The effects of the geothermal gradient on amino acid racemization     

Barry J. Katz  Christopher G.A. Harrison  Eugene H. Man 《Earth and Planetary Science Letters》1979,44(2):279-286

The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision.  相似文献   

Decomposition of hydroxy amino acids in foraminiferal tests; kinetics,mechanism and geochronological implications     

Jeffrey L. Bada  Ming-Yung Shou  Eugene H. Man  Roy A. Schroeder 《Earth and Planetary Science Letters》1978,41(1):67-76

The diagenesis of the hydroxy amino acids serine and threonine in foraminiferal tests has been investigated. The decomposition pathways of these amino acids are complex; the principal reactions appear to be dehydration, aldol cleavage and decarboxylation. Stereochemical studies indicate that the α-amino-n-butyric acid (ABA) detected in foraminiferal tests is the end product of threonine dehydration pathway. Decomposition of serine and threonine in foraminiferal tests from two well-dated Caribbean deep-sea cores, P6304-8 and -9, has been found to follow irreversible first-order kinetics. Three empirical equations were derived for the disappearance of serine and threonine and the appearance of ABA. These equations can be used as a new geochronological method for dating foraminiferal tests from other deep-sea sediments. Preliminary results suggest that ages deduced from the ABA kinetics equation are most reliable because “species effect” and contamination problems are not important for this nonbiological amino acid. Because of the variable serine and threonine contents of modern foraminiferal species, it is likely that the accurate age estimates can be obtained from the serine and threonine decomposition equations only if a homogeneous species assemblage or single species sample isolated from mixed natural assemblages is used.  相似文献   

Respiration rates and hypoxia on the Louisiana shelf     

Quay Dortch  Nancy N. Rabalais  R. Eugene Turner  Gilbert T. Rowe 《Estuaries and Coasts》1994,17(4):862-872

The spatial and temporal variation in water-column respiration, estimated from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf from May through October 1991. In July 1991, water-column respiration was also determined on an alongshore transect, and in situ benthic respiration and photosynthesis rates were determined at jour stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O₂<2 mg 1^?1) at most stations in July and August and sporadically hypoxic at other times. Water-column respiration rates were in the same range as earlier, less extensive studies and not unusually high for coastal and estuarine waters. They were highest in summer, decreased with distance offshore and depth, and increased with temperature. Their variation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur within days to months, depending on the season and location. In July, benthic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic: total (water column+benthic) respiration was high. Combined water-column and benthic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulfide), some mechanism(s) must exist to reaerate bottom waters. Most physical mechanisms are unlikely to provide significant reaeration at this time of year. Measured benthic and conservatively estimated bottom-water photosynthesis could resupply 23% of the oxygen lost daily by respiration. Although this is too limited a dataset from which to draw conclusions about the relative importance of bottom-water and benthic respiration and photosynthesis in determining bottom-water oxygen concentrations, it does suggest that all these processes must be considered.  相似文献   

Evaluation of an alternative method for numerically modeling nonhydrostatic flows over irregular terrain     

Jerome D. Fast  Eugene S. Takle 《Boundary-Layer Meteorology》1988,44(1-2):181-206

Modeling nonhydrostatic atmospheric flow requires the solution of the vertical equation of motion and a prognostic or diagnostic equation for pressure. If the nonhydrostatic components of the flow are relatively small, they can be approximated and incorporated into a purely hydrostatic model, which usually is conceptually simpler and computationally more efficient. A method to do this for a linear model of local thermally-induced circulations is further developed and adapted to a non-linear numerical model of the neutral atmospheric boundary layer. A hydrostatic model and the quasi-nonhydrostatic version were used to simulate neutral flow over simple terrain features. One set of observations taken over a simple change in roughness and another set taken over a change in both roughness and terrain were simulated by both models to assess the capabilities of the quasi-nonhydrostatic technique.It is found that (as expected) the pressure deviation from the hydrostatic state is negligible for the roughness change, but it is an important aspect of neutral flow over terrain. Thus, for flow encountering a simple roughness change, the hydrostatic approximation is good, even for small horizontal scales. However, the quasi-nonhydrostatic model qualitatively produces the features in the observations for flow over a terrain change that the hydrostatic model cannot produce.Journal Paper No. J-12737 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 2779.  相似文献   

Discovery of asteroid 1976 AA     

Eleanor F. Helin  Eugene M. Shoemaker 《Icarus》1977,31(4):415-419

Asteroid 1976 AA was discovered as a result of a continuing systematic search for planet-crossing asteroids. It is the first asteroid to be thoroughly investigated by means of photometry and radiometry on its discovery apparition. It is also the first asteroid found with a semimajor axis and period less than that of the Earth and the first Earth-crossing asteroid which does not cross the orbit of either Mars or Venus. We estimate that there might be several tens of objects to absolute magnitude 18, which are exclusively Earth crossing. Some of these objects might be exceptionally easy to reach by spacecraft.  相似文献   

Chemical analyses of ten stony meteorites     

Eugene Jarosewich 《Geochimica et cosmochimica acta》1966,30(12):1261-1265

A group of stony meteorites, mainly unequilibrated chondrites ( and , 1965), has been analysed chemically. The chemical analyses of ten meteorites are given in this paper. Detailed studies of the petrology and mineralogy of these meteorites are in progress.  相似文献   

Correlation Structures between Satellite All-Sky Infrared Brightness Temperatures and the Atmospheric State at Storm Scales     

Yunji ZHANG  Eugene E.CLOTHIAUX  David J.STENSRUD 《大气科学进展》2022,39(5):714-732

This study explores the structures of the correlations between infrared(IR)brightness temperatures(BTs)from the three water vapor channels of the Advanced Baseline Imager(ABI)onboard the GOES-16 satellite and the atmospheric state.Ensemble-based data assimilation techniques such as the ensemble Kalman filter(EnKF)rely on correlations to propagate innovations of BTs to increments of model state variables.Because the three water vapor channels are sensitive to moisture in different layers of the troposphere,the heights of the strongest correlations between these channels and moisture in clear-sky regions are closely related to the peaks of their respective weighting functions.In cloudy regions,the strongest correlations appear at the cloud tops of deep clouds,and ice hydrometeors generally have stronger correlations with BT than liquid hydrometeors.The magnitudes of the correlations decrease from the peak value in a column with both vertical and horizontal distance.Just how the correlations decrease depend on both the cloud scenes and the cloud structures,as well as the model variables.Horizontal correlations between BTs and moisture,as well as hydrometeors,in fully cloudy regions decrease to almost 0 at about 30 km.The horizontal correlations with atmospheric state variables in clear-sky regions are broader,maintaining non-zero values out to~100 km.The results in this study provide information on the proper choice of cut-off radii in horizontal and vertical localization schemes for the assimilation of BTs.They also provide insights on the most efficient and effective use of the different water vapor channels.  相似文献   

Doubt and the Values of an Ignorance-Based World View for Restoration: Coastal Louisiana Wetlands     

R. Eugene Turner 《Estuaries and Coasts》2009,32(6):1054-1068

Embracing doubt, a signature strength of science, is an essential core component of an ignorance-based-world view (IBWV) that assumes the areas of certainty are small relative to the large field of ignorance. The contrasting knowledge-based world view (KBWV) assumes that small and mostly insignificant knowledge gaps exist. When the KBWV is combined with a sense of urgency to “do something,” then the intellectual landscape is flattened, the introduction of new ideas is impeded, monitoring and adaptive management is marginalized, risky behaviors continue, and social learning is restricted. The history of three coastal Louisiana land-uses (agricultural impoundment, marsh management, and dredging) is one of ignored and untested assumptions that might have provided a cause-and-effect means to avoid catastrophic land losses—the result of a KBWV that remains the primary perspective of Louisiana’s current coastal restoration and management program that includes river diversions and a proposed expansion of hurricane protection levees into wetlands. I argue from the pathology of results that willful adoption of an IBWV in the administration, management, and implementation of restoration will reduce the scale and diversity of significant missteps in the future, improve project efficiencies, and cause fewer unintended consequences that cannot (again) be retracted.  相似文献   

A Comparison Of The Detailed Structure In Dispersing Tracer Plumes Measured In Grid-Generated Turbulence With A Meandering Plume Model Incorporating Internal Fluctuations     

Eugene Yee  David J. Wilson 《Boundary-Layer Meteorology》2000,94(2):253-296

A meandering plume model that explicitly incorporatesinternal fluctuations has been developed and used to model the evolutionof concentration fluctuations in point-source plumes in grid turbulenceobtained from a detailed water-channel simulation. This fluctuating plumemodel includes three physical parameters: the mean plume spread in fixedcoordinates, which represents the outer plume length scale; the meaninstantaneous plume spread in coordinates attached to the instantaneousplume centroid, which represents the inner plume length scale; and, theconcentration fluctuation intensity in the meandering reference frame,which represents the in-plume fluctuation scale. These parameters arespecified in terms of a set of coupled dynamical equations that modeltheir development with downstream distance from the source. Explicitexpressions for the concentration moments of arbitrary integral orderand the concentration probability density function have been obtainedfrom the fluctuating plume model. Detailed comparisons of model predictionsagainst water-channel measurements for the first four concentrationmoments and the concentration probability distributions generally showvery good overall quantitative agreement. Exact quantitative conditions,expressed in terms of the physical parameters of the fluctuating plumemodel, have been derived for the emergence of off-centreline peaks inthe concentration variance profile. These quantitative conditions havebeen illustrated in terms of a diagram of states of the dispersing plume,and the qualitatively different regimes of plume concentration variancebehaviour on this state diagram have been identified and characterized.  相似文献   

Features of the Non-Plane-Polarized Interaction ofMagnetohydrodynamic (MHD) Shock Waves     

Eugene A. Pushkar'  Alexei A. Barmin 《Astrophysics and Space Science》2000,274(1-2):439-447

The features of the non-plane-polarized interaction of two plane MHD shock waves colliding at an arbitrary nonzero angle in the presence of an arbitrary magnetic field are considered. The problem is solved over a wide range of key parameters. When the key parameters of the problem vary continuously, there are no sudden restructurings (catastropies) of the flow pattern characteristic of the plane-polarized case. As distinct from the plane-polarized case, in the flow developed always there are two Alfvén discontinuities of different intensities with circular polarization. The presence of a magnetic field along the line of intersection of the shock fronts smoothers variation of pressure on the contact discontinuity C as a function of the angles of inclination of the magnetic field, the pressure on C being less than that in the plane-polarized case by several times. The velocity acquired by the medium as a result of the interaction depends significantly on the inclination of the magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献