Lagoa de Santo André - The Holocene evolution of a coastal lake at the Atlantic coast of Portugal

The Lagoa de Santo André is a coastal lake located ca. 80 km south of Lisbon at the Atlantic coast of Portugal. The region is highly vulnerable to earthquake-triggered tsunami events due to its exposure to the very active tectonic boundary between the Eurasian and the African plates. Despite several natural and artificial breaches of beach barrier in the past, 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 lake by using a floating platform. Sedimentological, geochemical and micropalaeontological analyses were performed in order to unravel the former geological and climatic conditions, and to identify short-term events like tsunamis or storm surges that might have crossed or breached the shielding barrier, leaving their footprint in the sedimentary record of the lagoon.

The sediment core covers the palaeoenvironmental evolution of Lagoa de Santo André for at least the last eight millennia. The sandy deposits of the lowermost part of the core most likely represent the littoral phase of a palaeo-beach that had developed when the postglacial marine transgression had reached the area. Above alternating layers indicate varying environmental conditions that are characterised by peat growth, stagnant-water areas and intercalating sand layers that deposited during disconnections from and reconnections with the open sea. Since 3000 BC at the latest, the longshore transport had formed a beach barrier that separated the coastal lake from the open sea. As yet no deposits of an extreme wave event have been detected. However, the ongoing microfaunal analysis will clarify, if sudden changes of the salinity have occurred due to significant saltwater intrusions, which would hint at potential extreme wave events.