Discovery and characterization of a mud volcano field in the south Alboran Sea: New insights into Western Mediterranean mud volcanism

Mud volcanoes (MVs) are significant geologic and ecological features, widely distributed across the Gibraltar Strait region. While initial discoveries during UNESCO TTR expeditions in the 1990s documented mud volcanism on both sides of the Strait, subsequent studies east of Gibraltar have primarily concentrated on the Ceuta contourite drift, where rapidly deposited sediments created conditions for shale diapirism and related expulsion structures. Here, we report the discovery of a previously unknown field of mud volcanoes on the Moroccan continental slope, west of Melilla. This finding arises from the STRENGTH Leg 3 expedition aboard the R/V Sarmiento de Gamboa in April 2023. Collected bathymetric data and imagery from a towed side-scan sonar revealed 15 distinct MVs, characterized by conical morphologies, basal moats, and mud flows extruded from summit emission sites. These features, reaching up to 30 meters in height and 300 meters in diameter, are interpreted as dormant MVs due to the absence of active mud expulsion. Nevertheless, ROV observations documented extensive biological colonization, including corals, sponges, and other sessile organisms, highlighting their role as ecological hotspots, due to the more competent substrate MVs provide. Geophysical data, particularly sparker profiles, have provided detailed seismic imaging of the upper ~300 meters below the seafloor. These profiles revealed extensive fluid migration pathways feeding the MVs, with distinct gas-related wipeouts both beneath the MVs structures and laterally within surrounding sediments at a consistent depth. These subsurface anomalies combined with the presence of sessile organisms (colonizing carbonate crusts deposited by authigenic mineral precipitation) suggests ongoing fluid dynamics despite their apparent dormancy. Sediment cores from several MVs were retrieved for geochemical and geochronological analyses, which will shed light on the origin and evolution of these features. This discovery expands our understanding of mud volcanism in the region and provides a foundation for future interdisciplinary studies of fluid migration, tectonics, and cold-seeps associated ecosystems.