Testing the reliability of detrital cave sediments as recorders of paleomagnetic secular variations, Seso Cave System (Central Pyrenees, Spain).

摘要

A paleomagnetic study has been carried out on a waterlaid detrital sedimentary sequence of ~240 cm thick within the Seso Cave System (West-Central Pyrenees). In these sediments, seven charcoal samples were dated using 14C AMS ranging from 2080 to 650 cal yr BP (130 BC-1300 AD). Two levels of human occupation of the cave have been recognized by ceramics associated to the Iberian Period and to the Roman Period, respectively. The detrital sedimentary sequence is made of autochthonous (piping detached material from the Eocene marls host rock inside of the cavity) and allochthonous (stream transported sediments from the outside) sediments. The autochthonous material (first 100 cm), made of fine grain laminated sediments (lutites and marls) corresponds to pond facies; the allochthonous material (190-240 cm) is made of lutites and sands and corresponds to stream facies, and both facies are mixed from 100 to 190 cm. The increase in sedimentation rate towards the end of the sequence (stream facies) points to an intensification of the alluvial activity as a possible consequence of a more arid climate during the Medieval Climate Anomaly. For the paleomagnetic study, 44 discrete cylindrical samples were taken along the detrital sequence. The values of the natural remanent magnetization and magnetic susceptibility are significantly lower in the pond sediments than in the stream sediments. The declination and inclination of the paleomagnetic characteristic component (sister samples analyzed by both alternating field and temperature demagnetizing procedure) of each depth point is compared to the Spanish archeomagnetic catalog and available geomagnetic models (ARCH3k.1, CALS3k.4, CALS10k.1b and SCHA.DIF.3K) in order to determine the accuracy of these sediments recording the Earth's magnetic field. Results suggest that these sediments poorly record the Earth's magnetic field, however, paleomagnetic inclination shows similar results between both demagnetizing methods and the inclination is well recorded especially in the younger stream facies. The lack of archeological remains with absolute dates from 925 to 1545 cal yr BP in the Iberian paleomagnetic secular variation reference curve has prevented, up to now, the study of that time period. Therefore, the inclination data from the Seso Cave deposit is the first record of the Iberian paleomagnetic secular variation during most part of the Medieval time, and they are closer to the inclination values of one geomagnetic model (CALS10k.1b).