Vera Lopes

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.

Cryogenic processes and fire activity in a high Atlantic mountain area in NW Iberia (Picos de Europa) during the Mid–Late Holocene

Mid-Late Holocene environmental changes in the Cantabrian Mountains are a consequence of both climate variability and human activity. A 182cm-long sedimentary sequence was collected from Belbín depression, Western Massif of Picos de Europa (Cantabrian Mountains, NW Spain), in order to reconstruct Holocene environmental dynamics and the factors triggering landscape changes in the area. Using multi-proxy analysis of the uppermost 60cm of the sediments (texture, organic matter content, quartz grains microstructures, charcoal deposition) together with three 14C AMS dates, a sequence of alternating warmer and colder phases has been inferred for the last ca. 6.7kycalBP. Warm stages are defined by low to moderate chemical weathering of the quartz grain particles with relative increases of the C/N ratio, while colder phases show a moderate to intense physical weathering of the quartz grains and lower C/N ratios. Warmer temperatures were recorded in Belbín area between: 6.7-5, 3.7-3, 2.6-1.1, 0.87-0.51 and since 0.01kycalBP. A colder regime occurred between 5-3.7, 3-2.6, 1.1-0.87 and 0.51 to 0.01kycalBP. The increasing organic matter content during the Late Holocene may be associated with increasing temperatures. The charcoal particles do not show a higher or lower concentration during prevailing colder or warmer conditions, and therefore may be linked to human-induced fire management of the landscape. The most intense period with fire activity occurred between 3.5 and 3kycalBP during the Bronze Age.

Cyprideis torosa (Jones, 1850) in mainland Portugal: what do we know?

The distribution of Recent and Holocene Cyprideis torosa (Jones, 1850) from brackish estuaries and lagoons of mainland Portugal is presented; older Cenozoic C. torosa are also briefly mentioned. Around 550 specimens were found alive, in Melides lagoon (456) and salt marshes from large estuaries (90). Thousands of empty valves and carapaces, with different ornamentation, were found in the lagoon, the estuaries and in Holocene samples from boreholes, in the latter with higher concentration in sediments deposited in lagoons. The modern C. torosa was found living mainly in Melides lagoon, associated with brackish and freshwater ostracod species, in soft mud or muddy sand sediments from quiet zones, with salinities from 4.1 to 15.1; it is also found in soft mud to muddy sand sediments whose interstitial waters have salinities from 19.0 to 34.1, on salt marshes (tidal flat and low marsh) of large estuaries, similar to lagoons, in low hydrodynamic areas, together with other brackish ostracods. The main parameters controlling the distribution of C. torosa appear to be the substrate, water dynamics and emersion time, whereas water chemistry is more related to the morphology of valves. In general, C. torosa presents the same ecological characteristics in the Holocene and in modern times.

Holocene sea level and climate interactions on wet dune slack evolution in SW Portugal: A model for future scenarios?

We examine the Holocene environmental changes in a wet dune slack of the Portuguese coast, Poço do Barbarroxa de Baixo. Lithology, organic matter, biological proxies and high-resolution chronology provide estimations of sediment accumulation rates and changes in environmental conditions in relation to sea-level change and climate variability during the Holocene. Results show that the wet dune slack was formed 7.5 cal. ka BP, contemporaneous with the last stages of the rapid sea-level rise. This depositional environment formed under frequent freshwater flooding and water ponding that allowed the development and post-mortem accumulation of abundant plant remains. The wetland evolved into mostly palustrine conditions over the next 2000 years, until a phase of stabilization in relative sea-level rise, when sedimentation rates slowed down to 0.04 mm yr−1, between 5.3 and 2.5 cal. ka BP. Later, about 0.8 cal. ka BP, high-energy events, likely due to enhanced storminess and more frequent onshore winds, caused the collapse of the foredune above the wetlands’ seaward margin. The delicate balance between hydrology (controlled by sea-level rise and climate change), sediment supply and storminess modulates the habitat’s resilience and ecological stability. This underpins the relevance of integrating past records in coastal wet dune slacks management in a scenario of constant adaptation processes.

A GIS-assisted reconstruction of the Holocene transgressive paleosurface of Pederneira lowland (W Portugal)

ABSTRACT Lopes, V., Freitas, M.C., Andrade, C., Taborda, R., Ramos, R. and Oliveira, M.A. 2013. A GIS-assisted reconstruction of the Holocene transgressive paleosurface of Pederneira lowland (W Portugal) The Last Glacial Maximum low stand and coeval re-incision of the Portuguese hydrographic network created the space and defined the shape of the main morphological features available to accommodate the inundation resulting from the Holocene transgression and the marine sediments deposited in tune with changing base levels. The reconstruction of the transgressive paleosurface is relevant in paleoenvironmental studies, namely to establish the paleobasin morphology and evaluate accommodation volume. In the case of the study area (Pederneira lowland), the geological data set available to reconstruct that surface consists of borehole logs carried out for geotechnics and groundwater and three cores performed for paleoenvironmental reconstruction. The borehole data scatter non-uniformly across the lowland providing spatially biased information. The surveying of several Transient Electromagnetic Method (TEM) profiles yielded inconclusive results in separating sediment units below and above the transgressive surface due to poor contrast in sediment resistivity and high conductivity of groundwater. This study addresses a method for estimating that paleosurface using the combination of a standard geostatistical interpolation procedure (kriging) with a conceptual geomorphological model. The geomorphological model comprised the sketching of a paleoriver drainage system in which the position of the thalwegs was defined departing from present-day slope morphology and depths estimated by adjusting a mathematical curve describing their longitudinal profile to control points. Final adjustments to the paleodrainage system were constrained by borehole data and expert judgment. This methodology proved to be effective in an area where limited objective geological and geophysical data are available.