Fluvial response and recovery after the Permo-Triassic Mass Extinction revealed through quantitative paleohydrology

Xiang Yan , Alexander C Whittaker, Gary J Hampson

Anthropogenic climate change represents the greatest modern forcing on global river dynamics. Investigating the sensitivity of rivers to past climate in the geological record is thus crucial in understanding future landscapes in a warming world. The Sherwood Sandstone Group, UK, a unit of early-middle Triassic (c. 248-239 Ma) dryland fluvial deposits, records events during and after the Permo-Triassic Mass Extinction. We quantify the evolution of this fluvial system, exposed in the southwest UK, by combining facies-scale sedimentology with a numerical paleohydrologic workflow. We reconstruct key hydraulic and geomorphic characteristics, showing that gravel-bed lower Triassic rivers had paleoslopes between 1.0 × 10-3 and 2.0 × 10-3 m/m and bankfull water discharges of 760-810 m3/s. During the middle Triassic, rivers were sand-bedded, with paleoslopes between 8 × 10-4 and 5 × 10-4 m/m. Bankfull water discharges ranged between 380 and 480 m3/s: half those of the lower Triassic. This halving of bankfull discharge occurs despite an inferred eightfold increase in catchment area in the mid-Triassic and stable levels of long-term precipitation, indicating that catchments in the lower Triassic were generating highly intermittent, disproportionately large bankfull flow events. Coupled with the abnormally coarse grain size recorded in the Lower Triassic and regional stratigraphic correlations to similar deposits across NW Europe, we attribute this to a global hyperthermal event: the Late Smithian Thermal Maximum (c. 248 Ma). Rapid warming during a greenhouse interval triggered widespread landscape devegetation and bedrock erosion, and extreme, infrequent monsoonal precipitation. Our results demonstrate the ability of quantitative paleohydrology to produce numerically-informed depositional models, and identify the impacts of global warming during the greatest biotic-climatic turnover of the Phanerozoic.