The paleomagnetism and U-Th dating of Mexican stalagmite, DAS2

Abstract

A 72 cm high stalagmite from Mexico, DAS2, has been analyzed for its paleomagnetism, and has provided a record of secular variation beginning about 1200 years ago to present. The stalagmite did not grow uniformly, and a low content of magnetic material has rendered the record less precise in some parts. A topographic test involving the magnetic measurement of coeval specimens from the sides of the stalagmite has shown that the record is detectably free from depositional errors. Age data were provided by the U-Th method, and concordancy checked by 14C and 226Ra methods. Shallow inclinations at the base of the record are followed by a partial anticlockwise loop in the declination-inclination plot. There is good agreement between the DAS2 data and the southwest U.S.A. archeomagnetic record and with a lacustrine record from Oregon, from 750 to 1400 AD. Comparison of the record with the present field at the site latitude shows that westward drift of the field has persisted for at least 1200 years at a drift rate of about 0.30° yr −1. A close match in declinations with the present field at the site latitude suggests that the stalagmite may have recorded the passing of the northern edge of the Gulf of Guinea negative anomaly about 1000 years ago, though the corresponding inclination feature is not present in the record. An archeomagnetic synthesis by Merrill and McElhinny of worldwide data to estimate positions of the main dipole shows that it was tilted away from Mexico from 500 to 1250 AD, and this may account for the shallow inclinations seen at the start of the record.