Susana Costas

DISTRIBUTION OF BENTHIC FORAMINIFERA IN COARSE SEDIMENTS, RÍA DE VIGO, NW IBERIAN MARGIN

The distribution of live benthic foraminifera (>63 μm) in box core samples from coarse sediments of the Ria de Vigo (NW Iberian margin) is mainly related to food availability and bottom current range. Live benthic foraminifera are dominated by small forms (63–125 μm). Faunal densities and diversity values suggest that very coarse substrates provide a favorable settlement for live benthic foraminifera. It is suggested that microalgae or bacterial biofilms that colonize shell surfaces are the main food resource for foraminifera. During upwelling events, the food supply to the bottom increases and the environment is colonized by opportunistic species. Moderate- to low-velocity bottom currents favor such pathways. The upwelling not only enhances the density of benthic foraminifera, but also allows the arrival of new species from the shelf. Attached forms are related to relatively high-velocity bottom currents and free living forms to moderate- to low-velocity bottom currents. Live benthic foraminifera inhabit a wide range of sediment depths and are not restricted to the uppermost centimeter. Under relatively high-velocity bottom current regimes, the vertical distribution of live benthic foraminifera is controlled by current strength. In coarse sediments, attached forms commonly considered strict epifaunal, and inhabiting above the disturbance depth, are considered “pseudoepifauna.” Below the disturbance depth, the sediment is not affected by currents and a “true” infauna is recognized.

Coastal barrier stratigraphy for Holocene high-resolution sea-level reconstruction

The uncertainties surrounding present and future sea-level rise have revived the debate around sea-level changes through the deglaciation and mid- to late Holocene, from which arises a need for high-quality reconstructions of regional sea level. Here, we explore the stratigraphy of a sandy barrier to identify the best sea-level indicators and provide a new sea-level reconstruction for the central Portuguese coast over the past 6.5u2009ka. The selected indicators represent morphological features extracted from coastal barrier stratigraphy, beach berm and dune-beach contact. These features were mapped from high-resolution ground penetrating radar images of the subsurface and transformed into sea-level indicators through comparison with modern analogs and a chronology based on optically stimulated luminescence ages. Our reconstructions document a continuous but slow sea-level rise after 6.5u2009ka with an accumulated change in elevation of about 2u2009m. In the context of SW Europe, our results show good agreement with previous studies, including the Tagus isostatic model, with minor discrepancies that demand further improvement of regional models. This work reinforces the potential of barrier indicators to accurately reconstruct high-resolution mid- to late Holocene sea-level changes through simple approaches.

Climate-driven episodes of dune mobilization and barrier growth along the central coast of Portugal

Abstract Here, we explore the evolution of the coastal stretch between Mira Beach and Quiaios Beach in Portugal to understand how it adapted to climatic oscillations. To accomplish this, we integrate subsurface radar images, and sedimentological and chronological data, of the emerged coastal barrier. Our results show the installation and progradation of a stable barrier anchored to transgressive dunes 400 years ago. This is just the last pulse of barrier growth within a complex approximately 5000 year-long history of shoreline stability/instability. Episodes of inland dune mobility have been related to instabilities in the beach sediment budget driven by enhanced storminess and wave rotation around 4.25 and 1.14 ka ago. Conversely, lagoonal deposits documented in the literature suggest periods of relative barrier stability and growth around 4.3 and 2.7 cal ka BP. Wave and wind climate variability are driven by shifts in one of the major modes of atmospheric circulation in the North Atlantic, the North Atlantic Oscillation (NAO). Episodes of persistent positive mode of the NAO related to barrier growth and enhanced longshore sediment transport; those of persistent negative mode contributed to instabilities in the beach sediment budget and aeolian activity by enhancing storminess, but reduced effective longshore sediment transport.

8000 years of environmental evolution of barrier–lagoon systems emplaced in coastal embayments (NW Iberia)

The rocky and indented coast of NW Iberia is characterized by the presence of highly valuable and vulnerable, small and shallow barrier–lagoon systems structurally controlled. The case study was selected to analyse barrier–lagoon evolution based on detailed sedimentary architecture, chronology, geochemical and biological proxies. The main objective is to test the hypothesis of structural control and the significance at regional scale of any high-energy event recorded. This work is also aimed at identifying general patterns and conceptualizing the formation and evolution of this type of coastal systems. The results allowed us to establish a conceptual model of Holocene evolution that applies to rock-bounded barrier–lagoon systems. The initial stage (early Holocene) is characterized by freshwater peat sedimentation and ended by marine flooding. The timing of the marine flooding depends on the relation between the elevation of the basin and the relative mean sea-level position; the lower the topography, the earlier the marine inundation. Thus, the age of basin inundation ranged from 8 to 4 ka BP supporting significant structural differences. Once marine inundation occurred, all systems followed similar evolutionary patterns characterized by a phase of landward barrier migration and aeolian sedimentation towards the back-barrier (i.e. retrogradation) that extended circa 3.5 ka BP. The later phases of evolution are characterized by a general trend to the stabilization of the barriers and the infilling of the lagoons. This stabilization may be temporally interrupted by episodes of enhanced storminess or sediment scarcity. In this regard, washover deposits identified within the sedimentary architecture of the case study explored here suggest pervasive high-energy events coeval with some of the cooling events identified in the North Atlantic during the mid- to late Holocene.

Imprints of the 1755 tsunami in the Tróia Peninsula shoreline, Portugal

ABSTRACT Rebêlo, L., Costas, S., Brito, P., Ferraz, M., Prudêncio, M. I. and Burbidge, C., 2013. Imprints of the 1755 tsunami in the Tróia Peninsula shoreline, Portugal A large earthquake shook the offshore region of Portugal in November 1st 1755 with an estimated magnitude of 8.5±0.3 in the Richter scale. It lead to the massive destruction of the city of Lisbon, which was followed by intense fires. Although most of the detailed consequences of the earthquake and tsunami focused in Lisbon, the effect of tsunami destruction was also reported in other locations along the Peninsula coasts and Morocco. At Setúbal, located 30 km to the south of Lisbon in the Sado estuary, evidences of inundation were also documented, reporting wave heights that reached a first floor height. Despite the magnitude of the inundation in the interior of the estuary, there has not been reported any evidence of the impact of this event on the estuarine shorelines. In order to understand the consequences of such a wave on the sandy shores of the region, we explore the inlet shoreline and seaward exposed region of the estuary, the Tróia peninsula. To do so, we integrate multiple types of data and information: dune ridge position were extracted from aerial photographs, shorelines from the historical maps, shoreline progradation rates, Ground Penetrating Radar profiles and optically stimulated dating of barrier sediments. This information allows us to reconstruct the shoreline following the tsunami, documenting a greater impact towards the inlet where the water flow concentrate as it entered the estuary. The impact of the tsunami in Tróia peninsula translates on the erosion of about 1000 years of barrier deposits within the spit end.

The Joint History of Tróia Peninsula and Sado Ebb-Delta

Traditionally, the study of coastal evolution has focused on emergent barriers or stratigraphic sequences on the adjacent shelf, but seldom are these two systems studied holistically or the information combined into a single model. Here, we combine data sets from the emerged and submerged sectors of a prograding coast, from the coastal dune to the innermost continental shelf, to reconstruct the long-term history of shelf reworking and spit elongation of Troia Peninsula in Portugal. This analysis involves synthesizing high-resolution reflection seismic profiles from the shoreface, Ground Penetrating Radar images from the emerged sand barrier, high resolution digital terrain models, and Optically Stimulated Luminescence and radiocarbon dating of sediment samples from the emergent sand barrier and backbarrier. The results document the growth of the sandy peninsula in five major phases of progradation represented by massive foredunes separated by hiatuses of sedimentation and periods of shoreline stability. Formation of the peninsula began circa 6,500 years ago by spit elongation from the south as documented by the oldest beach sediments within the spit. The spit enlarged until a tidal inlet was formed around 3,300 years ago, which caused the construction of the ebb-tidal delta. The latter sequestrated the sand supplied to the spit inhibiting spit progradation until the ebb delta reached an equilibrium volume, allowing shoreline progradation out of phase with delta enlargement. Hiatuses in spit progradation are tentatively related to the onset of erosive conditions caused by enhanced storminess around 4,000 and 1,800 years ago and to a tsunami 250 years ago. These results suggest that the growth of the spit is largely controlled by self-adjustment processes and/or major climate shifts.