STEPWISE ONSET OF THE ICEHOUSE WORLD AND ITS IMPACT ON OLIGO- MIOCENE CENTRAL ASIAN MAMMALS

Central Asia is a key area to study the impact of Cenozoic climate cooling on continental ecosystems. One of the best places for that is the Valley of Lakes in Mongolia with its outstandingly fossil-rich Oligocene and Miocene terrestrial sediments. In an integrated study we gained sedimentological, geochemical, geophysical and paleontological data to reconstruct the early stage of Earth’s icehouse climate in Central Asia.

The mostly siliciclastic sediments with several levels of paleosols were studied for sedimentology, mineralogy, major and trace element composition, stable δ¹³C and δ¹⁸O isotopes and magnetic susceptibility. The results show that hydrothermal fluids related to the middle Miocene flood basalt flow induced a strong illitization of the fluviale and lacustrine sediments. This contradicts the current conceptual view that the fine fraction of the sediments is of aeolian origin and that the early Oligocene fine sediments are the oldest Loess sediments in Central Asia. The diagenetic processes strongly overprinted the sediments and disturbed any paleoclimatic information. Only the stable isotope values of authigenic carbonate in calcrete horizons still record the ambient climatic conditions during paleosol formation. Our novel δ¹³C and δ¹⁸O record suggests an early Oligocene aridification at 31 Ma, whereas the mid-Oligocene Glacial Maximum shows constant δ¹³C and no increase in aridification. A second, regional-scale aridification occurs at 25 Ma and corresponds to the Late Oligocene Extinction.

Based on statistical analyses of occurrence and abundance data of 18,608 specimens representing 175 mammal species we link shifts in diversities with major climatic variations. Our data document for the first time that the post-Eocene aridification of Central Asia happened in several steps, was interrupted by short episodes of increased precipitation, and was not a gradual process. We show that the timing of the major turnovers in Oligocene mammal communities is tightly linked with global climate events rather than slow tectonics processes. The most severe decline of up to 48% of total diversity is related to aridification during the maximum of the Late Oligocene Warming at 25 Ma. Its magnitude was distinctly larger than the community turnover linked to the mid-Oligocene Glacial Maximum.