Pushing back the frontiers of luminescence dating

Overview

Luminescence (especially optically stimulated luminescence – OSL) dating is an established and important tool for dating events during the Late Pleistocene and latter part of the Middle Pleistocene (Rhodes, 2011). This ‘trapped charge’ method is based on the use of crystalline minerals (typically, quartz or feldspar) to measure the cumulative radiation dose since deposition arising from the decay of naturally occurring lithogenic radioactive isotopes within the environment. The contemporary radiation dose rate in the sedimentary environment is also measured; dividing the cumulative dose by the dose rate gives the numerical age of the sample. However, in its present form, the age span of this technique when applied to sedimentary deposits is restricted by a limitation in the capacity of the crystal structure to store trapped charge, giving rise to a saturation effect that presents an upper limit of ages and hence underestimating the true depositional age. This is a common problem when applying OSL dating to Middle Pleistocene samples recovered from deposits with typical dose rates.

The objective of this project is to achieve a methodological development in OSL dating to enable a much broader range of application by extending its chronological reach to ca 1 Ma. The focus of the research will be the dating of lithics, in particular unheated quartzite/quartz pebbles and cobbles that, prior to burial had a prolonged history of cycles of exposure to daylight, such as from fluvial and coastal depositional contexts. Recently published work (e.g., Sohbati, 2011) has shown that it is feasible to date the burial of lithics of this type by analysis of the luminescence response as a function of depth in the lithic sample. Compared with sedimentary deposits, the use of this approach with lithics enables the dose rate to be substantially reduced, relative to typical surface environments, giving rise to a corresponding extension in chronometric range. Current work indicates that this approach can also be used with some redeposited Holocene deposits, substantailly widening the range of potentially suitable sites where the method can be applied.
This approach would significantly enhance an important chronometric tool in both environmental and archaeological research and in the support of geoconservation and heritage protection (Last et al., 2013).

Methodology

The project will combine techniques of geomorphology, Quaternary stratigraphy and luminescence dating to investigate the primary objective of extending the chronometric range of luminescence dating of fluvial (river-terrace) and coastal (raised-beach) gravel deposits. This will require an understanding of site formation processes and the interpretation of depositional sequences from environmental and archaeological perspectives.
Desk-based research
A survey of known and previously recorded gravel sites in the Trent and Thames fluvial systems will be performed to compile a prioritised list of locations for inspection and sampling. This will include an assessment of existing chronostratigraphic data; in the UK, potential sites within the chronological range of interest will include Cannon Court Farm Pit, Fern House Gravel Pit and Highlands Farm Pit, which expose Middle Pleistocene Thames gravel deposits of different ages. Ultimately the project will aim to build a group of test sites that range from the Holocene to deeper into the Middle Pleistocene, depending on the luminescence characteristics of the lithics. Previous NERC funded projects (as well as the Leverhulme Trust and the Aggregates Levy Sustainability Fund) have established a range of sites where chronometric data have been obtained, ranging from the Holocene to the Middle Pleistocene). Access to further sites is also potentially available through existing collaboration with two external research groups (CNRS, Paris and University of Coimbra; see below). The CNRS group is currently developing ESR dating of Pleistocene fluvial sediments in NW Europe and the Iberian Peninsula.

Geomorphology/Quaternary stratigraphy
The targeted Thames terrace deposits are in the middle reach of the river where the quartz content of the gravels will be high enough to obtain the required lithics. The terraces are rather poorly dated at present, relying on reconstruction of terrace long-profiles and correlation thereby with biostratigraphically dated deposits in other reaches. Their presumed ages range from MIS 12 (~450,000 years) to MIS 6 (~160,000 years). Sampled localities will be recorded and sampled for other methodologies as appropriate, including particle-size, micro-fossil and palaeosol analysis. Required field training will be provided by the supervisors.

Luminescence techniques
The project will focus initially on the application of OSL techniques to quartzite/quartz lithics of cobble and smaller sizes to take advantage of a radiation dose rate that is substantially lower than for clastic sediment deposits, arising from the elimination of the external dose rate from beta radiation. Where this mineral type is absent or has unsuitable properties, feldspar-bearing minerals will be tested. Irrespective of the mineral type, the approach applied so far in dating unheated lithics is to obtain drilled cores and cut thin slices, moving progressively into the inner part of the lithic sample, and this experimental approach will be applied to obtain measurement samples. The depth to which optical resetting of the trapped charge was obtained prior to burial is a key issue that requires the building of a depth-dose profile. A single aliquot procedure will be applied to each slice, including the extensive series of tests for robustness of dose determinations. With extension of chronometric range being a primary objective, the measurement and form of the dose response curves will require detailed consideration, including an assessment of trapped charge stability of the various components present in the measured OSL decay curves. Where the mineral transparency limits the optical depth to the outer volume of the lithic (e.g., 5-10 mm), models are available to deconvolute the profile and extract estimates of the burial dose (Freiesleben et al, 2015).
For some sites it may also be appropriate to obtain OSL depositional ages for either interstitial sediment within the gravel deposit or from overlying and underlying strata to provide markers for comparison with other chronostratigraphic data. Where only feldspar-bearing lithics are measurement protocols based on the use of infrared stimulation (IRSL) are available (although quartzite/quartz is the preferred mineral in this study). Appropriate procedures will be applied to establish the laboratory beta dose rate to sliced lithic samples (Bailiff, 2018).

The assessment of the burial dose rate to lithic samples will be performed by a combination of in situ measurements using a portable gamma ray spectrometer (gamma/cosmic activity) and laboratory measurements to determine the internal lithic alpha/beta dose rate employing direct and indirect dosimetry techniques. At the Durham laboratory we have available instrumentation for spatially-resolved measurement of beta dose rate, beta activity and also expertise in the use of radiation transport code for dosimetry simulations. The reduction of the total dose rate components external (gamma and cosmic radiation) and internal (alpha and beta radiation) to the lithic samples is expected to give rise to an annual dose rate of well below 1 mGy/a and consequently requires a detailed experimental approach and uncertainty assessment. The radioactive content of lithic (and sediment) samples will be determined using resolution gamma ray spectrometry. The effect of moisture content will also require a modelled approach, although the effects of fluctuations on the overall dose rate are reduced compared with sediment deposits by eliminating the external beta component of the dose rate. The OSL ages will be compared with the existing chronostratigraphic schemes for the sites selected and, where appropriate assessed, using Bayesian analytical tools.

Collaborators:
Coastal and fluvial geomorphology: Prof. Pedro Proença Cunha, Dept. Earth Sciences, University of Coimbra, Portugal.
https://www.uc.pt/fctuc/dct/pessoas/fichaspessoais/pedrocunha

Project Timeline

Year 1

Induction: desk-based and field-based research methods, health and safety and ethics assessment. Laboratory training in reconnaissance, fieldwork recording, luminescence dating techniques, including dosimetry for lithic samples and geomorphological analysis of depositional contexts. Sample collection from one or more sites with chronostratigraphic control; testing and analysis of lithic samples and contexts. Planning and execution of Stage 1 fieldwork to collect samples from the selected study sites. Reporting outcome of sites tested and evaluation. Identification of methodological improvements/ development required; assessment of potential of research for presentation and publication.

Year 2

Laboratory testing and analysis of lithic samples collected in Stage 1 fieldwork. Assessment of testing outcomes and evaluation, leading to planning of Stage 2 fieldwork. Execution of Stage 2 fieldwork. Laboratory testing and analysis of lithic samples collected in Stage 2 fieldwork. Production of report containing the outcome of sites tested and evaluation. Identification of further methodological improvements or development required; assessment of potential of research for presentation and publication. Planning/drafting publication(s).

Year 3

Continuation and completion of laboratory testing of Stage 2 samples. Production of report containing the outcome of the testing of Stage 2 samples and evaluation. Execution of Stage 3 fieldwork – application to new sites (as required).

Year 3.5

4.1 Writing up of thesis and publications.

Training
& Skills

This project will suit strong candidates with backgrounds in scientific archaeology, Earth Science, Physical Geography, Chemistry or Physics. Appropriate training will be provided to develop a multidisciplinary skill set, regardless of the candidate’s background.
In addition to IAPETUS and DU core training, the student will complete a range of specialist training in fieldwork and laboratory techniques.
– Luminescence dating: lithic and sediment sample assessment and preparation, lab- and field-based experimental dose rate assessment, data analysis and interpretation. Also training in radiation transport modelling of naturally radioactive environments will be available. Chronostratigraphic and site formation interpretation.
– Training in assessment of the geomorphological and Quaternary stratigraphic context will include standard recording and sampling techniques, with access to laser particle-size measuring equipment and micro-fossil analysis. Palaeosol study would require thin-section preparation, which would be separately funded if required, with training needed for evaluation.

References & further reading

Bailiff, I.K., 2018. An examination of beta dose attenuation effects in coarse grains located in sliced samples. Radiation Measurements, In press. https://www.sciencedirect.com/science/article/pii/S1350448717308739
Bridgland, D.R., 2006. The Middle and Upper Pleistocene sequence in the Lower Thames; a record of Milankovitch climatic fluctuation and early human occupation of southern Britain. Henry Stopes Memorial Lecture. Proceedings of the Geologists’ Association, 117, 281-305.
Bridgland, D.R., Westaway, R., 2014. Quaternary fluvial archives and landscape evolution: a global synthesis. Proceedings of the Geologists’ Association 125, 600-629.
Bridgland, D.R., Howard, A.J., White, M.J., White, T.S. & Westaway, R., 2015. New insight into the Quaternary evolution of the River Trent, UK. Proceedings of the Geologists’ Association, 126, 466-479.
Freiesleben, T., Sohbati, R., Murray, A., Jain, M., Al Khasawneh, S., Hvidt, S., Jakobsen, B., 2015. Mathematical model quantifies multiple daylight exposure and burial events for rock surfaces using luminescence dating. Radiation Measurements 81, 16- 22.
Last, J., Brown, E.J., Bridgland, D.R., Harding, P., 2013. Quaternary geoconservation and Palaeolithic heritage protection in the 21st century: developing a collaborative approach. Proc Geol. Assoc. 124, 625-637.
Rhodes E.J. 2011. Optically Stimulated Luminescence Dating of Sediments over the Past 200,000 Years. Annual Review of Earth and Planetary Sciences, 39, 461-488.
Sohbati, R., Murray, A., Jain, M., Buylaert, J.-P., Thomsen, K., 2011. Investigating the resetting of OSL signals in rock surfaces. Geochronometria 38, 249-258

Further Information

For further information contact
Ian Bailiff, Durham University.
Email Ian.bailiff@dur.ac.uk
Tel. (44)191 334 1124

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