Search

The precise location of the seismic source of 1755 CE Great Lisbon earthquake is still uncertain. The aim of this work is to use an onland sedimentary record in southern Portugal to test and validate seismic sources for the earthquake. To achieve this, tsunami deposit thicknesses from over 150 cores collected at Salgados in southern Portugal were compared to the results of a tsunami sediment transport model (Delft3D‐FLOW) that simulates tsunami propagation, inundation, erosion, and deposition. Five different hypothetical seismic sources were modeled with varying bed roughness coefficients to assess how well they reproduced observed patterns of tsunami deposit thicknesses and dune. Modeled and observed historical tsunami arrival times were also used to test different earthquake sources. Based on these comparisons, three modeled earthquake sources were able to reproduce the observed data, suggesting they should be regarded as somewhat more likely sources for the 1755 earthquake in contrast to four other modeled sources. The fault closest to shore (Marquês de Pombal) yielded the best correlations between model and observations.

Tsunami geoscience has evolved greatly since its seminal works in late 1980’s. Initially, tsunami deposits were merely identified in the stratigraphic record using its singularity and penetration inland. Later, especially after the 2004 and 2011 tsunami events, recognition of tsunami deposits progressed to interpretation on sediment dynamics and inundation phases based on the progressive application of different sedimentological, geochemical, paleontological and geophysical analytical techniques. Equally to other locations worldwide, tsunami deposits in Portugal were originally (early 1990’s) identified due to its geomorphological imprint or by its coarser sandy nature in muddy low-lying basins within the stratigraphic sequence of coastal sectors along the southern coast (Algarve). Many of these deposits were firstly studied in detail in terms of spatial distribution, texture and micropalaeontological composition. One aspect that was noticed was the uniqueness of the CE 1755 event in the top of the Holocene sequence. The CE 1755 tsunami is well-known for its consequences all over the Atlantic basin however its epicenter is yet to be established with certainty. In that sense, over the last decade, a multitude of analysis and new sites were studied (Salgados, Alcantarilha, Furnas, Barranco, Almargem) and contributed to shed new light on the CE 1755 and on other extreme events that impinged the Portuguese coastal fringe. For example, boulder analysis and the erosional signature in dune fields were used to model wave flow characteristics (run-up, flow velocity and flow depth). On the other hand, grain-size data and heavy mineral composition established a robust source-to-sink relationship between the CE 1755 tsunami deposits and dune sediments. Similarly, microtextural analysis corroborated these findings reaching similar conclusions. The application of geochemistry and high-resolution micropaleontological analysis brought new insights in terms of inundation extent and in the establishment of inundation phases. All these analyses contributed to a better understanding of the CE 1755 tsunami dynamic and its onshore sedimentological imprint. Very recently, state-of-art hydrodynamic and morphodynamic modelling exercises have been conducted using this unique geological database to be validated. They contribute to exclude potential generation zones and to narrow down the search for the CE 1755 epicenter.Another very innovative aspect is the recent study of the shelf area that is providing a ground-breaking opportunity to couple onshore and offshore palaeotsunami data and make inferences about the relevance of the backwash process on the depositional imprint.This work will summarize the present state-of-knowledge on the Portuguese tsunami geological record, a unique tsunami geoscience case-study in Europe. Authors acknowledge the financial support of FCT through projects UIDB/50019/2020–IDL and OnOff –PTDC/CTAGEO/28941/2017.

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.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.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 Ta division of the Bouma sequence or the S1, S2 and S3 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.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.

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.

Understanding past tsunami events is key to assess tsunami hazard and numerical modelling is a powerful tool to better understand these events. A rising number of studies focusing on palaeotsunami numerical modelling reinforce the potential of this tool to unravel unknown features of past tsunami events. The AD 1755 Lisbon is the largest historical tsunami registered on Western Europe and despite its relevance there are still uncertainties regarding its source, magnitude and wave characteristics. This work contributes to a better understanding of the AD 1755 tsunami through the evaluation of the wave characteristics and of the onshore and offshore sedimentary dynamic using Delft3D software (Flow module). The study site, Almargem lowland, is located on the Portuguese southern coast (central Algarve) where a two-dimensional vertical (2DV) morphodynamic modelling approach was carried out along 10 vertical layers. Bathymetry is represented by a across-shelf profile with 7 kilometers long thus, extending from the coast up to 50 m deep. Bottom sediments and roughness vary along the profile according to the sediment characteristics of each coastal sector (shelf, beach, dune and lowland area). Boundary conditions were imposed according to three scenarios represented by offshore sinusoidal tsunami waves of 3, 4- and 5-meters height. Tsunami-induced sedimentary dynamics results show that the 3 m wave scenario (50 m depth) present the most compatible scenario with the observed onshore deposit, in particular its 600 m inland extension. Moreover, the dune was singled-out as the main sediment source of the deposit. This source-to-sink relationship confirms previous field and sedimentary results (Costa et al., 2012, Sed Geol). A 3D morphodynamic modelling approach carried out by Dourado et al. (in press) on an adjacent study area (Salgados, central Algarve) also pointed to a 3-meter tsunami wave height (at 50 meters deep) and further corroborates the results of this work. Despite good initial results, with harmonization and validation of modelling results, further efforts are necessary to incorporate backwash influence on the onshore sediment dynamics as well as a detailed analysis on the vertical sediment distribution. The authors would like to acknowledge the financial support from FCT through projects UIDB/50019/2020 – IDL and PTDC/CTA-GEO/28941/2017 - OnOff.

This study examined temporal distributions of macrobenthic communities in relation to bottom depth and specifically addressed whether variation in sedimentary dynamics affects species composition and functional structure. Sampling was carried in 2014 (February, May and September) off Tavira Island (southern Portugal). In each sampling period, three replicates of sediment samples were collected at 7, 12, 17 and 22 m deep for both macrobenthic and sediment characterisation. Distributional patterns of macrofauna communities were a function of the interaction between depth and temporal variability, influenced by a gradient in hydrodynamics and species recruitment. The duration of sediment remobilisation and the organic matter content in sediments were the main features of the shoreface sedimentary dynamics responsible for differences in macrobenthic species composition and feeding habits. At low depths (7 m), under intense hydrodynamic conditions and longer period of sediment remobilisation, sediments were characterised by slightly gravelly sand with low contents of organic matter and calcium carbonate. Macrobenthic communities inhabiting shallow depths displayed low abundance and species richness, being dominated by grazers and suspension feeders. In contrast, at the highest studied depth (22 m), subject to low sediment remobilisation, sediments were composed by gravelly sand with high contents of organic matter and calcium carbonate. This depth (22 m) was characterised by high abundances and species richness, reflecting a more stable macrobenthic community composed mainly by surface deposit feeders. According to the benthic communities, the seaward limit of significant wave effects on the seabed is around 7 m in low-energy periods and over 17 m in high hydrodynamic energy periods.

The southwest of the Iberian peninsula is, due to its border position between Africa and Europe, a key territory of major geoarchaeological interest, as well as a reservoir of biodiversity and a wildlife refuge area during the Holocene. Bioclimatic conditions have been significantly unstable during this period in the Western Mediterranean. Therefore, further studies are still required to understand how abrupt climate changes such as the 8.2 and 4.2 ka cal BP events impacted societies and environment. In November 2018 the RV Meteor cruise M-152 retrieved 19 vibracores and 4 gravity cores along the Algarve coast after mapping the bathymetry. One of these cores, GeoB23519-01, was taken 65 m below present sea level and recovered 365 cm of sediment. Four potential event layers were identified over the last 11 ka cal BP and, at least two of them, are related to tsunami deposits (ca. 4370 cal BP and AD 1755). This sedimentary archive was analysed in a multi-proxy approach, including palynological and micropalaeontological analyses, which allow characterizing palaeoenvironmental changes along the core. However, considering the characteristics of these deposits, we raise questions about how complex this palynological record is and how it mirrors some short-term events, climate dynamics, and cultural disruptions.

The precise location of the seismic source of Great Lisbon Earthquake is still uncertain. The aim of this work is to use the sedimentary record to test and validate seismic sources of the AD 1755 e...

This paper reviews the evidence for onshore sedimentary imprints of tsunami inundation known from the geological record of the Atlantic basin. The central aim of the paper is to offer a broad overview on the main deposits and key localities that have been documented along the Atlantic coastlines, and which attest to the local or regional impact of tsunamis during historical, pre-historical and recent geological times. Considerable detail is devoted to summarising key diagnostic criteria used to identify the deposits as tsunami-derived, and to set each deposit in its own unique geomorphological setting and context, always referring to the latest scientific knowledge of the events that generated them. The paper also discusses the relationships between the different tsunamigenic sources that concurred to the formation of the deposits, as well as critical information on magnitude and frequency, as inferred from the sedimentary responses preserved in the sediment archives. Documented case studies range from the well-studied landslide-triggered and earthquake-triggered Storegga and 1755 Lisbon tsunamis, respectively, to collapse- and eruption-triggered tsunamis in the Atlantic archipelagos, to other less well-known events in the south Atlantic, Caribbean and Arctic. Despite its less frequent tsunami recurrence, the Atlantic coastal stratigraphy presents some world-class case studies and outstanding outcrops, from which critical knowledge can be gained with respect to some of the most enigmatic aspects of tsunami science.

Tese de doutoramento, Geologia (Geodinâmica Externa), Universidade de Lisboa, Faculdade de Ciências, 2017 Submitted by Amelia Janeiro (ajaneiro@reitoria.ul.pt) on 2018-10-09T15:39:51Z No. of bitstreams: 1 ulsd732233_td_Ivana_Coelho.pdf: 6149132 bytes, checksum: 334da2e05aac1a85bb81d6c5b6b80ef5 (MD5) Made available in DSpace on 2018-10-12T15:15:14Z (GMT). No. of bitstreams: 1 ulsd732233_td_Ivana_Coelho.pdf: 6149132 bytes, checksum: 334da2e05aac1a85bb81d6c5b6b80ef5 (MD5) Previous issue date: 2017 Fundação para a Ciência e a Tecnologia (FCT), SFRH/BD/77224/2011

The understanding of the sedimentary links between the beach and the continental shelf is crucial for the evaluation of the coastal sediment budget. However, the comprehension of this dynamics is still poorly understood owing greatly to the lack of direct sediment transport measurement at seasonal and longer time scales. This work aims at evaluating sediment transport just seaward of the closure depth through a sand tracer experiment coupled with wave–current monitoring and modelling. Observations were carried out over 1 year at 14 m depth over a sandy continental shelf offshore Tavira (southern Portugal). The sand tracer experiment was carried out by injecting 400 kg of fluorescent tracer followed by four sediment sampling surveys. Tracer results show a high dispersion of the tracer cloud with a net transport of low magnitude. Time-averaged alongshelf sediment transport rate was estimated in 0.61 m3/m/yr (southwestward) while the cross-shelf transport rate was estimated in 0.31 m3/m/yr (onshore). During the observational period nearbed currents were dominated by the northeastern component, thus flowing in the opposite direction of the tracer displacement. However, when wave-current bed shear stress exceeded the threshold of particle motion, nearbed currents were dominated by a southwestern component which is compatible with tracer displacement. Overall this study showed that seaward the closure depth bottom sediment dynamics is characterized by frequent remobilization but with very low net transport rates. info:eu-repo/semantics/publishedVersion