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121.
122.
The interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Niña years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Niño years. The difference in early season cyclone counts between La Niña and El Niño years was particularly pronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico. 相似文献
123.
C. F. M. Lewis C. W. HeilJr. J. B. Hubeny J. W. King T. C. MooreJr. D. K. Rea 《Journal of Paleolimnology》2007,37(3):435-452
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern
Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was
inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from
the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence
rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the
Lake Stanley phase.
A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates
the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated
lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is
attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in
the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley
lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows
that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest
of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows
that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The
late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time.
A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand,
late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes.
This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004),
held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special
issue. 相似文献
124.
We have developed a non-subjective technique for recording the occurrences of coronal mass ejection (CME) in data recorded by the Large Angle Spectrometric Coronagraph experiment (LASCO) aboard the Solar and Heliospheric Observatory spacecraft (SOHO). We have found evidence for, and quantified, an asymmetry in the apparent longitudes at which mass ejections occurred during the first year of LASCO synoptic observations and coinciding with the 1996–1997 solar minimum. Throughout this period the solar surface could loosely be characterized as having both an active and a quiet hemisphere and the observed mass ejection asymmetry is seen to relate closely with the longitudes of most persistent disc activity. However, our best estimate for the centroid of the CME distribution is 45 deg to the west of the brightest regions visible in Fe 195 Å emission on the disc and in an area of reduced coronal emission. This corresponds to the location of a trans-equatorial extension of the northern coronal hole which persisted to some degree throughout the year and was directly associated with the most active region on the disc. We suggest that this indicates magnetic reconnection, which is necessary at the boundaries of coronal holes to maintain their quasi-rigid rotation above the differentially rotating photosphere, could play an important role in triggering the destabilization of nearby structures and result in the observed prevalence of mass ejections. We estimate that the events included in the study could contribute around 8% to the total solar mass loss through the solar wind (which is around 1014 kg day–1) and find a scale of asymmetry indicating that close to 70% of this mass is ejected from within a single hemisphere. 相似文献
125.
Stephen Edward Lewis Robert A. Henderson Gerald R. Dickens Graham A. Shields Simon Coxhell 《Mineralium Deposita》2010,45(6):599-620
A significant resource of vanadium and molybdenum exists near Julia Creek, Australia, where the middle Cretaceous organic-rich
Toolebuc Formation lies between 0 and 25 m of the surface. We present and discuss a comprehensive geochemical study of the
Toolebuc Formation and its enclosing stratigraphy near Julia Creek to understand this ore deposit. V and Mo contents in fresh
facies are strongly associated with total organic carbon (TOC) contents, but not with Al or CaCO3; this suggests that V and Mo were originally concentrated in the organic fraction. However, chemical extractions using H2O2 indicate that Mo was originally concentrated in pyrite. The data also suggest that V was mobilised from organic matter during
early diagenesis and became associated with clays as little V was extracted by H2O2 in the fresh samples. TOC contents in the Toolebuc Formation were removed during weathering, residually enriching trace metals
including V and Mo, and as a result, the TOC relationship with V and Mo disintegrates. With weathering, both V and Mo predominantly
became associated with iron oxide/hydroxide phases (and possibly other unidentified phases) as these elements in the weathered
facies were highly soluble in the sodium citrate–sodium dithionite digestion. Large shale-hosted V and Mo deposits such as
Julia Creek offer a potentially viable alternative to the currently mined magnetite-hosted deposits. A thorough understanding
of the formation and host mineral phases for V and Mo of these shale deposits, however, is critical to ensure that these valuable
metals can be feasibly extracted. 相似文献
126.
Cynthia F. Lewis Stopher L. Slade Kerry E. Maxwell Thomas R Matthews 《新西兰海洋与淡水研究杂志》2013,47(1):271-282
Commercial fishers report finding their lobster traps often great distances from their original location following major hurricanes. But traps also move during lesser wind events, such as during winter cold fronts. To assess trap impact on coral communities following winter storms, lobster traps were placed in hardbottom and reef habitats commonly used by commercial fishers in the Florida Keys, United States. Trap movement, percentage benthic faunal cover, and benthic faunal damage were assessed after 26 wind events occuring over three winters. Traps moved when storms with sustained winds greater than 15 knots (27.8 km/h) persisted for more than 2 days. Winter storms above this threshold moved buoyed traps a mean (±SE) distance of 3.63 ± 0.62 m, 3.21 ± 0.36 m, and 0.73 ± 0.15 m per trap and affected a mean area of 4.66 ± 0.76 m2, 2.88 ± 0.29 m2, and 1.06 ± 0.17 m2, per trap at 4‐m, 8‐m, and 12‐m depths, respectively. Unbuoyed traps, simulating derelict traps, moved a mean distance of 0.43 ± 0.08 m and 0.44 ± 0.02 m, and affected a mean area of 0.77 ± 0.06 m2 and 0.90 ± 0.08 m2 per trap at 4‐m and 8‐m depths, respectively. Injuries caused by trap movement included scraped, fragmented, and dislodged sessile fauna, resulting in significant damage to stony coral, octocoral, and sponges. Overall, sessile fauna cover along the trap movement path was reduced from 45% to 31%, 51% to 41%, and 41% to 35% at the 4‐m, 8‐m, and 12‐m sites, respectively. Because of the large numbers of traps deployed and reported lost each season, damage to sessile fauna and loss of benthic faunal cover caused by traps needs to be considered to effectively protect coral reefs and manage essential fishery habitat in the future. 相似文献
127.
A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the 40Ar/36Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform 3He/4He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle. 相似文献
128.
A spectrum scanning Stokes polarimeter 总被引:1,自引:0,他引:1
129.
Lower slopes of the Sandia Mountains are characterized by granitic corestone topography and weathering-limited slopes with thin grusy colluvium and weakly developed soils. In contrast, thick soils with illuvial clay and pedogenic carbonate have developed below aplite outcrops. Aplite is resistant to chemical decomposition, but physically weathers to blocky clasts that enhance surface roughness and erosional resistance of colluvium, promoting accumulation of eolian fines. Thick B horizons on aplite slopes indicate limited erosion and prolonged periods of stability and soil development. Accretion of eolian material limits runoff and prevents attainment of a steady-state balance between soil production and downslope transport. 相似文献
130.
The Chaman left‐lateral strike‐slip fault bounds the rigid Indian plate boundary at the western end of the Himalayan‐Tibetan orogen and is marked by contrasting topographic relief. Deformed landforms along the fault provide an excellent record for understanding this actively evolving intra‐continental strike‐slip fault. The geomorphic response of an active transpessional stretch of the Chaman fault was studied using digital elevation model (DEM) data integrated with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Visible and Near Infrared/Short Wave Infrared (VNIR/SWIR) and images from GeoEye‐1. Geologic and geomorphic mapping helped in reconstructing the Late Quaternary landscape history of this transpessional strand of the Chaman strike‐slip fault and the associated Spinatizha thrust fault in western Pakistan. Topographic analysis of a part of the transpression (the thrust bounded Roghani ridge) revealed northward growth of the Spinatizha fault with the presence of three water gaps and two corresponding wind gaps. Geomorphic indices including stream length‐gradient index, mountain front sinuosity, valley floor width to valley height ratios, and entrenchment of recent alluvial fan deposits were used to define the lateral growth and direction of propagation of the Spinatizha fault. Left‐lateral displacement along Chaman fault and uplift along the Spinatizha fault was defined using topographic analysis of the Roghani ridge and geomorphic mapping of an impressive alluvial fan, the Bostankaul fan. The landforms and structures record slip partitioning along the Indian plate boundary, and account for the convergence resulting from the difference in the Chaman fault azimuth and orientation of the velocity vector of the Indian plate. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献