Relative Paleointensity of the geomagnetic field from Holocene-Pleistocene Sediments, lake El Trébol (Patagonia, Argentina)

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

Toulouse-Francia

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

IAGA

RELATIVE PALEOINTENSITY OF THE GEOMAGNETIC FIELD FROM HOLOCENE-PLEISTOCENE SEDIMENTS, LAKE EL TREBOL (PATAGONIA, ARGENTINA) C.S.G. Gogorza (1,2), M.A. Irurzun (1,2), M.A.E. Chaparro (1,2), A.M. Sinito (1,2) (1) CONICET; (2) Universidad Nacional del Centro, Argentina Four cores (labelled lt98-1, lt98-2, lt98-4, lt98-5, respectively) about 11m long from Lake El Trébol (Patagonia, Argentina) have been used to estimate regional geomagnetic paleointensity. Lake El Trébol, a closed basin (41°04’S 71°29’W) is a small lake (surface: 0.4km2, maximum depth: 11m) located at 758a.s.l. on the east side of the Andean Patagónica Cordillera in a wooded area with moderate human influence. Sedimentological studies indicate that Lake El Trébol sedimentary column shows strong similarities with other obtained by the authors from surrounding lakes. Rock magnetic studies, 14C and δ13C were carried out on samples from one core. 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, 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 8µm) and concentration (between 0.01 and 0.1%), thereby meeting established criteria for relative paleointensity studies. The samples whose concentrations and size are beyond this range were rejected. 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. Coherence function analysis indicates that the normalised records are free of environmental influences. This suggests that the variations in normalised remanence are mostly likely due to geomagnetic paleointensity fluctuations. Our paleointensity versus age curve is an a good agreement with published record from another lake of Argentina and with records from other parts of the world suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar, signal.