Experimental validation of the planktic foraminifera fragmentation index as proxy for the end-Cretaceous Ocean Acidification

Abstract

The final ~50 ky of the Maastrichtian leading up to the Cretaceous-Paleogene (K-Pg) boundary mass extinction at Bidart (France) and Gamsbach (Austria) record poor carbonate preservation indicative of ocean acidification preceding the mass extinction. Further evidence includes enhanced planktic foraminifera test fragmentation, anomalously low bulk-rock magnetic susceptibility (acidification) and peak mercury content (toxicity) related to peak Deccan volcanism in India. Here we present new experimental data that validates the Fragmentation Index of planktic foraminifera as a reliable proxy for the end-Cretaceous acidification. Pristine Cretaceous planktic foraminiferal shells (from DSDP 525A, palaeodepth ~1000 m) were exposed to buffers of pH 8.0, 7.5, 7.0 and 6.5 for 15 days each, and their preservation state was quantified as a function of time. The critical variables affecting test taphonomy are morphology, pH and time of exposure. Thin-walled fragile biserial species constitute an average ~60% of typical Late Maastrichtian assemblages (e.g Heterohelix globulosa and H. planata) and are most susceptible to dissolution, followed by simple coiled forms such as Rugoglobigerina sp. (~19% of the assemblage) and Hedbergella sp. (~6% of the assemblage). The globotruncanids (Globotruncana, Globotruncanita, ~12%) are least susceptible to chemical and physical damage. Lower pH conditions render tests more vulnerable to physical fragmentation. Caution is needed to assess taphonomic inflation of morphologically robust yet environmentally sensitive Cretaceous species (e.g. globotruncanids), that may result in an underestimation of the degree/nature of the faunal crisis and tempo of extinctions in the pre-extinction acidification interval.