Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars, Dendraster excentricus     

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   

Entrainment and emplacement of englacial debris bands near the margin of Storglaciären,Sweden     

Peter L. Moore  Neal R. Iverson  Kevin T. Uno  Matthew P. Dettinger  Keith A. Brugger  Peter Jansson 《Boreas: An International Journal of Quaternary Research》2013,42(1):71-83

Internal structure, stable isotope composition and tritium concentration were measured in and around debris‐bearing ice at the margin of Storglaciären, where englacial debris bands have previously been inferred to form by thrusting. Two types of debris bands were distinguished: (i) an unsorted diamicton band that is laterally continuous for more than 200 m, and (ii) well‐sorted sand and gravel bands that are lenticular and discontinuous. Above‐background tritium levels and enrichment of δ¹⁸ O and δD in ice from the diamicton band indicate entrainment by basal freeze‐on since 1952. Isotopic enrichment and tritium‐free ice in the sandy debris bands also indicate entrainment in freezing water, but prior to 1952. The lenticular cross‐section, sorting and stratification of the sandy bands suggest that they were deposited englacially. The basally accreted diamicton band has been elevated tens of metres above the bed and presently overlies the englacially deposited sandy bands, suggesting that the stratigraphy has been disrupted. Three interpretations could account for these observations: (i) thrusting of fast‐moving ice over slow, marginal ice uplifting recently accreted basal ice along the fault; (ii) folding near the margin, elevating young basal ice over older basal and englacial ice; and (iii) debris‐band formation by an unknown mechanism and subsequent contamination of ice geochemical properties by meltwater flow through debris bands. Although none of these interpretations is consistent with all measurements, folding is most compatible with observations and local ice‐flow kinematics.  相似文献   

A spectral optical flow method for determining velocities from digital imagery     

Neal Hurlburt  Steve Jaffey 《Earth Science Informatics》2015,8(4):959-965

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Accretion, feedback and galaxy bimodality: a comparison of the GalICS semi-analytic model and cosmological SPH simulations   总被引：1，自引：0，他引：1  

Andrea Cattaneo  Jérémy Blaizot  David H. Weinberg  Dusan Kere&#;  Stéphane Colombi  Romeel Davé  Julien Devriendt  Bruno Guiderdoni  Neal Katz 《Monthly notices of the Royal Astronomical Society》2007,377(1):63-76

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Constraining the structure of the non-spherical pre-protostellar core L1544     

Steven D. Doty  Sheila E. Everett  Yancy L. Shirley †  Neal J. Evans  II    Matthew L. Palotti 《Monthly notices of the Royal Astronomical Society》2005,359(1):228-236

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On the evolutionary history of stars and their fossil mass and light     

Mark A. Fardal  Neal Katz  David H. Weinberg  Romeel Davé 《Monthly notices of the Royal Astronomical Society》2007,379(3):985-1002

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An assessment of large-scale flood modelling based on LiDAR data     

Guénolé Choné  Pascale M. Biron  Thomas Buffin-Bélanger  Iulia Mazgareanu  Jeff C. Neal  Christopher C. Sampson 《水文研究》2021,35(8):e14333

Large-scale flood modelling approaches designed for regional to continental scales usually rely on relatively simple assumptions to represent the potentially highly complex river bathymetry at the watershed scale based on digital elevation models (DEMs) with a resolution in the range of 25–30 m. Here, high-resolution (1 m) LiDAR DEMs are employed to present a novel large-scale methodology using a more realistic estimation of bathymetry based on hydrogeomorphological GIS tools to extract water surface slope. The large-scale 1D/2D flood model LISFLOOD-FP is applied to validate the simulated flood levels using detailed water level data in four different watersheds in Quebec (Canada), including continuous profiles over extensive distances measured with the HydroBall technology. A GIS-automated procedure allows to obtain the average width required to run LISFLOOD-FP. The GIS-automated procedure to estimate bathymetry from LiDAR water surface data uses a hydraulic inverse problem based on discharge at the time of acquisition of LiDAR data. A tiling approach, allowing several small independent hydraulic simulations to cover an entire watershed, greatly improves processing time to simulate large watersheds with a 10-m resampled LiDAR DEM. Results show significant improvements to large-scale flood modelling at the watershed scale with standard deviation in the range of 0.30 m and an average fit of around 90%. The main advantage of the proposed approach is to avoid the need to collect expensive bathymetry data to efficiently and accurately simulate flood levels over extensive areas.  相似文献   

Exploring the Moon's surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin?          下载免费PDF全文

Charles K. Shearer  Paul V. Burger  Aaron S. Bell  Yunbin Guan  Clive R. Neal 《Meteoritics & planetary science》2015,50(8):1449-1467

Remotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan‐1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high‐Ti mare basalt 74275 are one of these ambiguous examples. In this high‐Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X‐ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe‐rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high‐pressure mineral assemblages exhibited by the Mg‐suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg‐suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low‐Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low‐Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus‐Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re‐equilibration. During the last episode of re‐equilibration, the dunite cumulate was sampled by the 74275 high‐Ti basalt and transported over a period of 30–70 days to the lunar surface.  相似文献   

A multi-proxy lacustrine record of Holocene climate change on northeastern Baffin Island, Arctic Canada   总被引：3，自引：0，他引：3  

Jason P. Briner  Neal Michelutti  Donna R. Francis  Gifford H. Miller  Yarrow Axford  Matthew J. Wooller  Alexander P. Wolfe 《Quaternary Research》2006,65(3):431-442

Reconstructions of past environmental changes are critical for understanding the natural variability of Earth's climate system and for providing a context for present and future global change. Radiocarbon-dated lake sediments from Lake CF3, northeastern Baffin Island, Arctic Canada, are used to reconstruct past environmental conditions over the last 11,200 years. Numerous proxies, including chironomid-inferred July air temperatures, diatom-inferred lakewater pH, and sediment organic matter, reveal a pronounced Holocene thermal maximum as much as 5°C warmer than historic summer temperatures from 10,000 to 8500 cal yr B.P. Following rapid cooling 8500 cal yr B.P., Lake CF3 proxies indicate cooling through the late Holocene. At many sites in northeastern Canada, the Holocene thermal maximum occurred later than at Lake CF3; this late onset of Holocene warmth is generally attributed to the impacts of the decaying Laurentide Ice Sheet on early Holocene temperatures in northeastern Canada. However, the lacustrine proxies in Lake CF3 apparently responded to insolation-driven warmth, despite the proximity of Lake CF3 to the Laurentide Ice Sheet and its meltwater. The magnitude and timing of the Holocene thermal maximum at Lake CF3 indicate that temperatures and environmental conditions at this site are highly sensitive to changes in radiative forcing.  相似文献   

Ice lines, planetesimal composition and solid surface density in the solar nebula     

Sarah E. Dodson-Robinson  Karen Willacy  Peter Bodenheimer  Neal J. Turner  Charles A. Beichman 《Icarus》2009,200(2):672-693

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2–3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133–1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] by a factor of 3–4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.  相似文献