Cryogenic wedges on the NE Qinghai-Tibet and Ordos Plateaus: Their characteristics,origin and OSL dating

Cryogenic wedges developed due to very cold, rather arid conditions during the maximum of the last cold event when the drying up of the neighboring China Sea resulted in the failure of the East Asian Monsoon. As the climate ameliorated and the Monsoon rains reappeared, ice-wedges developed. Further warming permitted thawing of the ice infillings accompanied by replacement of the ice by sediments partly from the host ground as well as from the surface by wind or sheet wash. In cases of extreme surface water flow on slopes after 10 ka B.P., small baydjarakhs typically c. 50 cm high developed, only to have the resulting hollows infilled by sediments carried by wind and/or sheet wash. These shallow structures form a network on top of many of the cryogenic wedges. This complex history makes dating the ages of the wedges difficult using OSL methodology. Unfortunately, past field work ignored the problem of the angle of the cut face to the direction of the wedge infilling when sampling the contents of the narrow wedges, resulting in potential contamination of the samples with the host sediment. Sampling of the larger deposits should be alright, but the likelihood of contamination makes the interpretation of the resulting OSL dates from the narrow wedges questionable. Primary wedges consisting of primary mineral infillings should still have similar OSL dates with depth for a given wedge, but the distinction between ice-wedge infillings and soil wedges is difficult since both can exhibit older dates of the infillings with depth. The available data suggests that ice-wedges were significantly more common than sediment-filled primary wedges. A protocol to avoid having to obtain large numbers of OSL dates by more careful field sampling and the use of grain size determinations is provided in the Appendix.