REVISITING MECHANISMS AND THRESHOLDS OF MASS EXTINCTIONS

Identifying what “causes” mass extinction events has long been a topic of debate amongst geoscientists across many sub-fields. Substantial effort has been made to characterize large changes to the Earth system (such as the emplacement of large igneous provinces, LIPs, or asteroid impacts) that might trigger a mass extinction. However, the unique circumstances associated with each of the Phanerozoic mass extinctions has made generalizing to specific predictors a challenge (i.e., not all LIPs or asteroids trigger mass extinction). Here we explore the utility of refocusing the investigation of mass extinctions away from triggers and towards kill mechanisms (sensu Knoll et al. 2007); that is, those specific environmental factors that directly lead to organismal deaths. There are far fewer of these for investigation (i.e., the “Four Horseman of the Evolutionary Apocalypse” including: ocean anoxia, ocean acidification/acid rain, temperature change, and habitat loss), and analyses that quantitatively test interactions between these mechanisms may uncover unique commonalities among mass extinction events. Additionally, since these factors are not “one-off” events but rather continually wax and wane in response to the dynamic Earth system, the explicit consideration of kill mechanisms potentially renews a long-standing discussion of Earth system thresholds that once surpassed, may push global biodiversity from a background to mass extinction state (e.g., continuity of cause, Wang 2003). Recent work finding that carbon cycle and sea level variability may enhance extinction rates when a mass extinction is triggered, support further analysis in this area. Finally, explicit study of Earth system thresholds may contribute to investigations of macroevolutionary scaling; that is, facilitate better integration of population- and ecosystem-level extinction dynamics with global-scale extinction phenomena. This approach will undoubtedly benefit from the application of novel mathematical modeling approaches which will open new avenues in mass extinction science going forward.