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During the Late Miocene, the climate patterns and ecosystems of continental land masses experienced crucial transitions, but whether the principal driver was regional tectonic forcing or a decline in CO2 concentrations remains debated. Here we present a terrestrial paleotemperature record spanning similar to 12.7-5.2 Ma based on tetraether lipids extracted from the northeastern Tibetan Plateau. Our results reveal a sharp cooling (similar to 8 degrees C) during similar to 10.5-8 Ma, asynchronous with minor fluctuations in global sea temperatures, suggesting a rapid tectonic uplift of similar to 1 km in extent. This event appears consistent with the simultaneous aridification and transitions of ecosystems experienced in adjacent regions. Moreover, the amplitude of the cooling over land is less than that which occurred over the ocean during the CO2-dominated Late Miocene cooling event (similar to 7-5.4 Ma). We therefore concluded that tectonic forcing, rather than a decline in CO2 levels, most likely dominated continental climate patterns and ecosystem transitions during the Late Miocene.

Plain Language Summary Animals and plants in many regions had to survive a gradual drying and cooling environment during the Late Miocene epoch (ca. 12-5 Ma). Exploring the drivers and mechanisms behind Late Miocene climate change is helpful when considering current global warming trends. Although the exact reason for these Late Miocene changes remains unclear, two suspects have been identified, that is, a decline in CO2 levels and/or regional tectonic activity. In this study we used microbial cell membrane lipid fossils to reconstruct the quantitative paleotemperatures and paleoaltitudes of the Xining Basin, on the northeastern Tibetan Plateau, during the Late Miocene. Our results revealed a surficial uplift event (similar to 10.5-8 Ma) delineated by the identification of a rapid cooling process during a period that experienced constant sea surface temperatures, leading to an increasingly dry climate and ecosystem changes over large areas. Moreover, the amplitude of this cooling over land appears to have been less than that observed over the ocean during the CO2-dominated Late Miocene cooling event (similar to 7-5.4 Ma). Therefore, we would posit that regional tectonic activities played a more important role than CO2 levels in the changes to terrestrial climate patterns and ecosystems during the Late Miocene.