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21.
By dissolving 30–400 kg of marine limestone in HCl and HF acid, our group has previously recovered common relict chromite grains (approximately 63–250 μm) from ordinary chondritic micrometeorites that fell on ancient sea floors, up to 500 Myr old. Here, we evaluate if CM group carbonaceous chondritic material, which makes up an important fraction of the micrometeorite flux today, contains analogous grains that can be searched for in acid residues. We dissolved 8 g of CM2 meteorite Acfer 331 in HF, which yielded a characteristic assemblage of both transparent Mg‐Al‐ and opaque Cr‐spinels >28 μm. We find on average 4.6 and 130 Mg‐Al‐spinel grains per gram in the 63–250 and 28–63 μm size fractions, respectively. These grains are mostly pink or colorless, and often characterized by heterogeneous Cr‐content. Black, opaque Cr‐spinel grains are absent from the >63 μm fraction, but in the 28–63 μm fraction we find approximately 65 such grains per gram meteorite. The individual grains have a characteristic composition, with heterogeneous major element compositions (e.g., 44.4–61.7 wt% Cr2O3), but narrow ranges for maximum TiO2 (0.6–1.6 wt%) and V2O3 (0.5–1.0 wt%) concentrations. The content of spinel grains in the 28–63 μm fraction of CM meteorites appears comparable at the order of magnitude level with the content of >63 μm sized chromite grains in fossil L‐chondrites from Ordovician limestone. Our approach of recovering meteoritic spinel from sediment may thus be extended to include CM meteorites, but the smaller size fraction of the acid residues should be searched. 相似文献
22.
Multivariate statistics are used to investigate sensitivity of the tropical atmospheric circulation to scenario-based global
land cover change (LCC), with the largest changes occurring in the tropics. Three simulations performed with the fully coupled
Parallel Climate Model (PCM) are compared: (1) a present day control run; (2) a simulation with present day land cover and
Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A2 greenhouse gas (GHG) projections;
and (3) a simulation with SRES A2 land cover and GHG projections. Dimensionality of PCM data is reduced by projection onto
a priori specified eigenvectors, consisting of Rossby and Kelvin waves produced by a linearized, reduced gravity model of
the tropical circulation. A Hotelling T
2 test is performed on projection amplitudes. Effects of LCC evaluated by this method are limited to diabatic heating. A statistically
significant and recurrent signal is detected for 33% of all tests performed for various combinations of parameters. Taking
into account uncertainties and limitations of the present methodology, this signal can be interpreted as a Rossby wave response
to prescribed LCC. The Rossby waves are shallow, large-scale motions, trapped at the equator and most pronounced in boreal
summer. Differences in mass and flow fields indicate a shift of the tropical Walker circulation patterns with an anomalous
subsidence over tropical South America. 相似文献
23.