Abstract
Abstract A new protocol using the viscous remanent magnetization (VRM) of boulders to date cataclysmic geological events such as tsunamis, glacial floods, and landslides is presented and its performance is assessed against two jökulhlaups (glacial floods) of known age in Iceland. High-intensity jökulhlaups have the ability to break off large boulders from bedrock and emplace and rotate them. These rocks originally carried a remanent magnetization parallel to the geomagnetic field during their formation. After being rotated by the flood, they acquire a VRM parallel with Earth’s magnetic field. In continuous thermal demagnetization experiments the unblocking temperature of the VRM can be determined, and subsequent rock magnetic VRM acquisition experiments can be used to establish a relationship between the unblocking temperature and the acquisition time, from which the time since the flood can be determined. The protocol was tested on 44 boulders from 2 historical jökulhlaups in Iceland and found to yield good order-of-magnitude estimates: 72 yr (confidence limits 11–360 yr) versus known 155 yr at the Sólheimajökull jökulhlaup and 290 yr (confidence limits 80–2300 yr) versus known 288 yr for the Kotarjökull jökulhlaup. The method can therefore be a valuable tool for future dating of cataclysmic events.
Highlights
Floods with recurrence periods on historic time scales may pose an important natural hazard
Directional analysis found that most boulders carried a primary non-north clustering magnetization and a secondary northward clustering remagnetization, i.e., a viscous remanent magnetization (VRM) (Item DR5), but 9 of 44 boulders were rejected because their VRMs did not carry a secondary northward clustering remagnetization
Hysteresis and first-order reversal curves (FORC) diagrams measured for most boulders generally indicated that samples from Sólheimajökull were more SD like, whereas those from Kotarjökull were more pseudo-SD and MD like, but no correlation between suitability for VRM dating and domain state was found (Item DR6)
Summary
Floods with recurrence periods on historic time scales may pose an important natural hazard. These include storm floods and tsunamis, and jökulhlaups, i.e., sudden high-volume glacial meltwater outbursts, which are common in Iceland, where they may have affected early settlement in medieval time (Smith and Dugmore, 2006). Single-domain (SD) particles, the size of a VRM is a function of time, temperature, mineralogy, and grain-size distribution (Néel, 1949). Both the VRM and the original NRM can be recovered by demagnetizing the samples to progressively higher temperatures and measuring the remaining remanent magnetization vector: first, removing and identifying the VRM, and the NRM. VRM dating intrinsically reliant on the rocks, and is independent of external factors
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