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51.
Huan Feng J. Kirk Cochran David J. Hirschberg Robert E. Wilson 《Estuaries and Coasts》1998,21(2):263-280
Multiple sediment cores were collected in June 1994 in the turbidity maximum zone of the Hudson River estuary off Manhattan, New York. Results from X-radiography of the sediments and measurements of natural radionuclides (234Th,7Be, and210Pb) and trace metals (Ag, Cd, Cu, Pb, and Zn) show significant spatial variability of sediment composition and structure and patchy distributions of radionuclides activities and trace metal concentrations in this small area (0.6 km × 0.5 km). Radionuclide and trace metal analyses confirm prior work (Olsen et al. 1978; Olsen et al. 1981; Hirschberg et al. 1996) that show the western margin area of the river acts as a repository of these chemical constituents at least for the short-term period (0.5–1 yr), and the mid-channel area is not a depositional area for sediments and associated chemical constituents.7Be profiles reveal short-term sediment deposition rates ranging from 6 cm yr?1 to 26 cm yr?1 in the western margin area. Significant spatial variations in excess234Th and7Be inventories (up to a factor of 10 and 5 for234Th and7Be, respectively) are found in the western margin depositional area, although the inventories are balanced, on average, with in situ production in water column and atmospheric supply. The spatial variation of surficial excess210Pb and trace metal concentrations in depositional areas of the western margin are ≤10% for Ag, Cu, Pb, and Zn and 29% for Cd. However, the variations in the transition zone range from 28% to 93%. This variability is likely related to variations in tidal current velocity, bottom shear stress, and river channel morphology. 相似文献
52.
Artie P. Hatzes & William D. Cochran 《Monthly notices of the Royal Astronomical Society》1998,293(4):469-478
The shape of the Ti i 6303.8-Å spectral line of Aldebaran as measured by the line bisector was investigated using high signal-to-noise ratio, high-resolution data. The goal of this study was to understand the nature of the 643-d period in the radial velocity for this star reported by Hatzes & Cochran. Variations in the line bisector with the radial velocity period would provide strong evidence in support of rotational modulation or stellar pulsations as the cause of the 643-d period. A lack of any bisector variability at this period would support the planet hypothesis.
Variations in the line asymmetries are found with a period of 49.93 d. These variations are uncorrelated with the 643-d period found previously in the radial velocity measurements. It is demonstrated that this 50-d period is consistent with an m =4 non-radial sectoral g-mode oscillation. The lack of spectral variability with the radial velocity period of 643 d may provide strong evidence in support of the hypothesis that this variability stems from the reflex motion of the central star due to a planetary companion having a mass of 11 Jupiter masses. However, this long-period variability may still be the result of a low-order ( m =2) pulsation mode as these would cause bisector variations of less than the error measurement. 相似文献
Variations in the line asymmetries are found with a period of 49.93 d. These variations are uncorrelated with the 643-d period found previously in the radial velocity measurements. It is demonstrated that this 50-d period is consistent with an m =4 non-radial sectoral g-mode oscillation. The lack of spectral variability with the radial velocity period of 643 d may provide strong evidence in support of the hypothesis that this variability stems from the reflex motion of the central star due to a planetary companion having a mass of 11 Jupiter masses. However, this long-period variability may still be the result of a low-order ( m =2) pulsation mode as these would cause bisector variations of less than the error measurement. 相似文献
53.
Near surface (<10 cm) sediment distributions of234Th sampled multiple times at five locations along the axis of the Hudson Estuary from the Upper Bay of New York to Haverstraw Bay are compared with210Pb data from longer cores at the same locations. The comparison indicates that while there is little net sediment accumulation anywhere except at one location in the Upper Bay, near surface sediment in this reach of the estuary is mobile on short (months) time scales. The sediment appears to be physically mixed rather than bioturbated. Comparison of the sediment inventories of234Th with calculated water column production indicates short time scale (months) variability in234Th deposition. Some parts of the bottom have234Th inventories in excess of local production but these appear to be balanced by234Th deficient areas, resulting in a general equilibrium. Sediment inventories of Pb, Cu, and Zn normalized to210Pb show no evidence of a uniquely urban source of these metals to the lower estuary. Silver distributions in sediment indicate a possible source of silver from New York City, probably related to sewage inputs. 相似文献
54.
55.
Ecological classification of macrophytes and phytobenthos for rivers in Germany according to the water framework directive 总被引:9,自引:0,他引:9
Jochen Schaumburg Christine Schranz Julia Foerster Antje Gutowski Gabriele Hofmann Petra Meilinger Susanne Schneider Ursula Schmedtje 《Limnologica》2004,34(4):283-301
A new assessment system for macrophytes and phytobenthos in German rivers meeting the requirements of the Water Framework Directive (WFD) of the European Community is described. Biocoenotic types based on biological, chemical and hydromorphological data from over 200 river sites covering the main ecoregions, hydromorphological stream types and degradation forms have been defined. For developing a classification system the quality element macrophytes and phytobenthos was divided into three components: macrophytes, benthic diatoms and remaining phytobenthos. For macrophytes seven types including one subtype, for benthic diatoms 14 types including three subtypes and for the remaining phytobenthos five river types were identified. The benthic vegetation at reference condition was described for most of the river types. Degradation is characterised as deviation in benthic vegetation species composition and abundance from the reference biocoenosis. For classification in five ecological status classes, several metrics were developed and used in combination with existing indices. For some of the described river types additional investigations are necessary before a classification system can be developed. 相似文献
56.
Spatially resolved absolute reflectivities of several regions of the Jovian disk in the wavelength region 3000 to 10760 Å are presented. Spectra were obtained of the central meridian and limbs of the Equatorial Region, North Equatorial Belt, and North Tropical Zone. Equivalent widths of several CH4 and NH3 bands are measured. The spatial variations of continuum reflectivity and absorption band profiles are shown in various ratio spectra. 相似文献
57.
Abstract— A meteoritic origin was proposed for the New Quebec Crater in 1949 on the basis of an aerial photograph showing its unique circularity and raised rim amid Precambrian gneisses of the Canadian Shield. At that time, only those few craters associated with meteorites were generally accepted as of impact origin. When the earliest field expeditions failed to find meteorites or impact products, two leading meteoriticists, Frederick C. Leonard and Lincoln LaPaz, cited the “Chubb” Crater as a flagrant example for which claims of meteoritic origin were advanced without valid proof. They also listed the Lake Bosumtwi Crater in Ashanti (now Ghana) among crater-like features, clearly of non-meteoritic origin, misidentified as meteorite craters. Controversy over the origin these two craters continued for decades. In Part I of this paper, we trace the investigations that led to the current acceptance of New Quebec as an authentic impact crater. We note that, for reasons that are not entirely clear, a meteoritic origin for the New Quebec Crater achieved wider acceptance at an earlier date than for the Lake Bosumtwi Crater, where petrographic and chemical evidence is more abundant and compelling. In Part II, we describe the petrography of two impact melt samples from the New Quebec Crater and present new evidence on the degrees of shock metamorphism affecting the accessory minerals: apatite, sphene, magnetite and zircon. Zircon, in particular, shows a range from euhedral grains with no signs of alteration to those decomposed to baddeleyite plus silica. 相似文献
58.
This paper presents an overview of the ways that natural radionuclides have been used as tracers of biogeochemical processes occurring in the coastal ocean. The radionuclides involved include those in the uranium and thorium decay series and those produced in the atmosphere by the interaction of cosmic rays with atmospheric gases (i.e., cosmogenic radionuclides). The property of radioactivity provides a chronometer with which to measure the rates of oceanic processes, and the fundamental biogeochemical behavior of the radionuclide determines which process(es) it may be used to trace. Examples from recent research will be presented in the talk. 相似文献
59.
60.
Cochran Anita Veverka Joseph Bell James Belton Michael Benkhoff Johannes Benkhoff Andrew Clark Benton Feldman Paul Kissel Jochen Mahaffy Paul Malin Michael Murchie Scott Neimann Hasso Owen Tobias Robinson Mark Schwehm Gerhard Squyres Steve Thomas Peter Whipple Fred Yeomans Donald 《Earth, Moon, and Planets》2000,89(1-4):289-300
In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished. 相似文献