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261.
Nancy G. Newberry Donald R. Peacor Eric J. Essene John W. Geissman 《Contributions to Mineralogy and Petrology》1982,80(4):334-340
Magnetite-ilmenite oxidation-exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy. Silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite. The average value in magnetite is 1.2 wt.% Si, equivalent to solid solution of 9 mole % Fe2SiO4. Silicon is also present in very small silicate inclusions and as unusual Si-rich domains of uncertain origin in magnetite. The inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe. Microprobe analyses for Si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution. The petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of Si in magnetite is better understood.Contribution No. 376 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan 相似文献
262.
Nancy E. Williams John A. Westgate D. Dudley Williams Anne Morgan Alan V. Morgan 《Quaternary Research》1981,16(2):146-166
Larval caddisfly, chironomid, and beetle remains have been recovered from the Pleistocene Scarborough Formation in the Toronto region of southern Ontario. Three stratigraphic levels were sampled at the northeastern end of the Scarborough Bluffs; the youngest horizon yielded 16 chironomid taxa, 33 caddisfly taxa, and 28 beetle taxa, whereas the two older levels yielded somewhat less diverse assemblages. Only one taxon in each of the caddisfly and chironomid groups was identified from the presumed correlative beds at Woodbridge, Ontario, but numerous beetle fragments were recovered, several of which have been specifically identified and match species found previously in the upper part of the Scarborough Formation. The youngest sampled assemblage in the Scarborough Formation at the northeastern end of the Scarborough Bluffs is interpreted as indicating cool climatic conditions in a boreal forest environment, given the present-day distributions and feeding habits of these river, lake, and terrestrial taxa. The mean July temperature at this time was probably about 15°C, as compared to the present-day value of 20.5°C. The forest was poorer in deciduous species during deposition of the older part of the formation as preserved here. These results agree well with previous interpretations based on plant remains. We suggest that both aquatic and terrestrial insects are good indicators of macroclimate. 相似文献
263.
Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf
of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late
summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement
peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more
affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement
ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement
ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae
were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables
on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered
cycles of settlement occur among years. 相似文献
264.
The importance of the El Niño-Southern Oscillation (ENSO) on regional-scale climate variability is well recognized although the associated effects on local weather patterns are poorly understood. Little work has addressed the ancillary impacts of climate variability at the community level, which require analysis at a local scale. In coastal communities water quality and public health effects are of particular interest. Here we describe the historical influence of ENSO events on coastal water quality in Tampa Bay, Florida (USA) as a test case. Using approximate randomized statistics, we show significant ENSO influences on water quality particularly during winter months, with significantly greater fecal pollution levels during strong El Niño winters and significantly lower levels during strong La Niña winters as compared to neutral conditions. Similar significant patterns were also noted for El Niño and La Niña fall periods. The success of the analysis demonstrates the feasibility of assessing local effects associated with large-scale climate variability. It also highlights the possibility of using ENSO forecasts to predict periods of poor coastal water quality in urban region which local agencies may use to make appropriate prepations. 相似文献
265.
Nancy E. Williams 《Journal of Paleolimnology》1988,1(4):241-248
The use of assemblages of caddisflies (Trichoptera) in paleoecology has been explored over the past ca. 12 years. During this time, sites in North America, Great Britain and Europe have been studied and progress has been made in both the mechanics of identification and in the understanding of factors relevant to the interpretation of assemblages of this wholly aquatic group. Quaternary caddisfly fossils are abundant and usually well-preserved in waterlaid sediments. Individual larval sclerites can be identified by reference to shapes, textures, colour patterns muscle scar patterns and setal distributions. The flat frontoclypeus is particularly easily identified.Study of the biological and distributional data relevant to a caddisfly assemblage yields information at two levels. First the probable local habitat and second the climate can be described. This information is derived from both modern collections and the literature as the morphology of species, and hence it is assumed their environmental requirements, have not changed during the Quaternary. Comparisons with fossil assemblages of other plant and animal groups suggest that there are important factors to be considered in interpreting caddisfly assemblages, particularly those from sediments deposited during cold periods: for example, caddisflies may be slower to migrate than certain terrestrial insects, in particular the beetles (Coleoptera), and some glacial assemblages may therefore be dominated by hangers on. Lotic and lentic species also may migrate at different rates.Although caddisflies have not yet been fully exploited as paleoecological indicators, they have already contributed to our understanding of past environments and warrant much greater use. 相似文献
266.
Rick A. Hagen Nancy A. Baker David F. Naar R. N. Hey 《Marine Geophysical Researches》1990,12(4):297-315
SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by
recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km
to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated
400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km2 and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar
to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation.
Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km2. The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby
island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter
indicates that they are also the products of relatively recent submarine volcanic activity.
The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the
various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of
Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion
of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate. 相似文献
267.
Nancy ArandaCirerol Jorge A. Herrera-Silveira Francisco A. Comín 《Estuarine, Coastal and Shelf Science》2006,68(3-4):445
This work is the beginning of a coastal water quality monitoring program to establish the baseline for the implementation of an integrated coastal management of the Yucatán Peninsula tropical ecosystem. Coastal water quality is affected by the increasing economic development. This area has no rivers because of its karst geomorphology, and the coastal freshwater comes from springs or seeps. Coastal water quality was studied in four towns from January to December 2000. Statistically significant differences among water quality variables and processes are discussed. Along with groundwater discharge, domestic and shrimp farming sewage are the main sources of nutrients, predominantly of nitrogen and silica. Salinity dilution is used to estimate the groundwater fraction that influences each area in northwestern Yucatán. 相似文献
268.
269.
Photometry obtained in 1973 on the uvby system yields high-precision rotational light curves for Io, Europa, and Ganymede at a mean phase angle of ~6°. By combining our observations with photometry obtained by others over a broader range of phase angle, we alsi derive improved values for the phase coefficients and opposition surges of the four Galilean satellites. The values of V(1, 0) obtained by linear extrapolation to zero phase are accurate to ±0.03 magnitudes. We also derive the colors of the sun of the uvby system and use these to obtain albedos of the satellites in four colors. 相似文献
270.
The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal-liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal-silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron-wüstite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor. 相似文献