TOWARD CONSTRAINING THE AGE AND DURATION OF THE LASCHAMP GEOMAGNETIC EXCURSION USING A SPELEOTHEM RECORD FROM CREVICE CAVE, MISSOURI

Speleothems have immense potential for archiving high quality, high resolution geomagnetic records, and can be dated with extreme precision using radiometric techniques. One of the frontiers of paleomagnetic research focuses on the understanding of the Earth’s magnetic field behavior during centennial to millennial events known as geomagnetic excursions. Here, we investigate a record of the Laschamp geomagnetic excursion preserved in a stalagmite from Crevice Cave, Missouri. The Laschamp excursion was the first geomagnetic excursion described in the paleomagnetic record, and remains the most studied geomagnetic event of its kind. However, the exact age and duration of the event are still a matter of debate. The most accurate assessment of the timing of the occurrence of the Laschamp excursion remains the analysis of Singer et al. (2009), who determined a mean age of 40.7 ± 1.0 ka for the event using a combination of radiometric dating techniques on volcanic rocks. Our speleothem sample from Crevice Cave formed during the Last Glacial (MIS 3 and 4), with growth rates of 5-20 mm/ka. The sample has been previously dated using the ²³⁰Th method, with age errors of 100-150 years at the time of the Laschamp (Dorale et al., 1998), an order of magnitude smaller than the errors of the Laschamp age determined by Singer et al. (2009). We have cut a 19-cm stalagmite slab into discrete samples using a wire saw at 5 mm intervals, with resulting volumes of ~0.5 cm³. These volumes have signal:noise ratios of at least 10 on a cryogenic rock magnetometer with a nominal sensitivity of 10^-12 Am², which allowed us to measure the natural remanent magnetization (NRM), and to determine its stability during progressive demagnetization. The NRM measurements of the discrete samples show a directional excursion expressed as an inclination departure of ~50°, and a declination departure of ~140° before returning to initial values. The excursion occurs between 2 and 4 cm from the top of the stalagmite, at ~40 ka BP (Dorale et al., 1998). Based on annual band counting from confocal micrographs, we estimate a maximum duration of 3500 years for the Laschamp excursion.