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991.
Scott A. Elias 《Quaternary Science Reviews》1997,16(10):1217-1225
The Mutual Climatic Range (MCR) method of palaeoclimate reconstruction has been employed in Europe for the last decade. A quantitative, calibrated method, MCR has many advantages over qualitative methods. More recent applications deal with eastern and central North America, and the method is also being developed for desert and arctic faunas. The climate envelopes for North American beetles have been compiled using a 25-km gridded North American climate database that pairs climate parameters with modern collection sites. Modern tests of the reliability of the MCR method for North American species yielded similar results to prior European tests. Linear regressions of predicted on observed values yielded equations used to calibrate the MCR estimates. Work is under way to develop MCR estimates of mean annual precipitation for fossil assemblages from the desert southwest, where moisture conditions may play a more important role in determining beetle species' ranges.An examination of British and North American mean July temperature reconstructions during the Late Wisconsinan glacial interval compares and contrasts three sets of records. The North American records show no indication of the Younger Dryas cooling that is clearly marked in records from northwest Europe. The MCR method adds vigour to our reconstructions, and allows us to compare between regions and with other palaeoenvironmental methods. 相似文献
992.
Sedimentary dykes hosted in ? Ordovician-Devonian strata on the Falkland Islands contain diamictite. The dykes, which are discordant to the host rocks, are sub-vertical, parallel-sided sheets formed by downward injection of a semi-fluidized sediment. On West Falkland palynomorphs present in 2 of the 11 dykes located demonstrate a Late Visean-Early Namurian age. This is older than the main Lafonian Diamictite Formation (Permian) and indicates that the dykes are a rare preservation of sediments formed during the main ice coverage of the Gondwana glaciation. Vitrinite reflectance from organic matter in the dykes indicates a shallow burial history (< 2–3 km) for the southwest Falkland Islands implying deposition of an incomplete or condensed sequence during Late Palaeozoic times, although the stratigraphical relationship of the dykes indicates that this may be extended back into the Mid Palaeozoic. The two diamictite dykes located on East Falkland are shown to be younger; probably Permian in age. 相似文献
993.
A.I. Olayinka 《Journal of African Earth Sciences》1997,24(4):635-647
The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the non-identification of a suppressed layer, has been investigated. The principal objective is to evaluate how the layer thicknesses and resistivities affect the accuracy of depth estimates. In the computations, the intermediate layer in a 3-layer model, in which the resistivity increases with depth, is removed and the 2-layer sounding curve that is electrically equivalent to the 3-layer curve is generated. The results indicate that there is a possibility for large depth underestimations when the resistivity contrast between layers 1 and 2 is very large. This is manifested in a steeply rising terminal branch on the sounding curve. There is a slight decrease in the depth underestimation as the resistivity contrast between layers 2 and 3 increases. Conversely, if the intermediate layer is fairly thick and the resistivity contrasts are not too large, the best-fit 2-layer curve shows large deviations from the 3-layer curve. In such cases, the intermediate layer can be identified, resulting in reliable depth estimates. A field example from Nigeria is presented in which the sounding data has been interpreted so as to account for a prebasement layer of intermediate resistivity, indicative of a fractured granite. 相似文献
994.
995.
A. L. Läufer W. Frisch G. Steinitz J. Loeschke 《International Journal of Earth Sciences》1997,86(3):612-626
The Eder unit in the Carnic Alps, which is situated immediately south of the Periadriatic lineament (PL), represents a fault-bounded block consisting of a low-grade (up to 400?°C, indicated by epizonal illite “crystallinity” values, recrystallized quartz, and non-recrystallized white mica) metamorphic Paleozoic metasedimentary sequence. Until now, it has been assumed to represent a separate Variscan nappe. The rocks of the Eder unit show a strong E- to W-oriented stretching lineation on steep foliation planes (D1) subparallel to the PL. D1 structures originated near the temperature peak of metamorphism, and shear sense indicators show dextral ductile shear parallel to the PL. Tight mesoscale D2 folds formed on the cooling path. K–Ar and Ar–Ar ages from newly formed white mica cluster around 32–28 and 18–13 Ma and suggest a two-stage Tertiary history of the Eder unit. We interpret the Eder unit as a fault-bounded block formed during Oligocene large-scale dextral shearing along the PL (near Tmax) and exhumed in mid-Miocene times during another phase of activity along the PL. Its nature as a separate Variscan nappe is questioned. 相似文献
996.
There is a need for research into bioindicators of stress in threatened plant communities such as coastal wetlands. Land subsidence, diversion of sediment, and salt-water intrusion produce stresses associated with waterlogging, elevated salinity, and nutrient depletion. Temporal and spatial environmental variation (soil redox potential, interstitial water salinity, pH, ammonium and phosphorus, and cation and trace metal concentrations) was analyzed near Lake de Cade, Louisiana, in a brackish marsh which is a mosaic of healthy plant communities interspersed with areas where wetland loss is occurring. Environmental variation was related to indicators of stress inSpartina patens, which included variables derived from the adenine nucleotide levels in plants, leaf spectral reflectance, leaf proline concentrations, and shoot elongation. In a comparison of burned and unburned sites, streamside and inland marsh, and along a salinity gradient, among-site differences were found in spectral reflectance and adenine-nucleotide-related indicators. Although it was difficult to relate a single causal environmental variable to the response of a specific indicator, spectral reflectance in the visible light range responded to salinity or to elements borne in seawater, and adenine-nucleotide indices were sensitive to nutrient availability. The ability of indicators to detect plant responses changed during the growing season, suggesting that they were responding to the changing importance of different environmental factors. In addition, some reflectance indicator responses occurred along salinity gradients when salinity differences were less than those that were found to have ecologically meaningful effects in greenhouse experiments. A multivariate numerical approach was used to relate environmental variation with indicator responses. We concluded that factors which in combination cause the degradation and loss of Louisiana wetlands produce environmental conditions that are only subtly different from those in vigorously growing marsh communities. 相似文献
997.
998.
P. K. Mukhopadhyay F. Goodarzi M. A. Kruge M. H. Alimi 《International Journal of Coal Geology》1997,34(3-4)
Organic-rich from the Schei Point group (middle to late Triassic in age) and the Ringnes formation (late Jurassic) from the Sverdrup basin of the Canadian arctic archipelago have been geochemically evaluated for source rock characterization. Most samples from the Schei Point group are organic-rich (> 2% TOC and are considered as immature to mature oil-prone source rocks [kerogen types I, I–II (IIA) and II (IIA)]. These kerogen types contain abundant AOM1, AOM2 and alginite (Tasmanales, Nostocopsis, Leiosphaeridia, acritarch and dinoflagellate) with variable amounts of vitrinite, inertinite and exinite. Samples from the Ringnes formation contain dominant vitrinite and inertinite with partially oxidized AOM2, alginite and exinite forming mostly immature to mature condensate- and gas-prone source rocks [kerogen type II–III (IIB), III and a few II (IIA)]. Schei Point samples contain higher bitumen extract, saturate hydrocarbons and saturate/ aromatic ratio than the Ringnes samples. Triterpane and sterane (dominant C30) distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the Schei Point group are at the onset of oil generation and contain a mixture of sapropelic (algal) and minor terrestrial humic organic matter. Sterane carbon number distributions in the Ringnes formation also suggest a mixed algal and terrestrial organic matter type. There are some variations in hopane carbon number distributions, but these are apparently a function of thermal maturity rather than significant genetic differences among samples. Pyrolysis-gas chromatography/mass spectrometry of the two samples with similar maturity shows that the Schei Point sample generates three times more pyrolyzate than the Ringnes sample. Both samples have a dominant aliphatic character, although the Ringnes sample contains phenol and an aromaticity that is higher than that of the Schei Point sample. 相似文献
999.
The Sakoa Group is the lowermost stratigraphical succession of the Karoo Supergroup and the oldest sedimentary unit in Madagascar, spanning the Late Carboniferous through Early Permian epochs. The Sakoa Group is exposed in the southern Morondava Basin. It is predominantly a siliciclastic sequence comprising seven lithofacies associations: (1) diamictites; (2) conglomeratic sandstones; (3) sandstones; (4) interbedded thin sandstones and mudstones; (5) mudstones; (6) coals; and (7) limestones. These facies represent deposition in the early extensional stages of continental rift development. The sediments were deposited predominantly on alluvial fans, and in braided to meandering stream and overbank environments. Locally lacustrine and coal swamp environments formed in low areas of the basin floor during rift initiation. Subsidence rates remained fairly constant throughout the Early Permian and were accompanied by a gradual reduction in relief of the basin margins and an increased geomorphic maturity of the fluvial systems flowing across the basin floor. Near the end of the Early Permian the southern Morondava Basin was inundated by a marine transgression , which resulted in deposition of the Vohitolia Limestone. Subsequent tectonic uplift and erosion resulted in a regional unconformity between the Sakoa Group and the overlying Sakamena Group. 相似文献
1000.
Stanislav V. Sinogeikin Jay D. Bass Bridget O'Neill Tibor Gasparik 《Physics and Chemistry of Minerals》1997,24(2):115-121
The adiabatic elastic moduli of polycrystalline En50Py50 and En80Py20 majorite-garnet solid solutions (where En=4MgSiO3, Py=Mg3Al2Si3O12) and the end-member En100 tetragonal majorite were determined at ambient conditions using Brillouin spectroscopy. The adiabatic bulk modulus, K, and
shear modulus, μ, of En50Py50 were found to be K=173.1 (20) and μ=92.3 (10) GPa, and are indistinguishable from those of the end-member pyrope, Py100. The moduli of the more majorite-rich samples are significantly lower and are virtually identical to each other (K=162.6(11)
and μ=85.7(7) for En80Py20; K=166(5) and μ=88(2) for En100). In combination with previously reported moduli for this system, we conclude that both K and μ are constant over the compositional
range from Py100 to a majorite content of about 70–80%, whereupon the moduli decrease substantially. For compositions ranging from En80Py20 to the end-member majorite, the moduli are also approximately independent of composition, but at a lower value. An alternative
model with a continuous decrease in moduli with increasing majorite content cannot be excluded, within the uncertainties of
existing measurements. The contrast in moduli between aluminous pyrope garnet and Al-free majorite are small compared with
the modulus changes accompanying the pyroxene – majorite phase transformation.
Received August 16, 1995 /Revised, accepted January 12, 1996 相似文献