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1.
Turnagain Arm is a macrotidal fjord‐style estuary. Glacier Creek is a small, glacially fed stream which enters the estuary tangentially near Girdwood, Alaska. Trenches and daily sedimentation measurements were made in a mudflat along the fluvio–estuarine transition of Glacier Creek during several summers since 2003. Each year, the flats appear to erode during the winter and then accrete vertically in the spring and summer. In each of the years studied, tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap–spring tidal cycles. In 2004, bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats. Five laminae bundles alternated between thicker and thinner bundles, corresponding to the perigean (high spring) and apogean (low spring) tides. Well‐preserved apogean–perigean cycles have rarely been documented in modern tidal flat sediments. At this location, vertical accretion of tidal rhythmites with well‐developed neap–spring cyclicity is possible because of the near‐complete removal of the flat from the previous year, which creates accommodation space for vertical accretion without significant reworking. Macrotidal conditions, no reworking by infaunal invertebrates, protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation. The position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles. In the summer of 2004, apogee and perigee were closely aligned with the new and full moons, resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick–thin spring cycles encoding the apogean and perigean tidal cycle. The apogean–perigean signal was not observed in subsequent years.  相似文献   
2.
As hypoxic conditions spread in our oceans, indices that quickly and efficiently assess oxygen content in sediment pore water, and habitat quality are increasingly becoming desirable. Depth to the appa...  相似文献   
3.
The Neem tree, the oil of which has a long history of pesticide, fertilizer and medicinal use in India, has been studied extensively for its organic compounds. Here we present a physical, mineralogical and geochemical database resulting from the analyses of two Neem soil profiles (epipedons) in India. Neem tree derivatives are used in the manufacture of a variety of products, from anti‐bacterial drugs and insecticides to fertilizers and animal feeds. A preliminary geochemical and mineralogical analysis of Neem soils is made to explore the potential for chemical links between Neem tree derivatives and soils. Physical soil characteristics, including colour, texture and clay mineralogy, suggest the two pedons formed under different hydrological regimes, and hence, are products of different leaching environments, one well‐drained site, the other poorly drained. Geochemically, the two Neem soils exhibit similarities, with elevated concentrations of Th and rare earth elements. These elements are of interest because of their association with phosphates, especially monazite and apatite, and the potential link to fertilizer derivatives. Higher concentrations of trace elements in the soils may be linked to nutritional derivatives and to cell growth in the Neem tree.  相似文献   
4.
In nature where unsteady flows prevail, the characters of bed configurations lag, or differ in phase from, the flow conditions. This is demonstrated from quantitative data for dune beds in the Fraser River (Canada) and the Gironde Estuary (France), where the flow conditions change ahead of the bed form by as much as 1/4 period. Qualitative evidence for lag abounds, for instance, substantially unmodified dunes exposed during low water. The existence of lag appears to confuse attempts to establish from field data the relationships between bed-form properties and flow conditions and, at the same time, makes difficult the confident application to field situations of formulae based on steady-state experiments or theories. Unfamiliar types of field study and experiments are needed to increase understanding of the degree and precise significance of lag.  相似文献   
5.
Detailed pollen, charcoal and loss on ignition profiles were analysed from Llyn Cororion, North Wales, UK. The chronology was based on 11 radiocarbon dates. This site is particularly important for this region because its high-resolution record improves the spatial and temporal resolution of records of Holocene vegetation change in an area characterized by a highly variable environment. An early Holocene phase of Juniperus-Betula scrub was succeeded by Betula-Corylus woodland. Quercus and Ulmus were established by c. 8600 14C yr BP, with Pinus dominating at c. 8430 14C yr BP. Local disturbance then allowed the spread of Alnus; Tilia was a common component of the forest by 5650 14C yr BP. Charcoal and pollen records suggest that by c. 2600 14C yr BP there was progressive deforestation, increased use of fire and spread of grassland; the first cereal grain was recorded at c. 2900 14C yr BP. Compared with data from upland Snowdonia, the results show that within a topographically diverse region there were significant local variations in forest composition. These variations developed as a response to interactions between many environmental parameters and were further complicated by the influence of human activity. In an area such as North Wales it is therefore unlikely that one site can be representative of regional Holocene vegetational development. The site is additionally important because it contributes to the data available for meta-analyses of environmental change in the North Atlantic region, particularly as detailed pollen diagrams from coastal lake sites around western Europe are rare.  相似文献   
6.
The imminent return of the Genesis Sample Return Capsule (SRC) from the Earth’s L1 point on September 8, 2004, represents the first opportunity since the Apollo era to study the atmospheric entry of a meter-sized body at or above the Earth’s escape speed. Until now, reentry heating models are based on only one successful reentry with an instrumented vehicle at higher than escape speed, the 22 May 1965 NASA “FIRE 2” experiment. In preparation of an instrumented airborne and ground-based observing campaign, we examined the expected bolide radiation for the reentry of the Genesis SRC. We find that the expected emission spectrum consists mostly of blackbody emission from the SRC surface (T∼ ∼2630 K@peak heating), slightly skewed in shape because of a range of surface temperatures. At high enough spectral resolution, shock emission from nitrogen and oxygen atoms, as well as the first positive and first negative bands of N2+, will stand out above this continuum. Carbon atom lines and the 389-nm CN band emission may also be detected, as well as the mid-IR 4.6-μm CO band. The ablation rate can be studied from the signature of trace sodium in the heat shield material, calibrated by the total amount of matter lost from the recovered shield. A pristine collection of the heat shield would also permit the sampling of products of ablation.  相似文献   
7.
8.
The plan shape of current ripples in relation to flow conditions   总被引:3,自引:0,他引:3  
Two sets of empirical data on the relationship of the plan shape of current ripples to flow conditions previously thought incompatible are reconciled by taking explicit and necessary account of the effects of flow width relative to flow depth. The aspects of plan shape considered are the mean streamwise ripple wavelength and the wavelength, measured across the flow, of longitudinal features due to centrifugal instability formed on the backs of the ripples. A regression analysis connecting these attributes with flow conditions including the width-effect shows that the amount of unexplained variability is of the same order as the accumulated experimental errors, and suggests that the two sets of data are in excellent agreement. The relationship of the plan geometry to flow conditions is less simple than has sometimes been supposed, but ways are suggested in which the experimental results can be used to establish the hydraulics of past environments.  相似文献   
9.
Arguments are developed to show that a factor termed preservation potential could bias the operational data of palaeocurrent analysis, and hence any palaeogeo- graphical inferences that are drawn. Flow systems and their directional sedimentary response elements are hierarchically organised in the geological present. A flow system can therefore be characterised (within the limitations imposed by preservation potential) only by reference to all the ranks of sedimentary structure generated therein. The operational rock structures generally employed in palaeocurrent analysis are usually low-ranking and subject to several sources of variance.  相似文献   
10.
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