Paleomagnetic, rock magnetic and geochemical study of the 1755 tsunami deposit at Boca do Rio (Algarve, Portugal)

A multiproxy study combining paleomagnetic, rock magnetic (magnetic susceptibility; anhysteretic remanent magnetization, ARM; isothermal remanent magnetization, IRM, anisotropy of magnetic susceptibility, AMS) and geochemical (XRF) data was carried out on trench sediments collected in the area of Boca do Rio (Algarve, South Portugal) known to have been stricken by a tsunami wave related to the 1755 Lisbon earthquake. The magnetic mineral assemblage is dominated by multidomain ferrimagnetic grains with a significant contribution (5-9%) of ultra-fine magnetic particles related to the presence of pedogenetic magnetite. The variable amount of this post-depositional magnetite plays a role on the characteristic remanent magnetization (ChRM) of the sediment. The primary magnetization recorded at T1, located farthest from the coast, is representative of the paleosecular variation of the Earth's magnetic field and a comparison of the results with the regional model for the geomagnetic field in Europe (SCHA.DIF.3K) was used to constrain the erosional signature of the tsunami layer. The result indicates that less than eighty years could be missing in the sedimentary column. The eroded sediment (thickness 27-40 cm) was incorporated in the tsunami layer that is characterized by a low magnetic content and a large increase in the Ca content representing allochthonous material. Sedimentological, geochemical, magnetic and AMS data show that the signature of the tsunami layer reflects at least two waves with an erosive capacity that is controlled by the distance from the shore.