Abrupt episode of mid-Cretaceous ocean acidification triggered by massive volcanism

Large igneous province volcanic activity during the mid-Cretaceous approximately 94.5 million years ago triggered a global-scale episode of reduced marine oxygen levels known as Oceanic Anoxic Event 2. It has been hypothesized that this geologically rapid degassing of volcanic carbon dioxide altered seawater carbonate chemistry, affecting marine ecosystems, geochemical cycles, and sedimentation. Here, we report on two sites drilled by the International Ocean Discovery Program offshore of southwest Australia that exhibit clear evidence for suppressed pelagic carbonate sedimentation in the form of a stratigraphic interval barren of carbonate, recording ocean acidification during the event. We then use the osmium isotopic composition of bulk sediments to directly link this protracted ~600- kiloyear shoaling of the marine calcite compensation depth to the onset of volcanic activity. This decrease in marine pH was prolonged by biogeochemical feedbacks in highly productive regions that elevated heterotrophic respiration of carbon dioxide to the water column. A compilation of mid- Cretaceous marine stratigraphic records reveals a contemporaneous decrease of sedimentary carbonate content at continental slope sites globally. Thus, we contend that changes in marine carbonate chemistry are a primary ecological stress and important consequence of rapid emission of carbon dioxide during many large igneous province eruptions in the geologic past