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An approximate analytical solution is presented for torsional vibrations of footings partially embedded into a semi-infinite medium or a stratum. Simple formulas derived for pure torsional motion make it possible to apply a correction for the effect of embedment to the known solutions of surface footings. The solution completes an approach to the analysis of all modes of footing vibrations, including the coupled modes. The approach to coupled modes is illustrated by the solution of coupled response involving horizontal translation, rocking and torsion. Formulas are presented for stiffness and damping coefficients that can be used in the analysis of embedded footings or structures supported by such footings Field experiments were conducted with concrete footings featuring circular, square and rectangular bases and variable embedment depths. The experimental results were compared with theoretical predictions of pure torsional vibrations. 相似文献
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Harvey Maurice Sachs 《Quaternary Research》1973,3(1):73-88
A quantitative study of the distribution of Radiolaria in surface sediments of the North Pacific has demonstrated the feasibility of utilizing complete radiolarian thanatocoenoses as indicators of past oceanographic conditions.In samples from 36 core tops from the region 33° N-54° N, 148° E-140° W, 87–96% of all radiolarians encountered could be assigned to one of 120 taxa recognized. Q-mode factor analysis of distributional data for the 57 most abundant species yields four independent assemblages. Three of these reflect near-surface oceanographic conditions, and the fourth responds to bottom conditions. Regression-developed transfer functions describe the relationship between the assemblages and oceanographic parameters. 相似文献
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Harvey Maurice Sachs 《Quaternary Research》1973,3(1):89-98
Objective quantitative estimates of paleo-oceanographic conditions in the North Pacific can be made by analyses of radiolarian assemblages. With appropriate computation, transfer functions developed in a study of surface sediments can be used to estimate oceanographic conditions in cores containing late Pleistocene radiolarian faunas. Analysis of core V21-173 indicates that conditions as warm as the Holocene were rare during the past 800,000 yr, and that the region experienced marked near-surface temperature drops correlative with Caribbean and continental records for the past 250,000 yr. A major world-wide warm event at about 400,000 yr is also indicated. 相似文献
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Julian P. Sachs 《Geochimica et cosmochimica acta》2011,75(2):444-459
The hydrogen isotope ratio of the dinoflagellate sterol dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) was measured in suspended particles and surface sediments from the Chesapeake Bay estuary in order to evaluate the influence of salinity on hydrogen isotope fractionation. D/H fractionation was found to decrease by 0.99 ± 0.23‰ per unit increase in salinity over the salinity range 10-29 PSU, a similar decrease to that observed in a variety of lipids from hypersaline ponds on Christmas Island (Kiribati). We hypothesize that the hydrogen isotopic response to salinity may result from diminished exchange of water between algal cells and their environment, lower growth rates and/or increased production of osmolytes at high salinities. Regardless of the mechanism, the consistent sign and magnitude of dinosterol δD response to changing salinity should permit qualitative to semi-quantitative reconstructions of past salinities from sedimentary dinosterol δD values. 相似文献
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Lipid biomarkers in lacustrine sediments provide valuable information about lake history and environmental change. Here we report the occurrence of a unique set of lipid biomarkers from a freshwater crater lake, El Junco, in the Galápagos. In addition to previously reported alkadienes, botryococcenes and lycopadienes indicative of Botryococcus braunii A, B and L races, we find highly branched C25 isoprenoids (HBIs) from diatoms, monomethyl alkanes likely from insect epicuticular waxes, long chain alkenols, diols and a triol, keto-ols, hydroxy acids and keto acids. Saturated and mono-unsaturated long chain diols from C30-C36 had terminal hydroxyl groups and hydroxyl groups between the ω16 and ω20 positions. Vicinal diols with hydroxyl groups at ω9 and ω10 were likely from the floating fern Azolla. C30-C36 keto-ols, mid-chain hydroxy and keto acids had mid-chain functional groups at similar positions to the diols, suggesting common origins. The predominance of ω20-hydroxy acids and diols, together with 20,21-dihydroxy-nonacosanoic acid is indicative of an Azolla source, while ω16 and ω18 hydroxy acids and diols imply a microalgal source. 相似文献
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Abstract Two fluid layers of constant density lying one over the other on top of a rigid horizontal lower boundary with either a free upper surface or a rigid upper boundary can support solitary waves. The existence of a unique branch of such waves emanating from the horizontal flow at a critical speed U ? is demonstrated in both cases by use of the Nash—Moser implicit function theorem. These results complement the global results of Amick and Turner (1986) and are analogous to the work of Friedrichs and Hyers (1954) and Beale (1977) for surface waves. It is also noted that the most obvious variational principle which characterizes these waves as constrained extremals (Benjamin, 1984) is of indefinite type, having a Hessian with infinitely many positive and infinitely many negative eigenvalues. 相似文献
27.
Sediments from 28 saline and hypersaline (salinity 13.6-149.2) ponds on Christmas Island (Kiritimati), in the Central tropical Pacific Ocean, were investigated for the effect of salinity on the D/H ratios of lipid biomarkers. Hydrogen isotope ratios (expressed as δD values) of total lipid extracts, and individual hydrocarbons heptadecane, heptadecene, octadecane, octadecene, diploptene, and phytene from cyanobacteria, became increasingly enriched in deuterium as salinity increased, spanning a range of 100‰, while lake water δD values spanned a range of just 12‰. Net D/H fractionation between lipids and source water thus decreased as salinity increased. Isotope fractionation factors (αlipid-water) were strongly correlated with salinity, and increased in all compound classes studied by up to 0.0967 over a salinity range of 136. Differences in the hydrogen isotopic composition of lipids derived from three biosynthetic pathways (acetogenic, mevalonate, and non-mevalonate) remained similar irrespective of the salinity. This suggests that the mechanism responsible for the observed αlipid-water-salinity relationship originates prior to the last common biosynthetic branching point, the Calvin Cycle. We propose that a decrease in the exchange of intra- and extra-cellular (ambient) water resulting from down-regulation or closure of water channels (aquaporins) within cyanobacterial cell membranes, and subsequent isotopic enrichment of the intracellular water, likely resulting from metabolic reactions. These findings imply that caution must be exercised when attempting to reconstruct source water δD values using lipid δD values from environments that may have experienced salinity variations. On the other, hand our results can be used to establish a paleo-salinity proxy based on lipid δD, if additional constraints on source water δD values can be made. 相似文献
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Boris Radosavljevic Hugues Lantuit Wayne Pollard Paul Overduin Nicole Couture Torsten Sachs Veit Helm Michael Fritz 《Estuaries and Coasts》2016,39(4):900-915
Arctic coastal infrastructure and cultural and archeological sites are increasingly vulnerable to erosion and flooding due to amplified warming of the Arctic, sea level rise, lengthening of open water periods, and a predicted increase in frequency of major storms. Mitigating these hazards necessitates decision-making tools at an appropriate scale. The objectives of this paper are to provide such a tool by assessing potential erosion and flood hazards at Herschel Island, a UNESCO World Heritage candidate site. This study focused on Simpson Point and the adjacent coastal sections because of their archeological, historical, and cultural significance. Shoreline movement was analyzed using the Digital Shoreline Analysis System (DSAS) after digitizing shorelines from 1952, 1970, 2000, and 2011. For purposes of this analysis, the coast was divided in seven coastal reaches (CRs) reflecting different morphologies and/or exposures. Using linear regression rates obtained from these data, projections of shoreline position were made for 20 and 50 years into the future. Flood hazard was assessed using a least cost path analysis based on a high-resolution light detection and ranging (LiDAR) dataset and current Intergovernmental Panel on Climate Change sea level estimates. Widespread erosion characterizes the study area. The rate of shoreline movement in different periods of the study ranges from ?5.5 to 2.7 m·a?1 (mean ?0.6 m·a?1). Mean coastal retreat decreased from ?0.6 m·a?1 to ?0.5 m·a?1, for 1952–1970 and 1970–2000, respectively, and increased to ?1.3 m·a?1 in the period 2000–2011. Ice-rich coastal sections most exposed to wave attack exhibited the highest rates of coastal retreat. The geohazard map combines shoreline projections and flood hazard analyses to show that most of the spit area has extreme or very high flood hazard potential, and some buildings are vulnerable to coastal erosion. This study demonstrates that transgressive forcing may provide ample sediment for the expansion of depositional landforms, while growing more susceptible to overwash and flooding. 相似文献
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