Salt Profile in Sedimentary Deposits: an archive of past climate and tectonics

In hyper-arid environments, paleoclimatic and paleoenvironmental studies typically evaluate paleosol-based proxies such as morphology, variation of remnant vegetations, and geochemical and isotopic signatures of minerals to reveal the plaeoclimatic changes. Limited works utilized geochemical/isotopic approaches in sedimentary deposits which barely experience pedogenic processes. The objectives of this work are to evaluate the isotopic systematics and total chemistry of salts as function of depth within unsolidated sediments in an effort to reconstruct paleoclimatic conditions of Atacama Desert. Results of this work will demonstrate techniques that can also be applied in other hyper-arid environments (e.g., Antarctica, Mars). We report a study on a ~75 meter thick core from alluvial sedimentary deposits (Atacama Gravels) in the eastern margin of the central depression of Atacama desert, North Chile. This work combines mineralogical/geochemical/isotopic approaches to decipher the depositional conditions and the climatic conditions during and between sedimentation events. Our main focus is on the soluble sulfate, nitrate, chloride profiles through the sediments. These salt deposits likely originated from seasalts, atmospherically produced salts and/or salts from nearby evaporite deposits. The sources of salts are likely affected by offshore oceanic currents that may vary the seasalt input, or by regional tectonic or orogenic changes that may vary the terrestrial sources and their ratio with seasalt input. The outflux of the salts in these deposits are mainly due to groundwater drainage, and microbial facilitated salt consumption (e.g. sulfate reduction, denitrification), both of which are indicative of paleoclimatic conditions. Thus, the salt profile within the sediments records regional paleo-climate, global oceanic current setting, and regional tectonic activity. Morphological and mineralogical studies show that the target "gravel" consists of two main suites of sediments with a top layer (~35 meters) of alluvial deposits with no sign of standing water, and a bottom layer (~40 meters) of possible fluvial deposits. Salt concentration profiles indicate that interaction between salt inventories in these two suites were negligible. The multiple isotope compositions (D17O, δ34S and δ18O) of sulfates show distinct depth dependent pattern with all three parameters within top layer close to those of modern surface materials, and with decreasing D17O , δ34S, and increasing δ18O within bottom layer. Furthermore, with age constraint by dating a volcanic ash layer imbedded in a top layer, the isotope data indicate that the current level of hyper-aridity of the eastern margin of the central depression has been present since at least 10Ma. Our study and future modeling work may provide an analog for the study of martian environments, e.g. the Aeolis Mons in the center of Gale Crater, where salt enriched alluvial or aeolian deposits are likely to have been deposited under hyper-arid conditions.