Mário Cachão

Coccolithus pelagicus, a productivity proxy related to moderate fronts off Western Iberia

Abstract The traditional interpretation of Coccolithus pelagicus as a cold water proxy is examined based on its distribution patterns in the water column off the Portuguese coast (using data from eleven cruises) and in Holocene surface sediment samples and Quaternary cores from the same region. Coccolithus pelagicus is common in the Portuguese upwelling system, an area where surface waters are predominantly of subtropical origin. Although revealing an affinity for low temperature upwelled waters, the species was found in waters up to 18°C associated with riverine plume and shelf-break fronts. C. pelagicus seemed to consistently occupy a particular ecological niche, between other phytoplankton groups, related to moderate turbulence conditions combined with nutrient availability. From this behaviour, it is proposed that C. pelagicus can be used as a tracer of the periphery of areas of enhanced productivity. Coccolithus pelagicus preferences for fronts of moderate temperature and salinity gradients are tentatively used to explain particular features of its sedimentary record. The repeated increase of C. pelagicus in thanatocoenoses (surface sediment assemblages) close to three river mouths, on the Portuguese shelf, are interpreted as a positive response to the development of riverine plumes. On the other hand, inconsistencies in the correlation between sea surface cooler-glacial and warmer-interglacial isotope stages and the relative abundance pattern of C. pelagicus during the Late Quaternary, as registered in two Galicia Bank piston cores (42°N), are tentatively explained in terms of shifts in the extent of the outer limit of the local palaeoproductivity belt off the Iberian Peninsula.

On the presence of Allosaurus fragilis (Theropoda: Carnosauria) in the Upper Jurassic of Portugal: first evidence of an intercontinental dinosaur species

The species Allosaurus fragilis, from the Morrison Formation of North America (Kimmeridgian–Tithonian, Upper Jurassic), is one of the best known members of the Theropoda, the group including all predatory non-avian dinosaurs and birds. Here, we report on the first diagnostic theropod remains discovered from the Jurassic of the Iberian Peninsula. The specimen is the first evidence of A. fragilis outside North America. Thus, this taxon represents the first dinosaur species found on two different continents, and suggests the existence of a ‘land bridge’ between North America and Europe during the Late Jurassic.

Lateglacial and Holocene environmental changes in Portuguese coastal lagoons 2: microfossil multiproxy reconstruction of the Santo André coastal area

Benthic foraminiferal, ostracod, calcareous nannoplankton and diatom assemblages have been analysed from the 25.45 sedimentary succession of a borehole drilled in the Santo André lagoon (southwestern Portugal). The zones and subzones recognized according to their microfossil content have been used to define six palaeoenvironmental stages extending from the Lateglacial to the present. Stage A and the lower part of stage B (14.2 to 10.0 ka) represent a postglacial terrestrial environment when sea level was below present. Following eustatic sea-level rise, a normal marine, open-bay environment established during the upper part of stage B (10.0 to 5.4 ka). Before 5.4 ka, a coastal lagoon developed as a consequence of a sandy barrier formation. Since then, natural variation in the permeability of the detrital barrier through time has been the primary control of the physical, chemical and biological features of the lagoonal environment: during stage C (5.4 to 3.6 ka) as a freshwater-dominated lagoon, stage D (3.6 to 1.6 ka) as a brackish lagoonal environment, and stage E (1.6 to 0.5 ka) as a return to a freshwater-dominated lagoon. Finally, in the last 400 years (stage F), human intervention on the sandy barrier (artificial open- and closed-inlet conditions) has provoked rapid and dramatic environmental changes in Santo André.

THALASSINOIDES AND ITS PRODUCER: POPULATIONS OF MECOCHIRUS BURIED WITHIN THEIR BURROW SYSTEMS, BOCA DO CHAPIM FORMATION (LOWER CRETACEOUS), PORTUGAL

Abstract This work describes the occurrence of numerous, nearly intact Mecochirus rapax populations within Thalassinoides suevicus burrows in the Lower Cretaceous Boca do Chapim Formation. This exceptional preservation allows for unquestionable attribution of a specific producer to a biogenic structure. Analysis of sedimentological features combined with functional morphology and ichnological inferences suggests that the unique preservation of the fauna resulted from river flood depositional events that buried successive Mecochirus populations in a shallow marine setting.

The emergence of volcanic oceanic islands on a slow‐moving plate: The example of Madeira Island, NE Atlantic

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow-moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeiras subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

The sedimentary record of recent (last 500 years) environmental changes in the Seixal Bay marsh, Tagus estuary, Portugal

The inner Tagus estuary is essentially a sedimentation basin that receives cohesive sediment from terrestrial, marine, biological and anthropogenic sources. Three short cores from one site in a marsh area of this estuary (Seixal Bay) were analysed for sedimentary, geochemical and micropalaeontological contents (benthic foraminifera and nannoplankton). The length of the cores represents about half a millennium of sedimentation. Textural analysis suggests a highly uniform mud sedimentation for most of the cores but geochemical, mineralogical and micropaleontological results indicate climatic and environmental changes and anthropogenic disturbance. Three Foraminifera zones were identified. The lower part of the lower zone indicates sedimentation in an open channel or a lower domain of an exposed high-energy sandflat. Sediments of the upper part of the lower zone and of the middle zone were deposited in a lower-energy environment, probably associated with a sheltered, vertically aggrading mudflat located within the Seixal Bay. Biological and mineralogical indicators suggest that periods of total or partial closure of this bay occurred. Clay minerals indicate that drier and colder conditions prevailed in the lower half of this zone evolving gradually to a wetter and warmer environment towards the top. The upper zone indicates persistence of low-energy sedimentation and evolution towards the present salt-marsh conditions. Anthropogenic pollution is clear in geochemical proxies at the top of the sedimentary column and was used for dating purposes.

Nannofossils in 2011 El Hierro eruptive products reinstate plume model for Canary Islands

The origin and life cycle of ocean islands have been debated since the early days of Geology. In the case of the Canary archipelago, its proximity to the Atlas orogen led to initial fracture-controlled models for island genesis, while later workers cited a Miocene-Quaternary east-west age-progression to support an underlying mantle-plume. The recent discovery of submarine Cretaceous volcanic rocks near the westernmost island of El Hierro now questions this systematic age-progression within the archipelago. If a mantle-plume is indeed responsible for the Canaries, the onshore volcanic age-progression should be complemented by progressively younger pre-island sedimentary strata towards the west, however, direct age constraints for the westernmost pre-island sediments are lacking. Here we report on new age data obtained from calcareous nannofossils in sedimentary xenoliths erupted during the 2011 El Hierro events, which date the sub-island sedimentary rocks to between late Cretaceous and Pliocene in age. This age-range includes substantially younger pre-volcanic sedimentary rocks than the Jurassic to Miocene strata known from the older eastern islands and now reinstate the mantle-plume hypothesis as the most plausible explanation for Canary volcanism. The recently discovered Cretaceous submarine volcanic rocks in the region are, in turn, part of an older, fracture-related tectonic episode.

Trace fossil assemblages on Miocene rocky shores of southern Iberia

The use of rocky palaeoshore bioerosion analysis in the study of palaeontological and geological questions is beginning to bear fruit. Five southern Iberian Neogene rocky shores have been analysed and their bioerosion structures have been identified. The observed ichnodiversity is rather low; eleven ichnospecies were identified. These include bioerosion structures produced by polychaete annelids (Caulostrepsis, Maeandropolydora), clionaid sponges (Entobia), echinoids (Circolites), and endolithic bivalves (Gastrochaenolites). The different ichnoassemblages present in Miocene rocky shores in both Portuguese and Spanish sectors correspond to the Entobiaichnofacies. Comparison with the northeastern counterparts of these shores has also been carried out. The study of southern Iberian Miocene rocky shores made it possible to correlate them with the regional tectonic evolution and the main Neogene transgressive events affecting the region.

Extreme habitat adaptation by boring bivalves on volcanically active paleoshores from North Atlantic Macaronesia

A bstractExtensive bivalve borings are described in detail for the first time from basalt rockgrounds in the North Atlantic volcanic islands of Macaronesia. They occur on a Middle Miocene rocky shore of a small islet of Porto Santo (Madeira Archipelago of Portugal), as well as on Plio-Pleistocene rocky shores on Santiago Island (Cape Verde). A basalt substrate is widely penetrated by clavate-shaped borings belonging to the ichnogenus Gastrochaenolites interpreted as dwelling structures of suspension-feeding bivalves. Some of these borings still retain evidence of the alleged trace-makers preserved as body fossils, while others are filled with their casts. The ichnofossil assemblage present on these bioeroded surfaces belongs to the Entobia ichnofacies. Recognition of Gastrochaenolites borings in volcanic rocks provides useful paleoenvironmental information regarding an expanded strategy for hard-substrate colonization. Preliminary results from fieldwork in the Cape Verde Archipelago indicate that such borings are more widespread through Macaronesia than previously thought.

What Darwin did not see: Pleistocene fossil assemblages on a high-energy coast at Ponta das Bicudas, Santiago, Cape Verde Islands

Funding for fieldwork on Santiago Island in June 2011 was provided under grant CGL2010-15372-BTE from the Spanish Ministry of Science and Innovation to project leader Eduardo Mayoral (University of Huelva). Financial support to A. Santos came from the Spanish Ministry of Science and Technology (Juan de la Cierva subprogram, Ref: JCI-2008-2431). Additional support by the Junta de Andalucia (Spanish government) to the Research Group RNM276 is also acknowledged. Partial funding to J. Ledesma-Vazquez on this project came from the Programma Integral de Fortalecimiento Institucional 2010. We thank Christopher K. Pham, Department of Oceanography and Fisheries, University of the Azores, Portugal, for help with identification of the fossil barnacles and Ricardo Ramalho, Institut fur Geophysik, Westphalishe-Wilhelms Universitat, Germany, for discussions about bioerosion by sea urchins on basalt surfaces.