Mid-Miocene climatic optimum: Clay mineral evidence from the red clay succession, Longzhong Basin, Northern China

The Mid-Miocene Climatic Optimum (MMCO) is one of Earth's most recent prolonged warming events, recorded in both the deep ocean and mid- to high-latitude continents. Thick Cenozoic deposits in the Longzhong Basin, Northwest China, provide great potential for characterizing the MMCO, but have not been well documented. In this study, we carry out clay mineralogical and geochemical analyses of the Miocene red clay succession from the eastern Longzhong Basin, employing X-ray diffraction and geochemistry, and discuss the climatic characteristics of the MMCO. The results reveal that the MMCO occurred at 16–14 Ma and is associated with relatively high kaolinite and smectite contents, high illite crystallinity values and kaolinite/(illite + chlorite) ratio, and lower chlorite and illite contents. These clay mineral assemblages confirm a warm and moist climate during the MMCO. Furthermore, the SiO₂/Al₂O₃ ratio declined with increasing Al₂O₃, indicative of greater precipitation and intensified weathering during this period. When compared with local and global records, we find that our clay mineral records reflect regional climatic change superimposed upon global climatic trends during the MMCO. It is suggested that a high concentration of CO₂, and associated global warming, may have been responsible for the MMCO, rather than the uplift of the Tibetan Plateau.