The continental shelf as an offshore archive for tsunami deposits – an example from southwest Iberia (RV METEOR cruise M152)

Abstract

<p>In AD 1755 a strong earthquake-generated tsunami destroyed large parts of the southwest Iberian coastline. Data for the study of the sedimentological characteristics and palaeo-ecological effects of the backwash of this well-known AD 1755 Lisbon tsunami and possible preceding events on the continental shelf was obtained during RV METEOR cruise M152 in November 2018, since the hydrodynamics of tsunami backwash currents are as yet poorly understood. Furthermore, the suitability of the shelf as a reliable sedimentary archive for tsunami deposits was investigated.</p><p>Along the Algarve coast, prominent AD 1755 Lisbon tsunami deposits have been detected onshore for quite some time. Cruise M152 conducted a geophysical survey on the corresponding shelf area to obtain bathymetry and sub-bottom profiles for the recognition of depositional basins. Subsequently, 19 sediment cores were retrieved from the most suitable depositional basins by vibracoring at water depths from 65 to 114 m. The cores were analysed in a multiproxy approach (granulometry, magnetic susceptibility, P-wave velocities, organic and inorganic geochemistry, micropalaeontology). Deposits of the AD 1755 Lisbon tsunami were identified in most of the cores as a thin layer at ca. 20 cm depth.</p><p>More surprisingly, a second event deposit dating to ca. 3700 years cal. BP was detected at core depths of 122 to 155 cm. It is even traceable in the sub-bottom profiles and consists of a distinctive ca. 30 cm thick well sorted medium-sized siliciclastic sand. Due to the thickness of the deposit an in-depth study of its characteristics was possible. It displays an erosive basal contact followed by a thin matrix-poor shell hash layer, a reversely graded fine sand layer and ultimately a massive, quite homogeneous medium sand resembling the T<sub>a</sub> division of the Bouma sequence or the S<sub>1</sub>, S<sub>2</sub> and S<sub>3</sub> divisions of the Lowe sequence. The deposit is distinguishable from the silt to silty sand-dominated background sedimentation not only due to the textural and compositional features, but also due to contrasting geophysical and geochemical properties. Terrestrial provenance for (at least parts of) the sediment is revealed by biomarker analysis. Based on these characteristics, the deposit is interpreted as the result of a high density hyperpycnal flow from the coast towards the offshore caused by tsunami backwash. This event layer may be correlated to onshore observations of tsunami deposits along the southwest coast of Spain but has never been identified in Portugal where the onshore record of tsunami deposits only covers the last three millennia.</p><p>The results of this multiproxy analysis strongly suggest the shallow offshore area below storm wave base to host reliable sedimentary archives for tsunami backwash deposits, which allow the discovery of as yet unknown events. Palaeotsunami research can benefit from the investigation of offshore archives, especially where onshore records are incomplete or sparse.</p>