The ages of most of the projectile point types described on this website have been determined, at least in part, through radiocarbon dating. This technique is based on the fact that living organisms incorporate the atmospheric carbon-14 isotope into their bodies while alive, and this isotope begins to decay at a known rate after the organism dies. Measuring the amount of carbon-14 that still survives in an organism’s remains tells you how long ago it died. Unfortunately, the amount of carbon-14 in the atmosphere has fluctuated over time, which complicates dating. To correct for these fluctuations, evidence from things like tree rings is used to calibrate the carbon-14 dates and convert them into a standard calendar date. A number of different calibration corrections have been developed, so archaeologists often just publish the raw radiocarbon dates. But this can be confusing for the general public. For the Projectile Points website, we have used a calibration model called IntCal04, published in the journal Radiocarbon in 2004 (http://researchcommons.waikato.ac.nz/bitstream/
Whenever possible, we used IntCal04 to come up with an approximate calendar year for a carbon-14 date, and included both in the type descriptions (radiocarbon dates that are less than about 1500 years old correspond pretty closely with calendar dates, so in those cases only one set of dates is included). Because radiocarbon dates are not absolutely accurate, they are published with an error range, +/- a certain number of years. That error range is included in our type descriptions, but for the calendar year calibration we just use the mid-point date, for simplicity. Radiocarbon dates are typically reported in years Before Present (B.P., where “the present” is 1950 A.D.), but are sometimes published as years B.C. The calendar dates in the type descriptions are all B.C. or A.D.
More information about radiocarbon date calibration can be found at https://www.nps.gov/history/seac/hnc/outline/02-paleoindian/index.htm.