Detailed Record of Paleomagnetic Field Change from Lake Sediments From Southern Argentina

GOGORZA, C.S.G.; SINITO, A.M.; LIRIO, J.M.; NUÑEZ, H.; VILAS, J.F.; CHAPARRO, M.A.E.

San Francisco (USA)

Conferencia; AGU 2001 Fall Meeting; 2001

AGU

Paleomagnetic, magnetic and sedimentological studies carried out on four cores, about 10m long, from the bottom sediments of Escondido Lake (southwestern Argentina) are described. Measurements of intensity and directions of Natural Remanent Magnetization (NRM), magnetic susceptibility, isothermal remanent magnetization, saturation isothermal remanent magnetization (SIRM), back field, anhysteric remanent magnetization and saturation anhysteric remanent magnetization (SARM) were carried out. Stability of the NRM was investigated by alternating-field demagnetization. One sample of each five were chosen as pilot samples, which were demagnetized successively until 70mT peak and the stable remanent magnetization was isolated. Most of the samples showed no systematic change in the direction of their remanent magnetization during AF demagnetization; few of them showed a viscous magnetization, probably picked up in laboratory fields, which could easily be removed by AF demagnetization at about 25 mT. The main carriers of magnetisation seems to be ferrimagnetic minerals, predominantly pseudo single domain magnetite Within-lake correlation was based on the magnetic susceptibility and intensity of remanent magnetization. The individual measurements were stretched. The tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs. Due to their different chronological meaning these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed, obtaining a shortened depth. Calibrated ages were calculated from radiocarbon dating and a transfer function shortened depth-age was built. SARM and SIRM measurements were used to obtain relative paleointensity profiles. The secular variation (SV) of the geomagnetic field (both in intensity and direction) was studied using spectral analysis and precession analysis of the magnetic vector. In order to test the stability of the obtained periodicities, spectral analysis has been applied to every 6000 yr. time span at intervals of 500 yr. for both records. Both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of VGP path and curvature of field motion path.