Accessing High-resolution Magnetic Records in Speleothems

Calcite speleothems may preserve continuous records of the geomagnetic field spanning tens of thousands of years with annual resolution through progressive encapsulation of magnetic minerals in a calcite matrix. Such records, coupled with accurate uranium-thorium dating, can help characterize short-term variability of the geomagnetic field beyond the modern era and constrain sub-millennial scale geomagnetic events (e.g., jerks and excursions). These, in turn, provide invaluable information for understanding and modeling fast core dynamics, small-scale core field processes, and lower mantle conductivity, which are poorly constrained by the existing paleomagnetic record. However, owing to the weakly magnetized nature of speleothems, the temporal resolution of measured magnetic field records has been limited by the minimum sample size required for magnetic moment detection in standard superconducting rock magnetometers (SRMs). Quantitative scanning magnetometry offers a new means for accessing high-spatial resolution magnetic records in speleothems. We anticipate that a variety of analytical approaches will be tested in the years ahead, ranging from high resolution (<10 micrometers) mapping with quantum diamond magnetometers (QDMs) to moment measurements of <1 mm contiguous samples using scanning SQUID microscopy (SSM). Here we show how QDM and SSM combined with a magnetic multipole model for retrieving net moment can be used to acquire magnetic records in speleothems with unprecedented temporal resolution (>5x better than that obtained with SRMs). In particular, we present high-resolution paleomagnetic records of the Laschamp event in samples of a stalagmite from Crevice Cave, Missouri, USA, and provide a comparison with data obtained with a commercial SRM. We observe a sharp decrease in magnetization while traversing stratigraphically downward into the speleothem from SSM data. This approximately corresponds to a decrease in magnetization observed with a commercial SRM and to the end of the Laschamp excursion at ~39 ky B.P as measured by uranium-thorium dating. We are now probing the fine structure of this event by fitting for the net magnetic moments of the samples using a multipole approach.