PALEOENVIRONMENTAL INFLUENCE OF DECCAN VOLCANISM RELATIVE TO THE KT EXTINCTION

Deccan Traps erupted in three main phases with 6% total Deccan volume in phase-1 (C30n), 80% in phase-2 (C29r) and 14% in phase-3 (C29n). Recent studies indicate that the bulk (80%) of Deccan trap eruptions (Phase-2) occurred over a relatively short time interval in magnetic polarity C29r. U-Pb zircon geochronology shows that the main Phase 2 began 250 ky before the Cretaceous-Tertiary (KT) mass extinction, suggesting a cause-and-effect relationship.

In India a strong floral response is observed as a direct consequence of volcanic phase-2. Shortly after the onset of Deccan Phase 2, the floral association dominated by gymnosperms and angiosperms was decimated as indicated by a sharp decrease in pollen and spores coupled with the appearance of fungi, which mark increasing stress conditions as a direct result of volcanic activity. The inter-trappean sediments deposited in Phase 2 are characterized by the highest alteration CIA index values suggesting increased acid rains due to SO2 emissions. Closer to the eruption center, the lava flows are generally separated by red weathered horizons known as red boles, marking quiescent periods between basalt flows.

Red boles consist mainly of red silty clays characterized by concentrations of immobile elements such as Al and Fe3+ ions, typical of paleo-laterite that probably developed during the short periods of weathering between eruptions. At least 15 thick red bole layers are present in C29r below the KT boundary. All of these were deposited during phase-2 volcanic eruptions that occurred over a time span of about 250 ky. The short duration exposures of these red boles are reflected in the mineralogical and geochemical data that indicate rapid weathering (high CIA) linked to increasing acid rains.

Global geochemical effects of Deccan volcanism have been detected in the marine sedimentary record. At Bidart (France), high mercury (Hg) of volcanic origin characterizes the sedimentary interval located below the Iridium-rich layer where low magnetic susceptibility was hypothesized to result from paleoenvironmental perturbations linked to paroxysmal Deccan phase-2. These elevated Hg concentrations correlate with high shell fragmentation and dissolution effects in planktic and benthic foraminifera indicating that ocean acidification drastically affected marine biodiversity.