D. Banerjee

Equivalent dose estimation using a single aliquot of polymineral fine grains

We have tested the suitability of a new single-aliquot regenerative-dose protocol for estimating the equivalent dose (De) in polymineral fine grains extracted from colluvia from various sites in Germany. First, we report the behaviour of three OSL signals: (i) blue-stimulated, (ii) infrared-stimulated luminescence, and (iii) blue-stimulated luminescence following infrared (IR) stimulation, using a near-UV (290–380 nm) detection window in each case. For these three signals, there is a significant change in sensitivity with regeneration cycle; this change can be compensated for using the response to a fixed test dose after each natural or regenerated measurement. The source of the three luminescence signals is then investigated using pulse-anneal and elevated-temperature experiments. Fading tests on laboratory-induced signals show that although the IR signals fade by up to 23% in 15 days at 100°C, the post-IR blue signals are stable. The preheat dependence of estimates of De obtained using fine grains is presented for the first time, for both blue- and IR-derived signals. Our results are compared with De estimates derived from multiple-aliquot additive-dose IR luminescence data, obtained using a blue detection window, and also with expected values of De based on independent age estimates and measured dose rates. We conclude that post-IR blue-stimulated luminescence provides reliable estimates of De, and that these are probably superior to the IRSL estimates obtained using both near-UV and blue detection windows.

Sea-level and environmental changes since the last interglacial in the Gulf of Carpentaria, Australia: an overview

Abstract The Gulf of Carpentaria is an epicontinental sea (maximum depth 70xa0m) between Australia and New Guinea, bordered to the east by Torres Strait (currently 12xa0m deep) and to the west by the Arafura Sill (53xa0m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf was separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, an isolation basin, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. A preliminary interpretation is presented of the palaeoenvironments recorded in six sediment cores collected by the IMAGES program in the Gulf of Carpentaria. The longest core (approx. 15xa0m) spans the past 130xa0ka and includes a record of sea-level/lake-level changes, with particular complexity between 80 and 40xa0ka when sea-level repeatedly breached and withdrew from Gulf/Lake Carpentaria. Evidence from biotic remains (foraminifers, ostracods, pollen), sedimentology and geochemistry clearly identifies a final marine transgression at about 9.7xa0ka (radiocarbon years). Before this transgression, Lake Carpentaria was surrounded by grassland, was near full, and may have had a surface area approaching 600xa0km×300xa0km and a depth of about 15xa0m. The earlier rise in sea-level which accompanied the Marine Isotopic Stage 6/5 transgression at about 130xa0ka is constrained by sedimentological and biotic evidence and dated by optical- and thermoluminescence and amino acid racemisation methods.

Optically stimulated luminescence dating of Holocene relict foredunes, Guichen Bay, South Australia.

Abstract Optically stimulated luminescence (OSL) dating of quartz sand (106–150 or 150–200xa0μm) in bioclastic carbonate–quartz sediments sampled from coastal relict foredunes (beach ridges) was undertaken to evaluate the utility of the OSL method for studies of dune dynamics and to quantify rates of coastal progradation. Twelve sediment samples from a 4xa0km transect across a Holocene embayment fill at Guichen Bay, South Australia, were measured for their luminescence characteristics. Apart from one age inversion attributed to recent disturbance associated with mining, the OSL ages are in sequential order when considered in the context of their associated error terms. The OSL ages indicate an extremely rapid initial phase of sedimentation (i.e. 1600xa0m within a few hundred years, approximately 5xa0ka ago) followed by a constant rate of progradation for the past 4xa0ka of 0.39xa0m/a, with a high level of association between distance across the embayment fill and luminescence age for this time interval. An average rate of dune development of one dune every 80xa0yr from 3900xa0yr ago to the present day is apparent. The OSL ages for the late Holocene indicate that the present beach state is largely in equilibrium with sediment supply.

Sedimentary records and luminescence chronology of Late Holocene palaeofloods in the Luni River, Thar Desert, Northwest India

River Luni is the only well-integrated river system in the Thar Desert of India. This river catastrophically flooded due to unusually heavy rainfall in the catchment area during July 1979. In order to establish whether floods of this magnitude have occurred in the recent geological past, sedimentary records of palaeofloods occurring in the Sindari Gorge, in the lower Luni Basin were investigated. The principal slackwater flood deposits were observed in a back-flooded tributary near Bhuka. Analysis of the slackwater flood deposits preserved in the back-flooded tributary and their luminescence dating suggests that the Luni River has experienced at least 17 extreme floods during the past millennium. Evidence from the Bhuka site also suggests that no floods comparable in magnitude to the July 1979 megaflood have occurred during this period. This observation is in conformity with the palaeoflood record of central India. Comparison of the long-term monsoon rainfall series for the Luni Basin and the Indian region reveals a clear link between the two, and indicates that the clustering of large floods in the last few decades and during the Medieval warming period is a regional phenomenon associated with wetter conditions. This correlates with a regionally extended episode of landscape stability denoted by stabilization of dunes in this region.

Using the OSL single-aliquot regenerative-dose protocol with quartz extracted from building materials in retrospective dosimetry

We report on the application of the single-aliquot regenerative-dose (SAR) protocol to the optically stimulated luminescence signal from quartz extracted from fired bricks and unfired mortar in retrospective dosimetry. The samples came from a radioactive materials storage facility, with ambient dose rates of about 0.1 mGy/h. A detailed dose-depth profile was analysed from one brick, and compared with dose records from area TL dosemeters. Small-aliquot dose-distributions were analysed from the mortar samples; one associated with the exposed brick, and one from a remote site exposed only to background radiation. We conclude that unfired materials have considerable potential in retrospective dosimetry.

New quartz SAR-OSL ages from the stranded beach dune sequence in south-east South Australia

A sequence of stranded coastal barriers in south-east South Australia preserves a record of sea-level variations over the past 800xa0ka. Huntley et al. (Quat. Sci. Rev. 12 (1993a) 1; Quat. Sci. Rev. 13 (1994a) 201) attempted to test thermoluminescence (TL) dating methods and found good agreement between quartz TL ages with independent ages for these dunes. We investigate the accuracy of the single-aliquot regenerative-dose (SAR) procedure (Radiat. Meas. 32 (2000) 57) over an extended age range of 0–250xa0ka, by comparing SAR-OSL ages determined on quartz extracts from these dunes with the existing chronology. We show that Robe II range is ∼60xa0ka, and that Robe III is 100xa0ka old. Not surprisingly, the OSL ages increase monotonically from the Robe II range to the West Naracoorte range. For the younger dunes (<240xa0ka), the SAR-OSL ages agree with the expected ages within 1σ errors, whereas for the older dunes the SAR ages are consistent with independent ages within 2σ error limits. We consider these results to be very promising, and lend support to the large number of quartz SAR-OSL ages being presented in the literature, where such comparisons with independent chronology are not usually possible.

Reconstruction of the late Quaternary environment of the lower Luni Plains, Thar Desert, India

Geomorphological processes in the Thar Desert of India are largely climate driven. In the lower reaches of the River Luni (the only major drainage system in the Thar Desert) a fluvio-aeolian sequence was located at a site called Khudala. Sediments of this sequence represented a variety of depositional environments, namely aeolian, fluvially reworked aeolian, overbank deposits, gravels, and occasional evidence of pedogenesis. This provided a good opportunity to study aeolian–fluvial interaction in the region and for deducing climatic records. From the luminescence dating standpoint these sequences offered a good opportunity for a comparative study of thermoluminescence (TL), blue-green light stimulated luminescence (BGSL) and infrared light stimulated luminescence (IRSL) on different mineral separates of identical provenance but deposited under different environments. Broadly, within experimental errors, the TL ages agreed with BGSL and IRSL ages on aeolian sands, but differed substantially in the case of fluvially reworked and proximally deposited sands and silts. The sequence provided a record spanning more than 100 ka, with an aeolian phase at > 100 ka, a channel activation phase between 70 and 30 ka and a phase of climate instability between 13 and 8 ka. This appears consistent with the records of monsoon performance during this period, which includes the Younger Dryas. It is also inferred that during the Last Glacial epoch, geomorphological processes in the Thar (both aeolian and fluvial) were dormant largely on account of their relationship with the southwest monsoon. Copyright

Luminescence studies on neotectonic events in south-central Kumaun Himalaya — a feasibility study

Abstract Results of a feasibility study on luminescence dating of neotectonic events as recorded in fault gouges and buried soils formed on landslide debris are reported. It is suggested that clay mineralogical data on illite and chlorite can provide a reasonable estimate on the thermal excursion (and hence on resetting of the luminescence clock) during faulting. The thermoluminescence ages are consistent with the available radiocarbon ages and have been used to infer that Naini Lake was formed at ca. 40–50 ka.

Supralinearity and sensitivity changes in optically stimulated luminescence of annealed quartz

Abstract The supralinear behaviour of the optically stimulated luminescence (OSL) signal is examined in an annealed quartz sample. Such investigations are important in luminescence dating and retrospective dosimetry, especially when the supralinearity intercept comprises a significant component of the paleodose in the sample. The sensitivity-uncorrected OSL growth curve of the annealed quartz sample 3-1/1 is observed to be supralinear in the dose range 0–5 Gy; it is then linear until about 20 Gy; and becomes sublinear beyond 20 Gy. The low-dose supralinearity is completely removed after a correction for sensitivity change. It is shown that the competing trap model (Kristianpoller, N., Chen, R., Israeli, M., 1974. Dose dependence of thermoluminescence peaks. Journal of Physics D: Applied Physics, 7, 1063–1072) and the competing centre model (McKeever, S.W.S., Chen, R., 1997. Luminescence models. Radiation Measurements, 27, 625–661) originally proposed to explain the supralinearity in thermoluminescence of quartz, can also be applied to explain supralinearity in the OSL response of the annealed quartz sample. Furthermore, cycle-dependent sensitivity changes are observed in another annealed quartz sample (NLT-28199) during repeated regeneration cycles. The OSL sensitivity decreases with cycle in this sample; this decrease is explained in terms of a recent model proposed by McKeever et al. (McKeever, S.W.S., Agersnap Larsen, N., Botter-Jensen, L., Mejdahl, V., 1997. OSL sensitivity changes during single aliquot procedures: computer simulations. Radiation Measurements 27, 75–82).

Concepts and approaches to in situ luminescence dating of martian sediments

In this paper we present the concept of a robotic instrument for in situ luminescence dating of near-surface sediments on Mars. The scientific objectives and advantages to be gained from the development of such an instrument are described, and the challenges presented by the Mars surface environment to the design and operation of the instrument are outlined.