THE DYNAMICS AND ROLE OF SOIL AIR DURING THE PERMO-TRIASSIC INTERVAL

Both low atmospheric oxygen levels and high temperatures have been inferred for the latest Permian and early Triassic periods, including the major biotic crisis at the boundary. The linkage between plant and soil microbe respiration, temperature, and weathering suggests that the composition of soil air could have greatly fluctuated during the Permo-Triassic interval, creating feedbacks. Specifically, under conditions of lower oxygen and higher temperature, higher root and microbial respiration could contribute to soil hypoxia or patchy anoxia. Terrestrial soils could have become a net source of methane to the atmosphere, creating a positive climate feedback. Vascular plants would have been outcompeted by bryophytes and microbes for limited soil oxygen. Die-off of vascular plants and subsequent thinning of soils by erosion would allow soil CO2 concentrations to relax towards the atmospheric level, increasing soil water pH, lowering weathering rates of the parent rock, and allowing atmospheric CO2 and temperature to increase (yet further) until weathering rates again compensated volcanogenic CO2 flux. Higher temperatures and erosion rates would have accelerated the oxidation of recalcitrant soil organic matter, further lowering atmospheric oxygen. This scenario predicts that indicators of soil methanogenesis would be followed by a change in erosion style and abundant pedogenic carbonates in the paleosol record. I elaborate this scenario using a coupled climate-biogeochemical model designed for the simulation of kyr-Myr fluctations in atmospheric oxygen. The model includes a fast analytical zonal energy-balance model that accounts for latitudinal and seasonal variation in surface temperatures. The distribution of land area with latitude is explicitly accounted for. Soil depth, soil air oxygen and carbon dioxide concentration, weathering rates, and recalcitrant soil organic matter content are calculated with latitude. Volcanogenic input of carbon dioxide and sulfur dioxide, and burial of organic matter in marine sediments are treated as independent, adjustable parameters. Alternatively, the organic matter burial rate is scaled to the release of phosphorus by weathering of continental rocks.