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Pavle I. Premović 《Die Naturwissenschaften》1982,69(10):479-482
The Middle Precambrian chert of Gunflint formation contains an exceptionally high amount of bitumen free radicals. The Gunflint chert appears to be the oldest formation in which the presence of these radicals has been found. Their presence in this deposit constitutes evidence for the existence of primitive protovascular plants in the early Proterozoic environment of the Lake Superior Region of Canada. 相似文献
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Nada Horvatinčić José Luis Briansó Bogomil Obelić Jadranka Barešić Ines Krajcar Bronić 《Water, Air, & Soil Pollution: Focus》2006,6(5-6):475-485
The process of eutrophication in form of intense plant growth has been observed in some lakes and water streams at the Plitvice
Lakes National Park in central Croatia. Here we investigate whether this phenomenon is a consequence of anthropogenic pollution
or due to naturally produced organic matter in the lakes. We applied chemical analysis of water at two springs and four lakes
(nutrients, dissolved organic carbon (DOC), trace elements) and measurements of surface lake sediments (mineral and organic
fraction analyses, trace elements) in four different lakes/five sites. The chemical composition of water does not indicate
recent anthropogenic pollution of water because the concentrations of most trace elements are below detection limits. The
concentrations of DOC and nutrients are slightly higher in the area of increased eutrophication-plant growth. Also the content
of organic matter in the sediment is at the highest level in areas with highest C/N ratio indicating that the organic fraction of this sediment is mainly of terrestrial origin. There is no significant difference
among the trace element concentration in the upper segment of all cores, deposited approximately during last 50 years when
higher anthropogenic influence is expected due to development and touristic activity, and the lower part of the cores, corresponding
to the period approximately 100–200 years before present. The content of trace elements and organic matter in sediments decreases
from the uppermost lake downstream. According to our results there is no indication of recent anthropogenic pollution in water
and sediment. Higher concentrations of DOC in water as well as phosphorus and some other elements in the lake sediment can
be a consequence of input of natural organic matter to the lake water. 相似文献
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Summary This paper is a comparative study of several antennas commonly used in cellular telephones. These include a monopole, a helix-monopole
and a patch antenna. Each one of these structures is modeled and numerically tested using finite-difference time-domain simulation
and human models based on magnetic-resonance images, which allow for inclusion of details of the human body in the simulation.
The testing procedure involves antenna simulation in the proximity of the human head. The behavior of each antenna is evaluated
for variable distances from the head geometry (0, 1, 2, 4, 8 and 16 mm). Continuous waveform, representative of the sources
used in mobile telephones, (250 mW, 1.8 GHz) is used as the form of the antenna excitation. The simulation outputs used as
measures for this comparative study include transmitting and receiving antenna characteristics and the specific absorption
rate (SAR). The SAR levels for the head tissues are calculated for and with accordance to the two currently accepted standards:
Federal Communications Commission (FCC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP). The computed
SAR levels within each of the considered tissues vary for the three antennas under investigation and are within the determined
health safety standards. Results suggest that the patch antenna may be the structure of choice when considering safety standards,
as its radiation yields the lowest local SAR in the head tissues. 相似文献