Be chronology of the last deglaciation of County Donegal, northwestern Ireland

A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8 1.0 Be kyr to 20.9 1.3 Be kyr, with an uncertainty-weighted mean age of 19.4 0.3 Be kyr (19.4 1.2 kyr accounting for production rate uncertainty). Our results confirm one previous Be age obtained from the moraine, with the combined data (n=7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3 0.3 Be kyr (19.3 1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (Z20– 18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS 19.5–20kyr ago.