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

Abstract

Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of cetral Argentina, is presently the largest saline lake in South America ("-6000km2). The availability of hitorical, 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 hydrlogical balances have severely modified lakewater levels, salinity and primary productivity, and have also controlled the isotopic composition of both the authigenic carbonate (&8Ooarb and f133Ccarb) and sedimetary organic matter (6"C0,,). Extensive evaporation during lowstand stages results in an enrichment of 180 and 13C in the lake waters, and is recorded in the sediments as the most positive &8O8arb and 613C carb compositions (0.0%o and-1.9Yoo, respectively). Conversely, more negative 6f'8Ocarb and 813Ccarb values (-1.8%o and-3.8Yoo, respectively) are the result of increasing freshwater input into the lake system. The fi3Com values are related to the isotopic composition of the dissolved inorganic carbon pool and the cabonate equilibrium of the lake water. Relatively low 613COm 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 613Com 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.