Detecting post-depositional sediment disturbance in sandy deposits using optical luminescence

Post-depositional mixing or exhumation is common in surficial sediments, yet may be unobservable from field evidence. However, any disturbance may have significant consquences in terms of establishing a reliable luminescence age determination. Optically stimulated luminescence (OSL) measurements, particularly measurements at the single grain level, can be used to gain an insight into both contemporary and past post-depositional processes.

This paper examines sites from Texas and Florida (USA) with independent chronological control to demonstrate the potential effects of varying degrees of bioturbation on OSL. Results show that contemporary soil forming processes clearly impact on the palaeodose (D_e) replicate distributions which are measured in order to derive an OSL age. Significant levels of scatter and apparently zero dose grains are observed in the upper-most sediments; declining with depth from the surface. D_e replicates from undisturbed and fully bleached sediments are unskewed, show low overdispersion (OD) and comparable single grain and single aliquot OSL ages. Bioturbated sediments, however, may show highly skewed multi-model D_e distributions with higher OD values, zero dose grains at depth, and significant diffences between single grain and single aliquot results. True burial ages may be derived from minimally bioturbated sediments through the application of statistical analysis such as finite mixture modelling to isolate D_e components. However, for significantly bioturbated sediments, the latter approach, even at the single grain level, produces inaccurate ages. In such cases we argue that additional evidence (both dating and contexual) may be required to identify with confidence the burial D_e population.     

Developing a protocol for selecting and dating sandy sites in East Central Texas: Preliminary results

Vast areas of East Central Texas are overlain by deep unconsolidated sands, previously assumed to have been formed by in situ weathering and pedoturbation of the bedrock. This hypothesis would imply that palaeoenvironmental and archaeological reconstructions are of limited value. However, recent research has hinted that some elements of the landscape may have undergone repeated phases of erosion and redeposition, suggesting firstly that a reliable record of past environmental conditions may be gleaned from these deposits, and secondly that optically stimulated luminescence (OSL) may provide the key to unlocking this record. This research examines the depositional history of sites shown to provide a reliable chronology. Dates from these sites show that aeolian deposition occurred in East Central Texas at a number of times during the Holocene and Late Glacial, whilst three episodes of gullying occurred between approximately 123,000 and 44,000 years ago. The concordance of some of these results indicates that some phases of deposition may be regional.     

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On the nature of the anti-tail of Comet Kohoutek (1973f). II. Comparison of the working model with ground-based photographic observations

Photographic observations of the anti-tail of Comet Kohoutek (1973f), obtained at the Cerro Tololo Inter-American Observatory, are photometrically reduced and the results compared with a recently formulated working model of the anti-tail. The applied technique of photometric reduction is described, and the radial and transverse profiles of the anti-tail, corrected for the effects of atmospheric extinction and the variable intensity of the ambient sky, are derived. Brightness variations in time are also studied. The most important result reached so far is a quantitative confirmation of the previously suggested hypothesis, arguing that dust particles in the anti-tail suffered a significant loss in radius due to evaporation near the perihelion passage. We find that only particles initially larger than 100–150 μm in diameter (at an assumed density of 1 gcm^?3) survived. Numerically, however, this result is still preliminary, because the dynamical effect associated with particle evaporation remains to be explained. It is also tentatively suggested that the emission rate of the dust from the comet was probably comparable with the rates derived earlier for Comets Arend-Roland (1957 III) and Bennett (1970 II).     

High resolution absorption cross section measurements and band oscillator strengths of the (1, 0)−(12, 0) Schumann-Runge bands of O2

Cross sections of O₂ at 300 K have been obtained from photoabsorption measurements at various pressures throughout the wavelength region 179.3–201.5 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm^?1 grating and having an instrumental width (FWHM) of 0.0013 nm. The measured absorption cross sections of the Schumann-Runge bands (12, 0) through (1, 0) in this wavelength region are absolute, i.e., independent of the instrumental width, a result not achieved previously. The measured cross sections are presented graphically and are available at wavenumber intervals of > sim; 0.1 cm^?1 as numerical complications stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Band oscillator strengths of the (12, 0) through (1, 0) bands have been determined by direct numerical integration of the measured cross sections.