Relative paleointensity of the geomagnetic field 12-20 Kyr. from sediments cores, Lake Moreno (Patagonia, Argentina)

CLAUDIA S.G. GOGORZA; MARÍA A. IRURZUN; MARCOS A.E. CHAPARRO; ANA M. SINITO

Acapulco, Mexico

Congreso; 2007 Joint Assembly, American Geophysical Union (AGU); 2007

AGU

Four cores labelled Lmor1, Lmor2, Lmor3, Lmor98-1, Lmor98-2 from the bottom sediments of Lake Moreno (south-western Argentina) have been used to estimate regional geomagnetic paleointensity. Lake Moreno is on the east side of the Andean Cordillera Patagónica; it is located in the Llao Llao area, San Carlos de Bariloche, Argentina (about 41°S, 71°30’W). The following measurements were performed: Natural Remanent Magnetisation (NRM), magnetic susceptibility at low and high frequency (specific, X and volumetric, k), Isothermal Remanent Magnetisation (IRM) reaching the Saturation Isothermal Remanent Magnetisation (SIRM), Back Field, Anhysteric Remanent Magnetisation with a direct field of 0.1mT and an alternating field between 2.5 and 100mt (ARM100mT). Associated parameters were calculated: S-ratio, Remanent Coercitive Field (BCR), anhysteric volumetric susceptibility (kanh), SIRM/k, ARM100mT/k, and SIRM/ ARM100mT. The rock magnetic studies indicate that the magnetic mineralogy of the clay-rich sediments is dominated by pseudo-single domain magnetite in a narrow range of grain size (between 1 and 4µm) and concentration (between 0.05 and 0.1%), thereby meeting established criteria for relative paleointensity studies. The remanent magnetisation at 20mT (NRM20mT) has been normalised using the anhysteric remanent magnetisation at 20mT (ARM20mT), the saturation of the isothermal remanent magnetisation at 20mT (SIRM20mT) and k. A comparison of these results with relative paleointensity records obtained in previous work, Lake Escondido (Gogorza et al., 2004) and Lake El Trébol (Gogorza et al., 2006) allows obtaining a detailed information about the disagreement observed in the period 12-20 kyr in both records.