The Mid-Miocene Pollen Record of the Xunhua Basin, NE Tibetan Plateau: Implications for Global Climate Change

The northeastern Tibetan Plateau is located at the convergence of the Asian winter and summer monsoons and westerlies; thus, this area has witnessed historic climate changes.The Xunhua basin is an intermontane basin on the northeastern margin of the Tibetan Plateau.The basin contains more than 2000 m of Cenozoic fluvial–lacustrine sediments, recording a long history of climate and environmental changes.We collected the mid-Miocene sediments from the Xunhua basin and used palynological methods to discuss the relationship between aridification in the interior of Asia, global cooling, and uplift of the Tibetan Plateau.Based on the palynological analysis of the Xigou section, Xunhua basin, the palynological diagram is subdivided into three pollen zones and past vegetation and climate are reconstructed.Zone I, Ephedripites–Nitraridites–Chenopodipollis–Quercoidites（14.0–12.5 Ma）, represents mixed shrub–steppe vegetation with a dry and cold climate.In zone II, Pinaceae–Betulaepollenites–Ephedripites–Chenopodipollis–Graminidites（12.5–8.0 Ma）, the vegetation and climate conditions improved, even though the vegetation was still dominated by shrub–steppe taxa.Zone III, Ephedripites–Nitrariadites–Chenopodipollis（8.0–5.0 Ma）, represents desert steppe vegetation with drier and colder climate.The palynological records suggest that shrub–steppe dominated the whole Xigou section and the content gradually increased, implying a protracted aridification process, although there was an obvious climate improvement during 12.5–8.0 Ma.The aridification in the Xunhua basin and surrounding mountains during 14.0–12.5 Ma was probably related to global cooling induced by the rapid expansion of the East Antarctic ice-sheets and the relatively higher evaporation rate.During the 12.5–8.0 Ma period, although topographic changes（uplift of Jishi Shan） decreased precipitation and strengthened aridification in the Xunhua basin on leeward slopes, the improved vegetation and climate conditions were probably controlled by the decrease in evaporation rates as a result of continuous cooling.From 8.0 to 5.0 Ma, the rapid development of the desert steppe can be attributed to global cooling and uplift of the Tibetan Plateau.

Cenozoic tectonic and sedimentary evolution of southern Qaidam Basin, NE Tibetan Plateau and its implication for the rejuvenation of Eastern Kunlun Mountains

The Eastern Kunlun Mountains play an important role in the growth and eastward extrusion of the Tibetan Plateau. Tectonic and sedimentary study of the Cenozoic Qaidam Basin, especially the southern part, provides key evidence for understanding their evolution. Here we present evidence including isopach maps, seismic sections and sedimentary analysis of single well to illustrate the sedimentary development of the basin and the structural features of its southern margin. The Qaidam Basin extended across Qiman Tagh-Eastern Kunlun Mountains in the early Cenozoic and withdrew northward at ca. 35.5 Ma, and then buckled as an EW striking elliptical depression since ca. 14.9 Ma, with the main depocenter migrating eastward. Our results support the view that the Kumukol and Hoh Xil basins joined the Qaidam Basin in the early Cenozoic time and we propose the Eastern Kunlun Mountains uplifted in the mid-Miocene.

Lake geochemistry reveals marked environmental change in Southwest China during the Mid Miocene Climatic Optimum

The Mid-Miocene Climatic Optimum （MMCO; 15-17 Ma） was one of the short-term climatic warm events that punctuated the Cenozoic long-term cooling trend. Because there are very few terrestrial records of this event, most of our understanding comes from marine cores. In this report, we first present new palaeomagnetic data that revises the dating of our 400 m-thick lacustrine section in Wenshan （Yunnan）, previously thought to be Late Mio- cene. These new data suggest an older age, ca. 15.2-16.5 Ma, coinciding with the MMCO. We measured δ13C on bulk organic matter （3 Corg）, total organic carbon （TOC）, total nitrogen （TN） and C/N ratios at a high sample resolution to： （1） reconstruct the palaeoenvironmental changes in the lake catchment area, and （2） infer mechanisms responsible for these changes. Our results show that all four geochemical parameters demonstrate that a strong environmental change occurred around the middle of the section, shortly after the C5Cn/C5Br geomagnetic reversal and the Early/Middle Miocene boundary at 15.97 Ma. We propose that the environmental shift may be due to a combination of a change in climate, which became cooler, together with a change in organic matter cycling within the lake. This study provides a new insight into the MMCO and demonstrates that although the MMCO was generally a warm event, it was also a time of climatic instability and abrupt environmental changes.