Radio-echo layering in polar ice sheets and past volcanic activity

Abstract

A recent study¹ has shown how acidity profiles along dated Greenland ice cores can reveal large volcanic eruptions in the Northern Hemisphere during the past 10,000 yr. It has been suggested that these layers of acidic ice in the polar ice sheets may also be detected by airborne radio-echo sounding (RES) techniques as stratification echoes^2–4. Explosive volcanic eruptions eject large amounts of SO₂ into the stratosphere⁵ where it forms an H₂SO₄ aerosol⁶. Studies of Antarctic snow⁷ and Greenland ice cores² show that this material can be deposited in layers of large areal extent on polar ice sheets. Calculations based on observations of these slightly acidic layers show that they should give rise to radar reflections of similar magnitude to those observed for stratification echoes. New RES data from the Antarctic enables the present comparison to be made between observed layer power reflection coefficients (PRCs), and calculated values for reflections from acidic ice layers and from layers of ice of changed density. A gap in layering has been identified which seems to coincide with a similar gap reported in Greenland at Crête⁸; profiles of layer PRC against age show a common pattern for many sites on the Antarctic ice sheet. This PRC/age profile may provide a record of explosive volcanic activity for the Southern Hemisphere over the past 150,000 yr.