Measurements of diffusivity near the sediment-water interface with a fine-scale resistivity probe     

Daniel Andrews  Andrew Bennett 《Geochimica et cosmochimica acta》1981,45(11):2169-2175

Fine scale profiles of the sedimentary formation factor, F, were obtained from measurements of resistivity at millimetre intervals in marine sediments. These profiles demonstrated that rapid decreases in diffusivity occurred in the upper few centimeters of both compact intertidal sands and fine silty muds. In the compact sands, a 70% decrease in the formation factor was observed within 2 cm of the interface. Close correlations were found between the formation factor and sediment porosity for both sediment types. Profiles of $\text{1}\text{F}$ predicted from these correlations and porosity profiles were very similar to those obtained from measurements of resistivity. These correlations also suggested that φ^?3 is a better approximation of the formation factor in fine muds than the Archie relation φ^?2. In both sediments, the interfacial value of the diffusion coefficient was estimated to be between ~74 and ~87% of its value in free solution, a value much higher than is normally assumed.  相似文献   

Eigenvector analysis of reconstructed Holocene July temperature departures over northern Canada     

John T. Andrews  Henry F. Diaz 《Quaternary Research》1981,16(3):373-389

July temperatures for the past 6000 yr at 11 sites in northern Canada have been predicted by transfer-function equations. Normalized departures from the mean of each time series at 250-yr intervals are analyzed by principal component (eigenvector) analysis. An initial analysis included 9 sites and the first three principal components accounted for 85.7% of the variance. Maps of the loadings on the principal components show broad spatial coherence on all three components. Temporal coefficients (principal component scores) illustrate major regional and local midsummer temperature variations. An additional 2 sites were then included but the spatial pattern of the loadings remained essentially unchanged. A further test of this approach, with a view toward predicting paleoclimates of northern regions, was to use the spatial coefficients (loadings) to estimate the July temperature departures at an “unknown” site (Long Lake, Keewatin). This reconstruction compares favorably with an independent transfer-function reconstruction (Kay, 1979). Power spectrum analysis of the significant principal component scores (temperature departures) over the 6000 yr showed that the temporal fluctuations associated with the first three principal components follow a “red noise” spectrum, indicative of strong persistence in the reconstructed climatic records. The scores on the fourth principal component approximate a “white noise” spectrum. A peak in power between 2000 and 3000 yr occurs in the variance spectrum of the second principal component (significance 10%). We conclude that eigenvector analysis of Holocene paleoclimatic data has considerable power and may be useful for identifying regional and local climatic variations.  相似文献   

Growth rate of the Laurentide Ice Sheet and sea level lowering (with emphasis on the 115,000 BP sea level low)     

J.T. Andrews  M.A.W. Mahaffy 《Quaternary Research》1976,6(2):167-183

A physically plausible three-dimensional numerical ice flow model is used to examine the rate at which the Laurentide Ice Sheet could spread and thicken using as input likely values for the rate of fall of snowline and the amount of net mass balance over the growing ice sheet. This provides then both a test of the hypothesis of “instantaneous glacierization” and of the suggested rapid fall of world sea level to between ?20 and ?70 m below present at 115,000 BP. Two experiments are described: The first terminated after 10,050 years of model run with ice sheets centered over Labrador-Ungava and Baffin Island with a total volume of 3.0 × 10⁶ km³ of ice, whereas the second was completed after 10,000 years and resulted in a significantly larger ice sheet (still with two main centers) with a volume of 7.78 × 10⁶ km³ of ice. This latter figure is equivalent to the mass required to lower world sea level by 19.4 m. Our results indicate that large ice sheets can develop in about 10,000 years under optimum conditions.  相似文献   

Sedimentology of a late Cambrian regressive sequence (bowers group), Northern Victoria land,Antarctica     

Peter B. Andrews  M.G. Laird 《Sedimentary Geology》1976,16(1):21-44

The late Cambrian Bowers Group is regressive marine to non-marine. On the basis of associations of textures and sedimentary structures (both primary and biotic), and of palaeocurrent measurements, the sediments of the newly defined Mariner Formation appear to have accumulated on a platform or continental shelf of low gradient and narrow tidal range. Trilobite- and brachiopod-bearing fissile mudstone with sparse channels and scattered stratified sandstone (open marine) is succeeded by wavy-bedded and lenticular-bedded mudstone, muddy sandstone, and rippled sandstone with bands rich in brachiopods (shallow open marine), then mudstone and laminated sandstone with lenticular bodies of oosparite (open marine shoals and protected inner shelf), and red and green, ripple- and parallel-laminated sandstone, interbedded with burrowed muddy sandstone and mudstone, with lenticular bedding (tidal flat). The sharply overlying Camp Ridge Quartzite of cross-bedded sandstone, pebbly sandstone, and sandy pebble conglomerate, is regarded as braided channel alluvium.The detritus appears to have been derived from veined low-grade metamorphic rocks and fine-grained detrital sedimentary rocks that lay south of the area.  相似文献   

The use of a new modifier,Kr6D,in differential sulphide flotation     

R.M. Manser  P.R.A. Andrews 《International Journal of Mineral Processing》1975,2(3):207-218

The suitability of a new reagent, Kr6D, as a depressant for galena and sphalerite in a differential sulphide flotation stage has been tested. Small-scale laboratory flotation tests on pure sulphide minerals indicated that the reagent is capable of depressing both galena and sphalerite when used in small doses. At higher concentrations, chalcopyrite is also depressed. Single-stage batch-scale flotation tests have been carried out on bulk sulphide concentrates containing the three sulphide minerals. Kr6D has been compared with starch as a depressant both in the presence and in the absence of sulphur dioxide. In each case the new modifier has been shown to be more effective. When using sodium dichromate as a differential depressant in similar tests, it has been shown that the quantity necessary to achieve selectivity between chalcopyrite and galena may be considerably reduced if small quantities of Kr6D are added.  相似文献   

Past and present glaciological responses to climate in eastern Baffin Island     

J.T. Andrews  R.G. Barry  R.S. Bradley  G.H. Miller  L.D. Williams 《Quaternary Research》1972

Much of Baffin Island is close to the modern glaciation limit and climatic changes within the last decade are already being reflected in snow cover extent. Statistical analysis of glacierized and ice-free corries indicates that changes in direct solar radiation due to astronomical factors are inadequate to account for glacierization of those at present ice-free. These and other sources of evidence demonstrate the need for augmented winter snowfall in order to increase the extent of glacierization. The pattern of glacial history in this area is for maximum ice extent during the early glacial phase (>68,000, <137,000 BP), folfxlowed by a reduction in ice volume during the cold pleniglacial (>24,000, < 68,000 BP) and then a limited late glacial advance (the Cockburn Stade, ca. 8,000 BP) due to increased precipitation. The Barnes Ice Cap did not disappear in the Holocene as it did in the last interglacial. The area is highly suitable for long-term monitoring of climatic change and glacial response.  相似文献   

Climatic change recorded by stable isotopes and trace elements in a British Holocene tufa   总被引：1，自引：0，他引：1  

E. R. Garnett  J. E. Andrews  R. C. Preece  P. F. Dennis 《第四纪科学杂志》2004,19(3):251-262

Combined stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg, Sr) geochemistry from bulk tufa calcite and ostracod shell calcite from an early Holocene British tufa reveal clear records of Holocene palaeoclimatic change. Variation in δ¹⁸O is caused principally by change in the isotopic composition of Holocene rainfall (recharge), itself caused mainly by change in air temperature. The δ¹³C variability through much of the deposit reflects increasing influence of soil‐zone CO₂, owing to progressive woodland soil development. Bulk tufa Mg/Ca and Sr/Ca are controlled by their concentrations in the spring water. Importantly, Mg/Ca ratios are not related to δ¹⁸O values and thus show no temperature dependence. First‐order sympathetic relationships between δ¹³C values and Mg/Ca and Sr/Ca are controlled by aquifer processes (residence times, CO₂ degassing and calcite dissolution/reprecipitation) and probably record intensity of palaeorainfall (recharge) effects. Stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data. The ostracod isotopic records are markedly ‘spiky’ because the ostracods record ‘snapshots’ of relatively short duration (years), whereas the bulk tufa samples record averages of longer time periods, probably decades. The δ¹⁸O record appears to show early Holocene warming, a thermal maximum at ca. 8900 cal. yr BP and the global 8200 yr BP cold event. Combined δ¹³C, Mg/Ca and Sr/Ca data suggest that early Holocene warming was accompanied by decreasing rainfall intensity. The Mg/Ca data suggest that the 8200 yr BP cold event was also dry. Warmer and wetter conditions were re‐established after the 8200 yr BP cold event until the top of the preserved tufa sequence at ca. 7100 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Partitioning of Cu, Ni, Au, and platinum-group elements between monosulfide solid solution and sulfide melt under controlled oxygen and sulfur fugacities   总被引：2，自引：0，他引：2  

James E. Mungall  David R.A. Andrews  Paul J. Sylvester 《Geochimica et cosmochimica acta》2005,69(17):4349-4360

We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ∼50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.  相似文献   

The implications of laboratoryRn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer     

D.M. Bonotto  J.N. Andrews 《Applied Geochemistry》1997,12(6):715-726

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of²²²Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm² g⁻¹ , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factork (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.  相似文献   

The Paleoclimates of Arctic Lakes and Estuaries (PALE): goals and rationale of an international research program     

J. T. Andrews  L. Brubaker 《Journal of Paleolimnology》1994,10(2):163-166

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