A spectroscopic study of the surfaces of Saturn's large satellites: H₂O ice, tholins, and minor constituents     

Dale P. Cruikshank  Tobias C. Owen  Thomas R. Geballe  Catherine de Bergh  Francois Poulet  Joshua P. Emery 《Icarus》2005,175(1):268-283

We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0-2.5 μm, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution (R=λ/Δλ) of the data shown here is in the range 800-1000, depending on the specific instrument and configuration used; this is higher than the resolution (R=225 at 3 μm) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal H₂O ice at 1.65 μm. We model the leading hemisphere of Rhea in the wavelength range 0.3-3.6 μm with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are H₂O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3-1.0 μm. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of CO₂ (a few weight percent, depending on particle size and mixing), CH₄ (same), and NH₄OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH₄OH are presented for the purpose of detection on icy bodies. These limits for CO₂, CH₄, and NH₄OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 μm.  相似文献   

Climatic change in Central Asia during MIS 3/2: a case study using biological responses from Lake Baikal   总被引：5，自引：0，他引：5  

George E.A. Swann  Anson W. Mackay  Melanie J. Leng  Francois Demory 《Global and Planetary Change》2005,46(1-4):235

A Marine Isotope Stage (MIS) 3/early MIS 2 section from a structural high along the east coast of the North Basin of Lake Baikal was analysed for diatoms, C/N ratios, and organic carbon isotope ratios. Diatoms were present throughout MIS 3 and early MIS 2, with high concentrations of the planktonic taxa Cyclotella sp. c.f. gracilis between 54 and 51.5 kyr BP indicating relatively warm, interstadial, conditions. Following a %TOC inferred climatic cooling between 43.2 and 39.1 kyr BP, evidence of a more muted δ¹³C_(organic) and %TOC inferred climatic warming from c. 39.1–34.7 kyr BP coincides with a period of very high diatom concentrations, indicating high aquatic productivity, at the Buguldeika Saddle in the South Basin of Lake Baikal. No evidence exists for a ‘Kuzmin’ catchment erosional event in the North Basin during MIS 3. This, however, may reflect the location of the coring site away from major riverine inputs. Abrupt climatic cooling at the culmination of both warm phases in the North Basin are associated, on the basis of the palaeomagnetic age-model and correlations to existing sites in Lake Baikal, with the initiation of Heinrich events 5 (c. 50 kyr BP) and 4 (c. 35 kyr BP), respectively, in the North Atlantic. The amount of organic material declines across the MIS 3/MIS 2 transition while constant C/N ratios suggest organic material to be predominantly derived from phytoplankton. An increase in δ¹³C_(organic) at the MIS 3/MIS 2 transition may therefore indicate changes in aquatic productivity, pCO₂ or the inorganic carbon pool.  相似文献   

Imaging the Seismic Crustal Structure of the Western Mexican Margin between 19°N and 21°N     

Rafael Bartolomé  Juanjo Dañobeitia  François Michaud  Diego Córdoba  Luis A. Delgado-Argote 《Pure and Applied Geophysics》2011,168(8-9):1373-1389

Three thousand kilometres of multichannel (MCS) and wide-angle seismic profiles, gravity and magnetic, multibeam bathymetry and backscatter data were recorded in the offshore area of the west coast of Mexico and the Gulf of California during the spring 1996 (CORTES survey). The seismic images obtained off Puerto Vallarta, Mexico, in the Jalisco subduction zone extend from the oceanic domain up to the continental shelf, and significantly improve the knowledge of the internal crustal structure of the subduction zone between the Rivera and North American (NA) Plates. Analyzing the crustal images, we differentiate: (1) An oceanic domain with an important variation in sediment thickness ranging from 2.5 to 1 km southwards; (2) an accretionary prism comprised of highly deformed sediments, extending for a maximum width of 15 km; (3) a deformed forearc basin domain which is 25 km wide in the northern section, and is not seen towards the south where the continental slope connects directly with the accretionary prism and trench, thus suggesting a different deformational process; and (4) a continental domain consisting of a continental slope and a mid slope terrace, with a bottom simulating reflector (BSR) identified in the first second of the MCS profiles. The existence of a developed accretionary prism suggests a subduction–accretion type tectonic regime. Detailed analysis of the seismic reflection data in the oceanic domain reveals high amplitude reflections at around 6 s [two way travel time (twtt)] that clearly define the subduction plane. At 2 s (twtt) depth we identify a strong reflection which we interpret as the Moho discontinuity. We have measured a mean dip angle of 7° ± 1° at the subduction zone where the Rivera Plate begins to subduct, with the dip angle gently increasing towards the south. The oceanic crust has a mean crustal thickness of 6.0–6.5 km. We also find evidence indicating that the Rivera Plate possibly subducts at very low angles beneath the Tres Marias Islands.  相似文献   

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California     

Thierry Calmus  Carlos Pallares  Ren�� C. Maury  Alfredo Aguill��n-Robles  Herv�� Bellon  Mathieu Benoit  Fran?ois Michaud 《Pure and Applied Geophysics》2011,168(8-9):1303-1330

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.  相似文献   

L’ouverture de l’Atlantique et son influence sur les peuplements des grands foraminifères des plates-formes péri-océaniques au Mésozoïque     

Eric Fourcade  François Michaud 《Geodinamica Acta》2013,26(4-5):247-262

Résumé

— Au Jurassique et au Crétacé, le peuplement des plates-formes des marges atlantiques par les grands foraminifères benthi-ques s’est opéré d’Ouest en Est. Caractéristiques des zones tropicales ces organismes n’ont atteint la marge américaine de l’océan qu’au Callovien ou à l’Oxfordien. Jusqu’au Sénonien, les grands forammifères de cette marge étaient peu diversifiés; ils appartenaient à des espèces bien connues sur la marge européenne et africaine ainsi que dans la Néoléthys méditerranéenne où les grands foraminifères benthiques sont très diversifiés. Néanmoins un faible endémisme est apparu pendant l’Albien sur la marge américaine de l’océan.

D’une façon générale, la faible diversité des formes américaines est sans doute liée à la faible largeur qu’avait alors l’Océan Atlantique petit bassin océanique entre deux super continents, et aux instabilités climatiques que cette situation pouvait engendrer.

Un important changement est intervenu au Campanien où le peuplement des plates-formes carbonatées américaines a montré une grande diversité et un fort taux d’endémisme. Ceci peut être lié pour partie à la largeur notable qu’atteignait alors l’Atlantique aussi bien dans sa partie centrale que méridionale, isolant ainsi la marge américaine de la marge européenne et africaine.

S’agissant du régime climatique, la distribution des grands foraminifères au Jurassique supérieur-Crétacé inférieur montre qu’un même régime tropical régnait alors du Guatemala à Terre Neuve et du Sénégal à la Bretagne sur les deux rivages. Au Crétacé supérieur, les grands foraminifères ne sont connus qu’aux basses latitudes sur la rive américaine (Texas, Floride, Bahamas) tandis qu’ils sont relativement abondants aux latitudes élevées sur la marge européenne (Hollande, Suède méridionale). Il est probable qu’un paléo-Gulstream réchauffait alors les côtes septentrionales de l’Europe.  相似文献   

The self-organizing map,a new approach to apprehend the Madden–Julian Oscillation influence on the intraseasonal variability of rainfall in the southern African region     

Pascal Oettli  Tomoki Tozuka  Takeshi Izumo  Francois A. Engelbrecht  Toshio Yamagata 《Climate Dynamics》2014,43(5-6):1557-1573

The Madden–Julian Oscillation (MJO) is the major mode of intraseasonal variability (30–60 days) in the tropics, having large rainfall impacts globally, and possibly on southern Africa. However, the latter impact is not well understood and needs to be further explored. The life cycle of the MJO, known to be asymmetric, has been nevertheless analyzed usually through methods constrained by both linearity and orthogonality, such as empirical orthogonal function analysis. Here we explore a non-linear classification method, the self-organizing map (SOM), a type of artificial neural network used to produce a low-dimensional representation of high-dimensional datasets, to capture more accurately the life cycle of the MJO and its global impacts. The classification is applied on intraseasonal anomalies of outgoing longwave radiation within the tropical region over the 1980–2009 period. Using the SOM to describe the MJO is a new approach, complimentary to the usual real-time multivariate MJO index. It efficiently captures this propagative phenomenon and its seasonality, and is shown to provide additional temporal and spatial information on MJO activity. For each node, the subtropical convection is analyzed, with a particular focus on the southern Africa region. Results show that the convection activity over the central tropical Indian Ocean is a key factor influencing the intraseasonal convective activity over the southern African region. Enhanced (suppressed) convection over the central Indian Ocean tends to suppress (enhance) convection over the southern African region with a 10-day lag by modulating the moisture transport.  相似文献   

Impacts of convection schemes on simulating tropical-temperate troughs over southern Africa     

Tomoki Tozuka  Babatunde J. Abiodun  Francois A. Engelbrecht 《Climate Dynamics》2014,42(1-2):433-451

This study examines southern African summer rainfall and tropical temperate troughs (TTTs) simulated with three versions of an atmospheric general circulation model differing only in the convection scheme. All three versions provide realistic simulations of key aspects of the summer (November–February) rainfall, such as the spatial distribution of total rainfall and the percentage of rainfall associated with TTTs. However, one version has a large bias in the onset of the rainy season. Results from self-organizing map (SOM) analysis on simulated daily precipitation data reveals that this is because the occurrence of TTTs is underestimated in November. This model bias is not related to westerly wind shear that provides favorable conditions for the development of TTTs. Rather, it is related to excessive upper level convergence and associated subsidence over southern Africa. Furthermore, the model versions are shown to be successful in capturing the observed drier (wetter) conditions over the southern African region during El Niño (La Niña) years. The SOM analysis reveals that nodes associated with TTTs in the southern (northern) part of the domain are observed less (more) often during El Niño years, while nodes associated with TTTs occur more frequently during La Niña years. Also, nodes associated with dry conditions over southern Africa are more (less) frequently observed during El Niño (La Niña) years. The models tend to perform better for La Niña events, because they are more successful in representing the observed frequency of different synoptic patterns.  相似文献   

Thermal and exhumation histories of the northern subalpine chains (Bauges and Bornes—France): Evidence from forward thermal modeling coupling clay mineral diagenesis,organic maturity and carbonate clumped isotope (Δ47) data     

Xavier Mangenot  Jean‐Francois Deoninck  Magali Bonifacie  Virgile Rouchon  Pierre‐Yves Collin  Didier Quesne  Marta Gasparrini  Jean‐Pierre Sizun 《Basin Research》2019,31(2):361-379

Assessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ₄₇ clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160–180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ₄₇ thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160–180°C. High temperatures are associated with the tectonic emplacement of up to 4 km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ₄₇, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.  相似文献   

关于“含水中基性大陆下地壳部分熔融能形成C型埃达克岩吗？”的回复     

张超  马昌前  Francois HOLTZ 《高校地质学报》2013,19(2):381

焦永玲等对笔者发表在《高校地质学报》2012年第18 卷第1 期的“含水大陆下地壳的部分熔融：大别山C型埃达克 岩成因探讨”一文提出质疑。针对他们的问题,笔者进行了回复,并提出：（1）大别山C 型埃达克岩的源岩可能是基性的 （SiO2 含量50% 左右）,但是比较准确的成分估计还需要进一步研究;（2）变质含水基性岩（角闪岩）在1.0~1.2 GPa 压力下（相当于35~40 km）可以形成含石榴石20% 以上的残余体;（3）中钾的角闪岩（K2O 含量1% 左右）部分熔融可以形成高钾钙碱性长英质熔体（K2O 含量3.5%~5%）。  相似文献   

Dust tail of the active distant Comet C/2003 WT42 (LINEAR) studied with photometric and spectroscopic observations     

Pavlo P. Korsun  Irina V. Kulyk  Oleksandra V. Ivanova  Viktor L. Afanasiev  Francois Kugel  Claudine Rinner  Yuriy M. Ivashchenko 《Icarus》2010,210(2):916-929

We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a^−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.  相似文献