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301.
Moninya Roughan Newell Garfield John Largier Edward Dever Clive Dorman Dwight Peterson Jeff Dorman 《Deep Sea Research Part II: Topical Studies in Oceanography》2006,53(25-26):2931
The paradox of upwelling is the relationship between strong wind forcing, nutrient enrichment, and shelf productivity. Here we investigate how across-shelf structure in velocity and hydrography plays a role in the retention (inshore) and export (offshore) of particles such as nutrients, plankton and larvae. We examine the spatial structure of the coastal currents during wind-driven upwelling and relaxation on the northern Californian Shelf. The field work was conducted as part of the Wind Events and Shelf Transport (WEST) project, a 5-year NSF/CoOP-funded study of the role of wind-driven transport in shelf productivity off Bodega Bay (northern California) from 2000 to 2003. We combine shipboard velocity profiles (ADCP) and water properties from hydrographic surveys during the upwelling season to examine the mean across-shelf structure of the hydrography and velocity fields during three contrasting upwelling seasons, and throughout the upwelling-relaxation cycle. We also present results from two winter seasons that serve as contrast to the upwelling seasons.During all three upwelling seasons clear spatial structure is evident in velocity and hydrography across the shelf, exemplified by current reversals inshore and the presence of a persistent upwelling jet at the shelf break. This jet feature changes in structure and distance from the coast under different wind forcing regimes. The jet also changes from the north of our region, where it is a single narrow jet, adjacent to the coast, and to the south of our region, where it broadens and at times two jets become evident. We present observations of the California Under Current, which was observed at the outer edge of our domain during all three upwelling seasons. The observed across-shelf structure could aid both in the retention of plankton inshore during periods of upwelling followed by relaxation and in the export of plankton offshore in the upwelling jet. 相似文献
302.
303.
Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured . Exposure ages calculated from and ratios and constant relative production rates show a bias between the two ages due to variations in . Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the and production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe. 相似文献
304.
Determinations of40Ar/39Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give40Ar/39Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts.206Pb measured in these clasts is only ~1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the207Pb/206Pb model ages. The Pb data indicate that the initial207Pb/206Pb was no more than 0.08±0.07% higher than this ratio in Can?on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The40Ar/39Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating. 相似文献
305.
Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene)
included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted
in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis
diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional
fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic
and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times
chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic).
Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic).
Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment.
The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for
kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets,
and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a
different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis,
those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known
examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes.
Received: 27 January 1996 / Accepted: 5 May 1997 相似文献
306.
Using litter bag experiments in the Fraser River estuary in British Columbia, we tested for differences in the relative decomposition rates between leaves of purple loosestrife (Lythrum salicaria), an introduced exotic and a native sedge (Carex lyngbyei). The difference in the mean decay rate coefficient for the two species was significantly defferent (p<0.01) and the coefficient for purple loosestrife (0.0110 d?1) was nearly four times higher than for Lyngbyei's sedge (0.0028 d?1). This is the first estimate of the decay rate coefficient for purple loosestrife from an estuary. The rapid decay rate of loosestrife leaves suggests that they supply detritus to the ecosystem in autumn whereas the much slower decay rate of sedge implies that it supplies detritus throughout the winter and early spring. Consumer organisms important in juvenile salmon food webs appear to be adapted to take advantage of the detritus provided in these seasons. The findings have implications for habitat management because purple loosestrife has recently invaded estuaries of the northeast Pacific and may be outcompeting native sedges important in detrital-based food webs. 相似文献
307.
High inside corners at ridge-transform intersections 总被引:1,自引:0,他引:1
A large topographic high commonly occurs near the intersection of a rifted spreading center and a transform fault. The high occurs at the inside of the 90° bend in the plate boundary, and is called the high inside corner, while the area across the spreading center, the outside corner, is often anomalously low. To better understand the origin of this topographic asymmetry, we examine topographic maps of 53 ridge-transform intersections. We conclude the following: (1) High inside corners occur at 41 out of 42 ridge-transform intersections at slow spreading ridges, and thus should be considered characteristic and persistent features of rifted slow spreading ridges. They are conspicuously absent at fast spreading ridges or at spreading centers that lack a rift valley. (2) High inside corners occur wherever an axial rift valley is present, and an approximate 1:1 correlation exists between the relief of the rift valley and the magnitude of the asymmetry. (3) Large high inside corners occur at both long and short transform offsets. (4) High inside corners at long offsets decay off-axis faster than predicted by the square root of age cooling model, precluding a thermalisostatic origin, but consistent with dynamic or flexural uplift models.These observations support the existing hypothesis that the asymmetry is due to the contrast in lithospheric coupling that occurs in the active transform versus the inactive fracture zone. Active faulting in the transform breaks the lithosphere along a high angle fault, permitting vertical movement of the inside corner block, whereas the inactive fracture zone forms a weld that couples the outside corner to the adjacent block, preventing it from rising. Large asymmetry at very short transform offsets appears to be caused by the added effect of a second uplift mechanism. Young lithosphere in the rift valley couples to the older plate, and when it leaves the rift valley it lifts the older plate with it. At very short offsets, this coupled uplift acts upon the high inside corner; at long offsets, it may upwarp the older plate or its expression may be muted. 相似文献
308.
Don Maurer Richard T. Keck Jeff C. Tinsman Wayne A. Leathem 《Marine environmental research》1981,4(4):299-319
Benthic invertebrates have many characteristics which make them prime candidates for burial studies in dredged material. A major concern in dredging and disposal projects is the effect of burial on the survival of benthic invertebrates. The purpose of the research reported in this paper was to determine the ability of estuarine benthos—in particular three species of molluscs (Mercenaria mercenaria, Nucula proxima and Ilyanassa obsoleta)—to migrate vertically in natural and exotic sediments and to determine the survival of benthos when exposed to particular amounts of simulated dredged material.Mortalities generally increased with increased sediment depth, with increased burial time and with overlying sediments whose particle size distribution differed from that of the species native sediment. Temperature affected mortalities and vertical migration. It was concluded that vertical migration is a viable process which can significantly affect rehabilitation of a dredged disposal area. Under certain conditions, vertical migration should be considered, together with larval settling and immigration from outside impacted areas, as a mechanism of recruiting a dredgedump site. 相似文献
309.
David Petruska Jeff Geyer Rick Macon Michael Craig Alex Ran Neil Schulz 《Ocean Engineering》2005,32(7):767-782
The Mad dog project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar. As such, there were many challenges, which include:
- • Regulatory approval.
- • Designing a mooring system which is dominated by current loadings.
- • Largest polyester rope break load ever required.
- • Rope design qualification and testing.
- • Quality control and assurance.
- • Inspection, Maintenance, Repair and Retirement (IMRR) of such a mooring system.
310.
Richard T. Di Giulio Jeff V. Behar David B. Carlson Bruce M. Hasspieler David E. Watson 《Marine environmental research》1995,39(1-4)
Xenobiotic-mediated productions of reactive oxygen species, via enzymemediated redox cycling, have been implicated in a variety of toxicological phenomena including lipid peroxidation, enzyme inactivation and oxidative DNA damage leading to cancer. A comparison was undertaken of two benthic freshwater fish species that appear to differ markedly in their susceptibility to chemical carcinogenesis—the channel catfish (Ictalurus punctatus) and the more cancer-prone brown bullhead (Ameriurus nebulosus)-in terms of basic biochemical characteristics related to oxidative stress. This has included analysis of microsomal redox cycling of model xenobiotics (e.g. menadione) as well as antioxidant and other detoxifying enzymes in hepatic tissue of the two species. In addition, endpoints of oxidative stress, such as altered glutathione status and oxidative DNA damage, were examined. These studies have revealed numerous qualitative and quantitative differences between the two species both in terms of basal enzyme activities and in species response to model prooxidants. For example, bullhead appear to possess a greater capacity for microsomal redox cycling of xenobiotics, but have glutathione-dependent defense systems less able to withstand oxidative challenge. These and other interspecific differences have allowed for an improved understanding of the basic mechanisms which may underly species susceptibility to oxidative stress and critical manifestations such as cancer. 相似文献