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111.
A. MCDONALD 《地球,A辑:动力气象学与海洋学》2009,61(2):227-231
The problem of how to adapt the so-called transparent boundary conditions to the physical reality that the horizontal advection velocity varies in the vertical, is addressed. A practical demonstration is presented of the transparency of the resulting 'shear' boundary conditions. 相似文献
112.
Saioa Suárez Hazel M. Prichard Francisco Velasco Peter C. Fisher Iain McDonald 《Mineralium Deposita》2010,45(4):331-350
The distribution, mineralogy and mobility of the platinum-group elements (PGE) in the surface environment are poorly understood.
This study of the lower, less altered and upper, more altered gossan, overlying the Aguablanca Ni–Cu-(PGE) magmatic deposit
(Spain), has shown that the platinum-group minerals (PGM) are progressively oxidised and dispersed into iron oxides that form
the gossan. A combination of the characterization of PGE in host PGM, using a scanning electron microscope, and measurement
of PGE at lower concentrations in host iron oxides, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS),
has for the first time allowed the total distribution of the PGE within a gossan to be documented. This study has revealed
a complete in situ alteration and dispersion sequence of the PGM including (1) breakdown of both the more stable Pt-arsenides,
Pt/Pd-tellurides and the less stable bismuthotellurides, (2) formation of partially oxidised PGM, (3) development of a wide
range of oxidised Pt- and Pd-bearing phases, (4) subsequent formation of Fe–PGE-oxides and PGE-hydroxides, (5) incorporation
of PGE into ferruginous supergene products and lastly (6) concentration of PGE at the edges of veins and iron oxides. Dispersion
of Pd is greater than for the other PGE with Pd being widely distributed throughout the iron oxides. This oxidising environment
produced PGE-oxides rather than PGE-alloys, also commonly found in the surface environment, especially in placers. These results
provide critical evidence for the stages of mineralogical change from PGE host mineralogy in magmatic ores to surface weathering
producing PGE-oxides. 相似文献
113.
Scott Andrew Wooldridge Terence J. Done Colette R. Thomas Iain I. Gordon Paul A. Marshall Roger N. Jones 《Climatic change》2012,112(3-4):945-961
The threats of wide-scale coral bleaching and reef demise associated with anthropogenic (global) climate change are widely known. Less well considered is the contributing role of conditions local to the reef, in particular reef water quality, in co-determining the physiological tolerance of corals to increasing sea temperatures and declining pH. Here, the modelled benefit of reduced exposure to dissolved inorganic nitrogen (DIN) in terrestrial runoff, which raises the thermal tolerance of coastal coral communities on the central Great Barrier Reef (Australia), is considered alongside alternative future warming scenarios. The simulations highlight that an 80% reduction in DIN ‘buys’ an additional ~50–60?years of reef-building capacity for No Mitigation (‘business-as-usual’) bleaching projections. Moreover, the integrated management benefits provided by: (i) local reductions of ~50% in DIN contained in river loads, and (ii) global stabilisation of atmospheric CO2 below 450?ppm can help ensure the persistence of hard-coral-dominated reefscapes beyond 2100. The simulations reinforce the message that beyond the global imperative to mitigate future atmospheric CO2 emissions there still remains the need for effective local management actions that enhance the resistance and resilience of coral reef communities to the impacts of climate change. 相似文献
114.
115.
The development of stratigraphic sequences has been demonstrated to be controlled by a set of factors including variations in subsidence, sediment input, eustatic sea level and physiography. Well and seismic data from the Jeanne d'Arc Basin, Grand Banks indicate that mid-Cretaceous tectonism controls at least three of these factors, namely subsidence, sediment input and physiography. North Atlantic rift tectonism was therefore the dominant factor in controlling the migration of coastal to shallow marine environments and the development of sequence stratigraphy in this basin during the mid-Cretaceous. The Avalon Formation respresents a mainly Barremian to Early Aptian regressive phase of clastic, marine to marginal marine sedimentation. This followed the deposition of a thick sequence of mainly marine limestones and shales of the Whiterose Formation above a mid-Valanginian sequence-bounding unconformity. The increased clastic input and northward progradation of coastal environments represented by the Avalon Formation occurred during uplift of a basement arch to the south with subsidence of the basin increasing to the north, accompanied by only relatively minor faulting. These features indicate that a period of epeirogenesis was initiated during the Barremian. Continuing uplift over an expanding area at the southern end of the basin is interpreted to have resulted in the development of an angular unconformity with incised valleys. This mid-Aptian unconformity defines the top of the Whiterose/Avalon sequence. Initiation of brittle fracturing of the sedimentary package and underlying basement (i.e. rifting) in mid-Aptian times resulted in rapid fault-controlled subsidence and fragmentation of the Jeanne d'Arc Basin. This great increase in subsidence rate caused retrogradation of coastal environments across the previously developed sequence-bounding unconformity, despite continuing high rates of sediment input from the uplifted basin margins. The transgressive, siliciclastic Ben Nevis Formation comprises two separate but related facies associations. A locally preserved basal association represents interfingering back-barrier environments and is herein defined as the Gambo Member. An upper, ubiquitous facies association comprises tidal-inlet channel, shoreface and lower shoreface/offshore transition sandstones. This upper facies association onlapped marine ravinement diastems above the laterally equivalent back-barrier facies. The rapid fault-controlled subsidence and high sediment input rate of this mid-Aptian to late Albian rift period resulted in the accumulation and preservation of very thick shoreface sandstones. The transgressive sandstones were buried by laterally equivalent offshore shales of the Nautilus Formation. Flooding of the basin margins induced by the onset of thermal subsidence in latest Albian or early Cenomanian times marks the top of the Ben Nevis/Nautilus syn-rift sequence. 相似文献
116.
117.
Mervyn Freeman, Gareth Chisham, Iain Coleman and Mike Pinnock report on the November discussion meeting "Temporal and spatial structure of magnetic reconnection on the Earth's magnetopause", the only natural environment in which this universal astrophysical phenomenon can be observed locally and globally. 相似文献
118.
Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the 210Pb and 137Cs dating methods. Here, we test the applicability of using 210Pb and 137Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The 210Pb and 137Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, 210Pb dating, corroborated by 137Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by 210Pb and 137Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the 14C-based estimated dates for the upper unit. At two other trenches 210Pb and 137Cs dating only provided minimum ages (based on the presence or absence of 210Pbexcess and 137Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset. 相似文献
119.
120.
J. M. ASS'AD R. H. TATHAM J. A. MCDONALD T. M. KUSKY J. JECH 《Geophysical Prospecting》1993,41(3):323-339
An approximation to plane-wave propagation through a composite material is examined using a physical model with oriented but randomly distributed penny-shaped rubber inclusions within an isotropic epoxy resin matrix. A pulse transmission method is used to determine velocities of shear and compressional waves as a function of angle of incidence and crack density. The experimental and theoretical results of Hudson were compared and limitations within the crack parameters used in this study have been determined. Results from both polarized shear waves (S1, S2) compare favourably with the theory for a composite with up to 7% crack density, but theory and experiment diverge at higher crack densities. On the other hand, compressional-wave velocities at low crack densities (1% and 3%) compare favourably with the theory. It is also shown that the velocity ratio Vp/Vs for two extreme cases, i.e. propagation normal and parallel to the cracks, as a function of crack density and porosity, has a strong directional dependence. 相似文献