UNIVERSITY OF WISCONSIN SYSTEM
The cause of the end-Permian mass extinction remains controversial. However, some combination of CO2 or H2S toxicity and/or climate change, triggered or catalyzed by the eruption of the Siberian Traps, appears to be the most likely kill mechanism. The biotic recovery from the end-Permian mass extinction has received less attention, despite the fact that the Early Triassic recovery was unusually long (~5m.y.) The occurrence of ?anachronistic facies? (e.g., seafloor precipitated crystals, subtidal stromatolite bioherms, and wrinkle structures), multiple carbon isotopic excursions, and elevated seawater sulfate sulfur isotopic values in Lower Triassic strata suggest a prolonged period of anomalous environmental conditions following the mass extinction that may have delayed the biotic recovery.
Multiple lines of evidence imply that the Early Triassic was a time of elevated atmospheric CO2 most likely resulting from the extreme volcanism of the Siberian Traps. At the same time, metazoan reefs disappeared from the fossil record until the Middle Triassic, while calcifying marine invertebrates tended to be small. These observations and experimental work on living organisms have raised concerns about the ecological consequences of ocean acidification, in reef ecosystems in particular. Laboratory experiments with extant corals have demonstrated that as pH drops, corals survive as polyps without skeletons until pH returns to favorable levels. It has been suggested that similar ocean acidification during the Early Triassic inhibited growth of large calcareous shells, favoring small organisms, and prevented corals from forming skeletons. Thus, if the protracted recovery reflects a suppressed ecosystem due to elevated CO2 levels, Early Triassic communities may represent an ancient analog for the effects of greenhouse gas-forced ocean acidification on Earth?s ecosystems. Considering the extreme importance of coral skeletal frameworks in supporting the biodiversity of modern reef ecosystems, the inability of corals to produce skeletons will most likely have severe and negative effects on fisheries and the economies that depend on them. Thus, unusual Early Triassic deposits may provide a preview of marine ecosystems under conditions of increased atmospheric pCO2 and reduced ocean pH.
The Lower Triassic of the western U.S. was deposited in two basins on the western coast of Pangaea, which together comprise a nearly complete record of the entire Early Triassic. Studies of the biotic recovery in the western U.S. have largely focused on fossil abundance and diversity patterns and the occurrence of anachronistic facies. These studies have demonstrated that anachronistic facies such as seafloor-precipitated aragonite fans and large subtidal stromatolite bioherms are found exclusively in deeper water settings, below fair-weather wave base (except for the immediate P-T boundary interval). Crystal fans have been reported only from slope or basinal sections while stromatolite bioherms occur on flooding surfaces at the bases of parasequences in more proximal environments. Both seafloor crystal fans and stromatolite bioherms are interpreted to have formed under anaerobic conditions. The facies-dependent occurrence of anachronistic facies has led to the conclusion that anoxia was a deep-ocean phenomenon during the Early Triassic, and that shallower settings were a refuge from anoxia in the aftermath of the mass extinction. The lack of evidence for shallow-water anoxia during the Early Triassic suggests that the biotic recovery in the shallow realm may have been hindered not by anoxia, but by hypercapnia and ocean-acidification due to high atmospheric carbon dioxide.