Submitted by Sammy Smith ([email protected])
This Weekly Ponder began with an attempt to explain “cal BP.” The abbreviation stands for a calibrated date Before Present. But, what does “cal BP” really mean?
Before Present assumes “present” is AD 1950. This is a scientific convention. Agreement on an arbritrary date is necessary because obviously if “present” was the day the notation was made, the date would be different each time the term was used.
The “cal” part is more complicated.
Archaeological dating is in two forms. They are called relative and absolute. (This is not double-dating!)
Relative dates reflect the relative order of events in the past. That is, geological and archaeological stratigraphy has layers, or deposits that change as you go deeper. A basic assumption is that older materials are deeper. That is relative dating.
Think of a layer cake. It has layers of cake separated by frosting. A typical three-layer cake may even have different flavors of cake or frosting. Even making the cake, you have to put one layer in place first, and build up from the bottom. Thus, the relative stratigraphy of the cake means that the lowest layer “came first.”
Relative dating begins with investigators finding artifacts, features, and other ancient remains in context, or in the deposit where they were abandoned. Relative dating can also rely on distinctive styles of artifacts, for example pottery decorations. Therefore, ceramics with identical (or nearly identical) decorative patterns and manufacturing techniques found in different locations are assumed to date to the same period or periods.
Consider clothing styles. Jeans are a particular kind of trousers made from a fabric called denim, for the style of weaving (called twill) and place the fabric was originally made (apparently), and, traditionally, a distinctive blue color made from indigo dye, which came from tropical plants of the genus Indigofera. In the mid- to late-1700s, Georgia and other southern colonies exported indigo to Britain, which encouraged the crop by offering a bounty on it. When the Revolutionary War began, of course that bounty (we might call it a subsidy today) was withdrawn, and prices fell. Also, indigo growers in more tropical locales in the Americas could grow indigo more efficiently (more crops per year). Thus, indigo ceased to be a commercial crop in Georgia.
As you are probably well aware, the cut or style of jeans has varied over time. Early jeans were work clothing and worn baggy enough to allow physical movement, including bending and stretching. This was the 1800s, when only a rare woman wore dungarees. In the 1950s, some people began wearing jeans that were tighter. Waistlines and hem styles have changed.
In general, if you know a lot about fabric and fashion, you can “date” a pair of jeans based on these features. This is an example of relative dating. Consider the two photographs shown here. The upper one shows a woman’s jean, in black not the traditional blue. The lower one is a man’s jean, in blue, with a looser fit. Both date to around AD 2000.
For a long time, relative dating is the only kind of dating archaeologists and geologists had. This was frustrating because they didn’t know how much older a layer was compared to the one above it. Is it just a few years, or many generations—or much longer? Prior to the development of absolute dates, having only relative dates was a major source of frustration for archaeologists.
Radiocarbon dating is the first absolute dating method that was developed, and is perhaps the best-known. In short, radiocarbon dating has to do with minute amounts of carbon-14 that are in living things, including you, which chemically decay at a regular rate to carbon-12. When a living thing dies, it stops taking in carbon-14, and the carbon-14 it contains begins decay.
Almost all carbon on earth is carbon-12, but a small percentage is carbon-14. “Normal” carbon atoms have a nucleus of six protons and six neutrons. Carbon-14, is radioactive, and has eight neutrons. Chemically, it is not stable, and seeks to become stable by ridding itself of those extra neutrons. This process is called decay. The decay is radioactive because some radiation is also emitted in this process.
But the carbon-14 is not chemically stable, and disintegrates, or throws off excess neutrons to become more stable. The rate of this disintegration is such that carbon-14’s half-life is now understood to be 5730 ± 40 years. This means that after a living thing dies and 5730 ± 40 years has passed, the dead organism will have only half the carbon-14 it originally had. In another 5730 ± 40 years, half of that half will remain (that is, one-quarter of what it had to begin with). However, for historical reasons, the half-life that radiocarbon laboratories use is 5568 ± 30 years, called the Libby half-life; it is named after one of the originators of the methods for radiocarbon dating. With any reported radiocarbon date, you need to know the half-life the lab used to be able to properly calibrate the date.
Radiocarbon laboratories extract small samples, for example from bone or textile fragments, and process them with great care to cleanse them and avoid contamination, and produce a radiocarbon date. Radiocarbon dating only works until about 50,000 years ago, maybe a little bit more. This is because the carbon-14 half-life means the sample gets too small to measure, or effectively disappears.
In slightly more detail, radiocarbon dating is based on the fact that as plants grow they “fix” carbon molecules from carbon dioxide in the atmosphere during the process of photosynthesis. The atmosphere has slight variations in the amount of the isotope of carbon called carbon-14 or radiocarbon. The carbon-14 is constantly produced in the stratosphere by cosmic rays colliding with nitrogen atoms in the outer atmosphere. This production, however, is not constant, which means dates need calibration to compensate for irregularities.
Scientists discovered that radiocarbon dates are not identical to calendar dates because of variations in atmospheric carbon. Therefore, to make the radiocarbon date “match” a calendar date, it must be calibrated. One of the major variations in atmospheric carbon came from detonation of atom bombs. Still, the atmosphere in the ancient past had variation, too. This variation is sometimes referred to as “wiggles” in the calibration curve.
All of this is by way of saying that “cal BP” indicates that the date designated this way is a calibrated date, and therefore matches the calendar, and is reported as years before 1950. Therefore 2000 cal BP indicates a the calendar year 50 BC, and 1000 cal BP indicates AD 950. Some dates are reported “uncalib,” which of course indicates that the date is not calibrated, or adjusted based on the calibration curve.
Because there is some imprecision in radiocarbon measurements, scientists report radiocarbon dates “plus or minus” a few years. Thus, a radiocarbon date is best reported as a range rather than a single year. Thus, a date might be reported as 2614 ± 48 years cal BP. This means that there is a high degree of probability that the sample dates between 2662 and 2566 BP, or the calendar years 712 to 616 BC. This probability is also sometimes called a one-sigma (often designated using the Greek letter: σ) or one standard deviation range. Sometimes the plus and minus ranges are not even, and the date might be reported like this: 750 +24/-44 cal BP, indicating a probable range of AD 1156 to 1224.
To add another wrinkle, calibration may also indicate more than one date range, because of “wiggles” in the calibration curve. For certain parts of the curve, there’s a multi-modal distribution. This means that a single sample being dated may have two, or sometimes, three dates, and with current information we cannot tell in the laboratory which is the most likely date. All of this indicates how important careful excavation is in the radiocarbon dating of archaeological remains.
A properly reported radiocarbon date in academic publications will include a date range rather than just a single date, so that anyone can check on the assumptions that were made in calculating the date, along with uncertainties. Proper reporting also notes the laboratory that did the dating. These days, the lab may also date more than one sample, to improve accuracy and precision. In addition, archaeologists try to submit more than one sample for dating, to amplify their understanding of the duration of occupation of a settlement or house, for example. In most articles aimed at the general public, however, these details are glossed over, or avoided—which can cause some confusion. Remember though, that however a calibrated radiocarbon date is reported, it is based on a probabilistic curve, and this dating method can never produce an absolute calendar date.
In fact, the radiocarbon curve is still being refined, as we noted elsewhere on this website.
Radiocarbon dating is not the only kind of absolute dating, but discussing the others is beyond the scope of this article. Stay tuned!
The links given in this article will lead you to more information on radiocarbon dating, and dating in archaeology in general. Comments? Login and leave yours!
Posted online on Friday, March 12th, 2010