Paleomagnetism and Relative Paleointensity of two stalagmites from the Estremenho Limestone Massif, Portugal

Abstract

Speleothems magnetism has been used for the study of short-lived variations in the Earth's magnetic field (CMT) as well as climate. In this work, two speleothems from the Estremenho Limestone Massif of Portugal were studied, to obtain information about the variation of the CMT. Paleomagnetic data (natural remanent magnetization), measurement of magnetic properties (FORC), and U-Th dating were performed, as well as the calculation of relative paleointensity, through the Pseudo-Thellier method. Paleomagnetic results indicate that stalagmites recorded a stable remnant magnetization of primary origin (contemporaneous with stalagmite precipitation) and indicate magnetite as the main carrier of magnetization in stalagmites. U-Th dating provided an age range in one of the stalagmites (114.7 ±1.5 ky BP; 109.6 ±1.7 ky BP; 106.3 ±1.6 ky BP), compatible with the Blake geomagnetic excursion age. The relative paleointensity curve shows a pattern of variation very similar to the reference curves for this time interval, however, the magnetic data obtained do not indicate the presence of the occurrence of the Blake excursion. This suggests an older age for this geomagnetic excursion, or a secondary magnetization origin. Further research is needed to unravel this discrepancy.