Dynamical and collisional evolution of Halley-type comets

The number of observed Halley-type comets is hundreds of times less than predicted by models (Levison, H.F., Dones, L., Duncan, M.J. [2001]. Astron. J. 121, 2253–2267). In this paper we investigate the impact of collisions with planetesimals on the evolution of Halley-type comets. First we compute the dynamical evolution of a sub-set of 21 comets using the Mercury integrator package over 100 Myr. The dynamical lifetime is determined to be of the order of 10⁵–10⁶ years in agreement with previous work. The collisional probability of Halley-type comets colliding with known asteroids, a simulated population of Kuiper-belt objects, and planets, is calculated using a modified, Öpik-based collision code. Our results show that the catastrophic disruption of the cometary nucleus has a very low probability of occurring, and disruption through cumulative minor impacts is concluded to be negligible. The dust mantle formed from ejected material falling back to the comet’s surface is calculated to be less than a few centimeters thick, which is insignificant compared to the mantle formed by volatile depletion, while planetary encounters were found to be a negligible disruption mechanism.     

Deterioration of an infrastructure cutting subjected to climate change

Acta Geotechnica - Observations show that many soils in linear geotechnical infrastructure including embankments and cuttings undergo seasonal volume changes, and different studies confirm that...     

Construction,management and maintenance of embankments used for road and rail infrastructure: implications of weather induced pore water pressures

Understanding the age and construction quality of embankments used for road and rail infrastructure is critical in the effective management and maintenance of our transport networks, worth £billions to the UK economy. This paper presents for the first time results from full-scale, carefully controlled experiments on a unique model embankment conducted over the 4-year period between 2008 and 2011. It combines point location and spatially distributed measurements of pore water pressures and water content with outputs from hydrological modelling to draw conclusions of significance to both ongoing research in this field and to the asset management practices of infrastructure owners. For researchers, the paper highlights the crucial importance of transient permeability and soil water retention behaviour of fill materials in controlling the magnitude and distribution of pore water pressure in response to climate and weather events. For practitioners, the work demonstrates that there are significant differences in pore water pressure behaviour across the embankment, which is influenced by construction-related issues such as compaction level, aspect and presence of a granular capping material. Permeability was also observed to vary across the embankment both spatially and with depth, being dependent on degree of saturation and macroscale effects, particularly within a ‘near surface zone’. It is proposed that this ‘near surface zone’ has a critical effect on embankment stability and should be the focus of both ongoing scientific research and inspection and monitoring as encompassed by asset management regimes.     

A method of evaluating effects of antecedent precipitation on duststorms and its application to Yuma,Arizona, 1981–1988

Precipitation causes several short- and long-term effects on wind-induced surface erodibility and subsequent dust emission. Among the principal effects considered by this paper are soil moisture, soil crusts, and vegetation. A quantitative method is developed to assess these effects using differences between the potential and the actual amounts of dust emitted from dust sources as inferred from surface meteorological measurements obtained downwind from those sources. The results of this assessment must be interpreted with caution, however, when the size and location of dust sources are unknown.Using meteorological data recorded near Yuma, Arizona at the Yuma Marine Corps Air Station (YMCAS), the method is applied to calculate the potential and actual amounts of dust emitted from upwind dust sources during the spring and fall/winter seasons between January 1, 1981 and May 31, 1988. (Spring is considered to be the period between February 1 and May 31; fall/winter, between October 1 and January 31.) Because summer precipitation is intermittent and wind patterns are localized, summer meteorological data are not used to evaluate regional correlations between precipitation and dust storms. For the period between 1981 and 1988, a correlation of -0.60 was found between fall/winter precipitation and the actual amount of dust emitted from sources upwind of YMCAS during the following spring. A particularly strong reduction in dust emission was noted during the springs of 1983 and 1984 following the start of an El Nino event in fall/winter 1982. Photographs taken at a geological and meteorological data-collection (Geomet) site, located in the natural desert 25 km southeast of YMCAS, show a correspondence between increased antecedent precipitation recorded at the site and increased vegetation. Whereas the annual precipitation totals at YMCAS and the Geomet site from the beginning of 1982 through 1984 are high, their seasonal totals, especially during the fall/winter seasons, are disparate. This fall/winter precipitation disparity may account for evidence suggesting that significant vegetation growth occurred at dust sources upwind of YMCAS by spring 1983, but that such growth did not occur at the Geomet site until fall/ winter of 1983. Spatial inhomogeneity in fall/winter precipitation probably contributed to the relatively low correlation (-0.60) between fall/winter precipitation recorded at YMCAS and the actual amount of dust emitted from upwind sources during the following spring.     

Quantification and causes of the terrigeneous sediment budget at the scale of a continental margin: a new method applied to the Namibia–South Africa margin

The terrigeneous sediment budget of passive margin basins records variations in continental relief triggered by either deformation or climate. Consequently, it becomes a major challenge to determine sediment accumulation histories in a large number of basins found in various geodynamic contexts. In this study, we developed a GIS‐based method to determine the sediment budget at the scale of a whole basin (from the upstream continental onlap to the most distal deepest marine deposits) and the associated uncertainties. The volume of sediments preserved in the basin for each time interval was estimated by interpolation between cross‐sections and then corrected from in situ production and porosity to obtain terrigeneous solid volumes. This approach was validated by applying it to Namibia–South African passive margin basins for which independent data are available. We determined by a statistical approach the variances associated with each parameter of the method: the geometrical extrapolation of the section (8–43%), the uncertainties on seismic velocities for the depth conversion (2–10%), on the absolute ages of stratigraphic horizons (0.2–12%), on the carbonate content (0.2–46%) and on remaining porosities estimation (3–5%). Our estimates of the accumulated volumes were validated by comparison with previous estimates at a lower temporal resolution in the same area. We discussed variations in accumulation rates observed in terms of relief variations triggered by climate and/or deformation. The high accumulation rates determined for the Lower Cretaceous, progressively decreasing to a minimum in the Mid‐Cretaceous, are consistent with the progressive relaxation of a rift‐related relief. The following increase to an Upper Cretaceous maximum is consistent with a major relief reorganization driven either by an uplift and/or a change to more humid climate conditions. The lower accumulation rate in the Cenozoic suggests a relief reorganization of lesser amplitude over that period.     

Interest in geological and palaeontological curiosities by southern African non-western societies: A review and perspectives for future study

Compared with other parts of the world, the study of geomythology in southern Africa, and the associated documentation of non-western awareness of palaeontological and geological phenomena, is in an early phase. We focus on examples of rocks and fossils as items of special interest and curiosity, and we search for evidence of an indigenous palaeontology and geology. We review twenty-one sites or cases for which published accounts exist, and we describe a newly identified trilobite manuport site. In combination these sites provide various levels of evidence of palaeontological and geological awareness exhibited by non-western cultures in southern Africa, and how these cultures incorporated this knowledge into their understanding of their world. We anticipate that in time a diverse heritage of such ‘natural knowledge’ may become evident in southern Africa, aided in part by recognition of the possibility that rock art images may be associated with awareness of body fossils and trace fossils. We suggest ways in which further analysis may bolster this contention.     

The Origin of late Devensian sands and gravels,southeast Anglesey,N. Wales

The Lleiniog Sand and Gravel Formation, of late Devensian age, has been divided into four Members. The sediments are interpreted as the sequential products of the wasting of a single ice lobe. Member I formed where subglacial streams debouched into a pro-glacial lake and is noteworthy for the occurrence of a bedform, hitherto undescribed but which may be common in many outwash sequences; Member 2 consists of channel sands and gravels deposited by pro-glacial streams; Member 3 has the characters of a melt-out till deposited after a readvance; and Member 4 accumulated in pools of meltwater on the irregular surface of Member 3. Clasts above granule grade are dominantly of locally-derived Carboniferous rocks although a far travelled component from northern England and southern Scotland can be distinguished as well as a small but constant input of Lower Palaeozoic rock types from the Conwy area. The sand grade is dominated by quartz grains released from Permo-Triassic sandstones. Clay grade material has been derived from Carboniferous shales.