Mid-Holocene emergence of southern Tunisian coasts

We present new sea-level data from the coasts of southern Tunisia, between the Gulf of Gabès and the Libyan border. The work tests, previously, published evidence on Holocene shorelines, and confirmed that a distinct emergence has occurred in this area during this time. The emergence peak lies at least 186 ± 11 cm above present and is inferred from: (1) AMS radiocarbon dates of subtidal vermetids and boring shells collected in growth position, and (2) careful assessment of tidal heights. Maximum emergence took place between about 6000 and 5000 ¹⁴C years BP; it cannot be ascribed to tectonics and is probably related to post-glacial hydro-isostatic effects. It challenges the inference of a 3-m global sea-level rise since 6000 years BP due to residual Antarctic melting.     

A general theory of three-dimensional wave groups Part II: Interaction with a breakwater

The equation obtained in Part I predicts how an exceptionally high wave occurs at any fixed point within a wind wave field. The equation may be applied with a theoretical spectrum or directly with the random time series obtained by an array of wave gauges in the field. From both approaches, it emerges that a very high wave at a breakwater occurs because a well-defined three-dimensional wave group at the apex of its development hits against the breakwater, and that a very high wave at some distance before the breakwater occurs because of the collision of two wave groups: the first one going back after having been reflected, and the second one approaching the breakwater. In order to test the theory, a special breakwater was assembled off the beach at Reggio-Calabria where the significant height of the wind waves typically ranges from 0.20 to 0.40 m. When an exceptionally high wave (H = 9.6 σ) occurred at a point before this breakwater, the records made by a gauge array confirmed all the essential features of the prediction.     

Semidiurnal Dynamics of Salinity, Nutrients and Suspended Particulate Matter in an Estuary in the Seto Inland Sea, Japan, during a Spring Tide Cycle

The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan, was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively. At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply after the lower low tide (>40 mg l⁻¹) and this coincided with a marked decrease of the C/SS content (<20 mg g⁻¹). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments as the tidal level rose, and particle transport selection in relation to the tidal state. This revised version was published online in August 2006 with corrections to the Cover Date.     

Groundwater flow systems in turbidites of the Northern Apennines (Italy): natural discharge and high speed railway tunnel drainage

Turbidites crop out extensively in the Northern Apennine mountains (Italy). The huge amounts of groundwater drained by tunnels, built for the high speed railway connection between Bologna and Florence, demonstrate the aquifer-like behaviour of these units, up to now considered as aquitards. A conceptual model of groundwater flow systems (GFS) in fractured aquifers of turbidites is proposed, taking into account both system natural state and the perturbation induced by tunnel drainage. Analysis of hydrological data (springs, streams and tunnel discharge), collected over 10 years, was integrated with analysis of hydrochemical and isotopic data and a stream-tunnel tracer test. Hydrologic recession analysis of undisturbed conditions is a key tool in studying turbiditic aquifer hydrogeology, permitting the discrimination of GFS, the estimation of recharge relative to the upstream reach portion and the identification of springs most vulnerable to tunnel drainage impacts. The groundwater budgeting analysis provides evidence that the natural aquifer discharge was stream-focused through GFS, developed downslope or connected to main extensional tectonic lineaments intersecting stream beds; now tunnels drain mainly active recharge groundwater and so cause a relevant stream baseflow deplenishment (approximately two-thirds of the natural value), possibly resulting in adverse effects on local ecosystems.     

A calibration system for superconducting gravimeters

A new method for the calibration of a superconducting gravity meter is described, in which a 273 Kg annular mass is placed around the meter and is moved up and down. The geometry of the apparatus is easy to model and the accuracy in the computation of the gravity variation induced by the mass, 6.7µgal, is limited only by the accuracy in the knowledge of value of the gravitational constant. Measurements done in 91 and 92 for the calibration of the instrument GWR-T015 are described. The calibration factor has been determined with a precision of about 0.3%.     

Dynamical evolution of NEAs: Close encounters, secular perturbations and resonances

We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v ₆ (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some fast-track dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.On leave from the Department of Mathematics, University of Pisa, Via Buonarroti 2, 56127 Pisa, Italy, thanks to the G. Colombo fellowships of the European Space Agency.     

Hamiltonian perturbation theory on a manifold

This paper deals with Hamiltonian perturbation theory for systems which, like Euler-Poinsot (the rigid body with a fixed point and no torques), are degenerate and do not possess a global system of action-angle coordinates. It turns out that the usual methods of perturbation theory, which are essentially local being based on the construction of normal forms within the domain of a local coordinate system, are not immediately usable to study perturbations of these systems, since degeneracy makes impossible to control that the system does not fall into a singularity of the coordinates. To overcome this difficulty, we develop a global formulation of Hamiltonian perturbation theory, in which the normal forms are globally defined on the phase space manifold. The key for this study lies in the geometry of the fibration by the invariant tori of an integrable degenerate Hamiltonian system, which is described by some generalizations of the Liouville-Arnol'd theorem and is reviewed in the paper. As an application, we provide a global formulation of Nekhoroshev's theorem on the stability for exponentially long times.     

Far-infrared observations of main sequence stars surrounded by dust shells

We present 50 and 100µm photometry and size information for several main sequence stars surrounded by dust shells. The observations from NASA's Kuiper Airborne Observatory include the Vega-like stars, Beta Pic, Fomalhaut, as well as four stars suggested by Walker and Wolstencroft to belong possibly to the same class. The results of our observations are best interpreted as upper limits to the far-infrared sizes of the dust clouds around all of the stars except Fomalhaut and Beta Pic. We have also fit simple, optically thin models to the Beta Pic data to explore the range of shell parameters consistent with our limits and with previous observations.     

Irregular extrasolar systems and dynamical instability

The astrometric search for extrasolar planetary systems can regard, at least in the first stage, only systems with high mass ratio (10^–2) of the largest planets to the central star. The formation of such systems is probably a common occurrence in the galaxy, as indicated by various theoretical arguments; these systems could have evolved in a regular way (near-circular and coplanar orbits, Titius-Bode law, etc.) until the most massive accumulating protoplanet exceeded the critical value of the mass ratio corresponding to dynamical instability. This phenomenon has been extensively investigated in recent years, showing that a three- orN-body system becomes unstable (for relative separations similar to the planetary ones) for >10^–2. Therefore, it seems likely that observations of systems within this range of mass ratio will show the irregular end-products of processes related to the instability (close encounters or ejections), which affected drastically the orbital configuration in the last phase of the planet formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.