The Zanclean megaflood of the Mediterranean – Searching for independent evidence

About six million years ago, the Mediterranean Sea underwent a period of isolation from the ocean and widespread salt deposition known as the Messinian Salinity Crisis (MSC), allegedly leading to a kilometer-scale level drawdown by evaporation. One of the competing scenarios proposed for the termination of this environmental crisis 5.3 million years ago consists of a megaflooding event refilling the Mediterranean Sea through the Strait of Gibraltar: the Zanclean flood. The main evidence supporting this hypothesis is a nearly 390 km long and several hundred meters deep erosion channel extending from the Gulf of Cádiz (Atlantic Ocean) to the Algerian Basin (Western Mediterranean), implying the excavation of ca. 1000 km3 of Miocene sediment and bedrock. Based on the understanding obtained from Pleistocene onshore megaflooding events and using ad-hoc hydrodynamic modeling, here we explore two predictions of the Zanclean outburst flood hypothesis: 1) The formation of similar erosion features at sills communicating sub-basins within the Mediterranean Sea, specifically at the Sicily Sill; and 2) the accumulation of the eroded materials as megaflood deposits in areas of low flow energy. Recent data show a 6-km-wide amphitheater-shaped canyon preserved at the Malta Escarpment that may represent the erosional expression of the Zanclean flood after filling the western Mediterranean and spilling into the Eastern Basin. Next to that canyon, a ~1600 km3 accumulation of chaotic, seismically transparent sediment has been found in the Ionian Sea, compatible in age and facies with megaflood deposits. Another candidate megaflood deposit has been identified in the Alborán Sea in the form of elongated sedimentary bodies that parallel the flooding channel and are seismically characterized by chaotic and discontinuous stratified reflections, that we interpret as equivalent to gravel and boulder megabars described in terrestrial megaflood settings. Numerical model predictions show that sand deposits found at the Miocene/Pliocene (M/P) boundary in ODP sites 974 and 975 (South Balearic and Tyrrhenian seas) are consistent with suspension transport from the Strait of Gibraltar during a flooding event at a peak water discharge of ~108 m3 s−1

Processing speed and the relationship between Trail Making Test-B performance, cortical thinning and white matter microstructure in older adults

Part B of the Trail Making Test (TMT-B) is widely used as a quick and easy to administer measure of executive dysfunction. The current study investigated the relationships between TMT-B performance, brain volumes, cortical thickness and white matter water diffusion characteristics in a large sample of older participants, before and after controlling for processing speed. Four hundred and eleven healthy, community-dwelling older adults who were all born in 1936 were assessed on TMT-B, 5 tests of processing speed, and provided contemporaneous structural and diffusion MRI data. Significant relationships were found between slower TMT-B completion times and thinner cortex in the frontal, temporal and inferior parietal regions as well as the Sylvian fissure/insula. Slower TMT-B completion time was also significantly associated with poorer white matter microstructure of the left anterior thalamic radiation, and the right uncinate fasciculus. The majority of these associations were markedly attenuated when additionally controlling for processing speed. These data suggest that individual differences in processing speed contribute to the associations between TMT-B completion time and the grey and white matter structure of older adults

Modelling radioactivity dispersion in the Alborán Sea, western Mediterranean

A numerical model which simulates the dispersion of non-conservative radionuclides in the Alboran Sea has been developed. Water currents are obtained from two hydrodynamic models working on the same domain: a 2D depth-averaged model is used two calculate tides and a 2-layer has been applied to obtain the residual (mean) circulation. The output of the hydrodynamic models is first applied to calculate suspended particle concentrations and sedimentation rates over the domain, which are also required by the radionuclide transport model. This assumes that radionuclides may be present in three phases: dissolved, suspended matter and bed sediments. Exchanges between the liquid and solid phases are described through kinetic transfer coefficients. The model has been applied to simulate the dispersion of 137-Cs and 239,240-Pu introduced in the sea from atmospheric fallout. Measured and computed radionuclide concentrations in the water column and in bed sediments are, in general, in good agreement

Modelling the flux of radionuclides from the Rhone River into the Mediterranean Sea

A numerical model has been developed to simulate the input of radionuclides into the Mediterranean Sea through the Rhone River plume. The model is fully 3D. It solves the hydrodynamic equations, which include baroclinic terms and a turbulence model, together with the suspended sediment equations and radionuclide dispersion equations. The suspended sediment sub-model includes four particle classes, settling and deposition. The radionuclide equations include the exchanges of radionuclides between water, suspended sediments and bed sediments, described using a kinetic model. The effect of salinity changes upon uptake kinetics has also been included. Computed water circulation, salinity pattern, suspended sediment distribution and sedimentation rates are in agreement with observations in the area. The model has been applied to simulate the dispersion of 137Cs and $^{239,240}$Pu through the plume. Computed specific activities in water, suspended matter and bottom sediments are in agreement with the measured distributions. The model can also be applied to calculate distribution coefficients. Although some water circulation models for the Rhone River plume exist in current literature, this is the first time in which a detailed hydrodynamic and suspended sediment model has been applied to simulate the dispersion of radionuclides in the plume

Models for radioactivity dispersion assessments in Andalusian coastal waters: Gulf of Cádiz and Alborán Sea

Rapid response models for the assessment of the consequences of a radioactive spill in the coastal waters of Andalusia (south of Spain) have been developed. These waters comprise the Gulf of Cadiz, GoC, (Atlantic Ocean) and the Alborán Sea, AS, (Mediterranean Sea) and two models are described, covering both sectors. This subject is of high relevance since these waters are the only connection between the Atlantic and the Mediterranean. Thus, there are intense shipping activities that for instance include the transit of nuclear submarines. The models include hydrodynamic sub-models appropriate to describe the water circulation in each area. Results from the hydrodynamic models have been carefully tested through comparisons with observed tides and currents. The dispersion models are based upon a particle-tracking technique. Thus, the radioactive spill is simulated by a number of discrete particles, each one equivalent to a number of units, whose paths are computed. Turbulent diffusion and radioactive decay are calculated using a stochastic Monte Carlo method. The radionuclide concentrations may be obtained at the desired time from the density of particles per water volume. Some applications and examples of results are given

Time scales for conservative and non-conservative radionuclide transport in the English Channel

A numerical model that simulates the dispersion of radionuclides in the English Channel has been applied to study the dispersion of conservative and non-conservative radionuclides released from La Hague nuclear fuel reprocessing plant. The model is based upon previous work and now is able to simulate dispersion over long time scales (decades), explicitly including transport by instantaneous tidal currents and variable wind conditions. Wind conditions are obtained from meteorological statistics using a stochastic method. Outputs from the model are treated using time-series analysis techniques. These techniques allow the determination of characteristic times of the system, transport velocities and dispersion factors. This information may be very useful to support the decision-making process after an emergency situation. Thus, we are proposing that time-series analysis can be integrated with numerical modelling for helping decision-making in response to an accident. The model is first validated through its application to actual releases of 99-Tc and 125-Sb, and characteristic times for several radionuclides (a conservative one, 137-Cs and 239,240-Pu) are given next. Characteristic transport velocities and dispersion factors have been calculated as well

Redissolution of caesium and plutonium from Irish Sea sediments: A comparison between different modelling approaches

Two models with different conceptual and numerical approaches have been used to simulate the dispersion of radionuclides released from Sellafield nuclear fuel reprocessing plant. The first is a long-term box-model for simulating dispersion from Sellafield to the Arctic environment in which water fluxes between boxes are obtained from basic oceanographic information. Water-sediment radionuclide exchanges are described through sedimentation and remobilisation processes on the basis of equilibrium distribution coefficients. The second model is a 3D high-resolution model that explicitly solves tidal mixing and suspended sediment transport in the Irish Sea. Exchanges of radionuclides between the liquid and solid phases are now described in terms of kinetic rates. Thus, this model has a general applicability and can be used in situations out from equilibrium. Both models have been used to simulate the dispersion of Cs and Pu in the Irish Sea. Results from the two models are comparable, being differences in the results smaller than should be expected from the very different modelling approaches