Abrupt transitions during sustained explosive eruptions: examples from the 1912 eruption of Novarupta, Alaska     

Nancy K. Adams  Bruce F. Houghton  Wes Hildreth 《Bulletin of Volcanology》2006,69(2):189-206

Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 10⁷–10⁹ cm^–3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h.  相似文献   

Variability of Sea-Ice Extent in Baffin Bay over the Last Millennium     

Nancy S. Grumet  Cameron P. Wake  Paul A. Mayewski  Gregory A. Zielinski  Sallie I. Whitlow  Roy M. Koerner  David A. Fisher  James M. Woollett 《Climatic change》2001,49(1-2):129-145

Comparison of an ice core glaciochemical time-series developed from thePenny Ice Cap (PIC), Baffin Island and monthly sea-ice extent reveals astatisticallysignificant inverse relationship between changes in Baffin Bay spring sea-iceextent andPenny Ice Cap sea-salt concentrations for the period 1901–1990 AD.Empiricalorthogonal function analysis demonstrates the joint behavior between changesin PICsea-salt concentrations, sea-ice extent, and changes in North Atlanticatmosphericcirculation. Our results suggest that sea-salt concentrations in snowpreserved on thePIC reflect local to regional springtime sea-ice coverage. The PIC sea-saltrecord/sea-ice relationship is further supported by decadal and century scalecomparisonwith other paleoclimate records of eastern Arctic climate change over the last700 years. Our sea-salt record suggests that, while the turn of the century wascharacterized bygenerally milder sea-ice conditions in Baffin Bay, the last few decades ofsea-ice extentlie within Little Ice Age variability and correspond to instrumental recordsof lowertemperatures in the Eastern Canadian Arctic over the past three decades.  相似文献   

Book reviews     

Nancy N. Rabalais  Gary R. Gaston  John W. Day  L. Harold Stevenson  James S. Latimer 《Estuaries and Coasts》1997,20(3):655-658

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Origin and significance of high nickel and chromium concentrations in Pliocene lignite of the Kosovo Basin, Serbia     

Leslie Ruppert  Robert Finkelman  Emilija Boti  Milan Milosavljevic  Susan Tewalt  Nancy Simon  Frank Dulong 《International Journal of Coal Geology》1996,29(4):235-258

Trace element data from 59 Pliocene lignite cores from the lignite field in the Kosovo Basin, southern Serbia, show localized enrichment of Ni and Cr (33–304 ppm and 8–176 ppm, respectively, whole-coal basis). Concentrations of both elements decrease from the western and southern boundaries of the lignite field. Low-temperature ash and polished coal pellets of selected bench and whole-coal samples were analyzed by X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray analyses. These analyses show that most of the Ni and Cr are incorporated in detrital and, to a lesser degree, in authigenic minerals. The Ni- and Cr-bearing detrital minerals include oxides, chromites, serpentine-group minerals and rare mixed-layer clays. Possible authigenic minerals include Ni---Fe sulfates and sulfides. Analyses of three lignite samples by a supercritical fluid extraction technique indicate that some (1–11%) of the Ni is organically bound. Ni- and Cr-bearing oxides, mixed-layer clays, chromites and serpentine-group minerals were also identified in weathered and fresh samples of laterite developed on serpentinized Paleozoic peridotite at the nearby Glavica and ikatovo Ni mines. These mines are located along the western and northwestern rim, respectively, of the Kosovo Basin, where Ni contents are highest. The detrital Ni- and Cr-bearing minerals identified in lignite samples from the western part of the Kosovo Basin may have been transported into the paleoswamp by rivers that drained the two Paleocene laterites. Some Ni may have been transported directly into the paleoswamp in solution or, alternatively, Ni may have been leached from detrital minerals by acidic peat water and adsorbed onto organic matter and included into authigenic mineral phases. No minable source of Ni and Cr is known in the southern part of the lignite field; however, the mineral and chemical data from the lignite and associated rocks suggest that such a source area may exist.  相似文献   

The effect of oxygen vacancies and aluminium substitution on the high-pressure properties of brownmillerite-structured Ca₂Fe_2?xAl_xO₅     

Carine B. Vanpeteghem  Ross J. Angel  Jing Zhao  Nancy L. Ross  Günther J. Redhammer  Friedrich Seifert 《Physics and Chemistry of Minerals》2008,35(9):493-504

The structural evolution with pressure and the equations of state of three members of the brownmillerite solid solution, Ca₂(Fe_2−x Al _x )O₅, have been determined by single-crystal X-ray diffraction up to a maximum pressure of 9.73 GPa. The compositions of the samples were x = 0.00 and x = 0.37 (with Pnma symmetry) and x = 0.55 (with I2mb symmetry). No phase transitions were observed in the experiments. The equation of state parameters determined from the pressure-volume data are K _0T = 128.0 (7) GPa, K′₀ = 5.8 (3) for the sample with x = 0.00, K _0T = 131 (2) GPa, K′₀ = 5.5 (4) for x = 0.37, and K _0T = 137.5 (6) GPa, K′₀ = 4 for x = 0.55. The bulk modulus therefore increases with Al content, being 11% higher in the x = 0.55 sample than in the Al-free sample. The unit-cell compression is anisotropic, with the c-axis being stiffer than a or b, and the anisotropy increases with increasing Al content of the structure. The structural response to pressure of all samples is similar. The (Al,Fe)O₄ tetrahedra and the (Al,Fe)O₆ octahedra undergo approximately isotropic compression. There is an increase in the twists of the chains of corner-sharing (Al,Fe)O₄ tetrahedra, and an increase in the tilts of the (Al,Fe)O₆ octahedra, because these framework polyhedra are stiffer than the Ca–O bonds to the extra-framework Ca site. The alignment of the two shortest Ca–O bonds sub-parallel to [001] accounts for the relative stiffness of the c-axis and thus the elastic anisotropy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Fragile empowerment: The dynamic cultural economy of British drum and bass music     

Alistair Fraser  Nancy Ettlinger   《Geoforum》2008,39(5):1647-1656

This paper discusses the dynamic cultural economy of British drum and bass (D&B) music, which emerged out of Britain’s rave culture in the early 1990s. We suggest that D&B offers insight into more general issues regarding the relation between alternative cultural economies and capitalism. We examine relations between D&B and the mainstream capitalist economy and argue that D&B calls attention to the possibility for alternatives to conventional capitalist relations to survive and possibly thrive without pursuing separation from capitalism. We also theorize D&B as a vehicle towards empowerment regarding the industry segment vis-à-vis the mainstream music industry and also regarding D&B’s practitioners, many of whom can be understood as marginalized discursively and/or materially. However, D&B empowerment is fragile, due in part to technological changes that threaten practices that have helped cultivate innovativeness as well as communal relations. The empowerment of alternative practices is fragile not only for D&B as an industry segment, but also from the vantage point of internal power relations – notably with respect to differences along axes of gender and generation/age. Our conclusions indicate the broader significance of the paper for critical social theory and propose how new research might build on our dynamic view of D&B’s cultural economy.  相似文献   

Conservation Opportunity and Risk Mapping for Carnivores Using Landowner Survey Data from the Greater Yellowstone Ecosystem     

Andra Bontrager  Heidi Kretser  Kirsten Leong  Nancy Connelly 《The Professional geographer》2017,69(2):225-238

Large landscape planning for wildlife corridors often requires inclusion of private lands and willing landowners to establish successful pathways to and from protected core habitats. Using mail survey data, we spatially assessed carnivore occurrences, perceptions of carnivores, and landowner preferences toward conservation planning tools from three communities to quantify conservation opportunity and risk related to carnivore movement on the landscape. The mapping of social data illustrates the importance of understanding individuals for conservation planning. The approach has the potential to identify areas that pose risks or present opportunities for the implementation of on-the-ground conservation actions to facilitate long-term wildlife movement across private lands.  相似文献   

Discussion of "Ground Water Monitoring Statistics Update: Parts I & II," by Charles B. Davis and Roger J. McNichols, Ground Water Monitoring & Remediation, v. 14, no. 4, pages 148–75, Fall 1994.     

Nancy Alyanak 《Ground Water Monitoring & Remediation》1995,15(2):184-187

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Resurgence of submersed aquatic macrophytes in the tidal Potomac River,Maryland, Virginia,and the District of Columbia     

Virginia Carter  Nancy Rybicki 《Estuaries and Coasts》1986,9(4):368-375

A 1978–81 survey of submersed aquatic macrophytes in the tidal Potomac River showed that there were virtually no plants in the freshwater tidal river between Chain Bridge and Quantico, Virginia, decades after the disappearance of plants in the late 1930’s. Plant populations were monitored in subsequent years (1983–85) using qualitative shoreline surveys and quantitative resampling of the original 1978–81 transects. In 1983, 12 species of submersed aquatic macrophytes were found in the tidal river. Population increases were dramatic; by fall 1985, plants had colonized all shallow areas between Alexandria and Gunston Cove, Virginia.Hydrilla verticillata dominated in Dyke Marsh-Hunting Creek and Swan Creek. Most other areas contained a variable mixture ofHeteranthera dubia, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria americana, Najas guadalupensis andHydrilla verticillata. No plants were found along the main river or in tidal embayments in the reach between Gunston Cove and Quantico, Virginia. Total dry weight collected in the upper tidal river in fall 1985 was 14.5 times that of spring 1985, and four times that of fall 1984.  相似文献   

The Mg2GeO4 olivine-spinel phase transition     

Nancy L. Ross  Alexandra Navrotsky 《Physics and Chemistry of Minerals》1987,14(5):473-481

Enthalpies and entropies of transition for the Mg₂GeO₄ olivine-spinel transformation have been determined from self-consistency analyses of Dachille and Roy's (1960), Hensen's (1977) and Shiota et al.'s (1981) phase boundary studies. When all three data sets are analyzed simultaneously,ΔH ₉₇₃ andΔS ₉₇₃ are constrained between ?14000 to ?15300 J mol^?1 and ?13.0 to ?14.1·J mol^?1 K^?1, respectively. High-temperature solution calorimetric experiments completed on both polymorpha yield a value of ?14046±1366 J mol^?1 forΔH ₉₇₃. Kieffer-type lattice vibrational models of Mg₂GeO₄ olivine and spinel based on newly-measured infrared and Raman spectra predict a value of ?13.3±0.6 J mol^?1 K^?1 forΔS ₁₀₀₀. The excellent agreement between these three independent determinations ofΔH andΔS suggests that the synthesis runs of Shiota et al. (1981) at high pressures and temperatures bracket equilibrium conditions. In addition, no configurational disorder of Mg and Ge was needed to obtain the consistent parameters quoted. The Raman spectrum and X-ray diffractogram show that little disorder, if any, is present in Mg₂GeO₄ spinel synthesized at 0.2 GPa and 973–1048 K.  相似文献