Injection of Oort Cloud comets: the fundamental role of stellar perturbations

We present Monte Carlo simulations of the dynamical evolution of the Oort cloud over the age of the Solar System, using an initial sample of one million test comets without any cloning. Our model includes perturbations due to the Galactic tide (radial and vertical) and passing stars. We present the first detailed analysis of the injection mechanism into observable orbits by comparing the complete model with separate models for tidal and stellar perturbations alone. We find that a fundamental role for injecting comets from the region outside the loss cone (perihelion distance q > 15 AU) into observable orbits (q < 5 AU) is played by stellar perturbations. These act in synergy with the tide such that the total injection rate is significantly larger than the sum of the two separate rates. This synergy is as important during comet showers as during quiescent periods and concerns comets with both small and large semi-major axes. We propose different dynamical mechanisms to explain the synergies in the inner and outer parts of the Oort Cloud. We find that the filling of the observable part of the loss cone under normal conditions in the present-day Solar System rises from <1% for a < 20 000 AU to about 100% for a ? 100 000 AU.     

Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

Abstract— We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous‐Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K‐edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si‐K impact spherule from the same impact layer. The pre‐edge peak of our high‐resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe³⁺. Yellow K/T glasses have significantly higher Fe³⁺/(Fe²⁺ + Fe³⁺) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe³⁺) and high Si‐K glass (20 mole% Fe³⁺). In particular, all the pre‐edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of ^[4]Fe²⁺ and ^[5]Fe²⁺) to ^[4]Fe³⁺ and ^[5]Fe³⁺, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of ^[4]Fe^{2+, [5]}Fe^{2+, [4]}Fe³⁺, and ^[5]Fe³⁺. Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe³⁺/(Fe²⁺ + Fe³⁺) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca‐sulfate‐ and Ca‐carbonate‐bearing sedimentary rocks at the impact site.     

Correlating Alpine glaciation with Adriatic sea‐level changes through lake and alluvial stratigraphy

We compare lake and alluvial stratigraphy along a frame connecting the southern Alpine foothills and the Adriatic Sea, with the aim of matching the effects of Alpine glaciation and sea‐level changes on sedimentation during the last glacial cycle. The palynostratigraphy of Lake Fimon provided proxies for regional vegetation and climate change and was coupled with sediment petrography, loss on ignition and magnetic susceptibility, disentangling alluvial phases from fluvioglacial activity related to culminations of the southeastern Alpine glaciers. The Fimon area was not reached by alluvial fans during the penultimate glacial maximum, nor by the sea transgression during the last interglacial, but a closed lake soon developed at the Eemian onset due to enhanced rainfall. Sea‐level fall at glacial inception triggered the entrenchment of the drainage network in the plain reaching the outer Fimon Basin. Slow aggradation, but no sign of fluvioglacial activity, lasted to 38.2 ± 1.45 cal. ka BP, when a major forest withdrawal took place, coeval to the spread of alluvial fans. By 27.5 ± 0.5 cal. ka BP the Fimon Basin was dammed by the Brenta outwash system. The main step of of forest recovery commenced at around (15.8) cal. ka BP, when apex trenching of the outwash fans was triggered by the glacier's decay. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural model of the Trieste Gulf: A proposal

The onshore geology of the Trieste Gulf is well known in Italian reliefs, as well as in Slovenian and Croatian ones (Istria peninsula). In the northeastern and southern part carbonate platforms outcrop mainly in shelf facies of the Cretaceous to the Paleogene ages and are covered by Eocene flysch deposits. In the northern part the Gulf is closed by the coastal line of the Friuli plain composed mainly of glacio-fluvial deposits of Quaternary age.From a structural point of view, overthrusts set in a typical Dinaric NW–SE direction dominate and where one can identifiy many tectonic units with a SW vergence, tending to overlap the Mesozoic–Paleogene carbonate units which lie over the Eocene terrigenous ones. In this general picture, some sub-vertical faults with an anti-Dinaric NE–SW orientation and a strike-slip activity, cut the previous overthrusts.On the other hand the offshore geological knowledge of the Gulf is scarce due also to the absence, in the inner part of the Gulf, of exploration wells.Only recent studies on some tectonic structures of the Trieste Gulf revealed by geophysical explorations made a few years ago, have suggested making a hypothesis on its formation that has never been studied before. The model proposed here starts from the hypothesis that all tectonic lines that come from the Italian, Slovene and Croatian coastline, continue offshore. This is validated by highlighting all the geometries and morphologic anomalies obtained by analyzing various documents that regard the buried subsoil and obtained through geophysical prospections. With this idea, the maps showing the isobaths of the top of the limestones, the depth and structures of the top of the flysch, the isobaths of the base of the Quaternary deposits and the morphology of the actual sea bed were compared.All these documents present many morphological elements, found repeatedly on maps with different finalities but which appear coherent with the general picture of the structural outcrop and with the starting hypothesis. Hence, the Gulf of Trieste, is situated on a graben defined by master faults in an anti-Dinaric NE–SE direction. These extend into the Gulf from two vertical faults documented in the onshore with a transtensional effect, and cut the NW–SE Dinaric thrusts that carry on length-wise into the offshore from the Istria coast. On the basis of the geometry of the top of the buried limestones and other pieces of evidence, we hypothesize also a new line, NE–SW oriented (Aquileia line) that could actually separate two buried sectors of limestone platform each with a different evolution.     

The contribution of “silent” faults to the seismic hazard of the northern Adriatic Sea

There is a difference in the seismogenic zonation used for seismic hazard assessment of ordinary buildings and that considered for critical facilities, because different levels of exceedence probability are taken into account. Consequently, in the second case tectonic structures with a low, or very low, likelihood of activation also need attention. The key factor in seismogenic zonation for seismic hazard assessment is investigated here considering some seismically undocumented faults of the northern Adriatic Sea area. Seismic hazard is evaluated for two constructions located around Trieste and close to the sea: an ordinary building and a critical facility. The results clearly show that the two constructions should be designed with quite a different level of expected ground motion in mind. Part of the difference, in the computation of the critical facility, is determined by the introduction of some faults without documented seismicity.