The Continental Shelf As an Offshore Archive for Tsunami Deposits

EGU2020-8504 152 - Lis R bo O n E 1 T 7 E 5 5 M The continental shelf as an offshore archive for tsunami deposits

u 8 n 1 c h 0 - an example from southwest Iberia (RV METEOR cruise M152) - a 2 l - V H O a N mb 13 urg 02 -

Lisa Feist1*, Klaus Reicherter1, Pedro J.M. Costa2,3, Piero Bellanova1,4, Juan I. Santisteban5, Ivana Bosnic2, Cristina Val-Péon1, *corresponding author Jan Schwarzbauer4, Mike Frenken1,4, Andreas Vött6, Helmut Brückner7, Holger Schüttrumpf8, César Andrade2, João F. Duarte9, Lisa Feist, M.Sc. Jannis Kuhlmann10, and the M152 scientific team +49 241 80-98237 [email protected] 1 Neotectonics and Natural Hazards Group, RWTH Aachen University, Germany 2 Instituto D. Luiz, Faculdade de Ciências, Universidade de Lisboa, Portugal Scan or click on 3 Earth Sciences Department, Faculty of Sciences and Technologies, University of Coimbra, Portugal 4 Laboratory for Organic-Geochemical Analysis, Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Germany the QR codes 5 Department of Geodynamics, Stratigraphy and Paleontology, Fac. Geological Sciences, Complutense University of Madrid, Spain for information 6 Institute of Geography, Natural Hazard Research and Geoarchaeology Group, Johannes Gutenberg-Universität Mainz, Germany 7 Institute of Geography, Department of Geosciences, University of Cologne, Germany 8 Institute of Hydraulic Engineering and Water Resource Management, RWTH Aachen University, Germany This work is part of project 9 Divisão de Geologia Marinha, Instituto Hidrográfico, Portugal 10 MARUM - Center for Marine Environmental Sciences, Research Faculty, University of Bremen, Germany

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400000 800000 500000 520000 540000 A B Portimão For more information ALGARVE Lagos see cruise report Vigo Boca do Rio

Porto Fig. 2 50 m 19 Sagres 20 21 17 16 13 15 22 11 14 9 Gravity cores 7 12 SPAIN 10 Portimão canyon PORTUGAL 23 100 m Number of cores: 4 24 6 Sampling sites: 3-5 Lisbon Water depths: 221 - 476 m 5 4 150 m Vibracores 25 200 m shelf (west & east transects) 4080000 4100000 26 Number of cores: 19 300 m 3 B Seville Sampling sites: 6, 7, 9-13, 19-23 Faro 400 m Water depths: 65 - 113 m 500 m 600 m gravity core Grab samples 27 700 m vibracore shelf (parallel), slope (west) 4000000 4400000 Number of cores: 8 0 10 20 grab sample cruise track Sampling sites: 14-17, 24-27 km Water depths: 69 - 625 m

Introduction: In AD 1755 a strong earthquake-generated tsunami destroyed large parts of the southwest Iberian coastline. Since the hydrodynamics of tsunami backwash currents are as yet poorly understood, 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 Algarve continental shelf were obtained during RV METEOR cruise M152 in November 2018. Along the Algarve coast, prominent AD 1755 Lisbon tsunami deposits have been detected onshore for quite some time (e.g., at Boca do Rio).

Figure 1: A Location of the study area offshore of southwestern Portugal. B Cruise track and sampling sites (gravity coring, vibracoring, grab sampling) of RV METEOR cruise M152 with bathymetry.

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510000 515000 A B NE SW N S A TWT (two-way-traveltime) TWT [s] 1920 of 0.1 s refers to appr. 75 m 0.070 21 0.080

4100000 0.090 ALGARVE 22 COAST 0.100 0.110 0.120 23 0.130 0.140 2 19 4 km 0.150 20

4095000 21 NE SW

cores GeoB23519-01/-02/-03 22 profiles in B organic-rich silt layer core GeoB23520-01 4090000

0 2 4 km

Legend for A: coarse silt rocky 0.25 - 0.5 mm 0.062 - 0.25 mm 25% < mud < 50% lithoclastic biolithoclastic sandy litho- ca. 3700 cal. BP event layer area medium sand fine sand bioclastic mud > 0.5 mm > 15% 0.25 - 0.5 mm 0.062 - 0.25 mm 25% < mud < 50% gravelly bio- lithobioclastic lithobioclastic sandy biolithoclastic sand medium sand fine sand clastic mud fine sand 0.25 - 0.5 mm mud < 25% mud < 25% 50% < mud < 90% bedrock bioclastic muddy biolitho- muddy lithobioclastic 1 km medium sand clastic sand bioclastic sand mud (drilled at site 19-02) Instituto Hidrográfico, 2009, Surface Sediments of the Portuguese Continental Shelf Chart - SED 7/8.

Figure 2: A Surface sediments in the area of the western transect (adapted from Instituto Hidrográfico, 2009). B Sub-bottom profiles (obtained by an Atlas Parasound P70 system) of the western transect with interpretation based on reflectors and stratigraphy of the retrieved vibracores.

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GeoB23519-01 GSD (%) Mean (Φ) Sorting (Φ) GeoB23519-01 - ca. 3700 cal. BP event layer 0 50 100 64 2 0 4 2 0 0 m background mud sand very fine sand medium 120 sedimentation silt gravel normal 0.5 m grading 125 coarse silt 1.0 m 130

siliciclastic relatively 135 medium sand homogeneous very fine 1.5 m massive sand deposit 140

2.0 m fine 145 sand

2.5 m 150 inverse medium grading fine sand sand 155 shell hash 3.0 m shell hash erosive base erosive base background shell 160 coarse silt cm sedimentation hash 3.5 m

Figure 3: Stratigraphy, grain size distribution (GDS, in %), mean grain size (in Φ) and sorting (in Φ) of core GeoB23519-01 (from 65 m water depth). Photo and sub-units of a ca. 3700 cal. BP event layer (1.21-1.55 m core depth) are shown in a close-up.

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GeoB23519-01 MS p-waves Ti Ca Si K 1969 tsunami? GeoB23519-02 n-alkanes ratios -700 70 1250 2050 00.6 0.12 03 6 0 0.025 0.05 0 0.07 0.14 1755 tsunami? 4x105 12x105 050 100 0 m 0 m 171 cal. BP (1779 cal. AD) 1057 cal. BP (893 cal. AD) 2220 cal. BP (271 cal. BC) pot. event? pot. event? 1.0 m 3656 cal. BP (1707 cal. BC) ca. 3700 cal. BP tsunami 1.5 m 3721 cal. BP (1772 cal. BC) terrestrial CPI aquatic OEP marine TAR 2.0 m 2.0 m Biomarker indicative ratios

2.5 m 8439 cal. BP CPI = carbon (6490 cal. BC) preference index

OEP = odd/even 3.0 m 3.0 m predominance

TAR = terrestrial/aquatic ratio 3.5 m 11,195 cal. BP -1.50 1.5-2 0 20 0.03 0.060 0.015 0.030 60 1800 10 20 (9246 cal. BC) log(Fe/Ca) log(Si/Ti) Ti/Ca Si/Ca Si/Al K/Si Radiocarbon ages: GeoB23519-01 GeoB23519-02

-5 Figure 4: Stratigraphy, magnetic susceptibility (MS, 10 SI), p-wave velocities (m/s), XRF results (cps/kcps and (log-) ratios), n-alkanes (ng/gTOC) and biomarker indicative ratios of GeoB23519-01. Potential event layers are marked, ultimate assignments to specific event types and correlations to confirmed events can as yet not be made - work in progress.

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Interpretation See Val-Péon et al. (EGU2020-13902) A tsunami backwash scenario can be assigned as most suitable for the generation of the ca. 3700 cal. BP event for foraminifera & pollen results deposit, based on internal structure (traction carpet, massive deposit), coastal provenance of the deposited sediments (based on XRF and biomarker), broad lateral distribution and likely correlation to Spanish tsunami records along the Gulf of Cádiz. Dry conditions related to the 4.2 ka climate event (see Val-Péon et al., EGU2020-13902) may have favoured sand availability both onshore and nearshore, thus generating increased sediment supply for the event deposit.

Conceptual model onshore continental shelf slope beach inner mid outer shelf break Backwash fine-grained surface suspension cloud onshore deposit suspension erosion

t sunami backwash deposition high-density flow

The continental shelf as an offshore archive? Acknowledgements: The authors are (a) Tsunami event deposit preservation is possible on the continental shelves below the storm-wave base deeply grateful to the research and technical crew under the command of Captain (b) Pre-historic event deposits may also be preserved considering the lower sea-levels Hammacher from RV METEOR cruise M152. The work benefited from the support Offshore sedimentary shelf archives show the potential to extend the understanding of the regional tsunami history, of Project OnOff (PTDC/CTAGEO/28941/ especially when coastal records are incomplete or sparse. However, offshore tsunami research is as yet limited (e.g., 2017) financed by FCT. The authors lateral distribution, access etc.) and focuses on specific locations along the shelf, such as depositional areas in mor- acknowledge financial support by the DFG and logistic support by the LDF. Martin phological sinks or channel structures, wherefore observed characteristics are strongly site-specific. Furthermore, Melles, University of Cologne, kindly facilitat- tsunami records of continental shelves face limitations due to, e.g., post-depositional bioturbation and current-induced ed access to an ITRAX core scanner for XRF sediment transport. analysis.

M152 shipboard scientific party: C. Andrade, G. Bartzke, P. Bellanova, H. Brückner, P.J.M. Costa, B. Deutschmann, J.F. Duarte, D. Eichner, F. Fatela, L. Feist, M. Frenken, H. Hadler, N. Höbig, L. Hönekopp, L. Koch, T. Köhler, J. Kuhlmann, N. Lapa, S. Lechthaler, M. Mathes-Schmidt, K. Pallapies, J. Pombo, J.I. Santisteban, H. Schüttrumpf, J. Schwarzbauer, A.N. Silva, A. Vött Chief scientist: K. Reicherter

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