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81.
Geochemical evidence for diversity of dust sources in the southwestern United States 总被引:1,自引:0,他引:1
Several potential dust sources, including generic sources of sparsely vegetated alluvium, playa deposits, and anthropogenic emissions, as well as the area around Owens Lake, California, affect the composition of modern dust in the southwestern United States. A comparison of geochemical analyses of modern and old (a few thousand years) dust with samples of potential local sources suggests that dusts reflect four primary sources: (1) alluvial sediments (represented by Hf, K, Rb, Zr, and rare-earth elements, (2) playas, most of which produce calcareous dust (Sr, associated with Ca), (3) the area of Owens (dry) Lake, a human-induced playa (As, Ba, Li, Pb, Sb, and Sr), and (4) anthropogenic and/or volcanic emissions (As, Cr, Ni, and Sb). A comparison of dust and source samples with previous analyses shows that Owens (dry) Lake and mining wastes from the adjacent Cerro Gordo mining district are the primary sources of As, Ba, Li, and Pb in dusts from Owens Valley. Decreases in dust contents of As, Ba, and Sb with distance from Owens Valley suggest that dust from southern Owens Valley is being transported at least 400 km to the east. Samples of old dust that accumulated before European settlement are distinctly lower in As, Ba, and Sb abundances relative to modern dust, likely due to modern transport of dust from Owens Valley. Thus, southern Owens Valley appears to be an important, geochemically distinct, point source for regional dust in the southwestern United States. 相似文献
82.
Andrew E. Laursen Sybil P. Seitzinger Robert Dekorsey James G. Sanders Denise L. Breitburg Richard W. Osman 《Estuaries and Coasts》2002,25(1):57-69
The effects of nutrients, trace elements, and trophic complexity on benthic photosynthesis and respriation were studied in the Paxtuxent River estuary near St. Leonard, Maryland. Experiments were conducted over three years (1995–1997) in mesocosms containing riverine sediment and water. The experimental design was 2×2×3 factorial with two levels of nutrients (ambient and + nutrients), two of trace elements (ambient and + trace elements) and three of trophic complexity (plankton, plankton + fish, and plankton + fish + benthos). Trace elements included arsenic (As), copper (Cu), and cadmium (Cd). The experiment was conducted three times in 1995 and 1997 and four times in 1996. In 1995 and 1996, sediments were muddy, while in the final year sediments were sandy. In mesocoms with sandy sediments, nutrient additions increased benthic photosynthesis overall, while trace element additions increased benthic photosynthesis in two of three experimental runs. Benthic photosynthesis in these mesocosms appeared to be related to nitrogen loading. Benthic respiration increased in nutrient and trace element amended mesocosms with sandy sediments, apparently in response to higher benthic photosynthesis. Increasing trophic complexity, particularly the presence of benthic organisms, also increased benthic respiration in mesocosms with sandy sediments. There were no effects of nutrient or trace element additions on benthic photosynthesis and respiration when the sediments were muddy. The lack of consistent responses to nutrient additions was surprising given that benthic respiration rates (and presumably nutrient regeneration) were similar in all three years, regardless of sediment type. Muddy, sediments did not mask, the effects of nutrient addition by supplying more nutrients to benthic microalgae than sandy sediments. In 1996, the presence of filter feeding bivalves increased the relative heterotrophy of sediments, measured as production: respiration. Consistent with increased heterotrophy, effluxes of ammonium and soluble reactive phosphorus from sediments were greater in mesocosms containing benthic organisms. Anthropogenically-induced changes in estuaries, such as loading of nutrients and trace elements or reduced trophic complexity, can have important effects on benthic processes and potentially pelagic processes through feedback mechanisms. 相似文献
83.
Sergey Fomel James G. Berryman Robert G. Clapp Marie Prucha 《Geophysical Prospecting》2002,50(6):577-588
We apply iterative resolution estimation to least‐squares Kirchhoff migration. Reviewing the theory of iterative optimization uncovers the common origin of different optimization methods. This allows us to reformulate the pseudo‐inverse, model resolution and data resolution operators in terms of effective iterative estimates. When applied to Kirchhoff migration, plots of the diagonal of the model resolution matrix reveal low illumination areas on seismic images and provide information about image uncertainties. Synthetic and real data examples illustrate the proposed technique and confirm the theoretical expectations. 相似文献
84.
Active tectonics of the South Caspian Basin 总被引:15,自引:0,他引:15
James Jackson Keith Priestley Mark Allen Manuel Berberian 《Geophysical Journal International》2002,148(2):214-245
85.
86.
James L. Etienne Neil F. Glasser Michael J. Hambrey 《Geografiska Annaler: Series A, Physical Geography》2003,85(2):149-164
Mapping and laboratory analysis of the sediment—landform associations in the proglacial area of polythermal Storglaciären, Tarfala, northern Sweden, reveal six distinct lithofacies. Sandy gravel, silty gravel, massive sand and silty sand are interpreted as glaciofluvial in origin. A variable, pervasively deformed to massive clast‐rich sandy diamicton is interpreted as the product of an actively deforming subglacial till layer. Massive block gravels, comprising two distinctive moraine ridges, reflect supraglacial sedimentation and ice‐marginal and subglacial reworking of heterogeneous proglacial sediments during the Little Ice Age and an earlier more extensive advance. Visual estimation of the relative abundance of these lithofacies suggests that the sandy gravel lithofacies is of the most volumetric importance, followed by the diamicton and block gravels. Sedimentological analysis suggests that the role of a deforming basal till layer has been the dominant factor controlling glacier flow throughout the Little Ice Age, punctuated by shorter (warmer and wetter climatic) periods where high water pressures may have played a more important role. These results contribute to the database that facilitates discrimination of past glacier thermal regimes and dynamics in areas that are no longer glacierized, as well as older glaciations in the geological record. 相似文献
87.
88.
James W. Head III Lionel Wilson 《Journal of Volcanology and Geothermal Research》2003,121(3-4):155-193
Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expected for the purely magmatic hawaiian case; cooling of descending pyroclasts would be more efficient, leading to different types of proximal deposits; and runout distances for density flows would be greater, potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer. In addition, flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles, and thus extremely poor in vesicles relative to typical flows associated with hawaiian-style eruptions in the primary magmatic gas case. We examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid-ocean ridges. Furthermore, the latter stages of development of volcanic edifices (seamounts) formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth, but also to: (1) the presence of a summit magma reservoir, which favors the buildup of magmatic foams (enhancing hawaiian-style activity) and episodic dike emplacement (which favors strombolian-style eruptions); and (2) the common occurrence of alkalic basalts, the CO2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts. These models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles. 相似文献
89.
90.