1. Introduction and Application 3. Field Supplies and Sampling
Luminescence dating is utilized in a number of geologic and archaeologic studies to obtain a depositional (burial) age on alluvium, colluvium, eolian, glacial, marine, paleontological, biological and anthropogenic sediment or rock.
Exposure to sufficient sunlight (290-3200nm) or heat (>500°C) will reset any previous luminescence signal to zero. After removal from the stimulation source, ionizing energy from radioactive decay in surrounding sediment/rock (15-30cm) and within the mineral grain will excite atomic orbital electrons- some will get trapped in mineral lattice defects. This trapping and storing effectively acts as a clock and accumulation of electrons will continue until the trap becomes saturated, or a stimulating source aids in their escape back to their original orbit. Upon trap departure, some electrons will Figure 1. Required gear used for tube-sample collection method produce a photon of light when the stored energy is released. in luminescence dating. (A) Measuring tape for burial depth, In the lab, this light energy (luminescence) is then calibrated to important for cosmic DR. (B) For DE sample, OSL sampling tube radiation doses for deriving a geologic radiation dose (metal or other opaque material) sharpened at one end and pre- equivalent, known as Equivalent Dose (DE) in grays (Gy) of loaded with a styrofoam plug on the sharpened end to limit radiation. The natural decay of radioelements in the sediment shaking during pounding. (C) Rubber end caps for tube sedimentary environment and from cosmogenic fall out (up to (tinfoil and duct tape can be substituted if not available). (D) 30 m depth) that drive the electron excitation (and thus trap- Pounding cap (2-in stainless steel outside threaded plug). Do not filling rate) is known as the environmental Dose Rate (DR), use pounding caps that fit tightly on tubes or with internal through chemical or spectral analysis and conversion factors, threads as they can get permanently seized onto pipes. (E) One- the DR is known in Gy/ka. Thus, quart zip-seal bag half-filled for DR sample collection. (F) Film canister for water-content samples (triple bagged zip-bags or Age (ka) = DE (Gy) / DR (Gy/ka) other air-tight containers also acceptable).
The repetition of light and/or heat exposure and subsequent Additional tools in sampling kit: trowel (or field knife, small shovel) burial, over geologic time through sedimentary cycling, is a for clearing back sediments from the trench or outcrop face and main driver for luminescence sensitization. Generally, the collection of sediment for DR samples; sledge hammer for pounding more sensitive a grain is to acquiring a burial dose, the better in sample tube (rubber mallets and light field hammers not the precision and accuracy of the age. One caveat is the carry- recommended for most sediments types); duct tape to seal ends of tubes; permanent marker and clear packing tape to cover labels; over of an older dose at a more recent burial phase, or partial field book to document stratigraphic context and GPS location and bleaching/ partial resetting of a signal from a preceding elevation. Other items not shown might include tinfoil for wrapping depositional event. If left uncorrected, partial bleached ages samples and securing tube ends if end caps are not available, camera will overestimate the age of the depositional event of interest. to document sample placement and light-proof tarps for use if Additionally, mixing of younger material through biologic, modified sample collection is necessary (e.g., for coarse-grained pedogenic, or cryogenic processes will cause age deposit or sampling under rocks). underestimation.
2. Age Range and Suitable Geologic Material
Datable range is dependent on mineralogy, DR, and technique. Standard protocols: Optically Stimulated Luminescence (OSL): 0 - 250 ka Infrared Stimulated Luminescence (IRSL): 0 - 500 ka Post Infrared- IRSL (pIRI-IRSL): 0->500 ka
Thermal Luminescence (TL): 0 - 800 ka = Quartz Figure 2. Illustration of traditional OSL sample collection by New and evolving protocols: = Feldspar pounding a tube into an outcrop exposure. Circle in (a) depicts Thermal Transfer (TT) OSL: ~100 ka- 1 Ma area of surrounding sediment that should be uniformly sampled Red TL (RTL) : up to 1.5 Ma for dose-rate analysis. (b) Measurement of the burial depth, Violet Stimulated Luminescence (VSL): up to 1.6 Ma indicate recent changes to depth through deposition or erosion.
Tammy Rittenour, Utah State University, Logan, UT, [email protected]
Michelle Nelson, USU Luminescence Lab, N. Logan, UT, [email protected] Shannon Mahan, USGS Luminescence Lab, Denver, CO, [email protected]
4. Sample Integrity and Considerations 5. Laboratories
Key Assumptions for Successful Luminescence Dating Baylor University, Steven Forman, [email protected], www.baylor.edu/geology/index.php?id=868082, o Materials have uniform and definable dose rates. geomorphology/geoarchaeology o Moisture content of the sample can be determined. California State University, Long Beach, Sachiko Sakai, [email protected], archaeology/geoarchaeology o Depth, altitude, intensity of cosmic rays can be calculated, known. Daybreak Nuclear and Medical Systems, Victor Bortolot, Radiation-induced signal has to be thermally or optically reset by o [email protected], instrument sales/authentication the event to be dated, completeness of "resetting" obtained. daybreaknuclear.us/daybreak_frameset.html o Luminescence signal must have been stable during the time span Desert Research Institute, Amanda Keen-Zebert, in question. Any loss of signal (fading) can be measured/ [email protected], www.dri.edu/luminescence-lab corrected. Quaternary geology/geomorphology Primary Considerations at the Outcrop East Carolina University, Regina DeWitt, [email protected], luminescence properties/Quaternary geology • Mineral and Grain-Size Composition Quartz and K-feldspar Illinois State Geological Survey, Sébastien Huot, [email protected] 4-11µm for fine-grain dating www.isgs.illinois.edu/research/geochemistry/labs/osl, 63-250 µm for coarse-grain dating luminescence properties/Quaternary geology
• Tectonic and sedimentary history Kansas State University, Joel Spencer, [email protected], Geologic Source Area Geologic context may play a key role in luminescence sensitivity, luminescence properties/Quaternary geology stability, and overall acquisition. Generally, sediments derived from McDaniel College, Vasilis Pagonis, [email protected] sedimentary bedrock units are better suited for luminescence dating www2.mcdaniel.edu/Physics/TLwebsite/thermo.html, than those sourced from young, igneous and metamorphic terranes. luminescence properties
• Previous Signal Resetting Partial bleaching McMaster University, Jack Rink, [email protected], coastal geomorphology/Quaternary geology Sufficient exposure required to reset previously-acquired signal. OSL/IRSL: sunlight exposure > 1 minute, or more North Dakota State University, Ken Lepper, [email protected] TL: > 500°C geomorphology/Quaternary geology
Oklahoma State University, Steve McKeever, Fluvial, colluvial, glacier-proximal settings are often plagued by partial [email protected], luminescence properties bleaching and may require additional statistical treatment to select the most recent bleaching event. University of California, Los Angeles, Ed Rhodes, [email protected], Quaternary geology/geomorphology • Post-Depositional Mixing Law of Original Horizontality University of Cincinnati, Lewis Owen, [email protected], Avoid deposits with signs of bioturbation (i.e. krotovina, insect Quaternary geology/geomorphology burrows, root casts, modern human debris), cryoturbation (i.e. vertical University of the Fraser Valley, Olav Lian, [email protected] cracks, frost wedges), pedoturbation (i.e. clay translocation) www.ufv.ca/geography/research/luminescence-dating-
laboratory/, luminescence properties/Quaternary geology • Dose Rate Homogeneity β and γ decay, H2O important to DR University of Georgia, Athens, George Brook, [email protected] Uniform lithology and grain size within 15-30cm of DE sample is geography.uga.edu/luminescence-dating-laboratory/, Quaternary preferred. Note any buried / missing soil horizons indicating change in geology/geomorphology burial depth. Water content in pore space attenuates radiation – must , Paul Hanson, [email protected] collect in-situ H2O sample and note changes over time. University of Nebraska, Lincoln snr.unl.edu/sandhills-biocomplexity/osl_dating.htm, • Bulk (night) sampling/ block samples No sand lens present geomorphology/Quaternary geology Clear back outcrop by 3-5 m, remove loose sediment above, avoid Univ of Quebec, Montreal, Michel Lamothe, lux.uqam.ca, mixing older and younger units, entire sample will be consumed for DE [email protected], Quaternary geology/geomorphology and DR. Necessary to contact the lab before sampling for additional guidance. University of Washington, James Feathers, [email protected] depts.washington.edu/lumlab/, archaeology/geoarchaeology • Heated Materials Ceramics, bricks, and chert USGS, Shannon Mahan, [email protected], Firing temperature and duration play key role in thermal resetting, can usgs.gov/centers/gecsc/science/luminescence-dating-laboratory be dated with OSL or TL, must have sufficient amount of material for Quaternary geology/ geomorphology processing and dating as outer 2 mm of ceramics and lithics are Utah State University, Tammy Rittenour, [email protected] removed for dosimetric reasons. Separate (additional) soil sample www.usu.edu/geo/luminlab, geomorphology/geoarchaeology needed for DR. See http://www.usu.edu/geo/luminlab/Lumin_Lab_List.pdf for a more detailed list of North American labs
Tammy Rittenour, Utah State University, Logan, UT, [email protected]
Michelle Nelson, USU Luminescence Lab, N. Logan, UT, [email protected] Shannon Mahan, USGS Luminescence Lab, Denver, CO, [email protected]