Analytic construction of periodic orbits about the collinear points

A third-order analytical solution for halo-type periodic motion about the collinear points of the circular-restricted problem is presented. The three-dimensional equations of motion are obtained by a Lagrangian formulation. The solution is constructed using the method of successive approximations in conjunction with a technique similar to the Lindstedt-Poincaré method. The theory is applied to the Sun-Earth system.     

Devensian glacial events in Buchan (NE Scotland): A progress report on new OSL dates and their implications

Optically stimulated luminescence (OSL) dating of a series of glaciofluvial/glaciodeltaic sediments in central Buchan and Aberdeen has recently been undertaken. The aims of this project are to test the chronological model proposed by the most recent regional review and the suggestion that parts of Buchan may have remained ice-free during the last glacial maximum (late marine isotope stage (MIS) 3 and MIS 2 29–15 ka). The preliminary results indicate that during the Devensian (ca. 116–12 ka), extensive areas of Buchan may have been glaciated earlier than previously believed (possibly during MIS 4, 72–60 ka), but parts of the region show no depositional evidence of later glaciation. Some waterlain sediments from the Ugie Valley have yielded OSL ages indicating deposition during MIS 5d to MIS 5a (116–72 ka). We discuss whether the absence of overlying glaciogenic sediments at these locations raises questions about the reliability of the OSL ages or about existing models of the Devensian glaciation of Buchan.     

Investigation of river network evolution using luminescence dating and heavy mineral analysis of Late-Quaternary fluvial sands from the Great Hungarian Plain

To reconstruct the evolution of Late-Quaternary river network in the southeastern part of the Great Hungarian Plain, we have used optically stimulated luminescence (OSL) and heavy mineral analysis of 25 sand samples from the upper 2–8 m of the fluvial units, complemented by four radiocarbon ages. The estimated OSL depositional ages vary between 10 and 47 ka. The heavy mineral composition of the OSL samples was compared to the compositional data of recent river sediments using cluster analysis. The new OSL and heavy mineral data show that from 47 to 10 ka ago the sediments were transported mainly from the northeast direction into the southeastern part of the Great Hungarian Plain by the ancestor of the Tisza river and its northern tributaries, and probably by another large river which also flowed northeast–southwest, parallel to the modern Tisza. Between 23 and 14 ka sediments periodically came from the east and reached the eastern part of our study area. Between 15 and 12 ka ago, sands transported from the southeast also occur in the southeastern and central part of the study area. These data suggest that the modern rivers occupied their present courses only in the last 10 ka.     

Resetting of sediments mobilised by the LGM ice-sheet in southern Norway

Former geological field investigations in the Rondane area, east-central southern Norway, have proposed that the maximum Fennoscandian ice-sheet coverage occurred during the Late Weichselian Glacial Maximum (LGM, ca. 20 ka) and that subsequent glaciofluvial sediments were first deposited in the early Holocene (after 10 ka). However, recent field investigations with ages from three internally consistent quartz optically stimulated luminescence (OSL) age series show an apparent deglaciation of northern Rondane in the period 20.0–13.8 ka. We examine here the possibility that these ages are too old because the sediment was not completely zeroed prior to deposition. Our investigations of incomplete bleaching use modern analogues, small aliquots, and single grains of quartz. First, the symmetric shape of small aliquot equivalent dose distributions suggests that the sediment was probably well bleached at deposition. This is supported by 5 modern analogue equivalent doses (D_e) of 0.6 Gy, 1.5% of the typical D_e from the deglaciation sediments. Finally, from single grain studies on three samples, we conclude that there is no evidence for poor bleaching in these samples; thus the weighted mean gives the best estimates of D_e, and these are completely consistent with both large and small aliquot estimates for these samples. These comparisons between large aliquots, modern analogues, small aliquots and single-grain analyses help to validate the OSL ages and confirm the complete resetting of these sediments prior to deposition.     

Geological controls on the sulphur content of coal seams in the Northumberland Coalfield, Northeast England

The sulphur content of coal is an important consideration when developing reserves for exploitation, driven by emission limits from power stations becoming more stringent. Variations in the sulphur content of Westphalian A and B coals from the predominantly freshwater Northumberland Coalfield, Northeast England, were studied according to their regional, stratigraphic and in-seam location. The observed variation in sulphur content spatially increases towards the source area away from more marine influenced areas, with increased sulphur content through time linked to changes in the general depositional environment as conditions became more marine-influenced. A model of basinal surface water and groundwater flow driven by post-depositional source area tectonism is thought to have played only a minor role in contributing secondary sulphur to the coal. However, the isotopic composition of coal pyrite shows a similar range in composition to that of pyrite and other sulphides from the North Pennine Orefield along the southern margin of the coalfield, suggesting an additional potential source of secondary sulphur, as sulphur-rich fluids were expelled northwards through the coal measures during early Permian Variscan transpression from the south. The Westphalian A and B are interpreted as third-order depositional sequences, defined by third-order maximum flooding surfaces. Each sequence is made up of several coal-bearing fourth-order parasequences, which tend to be more brackish to marine in character, on either side of the third-order maximum flooding surfaces when base level was relatively high. The lowest sulphur coals are confined to the lower to middle, relative low stand part of the Westphalian A third-order base level curve and the lowest part of the Westphalian B third-order base level curve. This difference is attributed to a more rapid rise of base level in the Westphalian B. The stratigraphic and spatial distribution of coal sulphur has been used as a guide to prediction of reserve identification for surface mining operations.     

The use of ultra-sound in the preparation of carbonate and clay sediments for chironomid analysis

Initial investigations of Holocene carbonate sediment from Hawes Water, Northwest England, yielded lower numbers of chironomid head capsules than anticipated. Standard techniques used to prepare sediment for chironomid analysis were ineffective in breaking up the coarse crystalline sediment structure sufficiently. This led to large amounts of sediment being retained and increased sample processing times. The low yield of head capsules also meant that more sediment was needed to produce adequate numbers of head capsules for analysis. The use of ultra-sound as part of the sediment processing was investigated. This technique reduced the amount of sediment left for sorting and yielded significantly more head capsules which were of equivalent structural condition and cleaner than those produced by conventional methods. The technique was extended to clay samples where similar results were obtained although shorter treatment times are recommended. The proportion of Tanytarsini and Tanypodinae heads increased significantly in carbonate and clay samples, respectively; both sediment types showed a significant decline in the proportion of Chironomini. The results indicate that ultra-sonic preparation of samples will yield a more accurate representation of chironomid assemblages in sediments leading to greater sensitivity and reliability in analysing past environmental conditions.     

Simulations of superhumps and superoutbursts

We numerically study the tidal instability of accretion discs in close binary systems using a two-dimensional SPH code. We find that the precession rate of tidally unstable, eccentric discs does not only depend upon the binary mass ratio q . Although the (prograde) disc precession rate increases with the strength of the tidal potential, we find that increasing the shear viscosity ν also has a significant prograde effect. Increasing the disc temperature has a retrograde impact upon the precession rate.   We find that motion relative to the binary potential results in superhump-like, periodic luminosity variations in the outer reaches of an eccentric disc. The nature and location of the luminosity modulation are functions of ν. Light curves most similar to observations are obtained for ν values appropriate for a dwarf nova in outburst.   We investigate the thermal–tidal instability model for superoutburst. A dwarf nova outburst is simulated by instantaneously increasing ν, which causes a rapid radial expansion of the disc. Should the disc encounter the 3: 1 eccentric inner Lindblad resonance and become tidally unstable, then tidal torques become much more efficient at removing angular momentum from the disc. The disc then shrinks and M _d increases. The resulting increase in disc luminosity is found to be consistent with the excess luminosity of a superoutburst.