Tammy Rittenour, Utah State University, Logan, UT, tammy.rittenour@usu.edu

Michelle Nelson, USU Luminescence Lab, N. Logan, UT, michelle.nelson@usu.edu Shannon Mahan, USGS Luminescence Lab, Denver, CO, smahan@usgs.gov

1. Introduction and Application 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 produce a photon of light when the stored energy is released. In the lab, this light energy (luminescence) is then calibrated to radiation doses for deriving a geologic radiation dose equivalent, known as Equivalent Dose (DE) in grays (Gy) of radiation. The natural decay of radioelements in the sedimentary environment and from cosmogenic fall out (up to 30 m depth) that drive the electron excitation (and thus trap-filling rate) is known as the environmental Dose Rate (DR), through chemical or spectral analysis and conversion factors, the DR is known in Gy/ka. Thus,

Age (ka) = DE (Gy) / DR (Gy/ka)

The repetition of light and/or heat exposure and subsequent burial, over geologic time through sedimentary cycling, is a main driver for luminescence sensitization. Generally, the more sensitive a grain is to acquiring a burial dose, the better the precision and accuracy of the age. One caveat is the carry-over of an older dose at a more recent burial phase, or partial bleaching/ partial resetting of a signal from a preceding depositional event. If left uncorrected, partial bleached ages will overestimate the age of the depositional event of interest. Additionally, mixing of younger material through biologic, pedogenic, or cryogenic processes will cause age 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

New and evolving protocols: Thermal Transfer (TT) OSL: ~100 ka- 1 Ma Red TL (RTL): up to 1.5 Ma Violet Stimulated Luminescence (VSL): up to 1.6 Ma

3. Field Supplies and Sampling

Figure 1. Required gear used for tube-sample collection method in luminescence dating. (A) Measuring tape for burial depth, important for cosmic DR. (B) For DE sample, OSL sampling tube (metal or other opaque material) sharpened at one end and pre-loaded with a styrofoam plug on the sharpened end to limit sediment shaking during pounding. (C) Rubber end caps for tube (tinfoil and duct tape can be substituted if not available). (D) Pounding cap (2-in stainless steel outside threaded plug). Do not use pounding caps that fit tightly on tubes or with internal threads as they can get permanently seized onto pipes. (E) One-quart zip-seal bag half-filled for DR sample collection. (F) Film canister for water-content samples (triple bagged zip-bags or other air-tight containers also acceptable).

Additional tools in sampling kit: trowel (or field knife, small shovel) for clearing back sediments from the trench or outcrop face and collection of sediment for DR samples; sledge hammer for pounding in sample tube (rubber mallets and light field hammers not recommended for most sediments types); duct tape to seal ends of tubes; permanent marker and clear packing tape to cover labels; field book to document stratigraphic context and GPS location and elevation. Other items not shown might include tinfoil for wrapping samples and securing tube ends if end caps are not available, camera to document sample placement and light-proof tarps for use if modified sample collection is necessary (e.g., for coarse-grained deposit or sampling under rocks).

Figure 2. Illustration of traditional OSL sample collection by pounding a tube into an outcrop exposure. Circle in (a) depicts area of surrounding sediment that should be uniformly sampled for dose-rate analysis. (b) Measurement of the burial depth, indicate recent changes to depth through deposition or erosion.

= Quartz

= Feldspar

Tammy Rittenour, Utah State University, Logan, UT, tammy.rittenour@usu.edu

Michelle Nelson, USU Luminescence Lab, N. Logan, UT, michelle.nelson@usu.edu Shannon Mahan, USGS Luminescence Lab, Denver, CO, smahan@usgs.gov

5. Laboratories

Baylor University, Steven Forman, Steven_Forman@baylor.edu, www.baylor.edu/geology/index.php?id=868082, geomorphology/geoarchaeology

California State University, Long Beach, Sachiko Sakai, sachikosak@gmail.edu, archaeology/geoarchaeology

Daybreak Nuclear and Medical Systems, Victor Bortolot, daybrk@concentric.net, instrument sales/authentication daybreaknuclear.us/daybreak_frameset.html

Desert Research Institute, Amanda Keen-Zebert, Amanda.KeenZebert@dri.edu, www.dri.edu/luminescence-lab Quaternary geology/geomorphology

East Carolina University, Regina DeWitt, dewittr@ecu.edu, luminescence properties/Quaternary geology

Illinois State Geological Survey, Sébastien Huot, shuot@illinois.edu www.isgs.illinois.edu/research/geochemistry/labs/osl, luminescence properties/Quaternary geology

Kansas State University, Joel Spencer, joelspen@ksu.edu, luminescence properties/Quaternary geology

McDaniel College, Vasilis Pagonis, vpagonis@mcdaniel.edu www2.mcdaniel.edu/Physics/TLwebsite/thermo.html, luminescence properties

McMaster University, Jack Rink, rinkwj@mcmaster.ca, coastal geomorphology/Quaternary geology

North Dakota State University, Ken Lepper, ken.lepper@ndsu.edu geomorphology/Quaternary geology

Oklahoma State University, Steve McKeever, stephen.mckeever@okstate.edu, luminescence properties

University of California, Los Angeles, Ed Rhodes, ed.rhodes@sheffield.ac.uk, Quaternary geology/geomorphology

University of Cincinnati, Lewis Owen, Lewis.Owen@uc.edu, Quaternary geology/geomorphology

University of the Fraser Valley, Olav Lian, olav.lian@ufv.ca www.ufv.ca/geography/research/luminescence-dating-laboratory/, luminescence properties/Quaternary geology

University of Georgia, Athens, George Brook, gabrook@uga.edu geography.uga.edu/luminescence-dating-laboratory/, Quaternary geology/geomorphology

University of Nebraska, Lincoln, Paul Hanson, phanson2@unl.edu snr.unl.edu/sandhills-biocomplexity/osl_dating.htm, geomorphology/Quaternary geology

Univ of Quebec, Montreal, Michel Lamothe, lux.uqam.ca, lamothe.michel@uqam.ca, Quaternary geology/geomorphology

University of Washington, James Feathers, jimf@u.washington.edu depts.washington.edu/lumlab/, archaeology/geoarchaeology

USGS, Shannon Mahan, smahan@usgs.gov, usgs.gov/centers/gecsc/science/luminescence-dating-laboratory Quaternary geology/ geomorphology

Utah State University, Tammy Rittenour, tammy.rittenour@usu.edu www.usu.edu/geo/luminlab, geomorphology/geoarchaeology

See http://www.usu.edu/geo/luminlab/Lumin_Lab_List.pdf for a more detailed list of North American labs

4. Sample Integrity and Considerations

Key Assumptions for Successful Luminescence Dating

o Materials have uniform and definable dose rates.

o Moisture content of the sample can be determined.

o Depth, altitude, intensity of cosmic rays can be calculated, known.

o Radiation-induced signal has to be thermally or optically reset by the event to be dated, completeness of "resetting" obtained.

o Luminescence signal must have been stable during the time span in question. Any loss of signal (fading) can be measured/ corrected.

Primary Considerations at the Outcrop

• Mineral and Grain-Size Composition Quartz and K-feldspar 4-11µm for fine-grain dating 63-250 µm for coarse-grain dating

• Geologic Source Area Tectonic and sedimentary history Geologic context may play a key role in luminescence sensitivity,

stability, and overall acquisition. Generally, sediments derived from sedimentary bedrock units are better suited for luminescence dating than those sourced from young, igneous and metamorphic terranes.

• Previous Signal Resetting Partial bleaching Sufficient exposure required to reset previously-acquired signal. OSL/IRSL: sunlight exposure > 1 minute, or more TL: > 500°C

Fluvial, colluvial, glacier-proximal settings are often plagued by partial bleaching and may require additional statistical treatment to select the most recent bleaching event.

• Post-Depositional Mixing Law of Original Horizontality Avoid deposits with signs of bioturbation (i.e. krotovina, insect

burrows, root casts, modern human debris), cryoturbation (i.e. vertical cracks, frost wedges), pedoturbation (i.e. clay translocation)

• Dose Rate Homogeneity β and γ decay, H2O important to DR Uniform lithology and grain size within 15-30cm of DE sample is

preferred. Note any buried / missing soil horizons indicating change in burial depth. Water content in pore space attenuates radiation – must collect in-situ H2O sample and note changes over time.

• Bulk (night) sampling/ block samples No sand lens present Clear back outcrop by 3-5 m, remove loose sediment above, avoid

mixing older and younger units, entire sample will be consumed for DE and DR. Necessary to contact the lab before sampling for additional guidance.

• Heated Materials Ceramics, bricks, and chert Firing temperature and duration play key role in thermal resetting, can

be dated with OSL or TL, must have sufficient amount of material for processing and dating as outer 2 mm of ceramics and lithics are removed for dosimetric reasons. Separate (additional) soil sample needed for DR.