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101.
We present a reconstruction of June–July minimum temperatures since AD 1245 for the Mackenzie Delta region based on a 29-site network of white spruce (Picea glauca) ring-width series. Most but not all trees experienced a divergent temperature–growth response, similar to the divergence that has affected other white spruce trees across Yukon and Alaska. However, divergence in the study region began as early as AD 1900 and we have documented our methods to avoid including divergent signals in the reconstruction. Calibration/verification testing based on local temperature data, and multi-century coherence with nearby and large-scale temperature proxy records, confirm that our reconstruction is robust. The reconstruction shows cool conditions in the late 13th, early 18th and early 19th centuries, corresponding with solar minima and increased volcanism. These cool periods are interrupted by warm periods consistent with early to mid-20th century warmth. The late 20th century is the warmest interval, and the last decade is estimated to be 1.4°C warmer than any decade before the mid-20th century. The reconstructed climate history corroborates other proxy-based inferences and supports the notion that high-latitude regions such as the Mackenzie Delta have experienced rapid warming in recent decades that is exceptional in the last eight centuries. 相似文献
102.
Trevor Fuller PhD student 《The Professional geographer》2013,65(2):272-274
A relatively simple modeling approach for estimating spatially distributed surface energy fluxes was applied to two small watersheds, one in a semi-arid climate region and one in a sub-humid region. This approach utilized a combination of ground-based meteorological data and remotely sensed data to estimate ‘instantaneous’ surface energy fluxes at the time of the satellite or aircraft overpasses. The spatial resolution in the watershed grid cells, which was on the order of 100-400 km, was selected to be compatible with ground measurements used for validation. The model estimates of surface energy fluxes compared well with ground-based measurements of surface flux (typically within approximately 40 Wm?2). The model accuracy may be slightly less for bare soil surfaces due to an overestimation of the soil heat flux. In addition to demonstrating the feasibility of computing spatially distributed values of surface energy fluxes, these maps were used to qualitatively infer the dominant factors controlling the energy fluxes for the time period shortly following precipitation events in the basins. For the semi-arid watershed, values of sensible heat flux varied considerably over the watershed and displayed a pattern very similar to that of the spatially variable cumulative precipitation for at least one to eight days prior to the image acquisition. Due to the large fraction of exposed bare soil in a semi-arid ecosystem, even very small precipitation events had a strong influence on the pattern of sensible heat fluxes observed shortly after the event (less than 24 hours). For the sub-humid watershed, the fluxes tended to be more uniform across the watershed, and were influenced by a combination of precipitation total and land cover type. 相似文献
103.
Trevor Birkenholtz 《The Professional geographer》2013,65(3):354-362
SHORE FEATURES. Patrick C. Moyer. LOKATE. Charles W. Smith. 相似文献
104.
Jesper Rasmussen † Trevor J. Ponman Lourdes Verdes-Montenegro Min S. Yun Sanchayeeta Borthakur 《Monthly notices of the Royal Astronomical Society》2008,388(3):1245-1264
Galaxies in compact groups tend to be deficient in neutral hydrogen compared to isolated galaxies of similar optical properties. In order to investigate the role played by a hot intragroup medium (IGM) for the removal and destruction of H i in these systems, we have performed a Chandra and XMM–Newton study of eight of the most H i deficient Hickson compact groups. Diffuse X-ray emission associated with an IGM is detected in four of the groups, suggesting that galaxy–IGM interactions are not the dominant mechanism driving cold gas out of the group members. No clear evidence is seen for any of the members being currently stripped of any hot gas, nor for galaxies to show enhanced nuclear X-ray activity in the X-ray bright or most H i deficient groups. Combining the inferred IGM distributions with analytical models of representative disc galaxies orbiting within each group, we estimate the H i mass-loss due to ram-pressure and viscous stripping. While these processes are generally insufficient to explain observed H i deficiencies, they could still be important for H i removal in the X-ray bright groups, potentially removing more than half of the interstellar medium in the X-ray bright HCG 97. Ram pressure may also have facilitated strangulation through the removal of galactic coronal gas. In X-ray undetected groups, tidal interactions could be playing a prominent role, but it remains an open question whether they can fully account for the observed H i deficiencies. 相似文献
105.
Biology of evechinus chloroticus (echinoidea: Echinometridae) from different localities 总被引:1,自引:1,他引:0
Trevor G. Dix 《新西兰海洋与淡水研究杂志》2013,47(2):91-116
The distribution, general habits, morphological variation, food, associates and predators ‘Of the common endemic New Zealand echiooid Evechinus chloroticus (Val.) are considered. Evechinus chloroticus is chiefly a rocky bottom dweller but adults, juveniles and very small individuals often occupy different microhabitats. The urchin is typically found in shallow subtidal situations (<12m) although considerable intertidal populations occur in some areas, particularly towards the north. A superimposed dispersion pattern of population and intra‐population clumping is typical and densities often exceed 50 adults/m2. Spine length, thickness and sharpness and test thickness vary between populations but no significant variation in test shape (height/diameter) was observed. Variations in spination are probably related to wave exposure while test thickness is at least in part related to nutrition. In the localities where macroscopic algae are diverse and abundant, Evechinus is almost exclusively algivorous, but it predominantly browses material from the substrate where algae are sparse. The rhabdocoele, Syndesmis sp. (in the gut) and various tiliates (in the gut and gonad) were commonly found in Evechinus. The urchin provides shelter and food for the clingfish Dellichthys morelandi, and the isopod Exosphaeroma sp. is probably also a close associate. Known predators include the cymaitiid gastropods Charonia capax and C. rubicunda, the seastar Coscinasterias calamaria, and the blue cod Parapercichthys colias and snapper Chrysophrys auratus. 相似文献
106.
Sediment grains in a bedrock‐alluvial river will be deposited within or adjacent to a sediment patch, or as isolated grains on the bedrock surface. Previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress. However, this analysis neglected potential interactions between the sediment patches, local hydraulics and grain entrainment. We present a series of flume experiments that measure the influence of sediment patches on grain entrainment. The flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains. In each experiment sediment was added either as individual grains or as a single sediment pulse. Flow was then increased until the sediment was entrained. Analysis of the experiments demonstrates that: (1) for individual grains, coarse grains are entrained at a higher discharge than fine grains; (2) once sediment patches are present, the different in entrainment discharge between coarse and fine grains is greatly reduced; (3) the sheltering effect of patches also increases the entrainment discharge of isolated grains; (4) entire sediment patches break‐up and are eroded quickly, rather than through progressive grain‐by‐grain erosion; (5) as discharge increases there is some tendency for patches to become more elongate and flow‐aligned, and more randomly distributed across the bed. One implication of this research is that the critical shear stress in bedrock‐alluvial channels will be a function of the extent of the sediment cover. Another is that the influence of sediment patches equalizes critical shear stresses between different grain sizes and grain locations, meaning that these factors may not need to be accounted for. Further research is needed to quantify interactions between sediment patches, grain entrainment and local hydraulics on rougher bedrock surfaces, and under different types of sediment supply. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
107.
Trevor L.L. Orr 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2019,13(3):205-213
ABSTRACTTen years after their publication, the Eurocodes, including Eurocode 7 for Geotechnical design, are being revised taking into account experiences with their use and developments in design concepts and practices since their publication to produce a second generation of the Eurocodes. The objectives for this second generation of Eurocode 7 are to improve its ease of use, harmonise its structure with the other Eurocodes, reduce the number of nationally determined parameters, include measures for robustness and include sections on topics not covered in the current Eurocode 7, such as numerical methods, reinforced soil and ground improvement. The evolution of Eurocode 7 aims to hone its safety and risk assessment aspects by developing the concept of Geotechnical Categories, expanding the supervision, monitoring and maintenance requirements, and explicitly allowing for reliability differentiation. 相似文献
108.
109.
Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula 总被引:1,自引:0,他引:1
Ming-Chang Liu Kevin D. McKeegan Kuljeet K. Marhas Trevor R. Ireland 《Geochimica et cosmochimica acta》2009,73(17):5051-1532
The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium-beryllium-boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta52, 2827-2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ26Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable 26Mg-excesses or exhibit 26Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying 26Al/27Al = (4.5 ± 0.2) × 10−5 at the time of isotopic closure, consistent with the “canonical” 26Al abundance characteristic of internal isochrons in many calcium-aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ26Mg∗ consistent with a “supracanonical” 26Al/27Al ratio of (6.4 ± 0.5) × 10−5. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ50Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies.Eight out of 11 26Al-free PLAC hibonite grains record 10B/11B excesses that correlate with Be/B; the inferred initial 10Be/9Be ratio of (5.1 ± 1.4) × 10−4 is lower than the best-constrained 10Be/9Be of (8.8 ± 0.6) × 10−4 in a CV CAI. The data demonstrate that 10Be cannot be used as a relative chronometer for these objects and that most of the 10Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of 26Al in PLAC hibonites indicates that significant amounts of 26Al were not formed in the same spallogenic processes that made 10Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of 26Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange.The oxygen isotopic compositions of SHIBs and PLACs are all highly 16O-enriched, but are not derived from a homogeneous reservoir: Δ17O values span a range of ∼−28‰ to −15‰. The ranges of 16O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most 16O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme 16O-rich chondrule in the early solar system. Geochem. J.37, 663-669]. Both PLACs and SHIBs formed in 16O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively 16O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 105 years based on 26Al records.One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett.7, 493-496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta56, 2503-2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large 26Mg-excess. Earth Planet. Sci. Lett.254, 115-126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including 26Al/27Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir. 相似文献
110.
Alexandru T. Codilean Paul Bishop Trevor B. Hoey Finlay M. Stuart Derek Fabel 《地球表面变化过程与地形》2010,35(1):16-27
We use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper Gaub River catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic 21Ne (21Nec) concentrations in individual detrital grains. The numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total 21Nec concentration that each grain acquires along its pathway. The model fully accounts for variations in nuclide production due to changes in latitude, altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates. Model results show that the form of the frequency distribution of 21Nec concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment's source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment. The results also show that lithology can affect the form of the 21Nec concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment. Model results further indicate that the form of the distribution of 21Nec concentrations in the exported sediment can also be affected by the acquisition of 21Nec after detachment from bedrock, in the diffusive (hillslope) and/or advective (fluvial) domains. However, for such post‐detachment nuclide acquisition to be important, this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock. Copyright © 2009 John Wiley and Sons, Ltd. 相似文献