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In 1755 CE, a strong earthquake followed by a transatlantic tsunami destroyed large coastal areas; it also left its sedimentary imprints in the Boca do Rio valley (western Algarve, Portugal). This tsunami layer is very well preserved and has been analysed in several studies. Deposits of preceding extreme wave events, however, have rarely been described for the entire Algarve coast. In this study, we present a multiproxy analysis of seven sediment cores from the Boca do Rio region, organized in two crossing transects, one parallel and the other perpendicular to the coastline. The geochronological framework has been established by combining radiocarbon and optically stimulated luminescence dating with sedimentological and geochemical analyses (XRF, C/N, magnetic susceptibility, granulometry, micropalaeontology), and covers the palaeogeographical evolution of that area for the last four millennia. As expected, the 1755 CE tsunami was easily identified at all coring sites, as a sandwiched stratum between fine-grained alluvium. This event layer presents several tsunami characteristics, such as erosive basal contact, rip-up clasts, a fining-upward sequence, and a mud cap. At one coring site, a second extreme wave event layer of marine origin was detected within floodplain deposits, due to its granulometric, XRF, magnetic susceptibility and micropalaeontological properties. It is stratigraphically located below the 1755 CE Lisbon tsunami layer and can be associated with another yet undocumented extreme wave event, most likely dating to the mid or late 1st millennium CE.

The Santo Andre lagoon is located on the southern west coast of Portugal, about 80 km south of Lisbon. Although the beach barrier separating the lagoon from the open sea was occasionally breached in the past and has artificially been opened on an annual basis for the last decades, the lagoon still represents an appropriate geo-bio-archive for reconstructing the Holocene palaeoenvironmental evolution. For this purpose, a 10-m-long sediment core was taken from the centre of the lagoon by using a floating platform. Sedimentological, geochemical and micropalaeontological analyses were performed in order to unravel past sedimentological, environmental and climatic conditions. Due to the lagoon's exposure to storms from the Atlantic and possible tsunamis triggered by earthquakes along the Eurasian-African plate boundary south of Portugal, it is of high interest to identify short-term high energy events that might have crossed or breached the shielding barrier, leaving their footprint in the sedimentary record of the lagoon. The sediment core archived the palaeoenvironmental evolution of the Santo Andre lagoon for at least the last eight millennia. The sandy deposits of the core's lowermost part most likely represent a former coastal flood plain that developed when the postglacial marine transgression had reached the area. The continued sea-level rise deposited alternating layers which indicate varying environmental conditions, characterized by peat growth, stagnant-water areas, as well as disconnections from and reconnections with the open sea. Since approximately 5000 BP, the longshore transport had formed a beach barrier, separating the marine embayment from the open sea and creating a lagoon. In addition, four sudden significant marine inundations between ~8500 and 6000 BP are indicated by the sedimentary and microfaunal analyses. Two of these layers can be correlated to extreme wave events (unit B-II), while for the other two layers an ingression caused by sea-level rise or extreme wave events remains debatable.