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541.
S. P. Littlefair V. S. Dhillon Steve B. Howell David R. Ciardi 《Monthly notices of the Royal Astronomical Society》2000,313(1):117-128
We present time-resolved, J ‐band (1.025–1.340 μm) infrared spectra of the short-period dwarf novae (DNe) WZ Sge and VY Aqr, and single spectra of the short-period DN EF Peg and the nova-like variable PX And. There is some evidence in the spectra of VY Aqr and EF Peg that we have detected the secondary star, both in the continuum slope and also through the possible presence of spectral features. The spectra of WZ Sge and PX And, on the other hand, show no evidence for the secondary star, with upper limits for its contribution to the J ‐band light of 10 and 20 per cent respectively. The spectral type of the secondary in WZ Sge is constrained to be later than M7.5V. Using skew mapping, we have been able to derive a value for the radial velocity semi-amplitude of the secondary star in VY Aqr of K R =320±70 km s−1 , which in conjunction with K W from Thorstensen & Taylor gives a mass ratio of q =0.15±0.04. 相似文献
542.
The southern Tien Shan metallogenic province of Central Asia hosts a number of important gold resources including the Jilau
gold–quartz vein system in western Tajikistan. These deposits were formed at the late stages of continent–continent collision
in association with subduction-related magmatism, metamorphism and continental margin deformation attributed to the Central
Asian Hercynian Orogeny. Jilau is hosted by a Hercynian syntectonic granitoid intrusive that was emplaced into bituminous
dolomite country rocks. Economic mineralisation is associated with a dilational jog within a high-angle, oblique dextral-reverse
slip shear zone that was undergoing brittle–ductile deformation. The orebody takes the form of shear-zone subparallel quartz
veins and lenses that emanate from a steeply plunging ore shoot of veins and stringers within a silicified and sulphidised
granodiorite core. It is thought to have formed by a dynamic process in which fluid flow was governed by a fault-valve mechanism.
Numerous cycles of fluid pressure build-up, fault failure, jog dilation, fluid flow, phase separation of low salinity H2O–CO2–CH4(–N2) fluids, and sealing took place. Gold appears together with scheelite and bismuth minerals predominantly as inclusions in
arsenopyrite in quartz veins and altered wall-rock, and is mainly associated with quartz containing fluid inclusions enriched
in CH4. The correlation between high gold grades and high CH4 concentrations suggests that components of the mineralising fluids were derived from, or passed through, the reducing, carbonaceous
rocks in the contact aureole of the intrusive. The occurrence of Au and W in an adjacent Hercynian skarn deposit and in the
Jilau orebody, infers that the ore metals in both these systems were ultimately derived from a magmatic source.
Received: 15 April 1999 / Accepted: 30 December 1999 相似文献
543.
544.
Paleoenvironments of the Torrey Pines State Reserve were reconstructed from a 3600-yr core from Los Peñasquitos Lagoon using fossil pollen, spores, charcoal, chemical stratigraphy, particle size, and magnetic susceptibility. Late Holocene sediments were radiocarbon dated, while the historical sediments were dated using sediment chemistry, fossil pollen, and historical records. At 3600 yr B.P., the estuary was a brackish-water lagoon. By 2800 yr B.P., Poaceae (grass) pollen increased to high levels, suggesting that the rising level of the core site led to its colonization by Spartina foliosa (cord-grass), the lowest-elevation plant type within regional estuaries. An increase in pollen and spores of moisture-dependent species suggests a climate with more available moisture after 2600 yr B.P. This change is similar to that found 280 km to the north at 3250 yr B.P., implying that regional climate changes were time-transgressive from north to south. Increased postsettlement sediment input resulted from nineteenth-century land disturbances caused by grazing and fire. Sedimentation rates increased further in the twentieth century due to closure of the estuarine mouth. The endemic Pinus torreyana (Torrey pine) was present at the site throughout this 3600-yr interval but was less numerous prior to 2100 yr B.P. This history may have contributed to the low genetic diversity of this species. 相似文献
545.
546.
The Matahina Ignimbrite (~160 km3 rhyolite magma, 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, Taupo Volcanic
Zone (TVZ), New Zealand. Juvenile clasts are divided into three groups: Group (1) the dominant crystal-poor rhyolite type,
Group (2) a minor coarse-grained, mingled/mixed intermediate type, and Group (3) a rare fine-grained basalt. The ignimbrite
consists of the Group 1 type and is divided into three members: a lower and middle member, which is high-silica, crystal-poor
(<10 vol.%) rhyolite, and the upper member, which is low-silica and slightly more crystal-rich (up to 21 vol.%). Cognate,
crystal-rich (up to 50 vol.%) basalt to intermediate pumice occurs on top of lag breccias and within lithic-rich pyroclastic
density current deposits along the caldera margin (Groups 2 and 3). Several lines of evidence indicate that the intermediate
clasts represent the cumulate complement to the melt-rich rhyolite: (1) continuity in the compositions of plagioclase, orthopyroxene,
hornblende, and oxides and normal zoning of individual phenocrysts; (2) the silicic glass from the intermediate magma (interstitial
melt) overlaps compositionally with the bulk rock rhyolite and glass; (3) high Zr and a slight positive Eu anomaly in the
intermediate magma relative to quenched enclaves from other intermediate TVZ eruptions indicates zircon and plagioclase accumulation,
respectively; (4) an increase in the Cl contents in glass from the least evolved to most evolved is consistent with the concentration
of volatiles during magma evolution. Most of the compositional variations in the low- to high-silica rhyolites can be accounted
for by continued Rayleigh fractionation (up to 15%), following melt extraction from the underlying mush, under varying fO2–fH2O conditions to form a slightly compositionally zoned rhyolitic cap. This link to the varying fO2–fH2O conditions is evidenced by the strong correlation between key geochemical parameters (e.g. Dy, Y), that qualitatively reflect
fH2O conditions (presence or absence of hornblende/biotite), and fO2 estimated from Fe–Ti oxide equilibrium. Magma mingling/mixing between the basalt–andesite and the main slightly compositionally
zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the lower portion of the reservoir
and effectively destroying any pre-eruptive gradients. 相似文献
547.
Stefan W. Ploum Steve W. Lyon Adriaan J. Teuling Hjalmar Laudon Ype van der Velde 《水文研究》2019,33(9):1304-1316
The Arctic is warming rapidly. Changing seasonal freezing and thawing cycles of the soil are expected to affect river run‐off substantially, but how soil frost influences river run‐off at catchment scales is still largely unknown. We hypothesize that soil frost alters flow paths and therefore affects storage–discharge relations in subarctic catchments. To test this hypothesis, we used an approach that combines meteorological records and recession analysis. We studied streamflow data (1986–2015) of Abiskojokka, a river that drains a mountainous catchment (560 km2) in the north of Sweden (68° latitude). Recessions were separated into frost periods (spring) and no‐frost periods (summer) and then compared. We observed a significant difference between recessions of the two periods: During spring, discharge was linearly related to storage, whereas storage–discharge relationships in summer were less linear. An analysis of explanatory factors showed that after winters with cold soil temperatures and low snowpack, storage–discharge relations approached linearity. On the other hand, relatively warm winter soil conditions resulted in storage–discharge relationships that were less linear. Even in summer, relatively cold antecedent winter soils and low snowpack levels had a propagating effect on streamflow. This could be an indication that soil frost controls recharge of deep groundwater flow paths, which affects storage–discharge relationships in summer. We interpret these findings as evidence for soil frost to have an important control over river run‐off dynamics. To our knowledge, this is the first study showing significant catchment‐integrated effects of soil frost on this spatiotemporal scale. 相似文献
548.
This paper reviews some of the main advances in our understanding of human evolution over the last 1 million years, presenting a holistic overview of a field defined by interdisciplinary approaches to studying the origins of our species. We begin by briefly summarizing the climatic context across the Old World for the last 1 million years before directly addressing the fossil and archaeological records. The main themes in this work explore (i) recent discoveries in the fossil record over the last 15 years, such as Homo naledi and Homo floresiensis; (ii) the implications of palaeogenetics for understanding the evolutionary history of, and relationships between, Neanderthals, Denisovans and Homo sapiens; (iii) the interplay between physiology and metabolic demand, landscape use, and behavioural adaptations in the evolution of morphological and behavioural innovation; and (iv) recent advances in archaeological understanding for the behavioural record, in particular that of the Neanderthals. This paper seeks to provide a broad‐scale, holistic perspective of our current understanding of human evolution for the last 1 Ma, providing a reference point for researchers that can be built upon as new discoveries continue to develop the landscapes of human evolution. © 2019 The Authors. Journal of Quaternary Science Published by Wiley Periodicals, Inc. 相似文献
549.
Katie Coluccio Isaac Santos Luke C. Jeffrey Marwan Katurji Steve Coluccio Leanne K. Morgan 《水文研究》2020,34(24):4592-4608
Coastal lagoons are significant wetland environments found on coastlines throughout the world. Groundwater seepage may be a key component of lagoon water balances, though only a few studies have investigated large (>100 km2) coastal lagoons. In this study, we combined airborne thermal infrared imagery with continuous measurements of radon (222Rn—a natural groundwater tracer), conductivity, water temperature and dissolved oxygen to map groundwater seepage to a large coastal lagoon in New Zealand. We found evidence of seepage along the margins of the lagoon but not away from the margins. Our findings confirmed previously known seepage zones and identified new potential locations of groundwater inflow. Both point source and diffuse seepage occurred on the western and northwestern margins of the lagoon and parallel to the barrier between the lagoon and sea. These observations imply geologic controls on seepage. The combination of remote sensing and in-situ radon measurements allowed us to effectively map groundwater discharge areas across the entire lagoon. Combined, broad-scale qualitative methods built confidence in our interpretation of groundwater discharge locations in a large, dynamic coastal lagoon. 相似文献
550.