The "Little Ice Age" and its Geomorphological Consequences in Mediterranean Europe   总被引：1，自引：0，他引：1  

A.T. Grove 《Climatic change》2001,48(1):121-136

Alpine glacier advances in the "Little Ice Age" took place in the decades around 1320, 1600, 1700 and 1810. They were the outcome of snowier winters and cooler summers than those of the twentieth century. Documentary records from Crete in particular, and also from Italy, southern France and southeast Spain point to a greater frequency in Mediterranean Europe's mountainous regions of severe floods, droughts and frosts at times of "Little Ice Age" Alpine glacier advances. Deluges, when more than 200 mm of rain fall within 24 hours, are most frequent on mountainous areas near the coast. An instance is given of the geomorphological consequences of a great deluge which struck the Tech valley in the eastern Pyrenees on 17 October 1940. An increased frequency of deluges, probably at times when Alpine glaciers were advancing in the "Little Ice Age" and earlier in the Holocene, in areas known to be tectonically unstable and underlain by soft sediments, could better explain the occurrence of fluvial terraces in Mediterranean Europe sometimes known as the "younger fill", than soil erosion resulting from deforestation.  相似文献   

The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study   总被引：2，自引：4，他引：2  

Othmar Müntener  Peter B. Kelemen  Timothy L. Grove 《Contributions to Mineralogy and Petrology》2001,141(6):643-658

Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H₂O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H₂O concentration in the liquid changes the crystallization sequence. Low H₂O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H₂O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm³. The combination of high pressure and high H₂O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO₂, Al₂O₃) of igneous crust in arcs.  相似文献   

Comment on “Multipath Ar transport in K-feldspar deduced from isothermal heating experiments” by Igor Villa     

Oscar M. Lovera  T. Mark Harrison  Marty Grove 《Earth and Planetary Science Letters》1996,140(1-4):281-283

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Oxygen isotopic composition of the Luna 20 soil     

Robert N Clayton 《Geochimica et cosmochimica acta》1973,37(4):811-813

The Luna 20 soil (< 125 μm fraction) has a relatively low δO¹⁸ (5.7%.), compared to other lunar soils (5.8 to 6.3%.). This implies either a low-O¹⁸ source material or an unusually small O¹⁸ enrichment in the processes of soil formation and maturation.  相似文献   

Chronology of late wisconsinan glaciation in middle North America     

Lee Clayton  Stephen R. Moran 《Quaternary Science Reviews》1982,1(1):55-82

We propose a chronology of late Wisconsinan glacial fluctuations in middle North America, from Alberta to Wisconsin, based on radiocarbon dates derived solely from wood. Previous chronologies of the southwestern margin of the North American Continental Ice Sheet have depended to a considerable degree on radiocarbon dates from fine-grained organic sediment. This material is commonly contaminated with older carbon, resulting in chronologic confusion. By using only dates from wood, much of the confusion disappears. However, because of the scarcity of wood dates, only four of the sixteen identified fluctuations are accurately dated: an advance into Iowa about 14,000 to 13,500 BP, an advance into South Dakota and Iowa about 12,300 BP, an advance into the Lake Michigan basin about 11,700 BP, and an advance into the Lake Superior basin about 9900 BP. In addition, the beginning of late Wisconsinan glaciation, before 20,000 BP, is fairly well documented. None of the fluctuations in the western part of the region are accurately dated.  相似文献   

Calculation of Uncertainty in the Variogram   总被引：6，自引：0，他引：6  

Julián Ortiz  Clayton V. Deutsch 《Mathematical Geology》2002,34(2):169-183

There are often limited data available in early stages of geostatistical modeling. This leads to considerable uncertainty in statistical parameters including the variogram. This article presents an approach to calculate the uncertainty in the variogram. A methodology to transfer this uncertainty through geostatistical simulation and decision making is also presented.The experimental variogram value for a separation lag vector h is a mean of squared differences. The variance of a mean can be calculated with a model of the correlation between the pairs of data used in the calculation. The data here are squared differences; therefore, we need a measure of a 4-point correlation. A theoretical multi-Gaussian approach is presented for this uncertainty assessment together with a number of examples. The theoretical results are validated by numerical simulation. The simulation approach permits generalization to non-Gaussian situations.Multiple plausible variograms may be fit knowing the uncertainty at each variogram point, . Multiple geostatistical realizations may then be constructed and subjected to process assessment to measure the impact of this uncertainty.  相似文献   

Lithium isotope fractionation in the southern Cascadia subduction zone   总被引：2，自引：0，他引：2  

Tom&#x; Magna  Uwe Wiechert  Timothy L. Grove  Alex N. Halliday 《Earth and Planetary Science Letters》2006,250(3-4):428-443

We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ⁷Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H₂O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ⁷Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust.  相似文献   

Deriving Constraints on Small-Scale Variograms due to Variograms of Large-Scale Data     

Hans Kupfersberger  Clayton V. Deutsch  André G. Journel 《Mathematical Geology》1998,30(7):837-852

The application of kriging-based geostatistical algorithms to integrate large-scale seismic data calls for direct and cross variograms of the seismic variable and primary variable (e.g., porosity) at the modeling scale, which is typically much smaller than the seismic data resolution. In order to ensure positive definiteness of the cokriging matrix, a licit small-scale coregionalization model has to be built. Since there are no small-scale secondary data, an analytical method is presented to infer small-scale seismic variograms. The method is applied to estimate the 3-D porosity distribution of a West Texas oil field given seismic data and porosity data at 62 wells.  相似文献   

A Field and Laboratory Investigation of Air Fingering During Air Sparging   总被引：2，自引：0，他引：2  

Wilson S. Clayton  Ph.D.  P.G. 《Ground Water Monitoring & Remediation》1998,18(3):134-145

Since the rejection of the bubble flow conceptual model for in situ air sparging, most practitioners have adopted the conceptual model of air channeling, which generally implies the development of widely spaced, discreet air channels that bypass large regions of the subsurface. While air channeling clearly develops in response to stratigraphic heterogeneity, the universality of widely spaced air channels in homogeneous media is not supported by available evidence. Air channeling results in low bulk air saturation due to bypassing, and field and laboratory measurements of air saturations and previously published studies were used to evaluate if air channeling is realistic. The results indicated that homogeneous coarse sands are prone to the development of air channeling, and that homogeneous fine sands show higher air saturations and are not prone to air channeling. Breakthrough air saturations, which represent the minimum air saturations, that will conduct air flow, of approximately 0.02 to 0.04 were observed in coarse sands. In contrast, breakthrough air saturations of 0.10 to 0.13 were observed in fine sands and medium sands. The transition between these behaviors falls at about 15 to 20 cm water air entry pressure. These result indicate that, at both the field and laboratory scale, coarse sands are more prone to air channeling and bypassing than fine sands. Additionally, the larger air gradients and capillary pressures in fine sands result in a less buoyancy-dominated flow pattern, with a larger lateral extent of air flow.  相似文献   

Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions   总被引：1，自引：0，他引：1  

Robert N. Clayton  M.L. Jackson  K. Sridhar 《Geochimica et cosmochimica acta》1978,42(10):1517-1522

Oxygen isotopic compositions of quartz from silty sediments buried to 5400 m depth from two wells in the Gulf of Mexico each showed δ5¹⁸O variations of less than 1.7%. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the 10–20 μm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the 10–20 μm fraction was found to have only 0.1–0.5%. greater δ¹⁸O than the corresponding 20–50 μm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eolian and fluvial additions of minerals to continental soils and pelagic sediments.  相似文献