Magnetic characterization of sediments from La Brava Lake (Buenos Aires Province, Argentina)

ANA M. SINITO; MARÍA A. IRURZUN; CLAUDIA S.G. GOGORZA; MARCOS A. E. CHAPARRO; JUAN M. LIRIO; HECTOR NUÑEZ

Foz do Iguaçu (Brazil)

Congreso; Joint Assembly 2010 The Meeting of the Americas, American Geophysical Union (AGU); 2010

American Geophysical Union

This study was carried out on bottom cores collected from Lake La Brava, Argentina (37º52'S 57º59'W). This elongated and shallow lake, of tectonic origin, has a surface area of about 4.5 km2. Four long cores were recovered in water depths of about 3.4 m using a push corer. The sedimentological description shows the presence of silt, clay and tephra layers. In order to characterize the sediments, a series of rock magnetic studies were conducted. The following measurements were carried out: magnetic susceptibility (mass-specific X, and volumetric, k); intensity of the natural remanent magnetization (NRM); isothermal remanent magnetization (IRM) in increasing steps up to 1.2 T, reaching the saturation (SIRM); back-field, in growing steps until canceling the magnetic remanence; anhysteretic remanent magnetization (ARM100mT), with a direct field of 0.1, 0.5 and 0.9 mT and a peak alternating field of 100 mT. Associated parameters were also calculated: S-ratio (IRM-300mT/SIRM), %soft IRM ((SIRM-IRM-40mT)*100/SIRM), coercivity of remanence (BCR), anhysteretic susceptibility (kARM) and the inter-parametric ratios: ARM100mT/k, ARM100mT/SIRM, kARM/k, kARM/SIRM and SIRM/k. Separation of magnetic phases using an experimental method (IRM-AF demagnetization) and thermal demagnetization of SIRM was carried out. The X and MRN profiles show lower values to the bottom and an increasing trend to the top. These parameters were used in order to correlate the different cores. The results of rock magnetic studies indicated pseudo single-domain (PSD) to elongated single-domain (ESD) (titano-) magnetite as the dominant magnetic carrier of remanence, with possible low proportion of antiferrimagnetic minerals (like hematite). The bilogarithmic plot of k vs. SIRM suggested that the majority of the magnetite is present as grains of about 4-8 um in size. The overall size ranges from 2 to ~ 8 um with occasional grains up to ~ 16 um, and the concentration of magnetite-like minerals is between 0.001 and 0.03%. The inter-parametric ratios showed an oscillation around a mean value superimposed to an increasing trend to the bottom and a decreasing trend to the top, which indicates increasing grain size to the top. The cores collected near tributaries and coastal reeds, showed higher magnetic concentration than those cores located in the central area of the lake. This behavior can be explained because of the presence of black silt  in the bottom sediment of most of the tectonic lakes of Buenos Aires Province, which are not totally spread out in this lake and it is only present in that areas