Acquisition of Viscous Remanent Magnetization in Speleothems
Speleothems are natural recorders of the Earth’s dynamic past. Isotope compositions of speleothem samples show millennial scale changes in processes related to the hydrological cycle and to overlying soil development. In addition to paleoclimate research, speleothems can also be used to understand dynamic processes that disturb the orientation of stalagmites, including seismic events and cave flooding. However, constraining the timing of such events can be challenging, as many stalagmites are not amenable to U-Th dating due to excess thorium associated with high clay contents. In these instances, paleomagnetic methods have the potential to provide a useful means for dating. As stalagmites grow, trace concentrations of magnetic minerals are incorporated into their matrix and are capable of recording the Earth’s magnetic field at the time of formation. The finest grain size portion of this magnetic mineral assemblage is capable of acquiring a secondary, viscous remanent magnetization (VRM). Here, we demonstrate that lab experiments using a laboratory field inside a magnetically shielded room can help us understand the rate at which a particular speleothem acquires a VRM. By applying different initial conditions to the samples, we were able to look at how they affect the rate and strength of acquisition. We’ve found the magnetic mineralogy did acquire a magnetization, similar to that of previous studies and that there is an initial-state, orientation, and time dependence that affects the VRM acquisition. This rate can be used as a tool to estimate when the position of a stalagmite was changed, thereby allowing us to constrain the age of a seismic event or paleoflood.