César Andrade

Sedimentation and hydrodynamic processes associated with the tsunami generated by the 1755 Lisbon earthquake

Abstract An unusual sedimentary horizon, believed to have been deposited by the tsunami associated with the 1755 Lisbon earthquake, has been identified at Boca do Rio in southern Portugal. This horizon marks a distinct sedimentological and micropalaeontological break from the deposits enclosing it. The sedimentary characteristics of the horizon, which consists of several distinct sub-units, vary greatly in both vertical and horizontal section. Detailed textural analysis through vertical sections of these sub-units shows abrupt changes in particle size characteristics. The highly variable nature of the horizon appears to be due to rapid variation in the hydrodynamic characteristics of the depositional event. The lowermost section of the deposit appears to have been deposited from a highly turbulent water mass which was able to transport gravel sized clasts as well as gravel sized mud balls eroded from underlying material. The water mass rapidly lost energy, leading to the deposition of predominantly sand, silt and clay particles. Sharp breaks in textural characteristics are likely to be due to changes in current direction caused by wave reflection or swash–backwash interference. The main sources of sand sized material within the deposit are the seaward barrier (beach–foredune system). The tsunami deposit at Boca do Rio shares many of the sedimentary characteristics of tsunami deposits from other sites around the world.

Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boca do Rio, Algarve, Portugal

A description is given of a distinctive marine deposit at Boca do Rio on the Algarve coast of Portugal. It is proposed here that the sediment accumulation was deposited by the tsunami generated by the Lisbon earthquake of 1 November, AD 1755. The deposit exhibits sedimentary characteristics quite unlike other coastal sediment accumulations that are deposited by more moderate wave régimes. These include laterally continuous sand layers, chaotic pebble horizons, large amounts of gravel-sized shell debris and distinctive assemblages of marine microfossils. A preliminary attempt is made here to use the empirical field and laboratory data to define more clearly the processes of coastal sedimentation that characterize tsunami runup. The use of tsunami sediments as time synchronous marker horizons is also discussed.

The geological record of environmental changes in southwestern Portuguese coastal lagoons since the Lateglacial

Abstract The multidisciplinary study (sedimentology, palaeoecology, geochemistry, isotopic dating) of the sedimentary infill of selected coastal lagoons of the southwestern Portuguese coast suggests that during the Early Holocene, eustasy was the dominating element forcing coastal change and controlled the sedimentation patterns of these areas. The strong decrease of the mean sea-level rise rate observed in the Middle Holocene defined an important threshold of regional significance. The flooded lowlands developed terminal barriers and started to silt up initially driven by natural forcing factors (e.g. climate, sediment supply) of local expression, which were later added or overwhelmed by anthropic influence. The latter relates with deforestation and agriculture, inducing changes of sedimentation rates and sediment delivery to the coast; human interference with the natural rhythms of barrier breaching generated significant change of the whole physico-chemical setting of the lagoonal basins that translated into sediments, water quality and depth and local ecosystems structure. This paper presents examples of the impacts of global versus local factors and natural versus anthropic influences as driving factors of coastal change since the Lateglacial. The available data suggest that this line of research is crucial to support objective and accurate models of past and future coastal evolution.

Separating eustatic from local environmental effects: a late-Holocene record of coastal change in Albufeira Lagoon, Portugal

Diatom and sedimentological analyses of a 7.6-m core of the Albufeira coastal lagoon, western Portugal, provide information about the forcing factors and environmental history of the evolution of this lagoon during the last 2500 years. At this timescale, eustasy is considered to be a minor environmental control on the dynamics of the barrier-lagoonal system. Instead, these dynamics are forced by local factors, such as changes in the sand-barrier permeability. Lithological and diatom facies are compared with present-day environ ments and show that the lagoon evolved essentially as a slightly brackish water body with a salinity of around 1‰. This pattern was disturbed by three major episodes of sedimentation associated with threshold responses in the permeability of the barrier. The first (c. 2370 BP) did not produce a specific lithological signal and is characterized by a peak in freshwater euplanktonic diatoms, implying an almost permanent isolation of the basin and freshwater flooding. A second episode was identified at c. 1600 BP, when a dramatic opening of the tidal inlet provoked a significant change in the sedimentation regime, which became dominated by inorganic, minerogenic sediments and allowed marine/brackish epiphytic diatoms to flourish together with allochthonous marine planktonic species. Finally, a third threshold occurred at c. 1225 BP corresponding to the establishment of the present-day muddy or sandy-muddy sedimentation pattern. Both the sedimentological features and the diatom assemblages indicate the establishment of a more tidally influenced environment. This reflects regular artificial breaching of the barrier associated with increasing anthropogenic intervention in the system.

Comparing historic records of storm frequency and the North Atlantic Oscillation (NAO) chronology for the Azores region

The storminess of the Azores region was investigated using newspaper records from AD 1836 onwards. The information obtained was rank-ordered for intensity and the time series of storm frequency analysed for interannual- to century-scale variability. The documentary data set was validated by comparison with objective cyclones intensity for the period AD 1958—2000. Results indicate that four periods of contrasting storm frequency are present (AD 1836—1870, 1870—1920, 1920—1940 and 1940—1998). The average storm lasts for 2.3 days and the average secular storm frequency is 3.1 storms/yr. Low intensity events occur four times every five years whereas an extreme storm occurs on average once every seven years. The documentary index of storminess is highly variable at different timescales, which is consistent with other studies of storminess in the North Atlantic. Nevertheless, an objective comparison between late nineteenth- and late twentieth-century storm frequency does not reveal a significant difference. Between AD 1865 and the late twentieth century the winter NAO and storminess indices show a statistically significant anti-correlation pattern at the monthly and seasonal scales. In the late nineteenth century and between AD 1950 and 1970 the NAO index was low and the storminess index high, whilst the opposite occurred from the early twentieth century until the middle 1950s; since AD 1970 both indexes reveal positive trends and are predominantly positive. The NAO mode of circulation is partially responsible for the storminess spatial pattern and temporal distribution over the Azores region since AD 1865 and for about a century, however this relation appears to have weakened since the 1960s.

Dune, Bluff and Beach Erosion due to Exhaustive Sand Mining – the Case of Santa Barbara Beach, São Miguel (Azores, Portugal).

ABSTRACT Coastal dunes are relatively rare features in the Azores islands and contribute to less then 1% to the total length of the archipelagos littoral zone, circa 844 km. Localised indications of aeolian activity, contemporaneous to the arrival of the first colonists in the 15th century have been found, but at present most of the coastal dunes are vegetated and stabilised. Dune sand has been exploited during the 20th century for industrial use, leading to severe damage or complete removal of a significant proportion of these features. In the 1950s the Santa Barbara area contained a rare and well-preserved example of an undisturbed active beach-foredune system, free of anthropogenic activities. It displayed a well-nourished beach profile margined by a coastal foredune and a climbing dune, which covered the adjacent bluff and extended landwards through an aeolian sand-sheet. Persistent sand mining between the early sixties and the late eighties, lead to volumetric depletion of the dune cover, lowering of the surface of the berm and triggering cliff erosion. In total, some 950,000 m3 of sand has been removed, the dunes making up half of this figure. Although this activity was stopped in 1995 by legal enforcement, the erosive process still currently persists indicating a surpassing of the natural resilience and maximum vulnerability of the coastal dune system. At present the bluff retreats at very high rates (0.6 m/yr), the dune features were totally obliterated and the sandy beach has been reduced in width to a single swash ramp which floods during half of the tidal cycle.

A tsunami record in the sedimentary archive of the central Algarve coast, Portugal: Characterizing sediment, reconstructing sources and inundation paths

This study describes sedimentation associated with the tsunami generated by the Lisbon earthquake of ad 1755. It is argued here that the tsunami deposited a sand sheet across the Lagoa dos Salgados (central Algarve, Portugal), that is intercalated with late-Holocene estuarine/lagoonal sediments. A wide range of proxies (sedimentological, exoscopic and palaeontological) are used to establish the provenance of the sandy material as well as to constrain the age of the deposit. Stratigraphic criteria are used to distinguish the uniqueness of the event layer. Exoscopic and textural analysis suggest that the source of the event deposit is mainly the dune, beach and underlying layer. Micropalaentological analysis (Foraminifera) indicates a conspicuous increase in diversity and dominance of marine species within the event sediment sheet. The spatial characteristics of the tsunami layer suggests that the barrier prevented widespread overtopping by the incoming tsunami allowing inference of c. 10 m as maximum height at the coast; they also indicate the inlet as the preferential route for both water and sediment transported inland. Dating results (14C, 210Pb and 137Cs) allow extrapolation of an age of deposition compatible with the ad 1755 Lisbon earthquake and tsunami, the most devastating event that affected this coastal area in historical times. Correlations with similar deposits detected in nearby lowlands strengthen the argument that the tsunami sediment layer represents a marker horizon in the coastal stratigraphy along the Portuguese Algarve coast. The stratigraphic uniqueness of this event might have implications in the establishment of millennial scale recurrence intervals for this type of high-energy marine inundation.

Wind-induced sand transport in Tagus estuarine beaches – First results

Sand beaches and spits of the southern embankment of the inner Tagus estuary indicate the influence of locally generated wind waves. The aim of this study is to evaluate the ability of these waves to resuspend bottom sediments, and sustain the longshore transport. For this purpose, local wave climates were obtained by running a wave forecasting numerical model for three selected target areas.Results suggest that local wave climate is largely responsible for sand movement and accumulation in the marginal zones of the inner estuary facing the longest effective fetch. Two main areas of longshore transport were identified in the inner Tagus estuary with similar annual gross transport rates, of about 12 000 m3 yr−1. Results obtained for the annual net drift amount and direction are in agreement with geomorphology and accretion direction of the beaches. Wave-induced longshore transport rates agree with field observations and erosional evolution of the Alfeite sand beach during the last century.

Modern foraminiferal record of alternating open and restricted environmental conditions in the Santo André lagoon, SW Portugal

Benthic foraminifera from eleven stations sampled seasonally were analyzed in order to examine the biological response to rapid and intense environmental changes taking place in the Santo André coastal lagoon. Foraminiferal assemblages show a very low species diversity and a high dominance of three euryhaline species throughout the year. Under closed-inlet conditions, foraminiferal assemblages exhibit low abundance of foraminiferal tests, whereas under open-inlet conditions absolute abundance greatly increases due to sea water entrance. Present-day dramatic environmental changes are thought to be responsible for deformed foraminiferal tests that are commonly found. Comparison of modern assemblages with those obtained from the Holocene sedimentary record indicates persistent restrictive environmental conditions after sandy barrier formation, circa 5000 years ago.

Onshore tsunami sediment transport mechanisms inferred from heavy mineral assemblages

The aim of this study is to discuss and to extend the characterization of (palaeo)tsunami deposits, and their source materials, based upon the detailed study of their heavy mineral assemblages. Results obtained from three distinct locations (Portugal, Scotland and Indonesia), different coastal contexts and chronologies (the tsunami events studied took place at 1500 cal. yr BP, AD 1755 and 2004) are summarized and discussed in order to contribute to the sedimentological study of onshore (palaeo)tsunami deposits. Results indicate that heavy mineral assemblages primarily reflect local specific conditions. For example, in the Portuguese sites, ca. 90% of the heavy mineral population consists of tourmaline+andalusite+staurolite, whereas in the Scottish samples garnet+amphiboles can be dominant in 90% of the assemblage, where at the Indonesian study site amphiboles+andalusite were the most frequent minerals. The application of Principal Component Analysis for each site reveals that the first two components explain at least 55% of the total variance. In the three studied areas, hydraulic sorting by density was observed and a higher presence of the denser heavy minerals of the assemblages was detected. However, it is important to stress that the sediment source plays a key role in the establishment of the heavy mineral assemblages of the (palaeo)tsunami deposits. In this study, relationships between the likely source sediments and the (palaeo)tsunami deposits were described and whenever possible sediment sources were clearly identified. Furthermore, it was also possible to detect the backwash signal using the analysis of the heavy minerals (e.g. higher frequency of denser minerals or variation in the presence of rounded or euhedral zircon). The work presented here, in contributing to the enhancement of sedimentological criteria presently available to recognize and differentiate extreme marine inundation deposits, also highlights new areas for future research.