Hydrochemistry, isotope studies and salt formation in saline lakes of arid regions: Extra-Andean Patagonia, Argentina.

The most favourable locations for the development of saline lakes are in the rain-shadow of mountain ranges, which provide large areas of precipitation catchment while the base of the basin is under arid climate and exposed to evaporation. These conditions are found in Extra-Andean Patagonia under the rain-shadow generated by the Andean cordillera. There, an endorheic basin with two shallow and saline lakes, Cari Laufquen Chica (CLC) and Cari Laufquen Grande (CLG), was studied with the aim of analysing the factors that condition the hydrochemical processes acting in the formation of evaporites associated with these environments. A monitoring network was installed and five surface and groundwater survey campaigns were carried out at different points in the basin to define groundwater flow and also to extract samples. In situ pH, electrical conductivity and temperature and laboratory physicochemical determinations of major ions and stable isotopes of the water were measured. SEM-EDS and XRD analyses of saline crusts from the edge of the lakes and adjacent sediments were carried out. The obtained results allowed identifying that CLC and CLG saline lakes have different surface water-groundwater configurations. CLC is a flow-through shallow lake, while CLG is a discharge shallow lake. The analysis of the saline precipitates and the chemical facies of the water allows identifying cycles of dissolution, evaporation and precipitation in both saline lakes. However, the different groundwater flow between the two lakes is reflected in their salinity as well as in the evolution and development of saline precipitates. CLC saline lake shows a dissolution-precipitation trend of thenardite and Ca-Mg carbonates, whereas CLG saline lake displays a trend of dissolution-precipitation of halite, thenardite and trona with strong dominance of halite. The present study identifies for the first time the main factors conditioning hydrochemical processes in these saline lakes of extra-Andean Patagonia. Our results indicate that the hydrological configuration with respect to the groundwater flow is the prevailing factor setting the hydrochemical processes that trigger the formation of salt crusts.

Wet tropical climate in SE Tibet during the Late Eocene.

Cenozoic climate cooling at the advent of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago, was stamped in the ocean by a series of climatic events albeit the impact of this global climatic transition on terrestrial environments is still fragmentary. Yet archival constraints on Late Eocene atmospheric circulation are scarce in (tropical) monsoonal Asia, and the paucity of terrestrial records hampers a meaningful comparison of the long-term climatic trends between oceanic and continental realms. Here we report new sedimentological data from the Jianchuan basin (SE Tibet) arguing for wetter climatic conditions in monsoonal Asia at ~35.5 Ma almost coevally to the aridification recognized northwards in the Xining basin. We show that the occurrence of flash-flood events in semi-arid to sub-humid palustrine-sublacustrine settings preceded the development of coal-bearing deposits in swampy-like environments, thus paving the way to a more humid climate in SE Tibet ahead from the EOT. We suggest that this moisture redistribution possibly reflects more northern and intensified ITCZ-induced tropical rainfall in monsoonal Asia around 35.5 Ma, in accordance with recent sea-surface temperature reconstructions from equatorial oceanic records. Our findings thus highlight an important period of climatic upheaval in terrestrial Asian environments ~2-4 millions years prior to the EOT.