River intermittency and Eocene climate change in the Castissent and Montllobat formations of the southern Pyrenean Foreland

Jonah S. McLeod , Alexander C Whittaker, Rebecca E. Bell, Gary J Hampson, Luis Valero, Ziqiang Zhou

Patterns of river water and sediment transport through time, or river intermittency factors, are generally considered to be highly sensitive to climate and tectonics. Determining the intermittency of rivers in ancient hothouse climates could provide a unique lens through which to investigate Earth’s response to climate change. However, this requires strong constraints on both mean and bankfull sediment and water discharges through time, which are rare in the stratigraphic archive due to the challenges of estimating material flux rates from source to sink. To address these challenges, we calculate basin-scale sediment volumetrics and palaeohydraulics to reconstruct water and sediment transport patterns in strata deposited under an ancient extreme climate punctuated by hyperthermal warming events: the Montllobat (52.0 – 50.5 Ma) and Castissent (50.5 – 49.7 Ma) formations of the Southern Pyrenees, during the Eocene hothouse. We reconstruct water intermittency factors (Iw) in these ancient river systems averaging 0.15-0.25, whereas sediment intermittency factors (Is) of 3x10-3 – 7x10-3 in the Montllobat Formation increased up to 3-fold in the overlying Castissent Formation. This implies that whilst rivers were likely perennial, sand-grade sediment was transported significantly more often in the Castissent rivers (p<10-3). A comparison with a global database of modern rivers also demonstrates that rivers during the Early Eocene Climatic Optimum were able to transport sediment more efficiently than most modern rivers of similar types and climate zones today. We hypothesise that a short-lived global warming event at c. 50.5 Ma enhanced monsoon seasonality in the Pyrenean foreland, causing heightened sediment transport efficiency which lasted up to 1 million years. This ancient example shows the long-lasting, basin-wide geomorphic consequences of short-term climatic change, resolved from river transport patterns in alluvial stratigraphy.