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Cosmogenic nuclide exposure dating of ancient shoreline terraces of the Quillagua-Llamara Soledad Lake in the central Atacama Desert of northern Chile provides new insights in the paleohydrology of the driest desert on Earth. The lake developed in a paleo-endorheic drainage system in the Central Depression prior to draining into the Pacific due to incision of the Río Loa canyon. The durations of lake stages were sufficiently long to form wave-erosion induced shoreline terraces on the wind-exposed slopes of former islands. Successively younger shoreline levels are preserved over an elevation range of 250 m due to progressive uplift of the islands coeval with the lake stages. Cosmogenic 10 Be- and 21 Ne-derived exposure ages of the shorelines reveals that the hyperarid conditions in the Río Loa catchment were interspersed by several pluvial stages during the Pliocene and Pleistocene, which generated a large and persistent lake in the Quillagua-Llamara basin. The exposure ages of the final lake stage provide the maximum age for the incision of the Río Loa canyon (274 ± 74 ka) and the subsequent breaching of the Coastal Cordillera. [ABSTRACT FROM AUTHOR]

Unconventional hydrocarbon resources such as shale oil/gas and coal-bed methane have become an increasingly important source of energy over the past decade. The Vaca Muerta Shale (Neuquén Basin, Argentina) contains the second largest technically recoverable quantity of shale gas in the world. Exploitation of the play has been complicated by elevated concentrations of CO 2 in several fields, the origin of which is currently poorly understood. Elevated CO 2 levels are consistently encountered when deep-rooted faults in the Auquilco Evaporite Formation, present below the Vaca Muerta Shale, overlap with shallower faults that propagate from the top of evaporites into the shale, indicating a sub-evaporate origin of the CO 2. Here we report new isotopic analysis of CO 2 -rich gases from two producing fields. CO 2 concentrations increase with C 1 /(C 2 + C 3) values (4.8–33.5) and fractionation of δ13C CO2 (−0.9 to −7.7‰), suggest that CH 4 have been displaced by CO 2 which entered the shale after hydrocarbon maturation. The noble gas composition (3He/4He of 3.43–3.95 R A , where R A is the atmospheric ratio of 1.399 × 10−6, 21Ne/22Ne of 0.0310–0.0455, 20Ne/22Ne of 9.89–10.52, 40Ar/36Ar of 2432–3674 and CO 2 /3He 6.8–20.2 × 107) of the gases is consistent with mixing of magmatic CO 2 with crustal hydrocarbon-rich gases and provides evidence for the loss of significant CO 2. Using inverse modelling techniques, we determine that the magmatic gas has a 3He/4He of 3.95–4.08 R A , CO 2 /3He of 8.8–16 × 108 and 20Ne/22Ne of 12.13 −0.10 +0.08, 21Ne/22Ne of 0.074 −0.003 +0.004. Based on the radiogenic He and Ne this is consistent with a depleted asthenosphere mantle source, which has been trapped in the crust since 6.0–22.8 Ma. This is significantly younger than Late Cretaceous maturation of the hydrocarbon source rocks. Mantle melting as a result of asthenosphere upwelling induced by the collision of the South Chile Ridge and the Chile Trench at ~14 Ma is the most likely source of the CO 2. Gases from below the shale contain two air saturated water-derived noble gas components, distinguished on the basis of 20Ne†/36Ar, 84Kr/36Ar, 132Xe/36Ar ratios. These are consistent with early and late stage open system Rayleigh fractionation of groundwater-derived noble gases. We find evidence that these mix with the magmatic component prior to entering the Vaca Muerta and mixing with an adsorption derived gas retained in the source kerogen. • Elevated CO 2 levels are consistently encountered when deep-rooted faults overlap with shallower faults in the basin. • Noble gases highlight a magmatic origin for CO 2 encountered in the Vaca Muerta Shale. • Noble gas isotopes trace source and evolution of gas composition in the Vaca Muerta Shale. [ABSTRACT FROM AUTHOR]