How radiocarbon dating works
In total we were able to extract almost 50 sub-samples of material from the 155m of core samples we collected last year; these were then sent to our colleagues at SUERC, in East Kilbride, for AMS* radiocarbon dating.Thankfully, we managed to extract multiple samples of datable material from each of the ten study sites, allowing us to produce objective “absolute” dates from these mounds for the first time.and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale.If you want more information about how radiocarbon dating works: The internet being what it is, there is plenty of nonsense online about radiocarbon dating, thankfully there are at least a few sensible sources which explain how the technique works – see here and here, for example.Wherever possible, we aimed to collect datable material (preserved plant remains, seeds, charcoal, bone etc.) from both the buried ground surface underneath each mound as well as from the actual makeup of the mound itself: Obtaining dates on the old ground surface can help to provide us with what archaeologists call a (“limit after which”) – a maximum age for the point in time that the old ground surface became buried underneath the mound.The radiocarbon evidence shows that Berkhamsted Castle was definitely built during the medieval period, but as of yet we cannot be more precise.“Castle 3” at Hamstead Marshall is another medieval mound, but is somewhat later than most of the other mounds.
In order to get the best estimate, we aimed to get as many dates as possible from each mound; the latest date would then provide us with the “best estimate” for the age of construction.Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed.The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.most castle mottes do seem to date to the medieval period.This in itself is an interesting result, since, on the eve of the 950th anniversary of the Norman Conquest of England, this is really the first time anyone has been able to scientifically date the massive impact of the Conquest on the English landscape!
In future posts (and forthcoming academic publications) we will look in more detail at the implications of these results…