EVOLUTIONARY CHARACTERISTICS OF LAKES AND PALAEOCLIMATIC UNDULATION IN THE Q AID AM BASIN,CHINA     

黄麒  Teh-Lung Ku  Fred M.Phillips 《中国海洋湖沼学报》1993,11(1):34-45

The Qaidam Basin is. a tectonically controlled depression at 2700 m altitude on the northern margin of the Tibet Plateau. A succession of Neogene clastic sediments indicates lake depocentre migration from NW to SE. Towards late Pliocene, evaporative conditions developed with formation of gypsum, halite, celestite, mirabilite and bloedite. Continued deformation through Pleistocene developed a series of NW- SE trending domes and troughs. This setting continued to exert a controlling influence on basin and brine evolution and the progressive development of sulfate to chloride brines as, dry conditions intensified in late Pleistocene, at the end of which, extensive flat floors were formed in Dalangtan, Qarhan and other playas.The, model, which was set up based on the synthetic analyses, divides the fluctuating palaeoclimate since 750ka,B. P. into 11 warm and humid stages and 10 dry and cold stages. The water of the lakes evaporated during the dry-cold periods and were freshened during the warm-humid periods  相似文献   

Trace-element and Sr,Nd, Pb,and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America   总被引：6，自引：0，他引：6  

Charles R. Stern  Fred A. Frey  Kiyoto Futa  Robert E. Zartman  Zicheng Peng  T. Kurtis Kyser 《Contributions to Mineralogy and Petrology》1990,104(3):294-308

The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The cratonic basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in ⁸⁷Sr/⁸⁶Sr (0.70316 to 0.70512), ¹⁴³Nd/¹⁴⁴Nd (_Nd) and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the cratonic basalts. In contrast, the transitional basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by ⁸⁷Sr/⁸⁶Sr=0.7039±0.0004, _Nd, ²⁰⁶Pb/²⁰⁴Pb=18.60±0.08, ²⁰⁷Pb/²⁰⁴Pb=15.60±0.01, and ²⁰⁸Pb/²⁰⁴Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the cratonic basalts, are displaced to higher ⁸⁷Sr/⁸⁶Sr at a given ¹⁴³Nd/¹⁴⁴Nd and to higher ²⁰⁷Pb/²⁰⁴Pb at a given ²⁰⁸Pb/²⁰⁴Pb. The transitional basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the cratonic and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, ¹⁸O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock ¹⁸O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock ¹⁸O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The cratonic basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, ⁸⁷Sr/⁸⁶Sr at a given ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁷Pb/²⁰⁴Pb at a given ²⁰⁸Pb/²⁰⁴Pb, and ¹⁸O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The transitional basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the transitional basalts earlier in the Cenozoic.  相似文献   

Understanding and Reducing False Alarms in Observational Fog Prediction     

Jonathan G. Izett  Bas J. H. van de Wiel  Peter Baas  Fred C. Bosveld 《Boundary-Layer Meteorology》2018,169(2):347-372

The reduction in visibility that accompanies fog events presents a hazard to human safety and navigation. However, accurate fog prediction remains elusive, with numerical methods often unable to capture the conditions of fog formation, and observational methods having high false-alarm rates in order to obtain high hit rates of prediction. In this work, 5 years of observations from the Cabauw Experimental Site for Atmospheric Research are used to further investigate how false alarms may be reduced using the statistical method for diagnosing radiation-fog events from observations developed by Menut et al. (Boundary-Layer Meteorol 150:277–297, 2014). The method is assessed for forecast lead times of 1–6 h and implementing four optimization schemes to tune the prediction for different needs, compromising between confidence and risk. Prediction scores improve significantly with decreased lead time, with the possibility of achieving a hit rate of over 90% and a false-alarm rate of just 13%. In total, a further 31 combinations of predictive variables beyond the original combination are explored (including mostly, e.g., variables related to moisture and static stability of the boundary layer). Little change to the prediction scores indicates any appropriate combination of variables that measure saturation, turbulence, and near-surface cooling can be used. The remaining false-alarm periods are manually assessed, identifying the lack of spatio–temporal information (such as the temporal evolution of the local conditions and the advective history of the airmass) as the ultimate limiting factor in the methodology’s predictive capabilities. Future observational studies are recommended that investigate the near-surface evolution of fog and the role of non-local heterogeneity on fog formation.  相似文献   

Wetland-water column exchanges of carbon, nitrogen, and phosphorus in a southern Everglades dwarf mangrove     

Stephen E. Davis  Daniel L. Childers  John W. Day  David T. Rudnick  Fred H. Sklar 《Estuaries and Coasts》2001,24(4):610-622

We used enclosures to quantify wetland-water column nutrient exchanges in a dwarf red mangrove, (Rhizophora mangle L.) system near Taylor River, an important hydraulic linkage between the southern Everglades and eastern Florida Bay, Florida, USA. Circular enclosures were constructed around small (2.5–4 m diam) mangrove islands (n=3) and sampled quarterly from August 1996 to May 1998 to quantify net exchanges of carbon, nitrogen, and phosphorus. The dwarf mangrove wetland was a net nitrifying environment with consistent uptake of ammonium (6.6–31.4 μmol m⁻² h⁻¹) and release of nitrite +nitrate (7.1–139.5 μmol m⁻² h⁻¹) to the water column. Significant flux of soluble reactive phosphorus was rarely detected in this nutrient-poor, P-limited environment. We did observe recurrent uptake of total phosphorus and nitrogen (2.1–8.3 and 98–502 μmol m⁻² h⁻¹, respectively), as well as dissolved organic carbon (1.8–6.9 μmol m⁻² h⁻¹) from the water column. Total organic carbon flux shifted unexplainably from uptake, during Year 1, to export, during Year 2. The use of unvegetated (control) enclosures during the second year allowed us to distinguish the influence of mangrove vegetation from soil-water column processes on these fluxes. Nutrient fluxes in control chambers typically paralleled the direction (uptake or release) of mangrove enclosure fluxes, but not the magnitude. In several instances, nutrient fluxes were more than twofold greater in the absence of mangroves, suggesting an influence of the vegetation on wetland-water column processes. Our findings characterize wetland nutrient exchanges, in a mangrove forest type that has received such little attention in the past, and serve as baseline data for a system undergoing hydrologic restoration.  相似文献   

Changes in Surface Morphology of Calcite Exposed to the Oceanic Water Column     

Paul J. Troy  Yuan-Hui Li  Fred T. Mackenzie 《Aquatic Geochemistry》1997,3(1):1-20

Reactions occurring on the surfaces of biogenic carbonate minerals can have important consequences for the biogeochemical cycle of carbon. In this study, carbonate mineral surface reactions with ambient seawater were investigated by atomic force microscopy (AFM). A sampling method was developed in which calcite surfaces were hung at discrete depths on a sediment trap array line for a three-day deployment period in subtropical North Pacific waters. Changes in surface morphologies were examined at nanometer resolution and evaluated using as a constraint the depth profile of calcite saturation in these waters. Evidence suggests that: (1) organic films which develop on carbonate surfaces exposed to shallow seawater may be responsible for the oversaturated state of the upper oceanic water column, (2) dissolution of carbonate minerals within the shallow warm layer of the ocean could be responsible for part of the alkalinity anomaly observed in the North Pacific.  相似文献   

Literature Review and Model (COMET) for Colloid/Metals Transport in Porous Media   总被引：8，自引：0，他引：8  

William B. Mills  Sally Liu  Fred K. Fong 《Ground water》1991,29(2):199-208

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Cosmogenic chlorine-36 production rates in terrestrial rocks   总被引：2，自引：0，他引：2  

Marek G. Zreda  Fred M. Phillips  David Elmore  Peter W. Kubik  Pankaj Sharma  Ronald I. Dorn 《Earth and Planetary Science Letters》1991,105(1-3)

Chlorine-36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of ³⁵Cl, spallation of ³⁹K and ⁴⁰Ca, and slow negative moun capture by ⁴⁰Ca. We have measured the ³⁶Cl content of ¹⁴C-dated glacial boulders from the White Mountains in eastern California and in a ¹⁴C-dated basalt flow from Utah. Effective, time-intergrated production parameters were calculated by simultaneous solution of the ³⁶Cl production equations. The production rates due to spallation are 4160 ± 310 and 3050 ± 210 atoms ³⁶Cl yr⁻¹ mol⁻¹³⁹K and ⁴⁰Ca, respectively. The thermal neutron capture rate was calculated to be (3.07 ± 0.24) × 10⁵ neutrons (kg of rock)⁻¹ yr⁻¹. The reported values are normalized to sea level and high geomagnetic latitudes. Production of ³⁶Cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates. Chlorine-36 dating was performed on carbonate ejecta from Meteor Crater, Arizona, and late Pleistocene morainal boulders from the Sierra Nevada, California. Calculated ³⁶Cl ages are in good agreement with previously reported ages obtained using independent methods.  相似文献   

A Proposal to Use Chlorine-36 for Monitoring the Movement of Radionuclides from Nuclear Explosions     

Fred M. Phillips  Stanley N. Davis  Peter Kubik 《Ground Water Monitoring & Remediation》1990,10(3):106-113

Chlorine-36 has been produced in large amounts by hundreds of nuclear explosions on the Nevada Test Site as well as 12 off-site explosions at eight locations in five states. Continued monitoring of the redistribution of radionuclides by subsurface water is of concern in most of the areas affected by the detonations. Chlorine-36 has the following advantages as a built-in tracer for this redistribution: its mobility is equal to or greater than water, its long half-life assures its continued usefulness over long periods, collection and storage of samples is simple, it is not subject to vapor transport at ordinary temperatures, its natural background is very low, and it does not form insoluble precipitates. Chlorine-36 from the Gnome event near Carlsbad, New Mexico, illustrates how ³⁶C1 can be used to help study the redistribution of radionuclides in the soil profile. Chlorine-36 is also potentially useful as a tracer to study movement of contaminants around large nuclear reactor complexes and near respositories for radioactive waste.  相似文献   

Estimation of surface heat and momentum fluxes using the flux-variance method above uniform and non-uniform terrain   总被引：4，自引：0，他引：4  

Gabriel Katul  Stewart M. Goltz  Cheng-I Hsieh  Yung Cheng  Fred Mowry  John Sigmon 《Boundary-Layer Meteorology》1995,74(3):237-260

Eddy-correlation measurements above an uneven-aged forest, a uniform-irrigated bare soil field, and within a grass-covered forest clearing were used to investigate the usefulness of the fluxvariance method above uniform and non-uniform terrain. For this purpose, the Monin and Obukhov (1954) variance similarity functions were compared with direct measurements. Such comparisons were in close agreement for momentum and heat but not for water vapor. Deviations between measured and predicted similarity functions for water vapor were attributed to three factors: 1) the active role of temperature in surface-layer turbulence, 2) dissimilarity between sources and sinks of heat and water vapor at the ground surface, and 3) the non-uniformity in water vapor sources and sinks. It was demonstrated that the latter non-uniformity contributed to horizontal gradients that do not scale with the vertical flux. These three factors resulted in a turbulence regime that appeared more efficient in transporting heat than water vapor for the dynamic convective sublayer but not for the dynamic sublayer. The agreement between eddy-correlation measured and flux-variance predicted sensible heat flux was better than that for latent heat flux at all three sites. The flux-variance method systematically overestimated the latent heat flux when compared to eddy-correlation measurements. It was demonstrated that the non-uniformity in water vapor sources reduced the surface flux when compared to an equivalent uniform terrain subjected to identical shear stress, sensible heat flux, and atmospheric water vapor variance. Finally, the correlation between the temperature and water vapor fluctuations was related to the relative efficiency of surface-layer turbulence in removing heat and water vapor. These relations were used to assess critical assumptions in the derivation of the flux-variance formulation.  相似文献   

On the aging of comets     

Fred L. Whipple 《Earth, Moon, and Planets》1995,71(3):69-78

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated ablation AGES. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater AGES tend to be less active and to show less dust in their spectra than comets of lesser AGES.Harvard-Smithsonian Center for Astrophysics  相似文献