The onset of Quaternary glaciations is a critical event in the climate and tectonic history of the Tibetan Plateau.The Shishapangma Glaciation,defined based on the till deposit from the northern slopes of Mt.Shishapangma,has been identified as the oldest glaciation on the Tibetan Plateau.However,the timing of this glaciation has not been constrained.We measured^(10)Be and^(26)Al concentrations of a set of boulders on top of this till and simulated their complex exposure-burial histories.The simulated results indicate that the formation age of this till is likely around 835.2±241.0 ka,representing the minimum timing of glacial onset on the Tibetan Plateau.The Shishapangma Glaciation is apparently much younger than the glacial onset in many other areas of the world,such as Europe and North America,and was likely driven by the coupled effect between tectonic uplift and climate cooling during the early–middle Pleistocene transition.
Yixin ChenYingkui LiMei ZhangZhijiu CuiGengnian Liu
Upfreezing is an important geomorphic process in the periglacial environment.It is a product of cold climate and thus an important part of the cryospheric processes.Based on the long-term positioning observations in the source area of the Urumqi River,this article represents an in-depth discussion of the characteristics of sorted circles and upfreezing mechanisms in the Tianshan Mountains.In the source area of the Urumqi River,the intensity of upfreezing is the highest within the top 25 cm near the surface,while targets with a diameter of 3 cm are least affected by upfreezing.There is no distinct difference between the centre with fine grains and the margin with coarse debris within the same sorted circle in terms of the intensity of upfreezing.The correlation analysis demonstrates that temperature plays an important role in upfreezing and the development of sorted circles.A long-time positioning observation of sorted circles reveals that periglacial landforms are sensitive to regional climate change and respond quickly to the temperature increase of the recent two decades.Enhanced upfreezing was found to be due to increased soil moisture content.
LI ChuanChuanCHEN YiXinNIE ZhenYuCUI ZhiJiuLIU GengNian
