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1.
David J. Dunlop Hironobu Hyodo Todd Knight Alan G. Steele 《Geophysical Journal International》1985,83(3):699-720
Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α95 = 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α95 = 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent 40 Ar/39 Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed. 相似文献
2.
W. R. Skinner M. D. Flannigan B. J. Stocks D. L. Martell B. M. Wotton J. B. Todd J. A. Mason K. A. Logan E. M. Bosch 《Theoretical and Applied Climatology》2002,71(3-4):157-169
Summary
In Canada, the average annual area of burned forest has increased from around 1 million ha in the 1970’s to over 2.5 million
ha in the 1990’s. A previous study has identified the link between anomalous mid-tropospheric circulation at 500 hPa over
northern North America and wildland fire severity activity in various large regions of Canada over the entire May to August
fire season. In this study, a northern North American study region of the hemispheric gridded 5° latitude by 10° longitude
500 hPa dataset is identified and analysed from 1959 to 1996 for a sequence of six monthly periods through the fire season,
beginning in April and ending in September. Synoptic types, or modes of upper air behavior, are determined objectively by
the eigenvector method employing K-means cluster analysis. Monthly burned areas from the Canadian Large Fire Database (LFDB)
for the same period, 1959 to 1996, are analysed in conjunction with the classified monthly 500 hPa synoptic types. Relationships
between common monthly patterns of anomalous upper flow and spatial patterns of large fire occurrence are examined at the
ecozone level. Average occurrence of a monthly synoptic type associated with very large area burned is approximately 18% of
the years from 1959 to 1996. The largest areas burned during the main fire (May to August) season occur in the western Boreal
and Taiga ecozones – the Taiga Plains, Taiga Shield, Boreal West Shield and Boreal Plains. Monthly burned areas are also analysed
temporally in conjunction with a calculated monthly zonal index (Zim) for two separate areas defined to cover western and eastern Canada. In both western and eastern Canada, high area burned
is associated with synoptic types with mid-tropospheric ridging in the proximity of the affected region and low Zim with weak westerlies and strong meridional flow over western Canada.
Received April 3, 2001 Revised July 13, 2001 相似文献
3.
E.C Todd 《Geochimica et cosmochimica acta》2003,67(5):881-893
The nature of the surface oxidation phase on pyrite, FeS2, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur LIII edge, and iron LII,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS2 can be detected in the X-ray absorption spectra. In a 1.667 × 10−3 mol/L Fe3+ solution with ferric iron present as FeCl3 in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH. 相似文献
4.
Steven M. Babin Todd D. Sikora Nathaniel S. Winstead 《Boundary-Layer Meteorology》2003,106(3):527-546
A case study of a particularly intense cold air outbreak over the northAtlantic Ocean extending from the northeast coast of the UnitedStates to the Gulf Stream is described. A RADARSAT satellite synthetic apertureradar (SAR) image of this outbreak dramatically illustrates the spatialevolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administration's Advanced Very HighResolution Radiometer are used to compare many interesting features.In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data arepresented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak.Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells becomenon-uniform with bright crescents and filled semi-circles on thedownwind sides of the cells. These are believed to be surface effectsof gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow. 相似文献
5.
Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California 总被引:13,自引:1,他引:13
Timothy L. Grove Julie M. Donnelly-Nolan Todd Housh 《Contributions to Mineralogy and Petrology》1997,127(3):205-223
Glass Mountain consists of a 1 km3, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust.
Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain.
Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H2O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional
crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated
at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5
km3) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and
provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled
from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two
distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of
the andesite at pressures of at least 200 MPa (6 km depth) under near H2O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing
magmatic inclusions indicates that crystallization under near H2O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence
indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction
with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the
crust.
Received: 26 March 1996 / Accepted: 14 November 1996 相似文献
6.
Critical rainfall statistics for predicting watershed flood responses: rethinking the design storm concept
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Recent advances have been made to modernize estimates of probable precipitation scenarios; however, researchers and engineers often continue to assume that rainfall events can be described by a small set of event statistics, typically average intensity and event duration. Given the easy availability of precipitation data and advances in desk‐top computational tools, we suggest that it is time to rethink the ‘design storm’ concept. Design storms should include more holistic characteristics of flood‐inducing rain events, which, in addition to describing specific hydrologic responses, may also be watershed or regionally specific. We present a sensitivity analysis of nine precipitation event statistics from observed precipitation events within a 60‐year record for Tompkins County, NY, USA. We perform a two‐sample Kolmogorov–Smirnov (KS) test to objectively identify precipitation event statistics of importance for two related hydrologic responses: (1) peak outflow from the Six Mile Creek watershed and (2) peak depth within the reservoir behind the Six Mile Creek Dam. We identify the total precipitation depth, peak hourly intensity, average intensity, event duration, interevent duration, and several statistics defining the temporal distribution of precipitation events to be important rainfall statistics to consider for predicting the watershed flood responses. We found that the two hydrologic responses had different sets of statistically significant parameters. We demonstrate through a stochastic precipitation generation analysis the effects of starting from a constrained parameter set (intensity and duration) when predicting hydrologic responses as opposed to utilizing an expanded suite of rainfall statistics. In particular, we note that the reduced precipitation parameter set may underestimate the probability of high stream flows and therefore underestimate flood hazard. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
7.
The Hammond Hill Research Catchment (HH) is a small (120 ha), temperate, second order tributary to Six Mile Creek, Cayuga Lake, and the Great Lakes (42.42°, −76.32°). The HH has been monitored since January 2017 for the purpose of understanding how recent infiltration mixes with antecedent soil water on hillslope forest floors and the spatial and temporal patterns of Root Water Uptake (RWU) by temperate northeastern US tree species (eastern hemlock [Tsuga canadensis], American beech [Fagus grandifolia], and sugar maple [Acer saccharum]). These data are informing us about the hydrologic consequences of anticipated tree species composition change and supporting the development of more refined ecohydrological models. The glaciated catchment is underlain by a shallow confining siltstone layer (1–1.5 m depth) and densely covered with an approximately 60 year old regrowth mixed species forest of hemlock, beech, and other deciduous tree species common to the northeastern US. Current datasets from the HH include precipitation snow water equivalent, discharge, and associated isotopic water compositions, δ2H & δ18O. Measurements of (top 10 cm) soil water content, as well as bulk soil water and hemlock and beech xylem isotopic compositions are made at several locations across a topographic wetness gradient. The near-term role of the HH is to support an understanding of the environmental and ecological drivers of plant RWU competition. All data from the HH are publicly available. 相似文献
8.
Keith E. Schilling Philip W. Gassman Catherine L. Kling Todd Campbell Manoj K. Jha Calvin F. Wolter Jeffrey G. Arnold 《水文研究》2014,28(8):3314-3325
Effects of agricultural land management practices on surface runoff are evident at local scales, but evidence for watershed‐scale impacts is limited. In this study, we used the Soil and Water Assessment Tool model to assess changes in downstream flood risks under different land uses for the large, intensely agricultural, Raccoon River watershed in Iowa. We first developed a baseline model for flood risk based on current land use and typical weather patterns and then simulated the effects of varying levels of increased perennials on the landscape under the same weather patterns. Results suggest that land use changes in the Raccoon River could reduce the likelihood of flood events, decreasing both the number of flood events and the frequency of severe floods. The duration of flood events were not substantially affected by land use change in our assessment. The greatest flood risk reduction was associated with converting all cropland to perennial vegetation, but we found that converting half of the land to perennial vegetation or extended rotations (and leaving the remaining area in cropland) could also have major effects on reducing downstream flooding potential. We discuss the potential costs of adopting the land use change in the watershed to illustrate the scale of subsidies required to induce large‐scale conversion to perennially based systems needed for flood risk reduction. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
9.
10.
Despite substantial survey effort and a large body of literature on abiotic and biotic factors in temperate reef ecosystems, knowledge of the complex and interactive effects of environmental variables on those communities is limited. Various survey methods have been developed to study environmental predictors of biodiversity, but there remains a gap in our understanding of how survey results are influenced by environmental factors. Here, we surveyed the fish assemblage associated with southeastern U.S. temperate marine reefs with simultaneous, paired trap, and camera gears throughout a ~50,000 km2 area during 2011–2013 and assessed the influence of environmental variables on the trap‐ and video‐surveyed assemblages. Predictor variables in the multivariate general linear models included depth, temperature, month, year, location, substrate relief, percent sessile biota, biota type, and turbidity. Depth and latitude had the greatest influence on the fish assemblage for both gears. The influence of habitat variables differed between methods and percent biota explained more variation in the fish assemblage when assessed by traps, while substrate relief and biota type explained more variation in the fish assemblage when assessed by video. In general, habitat complexity was positively related to the abundance of fishes in the video survey, but there was a negative relationship in the trap survey. Differences between gears were species‐specific and the influences of environmental variables were similar for some species such as Haemulon plumierii and Hyporthodus niveatus. The methods presented here can be used to assess method‐dependent differences in fish assemblages, which is a necessary precursor to assess the effect of environmental variables on the accuracy of surveys. 相似文献