Optimal Precursors Triggering the Kuroshio Extension State Transition Obtained by the Conditional Nonlinear Optimal Perturbation Approach

In this study, the initial perturbations that are the easiest to trigger the Kuroshio Extension(KE) transition connecting a basic weak jet state and a strong, fairly stable meandering state, are investigated using a reduced-gravity shallow water ocean model and the CNOP(Conditional Nonlinear Optimal Perturbation) approach. This kind of initial perturbation is called an optimal precursor(OPR). The spatial structures and evolutionary processes of the OPRs are analyzed in detail. The results show that most of the OPRs are in the form of negative sea surface height(SSH) anomalies mainly located in a narrow band region south of the KE jet, in basic agreement with altimetric observations. These negative SSH anomalies reduce the meridional SSH gradient within the KE, thus weakening the strength of the jet. The KE jet then becomes more convoluted, with a high-frequency and large-amplitude variability corresponding to a high eddy kinetic energy level; this gradually strengthens the KE jet through an inverse energy cascade. Eventually, the KE reaches a high-energy state characterized by two well defined and fairly stable anticyclonic meanders. Moreover, sensitivity experiments indicate that the spatial structures of the OPRs are not sensitive to the model parameters and to the optimization times used in the analysis.     

A smoothing algorithm to enhance finite-element tsunami modelling: An application to the 5 February 1783 Calabrian case,Italy

Numerical simulations are essential tools for studying tsunami generation and evolution and finite-element (FE) methods are widely used, especially because of their capability in modeling water waves in basins with complex bathymetry and irregular coastlines. This paper presents the numerical simulation of an historical Italian tsunami that affected the Tyrrhenian coasts of Calabria and Sicily on 5 February 1783 following a strong destructive earthquake that was the first of a terrible sequence of seismic shocks terrifying the Calabrian population for more than two months. The numerical model is an FE model based on the nonlinear nondispersive shallow-water approximation of the Navier-Stokes equations. Since FE discretization schemes may lead to solutions undesirably affected by noise over coarse grids, in this study numerical noise is controlled by suitably smoothing the FE solution at regular time steps t _s. The performance of our smoothing algorithm is tested for significant linear cases for which an analytical solution is available.     

Can Rock Composition Affect Sublittoral Epibenthic Communities?

Abstract. Biomineralogy, as the complex of interactions at different levels (cell, organism, species, and community) between organisms and minerals, may play a significant role in the spatial distribution and structure of marine communities. For instance, a negative influence of quartz has been underlined on the Mediterranean benthic communities, which show a species-poorer structure in quartz-rich environments. Excluding the role of quartz, the aim of this work is to verify whether the composition of various mineralogical substrata can affect a hard-bottom benthic community. In the Ligurian Sea, the Levanto area shows an astonishing complex of substrata with different petrographic characters: in a few kilometres, sandstones, serpentinites, gabbros, and basalts occur in geometric association. Consequently, this area represents a suitable frame for this type of study. Herein, algal photophilic assemblages growing on the four mineralogical substrata show significant differences in number of species and percentage cover or abundance. This suggests a significant influence of rock mineral composition on the hard bottom community. Therefore, rock composition must be taken into account for a better understanding of the processes at the basis of hard-bottom community development and diversity.     

The Messinian salinity crisis in Cyprus: a further step towards a new stratigraphic framework for Eastern Mediterranean

A revised stratigraphic framework for the Messinian succession of Cyprus is proposed demonstrating that the three‐stage model for the Messinian salinity crisis recently established for the Western Mediterranean also applies to the Eastern Mediterranean, at least for its marginal basins. This analysis is based on a multidisciplinary study of the Messinian evaporites and associated deposits exposed in the Polemi, Pissouri, Maroni/Psematismenos and Mesaoria basins. Here, we document for the first time that the base of the unit usually referred to the ‘Lower Evaporites’ in Cyprus does not actually correspond to the onset of the Messinian salinity crisis. The basal surface of this unit rather corresponds to a regional‐scale unconformity, locally associated with an angular discordance, and is related to the erosion and resedimentation of primary evaporites deposited during the first stage of the Messinian salinity crisis. This evidence suggests that the ‘Lower Evaporites’ of the southern basins of Cyprus actually belong to the second stage of the Messinian salinity crisis; they can be thus ascribed to the Resedimented Lower Gypsum unit that was deposited between 5.6 and 5.5 Ma and is possibly coeval to the halite deposited in the northern Mesaoria basin. Primary, in situ evaporites of the first stage of the Messinian salinity crisis were not preserved in Cyprus basins. Conversely, shallow‐water primary evaporites deposited during the third stage of the Messinian salinity crisis are well preserved; these deposits can be regarded as the equivalent of the Upper Gypsum of Sicily. Our study documents that the Messinian stratigraphy shows many similarities between the Western and Eastern Mediterranean marginal basins, implying a common and likely coeval development of the Messinian salinity crisis. This could be reflected also in intermediate and deep‐water basins; we infer that the Lower Evaporites seismic unit in the deep Eastern Mediterranean basins could well be mainly composed of clastic evaporites and that its base could correspond to the Messinian erosional surface.     

Evidence of positive tectonic inversion in the north-central sector of the Sicily Channel (Central Mediterranean)

In order to unravel the tectonic evolution of the north-central sector of the Sicily Channel (Central Mediterranean), a seismo-stratigraphic analysis of single- and multi-channel seismic reflection profiles has been carried out. This allowed to identify, between 20 and 50 km offshore the central-southern coast of Sicily, a ~80-km-long deformation belt, characterized by a set of WNW–ESE to NW–SE fault segments showing a poly-phasic activity. Within this belt, we observed: i) Miocene normal faults reactivated during Zanclean–Piacenzian time by dextral strike-slip motion, as a consequence of the Africa–Europe convergence; ii) releasing and restraining bend geometries forming well-developed pull-apart basins and compressive structures. In the central and western sectors of the belt, we identified local transpressional reactivations of Piacenzian time, attested by well-defined compressive features like push-up structures and fault-bend anticlines. The reconstruction of timing and style of tectonic deformation suggest a strike-slip reactivation of inherited normal faults and the local subsequent positive tectonic inversion, often documented along oblique thrust ramps. This pattern represents a key for an improved knowledge of the structural style of foreland fold-and-thrust belts propagating in a preexisting extensional domain. With regard to active tectonics and seismic hazards, recent GPS data and local seismicity events suggest that this deformation process could be still active and accomplished through deep-buried structures; moreover, several normal faults showing moderate displacements have been identified on top of the Madrepore Bank and Malta High, offsetting the Late Quaternary deposits. Finally, inside the northern part of the Gela Basin, multiple slope failures, originated during Pleistocene by the further advancing of the Gela Nappe, reveal tectonically induced potential instability processes.

     

Trace element geochemistry by laser ablation ICP-MS of micas associated with Ta mineralization in the Tanco pegmatite,Manitoba, Canada

In the Tanco pegmatite, one of the world’s major Ta deposits, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Micas are common in most of the pegmatite units and are intimately associated with the successive stages of Ta mineralization, from the wall zone to the central zones where micaceous replacement is pervasive. Different generations of micas, both primary and secondary, associated with Ta oxides, were selected for electron microprobe and laser ablation ICP-MS investigation. Their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated Ta mineralization. Micas range from dioctahedral muscovite to trioctahedral lepidolite through Al↔Li substitution. Unexpectedly, the most evolved compositions (low K/Rb ratios and high Li contents) occur in the wall zone; they are interpreted to reflect nonequilibrium crystallization from an undercooled melt, with or without boundary layer effects. In the central zones, the fine-grained mica–quartz assemblage hosts some coarser-grained Li-muscovite, which has the highest Ta contents (up to 400 ppm). These Li–F–a-rich micas are interpreted to originate from a magmatic metasomatic event, which was also at the origin of the MQM-style Ta mineralization at Tanco. However, the Li–Ta-poor, muscovite end-member compositions of fine-grained alteration micas suggest crystallization from an aqueous fluid, during a metasomatic (hydrothermal) event involving late pegmatitic fluids. The low Ta concentrations (around 50 ppm) of this fine-grained muscovite suggest that this fluid transported at least small amounts of Ta.     

New stratigraphical data on the Middle-Late Jurassic biosiliceous sediments from the Sicanian basin, Western Sicily (Italy)

The reported data present the stratigraphy of several sections across a Middle-Late Jurassic Radiolaritic Unit, well exposed in different thrust sheets pertaining to the Maghrebian chain of Southwestern Sicily. The aim was to define the chronostratigraphical distribution of the Jurassic biosiliceous sedimentation in the Sicanian palaeogeographical zone, a deep water basin belonging to the Southern Tethys continental margin. The radiolarian biostratigraphy indicates that the switching from carbonate to siliceous sedimentation in the Sicanian Basin is referable to the Bajocian, as shown by the section of Campofiorito, near Corleone. The biostratigraphical dataset allows the correlation between the onset of biosiliceous sedimentation and the fall of biodiversity in the Sicanian basin with the carbonate productivity crisis, indicated by the highest eutrophication that affected Western Tethys during Middle Jurassic times. Editorial handling: J.-P. Billon-Bruyat & M. Chiari (Guest)     

Geological evolution of Mount Etna volcano (Italy) from earliest products until the first central volcanism (between 500 and 100 ka ago) inferred from geochronological and stratigraphic data

We present an updated geological evolution of Mount Etna volcano based on new ⁴⁰Ar/³⁹Ar age determinations and stratigraphic data integrating the previous K/Ar ages. Volcanism began at about 500 ka ago through submarine eruptions on the Gela–Catania Foredeep basin. About 300 ka ago fissure-type eruptions occurred on the ancient alluvial plain of the Simeto River forming a lava plateau. From about 220 ka ago the eruptive activity was localised mainly along the Ionian coast where fissure-type eruptions built a shield volcano. Between 129 and 126 ka ago volcanism shifted westward toward the central portion of the present volcano (Val Calanna–Moscarello area). Furthermore, scattered effusive eruptions on the southern periphery of Etna edifice occurred until about 121 ka ago. The stabilization of the plumbing system on the Valle del Bove area is marked by the building of two small polygenic edifices, Tarderia and Rocche volcanoes. Their eruptive activity was rather coeval ending 106 and 102 ka ago, respectively. During the investigated time-span volcanism in Etna region was controlled by a main E–W extensional tectonic related to the reactivation of Malta Escarpment fault system in eastern Sicily. Electronic supplementary material The online version of this article () contains supplementary material, which is available to authorized users. An erratum to this article can be found at     

Forecasting the evolution in the mixing regime of a deep subalpine lake under climate change scenarios through numerical modelling (Lake Maggiore,Northern Italy/Southern Switzerland)

Climate Dynamics - The impact of air temperature rise is eminent for the large deep lakes in the Italian subalpine district, climate change being caused there by both natural phenomena and...