UNTANGLING THE GEOLOGICAL RECORD OF THE END CRETACEOUS OCEAN ACIDIFICATION LINKED WITH DECCAN VOLCANISM (Invited Presentation)

Multi-proxy evidence links a widespread ocean acidification event with intensified Deccan volcanic activity in the final ~50 ky of the Cretaceous ^[1]. This prelude to the Cretaceous-Paleogene (K-Pg) boundary mass extinction of planktic foraminifera marks ocean acidification as a likely kill-mechanism. Two complementary yet independent lines of evidence must converge to validate this hypothesis: (i) realistically low (surface ocean) pH’s that result in increased planktic test vulnerability and enhanced fragmentation, supporting the preservation trends observed in the geological record, and (ii) biological evidence of a calcification crisis recorded in the taphonomically defined “acidification” interval.

In vitro dissolution of Late Maastrichtian planktic foraminifera in controlled acidification scenarios (pH 8.0, 7.5, 7.0 and 6.5, 24-48 hours of exposure) does result in increased test fragmentation, validating the taphonomic proxy for ocean acidification. Heavily calcified planktic tests with robust morphologies (e.g. Globotruncana, Planoglobulina, Pseudotextularia) are observed to be more resistant to dissolution. This is consistent with a higher relative abundance of these same genera in the stratigraphic “acidification” interval at Bidart (France) and Gamsbach (Austria) indicating taphonomic bias. Similarly, dissolution resistant species also dominate the coeval high-stress assemblages of the Deccan intertrappean sediments in the Krishna-Godavari basin (India). The “acidification” intervals of Bidart (France) and Elles (Tunisia) record a measurable decrease in morphometric parameters (e.g. average length, width, diameter, area) of adult specimens of multiple species like Pseudoguembelina carsayae, Pseudotextularia elegans and Rugoglobigerina rugosa, confirming intraspecific dwarfing as a biological response to calcification crisis. The exceptional concordance of taphonomic and biological evidence therefore attests the validity of the ocean acidification hypothesis.

^[1] Font, E., Adatte, T., Sial, A.N., Drude de Lacerda, L., Keller, G. and Punekar, J., 2016. Mercury anomaly, Deccan volcanism, and the end-Cretaceous mass extinction. Geology, 44(2), pp.171-174.