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91.
This paper reviews the pattern of climate and environmental change in eastern England over the period of the Early and Middle Pleistocene, focussing especially upon northern East Anglia. Particular attention is given to the climate and tectonics that have brought about these changes and the distinctive geology, topography and biology that has developed. Throughout, an attempt is made to describe the new models that have been proposed for the Early and Middle Pleistocene of eastern England, and explain the reasons for these changes. The Early Pleistocene experienced relatively high insulation and relatively low magnitude climatic change and is represented primarily by non-climatically forced processes in the form of tidal current- and wave-activity which formed shallow marine deposits. It is possible to recognise a tectonic control in the distribution of deposits of this age because the surface processes do not have the power to remove this signature. The early Middle Pleistocene was dominated by higher magnitude climatic change involving, occasionally, climatic extremes that ranged from permafrost to mediterranean. The landscape at this time was dominated by the behaviour of major rivers (Thames, Bytham, Ancaster) and extensive coastal activity. In the latter part of the early Middle Pleistocene and the Late Middle Pleistocene the climate experienced major changes which resulted in periods of lowland glaciation and short intervals when the climate was warmer than the present. Details of tectonic activity are difficult to identify because they are removed by powerful surface processes, but it is possible to infer uplift focussed on the major interfluves of central England and subsidence in the North Seas basin. In the areas of glaciation the landscape changed radically from an organised terrain dominated by large rivers and extensive shallow coastal zones to complex, with small valleys, disrupted drainage and often discontinuous river, slope and coastal deposits. Likewise the switching off of the North Sea Delta and the opening of the Strait of Dover, separating Britain from continental Europe can be attributed to the onset of lowland glaciation. The case is made that eastern England was glaciated four times during the Middle Pleistocene: during MIS 16, 12, 10 and 6, and attention is given to recent evidence contradicting this model. Over the period of the Middle Pleistocene there is evidence for high biomass production occurring over short intervals coinciding with the climatic optima of MIS 19, 17, 15, 13, 11, and 7c, 7a and during most of these warmer periods, extending back to c. 750 ka (MIS 19/17), there is evidence in the region for the brief appearance of humans. 相似文献
92.
D. M. D. James 《Geological Journal》1990,25(2):199-204
Several modifications are proposed to the Ordovician stratigraphy recently proposed by Mackie and Smallwood (1987), based in particular on the recognition of a major Mynydd Trawsnant anticline on the northwest flank of the Tywi structure and on field relationships indicating the Foel Formation directly to overlay the Caradoc black shale succession and directly to underlay the Craig Las Formation. The Clyn Glas Formation is suggested to have accumulated at a slope base rather than a shelf–slope break. 相似文献
93.
94.
James F. Kasting 《Astrophysics and Space Science》1996,241(1):3-24
The goal of this conference is to consider whether it is possible within the next few decades to detect Earth-like planets around other stars using telescopes or interferometers on the ground or in space. Implicit in the term “Earth-like” is the idea that such planets might be habitable by Earth-like organisms, or that they might actually be inhabited. Here, I shall address two questions from the standpoint of planetary atmosphere evolution. First, what are the chances that habitable planets exist around other stars? And, second, if inhabited planets exist, what would be the best way to detect them? 相似文献
95.
96.
Day-night contrasts in the equatorial infrared (8- to 14-ωm) flux from Venus were observed in digital images obtained during the 1975 apparition. A systematic increase in 8- to 14-ωm flux associated with the emergence of the atmosphere from daylight into evening was observed and is shown to exhibit significant day-to-day variability. The data also show that the daytime side of the morning terminator is warmer than the dark side, but this night-day asymmetry is systematically smaller than the day-night brightening seen in the Venus evening. 相似文献
97.
James B. Pollack Donald W. Strecker Fred C. Witteborn Edwin F. Erickson Betty J. Baldwin 《Icarus》1978,34(1):28-45
We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H2SO4 and H2O contained in the cloud material both equal approximately 2× 10?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10?4 and 10?2. 相似文献
98.
James W. Head III 《Earth, Moon, and Planets》1979,21(4):439-462
New topographic data allow a reassessment of the ring structure of the Serenitatis basin and correlation with the younger Orientale basin. The northern Serenitatis basin is smaller and less well preserved than the southern Serenitatis basin. Three major rings of the main (southern) Serenitatis basin are mapped: ring 1, Linné ring, outlined by mare ridges, average diameter 420 km; ring 2, Haemus ring, outlined by basin-facing scarps and massifs with crenulated borders, 610 km; ring 3, Vitruvius ring, outlined by basin-facing linear scarps and massifs, 880 km. Ring 1 corresponds to the inner Rook Mountain ring of Orientale, ring 2 with the outer Rook ring, and ring 3 with the Cordillera Mountain ring. These ring identifications and assignments indicate that the Serenitatis basin is essentially the same size as the Orientale basin, rather than much larger, as previously proposed. The Apollo 17 site lies near the second ring, which is interpreted as the rim of the transient cavity. Apollo 15 lies at the junction of the Serenitatis and Imbrium third rings; Serenitatis ejecta should be present in significant amounts at the Apollo 15 site. The new reconstruction indicates that portions of the Serenitatis basin are better preserved than previously thought, consistent with recent stratigraphic and sample studies that suggest an age for Serenitatis which is older than, but close to, the time of formation of the Imbrium basin. 相似文献
99.
Andrew W. Leising James J. Pierson Claudia Halsband-Lenk Rita Horner James Postel 《Progress in Oceanography》2005,67(3-4):406
During late winter and spring of 2002 and 2003, 24 two- to three-day cruises were conducted to Dabob Bay, Washington State, USA, to examine the grazing, egg production, and hatching success rates of adult female Calanus pacificus and Pseudocalanus newmani. Here, we discuss the results of our grazing experiments for P. newmani. Each week, we conducted traditional microzooplankton dilution experiments and “copepod dilution” experiments, each from two different layers. Grazing was measured by changes in chlorophyll concentration and direct cell counts. Clearance rates on individual prey species, as calculated by cell counts, showed that Pseudocalanus are highly selective in their feeding, and may have much higher grazing rates on individual taxa than calculated from bulk chlorophyll disappearance. The grazing rates of the copepods, however, are typically an order of magnitude lower than the grazing rates of the microzooplankton community, or the growth rates of the phytoplankton. P. newmani ingested diatoms, but, at certain times fed preferentially on microzooplankton, such as ciliates, tintinnids, and larger dinoflagellates. Removal of the microzooplankton may have released the other phytoplankton species from grazing pressure, allowing those species’ abundance to increase, which was measured as an apparent “negative” grazing on those phytoplankton species. The net result of grazing on some phytoplankton species, while simultaneously releasing others from grazing pressure resulted in bulk chlorophyll-derived estimates of grazing which were essentially zero or slightly negative; thus bulk chlorophyll disappearance is a poor indicator of copepod grazing. Whether copepods can significantly release phytoplankton from the grazing pressure by microzooplankton in situ, thus causing a trophic cascade, remains to be verified, but is suggested by our study. 相似文献
100.
In the recent paper by Tai-Wen Hsu, John R.-C. Hsu, Wen-Kai Weng, Swun-Kwang Wang, and Shan-Hwei Ou (Coastal Engineering, 53, 865–877, 2006), the authors derived theoretical formulations for calculating the wave setup and setdown induced by obliquely incident waves on a beach. The derivation of an expression for setdown contains errors which would lead to an imbalance in longshore momentum flux outside the surfzone. We correct their derivation and give results in terms of the radiation stress concept in a general case including an oblique wave incidence. We also point out that the correct form of wave setdown is important to describe the zero-net force in the momentum balance outside the surfzone. 相似文献