Paleoenvironmental records from newly recovered sediment cores at the southeast margin of the Salar de Atacama, Chile

Abstract:

A suite of new cores recovered from recent boreholes in the southeastern margin of the Salar de Atacama, Chile span a modern environmental gradient from distal alluvial fans, groundwater discharge marshes, sulfate-rich playas, saline lagoons, and the halite nucleus of the salar. These same environments are preserved as stratigraphic records of environmental change in the cores. Cores from the salar nucleus are dominated by halite, and similarly alluvial cores provide a poor paleoenvironmental record. However, the cores from the transition zone between the salar margin and the halite nucleus document alluvial, lagoonal, and evaporite environments. Cores near the halite nucleus record inter-bedded carbonate, gypsum, and halite. Finely laminated carbonates inter-bedded with cm-thick halite beds are a target for U-series geochronology. Cores near modern lagoons contain 2–6 m thick diatomites in addition to microbially-mediated carbonate, organic-rich mud, and minor alluvium. The uppermost 20 cm of diatomite deposits are commonly rooted with vascular plant material which is being processed for ¹⁴C geochronology. Ignimbrite and tephra deposits are also encountered and will provide important chronological control. The presence and absence of the 3.5–4.0 Ma Tucucaro ignimbrite in various cores documents a complex pattern of subsidence near the salar margin, some areas have accumulated little sediment since its deposition while in other areas the cores likely record only late Pleistocene deposition. Preliminary interpretations of the stratigraphic records within a paleohydrologic context are tenable. The specific control on this paleohydrologic record is likely to be a combination of increased inflow due to wetter climates and migration of the freshwater/brine interface which underlies the margins of the Salar de Atacama. Stratigraphic variations in the lithium content of evaporite minerals is being explored as a potential indicator of water balance. Lithium concentrations are ~ 40 ug/g throughout 60 m of halite stratigraphy, suggesting minor hydrochemical fluctuation in the halite hosted brine. Hydrologic changes in the transition zone at the salar margin may record significant variation in lithium content.