Testing the use of viscous remanent magnetisation to date flood events

Abstract

Using erratics associated with large flood events, this paper assesses whether their viscous remanent magnetisation (VRM) can be used to date the flood events. We tested this method using flood erratics from three large events: (1) the Late Pleistocene Bonneville mega-flood in Idaho, USA, (~14-18 ka), (2) the 1918 A.D. Mt. Katla, Iceland, eruption and associated jokulhaup (meltwater flood) at Mýrdalssandur, and (3) the Markarfljot jokulhaup due to an earlier eruption of Mt. Katla (~2.5 ka). We measured 236 specimens, 66 of which yielded clear identifiable and measurable viscous magnetisation signals from erratics with clustered VRM directions. From the VRM unblocking temperatures, age estimates were made. The age estimate for the most recent event (Mýrdalssandur) worked well, with a median estimated age of 80 years (with individual erratic estimates distributed between 61 – 105 years) compared to the known age of 91 years. The ages of the other two events were over-estimated. The estimates for Markarfljot (15 ka (7 – 33 ka)) were based on the results of just one erratic. For the Bonneville flood the estimates were too old, however, this locality had the largest uncertainty in the ambient temperature used in the age determination; the VRM acquired is strongly dependent on the ambient temperature, the older the event the greater the uncertainty. Southern Idaho currently has hot summers, with average summer maximum temperatures of ~31 ˚C, but a mean annual temperature of only ~ 9 ˚C. It is suggested that the VRM dating method works best for recent events (< 2-3 ka) where the ambient temperature history can be constrained.