Using SQUID Microscopy to Obtain High Resolution Paleomagnetic Records from Speleothems

Speleothems are secondary mineral deposits formed by dripping and seeping water in caves that can record detrital or chemical remanent magnetizations at sub-annual timescales. They can provide high-resolution paleomagnetic records of short-term variations in Earth’s magnetic field, which are invaluable for understanding the dynamics of the geodynamo. Owing to the sensitivity limits of commercial cryogenic rock magnetometers (~10^-12 Am²), paleomagnetic studies of speleothems have been limited to samples with volumes of several hundreds of mm³, which correspond to averaging tens to hundreds of years of magnetic variation. Alternatively, smaller samples can be measured using superconducting quantum interference device (SQUID) microscopy, which has a sensitivity better than ~10^-15 Am². Nonetheless, it is necessary to show that smaller samples contain robust geomagnetic records with (1) detectable magnetic moment, (2) high-coercivity, and (3) an ability to reliably acquire magnetization. To address these limitations, we analyzed ~1 mm³ samples from previously studied speleothems, which represent averages of a few to several tens of years of magnetic variation. We applied an anhysteretic remanent magnetization (ARM) with bias fields of 50 and 10 µT, demagnetized the ARM using stepwise alternating fields (AFs), measured magnetic moments using the SQUID microscope at MIT, and determined the angular difference between the acquired ARM direction and that of the applied bias field. We found that speleothem samples from the Pau d’Alho cave system, Brazil, had magnetic moments between 5x10^-13 and 2x10^-11 Am² and were completely demagnetized by 60 mT. Using a 0.4 mm³ sample, we obtained a directional error of 4^o for a 50 µT ARM, and 45^o for a 10 µT ARM. Using a 1.5 mm³ sample, the directional error for a 10 µT ARM decreased to 18^o. These results indicate that SQUID microscopy may enable us to retrieve reliable high-resolution paleomagnetic directions from speleothems, with the required minimal sample volumes depending on the intensity of the ancient field and the magnetic mineral assemblage of the speleothem. Ongoing work is targeting high-resolution records from a speleothem from Crevice Cave, Missouri, which has recorded the Laschamp geomagnetic excursion.