Detrital subsidy to the supratidal zone provides feeding habitat for intertidal crabs     

Tyler L. Lewis  Malte Mews  Dennis E. Jelinski  Martin Zimmer 《Estuaries and Coasts》2007,30(3):451-458

Beach-cast wrack of marine origin is considered a spatial subsidy to the marine-terrestrial transition zone. We found that the wrack line on sand and gravel beaches of Vancouver Island was frequented by intertidal purple shore crabs,Hemigrapsus nudus (Dana 1851) and densely colonized by detritivorous talitrid amphipods. Amphipods spend the day buried in sand and forage on beach wrack during the night.H. nudus were found in supratidal wrack putches immediately after nightly high tides in field censuses, but spent most of the day and ebb tides either submerged subtidally or hidden underneath intertidal rocks and boulders. In feeding trials, intertidal shore crabs were capable of preying on talitrid amphipods. We considerH. nudus an omnivore feeding on both fresh and decaying macroalgae as well as animal prey. Although living supratidally, amphipods were significantly preferred over intertidal littorine snails by foraging shore crabs. Handling time of amphipods was significantly shorter than for littorine snails. While amphipods had a reduced risk of predation byH. nudus when buried in the sand, foraging undern eath wrack patches did not reduce predation pressure on amphipods by shore crabs. Rates of amphipod consumption by shore crabs were higher at darkness than daylight. In addition to an apparent day-night rhythm, tidal height and time elapsed since previous high tide had a significant influence on shore crab density wrack. We conclude that beach-cast wrack acts as a spatial subsidy by virtue of providing a valuable food source to talitrid amphipods, which are in turn consumed by shore crabs that ride the nightly high tide into supratidal wrack patches to reduce the risk of passing bare sand on theiry way to a feeding habitat rich in valuable prey.  相似文献   

Lake Winnipeg: geological setting and sediment seismostratigraphy     

B. J. Todd  C. F. M. Lewis  E. Nielsen  L. H. Thorleifson  R. K. Bezys  W. Weber 《Journal of Paleolimnology》1998,19(3):215-243

Lake Winnipeg, the seventh largest lake in North America, is located at the boundary between the Interior Plains and the Canadian Shield in Manitoba, Canada. Seismic profiles were obtained in Lake Winnipeg on two geoscientific cruises in 1994 and 1996. These data indicate the morphology of the bedrock surface. In most cases, a clear distinction between low relief Paleozoic carbonate rock and high relief Precambrian rock can be made. In northern Lake Winnipeg, the eastern limit of Paleozoic rock is clearly demarcated 30 km west of the previous estimate of its position. In southern Lake Winnipeg, all or most of the Paleozoic sequence terminates at a prominent buried escarpment in the centre of the lake. This indicates that Paleozoic rock on the eastern shore, known from drilling and outcrops, is an outlier. Major moraines are apparent as abrupt, large ridges having a chaotic internal reflection pattern. These include the Pearson Reef Moraine, the George Island Moraine and the offshore extension of The Pas Moraine. Little evidence for extensive or thick till was observed. Instead, fine-grained sediments deposited in glacial Lake Agassiz rest directly on bedrock over most of the lake basin. Hence an episode of erosion to bedrock was associated with glaciation and/or deglaciation. The Agassiz Sequence sediments are well-stratified, drape underlying relief and in some areas are over 100 m thick. In places, stratification in these sediments is disrupted, perhaps by dewatering. Evidence of erosion of Agassiz Sequence sediments by recent currents was observed. The contact between the Agassiz Sequence and the overlying Winnipeg Sequence sediments is a marked angular unconformity. The Agassiz Unconformity indicates up to 10 m of erosion in places. The low-relief character of this unconformity precludes subaerial erosion and the lack of till, moraines, or extensive deformation precludes glacial erosion. Waves appear to be the most likely erosional agent, either in waning Lake Agassiz or early Lake Winnipeg time. Winnipeg Sequence sediments, in places very thin, mantle most of the lakefloor. These sediments were deposited in the present Lake Winnipeg and are faintly stratified to massive and reach about 10 m in thickness in deep water. On the surface of the Winnipeg Sequence, vigorous, episodic currents are thought to contribute to the construction of flow-transverse sand waves as much as 6 m high in a deep, narrow constriction in the lake.  相似文献   

Microlensing of broad absorption line quasars     

Lewis  & Belle 《Monthly notices of the Royal Astronomical Society》1998,297(1):69-76

The physical nature of the material responsible for the high-velocity broad absorption line (BAL) features seen in a small fraction of quasar spectra has been the subject of debate since their discovery. This has been especially compounded by the lack of observational probes of the absorbing region. In this paper we examine the role of 'microlenses' in external galaxies in the observed variability in the profiles of BALs in multiply imaged quasars. Utilizing realistic models for both the BAL region and the action of an ensemble of microlensing masses, we demonstrate that stars at cosmological distances can provide an important probe of the physical state and structure of material at the heart of these complex systems. Applying these results to the macrolensed BAL quasar system H1413+117, the observed spectral variations are readily reproduced, but without the fine-tuning requirements of earlier studies which employ more simplistic models.  相似文献   

Factors controlling the noble gas abundance patterns of deep-sea basalts     

Jack Dymond  Lewis Hogan 《Earth and Planetary Science Letters》1978,38(1):117-128

A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the ⁴⁰Ar/³⁶Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform ³He/⁴He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle.  相似文献   

A rapid field method for detecting oil in sediments     

Lewis R. Brown  Gerard S. Pabst  Melvin Light 《Marine pollution bulletin》1978,9(3):81-82

A rapid thin-layer chromatographic technique for estimating the relative concentration of crude oil in sediments has been developed. The limit of detection on the chromatogram is 2 μg of Empire Mix or Saudi Arabian crude oil. The technique has been tested on 335 sediment samples collected from 67 stations in the Gulf of Mexico and has been utilized in assessing the location of oil entering the sediments from the Argo Merchant spill incident.  相似文献   

Debris flow deposits of early miocene age,deadman stream,Marlborough, New Zealand     

D.W. Lewis  M.G. Laird  R.D. Powell 《Sedimentary Geology》1980,27(2):83-118

Detailed analysis is presented of a conformable succession of Early Miocene conglomerates and sandstones lying between massive marine mudstones. The coarse sediments reflect deposition by a spectrum of subaqueous debris-flow mechanisms during an early pulse of tectonism that ultimately resulted in Plio-Pleistocene eversion of the Kaikoura Mountains.Sparse pebbly mudstones and rare sandy conglomerates show disoriented clasts and reflect high-viscosity flows and slurry-creep flow mechanisms. Other deposits have little mud matrix, hence appear to reflect low-viscosity flow processes. The largest clasts in these have a preferred planar orientation, probably reflecting dispersive grain pressure, and a preferred long-axis orientation parallel to flow direction. Common sorted sandstones and some conglomeratic sandstones show diffuse parallel lamination; with rare exceptions neither grading nor traction structures are present. Other conglomeratic sandstones show trough cross-bedding which we attribute to entrained bedload movement during intersurge episodes of debris flow.Microfossil data from the mudstones indicate sedimentation in an environment of outer neritic to upper bathyal aspect. Most detritus is abraded, suggesting derivation from terrestrial or inner neritic sources, but angular calcilutitic clasts and irregular sandstone and mudstone clasts and rafts were probably derived from submarine erosion between the emergent source area and the site of accumulation. Deposits generally appear to infill broad shallow channels. Paleocurrent and fabric analysis indicate a markedly uniform flow direction throughout succession, and suggest that the locus of channeling remained relatively fixed in space throughout accumulation of hundreds of metres of superimposed, commonly amalgamated debris-flow deposits. Although lateral control away from the measured sequence is limited, we infer that the locus of deposition lay shoreward of any submarine canyon or fan.  相似文献   

Simulated Effects of Irrigation on Salinity in the Arkansas River Valley in Colorado     

Karin Goff  Michael E. Lewis  Mark A. Person  Leonard F. Konikowd 《Ground water》1998,36(1):76-86

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Radiative Processes in Astrophysics: George B. Rybicki and Alan P. Lightman. Wiley,New York,NY, 1979, 382 pp., U.K. £ 19, ISBN 0-47-104815-1     

E.L. Lewis 《Physics of the Earth and Planetary Interiors》1981,25(4):432

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A finite-element study of fold propagation in a viscous layer     

R.W. Lewis  J.R. Williams 《Tectonophysics》1978,44(1-4)

A finite-element model of a viscous layer contained in a viscous matrix and undergoing layer-parallel compression is used to examine the hypothesis that a long chain of folds, as found in real rocks, can originate from one initial perturbation to the layer geometry. This hypothesis is tested by determining the velocity with which a perturbation spreads along layers of various viscosities.An insight is gained into the roles played by local strain and local layer strength in the folding mechanism. The results show that for layers with viscosity ratios comparable with those of real rocks it is impossible for long chains of folds to originate from one perturbation. The authors conclude that rock layers contain many initial perturbations and folding originates at all perturbation sites simultaneously. The growth of such folds depends on the amplitude and shape of the initial perturbation and on subsequent interference between folds.  相似文献   

The use of ²¹⁰Pb as a heavy metal tracer in the Susquehanna River system     

Dale M Lewis 《Geochimica et cosmochimica acta》1977,41(11):1557-1564

Analysis of soil profiles and shallow ground water in the Susquehanna River basin, northeastern U.S.A., indicates that the atmospheric flux of ²¹⁰Pb is efficiently scavenged by the organic-rich horizons of the soils. This atmospherically supplied ²¹⁰Pb in soil profiles can only be lost from the system by soil erosion. Based on the annual sediment yield of the Susquehanna River and the excess ²¹⁰Pb concentration in particulate matter, a mean residence time of 2000 yr is calculated for metals similar to Pb in soil profiles.The West Branch of the Susquehanna River (WBSR) is strongly affected by acid mine drainage and is low in pH and high in dissolved ( <0.4 μm) ²¹⁰Pb, Fe and Mn. Along its course iron hydroxide is precipitating at a pH of between 4 and 4.5 and the ²¹⁰Pb supplied by the acid mine water is diminished by about 25% as a result of dilution. As the WBSR enters the Valley and Ridge Province of the Appalachians it has a ²¹⁰Pb concentration of ~ 0.2 dpm/l. At this juncture it receives a considerable influx of alkalinity from tributaries draining carbonate terranes, resulting in neutralization of the sulfuric acid and increase of the river pH to around 6.5–7. This pH adjustment is accompanied by the precipitation of Fe and Mn. Due to the slow rate of Mn removal from solution, the Mn precipitation extends a considerable distance down river from the point of acid neutralization. Analyses for ²¹⁰Pb in the river at points in or below the region of Mn precipitation show that ²¹⁰Pb is rapidly scavenged from solution onto suspended particles. From the data it is possible to calculate the removal rate of Pb from water in the presence of Fe and Mn hydroxides and other particles. At a pH of 4–4.5 Pb removal is nonexistent relative to the river flow rate, but at a pH of 6.5–7 the ²¹⁰Pb data indicate a residence time of <0.7 day for dissolved Pb.  相似文献