Atlantic-Mediterranean Transition Research Articles

Perturbing effects of sub-lithospheric mass anomalies in GOCE gravity gradient and other gravity data modelling: Application to the Atlantic-Mediterranean transition zone

The GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission, launched on March 2009, included a new type of satellite instrument, an on-board three-axis gradiometer able to measure the Earth's gravity gradients at the satellite height (255km). The potential of this new type of measurement and its derived products (i.e., global gravity field models), together with other land-based geophysical observables (Bouguer and geoid anomalies), to image sub-lithospheric thermal and compositional anomalies is evaluated in this study. We focus on the Atlantic-Mediterranean Transition Region (AMTR), the diffuse, transpressive contact between the Iberian Peninsula and North Africa. The present-day lithospheric structure in the area is characterized by a wide band of active deformation, large lateral variations in the lithosphere and the presence of a positive seismic-velocity anomaly in the uppermost mantle beneath the Betics and the westernmost Alboran Basin-Gibraltar Arc. Here, the perturbing effects of deep, sub-lithospheric density anomalies in GOCE gravity gradients and other land-based geophysical data are assessed, and its impact in lithospheric-scale geophysical-petrological modeling within a thermodynamically consistent framework analyzed. Some of the gravity gradients computed at the satellite altitude are rather sensitive to the presence of even a relatively small sub-lithospheric cold slab, like the one observed in the AMTR, showing the potential of the new GOCE data to map upper mantle anomalies. Lithospheric models ignoring the AMTR sub-lithospheric heterogeneities could be significantly biased. In particular, extreme changes of 4–6km and 60–70km in the crustal and lithospheric thickness, respectively, are required to include sub-lithospheric mantle contributions to land-based and satellite gravity data.

A legacy of contrasting spatial genetic structure on either side of the Atlantic–Mediterranean transition zone in a marine protist

The mechanisms that underpin the varied spatial genetic structures exhibited by free-living marine microorganisms remain controversial, with most studies emphasizing a high dispersal capability that should redistribute genetic diversity in contrast to most macroorganisms whose populations often retain a genetic signature of demographic response to historic climate fluctuations. We quantified the European phylogeographic structure of the marine flagellate Oxyrrhis marina and found a marked difference in spatial genetic structure, population demography, and genetic diversity between the northwest Atlantic and Mediterranean Sea that reflects the persistent separation of these regions as well as context-dependent population responses to contrasting environments. We found similar geographic variation in the level of genetic diversity in the sister species Oxyrrhis maritima. Because the capacity for wide dispersal is not always realized, historic genetic footprints of range expansion and contraction persist in contemporary populations of marine microbes, as they do in larger species. Indeed, the well-described genetic effects of climatic variation on macroorganisms provide clear, testable hypotheses about the processes that drive genetic divergence in marine microbes and thus about the response to future environmental change.

Phylogeographic patterns of decapod crustaceans at the Atlantic–Mediterranean transition

Comparative multispecies studies allow contrasting the effect of past and present oceanographic processes on phylogeographic patterns. In the present study, a fragment of the COI gene was analyzed in seven decapod crustacean species from five families and with different bathymetric distributions. A total of 769 individuals were sampled along the Atlantic–Mediterranean transition area in order to test the effect of three putative barriers to gene flow: Strait of Gibraltar, Almeria–Oran Front and Ibiza Channel. A significant effect of the Strait of Gibraltar was found in the crabs Liocarcinus depurator and Macropipus tuberculatus. The Ibiza Channel had a significant effect for L. depurator. However, the Almeria–Oran front was not found to have a significant effect on any of the studied species. Higher levels of population structure were found in shallow-water species, although the number of species sampled should be increased to obtain a conclusive pattern. The haplotypes within the different species coalesced at times that could be related with past climatic events occurring before, during and after the last glacial maximum. Given the large diversity of phylogeographic patterns obtained within decapods, it is concluded that both historical and contemporary processes (marine current patterns, bathymetry and life-history traits) shape the phylogeographic patterns of these crustaceans.

The structure and evolution of the lithosphere–asthenosphere boundary beneath the Atlantic–Mediterranean Transition Region

The present-day thermal and compositional 3D structure of the lithosphere beneath the Atlantic–Mediterranean Transition Region and the lithosphere–asthenosphere interaction from Jurassic times to present has been studied. The Atlantic–Mediterranean Transition Region comprises the western segment of the Africa–Eurasia plate boundary, encompassing two main large-scale tectonic domains: the Gibraltar Arc System and the Atlas Mountains. An integrated and self-consistent geophysical–petrological methodology ( LitMod3D) has been applied that combines elevation, gravity, geoid, surface heat flow, and seismic data and allows modelling of compositional heterogeneities within the lithospheric mantle. Our results reveal large variations in the depth of the Moho and the lithosphere–asthenosphere boundary (LAB) as well as a lack of spatial correlation between the thicknesses of these two boundaries. The Moho essentially mimics the topography with depths ranging from ∼ 10 km beneath the oceanic domains of the Atlantic abyssal plains and the Algerian Basin to > 34 km in the Eastern Betics and Rif, the High Atlas mountains, and the Sahara Platform. In contrast, the LAB is shallower beneath the central and eastern Alboran Basin (∼ 70 km) and all along the High, Middle and Anti Atlas (< 100 km) coinciding with the loci of Cenozoic volcanism. Deeper LAB depths are found along the central and western Betics and the Moroccan Atlantic margin (> 140 km) with values exceeding 230 km beneath the Rif and the Sahara Platform. The average bulk composition of the lithospheric mantle corresponds to that of a typical Tecton (i.e. Phanerozoic) domain, with the exceptions of the Sahara Platform, the Alboran Basin, and Atlas Mountains. Distinct mantle compositions are required in these areas to make model predictions and geophysical observables compatible. It is proposed that the highly irregular LAB topography is the result of the superposition of three different geodynamic processes: i) shortening and thickening related to NW–SE Iberia–Africa convergence lasting from Late Cretaceous to Recent, ii) impingement of a baby-like mantle plume or small-scale convection beneath the High-Middle Atlas and Anti Atlas commencing in the mid Eocene, and iii) slab roll-back or mantle delamination in the Betic–Rif–Alboran realm acting from early to late Miocene.

Genetic differentiation and secondary contact zone in the seagrass Cymodocea nodosa across the Mediterranean–Atlantic transition region

AbstractAim A central question in evolutionary ecology is the nature of environmental barriers that can limit gene flow and induce population genetic divergence, a first step towards speciation. Here we study the geographical barrier constituted by the transition zone between the Atlantic Ocean and the Mediterranean Sea, using as our model Cymodocea nodosa, a seagrass distributed throughout the Mediterranean and in the Atlantic, from central Portugal to Mauritania. We also test predictions about the genetic footprints of Pleistocene glaciations.Location The Atlantic–Mediterranean transition region and adjacent areas in the Atlantic (Mauritania to south‐west Portugal) and the Mediterranean.Methods We used eight microsatellite markers to compare 20 seagrass meadows in the Atlantic and 27 meadows in the Mediterranean, focusing on the transition between these basins.Results Populations from these two regions form coherent groups containing several unique, high‐frequency alleles for the Atlantic and for the Mediterranean, with some admixture west of the Almeria–Oran Front (Portugal, south‐west Spain and Morocco). These are populations where only one or a few genotypes were found, for all but Cadiz, but remarkably still show the footprint of a contact zone. This extremely low genotypic richness at the Atlantic northern edge contrasts with the high values (low clonality) at the Atlantic southern edge and in most of the Mediterranean. The most divergent populations are those at the higher temperature range limits: the southernmost Atlantic populations and the easternmost Mediterranean, both potential footprints of vicariance.Main conclusions A biogeographical transition region occurs close to the Almeria–Oran front. A secondary contact zone in Atlantic Iberia and Morocco results from two distinct dispersal sources: the Mediterranean and southernmost Atlantic populations, possibly during warmer interglacial or post‐glacial periods. The presence of high‐frequency diagnostic alleles in present‐day disjunct populations from the southernmost Atlantic region indicates that their separation from all remaining populations is ancient, and suggests an old, stable rear edge.

FA2BOUG—A FORTRAN 90 code to compute Bouguer gravity anomalies from gridded free-air anomalies: Application to the Atlantic-Mediterranean transition zone

In this paper we present a computer program written in FORTRAN 90 specifically designed to determine the Bouguer anomaly from publicly available global gridded free-air anomaly and elevation database sets. FA2BOUG computes the complete Bouguer correction (i.e. Bullard A, B and C corrections) for both land and sea points in several spatial domains according to the distance between the topography and the calculation point. In each zone a different algorithm is used. In a distant zone we consider the harmonic spherical expansion of the potential of each right rectangular prism representing an elevation grid point. In an intermediate zone we compute the gravitational attraction produced by each prism using the analytic formula. Finally, an inner zone contribution is divided into two parts: a flat-topped prism with a height equal to the elevation of the calculation point, and four quadrants of a conic prism sloping continuously from each square of the inner zone to the calculation point. The program has been applied to the Atlantic-Mediterranean transition zone to obtain a complete Bouguer anomaly map of the area, integrating available onshore Bouguer anomaly with satellite-derived free-air anomaly data. Positive Bouguer anomalies are found in the Atlantic oceanic domain (240–300 mGal), central and eastern Alboran Basin (40–160 mGal) and SW Iberian Peninsula (>40 mGal). Major negative Bouguer anomalies are located beneath the west Alboran Basin (<−40 mGal), the Rif, the Rharb Basin and the Atlas Mountains (<−120 mGal). An isostatic residual anomaly map of the study area has been computed and compared with the crustal and lithospheric structure inferred from previous work.

Editorial and Retrospective 2008

Editorial and Retrospective 2008

Pillars of Hercules: is the Atlantic–Mediterranean transition a phylogeographical break?

The geological history of the Mediterranean Sea, its hydrography and connection with the Atlantic Ocean have been well documented. Despite a wealth of historical and oceanographic data, the Atlantic-Mediterranean transition remains controversial at the biological level as there are discordant results regarding the biogeographical separation between the Atlantic and Mediterranean biota. The opening of the Strait of Gibraltar at the end of the Messinian Salinity Crisis (some 5.33 million years ago), removed the land barrier that impeded the marine biota allowing it to disperse freely into the Mediterranean Sea. However, present day genetic patterns suggest a limitation to gene flow for some marine species, preventing population admixture. In the last few years, a large number of studies have challenged the hypothesis of the Strait of Gibraltar representing a phylogeographical break. A review of more than 70 papers reveals no obvious relationship between either dispersal ability or life history, and observed patterns of partial or complete genetic isolation between Atlantic and Mediterranean populations. We re-analysed a selection of this large body of data (20 studies in total) in order to provide a homogeneous and coherent view on the generality of the phylogeographical patterns and the presence of a phylogeographical barrier. This offered the opportunity to summarize the state of the art on this matter and reach some general conclusions on the evolutionary history across the Atlantic-Mediterranean range. Geographically, some species in the transition zone showed step changes of allele frequencies associated with the Almeria-Oran Front rather than with the Strait of Gibraltar itself. A major part of the data describe evolutionary events well within the time frame of the Quaternary age as very few taxa pre-date closure of the Tethys Sea. Results point to a combined signature of vicariance, palaeoclimate fluctuation and life-history traits on the Atlantic-Mediterranean phylogeographical patterns. Principal component analysis failed to show any particular association between biological traits and genetic variables. It would argue that organismal determinism may play a far less significant role than marine biogeographers have generally believed.

The structure of the Atlantic–Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia–Africa plate boundary)

The diffusive plate boundary between Iberia and Africa has been analyzed by means of already published and new 28 multichannel seismic profiles covering most of the Atlantic margins of SW Spain and NW Morocco. This region includes the prolongation of the Betic–Rifean tectonic units along the Gibraltar Arc. One of the most characteristic units in the study area is the Miocene seismically chaotic unit that has been divided in a tectonic and a gravitational domains in accordance to previous interpretation by Torelli et al. [Torelli, L., Sartori, R., Zitellini, N., 1997. The giant chaotic body in the Atlantic Ocean off Gibraltar: new results from a deep seismic reflection survey. Marine and Petroleum Geology 14, 125–138.]. In this work we redefine the limits of both domains and rename them as the Gulf of Cadiz Imbricate Wedge and the Horseshoe Gravitational Unit. We interpret the Gulf of Cadiz Imbricate Wedge as a west-migrating thick thrust system that build up in a relatively short period of time. This unit constitutes the continuation of the Miocene chaotic units observed onshore to the front of the External Units in the Betics and the Rif (the Guadalquivir Allochthonous Unit and the Rides Prerifaines). The Horseshoe Gravitational Unit, covering an approximate area of 18 000 km 2 with a volume of around 23 000 km 3, is interpreted as giant submarine debris flows sourced from the up-building of the Gulf of Cadiz Imbricate Wedge, surrounding basement highs and the continental Iberian Slope. The fast westward migration of the Flysch units, the External Units and the Gulf of Cadiz Imbricate Wedge, from the Early Miocene to the Late Tortonian corresponds to a different tectonic process than the protracted but slow NNW to NW convergence of Africa. Our interpretation favours the westward slab retreat theory to form the arcuate shape of the Atlantic side of the Iberia–Africa plate boundary. The subduction mechanism as well as potential roll-over was only active from late Early Miocene to Late Miocene spanning about 12 Myr.

Lithospheric structure in the Atlantic–Mediterranean transition zone (southern Spain, northern Morocco): a simple approach from regional elevation and geoid data

A new methodology for automatic modelling of crustal and lithospheric thickness or density variations that integrates both elevation and geoid anomalies is presented. We used this algorithm to image the main lithospheric heterogeneities within the Atlantic–Mediterranean transition zone. Main assumptions in the modelling are local isostasy and a two-layered model comprising crust and lithospheric mantle with constant densities, plus sea water and asthenosphere. The results show a wide zone of thickened lithosphere, oriented NE–SW and located beneath the western Betics and Rif, the Gulf of Cadiz and the northwestern Atlantic Moroccan margin. In addition, a zone of thinned lithosphere with the same orientation is shown beneath the Atlas Mountains and the eastern Alboran Basin. To cite this article: J. Fullea Urchulutegui et al., C. R. Geoscience 338 (2006).