlier phase of Deccan volcanism.