A DEEP TIME PERSPECTIVE ON OCEAN ACIDIFICATION

While demonstrating ocean acidification in the modern is relatively straightforward, identifying paleo-ocean acidification requires multiple lines of evidence. Aside from geochemical carbonate chemistry proxies (many of which are as yet ill-suited for deep time application), three main lines of evidence have been applied to fingerprint paleo-ocean acidification: 1) identification of a suitable acidification trigger; 2) a global sedimentary record of temporary carbonate depletion; 3) an extinction signature of selectivity against acidification-susceptible organisms. Each of these criteria is fraught with its own pitfalls when applied to deep time. In the first case, the timescales of acidification-relevant carbon cycle processes can challenge dating precision. In the second, the rock record is a constructive archive while ocean acidification is essentially a destructive (and/or inhibitory) phenomenon. In the third, the experimental biological data on susceptibility to acidification as yet lack robust consensus for many geologically-relevant clades.

Nonetheless, in combination these criteria can be applied to identify geological events consistent with ocean acidification. As a test case, the record of the end-Triassic mass extinction passes all three criteria. 1) The eruption of the Central Atlantic Magmatic Province and the associated massive and rapid release of CO₂ coincident with the end-Triassic mass extinction provide a suitable trigger for an acidification event (full carbonate undersaturation in the surface ocean is possible but improbable). 2) Tentative evidence for a global paucity of carbonate across the end-Triassic mass extinction versus the adjacent stratigraphy is consistent with a predicted sedimentary response to acidification. 3) The end-Triassic mass extinction was particularly selective against acidification-sensitive organisms (more so than perhaps any other extinction event).