Stable isotope record of hydrological changes in in subtropical Laguna Mar Chiquita (Argentina)over the last 230 years

PIOVANO, E., ARIZTEGUI, D., BERNASCONI, S.M, AND J. A. MCKENZIE

HOLOCENE (SEVENOAKS)

Año: 2004 p. 525 - 535

0959-6836

Abstract: Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of central Argentina, is presently the largest saline lake in South America (6000km2). The availability of historical, instrumental lake-level and salinity data for the past 100 years allows for the calibration of the isotopic archive recorded in the lake sediments. Prolonged intervals with either negative or positive hydrological balances have severely modified lakewater levels, salinity and primary productivity, and have also controlled the isotopic composition of both the authigenic carbonate (d18Ocarb and d13Ccarb) and sedimentary organic matter (d13Com). Extensive evaporation during lowstand stages results in an enrichment of 18O and 13C in the lake waters, and is recorded in the sediments as the most positive d18Ocarb and d13Ccarb compositions (0.0% and 1.9%, respectively). Conversely, more negative d18Ocarb and d13Ccarb values (1.8% and 3.8%, respectively) are the result of increasing freshwater input into the lake system. The d13Com values are related to the isotopic composition of the dissolved inorganic carbon pool and the carbonate equilibrium of the lake water. Relatively low d13Com values correspond with high lake levels, low salinity, low alkalinity and high lake productivity. High salinity during lowstands diminishes the amount of primary production and the d13Com value is correspondingly high. The calibrated isotopic model was extrapolated to reconstruct precipitation-evaporation variability from the end of the ?Little Ice Age? (c. AD 1770) to the present. Low water levels predominated until the last quarter of the twentieth century, when a positive hydrological balance without equivalent in the previous history of Laguna Mar Chiquita became dominant.