Recent environmental changes in Laguna Mar Chiquita (central Argentina): a sedimentary model for a highly variable saline lake

摘要

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 (≈6000 km~2). Recent variations in its hydrological budget have produced dry and wet intervals that resulted in distinctive lake level fluctuations. Results of a multiproxy study of a set of sedimentary cores indicate that the system has clearly recorded these hydrological variations from the end of the Little Ice Age (≈ AD 1770) to the present. Sedimentological and geochemical data combined with a robust chronology based on ~(210)Pb profiles and historical data provide the framework for a sedimentary model of a lacustrine basin with highly variable water depth and salinity. Lake level drops and concurrent increases in salinity promoted the development of gypsum-calcite-halite layers and a marked decrease in primary productivity. The deposits of these dry stages are evaporite-bearing sediments with a low organic matter content. Conversely, highstands are recorded as diatomaceous organic matter-rich muds. Average bulk sediment accumulation rose from 0.22 g cm~(-2) year~(-1) in lowstands to 0.32 g cm~(-2) year~(-1) during highstands. These results show that Laguna Mar Chiquita is a good sensor of high- and low-frequency changes in the recent hydrological budget and, therefore, document climatic changes at middle latitudes in south-eastern South America. Dry conditions were mostly dominant until the last quarter of the twentieth century, when a humid interval without precedent during the last 240 years of the lake's recorded history started. Thus, it is an ideal system to model sedimentary and geochemical response to environmental changes in a saline lacustrine basin.