Paleo extreme storm waves in the North Atlantic: geological evidence from Sal Island, Cape Verde Archipelago

The northwestern coast of Sal Island (Cape Verde Archipelago) is characterized by a rocky shoreline that is regularly impacted by Atlantic swells exceeding 4 m in height and 20 s in period. Yet, the only significant geomorphic expression of wave action is an extensive boulder ridge situated atop a rocky cliff, up to 80–100 m inland and between 10 and 15 m above present sea level. The presence of meter-scale boulders within this ridge raises a key question: is it actively shaped by modern storm waves, or is it a relic of paleo storms, impacting the shoreline during an interglacial period when sea level was significantly higher than today? To test this hypothesis, we apply a multidisciplinary approach combining satellite and drone imagery, topographic analysis, hydrodynamic modelling, and empirical boulder transport thresholds. Our results show that under current conditions, storm waves do not reach the ridge and cannot generate sufficient flow to mobilize its largest boulders. However, under modeled higher sea-level scenarios exceeding +5 m, wave runup reaches the ridge, and flow velocities are sufficient to initiate boulder transport by sliding and overturning. We therefore conclude that the ridge is a relict feature, most likely emplaced during Marine Isotope Stage 5e, when relative sea level in Sal Island was 5–7 m higher than today.