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At least 1,657 records · Page 92Linked to original sources
USGS2012

Characterization of tropospheric desert aerosols at solar wavelengths by multispectral radiometry from Landsat

Characteristics of tropospheric desert aerosols are derived by comparing nadir spectral reflectivities computed from the radiative transfer models with reflectivities measured from Landsat. Over the ocean, reflectivities are compared, but over land the comparison is carried out by determining the ratios of the nadir reflectivity of the surface-atmosphere system over heavy aerosol concentration to the reflectivity of the underlying surface. This remote sensing technique is found to be a sensitive approach for measuring n 2 , the imaginary part of the refractive index. The desert aerosols under study, in the Iran and Pakistan area, are essentially pure scatterers, inasmuch as an n 2 value of 0.001±0.001 was determined for each of the four Landsat spectral bands, that is, for a spectral interval from 0.5 to 1.1 μm.

Journal of Geophysical Research C: Oceans↗
USGS2012

Measurements of storm-generated bottom stresses on the continental shelf

Large values of bottom friction velocity, u *, and roughness length, z 0 , determined from burst-averaged speed data taken on the continental shelf in outer Norton Sound, Alaska, with the GEOPROBE tripod during a storm in September 1977 are correlated with extremely large values of near-bottom concentration of total suspended particulate matter (TSM). Combined wind-driven and tidal currents exceeding 30 cm/s at 1 m above the bottom and intense oscillatory bottom currents with maxima above 45 cm/s were associated with the largest measured values of TSM at 2 m above the sea floor. The values of u * and z 0 obtained from the ‘law of the wall’ velocity-depth relationship are diminished substantially throughout the storm period (average reduction of 44%) when the turbulence reducing effects of the vertical concentration gradient of TSM are considered. The form of the latter correction was adapted from Smith and McLean (1977 a ). Values of the mean u * computed from the theory of Grant and Madsen (1979), which predicts an enhanced shear stress due to nonlinear wave-current interactions, compare favorably with the u * values determined from the measured velocity profiles. The measured values of z 0 , however, are considerably larger than any of the estimates based on (1) the actual scales of the physical roughness elements; (2) the apparent roughness of Grant and Madsen (1979); or (3) the thickness of the bed-load layer as formulated by Smith and McLean (1977 a ).

Journal of Geophysical Research - Oceans↗
USGS2012

Water-soluble material on aerosols collected within volcanic eruption clouds

In February and March of 1978, filter samplers mounted on an aircraft were used to collect the aerosol fraction of the eruption clouds from three active Guatemalan volcanoes (Fuego, Pacaya, and Santiaguito). The samples were collected on Teflon (Fluoropore) filters with a nominal pore diameter of 0.5μm. The mass of air sampled by the filters ranged from 0.15 to 6.6 kg. The particulate material collected consisted of fragments of angular silicate ash and droplets of what is interpreted as dilute H 2 SO 4 and HCl. After collection of the samples, each filter was rinsed with 60 ml of distilled-deionized water. Splits of each extract were centrifuged to remove particles greater than or equal to 0.1 μm in diameter, acidified, and analyzed for B, Ba, Be, Ca, Cd, Co, Cu, Fe, Li, Mg, Mn, Mo, Na, Pb, Si, Sr, V, and Zn by inductively coupled plasma—optical emission spectroscopy. Separate splits were analyzed for F and Cl by specific-ion-electrode methods and for U by a fission track technique. The elements dissolved in the aqueous extracts represent components of water-soluble material either formed directly in the eruption cloud or derived from interaction of ash particles and aerosol components of the plume. Calculations of enrichment factors, based upon concentration ratios, showed the elements most enriched in the extracts relative to bulk ash composition were Cd, Cu, V, F, Cl, Zn, and Pb. These elements represent a subset (with the addition of Cl and F) of elements previously reported enriched in atmospheric aerosols in remote regions as well as in volcanic areas. This suggests that some of the enriched elements were widely dispersed as volatile halides emitted from a volcanic source.

Journal of Geophysical Research - Oceans↗
USGS2012

Seasat synthetic aperture radar ( SAR) response to lowland vegetation types in eastern Maryland and Virginia

Examination of SEASAT SAR images of eastern Maryland and Virginia reveals botanical distinctions between vegetated lowland areas and adjacent upland areas. Radar returns from the lowland areas can be either brighter or darker than returns from the upland forests. Scattering models and scatterometer measurements predict an increase of 6 dB in backscatter from vegetation over standing water. This agrees with the SCWigital number (DN) increase observed in the digital SEASAT data. The brightest areas in the Chickahominy, Virginia, drainage, containing P. virginica about 0.4 m high, contrast with the brightest areas in the Blackwater, Maryland, marshes, which contain mature loblolly pine in standing water. The darkest vegetated area in the Chickahominy drainage contains a forest of Nyssa aquatica (water tupelo) about 18 m high, while the darkest vegetated area in the Blackwater marshes contains the marsh plant Spartina alterniflora, 0.3 m high. The density, morphology, and relative geometry of the lowland vegetation with respect to standing water can all affect the strength of the return L band signal.

Journal of Geophysical Research - Oceans↗
USGS2012

Sea ice melting in the marginal ice zone

The heat and salt flux boundary conditions together with the freezing curve relationship are a necessary component of any ice-sea water thermodynamic model. A neutral two-layer oceanic planetary boundary layer model that incorporates these boundary conditions gives the following results: The interfacial salinity is within 10% of the far-field salinity for conditions commonly encountered in the MIZ and depends only on the turbulent Lewis number and the far-field temperature and salinity. The predicted melt rates agree with the limited field observations, of the order of 1 m day −1 . The Obukov lengths, determined from the predicted interface conditions and melt rates, are generally much greater than the Ekman layer thicknesses; hence, the surface buoyancy flux has little effect on the turbulence in the planetary boundary layer.

Journal of Geophysical Research - Oceans↗
USGS2012

Gas exchange rates across the sediment-water and air-water interfaces in south San Francisco Bay

Radon 222 concentrations in the water and sedimentary columns and radon exchange rates across the sediment-water and air-water interfaces have been measured in a section of south San Francisco Bay. Two independent methods have been used to determine sediment-water exchange rates, and the annual averages of these methods agree within the uncertainty of the determinations, about 20%. The annual average of benthic fluxes from shoal areas is nearly a factor of 2 greater than fluxes from the channel areas. Fluxes from the shoal and channel areas exceed those expected from simple molecular diffusion by factors of 4 and 2, respectively, apparently due to macrofaunal irrigation. Values of the gas transfer coefficient for radon exchange across the air-water interface were determined by constructing a radon mass balance for the water column and by direct measurement using floating chambers. The chamber method appears to yield results which are too high. Transfer coefficients computed using the mass balance method range from 0.4 m/day to 1.8 m/day, with a 6-year average of 1.0 m/day. Gas exchange is linearly dependent upon wind speed over a wind speed range of 3.2–6.4 m/s, but shows no dependence upon current velocity. Gas transfer coefficients predicted from an empirical relationship between gas exchange rates and wind speed observed in lakes and the oceans are within 30% of the coefficients determined from the radon mass balance and are considerably more accurate than coefficients predicted from theoretical gas exchange models.

California↗
USGS2012

Wave energy saturation on a natural beach of variable slope

Time series of flow were measured across the inner surf zone during a storm. These data were used to quantify the dependence of wave height (transformed from measured flow) and velocity on local slope and depth. Similar to previous studies, as incident waves broke and propagated into the surf zone, wave energy became saturated, and wave height was strongly dependent on depth. However, the ratio of rms wave height to local depth (γ rms ) was found not to be constant but to vary between 0.29 and 0.55; γ rms increased with local slope and was independent of deepwater wave steepness. Thus the surf zone similarity parameter (the ratio of slope to the square root of steepness) did not adequately parameterize γ rms .

Journal of Geophysical Research - Oceans↗
USGS2012

Storm‐dominated bottom boundary layer dynamics on the Northern California Continental Shelf: Measurements and predictions

Measurements of near‐bottom velocity profiles in 85 m water depth during a storm on the continental shelf off northern California using the GEOPROBE tripod in December 1979 provided estimates of shear velocities, , and roughness lengths, , when the near‐bottom velocity profiles were logarithmic. These estimates agree within 90% confidence intervals with values computed from a simple near‐bottom combined wave‐current model that includes movable bed effects. The reasonably good comparison between model and profile estimates of suggests that such models can be used to predict bed shear stresses (or shear velocities) under combined flows of waves and currents typical of stormy conditions on continental shelves if stratification corrections to the velocity profile resulting from suspended sediment are small. The repeated occurrences of storms of similar intensities and wind velocities during the winter months off northern California suggest that the resultant high bottom stresses due to the combined effects of waves and currents are major factors in controlling the distribution of surficial sediment on the central portion of the northern California shelf.

California↗
USGS2012

Infragravity waves over a natural barred profile

Measurements of cross-shore flow were made across the surf zone during a storm as a nearshore bar became better developed and migrated offshore. Measured infragravity band spectra were compared to synthetic spectra calculated numerically over the natural barred profile assuming a white run-up spectrum of leaky mode or high-mode edge waves. As in earlier studies, the spectra compared closely; however, for some frequencies the energy of the measured spectrum exceeded the energy of the synthetic spectrum, suggesting that the run-up spectrum was not white but had dominating frequencies. Utilizing cross-shore flow data and synthetic spectra from a number of cross-shore locations, an equivalent run-up spectrum was calculated for each day. On the first day of the storm, the equivalent run-up spectrum indicated a dominant wave that had a node in velocity reasonably close to the bar crest. Later during the storm, when the bar had migrated farther offshore, there was no evidence for a dominant motion having a velocity node at the bar crest. The structure of the equivalent run-up spectrum compared well with spectra of direct measurements of run-up obtained several hundred meters away. We have no clear evidence in support of the theory that infragravity waves might form or force the offshore migration of a bar. To confirm this finding, longer records obtained synoptically over a developing bar are required. The dominant wave observed early in the storm was consistent with Symond and Bowen's (1984) theoretical prediction of resonant amplification of discrete frequencies over a barred profile.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deuterium in interstitial water from deep-sea cores

As part of the Joint Oceanographic Institutions Deep Earth Sampling project, the interstitial waters of cores from 69 holes were sampled for deuterium analysis. Sixteen of the cores penetrated sediments as old as Eocene, and several sampled Cretaceous sediments, which allowed us to examine changes in the deuterium content of the oceans with time. Deuterium is shown to be a conservative constituent of the pore water. Its abundance in the pore fluids can be changed by diffusion, but the rate is slow, and corrections for this effect have been made. Changes in the abundance of deuterium can be related to changes in the amount of ice stored in continental glaciers, inasmuch as precipitation in the form of snow is highly depleted in deuterium compared with the oceans. Many of the cores show a change in isotopic composition of samples from early to late Miocene that can be ascribed to the buildup of the Antarctic ice sheets. After correcting for the role of diffusion in reducing the isotopic contrast between samples from a single core, we estimate an increase of 10 per mil (‰) δD (corresponding to a° δ 18 O change of about 1.2‰) between the early and late Miocene. A similiar analysis of Pleistocene to Holocene changes indicates a δD rise of 8‰ during the time of maximum continental ice, which corresponds to a δ 18 O increase of about 1.0‰. On the basis of limited data, we find no δD change in the oceans from Cretaceous to Miocene.

Journal of Geophysical Research Solid Earth↗
USGS2012

The structure of subtidal currents within and around Lydonia Canyon: Evidence for enhanced cross-shelf fluctuations over the mouth of the canyon

Between October 1980 and April 1981, currents were measured within Lydonia Canyon and on the adjacent shelf and slope. The amplitude of the subtidal currents over the shelf and slope ranged between 10 and 30 cm s −1 , but within the canyon, they were typically smaller than 5 cm s −1 . The subtidal currents had well-defined spatial structures over the shelf and the slope and in the middle and outer portions of the canyon. The along-isobath flow over the shelf and slope was unaltered by the canyon. Currents within the canyon and just above it were driven up and down the canyon by the cross-shelf pressure gradient in geostrophic equilibrium with the along-shelf flow. The measurements suggest that the Coriolis force on the cross-canyon flow, turbulent Reynolds stresses, and acceleration of the along-canyon flow balanced the imposed pressure gradient for flow near the rim of Lydonia Canyon. The Coriolis force was not important in the deeper portions of the canyon, where baroclinic adjustments of the density field began to be an important factor in the momentum balance. A simple model indicates that the magnitude of the horizontal turbulent viscosity coefficient for subtidal flow in this narrow canyon is 10 6 to 10 7 cm 2 s −1 . The mixing indicated by the large amplitude of the viscosity coefficient was probably caused by the strong tidal currents present within Lydonia Canyon. On the shelf, along-isobath currents were locally driven by the large-scale component of the wind field; along-shelf currents were equally correlated with local winds and with winds from sites 700 km northeast of Lydonia Canyon. Wind stress was not correlated with currents over the slope in water deeper than 450 m or with currents within the canyon.

Journal of Geophysical Research - Oceans↗
USGS2012

Effect of wave-current interaction on wind-driven circulation in narrow, shallow embayments

The effect of wind waves on the steady wind-driven circulation in a narrow, shallow bay is investigated with a two-dimensional ( y , z ) circulation model and the Grant and Madsen [1979] bottom-boundary layer model, which includes wave-current interaction. A constant wind stress is applied in the along-channel x direction to a channel with a constant cross-sectional profile h ( y ). The wind-induced flushing of shallow bays is shown to be sensitive to both the shape of the cross section and the effects of surface waves. The flushing increases with increasing , where h ′ is the standard deviation of cross-channel depth and is the mean depth. This is consistent with the findings of Hearn et al. [1987]. The flushing decreases, however, with the inclusion of surface wave effects which act to increase the bottom drag felt by the currents. Increasing effective bottom friction reduces the strength of the circulation, while the along-bay surface slope, bottom stress and the structure of current profiles remain nearly unchanged. An implication of the circulation dependence on wave-current interaction is that low-frequency oscillatory winds may drive a mean circulation when the wave field changes with wind direction. x

Journal of Geophysical Research C: Oceans↗
USGS2012

Sulfur dioxide and particles in quiescent volcanic plumes from Poás, Arenal, and Colima Volcanos, Costa Rica and Mexico

Measurements of SO 2 emission rates and concentrations and of particle distribution, size, shape, and composition were made in quiescent volcanic plumes emitted into the troposphere from Poás and Arenal volcanos, Costa Rica, and Colima volcano, Mexico. SO 2 emission rates were 700±180 metric tons per day (t/d) for Poás, 210±30 t/d for Arenal, and 320±50 t/d for Colima. The concentrations of SO 2 calculated from the COSPEC/lidar data were 5–380 ppb. Concentrations of SO 2 measured directly by flame photometry were 10–250 ppb. Particles collected in the plumes with a quartz crystal microbalance impactor were mostly less than 3 μm in diameter and consisted of droplets of dilute sulfur-bearing solutions and minor amounts of larger silicate particles coated with a sulfur-bearing film or crust. Total particle concentrations were 4.7 μg/m 3 for Poás and 18.8 μg/m 3 for Colima. Comparison of concentrations of SO 2 in the plumes with gas samples collected at fumaroles on the ground suggests that the plumes are diluted by the atmosphere by factors of up to 10 5 .

Journal of Geophysical Research Atmospheres↗
USGS2010

Influence of tidal range on the stability of coastal marshland

Early comparisons between rates of vertical accretion and sea level rise across marshes in different tidal ranges inspired a paradigm that marshes in high tidal range environments are more resilient to sea level rise than marshes in low tidal range environments. We use field‐based observations to propose a relationship between vegetation growth and tidal range and to adapt two numerical models of marsh evolution to explicitly consider the effect of tidal range on the response of the marsh platform channel network system to accelerating rates of sea level rise. We find that the stability of both the channel network and vegetated platform increases with increasing tidal range. Our results support earlier hypotheses that suggest enhanced stability can be directly attributable to a vegetation growth range that expands with tidal range. Accretion rates equilibrate to the rate of sea level rise in all experiments regardless of tidal range, suggesting that comparisons between accretion rate and tidal range will not likely produce a significant relationship. Therefore, our model results offer an explanation to widely inconsistent field‐based attempts to quantify this relationship while still supporting the long‐held paradigm that high tidal range marshes are indeed more stable.

Journal of Geophysical Research↗
USGS1987

Role of geophysics in identifying and characterizing sites for high-level nuclear waste repositories.

Evaluation of potential high-level nuclear waste repository sites is an area where geophysical capabilities and limitations may significantly impact a major governmental program. Since there is concern that extensive exploratory drilling might degrade most potential disposal sites, geophysical methods become crucial as the only nondestructive means to examine large volumes of rock in three dimensions. Characterization of potential sites requires geophysicists to alter their usual mode of thinking: no longer are anomalies being sought, as in mineral exploration, but rather their absence. Thus the size of features that might go undetected by a particular method take on new significance. Legal and regulatory considerations that stem from this different outlook, most notably the requirements of quality assurance (necessary for any data used in support of a repository license application), are forcing changes in the manner in which geophysicists collect and document their data. -Authors

Journal of Geophysical Research↗
USGS1991

Crustal structure of the western New England Appalachians and the Adirondack Mountains

Presents an interpretation of the crustal velocity structure based on a seismic refraction/wide-angle reflection experiment in eastern North America extending from the Adirondacks in New York State through the northern Appalachians in Vermont and New Hampshire to central Maine. Modeling of the eastern portion of the profile within the New England Appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5.5 to 6.2 km/s, a midcrustal velocity of 6.4 km/s, and a lower crustal velocity of approximately 6.8 km/s. Crustal thickness increases from 36 km beneath Maine to 40 km in Vermont. This experiment provides further evidence for the obduction of the allochthonous western Appalachian units onto Grenvillian crust above the azone of detachment that penetrates at least to midcrustal depths and was the locus of successive Paleozoic thrusting. -from Authors

Journal of Geophysical Research↗
USGS1996

Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

Wide-angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at-15-18 km depth is marked by a strong wide-angle reflector, beneath which lies an 8- to 10-km-thick lower crust with an average velocity of 6.75??0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide-angle reflections define two layers within the lower crust, with velocities of 6.4-6.6 km/s and 6.9-7.3 km/s. Prominent subhorizontal reflectivity observed at near-vertical incidence resides principally in the lowermost layer, the top of which corresponds to the "6-s reflector" of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6-s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide-angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40-50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower-crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15-km-thick layer of velocity 6.0-6.2 km/s overlying a 5-km-thick, high-velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc-derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike-slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma. Copyright 1996 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm-Nd two-mineral "isochrons" yield apparent ages for petrographically identical rocks believed to be coeval ranging from -0 to 113 ?? 26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr-diopside group isotopic compositions do not overlap with those of the host basalts. Copyright 1997 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗