Search

Feldspars occur with a range of major element chemistry and mineral structure. When dating sediments using coarse-grained feldspars, samples are prepared using density separation to make a crude distinction between potassium-rich and potassium-poor feldspar grains and other minerals. When applying luminescence dating techniques to rocks, even such relatively crude physical separation of minerals is not possible where intact rock slices are used. However, for both sediment and rock dating of feldspars, use of imaging detectors such as Electron-Multiplying Charge Coupled Devices (EMCCD) presents the opportunity to spatially resolve different mineral grains, offering potential advantages over the more conventional photomultiplier tubes that are typically used. Furthermore, scanning micro X-ray Fluorescence (µXRF) devices offer the opportunity to resolve major element chemistry at a similar resolution (~20µm) to EMCCD data, revealing different feldspar types and enabling these to be linked to luminescence behaviour. Such an approach would allow us to assess whether some feldspar types are better suited to dating than others. This study explores spatially-resolved optical resetting, dose response and fading properties for IRSL and IRPL signals in a selection of chemically and structurally characterized museum specimens of alkali and plagioclase feldspar. These feldspars were prepared as ~300 µm thick slices, and exposure to a SOL- 2 solar simulator was used to test optical resetting of the various luminescence signals examined. Spatiallyresolved detection of each luminescence signal was achieved using sensitive EMCCD detectors on SIRIOL [1] and DASH [2] heads mounted on a TL/OSL DA20 Risø reader. Maps of major element distributions for each feldspar slice were produced with a Bruker M4 Tornado Plus µXRF spectrometer. EMCCD and µXRF data were processed and analyzed using various packages in the Fiji distribution of the ImageJ software [3]. Bulk mineral structure was characterized for the feldspar samples by X-ray Diffraction (XRD) on powdered sample using a PANalytical X'Pert PRO Materials Powder Diffractometer, and bulk chemistry by XRF on pressed pellets of sample and cellulose binder using a Rigaku Primus IV XRF spectrometer. Spatial variability was observed in the luminescence properties across the signals and feldspars investigated. Relationships between the behaviour of these luminescence properties and major element chemistry are explored. The benefits of high spatial resolution measurements, and different feldspar types, are discussed in terms of luminescence dating of rocks and sediments. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar and luminescence dating studies of a large, complex, linear dune in the northern part of the Namib Sand Sea provide new information on the age and internal sedimentary structures of these dunes, with important implications for interpretations of paleoclimates and the rock record of eolian sandstones. The dune is a composite feature formed during several episodes of construction, including a hiatus of almost 2000 yr. The oldest sands within the dune are 5700 yr old, indicating complete turnover of sand during the Holocene. The dune has moved laterally by ∼300 m during the past 2500 yr, proving lateral migration of a large linear dune. Dune construction has been affected by climate change, and we attribute the hiatus to increased rainfall and vegetation, which largely halted sand movement and dune building in the Namib Desert during the middle Holocene. [ABSTRACT FROM AUTHOR]

Problems are often encountered in accurately establishing the rates of processes associated with river channel change. In this study, optical dating was used to obtain the lateral migration rate of an abandoned meander bend on the Klip River, eastern Free State, South Africa. Five samples were collected by augering along a transect across an approximately 200-m-wide scroll-bar sequence on the inner meander bend, and an additional sample was collected from the channel fill of the related oxbow lake. Optically stimulated luminescence (OSL) ages were derived for each sample using the single-aliquot regenerative-dose protocol on small aliquots of quartz. Incomplete bleaching was evident in some of the samples and thus the appropriate statistical analysis was performed using the 'finite mixture model' for the scroll-bar samples and the 'minimum age model' for the channel-fill sample. The ages calculated were in the correct chronological order for the depositional sequence, which was formed over the last ∼ 1000 years. The average rate of lateral migration across the scroll-bar sequence was found to be ∼ 0.16 m/yr but with an apparent increase in the rate occurring during the early sixteenth century. Where quartz-rich alluvium exists, the methods used in this study can potentially be applied to other river systems, enabling rates of channel change processes to be determined. [ABSTRACT FROM AUTHOR]

Rates of ice-stream retreat over decades can be determined from repeated satellite surveys and over millennia by paleoenvironmental reconstructions. Centennial time scales are an important temporal gap in geological observations of value in process understanding and evaluation of numerical models. We address this temporal gap by developing a 3 ka and 123 km retreat time series for the Irish Sea ice stream (ISIS), a major outlet draining the last British-Irish ice sheet. The Llŷn Peninsula (northwest Wales, UK) contains numerous ice-marginal indicators from which we reconstructed a robust chronological framework of margin oscillations. The landscape documents the retreat of a former marine-terminating ice stream through a topographic constriction, across a reverse bed slope, and across variations in calving margin width. New dating constraints for this sequence were integrated in a Bayesian sequence model to develop a high-resolution ice-retreat chronology. Our results show that retreat of the ISIS during the period 24-20 ka displayed centennial-scale oscillatory behavior of the margin despite relatively stable climatic, oceanic, and relative sea-level forcing mechanisms. Faster retreat rates coincided with greater axial trough depths as the ice passed over a reverse bed slope and the calving margin widened (from 65 to 139 km). The geological observations presented here over a 123-km-long ice-retreat sequence are consistent with theory that marine-based ice can be inherently unstable when passing over a reverse bed slope, but also that wider calving margins lead to much faster ice retreat. [ABSTRACT FROM AUTHOR]

A sequence of optically stimulated luminescence measurements was made on each of 8,961 grains from three sand samples from Blombos on the southern Cape coast. One sand unit overlay Middle Stone Age deposits in Blombos Cave. The measurement sequence, the single aliquot regenerative dose protocol, was used to obtain values for the total effective radiation dose to which each grain had been exposed since burial. A series of checks was carried out on each grain to ensure that the luminescence signals were reproducible, and that they were derived from quartz. This led to acceptance of less than 5% of the grains. An estimate of the radiation dose for the sand unit was obtained by combining the values using the central age model. In order to use a larger number of grains that might be representative of the sand unit, the radiation dose was also estimated by using the signal from the above grains, combined with the signals from those grains that had lower signals, but nonetheless contributed to the total light sum; this utilised between 9 and 18% of the grains. This enables us to obtain estimates of the ages as 67.3+/-3.8 ka, 65.6+/-2.8 ka and 68.8+/-3.0 ka for the three samples. These values agree with ages obtained using the single aliquot regenerative dose protocol for aliquots composed of several hundred grains.

An aeolian sand unit overlies the Middle Stone Age deposits at Blombos Cave on the southern Cape coast. These deposits contained culturally-important artefacts, including bone tools and pieces of engraved ochre, as well as a large number of worked lithics. The aeolian sand and two other remnants of the sand dune formed against the coastal cliff were dated using optical dating. To determine the dose received since deposition, measurements were made on 5mg aliquots of purified quartz grains using the single-aliquot regenerative-dose (SAR) protocol. The results of several internal check procedures are reported and at least 15 replicate dose determinations are presented for each sample. Combining these dose values with measurements of the radioactive content of each sample resulted in an age of 69.2+/-3.9 ka for the unit within the cave, and a mean age of 70.1+/-1.9 ka for all three dune samples. This provides a minimum age for the Middle Stone Age material at Blombos Cave.