Radiocarbon dating belfast
“The authors have done an excellent job in reconstructing the chronology of the Lake Sugietsu cores,” said A. Timothy Jull from the National Science Foundation’s Arizona Accelerator Mass Spectrometry Laboratory.
“We must exercise some caution about any lake sediment record as it's always possible that there are missing layers.
By providing a more precise record of this element in the atmosphere, the new data will make the process of carbon-dating more accurate, refining estimates by hundreds of years.
The data will allow archaeologists to better gauge the age of their samples and estimate the timing of important events such as the extinction of Neanderthals or the spread of modern humans through Europe.
His team took three cores that overlap in several places, and used two different approaches to count the varves: they looked at them under a microscope and also tracked the chemical changes along them using X-rays.
Finally, they compared their data with previous records, including tree rings and cave samples, to account for any uncertainties due to ambiguous layers.
"By 1685, however, the round tower is no longer seen on any historic maps, but a windmill is shown on the city's outskirts.
"The radiocarbon dates are not saying that the tower wasn't reused as a windmill in the 17th century, but it seems builders were making-do and mending, using the stump of the old round tower for a new purpose." The monument is now the only medieval structure still standing in Derry; others that did exist have been lost during more than 400 years of building that has gone on across the city.
The sediments are full of plant remains that, like tree rings, took their carbon-14 directly from the atmosphere, and can be accurately matched to a specific year using the varves as a mineral calendar.Dr Barrett explained: "The radiocarbon dates we obtained suggested the fabric of the tower was from the medieval period."The work that local historians in the Derry Tower Heritage Group had previously carried out suggests that a medieval round tower once existed in this general location in 1600.Hiroyuki Kitagawa from Nagoya University and Johannes van der Plicht from the University of Groningen found the annual varves in the 1990s.They extracted a core (a column of sediment), did some radiocarbon testing, and published their analysis in in 1998.