We welcome the comment by Berner and Ward on our 2005 paper, and appreciate the double meaning of “positive reinforcement” in its title, reflecting the absence of criticism of our work. The feedbacks they describe are likely components of the end-Permian biosphere upheaval, and the positive sign of the feedback loops are likely correct. However, the strength and response time of individual couplings among system components in their diagram are uncertain, but these are amenable to evaluation through numerical modeling. Perhaps the long biotic recovery time and persistence of extreme oceanic conditions through the early Triassic (e.g., Woods et al., 1999; Payne et al., 2004) was at least in part a consequence of the stifling effects of these self-enforcing feedback loops.
One subtle issue is the cause of oceanic anoxia. Berner and Ward link ocean warming and anoxia through the vigor of ocean circulation, but we, among others, have shown that the more important factor is warming of deep-water source regions, because of the inverse solubility of O2 in seawater as a function of temperature (Hotinski et al., 2001). Reductions in meridional overturning rate, in and of themselves, have less effect on deep-water oxygen concentrations because the reduction in O2 supply is matched by a reduction in O2 demand: reduced upwelling limits the export of organic material from the surface ocean and thus the rate of consumption in the deep sea (Broecker and Peng, 1982). Moreover, stagnation of circulation is not synonymous with ocean stratification; the modern ocean is vigorously circulating but strongly stratified in nutrient concentrations and modestly stratified in O2 concentration. The biological pump is a worthy competitor to the thermohaline circulation of the ocean.
A number of important issues remain to be resolved before the hypothesis of H2S as the proximal kill mechanism for the end-Permian mass extinction will be widely accepted, including 1) whether phosphate concentrations can build up to the necessarily very high concentrations needed to sustain such deep anoxia (the ocean would have to be highly supersaturated with respect to various apatite phases; however, Framvaren Fjord and the Black Sea currently demonstrate high levels of supersaturation); 2) whether during the transition from oxic to anoxic conditions, de-nitrification would rob the ocean of all nutrient nitrogen (however, recent analysis indicates that nitrogen fixation may well be able to keep pace with denitrification, given a much lower Fe requirement than previously thought; Arrigo, 2005); and 3) whether H2S has a selective lethality (relatively little is known about H2S toxicity, especially among important groups such as insects, which suffered their only mass extinction in geologic history during the Late Permian; Erwin, 2006). Moreover, fundamental uncertainty exists concerning the nature of the pre-Mesozoic biological pump: in the absence of planktonic calcifiers that produce the bulk of the ballast for settling bio-genic particles in the ocean today, would there be an effective transfer of organic matter to the deep sea that could keep pace with increased upwell-ing of nutrients from below? All of these issues are prime for research, and we look forward to the results of ongoing and future investigations.