Unraveling Viscous Remanent Magnetization of Cave Formations
The efficacy of using viscous remanent magnetization (VRM) as a tool for dating geological materials has been debated since no general method has been established. Successful applications involve Hadrian’s Wall, Icelandic glacial floods, and movements of boulders due to tsunamis. The technique relies on the presence of the fine scale grain sizes of magnetic minerals that are able to acquire new magnetization when a new orientation is established. One of the materials that has not been explored are speleothems. Speleothems are secondary carbonate formations in caves, such as stalagmites and stalactites, and are natural recorders of the Earth’s dynamic past. During formation, trace concentrations of magnetic minerals are incorporated into their matrix and record the Earth’s magnetic field at the time of formation. Results from all over the globe show the ubiquitous presence of a secondary magnetic component. Although they have not been studied, these secondary magnetizations offer the possibility to be used as a dating tool for past tectonic events, past human occupation in caves, and paleofloods.
In this study, we explored the magnetic mineralogy and the acquisition of viscous remanent magnetization of a stalagmite sample. Our goal was to establish a robust analytical time-dependent expression for the acquisition of VRM. We applied two protocols to unravel the viscous component. We demagnetized at small temperature steps to define the temperature range where the viscous component is defined, and calculated the grain size distribution that is required for this acquisition. Next, we simulated the acquisition of VRM in a controlled laboratory environment, by changing the initial states of the specimens. We were able to define the time required to gain a new vector magnetization. By fitting a curve to this acquisition, the mathematical model can be used to date orientation changes in the sample.