DELAYED BIOTIC RECOVERY FROM THE PERMIAN-TRIASSIC EXTINCTION MAY HAVE BEEN INFLUENCED BY A REDOX-DRIVEN REORGANIZATION OF THE MARINE CARBONATE SYSTEM

Organisms that secreted robust calcareous skeletons (e.g., corals, sponges, and algae) experienced proportionally high rates of extinction at the Permian-Triassic boundary (Knoll et al. 2007).  Additionally, organisms with robust skeletons are notably absent from the geologic record during ~5 million years of Lower Triassic time.  Because calcifying organisms incur and energetic cost proportional to available [CO₃^2-], a decrease in CaCO₃ saturation state due to a rapid influx of CO₂ into the ocean-atmosphere system is an attractive mechanism to explain this selective pattern.  However, the general lack of robust skeletons for several million years implies that calcification stress was long lived, too long for a P-T boundary carbon cycle perturbation to accommodate.  We argue, rather, that the Lower Triassic carbonate system was linked to longer-lived trends in water column oxygen, for which growing geological evidence exists.  We hypothesize that, following a P-T boundary CaCO₃ saturation state perturbation (perhaps triggered by Siberian Traps volcanism), [CO₃^2-] remained depressed in shallow-water shelf environments (despite continuing carbonate deposition) due to the effect of anaerobic cycling of organic carbon within anoxic ocean basins.  Enhanced anaerobic recycling led to reduced water column and pore fluid carbonate ion gradients, enhanced carbonate preservation, increased early cementation and precipitation of carbonate minerals directly on the seafloor (i.e., microbialites and seafloor crystal fans), and, ultimately, lower CaCO₃ saturation states in surface seawater.  Under such conditions, calcification may have become prohibitively expensive for many organisms.  In principle, this state was effectively stable, as long as a significant proportion of electrons in Lower Triassic oceans were channeled through anaerobic metabolisms.  When anoxia subsided toward the end of the Lower Triassic, CaCO₃ saturation states rebounded, and calcareous skeletons again began to constitute a conspicuous sink of marine carbonate.  If correct, the mechanism proposed here makes several predictions for additional intervals in Earth history, where intense episodes of anoxia might, likewise, have inhibited biological calcification.