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921.
Yardangs are streamlined ridges that form in arid environments on Earth and Mars through wind-driven abrasion of consolidated substrates. Currently, there is limited consensus on the mechanisms that initiate and establish patterns of yardangs on the landscape. In this work, we examine the spatial organization of yardangs in the Campo de Piedra Pómez ignimbrite deposit of north-western Argentina and identify evidence of antecedent controls on yardang patterns and formation. We mapped 14,826 yardangs in the region using a high-resolution digital elevation model (DEM) and satellite imagery. We classified yardangs as points using a two-stage decision rule based on morphology and spectral characteristics. Point pattern analysis shows that yardangs in the study area are not randomly distributed and commonly exhibit directional anisotropy in point pattern. The anisotropic pattern manifests as bands of closely-spaced yardangs oriented transverse to the dominant northwesterly wind direction. We hypothesize that banding is controlled by pre-existing antecedent topography in the bedrock, such as fumaroles or ridges associated with pyroclastic flow deposits. We present evidence from other locations on Earth and Mars to illustrate that the transverse banding is a common pattern in yardang landscapes. 相似文献
922.
Emanuele Perinati Chris Tenzer Andrea Santangelo Konrad Dennerl Michael Freyberg Peter Predehl 《Experimental Astronomy》2012,33(1):39-53
The pn-CCD cameras at the focal plane of the eROSITA space observatory will be the first X-ray (0.2?C12?keV) detector to operate in a L-2 orbit. Therefore, no direct information of instrumental non X-ray background (NXB) is available to make predictions for eROSITA. Since, in general, the instrumental NXB experienced in orbit has a major impact on the overall sensitivity of the cameras, we investigated and modeled the L-2 radiation environment and its interaction with the eROSITA mass distribution and cameras, in order to quantify the expected pn-CCD NXB level. We obtain an average value of NXB ??25% lower than that observed by the pn-CCDs on-board the XMM-Newton satellite, which is placed in a Highly Elliptical Orbit (HEO). We discuss this result in light of the differences between the L-2 and HEO space environments. 相似文献
923.
This paper documents and assesses emerging efforts to resist and subvert deep-seated and long-held governmental secrecy over geographical spaces of military/security activities and other sites deemed sensitive by the state. It explores tensions in new web-served mapping and high-resolution imagery of these sites, which view them though ‘pin holes’ of publicly available data. These ‘counter-mappings’ focus attention on the significance of sites that are either buried unnoticed in seamless global image coverage, or else censored on official mapping. Some reveal a tenaciously anti-hegemonic and oppositional discourse, others a more playful set of cultural practices, one that ridicules as much as directly resists. We situate these newly witnessed secret sites in contemporary visual culture, exploring the spectacular and Debordian possibilities of resistance that they offer, and evaluate the significance and ironies of these diverse imaging practices. 相似文献
924.
We compare melting of potassic alteration zones in metamorphosed gold deposits with that of unaltered rocks of the same protolith
to examine their relative contributions to crust-derived magmas and to investigate the implications for ore genesis. Potassic
hydrothermal alteration, at the crustal levels where orogenic gold deposits form, stabilizes a higher proportion of muscovite
and biotite than is possible in unaltered rocks at high metamorphic grades. Because these micas contain water, they control
the melt fraction generated through dehydration melting in that a greater proportion of micas permits more extensive melting.
Orogenic gold deposits, in which mineralization is typically encapsulated by potassic alteration, form at deep-enough crustal
levels to survive repeated tectonic activity, which can lead to their being metamorphosed. In the vicinity of this metamorphosed
gold mineralization, the greatest proportion of felsic melt is generated in the more metal- and sulfur-rich rocks because
of the associated potassic alteration. Ore minerals dissolve and are physically incorporated into the resulting felsic melt,
which thereby becomes metal- and sulfur-enriched. Since melt fraction is the dominant control on strain partitioning and melt
mobilization, increased melting in K-altered mineralized rocks implies that these sites will be the first to experience melt
escape and will continue to be the focus of melt escape during ongoing metamorphism. This strain partitioning promotes shear
zone development, and once shearing is localized to K-altered mineralized domains, it may attract external magma, allowing
extension and linking with nearby active shear zones. In this way, mineralized zones may connect to a regional network of
magma transfer, allowing metal enrichment of migrating magmas. Terrains that underwent widespread K alteration associated
with mid-crustal gold enrichment are likely, when metamorphosed, to produce significant volumes of reduced, relatively metal-
and sulfur-enriched felsic magma. The ages and relative tectonic preservation potential of different K alteration-associated
ore types implies that Au, Ag, As, Sb, Bi, Te, and W may be recycled within the crust through this mechanism, whereas Cu and
Mo are unlikely to be recycled and require mantle sourcing to form new intrusion-related ores. Silicate melt derived from
preexisting zones of gold enrichment in the lower crust may contribute significantly to the metal budget of intrusion-related
gold systems, and possibly some gold-rich porphyry deposits. 相似文献
925.
Marie-Pierre Aubry William A. Berggren Christian Dupuis Holeil Ghaly David Ward Chris King Robert W. O'B. Knox Khaled Ouda Moustafa Youssef Wael Fathi Galal 《地学学报》2009,21(4):237-256
We present a review of archaeological and geological studies on the West Bank as a basis for discussing the geological setting of the tombs and geologically related problems with a view to providing archaeologists with a framework in which to conduct their investigations on the restoration, preservation and management of the antique monuments. Whereas the geology of the Upper Nile Valley appears to be deceptively simple, the lithological succession is vertically variable, and we have recognized and defined several new lithological units within the upper Esna Shale Formation. We have been able to delineate lithological (shale/limestone) contacts in several tombs and observed that the main chambers in some were excavated below the Esna Shale in the Tarawan Chalk Formation. We have been able to document changing dip in the strata (warping) in several tombs, and to delineate two major orientations of fractures in the field. Investigations behind the Temple of Hatshepsut, in the Valley of the Kings and around Deir El Medina have revealed four broad regional structures. We confirm that the hills located near the Nile Valley, such as Sheik Abdel Qurna, do not belong to the tabular structure of the Theban Mountain, but are discrete displaced blocks including the Thebes Limestone, as supported by Google Earth photographs. 相似文献
926.
OluKayode J. Samuel Chris Cornford Martin Jones Olabisi A. Adekeye Samuel O. Akande 《Organic Geochemistry》2009,40(4):461-483
Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C27 steranes, C30 24-n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C12–C30 n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C29 steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta. 相似文献
927.
The issue of the social geographical dimensions of climate change is timely and important. This paper sets out to explore one example of this: how people living in the Pacific who are most at risk of being made landless by climate change are portrayed in policy discourse, and how high-level international representatives of Pacific nations have responded to these portrayals. At the heart of this is contention over the portrayal of Pacific Island peoples as ‘climate refugees’. This paper analyses a number of documents since the 1980s, largely from non-governmental organisations (NGOs) that deploy the identity construct of ‘climate refugees’. Fieldwork undertaken at the United Nations in New York in 2004 also enabled seven interviews with national ambassadors representing Pacific small island states. Interviews revealed how Pacific ambassadors have responded to the category of ‘climate refugees’, and positioned themselves in the discursive field surrounding the climate change debate. A poststructuralist framework, drawing on Foucault’s ideas of discourse and subject categories provided a means to critically scrutinise and better understand how people from Pacific countries are imagined in the wider, global geopolitical arena, but crucially, how leaders from these nations also construct themselves in relation to climate change and its associated impacts. 相似文献
928.
Marek Gierliski Chris Done Kim Page 《Monthly notices of the Royal Astronomical Society》2009,392(3):1106-1114
We build a simple model of the optical/ultraviolet (UV) emission from irradiation of the outer disc by the inner disc and coronal emission in black hole binaries. We apply this to the broad-band Swift data from the outburst of the black hole binary XTE J1817−330 to confirm previous results that the optical/UV emission in the soft state is consistent with a reprocessing a constant fraction of the bolometric X-ray luminosity. However, this is very surprising as the disc temperature drops by more than a factor of 3 in the soft state, which should produce a marked change in the reprocessing efficiency. The easiest way to match the observed constant reprocessed fraction is for the disc skin to be highly ionized (as suggested 30 yr ago by van Paradijs), so that the bulk of the disc flux is reflected and only the hardest X-rays heat the disc. The constant reprocessed fraction also favours direct illumination of the disc over a scattering origin as the optical depth/solid angle of any scattering material (wind/corona) over the disc should decrease as the source luminosity declines. By contrast, the reprocessed fraction increases very significantly (by a factor of ∼6) as the source enters the hard state. This dramatic change is not evident from X-ray/UV flux correlations as it is masked by bandpass effects. However, it does not necessarily signal a change in emission, for example, the emergence of the jet dominating the optical/UV flux as the reflection albedo must change with the dramatic change in spectral shape. 相似文献
929.
Chris Koen 《Monthly notices of the Royal Astronomical Society》2009,395(1):531-536
Systematic variability in stellar magnitudes, as derived from profile fitting to CCD images, may in some instances be due to variable seeing. It is suggested that this happens in cases where the stars are unresolved pairs, typically with sub-arcsecond separation between the components. It is shown that the fitting of suitable Generalised Additive Models to time series photometry can disentangle intrinsic stellar variability and seeing-induced brightness changes. It is possible that there will be a fixed seeing response associated with a given star which exhibits the effect: estimation of this response from several long photometric runs is demonstrated. 相似文献
930.
Caroline van Breukelen Chris Simpson Steve Rawlings Masayuki Akiyama David Bonfield Lee Clewley Matt J. Jarvis Tom Mauch Tony Readhead Ann-Marie Stobbart Mark Swinbank Mike Watson 《Monthly notices of the Royal Astronomical Society》2009,395(1):11-27
We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at z ∼ 1 in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ∼200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K -band luminosities of LK = 1.7 ± 0.7 L * for the X-ray sources and LK = 2.3 ± 0.1 L * for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage. 相似文献