1. Introduction 4. Suitable Materials Radiocarbon dating is used to determine the age of organic and inorganic materials containing carbon that are up to ~40,000 years old, and occasionally older. It is an isotopic dating method based on the radioactive decay of 14C to 14N with a half-life of 5,730 years. Radiocarbon dating has been used successfully to acquire ages from a variety of sample types, including wood, charcoal, seeds, pollen, plant remains, diffuse organic material, bones, shells, insect parts, and cloth. The technique was developed in the late 1940s by Willard Libby at the University of Chicago, and is now used routinely in geologic, archaeologic, and paleontologic studies around the Figure 2. Left, Charcoal in paleowetland deposits in the Las Vegas world. The advent of accelerator mass spectrometry (AMS) in Valley, Nevada. Right, Succineidae shells in loess deposits near the 14 the late 1980s revolutionized 14C dating, allowing for improved Yukon River, Alaska. Both materials are ideally suited for C dating. age determinations on samples that are much smaller than Some materials, such as charcoal, plant macrofossils, and what was previously required. Today, ages can be obtained for certain types of terrestrial snail shells, consistently yield samples that weigh a milligram or less, which is both reliable 14C ages. These are considered the “gold standards,” remarkable and potentially fraught with problems related to as their usage can result in exceptionally clear records if their context and contamination. context is well understood. Other substances, including bulk organic matter, humic acids, and aquatic snail shells, are less 2. What makes a 14C age reliable? than ideal, and often yield spurious results. Thus, it is Three criteria must be satisfied for an age to be considered important to realize that 14C ages are not always equal in terms reliable. First, the geologic and stratigraphic context of the of their quality, which has significant implications for sample must be known and understood. Second, the 14C comparing or synthesizing ages from disparate studies. concentration of the sample material (a plant, for example) must be in equilibrium with atmospheric 14C when it was alive. Finally, after burial, the sample material must behave as a closed system with respect to carbon, such that contaminants are excluded and only the targeted carbon pool is analyzed. If all three criteria are met, then radiocarbon dating can provide ages that are both accurate and precise. 3. Calibration Radiocarbon ages are not same as calendar ages. Changes in 14 the production rates of C in the atmosphere have occurred through time due to variations in the cosmic ray flux, the Figure 3. Point of Rocks Spring, Ash Meadows National Wildlife strength and orientation of the Earth’s magnetic field, and the Refuge, southern Nevada. distribution of carbon among the ocean, terrestrial biosphere, and atmospheric reservoirs. This requires 14C ages to be An example of extremely spurious 14C ages comes from aquatic converted, or “calibrated,” to calendar years so they can be snails living in spring pools in the Amargosa Valley of southern compared directly with ages derived from other techniques. Nevada. There, snails obtain their carbon from groundwater Calibration curves, including the most recent curve – IntCal13 that travels long distances through a deep, carbonate aquifer – are based on a combination of tree rings, varves, corals, plant system. Old carbon from the rocks is transferred to the water macrofossils, speleothems, and foraminifera. Calibration of through chemical interactions that occur along flow paths. ages can be done using publicly available software programs, Combined with long travel times and isotopic decay, the old such as OxCal (https://c14.arch.ox.ac.uk/oxcal.html) or CALIB carbon can lead to significant disequilibrium conditions and (http://calib.org/calib/). Finally, by convention, all 14C ages and wildly erroneous ages – they can be as much as 20,000 years calibrated ages are reported relative to the year 1950 A.D. too old! Jeff Pigati, U.S. Geological Survey, Denver CO, [email protected] Kathleen Springer, U.S. Geological Survey, Denver CO, [email protected] 5. Sample Processing 6. 14C Laboratories in the U.S. What happens to your sample in the laboratory after you Aeon Laboratories, L.L.C. submit it for radiocarbon dating? www.aeonlaboratories.com Step 1. Physical selection Arizona AMS Facility Laboratory scientists carefully examine each sample under University of Arizona www.physics.arizona.edu/ams magnification and make note of samples that could potentially contain carbon from multiple sources. Proper selection is Beta Analytic, Inc. www.radiocarbon.com especially important when submitting "bulk" samples that may contain a mixture of charcoal, fragments of wood, soil, plant Center for Accelerator Mass Spectrometry debris, etc., which could yield different ages. Lawrence Livermore National Laboratory https://cams.llnl.gov Step 2. Chemical isolation Contamination comes in many forms, from the addition of Center for Applied Isotope Studies secondary organic molecules to atmospheric gases that sorb University of Georgia onto the sample itself. The targeted carbon pool in a sample is www.cais.uga.edu isolated from contaminants using chemical pretreatment procedures that are specifically designed for the type of Direct AMS www.directams.com sample submitted. Some treatments, such as the acid-base- acid method for organic materials, are well known and used by Geochron Laboratories every 14C lab in the world. Isolation of cellulose from wood or www.geochronlabs.com collagen from bone, in contrast, have been more difficult to standardize and are the subject of ongoing research. INSTAAR Lab for AMS Radiocarbon Preparation and Research University of Colorado Step 3. CO2 extraction and purification www.colorado.edu/INSTAAR/RadiocarbonDatingLab Once the sample has been chemically treated, it is dried and International Chemical Analysis, Inc. weighed. The carbon content is estimated based on the www.radiocdating.com sample mass and type, and an appropriately sized portion is selected for extraction. Organic materials are combusted in Illinois State Geological Survey’s Radiocarbon Dating Laboratory the presence of high-purity oxygen to form CO2, whereas http://isgs.illinois.edu/research/geochemistry/labs/14C carbonate samples are converted to CO2 using H3PO4. Water Keck Carbon Cycle AMS Laboratory and other contaminants, including SOx, NOx, and halide University of California, Irvine species, are removed using cryogenic methods and/or high- www.ess.uci.edu/groupr/ams/home temperature copper and silver traps, resulting in pure CO2 gas. National Ocean Sciences AMS Facility Step 4. Graphitization Woods Hole Oceanographic Institution 14 In most C labs, the CO2 is converted to graphite using an iron www.whoi.edu/nosams catalyst and a standard hydrogen reduction technique. Other Purdue Rare Isotope Measurement Laboratory labs combine the same CO2 gas and iron catalyst with zinc Purdue University powder to produce graphite. In both cases, the resulting www.physics.purdue.edu/primelab 14 graphite powder is pressed into target cartridges and the C activity (or concentration) is measured by AMS. PaleoResearch Institute www.paleoresearch.com/analysis/radiocarbon Step 5. Reporting Radiocarbon laboratories typically report the sample number, U.S. Geological Survey’s (New) Radiocarbon Laboratory Coming in 2018! AMS number, 14C activity, and 14C age. They may also report the δ13C value and/or calibrated age of the sample. The journal Radiocarbon maintains a comprehensive list of active Uncertainties associated with 14C activities and 14C ages are radiocarbon laboratories both within and outside the United typically reported at the 1σ (68%) confidence level; States. See http://www.radiocarbon.org/Info/Labs.pdf for details. uncertainties associated with calibrated ages should be reported at the 2σ (95%) confidence level. Jeff Pigati, U.S. Geological Survey, Denver CO, [email protected] Kathleen Springer, U.S. Geological Survey, Denver CO, [email protected] .