Quantitative palaeohydrology reveals fluvial response to early-mid Triassic climate change

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  the protracted climatic-biotic recovery period following 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  palaeohydraulic workflow. We reconstruct key hydraulic and geomorphic characteristics, showing that gravel-bed lower Triassic rivers had palaeoslopes between 1.0 × 10⁻³ and 2.0 × 10⁻³ m/m and bankfull water discharges of  c. 600 m³/s. During the middle Triassic, rivers were sand-bedded, with palaeoslopes between 8 × 10⁻² and 5 × 10⁻² m/m. Bankfull water discharges ranged between 290 and 360 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, and is attributed to lower Triassic catchments generating highly intermittent, disproportionately large bankfull flow events. Coupled with the very coarse grain size recorded in the Lower Triassic and regional stratigraphic correlations to similar conglomerate sheets 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, bedrock erosion, and extreme, infrequent monsoonal precipitation. Our results demonstrate the ability of quantitative palaeohydrology to produce numerically-informed depositional models, and to identify the impacts of extreme global warming.