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

Abstract

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.

Paleodietary and Paleoclimatic Reconstruction of Hipparion Fauna at ∼9 Ma From the Xunhua Basin on the Northeastern Tibetan Plateau

“…and Gazella dorcadoides had high-crown teeth and were representative of grassland taxa (Zhang et al, 2002). Moreover, palynological data reveal that the Xunhua Basin contained mainly shrubs and herbs (e.g., Chenopodiaceae, Ephedra and Gramineae), broadleaved trees, and conifers at 12.5-8 Ma (Xu et al, 2015). Combined with enamel-δ 13 C data, the presence of the Hipparion fauna in the Xunhua Basin at ∼9 Ma is indicative of a mixed forest-grassland setting.…”

“…Palynological studies in the Jiuxi Basin, Qaidam Basin and Guyuan area indicate a long-lasting cooling trend beginning at ∼13-12 Ma (Ma et al, 2005;Jiang and Ding, 2008;Miao et al, 2011). Sporopollen and geochemical records of paleoclimate in the Xunhua Basin demonstrate a gradual cooling and drying trend from ∼14 Ma to 5 Ma (Xu et al, 2015). However, differences in calculated δ 18 O w between the Xunhua and Linxia basins, despite their proximity (Figure 6), suggest that global cooling was not the sole, or even the dominant, control on climate change on the northeastern Tibetan Plateau at ∼9 Ma.…”

“…The primary factors controlling Late Miocene aridification in the Xunhua and Linxia basins were global cooling (Miao et al, 2011) and tectonic uplift of the Tibetan Plateau (Zhisheng et al, 2001;Zhang et al, 2012b;Yang et al, 2021). Many studies have inferred that increasing aridification of the northeastern Tibetan Plateau after 14 Ma was the result of global climatic cooling (Zachos et al, 2001;Ma et al, 2005;Jiang and Ding, 2008;Miao et al, 2011;Xu et al, 2015). Global cooling is likely to have reduced the amount of water vapor in the atmosphere, weakening precession-related moisture cycles (Zachos et al, 2001).…”

“…The Xunhua Basin is located on the northeastern Tibetan Plateau in a climatically sensitive area, representing the transitional zone between the monsoonal coastal region to the east and the arid continental interior to the west. Much research has been conducted on climate change in the Xunhua Basin using stable isotopes (Hough et al, 2011), magnetostratigraphy (Ji et al, 2010), palynological data (Xu et al, 2015), ostracod data (Luo et al, 2010), clay mineralogy (Wang et al, 2010), and chemical weathering indices (Ye et al, 2010). Palynological studies in the Xunhua Basin indicate a long-lasting cooling trend beginning at ∼14 Ma.…”