968 resultados para odd-odd nuclei


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The quantity, type, and maturity of the organic matter of Quaternary and Tertiary sediments from the Japan Trench (DSDP Leg 56, Sites 434 and 436; and Leg 57, Sites 438, 439 and 440) were determined. The hydrocarbons in lipid extracts were analyzed by capillary- column gas chromatography and combined gas chromatography/ mass spectrometry. Kerogen concentrates were investigated by microscopy, and vitrinite-reflectance values were determined. Measured organic-carbon values were in the range of 0.13 to 1.00 per cent. Extract yields, however, were extremely low. Normalized to organic carbon, total extracts ranged from 4.1 to 15.7 mg/g Corg. Gas chromatography of non-aromatic hydrocarbons showed that all sediments, except one Oligocene sample, contained very immature, mainly terrigenous organic material. This was indicated by n-alkane maxima at C29 and C31 and high odd-carbon-number predominances. Unsaturated steroid hydrocarbons were found to be major cyclic compounds in lower- and middle-Miocene samples from the upper inner trench slope (Sites 438 and 439). Perylene was the dominating aromatic hydrocarbon in all but the Oligocene sample. Microscopy showed kerogens rich in terrigenous organic particles, with a major portion of recycled vitrinite. Nevertheless, almost all the liptinite particles appeared to be primary. This is a paradox, as the bulk of the samples were composed of hemipelagic mineral matter with a major siliceous biogenic (planktonic) component. A trend of reduced size and increased roundness can be seen for the vitrinite/ inertinite particles from west to east (from upper inner trench slope to outer trench slope). All sediments but one are relatively immature, with mean huminite-reflectance values (Ro)in the range of 0.30 to 0.45 per cent. The oldest and deepest sediment investigated, an Oligocene sandstone from Site 439, yielded a mean vitrinitereflectance value of 0.74 per cent and a mature n-alkane distribution. This sample may indicate a geothermal event in late Oligocene time. It failed to affect the overlying lower Miocene and may have been caused by an intrusion. Boulders of acidic igneous rocks in the Oligocene can be interpreted as witnesses of nearby volcanic activity accompanied by intrusions.

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This study investigates organic-rich sedimentary sequences deposited during the early Aptian Oceanic Anoxic Event (OAE1a) at Sites 1207 and 1213 on Shatsky Rise (ODP Leg 198) in the west-central Pacific. Biomarker analyses provide evidence of the algal and bacterial origin of organic matter (OM) in these sediments where the abundance of steroidal components, particularly sterenes and sterones, suggests that the OM includes major contributions from eukaryotic sources in an environment characterized by high phytoplankton productivity. The presence of alkenones at Site 1213B is diagnostic of OM derived from representatives of haptophyte algae among the calcareous nannoplankton and their d13C values (average -31.6 per mil) are consistent with those expected during elevated pCO2. The occurrence and prominence of 2b-methylhopanes and 2b-methylhopanones indicates significant contributions to the OM from cyanobacteria, which are also likely contributors of hopanoids based on their d13C compositions. These biomarker data suggest that oceanic conditions, perhaps nitrate- or iron-limited, were conducive to cyanobacteria production during OAE1a, which appears to distinguish this event from other Cretaceous OAE.

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A valid assessment of selective aerobic degradation on organic matter (OM) and its impact on OM-based proxies is vital to produce accurate environmental reconstructions. However, most studies investigating these effects suffer from inherent environmental heterogeneities. In this study, we used surface samples collected along two meter-scale transects and one longer transect in the northeastern Arabian Sea to constrain initial OM heterogeneity, in order to evaluate selective aerobic degradation on temperature, productivity and alteration indices at the sediment-water interface. All of the studied alteration indices, the higher plant alkane index, alcohol preservation index, and diol oxidation index, demonstrated that they are sensitive indicators for changes in the oxygen regime. Several export production indices, a cholesterol-based stanol/stenol index and dinoflagellate lipid- and cyst-based ratios, showed significant (more than 20%) change only over the lateral oxygen gradients. Therefore, these compounds do not exclusively reflect surface water productivity, but are significantly altered after deposition. Two of the proxies, glycerol dibiphytanyl glycerol tetraether-based TEX86 sea surface temperature indices and indices based on phytol, phytane and pristane, did not show any trends related to oxygen. Nevertheless, unrealistic sea surface temperatures were obtained after application of the TEX86, TEX86L, and TEX86H proxies. The phytol-based ratios were likely affected by the sedimentary production of pristane. Our results demonstrate the selective impact of aerobic organic matter degradation on the lipid and palynomorph composition of surface sediments along a short lateral oxygen gradient and suggest that some of the investigated proxies may be useful tracers of changing redox conditions at the sediment-water interface.

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We report on diatom abundance and preservation stratigraphy in the uppermost four cores of Ocean Drilling Program Hole 806B, which span the Quaternary period. Changes in diatom abundance and preservation show a rather complicated pattern, with much noise at high frequencies. However, in the cycles corresponding to eccentricity and obliquity variations, the picture is quite clear. Abundance and preservation follow glacial-interglacial cycles, with lowest abundances and poorest preservation observed in sediments that correspond to glacial stages. Seventy taxa compose the diatom assemblage of Hole 806B (from Samples 130-806B-1H-1, 8-9 cm, to -4H-7, 73-74 cm) with Azpeitia nodulifera as the dominant member. This species exhibits significant size variations related to glacial and interglacial stages during the Pleistocene. The distribution of power in the Fourier spectrum of the diatom signal (in the time domain) displays the expected Milankovitch frequencies (at 100,41, and 24-18 k.y.). It also shows concentration at various "odd" frequencies, especially at 62 k.y., suggesting a complicated response of productivity (and silicate chemistry) to climatic forcing.

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Bituminologic analysis of sediment cores from the Black Sea (water depth up to 2000 m, drilling depth up to 625 m) has revealed all components typical for fossilized rocks, viz. hydrocarbons, resins, asphaltenes, and insoluble matter. Proportions of these components, their composition and properties do not display any dependence on depth in hole and seem to be governed by composition of organic matter and conditions and degree of its transformation at early stages of lithogenesis.

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We generated preliminary downcore records of total organic carbon content, calcium carbonate, long-chain n-alkane concentration, total alkenone concentration, and alkenone-based sea-surface temperature for samples from the easternmost flank of Nazca Ridge (Site 1237) and the eastern crest of Carnegie Ridge (Site 1239). Total organic carbon and long-chain n-alkane concentrations will be used to evaluate terrestrial sediment sources. Downcore records of alkenone sea-surface temperature will benefit studies of paleoceanography of the southeastern Pacific. Since these sites are located under the influence of major tectonic events, such as the uplift of the Andes Mountains and the closure of the Isthmus of Panama, the records will help us to examine the effects of the tectonic events on the oceanic environment.

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The measurement of 28O-Mev neutron cross sections of various nuclei is described. The neutron beam used results from 340-Mev protons in the 184-in. cyclotron striking a 2-in. Be target. The neutron detector consists of a double coincidence anthracene scintillation counter telescope which counts recoil protons scattered at 15 deg from a paraffin cylinder placed in the collimated neutron beam. A 2-in. Cu absorber placed between the counters assures that only protons of energy greater than 250 Mev are counted. The cross sections for all nuclei measured from Li to Pb are smaller than the corresponding cross sections measured at 90 Mev by factors between 0.5 and 0.6.

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"Answers to odd-numbered problems": p. 375-393.

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Federal Highway Administration, Office of Safety and Traffic Operations Research and Development, McLean, Va.

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Advertisement for Fred. Lundberg Florist on p. [1] at end.

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Thesis statement on label affixed to inside front cover.

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Description based on: 156. Bd.