Late Holocene hydrology inferred from lacustrine sediments of Laguna Cháltel (southeastern Argentina)

Abstract

Hydrological changes that occurred during the last 4700 years have been reconstructed using multi-proxy analyses of sediment cores from the volcanic crater lake of Laguna Cháltel (50°S, 71°W). The chronology is based on AMS 14C age modeling constrained by paleomagnetic secular variations. Chemical and physical properties of the lake water together with results of lake-water surface-temperature monitoring, as well as sediment characteristics reveal conspicuous features archived as different sedimentary carbonate phases and morphologies which are attributed to lake-level changes. Sedimentological, geochemical and biological proxies together suggest the development from an initial playa lake-phase towards a system with progressively rising lake level. In detail, proxies indicate the existence of an ephemeral lake since 4700 cal BP until a glauberite-bearing carbonate crust formed around 4040 cal BP which probably is associated to the globally recognized 4.2 ka event. This crust is interpreted as a desiccation event terminating the ephemeral lake phase. Following this desiccation a shift towards conditions with a positive hydrological balance of Laguna Cháltel occurs, which leads to the development of a saline lake with ooid formation between 4040 and 3200 cal BP. Further lake-level increase with initially high minerogenic input until 2700 cal BP resulted in a lake freshening which allowed the preservation of diatoms. Sigmoidal and star shaped carbonate crystals occurred until 1720 cal BP indicating a syn- or post-depositional formation of ikaite. Anoxic conditions and increased deposition of clay and sand through fluvial and eolian input are interpreted as a further lake-level rise and/or a prolonged winter ice cover culminating during the Little Ice Age. The highest lake level was probably reached at that time and since then dropped to its present day height. Previous studies have shown that the southern hemisphere westerly winds (SWW) exert an oppositional control on hydrological regimes at the eastern and the western sides of the Patagonian Andes. At Laguna Cháltel SWW forcing is changing evaporation rates by varying wind intensities, air temperatures and lake ice coverages as well as by precipitation rates (easterly vs. westerly sources of moisture). Our data suggests that the lake-level history of Laguna Cháltel reflects changes in the SWW during the last 4.7 ka on the eastern side of the Andes. However, the elevated location of Laguna Cháltel on an 800 m high plateau at the leeward side of the Andes potentially leads to a local overprint of the SWW influence on the hydrological balance.