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101.
102.
Joseph A. Mason Peter M. Jacobs Paul R. Hanson Xiaodong Miao Ronald J. Goble 《Quaternary Research》2003,60(3):330-339
This paper reexamines the stratigraphy, sources, and paleoclimatic significance of Holocene Bignell Loess in the central Great Plains. A broadly similar sequence of loess depositional units and paleosols was observed in thick Bignell Loess sections up to 300 km apart, suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis. New optical ages, together with previously reported radiocarbon ages, indicate Bignell Loess deposition began 9000–11,000 yr ago and continued into the late Holocene; some Bignell Loess is <1000 yr old. There is little evidence that Holocene Loess was derived from flood plain sources, as previously proposed. Instead, thick Bignell Loess occurs mainly near the downwind margins of inactive dune fields, particularly atop escarpments facing the dunes. Thus, the immediate loess source was dust produced when the dunes were active. Previous work indicates that widespread episodes of dune activity are likely to have resulted from drier-than-present climatic conditions. The regionally coherent stratigraphy of Bignell Loess can be interpreted as a near-continuous record of climatically driven variation in dune field activity throughout the Holocene. 相似文献
103.
The following hardsphere modified Redlich-Kwong (HSMRK) equation of state was obtained by least squares fitting to available P-V-T data for methane (P in bars; T in Kelvins; v in cm3 mol?1; b = 60.00 cm3 mol?1; R = 83.14 cm3barmol?1K?1): c(T) = 13.403 × 106 + (9.28 × 104)T + 2.7 T2d(T) = 5.216 × 109 ? (6.8 × 106)T + (3.28 × 103)T2e(T) = (?2.3322 × 1011) + (6.738 × 108)T + (3.179 × 105)T2 For the P-T range of experimental data used in the fit (50 to 8600 bars and from 320 to 670 K), calculated volumes and fugacity coefficients for CH4 relative to experimentally determined volumes and fugacity coefficients have average percent deviations of 0.279 and 1.373, respectively. The HSMRK equation, which predicts linear isochores over a wide P-T range, should yield reasonable estimates of fugacity coefficients for CH4 to pressures and temperatures well outside the P-T range of available P-V-T data. Calculations for the system H2O-CO2-CH4, using the HSMRK equations for H2O and CO2 of Kerrick and Jacobs (1981) and the HSMRK equation for CH4 of this study, indicate that compared to the binary H2O-CO2 system, small amounts of CH4 in the ternary system H2O-CO2-CH4 slightly increases the activity of H2O, and significantly decreases the activity of CO2. 相似文献
104.
A. Günter Herrmann Douglas P. Blanchard Larry A. Haskin Jeffrey W. Jacobs Doris Knake Randy L. Korotev Joyce C. Brannon 《Contributions to Mineralogy and Petrology》1976,59(1):1-12
Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earth's crust (3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue. 相似文献
105.
Philip A. Helmke Douglas P. Blanchard Jeffrey W. Jacobs Larry A. Haskin 《Geochimica et cosmochimica acta》1973,37(4):869-874
The results of the analysis by neutron activation of six samples from the Luna 20 mission and one sample of < 1 mm fines from Apollo 16 are reported. The concentrations of the rare-earth elements (REE) in the samples of fines from Luna 20 and Apollo 16 are less than those found for corresponding materials from the mare areas but a negative Eu anomaly is still present. The concentrations of the REE in fines from Luna 20 are only about two-thirds as great as in the sample of Apollo 16 fines, but the concentrations of Co, Sc and Cr are greater by factors ranging from 1.5 to 2.3. 相似文献
106.
107.
108.
A simple model for potential dewfall in an arid region 总被引:4,自引:0,他引:4
It is not always easy to know, post-facto, whether both dewfall and fog may have occurred over a given evening period. Instrumentation limitations make it difficult to quantify dew deposition since they rely on artificial sensing surfaces that are either visually examined on a daily basis or recorded. In arid to Mediterranean regions, both dew and fog can play significant ecological roles as suppliers of moisture. Long-term observation records of dew and fog in such regions tend to be limited, however, due partly to a lack of interest and limited distribution of well-instrumented meteorological stations. Simple meteorological criteria are suggested here to calculate potential dewfall and to indicate whether fog was likely to have occurred over a given evening. A field campaign was carried out in the NW Negev desert, Israel, in September and October 1997, to collect meteorological data and carry out dewfall measurements. 相似文献
109.
Maria Alfredsson Furio Corà John P. Brodholt Steve C. Parker G. David Price 《Physics and Chemistry of Minerals》2005,32(5-6):379-387
Many rheological and transport properties of rocks are determined by the grain boundary structures of their constituent minerals.
These grain boundaries often also hold a high concentration of dopant ions. Here, as a first step towards modelling the transport
and rheological behaviour of the lower mantle, we report the results of lattice static simulations on the surface structures
of Fe2+ and Ca2+-doped orthorhombic MgSiO3-perovskite. For all the surfaces we studied, the energies of the doped structures are lowered, sometimes by more than 1 J/m2, with respect to the pure surfaces. From our calculated crystal morphologies, we predict that the grains become more tabular
as the concentration of Fe2+ ions increases, while under equilibrium conditions the grains are cubic. By calculating the replacement energies of Mg2+ by Fe2+ and Ca2+ ions in the six outermost surface layers, we conclude that these divalent ions would tend to segregate onto the crystal surfaces.
We suggest, therefore, that the grain boundary structure and rheology of MgSiO3-perovskite dominated rocks will be strongly affected by the presence of minor elements in the lower mantle. 相似文献
110.
X-ray computed microtomography (µCT) is a promising non-destructive imaging technique based on measurements of the attenuation of X-rays. It provides information on the internal structure of small samples with a maximum resolution of 10 µm. For this study, two porous local natural building stones, the sandstone of Bray and the limestone of Maastricht, were selected. Possible applications of the µCT-technique for qualitative monitoring of changes comprise (1) to non-destructively determine porosity based on 3-D images, (2) to visualise weathering phenomena at the micron-scale, (3) to understand the rationale of weathering processes, (4) to visualise the presence of water repellents and consolidation products, (5) to monitor their protective effects during decay in order to understand the operating mechanisms and (6) to provide advise on the suitability of products for the treatment of a particular rock type. The µCT-technique proves to be a powerful monitoring tool for the future as the combination of 3-D visualisation and 3-D data provide new insights that could optimise conservation and restoration techniques of building materials. 相似文献