Multiproxy paleosol evidence for a rain shadow effect linked to Miocene uplift of the North Patagonian Andes

Abstract

Climate changes related to orographic barrier uplift have been in the research spotlight over recent years. Several works have focused on the interactions between climatic and tectonic processes in order to understand the development of a rain shadow. Patagonia is an ideal region in which to analyze such interactions, since a Miocene climate change, from wetter to drier conditions, has mainly been associated with Andean uplift. In this work, we analyzed a succession of stacked paleosols recorded in a Miocene North Patagonian foreland basin in order to understand how the paleosol moisture regime related to the atmospheric humidity changes caused by the uplift of the Patagonian Andes. Based on macromorphological, micromorphological, and geochemical studies, and supported by a high-resolution chronostratigraphic scheme based on U-Pb geochronology and magnetostratigraphy, the paleosols were characterized with corresponding mean annual paleoprecipitation (MAP) and mean annual temperature values. Alfisol-like paleosols were identified at the base of the foreland infill (15–14.6 Ma) with a MAP of 1229 ± 108 mm/yr. The Andisol-like paleosols recognized in the middle section of the sequence (14.6–12.75 Ma) exhibited a MAP of 1053 ± 108 mm/yr, whereas the Aridisol-like paleosols occurring in the upper section of the infill (12.75–11.5 Ma) presented a MAP of 677 ± 108 mm/yr. The determined Miocene mean annual temperatures (∼11 ± 2.1 °C) were similar to the present-day values (11 °C). Based on the complete tectonic record of the Patagonian Andes, the observed decrease in MAP was assigned to the rain shadow effect created by the uplift of the North Patagonian Andes. Results indicate that although the process started around 19 Ma, the rain shadow effect was not effectively recorded before ca. 14.6 Ma.