“This dataset is the only continuous atmospheric record beyond the end of the tree rings,” said Paula Reimer, an archaeologist from Queen’s University Belfast in Northern Ireland who was not involved in the study.
Stephen Doherty, a member of the Heritage Group and teacher at Lumen Christi College said: "The new discovery is set to change our understanding of the early history of Derry.
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.
But tree ring data only go back 13,000 years, and thus cannot be used to calibrate older dates.
“The hope has always been that we’d find records that we could use for the whole period of radiocarbon dating,” said Bronk Ramsey. Due to yearly changes in the lake’s surrounding vegetation, different types of organic material settled on its bottom in summer and winter."By 1685, however, the round tower is no longer seen on any historic maps, but a windmill is shown on the city's outskirts.