What Controls the Duration and Intensity of Ocean Anoxic Events in the Paleozoic and the Mesozoic?

The sedimentological and geochemical expression of Mesozoic ocean anoxic events (OAEs) in the geologic record is reasonably well-characterized and the causal mechanisms driving these events are generally agreed upon. By contrast, OAE-type intervals in the Paleozoic are relatively understudied and less well constrained. Here, we review literature published on three intervals that may be termed OAEs in the Paleozoic: the Late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE) event, the Late Ordovician – Early Silurian Hirnantian Ocean Anoxic Event (HOAE), and the Late Devonian Frasnian-Famennian, or Kellwasser events. Comparing them to Mesozoic OAEs reveals that Paleozoic OAEs are typically longer in duration than Mesozoic OAEs, and it is also likely that anoxic waters were more areally extensive during these Paleozoic events. With the traditional framework for OAEs the potential for a very extended timeframe of anoxia is problematic, given that more extensive bottom-water anoxia should accelerate reoxygenation of the atmosphere-ocean by promoting more efficient phosphorous recycling in the ocean and therefore boosting primary productivity. However, benthic phosphorous retention could have been enhanced under anoxic and iron-rich (ferruginous) marine conditions. Therefore, we propose a role for ferruginous conditions with more widespread anoxia in driving longer OAEs. Ultimately, a protracted increase in atmospheric oxygen concentrations through the Phanerozoic may have been a key factor controlling the expression of OAEs through time.