Miocene evolution of the environment and topography on the Tibetan Plateau and ocean ventilation

Abstract

Knowledge of pre-Quaternary warm climates is highly important for understanding global changes that we will face in the warming future. The Miocene epoch, ~23-5 million years ago (Ma), is one of the substantially warm periods in Earth’s climate history that can be a strong candidate as a possible future climate analog. However, important issues including the environmental and topographic evolution on the Tibetan Plateau and ventilation changes in the ocean remain poorly resolved. This thesis presents multi-biomarker proxy records from the Qaidam Basin, Lunpola Basin, and South China Sea to address these outstanding issues. Firstly, in the Qaidam Basin on the northern Tibetan Plateau, β-carotane/n-C36, gammacerane index, terrigenous/aquatic ratio and long-chain saturated ketone content, altogether indicate lower lake level at the intervals of ~18-17 Ma, 13.5-11.5 Ma, 10.5-8 Ma and 7-6 Ma, and higher level at the intervals of ~17-13.5 Ma, 11.5-10.5 Ma, 8-7 Ma and 6-5 Ma, superimposed on the long-term shrinking trend. Comparison with existing regional and global temperature records suggests that the lake evolution was largely associated with global climatic conditions, i.e., shrinking under relatively cool conditions and vice versa, for both its long-term evolution and secondary fluctuations. Such global climate forcing can also be observed in detailed water-depth reconstructions in the Lunpola Basin on the central Plateau during the middle Miocene climate optimum. It thus appears that global climatic conditions had controlled the lake status during the period, whereas tectonic activities might have also contributed to its long-term gradual demise. Further, biomarker Group III alkenones and anomalous carbonate carbon isotope have been identified from the Honggou section in the Qaidam Basin. These results, together with existing records, consistently show the occurrence of marine indicators across the basin since the mid-Miocene, but not at earlier stages. It appears to be plausible that seawaters occasionally intruded into the basin in the mid-Miocene, indicative of a lowland basin floor close to the sea-level height. Meanwhile, the hydrogen isotope of long-chain n-alkanes shows prominent negative excursion afterward, possibly linked to the onset of tectonic uplift. Incorporating our findings and paleoelevation reconstructions in the southern-central plateau allows us to suggest a stepwise development of the Tibetan Plateau, with delayed surface growth of the Qaidam Basin on the northern plateau for dozens of million years. Lastly, glycerol dialkyl glycerol tetraether- and alkenone-based indices were used to reconstruct the ocean ventilation in the South China Sea. Together with existing records, we show contrasting ventilation varieties in global oceans across the middle Miocene climate transition, with strengthened ventilation in low latitudes and weakened ventilation in middle to high latitudes. Due to the close correlation with global climate, we infer that the improvement of ventilation in low latitudes might have resulted from a lower thermal gradient during the global cooling period, whereas reduced ventilation in mid- to high latitudes appears to be linked to a more sluggish meridional overturning circulation. Our results also suggest that the warm climate might be more favorable for overturning circulation enhancement at this time scale.