Hothouse hydrology: new insights from water and sediment transport patterns in the Eocene Pyrenees

Deciphering how Earth’s surface has responded to the extreme climates of the past is vital for understanding the impacts of global warming on our planet in the present and future, including flood and drought risk worldwide. Rivers are the most significant conduits of water, sediment and nutrients across Earth’s continents, and the patterns of river water and sediment transport through time, or river intermittency, are thought to be highly sensitive to geomorphic bounding conditions such as climate and tectonics. Determining the intermittency of rivers in ancient hothouse climates provides a unique lens through which to investigate this question. However, this requires strong constraints on sediment and water discharges and volumetrics from source to sink, which are rare due to the challenges of estimating bankfull and average water and sediment fluxes at continental scale. We reconstruct the evolving source-to-sink dynamics of the lower Eocene Montllobat (52.0 – 50.5 Ma) and Castissent (50.5 – 49.7 Ma) Formations, in the dynamic tectono-climatic setting of Southern Pyrenees during the Eocene Hothouse. By estimating fluvial morphodynamics and discharges in the Tremp and Ager Basins, in addition to depositional fluxes in the underfilled Ainsa and Jaca Basins, we estimate water and sediment intermittency in these systems for the first time. Sediment intermittency factors (I_s) in the Montllobat Formation average 0.009-0.029, implying annual sediment loads could have been completed with as little as 1 week of transport at bankfull capacity. The overlying Castissent Formation, characterized by enhanced braiding and sediment discharge, has higher I_s values of 0.012-0.036. Water intermittency factors (I_w), on the other hand, decreased from 0.25 in the Montllobat interval to 0.15 in the Castissent interval, implying perennial rivers almost halved their activity at c. 50.5 Ma. This suggests river discharge rapidly became more extreme and infrequent, whilst sediment became transported more efficiently. Coeval to a pulse of uplift in the Pyrenean hinterland, we reveal the deposits of the Montllobat and Castissent rivers record strong competing climatic and tectonic signals which drove over 20 km of fluvial progradation. Further, in comparison to modern systems, the Eocene rivers of the Pyrenean foreland have higher sediment intermittency factors than anticipated, transporting sediment more efficiently than similar rivers today. This suggests hothouse climates can cause reduced sediment export timescales, with important implications for source-to-sink dynamics in today’s evolving climate.