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91.
A 14-m-thick section of marine and nonmarine sediments of the Gubik Formation of northern Alaska, exposed in bluffs near Ocean Point on the Colville River, has been studied by means of pollen analysis. Pollen from the marine sediments, of probable late Pliocene age, records a boreal forest of spruce and birch with minor amounts of alder in the adjacent terrestrial vegetation. Pine and perhaps true fir were probably at or near their northern limit here, but hemlocks and hardwoods were absent. The suggested environment for the Arctic Slope during the time represented by the marine sediments is similar to that of present-day Anchorage. Pollen floras from the overlying fluvial strata, of early or middle Pleistocene age, record predominantly herbaceous taxa indicating tundra conditions probably more severe than those of the present day. These deposits were most likely contemporaneous with glacial conditions in the Brooks Range to the south. Pollen of woody taxa (spruce, alder, birch, heaths) is rare through most of the section, although birch and alder percentages similar to those found in modern river sediments indicate an interstadial or interglacial warming in midsection. Inland climates during glacial episodes may have been similar to those of the present Arctic coast. 相似文献
92.
A JHKK survey camera based on a 1040×1040 PtSi Charge Sweep Device has been constructed in a joint programme of the University of Tokyo, the National Astronomical Observatory of Japan and the South African Astronomical Observatory. We describe its design, construction and initial testing. 相似文献
93.
Paul W. Carter 《Geochimica et cosmochimica acta》1978,42(8):1239-1242
Adsorption experiments using fulvic acid with calcite and quartz demonstrate that carbonate surfaces are able to selectively adsorb aspartic acid-rich organic matter. Quartz does not have this ability, instead exhibiting a preference for the aspartic acid-poor fraction. The relative abundance of aspartic acid in the fraction adsorbed onto calcite is highly dependent on the ratio of fulvic acid to calcite. At high ratios, fulvic acid is adsorbed with a relative abundance of aspartic acid that is more than twice that in the original fulvic acid. With decreasing ratios, there is a concomitant decrease in the relative abundance of aspartic acid in the adsorbed fraction. Selective adsorption may be the most likely mechanism by which aspartic acid-rich organic matter is included in non-biogenic car- bonates such as ooids. This process may be a factor influencing the distribution of amino acids within ooids, their rate of growth, and may ultimately be a factor in limiting their size. 相似文献
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An evaluation of the contribution of soil micromorphology to the study of ancient arable agriculture
This article provides an overview of the development of soil micromorphological studies of ancient agriculture and the current position of research in this field. The stance adopted by the authors is deliberately combative, and it is hoped that the article will stimulate discussion of the current limitations and potentialities of the technique. The findings of this review may be summarized in three proposals: (1) Soil micromorphology cannot be used at present to identify cultivation in ancient soils. Ambiguous or multiple interpretations of soil micromorphological features are unlikely to be resolved, so that the detection of large scale agricultural features in the field (cultivation marks, lynchets) will remain the most secure and direct guides to the presence of a former cultivated soil. Indirect evidence on inferred land use can be provided from pollen and land snail evidence. (2) Micromorphology should retain a key role in the determination of the impact of agriculture on soils, encompassing issues such as erosion, soil structure, fertility, and biological activity. These can be related to agricultural practice: methods of tillage, manuring, soil conservation, and fallowing. (3) Three areas of micromorphological research can be proposed that address the impact of agriculture on soil: (a) the study of well-sealed, buried agricultural soils which have a clearly understood archaeological context; (b) the study of recent soils with known agricultural histories; (c) the establishment of controlled experiments with ancient agricultural techniques. © 1998 John Wiley & Sons, Inc. 相似文献
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A spherical cavity expansion model has been used to study the bearing capacity of foundations on calcareous and silica sands. Two constitutive models have been used to represent each sand: an elastoplastic, collapse-softening model, and a computationally simpler, elastic/perfectly plastic material. The predictions of the limit pressures for spherical cavity expansion are compared with laboratory data on the bearing capacities obtained from model footing tests. It is shown that the numerical predictions of the limit pressures correlate well with the experimental values of bearing capacity, particularly at higher effective overburden pressures. 相似文献
99.
In this paper, an arbitrary Lagrangian–Eulerian (ALE) method is generalized to solve consolidation problems involving large deformation. Special issues such as pore‐water pressure convection, permeability and void ratio updates due to rotation and convection, mesh refinement and equilibrium checks are discussed. A simple and effective mesh refinement scheme is presented for the ALE method. The ALE method as well as an updated‐Lagrangian method is then used to solve some classical consolidation problems involving large deformations with different constitutive laws. The results clearly show the advantage and efficiency of the ALE method for these examples. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
100.