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991.
We compute the growth of isolated gaseous giant planets for several values of the density of the protoplanetary disk, several distances from the central star and two values for the (fixed) radii of accreted planetesimals. Calculations were performed in the frame of the core instability mechanism and the solids accretion rate adopted is that corresponding to the oligarchic growth regime. We find that for massive disks and/or for protoplanets far from the star and/or for large planetesimals, the planetary growth occurs smoothly. However, notably, there are some cases for which we find an envelope instability in which the planet exchanges gas with the surrounding protoplanetary nebula. The timescale of this instability shows that it is associated with the process of planetesimals accretion. The presence of this instability makes it more difficult the formation of gaseous giant planets. 相似文献
992.
During the last decade there was a change in paradigm, which led to consider that terrestrial-type planets within liquid-water habitable zones (LW-HZ) around M stars can also be suitable places for the emergence and evolution of life. Since many dMe stars emit large amount of UV radiation during flares, in this work we analyze the UV constrains for living systems on Earth-like planets around dM stars. We apply our model of UV habitable zone (UV-HZ; Buccino, A.P., Lemarchand, G.A., Mauas, P.J.D., 2006. Icarus 183, 491–503) to the three planetary systems around dM stars (HIP 74995, HIP 109388 and HIP 113020) observed by IUE and to two M-flare stars (AD Leo and EV Lac). In particular, HIP 74995 hosts a terrestrial planet in the LW-HZ, which is the exoplanet that most resembles our own Earth. We show, in general, that during the quiescent state there would not be enough UV radiation within the LW-HZ to trigger the biogenic processes and that this energy could be provided by flares of moderate intensity, while strong flares do not necessarily rule-out the possibility of life-bearing planets. 相似文献
993.
Andrea Patzer Dolores H. Hill William V. Boynton 《Meteoritics & planetary science》2003,38(5):783-794
Abstract— Dar al Gani 872 (DaG 872) is a new meteorite from Libya that we classified by means of Instrumental Neutron Activation Analysis (INAA), electron microprobe, and optical microscopy. According to our results, DaG 872 is a Mg‐rich main group eucrite, i.e., a monomict noncumulate basaltic eucrite displaying a predominant coarse‐grained relict subophitic and a fine‐grained granulitic texture. The meteorite also shows pockets of late‐stage mesostasis and is penetrated by several calcite veins due to terrestrial weathering. Finally, it exhibits shock phenomena of stage 1–2 including heavily fractured mineral components, undulose extinction of plagioclase, kinked lamellae, and mosaicism in pyroxenes corresponding to peak pressures of ?20 GPa. In view of petrographic criteria as well as compositional and exsolution characteristics of its pyroxenes, the sample represents a metamorphic type 5 eucrite. Assuming the metamorphic type to be a function of burial depth on the parent body and taking into account the relatively high shock stage, the excavation of DaG 872 was likely induced by a major impact event. Prior to this point, DaG 872 apparently underwent a 4‐stage geological evolution that is reflected by intricate textural and mineralogical features. 相似文献
994.
Abstract— We examined 15 bulk samples of the acapulcoite‐lodranite clan for their major, minor, and trace element concentrations using INAA techniques. Among the analyzed meteorites are 2 new acapulcoites (Dhofar [Dho] 290, Thiel Mountains [TIL] 99002) as well as an additional acapulcoite that has been described previously only in very brief form (Graves Nunataks [GRA] 98028). The petrographic attributes of these 3 samples are addressed thoroughly. We also include petrographic information on 2 acapulcoites from Africa: Northwest Africa (NWA) 725 and NWA 1058. In general, our study strongly supports the widely accepted idea that acapulcoites and lodranites evolved through partial melting and melt migration of metal/sulfide phases and plagioclase. Furthermore, we concur with previous researchers that the original bimodal classification scheme for acapulcoites and lodranites proves to be too simple. Based on our data set, we introduce an alternative, extended scheme. With respect to their elemental distribution patterns, we distinguish 5 subtypes comprising primitive, typical, transitional, and enriched acapulcoites on one hand and lodranites on the other. The chemical distinction between the primitive, typical, and transitional acapulcoites is rather subtle and gradual. It stands in contrast to the clear modifications observed for the signatures of the enriched acapulcoites and the lodranites. The definition of subcategories basically reflects the concentrations of 2 key elements: K and Se. We note, however, that the assignment of subgroups may not be exclusively inferred from elemental abundances but should also consider additional petrographic information. 相似文献
995.
Mollusc and brachiopod skeletal hard parts: Intricate archives of their marine environment 下载免费PDF全文
The biogenic carbonate hard parts of fossil bivalves, cephalopods and brachiopods are among the most widely exploited marine archives of Phanerozoic environmental and climate dynamics research. The advent of novel analytical tools has led many workers to explore non‐traditional geochemical and petrographic proxies, and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms. These considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve, cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer. Specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral. Given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum, this paper evaluates differences and similarities of seawater versus that of body fluids. Cephalopods, bivalves and brachiopods exert a strong biological control on biomineralization that, to some degree, may buffer their shell geochemistry against secular changes in seawater chemistry. Disordered (amorphous) calcium carbonate precursor phases, later transformed to crystalline biominerals, may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases. A reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo‐environment. The impact of what is commonly referred to as ‘biological factors’ on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon, oxygen, strontium and traditionally used element to calcium ratios. In particular, the often used δ13Ccarb or the Mg/Ca and Sr/Ca elemental proxies are fraught with problems. An interesting new research field represents the analysis, calibration and application of non‐traditional proxies to mollusc and brachiopod hard parts. Examples include the carbonate clumped isotope (Δ47) approach and the analysis of the isotopes of Ca, Mg, N, Li, S or element to Ca ratios such as Li/Ca or B/Ca and rare earth elements. Based on considerations discussed here, a series of “do's and don'ts” in mollusc and brachiopod archive research are proposed and suggestions for future work are presented. In essence, the suggestions proposed here include experimental work (also field experiments) making use of recent archive organisms or, where possible, a reasonable recent analogue in the case of extinct groups. Moreover, the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra‐structures must guide sampling strategies for geochemical analyses. Where feasible, a detailed understanding of the diagenetic pathways and the application of multi‐proxy and multi‐archive approaches should form the foundation of fossil carbonate archive research. The uncritical compilation of large data sets from various carbonate‐shelled organisms collected at different locations is not encouraged. 相似文献
996.
Settlement and colonization of epi‐endobenthic communities on calcareous substrata in an underwater archaeological site 下载免费PDF全文
Edoardo Casoli Sandra Ricci Andrea Belluscio Maria Flavia Gravina Giandomenico Ardizzone 《Marine Ecology》2015,36(4):1060-1074
Underwater archaeological sites represent assets of great value that are subjected to physical and biological stresses. The study of biological settlement on calcareous substrata is the first step to understanding the biodeterioration processes that take place on man‐made structures (marble, statues, columns and other architectonic elements) in marine environments. To accomplish this objective, calcareous panels (30 × 30 × 2 cm) were immersed for 3, 6, 9 and 12 months along a north?south transect in marine protected areas of Baia (Naples). Both epilithic and endolithic assemblages were analysed: the latter was observed by scanning electron microscope after the cast‐embedding technique had been used. After 1 year the epilithic community was still changing, with algae and pioneer species overgrown by encrusting organisms, and the endolithic assemblage dominated by cyanobacteria/chlorophyte traces. Changes of epilithics lead changes on endolithics. The initial phototrophic borers were replaced by more sciophilous and competitive species; moreover, the first macroboring organisms arrived after the settlement of grazers. After 12 months of immersion, the rate of bioerosion was high, the material was deeply bored and all of the limestone surface was heavily covered. 相似文献
997.
Climate change and warm‐water species at the north‐western boundary of the Mediterranean Sea 下载免费PDF全文
Valeriano Parravicini Luisa Mangialajo Laure Mousseau Andrea Peirano Carla Morri Monica Montefalcone Patrice Francour Michel Kulbicki Carlo Nike Bianchi 《Marine Ecology》2015,36(4):897-909
The effects of global change are particularly serious in areas where range shifts of species are physically constrained such as the Ligurian Sea, which is one of the coldest sectors of the Mediterranean. In this basin, historical information on water temperature (from the sea surface down to 75 m depth) dates back to the 1950s. Early studies also recorded warm‐water species occurrence. Thanks to these data we provide the first detailed characterization of water temperature variation from 1958 up to 2010 in the layer 0–75 m depth. We coupled this analysis with the available information on rocky reef epibenthic communities (literature review from 1955 to 1964 and field data from 1980 to 2010). The analysis of water temperature revealed several patterns of variation: a cooling phase from 1958 to 1980, a phase of rapid warming from 1980 to 1990 and a phase of slower warming from 1990 to 2010. Inter‐annual variation in temperature increased over the entire period for the water layer down to 20 m. Warm‐water native and alien species richness increased during the warming phases. Literature estimates suggest a decrease in warm‐water native species richness during the cooling phase. The analysis of quantitative data collected in the early 1990s and late 2000s indicated a decrease in the cover of warm‐water native species on shallow rocky reefs and an increase in deeper waters. We argue that increased inter‐annual variation in water temperature may disadvantage native warm‐water species in shallow waters. Our results indicate that the effect of temperature rises in cold, constrained basins may be more complex than the simple prediction of species changing their geographical range according to their thermal limits. 相似文献
998.
Magnetite in the unequilibrated CK chondrites: Implications for metamorphism and new insights into the relationship between the CV and CK chondrites 下载免费PDF全文
Tasha L. Dunn Juliane Gross Marina A. Ivanova Simone E. Runyon Andrea M. Bruck 《Meteoritics & planetary science》2016,51(9):1701-1720
Bulk isotopic and elemental compositions of CV and CK chondrites have led to the suggestion that both originate from the same asteroid. It has been argued that magnetite compositions also support this model; however, magnetite has been studied almost exclusively in the equilibrated (type 4‐6) CKs. Magnetite in seven unequilibrated CKs analyzed here is enriched in MgO, TiO2, and Al2O3 relative to the equilibrated CKs, suggesting that magnetite compositions are affected by metamorphism. Magnetite in CKs is compositionally distinct from CVs, particularly in abundances of Cr2O3, NiO, and TiO2. Although there are minor similarities between CV and equilibrated CK chondrite magnetite, this is contrary to what we would expect if the CVs and CKs represent a single metamorphic sequence. CV magnetite should resemble CK3 magnetite, as both were metamorphosed to type 3 conditions. Oxygen fugacities and temperatures of CVox and CK chondrites are also difficult to reconcile using existing CV‐CK parent body models. Mineral chemistries, which eliminate issues of bulk sample heterogeneity, provide a reliable alternative to techniques that involve a small amount of sample material. CV and CK chondrite magnetite has distinct compositional differences that cannot be explained by metamorphism. 相似文献
999.
Hannah E. Glover Andrea S. Ogston Aaron T. Fricke Chuck A. Nittrouer Cherry Aung Thet Naing Evan J. Lahr 《Sedimentology》2023,70(1):214-234
Vegetated mid-channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide-dominated deltas. Meinmahla Island is a mangrove-forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid-channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi-continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single-entrance (dead-end/blind) and multi-entrance tidal channels of the island. The morphological evolution was evaluated using grain size, 210Pb geochronology, remote sensing and channel surveys. The data show that ebb-dominant, single-entrance channels along the island exterior import sediment year-round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multi-entrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal-channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi-entrance channel is responding to a drainage-network change by narrowing and shoaling. Overall, mid-channel islands trap sediment and associated nutrients at the river–ocean interface, and these resilient landscape features evolve in response to changes in drainage-network connectivity. 相似文献
1000.
An analysis of intersystem biases for multi-GNSS positioning 总被引:4,自引:3,他引:1