EGU2020-7686
https://doi.org/10.5194/egusphere-egu2020-7686
EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Uncharted archives – imprints of tsunami backwash deposits on the Algarve shelf (Portugal)

Piero Bellanova1,2, Klaus Reicherter1, Pedro J.M. Costa3,4, Mike Frenken1,2, Lisa Feist1, Jan Schwarzbauer2, Juan I. Santisteban5, Andreas Vött6, Ivana Bosnic3, Helmut Brückner7, Holger Schüttrumpf8, César Andrade3, João F Duarte9, Jannis Kuhlmann10, and the M152 scientfic Team
Piero Bellanova et al.
  • 1RWTH Aachen University, Neotectonics and Natural Hazards Group, Aachen, Germany (p.bellanova@nug.rwth-aachen.de)
  • 2RWTH Aachen University, Laboratory for Organic-Geochemical Analysis, Institute of Geology and Geochemistry of Petroleum and Coal
  • 3Universidade de Lisboa, Instituto D. Luiz, Faculdade de Ciências
  • 4University of Coimbra, Earth Sciences Department, Faculty of Sciences and Technologies
  • 5Complutense University of Madrid, Department of Geodynamics, Stratigraphy and Paleontology, Fac. Geological Sciences
  • 6Johannes Gutenberg-Universität Mainz, Institute of Geography, Natural Hazard Research and Geoarchaeology Group
  • 7University of Cologne, Institute of Geography, Department of Geosciences
  • 8RWTH Aachen University, Institute of Hydraulic Engineering and Water Resource Management
  • 9Instituto Hidrográfico, Divisão de Geologia Marinha
  • 10MARUM - Center for Marine Environmental Sciences, Research Faculty, University of Bremen

Research on offshore tsunami deposits is scarce and their depositional processes and preservation potential are virtually unexplored. Therefore, the RV Meteor cruise M152 mapped and sampled one coast-parallel and two coast-perpendicular transects at water depths from 65 to 114 m off the Algarve coast (Portugal). This coast was strongly affected by the well-known Lisbon earthquake and tsunami of November 1st, 1755 AD. Numerous onshore locations have been well documented and studied with historic damage reports and modern scientific investigations of the onshore tsunami deposits. However, very scarce information about the backwash, the water masses flowing back into the sea, exists and their imprint on the shelf is unexplored.

In order to fill this gap, a total of 19 vibracores were recovered during the RV Meteor cruise M152. For tracing the sedimentary imprint of the AD 1755 tsunami and potential predecessors, a multi-proxy analysis was carried out (sedimentology, micropaleontology, inorganic and organic geochemistry, radiocarbon and OSL dating). Within the offshore Holocene stratigraphic record, at least two event layers of likely tsunami backwash origin were identified based on their significantly different properties compared to the background shelf sediments. The uppermost tsunami layer (at a depth of 16-25 cm in most cores) displays an erosional contact at the base with heterogeneous compositional changes; its bounding radiocarbon ages allow a correlation with the AD 1755 Lisbon tsunami. Organic-geochemical markers, such as n-alkanes, polycyclic aromatic hydrocarbons, steroids and fatty acids, show an increased input of terrestrial matter in this offshore AD 1755 event layer.

A surprising discovery was another distinct high-energy deposit, i.e. a potential predecessor to the AD 1755 Lisbon tsunami, at a core depth of about 122-155 cm, which was 14C-dated to approx. 3700 yrs cal BP. Due to its erosional base and coarse-grained composition (well-sorted medium sand), as well as the increased terrestrial influence (displayed by biomarkers), it can be assumed that this deposit originates from the backwash of a paleo-tsunami.

This multi-proxy approach with sedimentological, micropaleontological, inorganic and organic-geochemical criteria, enabled us to (1) identify of backwash tsunami deposits; (2) establish a recurrence interval; and (3) estimate the hazard potential for the related coastal areas. Results of the M152 cruise demonstrate for the first time that the depositional basins on the Algarve shelf have the potential to reliably archive Holocene tsunami backwash deposits. The low-energy environment of the outer Algarve shelf sets prime conditions for the preservation of tsunami backwash deposits. Thus, these geoarchives offer the possibility to study the mechanisms and hydrodynamics of backwash currents, and to investigate tsunami strata that are not preserved elsewhere.

How to cite: Bellanova, P., Reicherter, K., Costa, P. J. M., Frenken, M., Feist, L., Schwarzbauer, J., Santisteban, J. I., Vött, A., Bosnic, I., Brückner, H., Schüttrumpf, H., Andrade, C., Duarte, J. F., Kuhlmann, J., and M152 scientfic Team, T.: Uncharted archives – imprints of tsunami backwash deposits on the Algarve shelf (Portugal), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7686, https://doi.org/10.5194/egusphere-egu2020-7686, 2020.

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