Paleomagnetic Records from Holocene-Pleistocene Sediments from South Argentina

GOGORZA, C.S.G., IRURZUN, M.A., LIRIO, J.M., NUÑEZ, H., VILAS, J.F., SINITO, A. M.

Toulouse-Francia

Congreso; 10th Scientific Assembly of International Association of Geomagnetism and Aeronomy-IAGA; 2005

IAGA

PALEOMAGNETIC RECORDS FROM HOLOCENE-PLEISTOCENE SEDIMENTS FROM SOUTH ARGENTINA C.S.G. Gogorza (1,2), M.A. Irurzun (1,2), M.A.E. Chaparro (1,2), J.M. Lirio(3), H. Nuñez (3), J.F. Vilas (1,4), A.M. Sinito (1,2) (1) CONICET; (2) Universidad Nacional del Centro, Argentina; (3) Instituto Antártico Argentino; (4) Universidad de Buenos Aires, Argentina Paleomagnetic studies carried out on six cores from the bottom sediments from lake El Trébol, South Argentina are described. Measurements of intensity (NRM) and directions (D and I) of Natural Remanent Magnetisation, magnetic susceptibility at low and high frequency (specific, X and volumetric, k), Isothermal Remanent Magnetisation (IRM), Saturation Isothermal Remanent Magnetisation (SIRM), back field and Anhysteric Remanent Magnetisation (ARM) were carried out. Stability of the NRM was investigated by alternating-field demagnetisation. The studies of rock magnetism suggest that the main carriers of magnetisation are ferrimagnetic minerals, predominantly pseudo single domain magnetite. Records of NRM and k show a good correlation with the sedimentological description, and also with those of shorter cores previously studied. The tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs. Due to their different chronological meaning and their rather bad behaviour as magnetic recorder, 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. Radiocarbons dating from these cores and from earlier studies help us to construct a paleosecular variation (PSV) record for the past 24,000 years. These results show good correlation with the previous one. The spectrum obtained from spectral analysis shows major peaks at periodicities of about 1,550; 2,100; 2,900; 3,500; 4,400; 6,200 and 10,600 years for inclination data and at periodicities of about 4,000; 5,450 and 10,300 years for declination data. Both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of the Bauer plots, with a preponderance of clockwise movement.