Spatial and temporal changes in dry-snow line altitude on the Antarctic Peninsula

Drastic changes were detected in glacial systems of the Antarctic Peninsula in the last decades. The observed phenomena comprise the disintegration of ice shelves, acceleration and thinning of glaciers, and retreat of glacier fronts. However, due to the lack of consistent systematic observations in particular of the higher parts of the glacial systems, it is difficult to predict further responses of the Antarctic Peninsula glaciers to climatic change. The present paper analyses spatial and temporal variations of changes in the dry-snow line altitude on the Antarctic Peninsula as extracted from a time series (1992–2005) of ERS-1/2 SAR and Envisat ASAR data. Upward changes in dry-snow line altitude were observed in general, and are attributed to extreme high-temperature events impacting the central plateaus of the Antarctic Peninsula and the increasing duration of warming periods. A mean decrease in dry-snow line altitude was detected on the west side of the peninsula and is identified as a response to recorded increase in precipitation and accumulation. These results validate the capability of SAR data for deriving superficial parameters of glaciers to be used as indicators of climatic changes in high-latitude regions where operational restrictions limit conventional meteorological observations.     

A meteorite crater on Earth formed on September 15, 2007: The Carancas hypervelocity impact

Abstract— On September 15, 2007, a bright fireball was observed and a big explosion was heard by many inhabitants near the southern shore of Lake Titicaca. In the community of Carancas (Peru), a 13.5 m crater and several fragments of a stony meteorite were found close to the site of the impact. The Carancas event is the first impact crater whose formation was directly observed by several witnesses as well as the first unambiguous seismic recording of a crater‐forming meteorite impact on Earth. We present several lines of evidence that suggest that the Carancas crater was a hypervelocity impact. An event like this should have not occurred according to the accepted picture of stony meteoroids ablating in the Earth's atmosphere, therefore it challenges our present models of entry dynamics. We discuss alternatives to explain this particular event. This emphasizes the weakness in the pervasive use of “average” parameters (such as tensile strength, fragmentation behavior and ablation behavior) in current modeling efforts. This underscores the need to examine a full range of possible values for these parameters when drawing general conclusions from models about impact processes.     

Diffusion of divalent cations in garnet: multi-couple experiments

We demonstrate the possibility of studying several diffusion couples in a single run, i.e. under almost similar P–T–t– conditions, allowing direct comparison of the diffusion rates in different diffusion couples. Thus the duration of experimental study and the risk of failure of expensive experimental equipment can be decreased considerably. The diffusion experiments were carried out in piston-cylinder apparatus. Gem-quality garnets of almandine, spessartine and grossular compositions together with inclusion-rich eclogitic garnets were embedded in a powder of natural pyrope and annealed together under dry conditions at P = 1.9–3.2 GPa and T = 1,070–1,400°C. Diffusion profiles were measured by electron microprobe and fitted numerically on the basis of multicomponent diffusion theory. The datasets derived from different diffusion couples yields parameters of the Arrhenius equation for Ca, Mg and Fe in natural eclogitic garnets and Mg, Mn and Fe in gem-quality garnets. We have also studied the effect of grain-boundary diffusion in the sintered pyrope matrix on interdiffusion on the basis of 2D modeling. Under conditions analogous to those of our experimental runs, we show that observed irregularities in some measured diffusion profiles (not applied for the diffusion modeling) can be directly related to the superposition of local grain-boundary diffusion on dominant volume diffusion.     

Luminescence-dated aeolian deposits of late Quaternary age in the southern Tibetan Plateau and their implications for landscape history

Aeolian deposits are widely distributed in the interior of the Tibetan Plateau, and their chronology is poorly known. It is not yet clear whether they accumulated only after the last deglaciation, or over a longer time. We applied quartz OSL dating to aeolian samples from the Lhasa area with OSL ages ranging from 2.9 ± 0.2 to at least 118 ± 11 ka. The probability density frequency (PDF) distribution of 24 ages reveals age clusters at about 3, 8, 16–21, 33, and 79–83 ka, indicating enhanced sediment accumulation then. The results show that aeolian deposition occurred throughout most of the last 100 ka. This implies that: 1) an ice sheet covering the whole Tibetan Plateau during the last glacial maximum (LGM) could not have existed; and 2) erosion during the last deglaciation was not as strong as previously proposed, such that not all pre-Holocene loess was removed. The age distribution shown in the PDF indicates that aeolian accumulation is episodic. Sand-formation events revealed by age clusters at 3, 8, and 16–21 ka imply roughly synchronous environmental responses to corresponding global-scale arid events.     

Geometric interpretation for the spectral stability in the charged three-body problem

We present a geometric interpretation of the spectral stability of the triangular libration points in the charged three-body problem. We obtain that the spectral stability varies with the position of the center of mass of the three charges with respect to the circumcenter of the triangle configuration, which does not depend directly of the charges. If the center of mass is outside or on the circumference of a well defined radius ??, then spectral stability occurs. In addition, we analyze the existence of resonances within the spectral region of stability under this geometric interpretation, determining resonance curves of order 2, 3, 4, . . ., some of them with multiple resonances.     

P–T–t estimation of deformation in low‐grade quartz‐feldspar‐bearing rocks using thermodynamic modelling and 40Ar/39Ar dating techniques: example of the Plan‐de‐Phasy shear zone unit (Briançonnais Zone,Western Alps)

Pressure–Temperature–time (P–T–t) estimates of the syn‐kinematic strain at the peak‐pressure conditions reached during shallow underthrusting of the Briançonnais Zone in the Alpine subduction zone was made by thermodynamic modelling and ⁴⁰Ar/³⁹Ar dating in the Plan‐de‐Phasy unit (SE of the Pelvoux Massif, Western Alps). The dated phengite minerals crystallized syn‐kinematically in a shear zone indicating top‐to‐the‐N motion. By combining X‐ray mapping with multi‐equilibrium calculations, we estimate the phengite crystallization conditions at 270 ± 50 °C and 8.1 ± 2 kbar at an age of 45.9 ± 1.1 Ma. Combining this P–T–t estimate with data from the literature allows us to constrain the timing and geometry of Alpine continental subduction. We propose that the Briançonnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision.     

Vorhersage von Niveauänderungen der 500 mb Fläche mittels numerischer Integration der Wellengleichung

Zusammenfassung Es wurde versucht mittels der numerischen Integration der zweidimensionalen homogenen Wellengleichung bei bekannter Phasengeschwindigkeit Niveauänderungen im 500 mb Niveau vorherzusagen. Gleichzeitig wurde das Ergebnis mit der prognostizierten Niveauänderung, die mittels graphischer Integration der barotropen Wirbelgleichung gewonnen wurde, verglichen. Dabei hat es sich gezeigt, dass die Übereinstimmung der beiden vorhergesagten Änderungen sowohl untereinander, als auch mit der tatsächlich eingetroffenen Änderung recht gut ist. Offenbar kann man in einfachen Fällen die physikalischen Vorgänge in der Atmosphäre mittels der homogenen Wellengleichung beschreiben.

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Summary The attempt is made to forecast height changes of the 500 mb contours by a numerical integration of the twodimensional homogenous wave equation provided the phase velocity is known. The result is compared with a forecast obtained by graphical integration of the barotropic vorticity equation. In the case investigated both methods show almost the same accuracy with respect to the contour changes with have actually occured. Appearently in some case it is possible to discribe height variations of an isobaric surface with the aid of the linear twodimensional wave equation with the same success as using the nonlinear vorticity equation.