Assessment of the record of the 1982 El Chichón eruption as preserved in Greenland snow

Variability in the SO42− and Cl− time series for the 1980s from 12 shallow snow pits across the Greenland ice sheet is used to evaluate the record of the 1982 El Chichon eruption and the potential for recording a moderate northern equatorial eruption in a single Greenland ice core. Composition of volcanic glass found in spring 1983 snow in one of the pits in the Summit region matches that from El Chichon glass, thereby verifying the deposition of material from the eruption. High Na+ and Cl− concentrations in this same layer probably represent deposition of the reaction products of halite and H2SO4 as observed in the stratosphere following the eruption. These findings and the presence of a Cl− signal in five of the other pits indicate that the Cl− aerosol component of some eruptions has the potential to remain aloft for at least 1 year after the eruption. Some of these aerosols may be adsorbed onto tephra particles. Distinct SO42− peaks that can be confidently linked to El Chichon were found in only 50% of the records developed through subseasonal sampling. However, in several other pits an elevated baseline, thought to represent more lengthy periods of El Chichon aerosol deposition as opposed to deposition from a single snowfall event, were observed. Smoothing of the original data by the calculation of annual SO42− flux resulted in the presence of high flux values between 1982 and 1984 (years thought to be affected by El Chichon aerosol deposition) in 9 of the 12 pits. These results suggest that a single ice core from anywhere in Greenland may record a signal from a northern equatorial eruption of magnitude similar to that of El Chichon about 75% of the time; this is despite the overall high levels of SO2−4 deposition from anthropogenic sources that now make identification and quantification of the volcanic SO2− portion of the record more difficult than obtaining the same data for preindustrial volcanism. Nevertheless, composite records from all the pits sampled yielded stratospheric loading (∼20 Mt) and optical depth (τ=0.13) estimates similar to stratospheric and satellite-based measurements following the eruption. Equally high SO42− concentration and flux values in snow from 1980 to 1982 reflect deposition from the series of middle to high northern latitude volcanic eruptions 1979–1981.