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

C. GOGORZA; A. IRURZUN; E. TERAN ; M. CORDOBA; M. SERRANO; QUIMEY GOMEZ; A. FARAMIÑAN; E. ADARO; B. ALBA; L. CHIAVARINO; S. DELGADO

Puerto Vallarta

Congreso; Reunión Anual 2016 de la Unión Geofísica Mexicana; 2016

A set of sediment cores from Laguna La Barrancosa is studied. The lake is located in the pampean region, Pcia. Buenos Aires, Argentina. The cores were extracted from several sites in the lake from the shore until the center during 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 cores 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 magnitude were 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 in the shore were also correlated but a hiatus found around 9 cm, indicate a low stand of the lake level and/or a dry environment in the catchment area. The top half of the core show low k values unlike the bottom half which has one order of magnitude higher values. The mean coercivity of the remanence (BCR) is 30 mT with a small variations around the media. High values are coincident with low Sratio (-IRM-300mT/SIRM) suggesting (titano-)magnetite as the main carrier of the remanence but the presence of low amount of antiferromagnetic minerals in a few samples cannot be discarded. The k vs. T curves are in agreement with this results. The Curie temperatures obtained were 296, 506 and 590ºC. The magnetic grain size were estimated from the inter-parametric ratios ARM/SIRM and SIRM/k. Both ratios indicate the presence of magnetite with different grain sizes along the cores. Coarse magnetic grain size were found in samples with high concentratión of magnetic parameters suggesting cold environments with low vegetation cover. The suggested climatic and environmental changes are in agreement with local documented droughts and global climatic events as the Little Ice Age.