Present‐day sedimentary processes on the shelves of inactive volcanic ocean islands: The case study of Porto Santo Island (Madeira Archipelago)

Abstract Shelves surrounding reefless volcanic islands usually host sediment that is considered to result mostly from erosion of pre‐existing volcanic rocks, with minor contributions from active volcanism and biogenic production. However, due to the scarcity of studies on volcanic islands, the way in which sediment is produced and deposited in these settings is still relatively poorly understood. In this study, a source‐to‐sink approach is used to understand how sediments on the southern shelf of the inactive volcanic island of Porto Santo (Madeira Archipelago) have formed. To achieve this aim, sediment texture, composition (carbon content) and morphoscopic characterization are integrated with acoustic information from the shelf (multibeam bathymetry, backscatter imagery and sediment thickness). Seafloor sediments are mostly moderately sorted sands with carbonate skeletal particles representing around 80% of the grains and minor volcaniclasts and aeolianite particles from inland subaerial erosion. Multibeam and seismic data allow recognizing some elongated erosive–depositional features that give insights into present‐day and past currents on the shelf. Additionally, an assessment of the subaerial sediment contribution to the shelf has been compared to the overall sediment volume present there. The findings indicate that the current sediment volume significantly exceeds the terrigenous supply, suggesting a significant primary biogenic carbonate production on the shelf. The sediment microscopic characterization supports this hypothesis, which leads to the conclusion that on low‐relief inactive volcanic islands, like Porto Santo, the contribution of carbonate factories to shelf sediment has a bigger role than previously considered.