Asking awkward questions: querying assumptions about ice core acid signals using tephra

Since the first comprehensive records of past volcanic activity were constructed from ice core records in the 1980s, there has been a temptation to deduce the source of acid signals based on known volcanic eruptions of a similar age. The conjectured sources have on occasion been used to interpret the timing and climate impact of specific eruptions, as well as to corroborate the chronological precision of ice core chronologies. Only the presence of tephra can, however, confirm the origin of an acid signal, although not all eruptions will disperse ash towards the polar ice caps. Here we review Late Holocene tephrochronological investigations focused on a number of targeted volcanic events in Greenland ice cores and highlight the risks associated with “guesstimating” the source of acid signals. We demonstrate that the correlation of acid signals to historically documented eruptions based on age equivalence lends itself to a misinformed understanding of volcanic impacts on climate, environment and society. Specifically, we draw attention to the need to consider independently-derived sulphate emissions of a given eruption, rather than the magnitude of the event, to understand its radiative forcing potential. We show that simultaneous eruptions can and do emit products that arrive concurrently in ice cores, which has implications for modelling regional, hemispheric and global impacts based on ice core-derived estimates of atmospheric aerosol loading and the assumed latitude of the source eruption. We recommend that tephra investigations be an integral component of all future ice core research in order to build a more robust framework with which to model volcanic climate impacts.