The world largest alpine permafrost region, the Tibetan Plateau, will experience amplified warming under global climate change scenarios. Studying its environmental evolution in the geological past is crucial to further our understanding of mechanism and thresholds of climate change. Here we present a 3.5-million-year-long, high-altitude vegetation and climate record from the Kunlun Pass Basin to reconstruct the transition from the sustained warm, high carbon dioxide environment of the Pliocene to the cool glacial and interglacial periods of the Pleistocene, 4.31 to 0.85 million years (Ma) ago. The Early Pliocene pollen record indicates the occurrence of patches of broadleaved and coniferous forests in a semi-desert shrubland at this high-altitude site. Pollen derived quantitative climate estimates based on modern pollen rain transfer functions suggest a moister climate before 4.0 Ma with mean annual temperatures (MATs) > 14 °C warmer than today. The retreat of broadleaved and coniferous forests from the Kunlun Pass Basin area is linked to stepwise cooling at ∼4.0 Ma and during the Plio-Pleistocene transition between 2.7 and 2.6 Ma. Pollen derived climate estimates and Δ47-palaeothermometry indicate an abrupt cooling of ∼4–8 °C resulting in the onset of permafrost condition on the Tibetan Plateau after 2.7 Ma. An expansion of Chenopodioideae dominated xerophytic shrublands after 3.8 Ma and 2.15 Ma indicates a reduction in precipitation on the NE Tibetan Plateau, which appears to be linked to a weakening of the East Asian Summer Monsoon at the Plio-Pleistocene transition. The reconstructed cooling at the Kunlun Pass Basin site reflects the regional expression of global climate change with high-elevation temperature amplification hence rejecting the notion of a major tectonic uplift of the Tibetan Plateau during the Plio-Pleistocene.
Abstract The early Cenozoic topography of the northern Tibetan plateau remains enigmatic because of the paucity of independent paleoelevation constraints. Long‐held views of northward propagating deformation imply a low Paleogene elevation, but this prediction is speculative. We apply flexural modeling to reconstructed Paleogene isopach data obtained from the Qaidam basin, which requires a larger topographic load in the Qilian Shan and a smaller load in the Eastern Kunlun Shan. Incorporating knowledge of proto‐Paratethys marine incursions in the Paleogene Qaidam basin, we infer a topographically low (0.4–1.0 km) Eastern Kunlun Shan and a higher (0.4–1.5 km) Qilian Shan during the Paleogene. This implied paleo‐relief contrasts with previous predictions and suggests more recently, Neogene surface uplift in the Eastern Kunlun Shan has been more significant than in Qilian Shan, highlighting diachronous growth of the northern Tibetan plateau. The low‐moderate paleoelevation implies a warmer and more humid climate in Northern Tibet during the Paleogene.
Culture experiments under controlled temperatures were conducted to study the carbon and oxygen isotopic fractionation between water and ostracod shells,as well as its significance in paleoclimatic reconstructions.The carbon isotopic composition of ostracod shells is mainly controlled by the carbon-isotope of water DIC in which the shells formed.The average offsets between δ13C of shells and the co-existing water DIC are-0.48‰,-0.59‰ and 0.7‰ for culture temperature 10 ℃,15 ℃and 19 ℃ respectively.Our experiments show that temperature is probably not a significant factor controlling the carbon isotopic fractionation between ostracod shells and DIC.However,the carbon fractionation factors of E.mareotica are very close to those of the synthetic inorganic calcite formed in isotopic equilibrium with the increase in water pH.So,the carbon isotopic fractionation of different species living in different water environment should be considered,if the measurements of 13C/12C ratios of non-marine ostracod valves are used in palaeoenvironmental reconstruction.
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