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101.
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
102.
Measurement of sediment resuspension rates in Long Island Sound 总被引:1,自引:0,他引:1
Resuspension rates have been measured on Long Island Sound sediments (New York/Connecticut, USA) under an oscillating grid,
using a flume, and in place, using high frequency acoustics, transmissometers, and sediment traps. Despite differences, the
results compare favorably. The methods all provide an order-of-magnitude estimate of the resuspension rate and increases under
storm conditions can be estimated. 相似文献
103.
Larry P. Atkinson Jackson O. Blanton Charles McClain Thomas N. Lee Masayuki Takahashi Takashi Ishimaru Joji Ishizaka 《Journal of Oceanography》1987,43(2):89-103
Hydrographic observations between the Izu Peninsula and Oshima Island, Japan, in May 1982, showed upwelling around the tip of the Peninsula and possibly also in the lee of Oshima Island. The upwelling introduced water as cold as 18 C with nitrate concentrations of 3M to the surface. Temperature/salinity analyses indicated that the upwelled water was of Kuroshio characteristics. Slightly fresher water was advected out of Sagami Bay in a coastal counter current.Contribution number 470 from the Shimoda Marine Research Center, University of Tsukuba. 相似文献
104.
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Lori E. Fenstermacher Gregory B. Crawford Jeffry C. Borgeld Tristan Britt Douglas A. George Meredith A. Klein Neal W. Driscoll Larry A. Mayer 《Marine Georesources & Geotechnology》2001,19(2):135-145
Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16-24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars ( Dendraster excentricus ) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars. The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the nearshore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8-15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance. 相似文献
107.
Laura S. L. Kong Robert S. Detrick Paul J. Fox Larry A. Mayer W. B. F. Ryan 《Marine Geophysical Researches》1988,10(1-2):59-90
High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long
portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift
valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or
discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that
extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated
by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal
activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that
are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted.
This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells
overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and
may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite
the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor.
Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less
than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the
rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely
intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor
features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity
within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading
cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production
rates. 相似文献
108.
Leslie Hebb Larry Petro Holland C. Ford David R. Ardila Ignacio Toledo Dante Minniti David A. Golimowski Mark Clampin 《Monthly notices of the Royal Astronomical Society》2007,379(1):63-72
We present high-cadence, high-precision multiband photometry of the young, M1Ve, debris disc star, AU Microscopii. The data were obtained in three continuum filters spanning a wavelength range from 4500 to 6600 Å, plus Hα, over 28 nights in 2005. The light curves show intrinsic stellar variability due to star-spots with an amplitude in the blue band of 0.051 mag and a period of 4.847 d. In addition, three large flares were detected in the data which all occur near the minimum brightness of the star. We remove the intrinsic stellar variability and combine the light curves of all the filters in order to search for transits by possible planetary companions orbiting in the plane of the nearly edge-on debris disc. The combined final light curve has a sampling of 0.35 min and a standard deviation of 6.8 mmag. We performed Monte Carlo simulations by adding fake transits to the observed light curve and find with 95 per cent significance that there are no Jupiter mass planets orbiting in the plane of the debris disc on circular orbits with periods, P ≤ 5 d. In addition, there are no young Neptune like planets (with radii 2.5 times smaller than the young Jupiter) on circular orbits with periods, P ≤ 3 d. 相似文献
109.
110.
Hydrogen and major element concentrations on 433 Eros: Evidence for an L‐ or LL‐chondrite‐like surface composition 下载免费PDF全文
Patrick N. Peplowski David Bazell Larry G. Evans John O. Goldsten David J. Lawrence Larry R. Nittler 《Meteoritics & planetary science》2015,50(3):353-367
A reanalysis of NEAR X‐ray/gamma‐ray spectrometer (XGRS) data provides robust evidence that the elemental composition of the near‐Earth asteroid 433 Eros is consistent with the L and LL ordinary chondrites. These results facilitated the use of the gamma‐ray measurements to produce the first in situ measurement of hydrogen concentrations on an asteroid. The measured value, ppm, is consistent with hydrogen concentrations measured in L and LL chondrite meteorite falls. Gamma‐ray derived abundances of hydrogen and potassium show no evidence for depletion of volatiles relative to ordinary chondrites, suggesting that the sulfur depletion observed in X‐ray data is a surficial effect, consistent with a space‐weathering origin. The newfound agreement between the X‐ray, gamma‐ray, and spectral data suggests that the NEAR landing site, a ponded regolith deposit, has an elemental composition that is indistinguishable from the mean surface. This observation argues against a pond formation process that segregates metals from silicates, and instead suggests that the differences observed in reflectance spectra between the ponds and bulk Eros are due to grain size differences resulting from granular sorting of ponded material. 相似文献