Paleosecular variations 12-20kyr as recorded by sediments from Moreno Lake (South of Argentina)

IRURZUN, M.A., GOGORZA, C.S.G., SINITO, A.M., CHAPARRO, M.A.E.

República Checa

Congreso; 10th. Castle meeting Paleo, Rock and Environmental Magnetism; 2006

Geophysical Institute of the Academy of Sciences of the Czech Republic and Geophysical Institute of the Slovak Academy of Sciences

PALEOSECULAR VARIATIONS 12-20 KYR. AS RECORDED BY SEDIMENTS FROM MORENO LAKE (SOUTHERN ARGENTINA) María alicia irurzun1,2, claudia gogorza1,2, ana maría sinito1,2, marcos chaparro1,2 1    Instituto de Física Arroyo Seco. Universidad Nacional del Centro de la Pcia. de Bs. As. Pinto 399, 7000 Tandil, Argentina (airurzun@exa.unicen.edu.ar) 2    Consejo Nacional de Investigaciones Científicas y Técnicas. Avda. Rivadavia 1917, C1033AAJ, Buenos Aires, Argentina. Paleomagnetic and sedimentological studies carried out on five cores Lmor1, Lmor2, Lmor3, Lmo98-1, Lmor98-2 from the bottom sediments of Moreno lake (southwestern Argentina) are described. The results obtained are integrated with the data from Gogorza et al. (2000). The length of the cores varies from 0.5 m to 4 m and 450 samples were collected. Measurements of directions (declination D and inclination I) and intensity of natural remanent magnetisation (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), isothermal remanent magnetisation (IRM), saturation isothermal remanent magnetisation (SIRM), and back field were carried out. Stability of the NRM was investigated by alternating-field demagnetisation. The correlation between cores was based on magnetic parameters as X and NRM. 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”. The Declination and Inclination logs of the characteristic remanent magnetization for the cores as function of shortened depth are obtained. Comparison between stacked inclination and declination records of Moreno lake and results obtained in previous works, Escondido lake (Gogorza et al., 1999) and El Trébol lake (Irurzun et al., 2006), shows a good agreement. This agreement allowed adjusting the new obtained records with those from Escondido and El Trébol  lakes in order to transform them into time series. The secular variation (SV) of the geomagnetic field 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 at intervals of 500 yr. for both records. Both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of Bauer plots.  References Gogorza C.S.G., Sinito A.M., Di Tomasso I., Vilas J.F., Creer K.M., Nuñez H., 1999. Holocene secular variation recorded by sediments from EscondidoLake (South Argentina). Earth Planets Space, 51, 93–106. Gogorza C.S.G., Sinito A.M., Di Tomasso I., Vilas J.F., Creer K.M., Nuñez, H., 2000. Geomagnetic secular variations 0–12000 year as recorded by sediments from Moreno Lake (South Argentina). J. South Am. Earth Sci., 13 (7), 627–645. Irurzun M.A. Gogorza C.S.G, Chaparro M.A.E., Lirio J.M., Nuñez H., Vilas J.F., Sinito A.M., 2006. Paleosecular variations recorded by Holocene-Pleistocene sediments from Lake El Trébol (Patagonia, Argentina). Physics of the Earth and Planetary Interiors, 154, 1, 1-17.