Preliminary rock magnetic and paleomagnetic studies on sediment cores from Laguna La Barrancosa, Argentina

EQUIPO OPTATIVA 2015; CLAUDIA GOGORZA; IRURZUN, M A; EQUIPO OPTATIVA 2016

Puerto Vallarta

Congreso; Unión Geofísica Mexicana, Reunión Anual 2016; 2016

Unión Geofísica Mexicana

A set of sediment cores from Laguna La Barrancosa was studied. The lake is located in the pampean region, Buenos Aires Province, Argentina. Several cores were extracted from the shore to the center of the lake in 2015 and 2016. Measurements of magnetic susceptibility (k), natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM) and variations of k with temperature (T) were performed. Associated parameters were also calculated and inclination (I) and declination (D) were estimated. The concentration of magnetic minerals, the magnetic mineralogy and their grain size were analyzed.The core lengths are between 25 and 65 cm. Previous studies suggest a sedimentation rate of around 1mm/yrs, indicating a maximum extension in time of 650 yrs before present (yrs BP). Viscous remanent magnetization of different magnitudes was found during NRM desmagnetization experiments. The inclination I varies between -20° and -40º while the inclination of the geocentric axial dipole (GAD) is XX. Since the cores were not orientated, the declination D was centered around 0º. Concentration parameters allow a correlation of the cores,with more correlation tie lines between the cores located in the center of the lake. The core located on the shore was also correlated but a hiatus found around 9 cm indicates a low stand of the lake level and/or a dry environment in the catchment area. The top half of the core shows low k values unlike the bottom half which has a higher order of magnitude. The mean coercivity of the remanence (B CR ) is 30 mT with small variations around the media. High values coincide with low S ratio (IRM300mT/SIRM) suggesting (titano) magnetite asthe main carrier of the remanence but the presence of a low amount of antiferromagnetic minerals in a few samples cannot be discarded. The k vs. T curves are in agreement with these results. The Curie temperatures obtained were 296°, 506° and 590ºC. The magnetic grain sizes were estimated from the interparametric ratios ARM/SIRM and SIRM/k. Both ratios indicate the presence of magnetite with different grain sizes along the cores. Coarse magnetic grain sizes were found in samples with high concentration of magnetic mineralssuggesting cold environments with low vegetation cover. The suggested climatic and environmental changes are in agreement with local documented droughts and global climatic events such as the Little Ice Age.