Pre-Treatment Methods of Samples for Radiocarbon Dating
Collagen is extracted from the bone samples based on the method of Brown et al. 1988 using a Vivaspin® filter cleaning method introduced by Bronk Ramsey et al. (2004). An alkali step has been added since 2014 following Brock et al. (2010). Measurement of C:N ratios, δ13C, and δ15N is described in ‘Stable Isotopes’.
Cremated bone procedure follows the method of Lanting and Brindley (1998) and Lanting et al. (2001). As sulfur in bone interferes with the graphitisation, the CO2 gas generated by hydrolysing the sample is heated with silver under vacuum to remove the sulfur.
Charcoal and other plant material
AAA (Acid-Alkali-Acid) pre-treatment is the standard treatment for charcoal, wood, peat and most plant macrofossils as in de Vries and Barendsen (1958) and Fischer and Heinemeier (2003). For macrofossils that have been separated from sediment using alkali treatment, an acid-only treatment is sufficient.
Charred organic residues on potsherds
For sufficiently robust samples, charred organic residues on potsherds will normally be treated to remove lipids to avoid reservoir offsets from utilization of marine or freshwater resources. The samples are treated with a 2:1 (v/v) mixture of chloroform and methanol and sonicated for 30 minutes, then filtered and the remaining solvent evaporated following the method of Boudin et al. (2010). The samples are then given the standard acid, alkali, acid (AAA) treatment. Samples that cannot withstand such treatment are normally treated using the AAA method only.
Bulk sediments (various fractions)
For peat or lake sediments various fractions can be separated and dated. Separation of humic acid (alkaline soluble) and humin (non-soluble) fractions of bulk sediments follow the method of Lowe et al. (2004). Bulk sediments are given an acid-only or acid fumigation pre-treatment.
Shells and other carbonates
Mollusc shells and other biogenic carbonates are cleaned in Milli-Q water and etched with HCl to remove some of the outer surface. The samples are then hydrolysed with phosphoric acid to evolve carbon dioxide as described as described in Santos et al. (2004). The CO2 is drawn out under liquid nitrogen on the graphite line.
Note: Mollusc samples with aragonitic shells should generally be analysed by x-ray diffraction (XRD) to determine if they have been recrystallized to calcite before dating. This is not possible for molluscs with calcite shells. This analysis can be arranged through the British Geological Survey, if desired.
Insect chitin pre-treatment follows the Tripp et al. (2004) procedure for fragile samples, using acetone, methylene chloride, and acetone again, freeze-dried and placed in HCl for 3 days. The sample is filtered, rinsed with Milli-Q water and freeze-dried overnight. The procedure has been found to be successful on insect remains from archaeological sites provided they have not been stored in organic solvents such as alcohol (Panagiotakopulu et al. 2015).
Consolidated or otherwise contaminated samples
For samples consolidated or contaminated with various organic substances such as PEG or PVA, a Soxhlet extraction is done with increasing polarity solvents ending in distilled water (Bruhn et al. 2001). The thoroughness of the extraction of the contaminants by solvent extraction is then tested by analysing the treated sample using Fourier transform infrared spectroscopy (FTIR) (D’Elia et al. 2007).
Fourier Transform Infrared Spectrometer (FTIR)
Samples that have been treated with preservatives or consolidating agents can be analysed using FTIR after removal using organic solvents in a Soxhlet extraction apparatus. Cremated bone samples will be analysed using FTIR to assess suitability for dating, based on the crystallinity index. Approximately 1mg of sample <0.63µm is mixed with approximately 0.25g potassium bromide. They are ground and mixed together using a pestle and mortar. A pellet is made using a manually operated hydraulic press at 10 tons for 3 minutes. The samples are analysed using a ThermoFisher ATR FTIR and the spectrum compared to standard material.
Measurement & Analysis of Samples
There are 2 combustion methods currently employed at 14CHRONO.
Dried organic samples are weighed into pre-cleaned tin capsules and combusted in oxygen with a helium carrier gas in an Elemental Analyser (Elementar Vario Isotope) before transfer to the AGE3 automated graphite system.
Dried organic samples are weighed into pre-combusted quartz tubes with an excess of copper oxide (CuO) and silver (Ag) foil, sealed under vacuum, and combusted to carbon dioxide (CO2) at 8500C for 8 hours.
Small to standard sized organic samples (0.3 – 1.0 mg C) are reduced to graphite on the AGE3 automated graphite system, which uses the hydrogen reduction method (Wacker et al. 2010).
Small to standard sized organic samples are combusted in quartz tubes and reduced to graphite on iron catalyst using the hydrogen reduction method on a graphitisation line (Vogel et al. 1987). Carbonate samples are hydrolysed and reduced with the hydrogen method on this line.
Graphite is pressed into vacuum cleaned aluminium holders (known as targets or cathodes) using an automated hydraulic press with a clean pin mounted in it. Once the targets have been transferred to the magazine, it is immediately loaded into the AMS.
The 14C/12C and 13C/12C ratios are measured by accelerator mass spectrometry (AMS) on an Ionplus Mini Carbon Dating System. A description of our AMS and measurement methodology is detailed here.
Background (blank) samples are processed along with the samples. In general anthracite is used for organic samples, Icelandic spar calcite for carbonates and mammoth bones for bone sample backgrounds. However, for non-bone organic samples expected to be >20,000 BP we use a background more closely matched to the sample material type, e.g. MOIS7 kauri wood. For foraminifera samples, it is preferable to use samples of the same species that are from sediment older than 50,000 BP and ideally from the same core (Nadeau et al. 2001).
The calculations used to produce the radiocarbon ages and uncertainties are detailed here.
Interpretation of results
Radiocarbon ages are not the same as calendar time due to past fluctuations of carbon-14 in the atmosphere. Calibration is necessary to convert radiocarbon ages into calendar equivalents (calibrated ages). For example, a radiocarbon age of 4520 ± 30 BP calibrates to 3356−3101 cal BC with the IntCal20 calibration curve at two sigma (95.4 %). The two-sigma range is generally preferred since there is a 95% probability that the true age of the sample falls within this range. In most publications the raw radiocarbon age should be reported as well as the calibrated age range.
Our reporting process includes an automatic attempt at calibrating your radiocarbon result with an internationally agreed calibration curve. No account is taken of whether your samples are from the Southern Hemisphere or if there is any marine component in your sample material. No attempt will be made to determine if your sample lies within the limits of the calibration curve. Thus, the automatic calibration result may contain warnings of the sample age being unsuitable for the calibration curve.
In addition to reporting radiocarbon ages we also report a quantity called F14C. Roughly speaking, this is the same as fraction modern, but with C13 fractionation and background corrections clearly implied.
The convention in reporting radiocarbon ages is that samples dating to a more recent time than 1950 AD are reported as “Greater than Modern”, although their F14C is still reported. Samples whose F14C is less than the background are reported as “Greater than” the age equivalent value of twice the background uncertainty.
If your samples are terrestrial samples from the Southern Hemisphere, these will need to be calibrated with SHCal20. Marine samples require special consideration of regional reservoir effects in calibration and should be calibrated with Marine20 and a regional calibration. For more information on our marine reservoir corrections please see our website at http://calib.org/marine/.
To use our on-line radiocarbon calibration program please go to http://calib.org/calib/. There is also a link to a downloadable version for MS Windows.