Preliminary Results from Paleoclimatic Studies on Lake Sediments from Laguna La Brava (Argentina)

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

Bojnice Castle, Slovak Republic

Congreso; 11th Castle Meeting “New Trends in Geomagnetism Paleo, Rock and Environmental Magnetism”; 2008

Geophysical Institute of the Slovak Academy of Science

 The only way to explore climate system dynamics beyond instrumental records is to study natural archives such as lacustrine (Zolitschka, 2003) and marine sediments (von Dobeneck and Schmieder, 1998), ice, trees and corals. Lake sediments are widely used as high-resolution of past environmental and climatic conditions on local, regional and global scales. Magnetic methods have the advantage of being rapid, simple and non-destructive. Material setting on a lake bed may have originated from the atmosphere, the surrounding catchment or the lake itself, and samples of accumulated sediment may be examined in a variety of ways to reconstruct past environmental conditions (Maher and Thompson, 1999). Six cores (labelled Br1 to Br5 and LB, respectively) were collected in water depths of about 5m using a piston-corer installed on a raft with a central hole, recovering six sediment columns of 6 cm diameter and up to 7m thick in Laguna La Brava (37°52´S 57°59´, Argentina). In the cores there are several tephra layers, possibly from volcanic eruption at the Andes. The following measurements were performed: magnetic susceptibility at low frequency (specific, X and volumetric, k) and high frequency; intensity of the natural remanent magnetisation (NRM), isothermal remanent magnetisation (IRM) in increasing steps up to 1.2 T, reaching the SIRM; back-field, in growing steps until cancelling the magnetic remanence; anhysteric remanent magnetisation (ARM100mT), with a direct field of 0.1mT and a peak alternating field of 100mT and anhysteric susceptibility (kanh). Associated parameters also were calculated: S (IRM−300mT/SIRM), %soft IRM (IRM−20mT/SIRM), %hard IRM (IRM−200mT/SIRM), coercivity remanence (BCR) and frequency-dependence magnetic susceptibility (F). The results from the studies of the previous parameters and the organic matter content suggest periodic changes from cooler to moister climatic conditions. The increasing trend in susceptibility record could be related to changes in sediment delivery and depositional process caused by human impact, particularly deforestation, in the lake catchment. Radiocarbon age determinations were made on samples of organic-rich clay from Br4. Results were calibrated by the CALIB Version 3.0 program (Stuiver and Reimer, 1993). A linear regression to the calibrated ages indicates that sediment accumulation rate averaged 1.4 mm/yr and a temporal extent of about 4200 years. Key words: lake sediments, Laguna La Brava, paleoclimate