A high-resolution palaeoclimate record for the last 4800 cal. years BP on lake la brava se pampas plains, Argentina

IRURZUN MA, GOGORZA CSG, SINITO, AM, CHAPARRO MAE, PRIETO AR, LAPRIDA C, LIRIO JM, NAVAS AM, NUÑEZ, H.

GEOFíSICA INTERNACIONAL

INST GEOPHYSICS UNAM

Año: 2013

0016-7169

The frequencies of rainfall and drought periods, temperature changes, abundance of vegetation, etc. are reflected in the variations of magnetic parameters. Sequences of lake sediments are good sources of information because they provide continuous and detailed records of paleoclimatic changes. This study was carried out on bottom cores collected from Lake La Brava, Argentina (37º52?S 57º59?W), an elongated lake of about 4.5 km2 area. The following measurements were performed: magnetic susceptibility (specific χ, and volumetric, k); intensity of the natural remanent magnetisation (NRM), isothermal remanent magnetisation (IRM) in increasing steps up to 1.2T, reaching the saturation (SIRM); back-field, in growing steps until cancelling the magnetic remanence; anhysteretic remanent magnetisation (ARM95mT), with a direct field of 0.1, 0.5 and 0.9 mT and a peak alternating field of 95 mT. Associated parameters also were calculated: S ratio, %soft IRM, coercivity remanent (BCR) and anhysteretic susceptibility (kanh). Total organic and inorganic carbon (TOC and TIC) and total sulphur (TS) were analysed. Alkaline elements, light and heavy metals were extracted after total acid digestion. Changes in vegetation communities were determined based on palynological studies. Relationships between pollen, geochemical and magnetic parameters were analysed. Five radiocarbon age determinations were made on samples of organic-rich clay and calibrated ages were calculated. A linear regression to the calibrated ages indicates that sediment accumulation rate averaged 1.22 mm/yr and a temporal extent of about 4700 calendar years. The results from this work and previous studies suggest periodic changes from cooler conditions to climate improvement, changes in sediment contribution and depositional processes possibly caused by human impact, particularly use of natural resources. Some floods and lower lake level events were identified. Keywords: Paleoclimatic changes, rock magnetism, sediment cores, South America, Lake La Brava.