Quaternary Glaciations Of Dalijia Shan Constrained By ¹⁰Be Exposure Ages

The Dalijia Shan is located on the northeast border of the Tibetan Plateau and the boundary between the Xunhua County of Qinghai Province and the Linxia County of Gansu Province.It is situated in the transition region between the Loess Plateau and the Tibetan Plateau.There is no glacier developed at present in the Dalijia Shan,but it was extensively and repeatedly glaciated during the Quaternary and glacial landforms from multiple glaciations are well preserved.In this paper,based on glacial deposits and stratigraphy,10Be exposure ages of 22 samples collected on four moraine groups in the Dalijia Shan were used to discuss the chronological framework of Quaternary glaciations in the Dalijia Shan.In addition,the equilibrium-line altitude(ELA)during mid-MIS3 was calculated by using an integrated method and then the estimated ELA value was input into statistic models to derive the forcing paleoclimatic conditions.There are three conclusions as follows:(1)The moraine groups identified from field and remote sensing imagery record four glacial events at 37.14±3.32?52.96±4.70 ka(MIS3)?20.17± 1.79?26.18±2.47 ka(MIS2)?16.92±1.49?17.24?1.53ka(MIS2)and 11.56±1.03?11.89±1.06 ka(Younger Dryas).These ages in dicate that glaciation in the northeastern Tibet Plateau is much younger than previously th ought.In addition,its local last glacial maximum(LGML)is asynchronous with the global last glacial maximum(LGMG)(MIS2),and the maximum glacial extent occurred during MIS 3,?20ka prior to the LGMG.During mid-MIS3,the Dalijia Shan was occupied by an ice cap which area was about 230 km2.Subsequently,this region experienced two more limit ed advances druing LGMG(MIS2).The youngest event,?11.7 ka records either a small ad vance or a period of time when the glacier reached a stillstand during retreat,possibly rec ording a Younger Dryas event,but this needs to be tested in future studies.(2)Based on the Ohmura's formula of the relationship between summer atmospheric temperature and the annual precipitation at ELA,the present theoretical ELA in the Dalijia Shan should be 4788±100m.The ELA of the ice cap on during mid-MIS3 was 4025m calculated by maximum elevation of lateral moraines(MELM)?toe-to headwall altitude ratio(THAR)?accumulation area ratio(AAR)and area altitude balance ratio(AABR)methods.With adjustments for the tectonic uplift(uplift rate is 0.75 m/ka)since MIS3(-39ka),the true ELA was 3996 m.Therefore,the ELA of mid-MIS3 was 792±100m lower than the present theoretical ELA.(3)Considering the annual precipitation of the Dalijia Shan during mid-MIS3 was 53?96%compared to the present,the temperature of mid-MIS3 was 4.3?5.6? lower than today based on the Degree-day model and the Shi's formula of the relationship between the annual atmospheric temperature and the annual precipitation at ELA.The result and other records sugguested that the temperature during mid-MIS3 was higher than that of LGMG.However the LGML of the Dalijia Shan occurred during mid-MIS3,so the LGML(mid-MIS3)of the Dalijia Shan should be the result of the coupling of the low temperature and abundant precipitation.