Critical Self-Organization of Astrophysical Shocks

There are two distinct regimes of the first-order Fermi acceleration of shocks. The first is a linear (test-particle) regime in which most of the shock energy goes into thermal and bulk motions of the plasma. The second is an efficient regime in which the shock energy goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system toward a self-organized critical state between those two regimes. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles.     

A new Late Holocene sea-level record from the Mississippi Delta: evidence for a climate/sea level connection?

A detailed relative sea-level (RSL) record was constructed for the time interval 600–1600 AD, using basal peat to track sea level and containing 16 sea-level index points that capture ～60 cm of RSL rise. The study area is in the Mississippi Delta where the spring tidal range is ～0.47 m, the impact of ocean currents on sea-surface topography is limited, and crustal motions are well constrained. Age control was obtained by AMS ¹⁴C dating and most ages represent weighted means of two subsamples. Sample elevations were determined by combining differential GPS measurements with optical surveying. All index points were plotted as error boxes using 2σ confidence intervals for the ages, plus all vertical errors involved in sampling and surveying, as well as the indicative range of the samples. A striking clustering of sea-level index points between ～1000 and ～1200 AD suggests a possible acceleration in the rate of RSL rise. Removal of the long-term trend (0.60 mm yr^?1) allows for the possibility of a sea-level oscillation with a maximum amplitude of ～55 cm. However, given the size of the error boxes the possibility that oscillations did not occur cannot be entirely ruled out. Comparison of the new RSL record with various proxy climate records suggests that sea level in this area may have responded to hemispheric temperature changes, including the Medieval Warm Period and the Little Ice Age. However, given the error margins associated with this reconstruction, it is stressed that this causal mechanism is tentative and requires corroboration by high-resolution sea-level reconstructions elsewhere.     

North Atlantic climate impact on early late‐glacial climate oscillations in the south‐eastern Alps inferred from a multi‐proxy lake sediment record

High‐resolution multi‐proxy analyses of a sediment core section from Lake Jeserzersee (Saissersee) in the piedmont lobe of the Würmian Drau glacier (Carinthia, Austria) reveal pronounced climatic oscillations during the early late glacial (ca. 18.5–16.0k cal a BP). Diatom‐inferred epilimnetic summer water temperatures show a close correspondence with temperature reconstructions from the adjacent Lake Längsee record and, on a hemispheric scale, with fluctuations of ice‐rafted debris in the North Atlantic. This suggests that North Atlantic climate triggered summer climate variability in the Alps during the early late glacial. The expansion of pine (mainly dwarf pine) between ca. 18.5 and 18.1k cal a BP indicates warming during the so‐called ‘Längsee oscillation’. The subsequent stepwise climate deterioration between ca. 18.1 and 17.6k cal a BP culminated in a tripartite cold period between ca. 17.6 and 16.9k cal a BP with diatom‐inferred summer water temperatures 8.5–10 °C below modern values and a shift from wet to dry conditions. This period probably coincides with a major Alpine glacier advance termed the Gschnitz stadial. A warmer interval between ca. 16.9 and 16.4k cal a BP separates this cold phase from a second, shorter and less pronounced cold phase between ca. 16.4 and 16.0k cal a BP, which is thought to correlate with the Clavadel/Senders glacier advance in the Alps. The following temperature increase, coupled with wet (probably snow‐rich) conditions, caused the expansion of birch during the transition period to the late glacial interstadial. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental impacts into chondritic targets,part I: Disruption of an L6 chondrite by multiple impacts

Abstract— A fragment of an L6 chondrite (Allan Hills [ALH] 85017,13) with an initial mass (M₀) of 464.1 g was the target in a series of experimental impacts in which the largest remaining fragment (M_R) after each shot was impacted by a 3.18 mm ceramic sphere at a nominal speed of 2 km s^?1. This continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot (M_S). Two chunks of Bushveldt gabbro with similar initial masses were also impacted under the same conditions until M_R was less than half M₀. The two gabbro targets required a total of 1.51 × 10⁷ and 1.75 × 10⁷ erg g^?1 to attain 0.27 and 0.33 M_R/M₀, respectively; the chondrite, however, was considerably tougher, reaching 0.40 and 0.21 M_R/M₀ only after receiving 2.37 × 10⁷ and 3.10 × 10⁷ erg g^?1, respectively. The combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series, which was the number of impacts required to disrupt each gabbro target (i.e., M_R/M₀ ≤ 0.5). Unlike the behavior shown in previous regolith‐evolution series, neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock (about 3 mm), an effect possibly related to the width of the shock pulse. The original chondrite was so fine‐grained and fractured, and the variance in its grain‐size distribution so large, that effects related to grain size were relegated to the<63 μm fraction. Impacts into ALH 85017 produced abundant, fine‐grained debris, but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets. The characteristic slopes of the chondrite's size distributions, however, were notably more constant over the entire nine‐impact series than those from any of the terrestrial targets, a testament to the control over comminution apparently exerted by pre‐existing fractures and other, microscopic damage in the meteorite. The enhancement in the finer fraction of debris from ALH 85017 indicates that ordinary chondrites in solar orbit would be very efficient contributors to the cosmic‐dust complex. At the same time, the greater resistance to disruption displayed by ordinary chondrites relative to that exhibited by igneous rocks indicates that a selection effect could be operative between the annealed, ordinary‐chondritic breccias and relatively weaker, differentiated meteorites. Preferential survival from their time in the regoliths of their parent bodies through their transit to Earth and passage through the atmosphere suggests that meteorite collections could be biased in favor of the ordinary chondrites.     

Noble gas investigations of lunar rocks 10017 and 10071

The noble gases He, Ne, Ar, Kr and Xe and also K and Ba were measured in the Apollo 11 igneous rocks 10017 and 10071, and in an ilmenite and two feldspar concentrates separated from rock 10071. Whole rock K/Ar ages of rocks 10017 and 10071 are (2350 ± 60) × 10⁶ yr and (2880 ± 60) × 10⁶ yr, respectively. The two feldspar concentrates of rock 10071 have distinctly higher ages: (3260 ± 60) × 10⁶ yr and (3350 ± 70) × 10⁶ yr. These ages are still 10 per cent lower than the Rb/Sr age obtained by Papanastassiouet al. (1970) and some Ar⁴⁰ diffusion loss must have occurred even in the relatively coarse-grained feldspar.The relative abundance patterns of spallation Ne, Ar, Kr and Xe are in agreement with the ratios predicted from meteoritic production rates. However, diffusion loss of spallation He³ is evident in the whole rock samples, and even more in the feldspar concentrates. The ilmenite shows little or no diffusion loss. The isotopic composition of spallation Kr and Xe is similar to the one observed in meteorites. Small, systematic differences in the spallation Kr spectra of rocks 10017 and 10071 are due to variations in the irradiation hardness (shielding). The Kr spallation spectra in the mineral concentrates are different from the whole rock spectra and also show individual variations, reflecting the differences in target element composition. The relative abundance of cosmic ray produced Xe¹³¹ differs by nearly 50 per cent in the two rocks. The other Xe isotopes show no variations of similar magnitude. The origin of the Xe¹³¹ yield variability is discussed.Kr⁸¹ was measured in all the samples investigated. The Kr⁸¹/Kr exposure ages of rocks 10017 and 10071 are (480 ± 25) × 10⁶ yr and (350 ± 15) × 10⁶ yr, respectively. Exposure ages derived from spallation Ne²¹, Ar³⁸, Kr⁸³ and Xe¹²⁶ are essentially in agreement with the Kr⁸¹/Kr ages. The age of rock 10071 might be somewhat low because of a possible recent exposure of our sample to solar flare particles.     

Geological development of Jabal al Akhdar,Libya

The northern Cyrenaic headland situated north of 32 degrees latitude between Benghází and Tubruq was subject of a geological mapping. This paper draws attention to the results of this geological survey concerning Upper Cretaceous and Tertiary facies development, paleogeography, tectonic and geomorphologic evolution.The essential part of the investigated area corresponds to Jabal al Akhdar (Green Mountains), a gently uparched plateau built of Upper Cretaceous and Tertiary sediments (mostly limestones, subordinate dolomites and marls). These sediments were deposited at the southern margin of the Tethys sea and were moderately folded, mainly during the intra-Senonian (pre-Campanian) and early Ypresian intervals.From Middle Eocene till Middle Miocene, the area was subject to a slight warping followed by oscillating transgressions of a shallow sea.The youngest tectonic movements resulted in a gentle doming of the area associated with downfaulting of certain zones. Some prominent faults revealed rejuvenated activity, partly with movement inversion. Deep faulting probably controlled the paleogeographic and tectonic development of certain zones.The present gross geomorphology of Jabal al Akhdar roughly corresponds to its final uparching dated after the Middle Miocene. Topographic evolution of the northern slope of the mountains has been importantly influenced by marine erosion. Two broad littoral terraces bordered by cliffs were formed successively.

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Zusammenfassung Die nördliche Cyrenaica nördlich des 32. Breitengrades, zwischen Benghazi und Tobruk, wurde geologisch kartiert. Ergebnisse dieser Aufnahmen sind unten wiedergegeben und umfassen die fazielle Entwicklung der Oberkreide und des Tertiär sowie die paläogeographische, tektonische und geomorphologische Entwicklung.Der wesentliche Teil des untersuchten Gebietes liegt im Djebel el Akhdar. Er repräsentiert ein herausgehobenes Plateau, das sich aus Oberkreide und Tertiärsedimenten (meistens Kalke, untergeordnet Dolomite und Mergel) zusammensetzt. Diese Sedimente wurden am Südrand der Tethys abgelagert und sind vorwiegend während des Senon (Prä-Campan) und frühen Ypresien schwach verfaltet worden.Vom mittleren Eozän bis ins mittlere Miozän wurde das Gebiet teilweise gehoben und von oszillierenden Transgressionen einer Flachsee erfaßt.Die jüngsten tektonischen Bewegungen führten zu einer domartigen Aufwölbung und zu Bruchtektonik entlang bestimmter Zonen. Dabei wurden markante Störungen mehrfach regeneriert, wobei auch inverse Bewegungen nachgewiesen werden konnten. Tiefgreifende Lineamente kontrollierten wahrscheinlich die paläogeographische Situation wie auch die tektonische Entwicklung entlang bestimmter Zonen.Die augenblickliche geomorphologische Ausgestaltung des Djebel el Akhdar korrespondiert mit der zuletzt stattgefundenen Aufwölbung nach dem mittleren Miozän. Die topographische Entwicklung seines nördlichen Randes ist weitgehend beeinflußt durch marine Erosionen. Zwei breite Küstenterrassen — durch Steilküsten begrenzt — haben sich dabei sukzessive herausgebildet.

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Résumé La Cyrénaique septentrionale au Nord du 32ème parallèle, entre Benghazi et Tobrouk, a été l'objet d'un lever géologique. Les résultats donnés ici concernent le développement des facies au Crétacé supérieur et au Tertiaire, ainsi que l'évolution paléogéographique, tectonique et géomorphologique.La plus grande partie du territoire se trouve dans le Djebel el Akhdar, plateau soulevé composé de sédiments du Crétacé supérieur et du Tertiaire (la plupart des calcaires, plus rarement des dolomites et des marnes). Ces sédiments se sont déposés dans la bordure meridionale de la Téthys et furent faiblement plissés principalement au cours du Sénonien (Précampanien) et au début de l'Yprésien.De l'Eocène moyen au Miocène moyen, cette région fut en partie soulevée et soumise à des transgressions oscillatives de mer peu profonde.Les mouvements tectoniques les plus récents ont eu pour conséquence un faible bombement de la région accompagné d'affaissements de certaines zones le long de failles. Certaines failles importantes ont rejoué plusieurs fois, partiellement avec inversion de mouvement. Des failles profondes ont influencé l'évolution paléogéographique et tectonique de certaines zones.La géomorphologie générale actuelle du Djebel el Akhdar correspond grosso-modo a son bombement final d'après le Miocène moyen. L'évolution topographique du versant septentrional de la chaîne a été influencée d'une façon importante par des érosions marines. Il s'est formé successivement deux terraces littorales bordées de falaises.

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Cu- and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations

Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.5 wt%). Structurally investigated samples show a pronounced positive correlation between the <Y-O> distances and the (Li + Mn²⁺ + Cu + Fe²⁺) content (apfu) at this site with R ² = 0.90. An excellent negative correlation exists between the <Y-O> distances and the Al₂O₃ content (R ² = 0.94). The samples at each locality generally show a strong negative correlation between the X-site vacancies and the (MnO + FeO) content. The Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between MnO and CuO we believe that the Cu content in tourmaline is essentially dependent on the availability of Cu and on stereochemical constraints.