Constraining intermittency in the geological past: Implications for reconstructing water discharges and sediment fluxes in ancient source-to-sink systems

Quantitative investigations of ancient source-to-sink systems usually provide insights into either instantaneous conditions or mean conditions. Field-based channel palaeohydraulic approaches effectively recover instantaneous water discharges and sediment fluxes, whereas catchment- and regional-scale modelling approaches often recover mean annual water discharges and sediment fluxes, or mean discharges and fluxes on longer, million-year timescales. There is a critical gap between these timescales of investigation which reflects the intermittency of flow and sediment transport. However, at present, this gap is difficult to reconcile. In ancient source-to-sink systems, constraining the intermittency of flow and sediment transport is necessary to reconstruct water discharges and sediment fluxes and, therefore, investigate: (1) magnitudes and characteristics of ancient floods; (2) river behaviour in warmer palaeoclimates; (3) river response to climatic perturbation; and (4) catchment hydroclimate. Intermittency constraints are therefore crucial to decipher how landscapes evolved in response to tectonic and climatic forcing in the geological past. Here we take a multi-proxy approach to address this challenge. We combine fluvial stratigraphic datasets, flow and sediment transport models, and general circulation model (GCM) results to develop new methods to constrain intermittency in ancient source-to-sink systems. We illustrate these methods for ancient systems preserved in the Turonian Ferron Sandstone, USA, which records a Cretaceous greenhouse climate, and the Paleocene–Eocene Esplugafreda Formation and Claret Formation, Spain, which record the Paleocene–Eocene Thermal Maximum. To evaluate our methods, we compare our intermittency estimates with facies observations that reflect discharge regime and surface runoff regime, as well as terrestrial palaeoclimate proxies. We find these methods are effective and, further, we outline the necessary next steps to advance these methods. Our results demonstrate the potential to use multidisciplinary datasets to constrain intermittency and, therefore, the dynamics and evolution of ancient source-to-sink systems in response to tectonic and climatic forcing. Further, with continual advances in the use of GCMs to model palaeoclimates, our results highlight the potential to use GCMs to explore water discharges and sediment fluxes in systems where the rock record is incomplete or inaccessible.