How accurate is ‘‘paleomagnetic dating’’? New evidence from historical lavas from Mount Etna

In the last years, paleomagnetism has been increasingly used to provide emplacement
ages of loosely dated volcanics. Dating is achieved by comparison of paleomagnetic
directions with a given reference curve of the paleosecular variation (PSV) of the
geomagnetic field. Recently, a debate has developed on the achievable precision (the a95
value) of the paleomagnetic directions and hence on the accuracy that ‘‘paleomagnetic
dating’’ can yield. At 39 different sites from Etna we paleomagnetically investigated 13
flows (four ‘‘test flows’’ with known age, and nine loosely dated flows), emplaced
between 122 B.C. and 1865 A.D. We systematically drilled 12 cores per flow spaced in
three (far from each other) sites and demagnetized one specimen per core by alternating
field cleaning. Results from the four test flows yield age windows effectively
encompassing the respective true flow ages, when dating based on Bayesian statistics at a
95% confidence level is adopted. We find a95 values for the flow mean directions ranging
between 3.3 and 5.7 (4.5 on average), which translate into accuracies of age
determinations of 136–661 years (307 years on average). Such dating uncertainty is likely
underestimated, as we disregarded several kinds of errors that might affect both the fidelity
of paleomagnetic recording and the PSV reference curve. The strong magnetization of
both the underlying terrain and the cooling flow itself and mineral magnetic variations
across the flows are the most likely sources for the scatter characterizing the recording
process of the magnetic field in volcanic rocks.