Search

During the last 50 ka, cryogenic wedges on the Ordos Plateau formed during three major periods: (i) early local Last Glaciation, ca. 50 ka BP; (iii) local Last Permafrost Maximum(local LPM), 25–19 ka BP; and (v) post-local LPM, 16–9 ka BP. Cryoturbations mainly formed in the following periods: (ii) pre-local LPM, 45–30 ka BP and (iv) ~ 20 ka BP. The coldest periods with well-developed permafrost (i and iii) were most conducive for forming cryogenic wedges. The following periods of warming climate and degrading permafrost favored the formation of cryoturbations. During the local LPM, sand wedges and polygons were well developed and widely distributed on the Ordos Plateau when mean annual air temperatures (MAATs) were approximately 12°C lower than that at present. At ~30 ka BP, MAAT was 6–7°C lower than that at present. Paleoclimate conditions on the Ordos Plateau were reconstructed since 50 ka BP as follows: cold (ca. 50 ka BP) → cool (45–30 ka BP) → very cold (25–19 ka BP) → cool (19–9 ka BP) → intermittent warming until the present day. The amount of precipitation fluctuated, but with a general trend of drying since 50 ka BP. Under the next generally warming climate (after 9 ka BP), permafrost gradually degraded and eventually disappeared from the Ordos Plateau.

This paper presents new field observations that assist in further defining the extent of Last Permafrost Maximum (LPM)permafrost and frozen ground in north China. In that respect, this region is specific by its arid environment and continental position. Bedrock exposures and sand and gravel pits were examined along a south-north transect from southern Ordos as far as the vicinity of Dongsheng District, Ordos City on the Ordos Plateau (North China). Attention focussed upon the recognition of sand-wedge remains and possible cryoturbations. Two types of wedge structures were recognized: type A wedges extend to depths in excess of 3.0 m and are thought to have formed under conditions of continuous permafrost. Type B wedges are restricted to the upper 1.5 m and are thought to reflect discontinuous permafrost and/or deep seasonal frost. OSL dating of wedge infills indicates that type A wedges formed between 23–19 ka and younger type B wedges formed until the early Holocene. It is now possible to delineate a more exact position of the boundary between continuous permafrost and discontinuous permafrost and/or deep seasonal frost on the Ordos Plateau during the LPM. This boundaries occur between Dongsheng and Jinbian at 37°-39°N.