A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32     

Katerina E. Petronotis  & Richard G. Gordon 《Geophysical Journal International》1999,139(1):227-247

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.  相似文献   

Summary findings of the fourth international radiocarbon intercomparison (FIRI)(1998–2001)     

Elisabetta Boaretto  Charlotte Bryant  Israel Carmi  Gordon Cook  Steinar Gulliksen  Doug Harkness  Jan Heinemeier  John McClure  Edward McGee  Philip Naysmith  Goran Possnert  Marian Scott  Hans van der Plicht  Mark van Strydonck 《第四纪科学杂志》2002,17(7):633-637

Interlaboratory comparisons have been widely used in applied radiocarbon science. These are an important part of ongoing quality assurance (QA) programmes, which are vital to the appropriate interpretation of the evidence provided by the ¹⁴C record in Quaternary applications (including climate change and environmental reconstruction). International comparisons of laboratory performance are an essential component of the quality assurance process in radiocarbon dating. If the user community is to have confidence in radiocarbon results, it needs to be assured that laboratories world wide are producing measurements that are reliable and in accordance with ‘good practice’. The findings from the most recent (completed in 2001) and extensive (more than 90 participating laboratories) radiocarbon intercomparison (FIRI) are reported here. This study was designed (i) to assess comparability, or otherwise, of the results from different laboratories and (ii) to quantify the extent and possible causes of any interlaboratory variation. The results demonstrate that there are no significant differences amongst the main measurement techniques (gas proportional counting, liquid scintillation counting and accelerator mass spectrometry (AMS)) but there is evidence of small laboratory offsets relative to known age samples for some laboratories. There is also evidence in some cases of underestimation of measurement precision. Approximately 10% of all results were classified as extreme (outliers) and these results were generated by 14% of the laboratories. Overall, the evidence supports the fact that radiocarbon laboratories are generally accurate and precise but that, notwithstanding internal QA procedures, some problems still occur, which can best be detected by participation in independent intercomparisons such as FIRI, where the results allow individual laboratories to assess their performance and to take remedial measures where necessary. The results from FIRI are significant in that they show a broad measure of agreement between measurements made in different laboratories on a wide range of materials and they also demonstrate no statistically significant difference between measurements made by radiometric or AMS techniques. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Glide twinning and pseudotwinning in peristerite: Si,Al diffusional stabilization and implications for the peristerite solvus     

William L. Brown 《Contributions to Mineralogy and Petrology》1989,102(3):313-320

Albite glide pseudotwins related to grain-boundary stresses have been observed in an exsolved peristerite (Brown 1989). The glide operation transposes the pre-existing periodic oligoclase/albite lamellae and interfaces into a position rotated by only 0.5° in the pseudotwins, but transforms the indices from outside to (081) inside the pseudotwin. The pseudotwin is anti-ordered with respect to Al and Si and both it and the transposed interface are unstable. They should revert to the initial state on stress removal. If however the stresses are maintained for a sufficiently long time, the pseudotwins are stabilized by inversion of Si,Al order and re-orientation of the interface by an angle of about 30° into a position close to . The continuous lamellae break up into a series of discs by diffusion of NaSi and CaAl, the minimum diffusion path being about the same as the thickness of the lamellae. On extrapolating available interdiffusion data in Ab-rich plagioclases to low temperatures, possible diffusion times may be calculated. The calculated times are long so that either the peristerite miscibility gap must be at a higher temperature than previously supposed or the low-temperature interdiffusion coefficients must be higher than the extrapolated experimental ones, or both. From recent data on ordering in albite, the crest of the gap is estimated to lie close to 650–625° C at low pressure and it is possible that interdiffusion under natural conditions is facilitated by hydrogen (protons) in feldspars.C.R.P.G. contribution 800  相似文献   

Turbulence characteristics of a small subtropical estuary during and after some moderate rainfall   总被引：1，自引：0，他引：1  

Mark Trevethan  Hubert Chanson  Richard Brown 《Estuarine, Coastal and Shelf Science》2008,79(4):661-670

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Depth-stratified phytoplankton dynamics in Cyclone Opal, a subtropical mesoscale eddy     

Michael R. Landry  Susan L. Brown  Yoshimi M. Rii  Karen E. Selph  Robert R. Bidigare  Eun Jin Yang  Melinda P. Simmons 《Deep Sea Research Part II: Topical Studies in Oceanography》2008,55(10-13):1348

As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80–100 m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0–40 m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50–60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70–90 m showed little evidence of growth rate enhancement and was principally defined by a 100-fold increase of large (>20-μm) diatoms and a shift from Prochlorococcus to diatom dominance (80%) of production. The intermediate layer of 50–60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal.  相似文献   

Near-infrared spectroscopy of Himalia     

Matthew A. Chamberlain  Robert H. Brown 《Icarus》2004,172(1):163-169

We present the first spectrum through the L band of an irregular satellite from the outer Solar System. Spectra of Himalia (JVI) were obtained with the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. The Himalia spectrum is essentially featureless, showing a slight red slope and the suggestion of an absorption at 3 μm that might indicate the presence of water in some form. Better measurements of the spectrum of Himalia, particularly in the region of the apparent 3-μm band, could help determine whether water is present, and if so, in what form.  相似文献   

Palaeomagnetism and chronology of Caledonian igneous episodes in the Cross Fell Inlier and northern Lake District     

J. D. A. Piper  A. S. Mccook  K. P. Watkins  G. C. Brown  W. A. Morris 《Geological Journal》1978,13(1):73-92

This study employs palaeomagnetism to differentiate between Palaeozoic minor intrusions in northern England. The lamprophyres of the Cross Fell Inlier subdivide petrographically into kersantites and minettes. All the kersantites have steep directions of magnetisation probably correlating with an interval of anomalous geomagnetic field in Caradocian times. The Threlkeld–St John's microgranodiorite (445 m.y.) and Carrock Fell dolerite dyke swarm also possess steep magnetisations assigned to the same interval. The minettes have shallow directions of magnetisation similar to the Shap Granite (393 m.y.) and correlate with the later Caledonian intrusions of northern England. These episodes of intrusion are separated by late-Silurian tectonism and define a trend of increasing K₂O/Na₂O in the later magmas of the Caledonian orogeny. Miscellaneous palaeomagnetic results from the Borrowdale Volcanic Group, Embleton diorites and Haweswater dolerite are also reported.  相似文献   

Lake ice and temperature trends for Ontario and Manitoba: 2001 to 2014          下载免费PDF全文

Justin Murfitt  Laura C. Brown 《水文研究》2017,31(21):3596-3609

Lakes are a prominent geographic feature in northern landscapes and play an important role in understanding regional climate systems. In order to better model changes within climate systems, it is important to study lake ice processes. Although the availability of records for lake ice through ground measurements has declined in recent years, the increased use of remote sensing provides an alternative to this. Using a preclassified snow and ice remote sensing product with a 500‐m resolution, based on images from the Moderate Resolution Imaging Spectroradiometer (MODIS/MOD10A1), and the use of measured and reanalysis temperature data, this study evaluated lake ice phenology dates in connection to recent trends in temperature and 0 °C isotherms within Ontario and Manitoba between 2001 and 2014. Temperature trends indicated both regional warming and cooling, with significant cooling observed in Southern Ontario (p < .05) and significant warming in Southern Manitoba (p < .1) during the fall. Spatial analysis of the trends in the lake ice data showed significant clustering of significant trends in ice on dates (p < .01). When analysing the trends in ice phenology in connection to the trends in temperature, it was found that 70% of lakes experienced a change in the ice on date with the expected change in temperature and 85% of lakes for ice off date. When shifting ice on and ice off dates are investigated in relation to 0 °C isotherms, it was seen that 80% of ice on dates and 100% of ice off dates shifted in sync with the isotherm dates. This demonstrates that the ice phenology of lakes in Ontario and Manitoba, Canada, is responding to short‐term variability in temperature. The MODIS product could be used to investigate ice phenology on a large scale and contribute towards expanding existing records of ice phenology. Establishing long‐term ice records could be a valuable asset for other research ranging from water balance studies to the response of lake biota under changing climate.  相似文献   

Devonshire marbles—the greatest British decorative stones?     

Gordon M. Walkden 《Geology Today》2016,32(4):135-142

‘Devonshire marbles’ were once the pre‐eminent British decorative stones, installed in dozens of Victorian buildings in Britain and elsewhere. There are numerous varieties, none of which are true marbles for they retain the fossils and sedimentary textures of the original Devonian limestones. Quarried and manufactured across south Devon and used both internally and externally, the stones were part of a passing nineteenth century architectural fashion. There are 20 or so main varieties, mostly now unobtainable, but excellent examples are found in the panels, shafts and floors of some exceptionally fine buildings. These reveal much about the fossils, their depositional environments and the changes that affected them during the Variscan orogeny. Now rediscovered and distinguished for the first time using geological criteria, Devonshire marbles represent a valuable resource for geological, conservation, heritage and architectural research.  相似文献   

Incorporating landscape depression heterogeneity into the Soil and Water Assessment Tool (SWAT) using a probability distribution          下载免费PDF全文

Balew A. Mekonnen  Kerry A. Mazurek  Gordon Putz 《水文研究》2016,30(13):2373-2389

Modelling the hydrology of North American Prairie watersheds is complicated because of the existence of numerous landscape depressions that vary in storage capacity. The Soil and Water Assessment Tool (SWAT) is a widely applied model for long‐term hydrological simulations in watersheds dominated by agricultural land uses. However, several studies show that the SWAT model has had limited success in handling prairie watersheds. In past works using SWAT, landscape depression storage heterogeneity has largely been neglected or lumped. In this study, a probability distributed model of depression storage is introduced into the SWAT model to better handle landscape storage heterogeneity. The work utilizes a probability density function to describe the spatial heterogeneity of the landscape depression storages that was developed from topographic characteristics. The integrated SWAT–PDLD model is tested using datasets for two prairie depression dominated watersheds in Canada: the Moose Jaw River watershed, Saskatchewan; and the Assiniboine River watershed, Saskatchewan. Simulation results were compared to observed streamflow using graphical and multiple statistical criterions. Representation of landscape depressions within SWAT using a probability distribution (SWAT–PDLD) provides improved estimations of streamflow for large prairie watersheds in comparison to results using a lumped, single storage approach. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献