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41.
The 1883 eruption of Augustine Volcano produced a tsunami when a debris avalanche traveled into the waters of Cook Inlet. Older debris avalanches and coeval paleotsunami deposits from sites around Cook Inlet record several older volcanic tsunamis. A debris avalanche into the sea on the west side of Augustine Island ca. 450 years ago produced a wave that affected areas 17 m above high tide on Augustine Island. A large volcanic tsunami was generated by a debris avalanche on the east side of Augustine Island ca. 1600 yr BP, and affected areas more than 7 m above high tide at distances of 80 km from the volcano on the Kenai Peninsula. A tsunami deposit dated to ca. 3600 yr BP is tentatively correlated with a southward directed collapse of the summit of Redoubt Volcano, although little is known about the magnitude of the tsunami. The 1600 yr BP tsunami from Augustine Volcano occurred about the same time as the collapse of the well-developed Kachemak culture in the southern Cook Inlet area, suggesting a link between volcanic tsunamis and prehistoric cultural changes in this region of Alaska. 相似文献
42.
Mark J. McCarthy Karen S. McNeal John W. Morse Wayne S. Gardner 《Estuaries and Coasts》2008,31(3):521-531
Bottom-water hypoxia effects on sediment–water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were
examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August
2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O2) and hydrogen sulfide (H2S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O2 demand and net N2 flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate
reduction to ammonium (DNRA) was measured using high-performance liquid chromatography. Sediment O2 penetration depths ranged from 5 to 10 mm. H2S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying
water or sediment in April. Sediment O2 demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of
\textNH\text4\text + {\text{NH}}_{\text{4}}^{\text{ + }} to overlying water during hypoxia but not during normoxia. Net N2 fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher
during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher
during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with
\text14 \textNH\text4\text + ^{{\text{14}}} {\text{NH}}_{\text{4}}^{\text{ + }} . DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that
N transformation pathways and rates change when bottom-water O2 concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results
indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between
episodic runoff events. 相似文献
43.
Geomorphology,acoustic backscatter,and processes in Santa Monica Bay from multibeam mapping 总被引:2,自引:0,他引:2
Santa Monica Bay was mapped in 1996 using a high-resolution multibeam system, providing the first substantial update of the submarine geomorphology since the initial compilation by Shepard and Emery [(1941) Geol. Soc. Amer. Spec. Paper 31]. The multibeam mapping generated not only high-resolution bathymetry, but also coregistered, calibrated acoustic backscatter at 95 kHz. The geomorphology has been subdivided into six provinces; shelf, marginal plateau, submarine canyon, basin slope, apron, and basin. The dimensions, gradients, and backscatter characteristics of each province is described and related to a combination of tectonics, climate, sea level, and sediment supply. Fluctuations of eustatic sea level have had a profound effect on the area; by periodically eroding the surface of Santa Monica plateau, extending the mouth of the Los Angeles River to various locations along the shelf break, and by connecting submarine canyons to rivers. A wetter glacial climate undoubtedly generated more sediment to the rivers that then transported the increased sediment load to the low-stand coastline and canyon heads. The trends of Santa Monica Canyon and several bathymetric highs suggest a complex tectonic stress field that has controlled the various segments. There is no geomorphic evidence to suggest Redondo Canyon is fault controlled. The San Pedro fault can be extended more than 30 km to the northwest by the alignment of a series of bathymetric highs and abrupt changes in direction of channel thalwegs. 相似文献
44.
We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H i Lyα emission line, but they are applicable to other spectral lines as well. 相似文献
45.
P. R. Vogt J. Gardner K. Crane E. Sundvor F. Bowles G. Cherkashev 《Geo-Marine Letters》1999,19(1-2):97-110
Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa
Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified
most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet
bulk density: 1400–1600 kg m-3; sound speed: 1480– 1505 m s-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m-1 deg C-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but
the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local. 相似文献
46.
Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano 总被引:2,自引:0,他引:2
Compositionally diverse dacitic magmas have erupted from Mount St Helens over the last 4000 years. Phase assemblages and their compositions in these dacites provide information about the composition of the pre-eruptive melt, the phases in equilibrium with that melt and the magmatic temperature. From this information pre-eruptive pressures and water fugacities of many of the dacites have been inferred. This was done by conducting hydrothermal experiments at 850°C and a range of pressures and water fugacities and combining the results with those from experiments at temperatures of 780 and 920°C, to cover the likely range in equilibration conditions of the dacites. Natural phase assemblages and compositions were compared with the experimental results to infer the most likely conditions for the magmas prior to eruption. Water contents disolved in the melts of the dacites were then estimated from the inferred conditions. Water contents in the dacites have varied greatly, from 3.7 to 6.5 wt.%, in the last 4000 years. Between 4000 and about 3000 years ago the dacites tended to be water saturated and contained 5.5 to 6.5 wt.% water. Since then, however, the dacites have been significantly water-undersaturated and contained less than 5.0 wt.% water. These dacites have tended to be hotter and more mafic, and andesitic and basaltic magmas have erupted. These changes can be explained by variable amounts of mixing between felsic dacite and basalt, to produce hotter, drier and more mafic dacites and andesites. The magma storage region of the dacitic magmas has also varied significantly during the 4000 years, with shifts to shallower levels in the crust occurring within very short time periods, possibly even two years. These shifts may be related to fracturing of overlying roof rock as a result of magma with-drawal during larger volume eruptions. 相似文献
47.
James E. Gardner Steve Carey Malcolm J. Rutherford Haraldur Sigurdsson 《Contributions to Mineralogy and Petrology》1995,119(2-3):224-238
Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO2) to felsic (65–67 wt% SiO2), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period. 相似文献
48.
49.
The magnitude of sheet flow in a small South Carolina salt marsh was evaluated by comparing the storage curve for the basin based on topographic data with that based on current measurements in the main channel. The results indicate that on spring tides more than 50% of the volume of water that enters the basin enters as sheet flow over the grassy flats of the marsh. The current data also suggest that the actual storage curve may be a hysteresis loop with separate flood and ebb segments. 相似文献
50.
James S. Gardner 《地球表面变化过程与地形》1983,8(4):347-355
Accretion of debris was monitored on six slopes in the Mt. Rae area of the Canadian Rocky Mountains for the period 1975–1982. Monitoring was accomplished by measuring annually the accumulated debris on fabric mats which were placed on the long profile of the slopes. Annual rates of accretion were extremely variable between points on the same slope, between slopes, and between years. The data do not show a systematic decrease in accretion downslope but indicate that an absence of accretion is more likely on the lower half of the slopes. In general, annual rates are comparable to previously published values from the Rockies and the maximum rates of 30 to 40 mm per year are comparable to those from Karkevagge in Scandinavia. Extremely high rates of accretion were all related to extreme events such as debris flows or full depth snow avalanches. The variation in accretion rates is not explained by single environmental factors such as lithology and slope exposure. 相似文献