EARLY TO LATE MAASTRICHTIAN ENVIRONMENTAL CHANGES LINKED TO DECCAN TRAPS AND NINETYEAST RIDGE VOLCANISM: CASE STUDY OF THE INDIAN OCEAN

The Maastrichtian (C31-C30n) was a time of major environmental changes that record evolutionary diversification in planktic foraminifera as well as a minor extinction associated with climate change and high carbonate dissolution. Although these changes have been observed worldwide, their cause(s) remain speculative. Here we report on these events based on Ninetyeast Ridge sites, Indian Ocean, investigate their links to Deccan and Ninetyeast Ridge volcanism and correlate the results with sections around the world. Methods include high-resolution quantitative planktonic foraminiferal biostratigraphy and oxygen and carbon stable isotopes analysis.

The Campanian cooling reached its maximum near the Campanian-Maastrichtian boundary (planktic foraminifera zone CF8, base C31r) and was followed by species diversification and a gradual increase in δ¹³C marking enhanced primary productivity (Event I, zones CF8-CF7). Volcanism associated with the Ninetyeast Ridge may have been an important source of nutrients that created favorable environmental conditions that led to this diversification.

Maximum δ¹³C and negative δ¹⁸O values in zone CF6 (top C31r) mark high productivity and climate warming correlative with a rapid increase in species diversity (Event II). The subsequent cooling and decrease in δ¹³C marks maximum Cretaceous diversity around the CF5-CF4 transition (Event III, C31r-C30n transition). In zone CF4, rapid warming (top C31n) followed by cooling (base C30n) led to a rapid increase in the relative abundance of the most robust planktic and benthic species (large, thick walls) and gradual extinctions of Campanian survivor species (Event IV, C31n-C30n transition). In zone CF3 (top C30n), an increase in the relative abundance of generalist and disaster opportunist species, a rapid decrease in diversity, species dwarfing, high fragmentation index and decreased primary productivity indicate enhanced environmental stress conditions (Event V). During this time, both Ninetyeast Ridge and Deccan phase-1 volcanism were active, suggesting that the environmental changes were likely linked to intense volcanic eruptions that released large amounts of volcanogenic SO₂ and CO₂ poisoning the environment, causing ocean acidification and ultimately extinctions.