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81.
Previous interdisciplinary paleoenvironmental and archaeological research along the Río Verde Valley of Oaxaca, Mexico, showed that Holocene erosion in the highland valleys of the upper drainage basin triggered geomorphic changes in the river's coastal floodplain. This article uses stratigraphic data from sediment cores extracted from Laguna Pastoría, an estuary in the lower Río Verde Valley, to examine changes in coastal geomorphology potentially triggered by highland erosion. Coastal lagoon sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late Holocene times. Three distinct storm facies are identified from sediment cores obtained from Laguna Pastoría, which indicate that profound coastal environmental changes occurred within the region and are correlated with increased sediment supplied from highland erosion. The Chione/Laevicardium facies was deposited in an open bay while the Mytella/barnacle facies and sand facies were deposited in an enclosed lagoon following bay barrier formation. We argue that highland erosion triggered major geomorphic changes in the lowlands including bay barrier formation by 2500 cal yr B.P. These environmental changes may have had significant effects on human populations in the region. The lagoon stratigraphy further indicates an increase in mid–late Holocene hurricane activity, possibly caused by increased El Niño frequencies. 相似文献
82.
J. Arthur Snoke I. Selwyn Sacks David E. James 《Geophysical Journal International》1979,59(1):219-225
Summary. There are two conflicting models for the dip of the subduction zone beneath central Peru and beneath central Chile. One model, based primarily on the distribution of hypocentres thought to be most reliably located, postulates a shallow (∼ 10°) dip for the downgoing plate. The second model, re-examined in this paper and based chiefly on an analysis of ScSp converted phases, postulates a normal (∼ 30°) dip for the subduction.
A detailed examination of ScSp data for central Peru and central Chile shows that ScSp arrivals can be identified on all seismograms on which the predicted signal/noise ratio is greater than 1, and the measured amplitude ratios of ScSp to ScS imply a normal dip for the conversion interfaces. The characteristics of the ScSp arrivals in these regions are virtually identical to those for ScSp arrivals in southern Peru, where a well-defined Benioff zone and the calculated ScSp -conversion region both imply a normal dip for the subduction. Hence ScSp observations in South America support the model of an approximately constant dip of subduction from central Peru to central Chile to a depth of at least 120 km, and impose a constraint that must be satisfied by any satisfactory tectonic model for these regions. 相似文献
A detailed examination of ScSp data for central Peru and central Chile shows that ScSp arrivals can be identified on all seismograms on which the predicted signal/noise ratio is greater than 1, and the measured amplitude ratios of ScSp to ScS imply a normal dip for the conversion interfaces. The characteristics of the ScSp arrivals in these regions are virtually identical to those for ScSp arrivals in southern Peru, where a well-defined Benioff zone and the calculated ScSp -conversion region both imply a normal dip for the subduction. Hence ScSp observations in South America support the model of an approximately constant dip of subduction from central Peru to central Chile to a depth of at least 120 km, and impose a constraint that must be satisfied by any satisfactory tectonic model for these regions. 相似文献
83.
84.
Niimi AJ 《Marine pollution bulletin》2004,49(9-10):778-782
Ballast water exchange practices were monitored on 28 incoming container vessels at the Port of Montreal. Measurements on 15 vessels indicated 13 of 32 tanks had salinities of <30 per thousand. The 16 transits with a North Atlantic route visited 31 of 37 ports located on freshwater or near freshwater outflows. Ballast carried by this vessel type represents an important means for the introduction of species on a global scale because of its transit routes, dockside discharge and moving ballast between tanks. Container vessels represent about 15% of the world fleet, but account for 32% of all visits to global ports, and 46% of visits to the 25 largest ports. The 10 ports that handled the largest volumes of international cargo also included 8 that handled the most cargo containers. Large ports can receive over 100,000 visits by all vessel types annually, and serve as hubs for over 500 ports in 100 countries. Secondary transport of exotic species is also a concern because of frequent visits by regional vessels. 相似文献
85.
Diffusive boundary layers (DBL), sediment oxygen flux, and natural passive flow through model macrofauna burrows were compared
in two benthic chambers: one with a conventional rotational stirrer and the other with a two-dimensional flow diffuser system.
Oxygen microprobe profiles showed that at similar velocities the mean diffusive boundary layer (DBL) thickness induced by
a conventional rotor stirrer (453±118 μm) was not significantly different to that produced by the diffuser system (403±53
μm). The rotor produced twice as much DBL spatial variability (coefficient of variation 27%) as the diffuser (CV 11%). Variability
between the rotor system’s DBL transect replicates was also two times greater (average CV of 22%) compared to the diffuser
(CV 14%). At equivalent stirring speeds over experimental sediments, mean O2 consumption rates were also not significantly different between the two systems. The diffuser induced consistently greater
(16–37%) passive Bernoullian flow through model macrofauna borrows irrespective of the position of inhalant-exhalent openings.
The rotor stimulated anomalous burrow flow regimes over a greater area of the chamber floor (36%) compared to the diffuser
(23%). Depending on vent orientation the rotor was shown to reverse (exhalent to inhalant) burrow flow regimes in the central
9% of the chamber floor. This artifact of radial pressure and velocity differentials may have severe implications for tube
dwelling infauna that rely on unidirectional flow. The diffuser system more closely mimicked natural two-dimensional water
flow over the sediment surface and structures therein and is likely to give more representative results when measuring benthic
processes within incubation devices. 相似文献
86.
Detlef P. van Vuuren Bert de Vries Arthur Beusen Peter S.C. Heuberger 《Global Environmental Change》2008,18(4):635
The conditional probabilistic scenario analysis combines statistical methods of uncertainty analysis at parameter level with storylines which recognize the deep uncertainty that exists for several underlying trends. The model calculations indicate that cumulative 21st century emissions could range from 800 to 2500 GtC in the absence of climate policy. This range originates partly from the underlying storylines, and partly from the probabilistic analysis. Among the most important parameters contributing to the uncertainty range are uncertainty in income growth, population growth, parameters determining energy demand, oil resources and fuel preferences. The contribution of these factors is also scenario-dependent. 相似文献
87.
Numerical models for wide triplets of galaxies are developed, and a statistical estimate of the dark matter content in these systems is derived. In a typical system, the mass of dark matter proves to be (1.5 - 3) × 1012h-1 M. 相似文献
88.
D. W. G. Arthur 《The Photogrammetric Record》1972,7(40):442-453
Close up photographs of the lunar surface obtained by the Apollo 14 astronauts presented unusual problems in relative orientation caused by essentially different directions of the camera axes. The problem was resolved by dividing each large unknown rotation into two parts, one of which was a large known part and the other was a small unknown part. The method succeeds when the unknown rotation elements can be initially estimated to about 10°. 相似文献
89.
90.