The accumulation of ²¹⁰Pb at Summit, Greenland since 1855

Detailed (202 samples) profiles of total beta and ²¹⁰Pb activity were determined from a 32 m firn core collected at the GISP2 site (72°20’N, 38°45’W) near Summit, Greenland in 1989. The beta radioactivity profile verifies year by year dating based on recognition of annual hoar layers back to 1955, and lends a high degree of confidence to the 74 years age assigned to the bottom of the core by this stratigraphic dating technique. The decay corrected activity of ²¹⁰Pb at the time of deposition shows considerable short term variability, but no clear seasonal or annual periodicity. ²¹⁰Pb activity in surface snow at this site has averaged 0.7 pCi kg^-1 since 1927, but the period 1915-1927 is characterized by a steady decline from higher levels. The average annual accumulation of ²¹⁰Pb has markedly declined since at least 1870. Similar observations at Dye 3 suggest that ²¹⁰Pb accumulation has decreased throughout this century over much of the Greenland Ice Sheet. If the records of ²¹⁰Pb in the firn on the Greenland Ice Sheet are mainly reflecting the northern hemisphere atmospheric burden of ²¹⁰Pb, these results will demand careful reassessment of ²¹⁰Pb-based radiochronologies. However, our current lack of understanding of the linkages between atmospheric and snow chemistry makes the widespread applicability of these findings an open question.