Lower Triassic large sea-floor carbonate cements: Their origin and a mechanism for the prolonged biotic recovery from the end-Permian mass extinction

Precipitation of inorganic calcium carbonate is a common occurrence in both modern and ancient marine environments. However, synsedimentary growth of large (>5-10 cm) crystalline carbonate cements directly onto the sea floor has been thought to be limited to the Proterozoic, when seawaters mere high ly oversaturated with calcium carbonate compared to average Phanerozoic val ues. Outer shelf to slope deposits of the Lower Triassic Union Wash Formati on in east-central California, deposited in oxygen-restricted settings, con tain crystalline calcium carbonate cements that appear to have grown direct ly on the sea floor. Paleoenvironmental analyses indicate that these large calcium carbonate cements grew under conditions that were similar to those proposed for the precipitation of inorganic calcite in the Black Sea. Sulfa te reduction of organic matter led to an increase in the amount of bicarbon ate ion in deep waters and a concomitant increase in Sigma CO2 and alkalini ty. Mixing with surface waters led to CO2 degassing, and precipitation of c ements from waters supersaturated with calcium carbonate. The presence of t hese cements and associated facies thus provides evidence of harsh environm ental conditions in the Early Triassic at the regional level, which may hav e acted in concert with biotic effects of the end-Permian mass extinction, as well as similar deleterious conditions (e.g., shelf anoxia) in other reg ions, to produce a prolonged as well as temporally and geographically varia ble biotic recovery from this mass extinction.