The shape of galaxy cluster dark matter haloes: systematics of its imprint on cluster gas and comparison to observations     

Ricardo A. Flores  Brandon Allgood  rey V. Kravtsov  Joel R. Primack  David A. Buote  James S. Bullock 《Monthly notices of the Royal Astronomical Society》2007,377(2):883-896

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Small‐scale variability in surface moisture on a fine‐grained beach: implications for modeling aeolian transport     

Brandon L. Edwards  Steven L. Namikas 《地球表面变化过程与地形》2009,34(10):1333-1338

Small‐scale variations in surface moisture content were measured on a fine‐grained beach using a Delta‐T Theta probe. The resulting data set was used to examine the implications of small‐scale variability for estimating aeolian transport potential. Surface moisture measurements were collected on a 40 cm × 40 cm grid at 10 cm intervals, providing a total of 25 measurements for each grid data set. A total of 44 grid data sets were obtained from a representative set of beach sub‐environments. Measured moisture contents ranged from about 0% (dry) to 25% (saturated), by weight. The moisture content range within a grid data set was found to vary from less than 1% to almost 15%. The magnitude of within‐grid variability varied consistently with the mean moisture content of the grid sets, following an approximately normal distribution. Both very wet and very dry grid data sets exhibited little internal variability in moisture content, while intermediate moisture contents were associated with higher levels of variability. Thus, at intermediate moisture contents it was apparent that some portions of the beach surface could be dry enough to allow aeolian transport (i.e. moisture content is below the critical threshold), while adjacent portions are too wet for transport to occur. To examine the implications of this finding, cumulative distribution functions were calculated to model the relative proportions of beach surface area expected to be above or below specified threshold moisture levels (4%, 7%, and 14%). It was found that the implicit inclusion of small‐scale variability in surface moisture levels typically resulted in changes of less than 1% in the beach area available for transport, suggesting that this parameter can be ignored at larger spatial scales. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Exhuming the Alps through time: clues from detrital zircon fission-track thermochronology     

M. Bernet  M. Brandon  J. Garver†  M. L. Balestieri‡  B. Ventura§  M. Zattin¶ 《Basin Research》2009,21(6):781-798

The European Alps are a mountain belt that is characterized by a series of discrete orogenic events, which have long been recognized. Despite the inherent episodic nature of orogenic evolution, the Alps have been continuously exhumed, mainly by erosion, but also by normal faulting. Since continental collision started in the late Eocene/Early Oligocene evidence for ongoing erosional exhumation has been preserved in synorogenic sediments that accumulated in basins adjacent to the pro- and retro-side of this double-vergent mountain belt. This long-term erosion record can be used to determine exhumation rates. Lag-times calculated from fission-track (FT) ages of detrital zircon from synorogenic sediments are fairly constant for the European Alps since the Oligocene–Late Miocene. Although the fast exhuming areas were unroofed at rates of 0.4–0.7 km Myr⁻¹, the overall average exhumation rate is between 0.2 and 0.3 km Myr⁻¹ on a regional scale. The detrital and bedrock zircon FT data of the Alps do not detect the increase in erosion rates since the Pliocene over the past ∼5 Myr, as shown elsewhere. This increase cannot be detected yet with the detrital zircon FT method because not enough rock has been removed to widely expose zircons with Pliocene or younger cooling ages in the Alps. Long term (30 Myr) exhumation rates appear to have been approximately constant when averaged over a sliding time window of about 8 Myr, or depth window of 5 to 10 km (ZFT closure depths); shorter-term fluctuations are not identified using this method.  相似文献   

Test of the climatological dispersion model in the strait of canso area,Nova Scotia     

R.W. Shaw  E.W. Brandon 《大气与海洋》2013,51(4):265-276

Abstract

The Climatological Dispersion Mode1 is used to estimate mean annual and mean monthly concentrations of sulphur dioxide in the Strait of Canso area of Noua Scotia. It is found that the wind distribution at the southern end of the Strait is more likely to produce a reliable estimate of the concentration of pollutants from the taller stacks, whereas the surface wind distribution from a central location in the Strait can be used, because of low‐level channelling, to estimate concentrations of pollutants from the lower sources. Data on winds aloft over the Strait, are utilized in support of this thesis. The procedure does not apply well to the summer months; otherwise the resulting patterns of predicted concentrations of sulphur dioxide are generally consistent with observed values. More accurate and sensitive instruments are needed in future studies of long‐term average conditions.  相似文献   

Direct shear testing of polished slickensided surfaces     

Christopher L. Meehan  Thomas L. Brandon  J. Michael Duncan  Binod Tiwari 《Landslides》2010,7(2):157-167

A series of ring shear and direct shear tests were performed to measure the drained residual strength of three clay soils. For each of the soils, slickensided direct shear specimens were prepared by wire-cutting intact specimens, and polishing the resulting shear plane on a variety of surfaces to align the clay particles in the direction of shear. Drained direct shear tests were then conducted on each of the polished specimens. The resulting shear strengths were compared with the residual strengths measured in the ring shear device to evaluate the effectiveness of the different polishing techniques for creating slickensided surfaces. Test results indicated that the measurement of residual strengths along preformed slickensided surfaces is extremely sensitive to both the soil type and the slickenside preparation technique that is used. Consequently, this approach does not appear to be a viable alternative to conventional repeated direct shear or ring shear tests to measure residual shear strengths.  相似文献   

Provenance, age, and environment of mid-Wisconsinan slackwater lake sediment in the St. Louis Metro East area, USA     

B. Brandon Curry  David A. Grimley 《Quaternary Research》2006,65(1):108-122

Valleys tributary to the Mississippi River contain fossiliferous slackwater lake sediment (Equality Formation) deposited in response to aggradation of the Mississippi River valley during the last glaciation. In the St. Louis Metro East area, the lower part of the Equality Formation is primarily laminated, fossiliferous silt and clay deposited from about 44,150 to 24,310 ¹⁴C yr B.P. The upper Equality Formation is primarily very fine sand to silt deposited from about 21,200 to 17,000 ¹⁴C yr B.P. Among the four cores that sample this succession in the St. Louis Metro East area, core MNK-3 (38.64EN, 90.01EW) was selected for detailed study. Three sources are distinguished by the following characteristics: (1) gray smectite-quartz-Se-rich, feldspar-poor material of the Des Moines, Wadena, and James lobes; (2) reddish brown kaolinite-Cu-Fe-rich sediment of the Superior and Rainy lobes; and (3) brown illite-dolomite-Sr-rich sediment of the Lake Michigan and Green Bay lobes. The earliest sediments (44,150 to 41,700 ¹⁴C yr B.P.) were derived from the central and western provenances and are chronocorrelative with the lower Roxana Silt. A hiatus occurred from about 41,700 to 29,030 ¹⁴C yr B.P. when much of the middle Roxana Silt (Meadow Member) was deposited on adjacent uplands. The youngest sediment includes evidence of heightened activity of the Superior Lobe at about 29,000 ¹⁴C yr B.P., the Lake Michigan and Green Bay lobes from about 25,000 to 24,000 ¹⁴C yr B.P., and the Wadena-Des Moines-James lobes at about 21,000 ¹⁴C yr B.P.  相似文献   

Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite     

Alan D. Brandon  Munir Humayun  Igor S. Puchtel  Michael E. Zolensky 《Geochimica et cosmochimica acta》2005,69(6):1619-1631

The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 ± 0.08), Pd/Ir (1.19 ± 0.06) and ¹⁸⁷Os/¹⁸⁸Os (0.12564-0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.The unique elevated Ru/Ir combined with elevated ¹⁸⁷Os/¹⁸⁸Os of Tagish Lake may be attributed to Ru and Re mobility during aqueous alteration very early in its parent body history. The Os, Ir, Pt, and Pd abundances of Tagish Lake are similar to CI chondrites. The elevated Ru/Ir and the higher Re/Os and consequent ¹⁸⁷Os/¹⁸⁸Os in Tagish Lake, are balanced by a lower Ru/Ir and lower Re/Os and ¹⁸⁷Os/¹⁸⁸Os in CM-chondrites, relative to CI chondrites. A model that links Tagish Lake with CI and CM chondrites in the same parent body may explain the observed systematics. In this scenario, CM chondrite material comprises the exterior, grading downward to Tagish Lake material, which grades to CI material in the interior of the parent body. Aqueous alteration intensifies towards the interior with increasing temperature. Ruthenium and Re are mobilized from the CM layer into the Tagish Lake layer. This model may thus provide a potential direct parent body relationship between three separate groups of carbonaceous chondrites.  相似文献   

U-Th zonation-dependent alpha-ejection in (U-Th)/He chronometry   总被引：4，自引：0，他引：4  

Jeremy K. Hourigan  Peter W. Reiners  Mark T. Brandon 《Geochimica et cosmochimica acta》2005,69(13):3349-3365

Both theoretical and empirical evidence shows that intracrystalline U-Th heterogeneity in zircon can lead to biases in (U-Th)/He ages if not accurately accounted for in α-ejection corrections. We present a model for age correction for U-Th zoned crystals. We apply this to spherical and prismatic grains with bipyramidal terminations. The spherical calculation is simplistic but allows rapid calculation of the approximate effects of a wide variety of U-Th zoning patterns. The bipyramidal calculation is computationally intensive but permits an more complete estimate of the combined effects of crystal morphology and source zoning as relevant to zircon. Our principle findings are: (1) the assumption of U-Th homogeneity can result in errors of up to ∼30% (in rare cases, higher) for ejection-corrected ages for typical grain sizes and realistic zonation. (2) Tetragonal prisms with bipyramidal terminations, which are typical of most zircons, exhibit bulk retentivities that can differ by several percent from models considering crystals with pinacoidal terminations. When extreme cases, such as dipyramids, are considered, the bias can exceed 10% or more. (3) Morphologic effects can be accounted for to better than 1% precision by using new second-order polynomial parameters that describe retentivity as a function of surface-area-to-volume ratio calculated through more complete analysis of crystal dimensions including the height of pyramidal crystal sections. We illustrate application of our model using U-Th concentration profiles determined from single zircons by laser ablation ICP-MS from zoned Tardree Rhyolite zircons.  相似文献   

Osmium isotope systematics of ureilites     

K. Rankenburg  A.D. Brandon 《Geochimica et cosmochimica acta》2007,71(9):2402-2413

The ¹⁸⁷Os/¹⁸⁸Os for 22 ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite ¹⁸⁷Os/¹⁸⁸Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1σ), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of ¹⁸⁷Os/¹⁸⁸Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the ¹⁸⁷Os/¹⁸⁸Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.  相似文献   

Climate change in southern Illinois, USA, based on the age and δC of organic matter in cave sediments     

Samuel V. Panno  B. Brandon Curry  Hong Wang  Keith C. Hackley  Chao-Li Liu  Craig Lundstrom  Juanzuo Zhou 《Quaternary Research》2004,61(3):301-313

Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P., floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and, in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four ¹⁴C ages on humified organic matter provide chronologic control. The δ¹³C values of the organic matter reflect the proportion of C₄-type to C₃-type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C₄-type vegetation was more prevalent relative to C₃-type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ¹³C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.  相似文献