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

Abstract

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 were highly oversaturated with calcium carbonate compared to average Phanerozoic values. Outer shelf to slope deposits of the Lower Triassic Union Wash Formation in east-central California, deposited in oxygen-restricted settings, contain crystalline calcium carbonate cements that appear to have grown directly 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. Sulfate reduction of organic matter led to an increase in the amount of bicarbonate ion in deep waters and a concomitant increase in ΣCO 2 and alkalinity. Mixing with surface waters led to CO 2 degassing, and precipitation of cements from waters supersaturated with calcium carbonate. The presence of these cements and associated facies thus provides evidence of harsh environmental conditions in the Early Triassic at the regional level, which may have acted in concert with biotic effects of the end-Permian mass extinction, as well as similar deleterious conditions (e.g., shelf anoxia) in other regions, to produce a prolonged as well as temporally and geographically variable biotic recovery from this mass extinction.