A 200-year 210Pb record from GreenlandJournal of Geophysical Research, Vol. 102, No. D4 p. 4325-4332, 1997
Glacier Research Group, Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham.
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.
A continuous profile of 210Pb activity extending back to 1766 has been developed for a firn/ice core collected at Site D in central Greenland in 1984. Unexpectedly high activities of 210Pb were found at the base of this core (0.032 pCi kg-1 in samples more than 200 years old), calling into question the common assumption that supported 210Pb can be neglected when constructing chronologies in glacial snow and ice. It is problematic to assert that all of the 210Pb measured at depth should be attributed to the supported fraction, given previous estimates of dust loading in Greenland ice cores. However, even if an estimated constant value of 0.032 pCi supported 210Pb kg-1 is subtracted from the measured values to estimate excess 210Pb, the 210Pb chronology for Site D yields ages that are significantly younger (mean accumulation rate too high) than an independent depth-age scale based on annual layer counting. It is apparent that the flux of excess and/or supported 210Pb to this site must have decreased over the past 2 centuries, with decreasing trends in both fractions most likely. Previously published 210Pb profiles for cores from Summit and Dye 3, Greenland, show similar trends, which had been interpreted as decreasing fluxes of excess 210Pb only. For all three sites, it is possible to separate variations in the fluxes of the excess and supported fractions of 210Pb, but variations in the total 210Pb flux will impact 210Pb-based chronologies generally if these variations have not been restricted to the Greenland ice sheet.