A 200-year ²¹⁰Pb record from Greenland

Journal of Geophysical Research, Vol. 102, No. D4 p. 4325-4332, 1997 

J.E. Dibb
Glacier Research Group, Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham.
H.B. Clausen
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.

ABSTRACT.
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.