Cryogenic wedges and cryoturbations on the Ordos Plateau in North China since 50 ka BP and their paleoenvironmental implications

Abstract

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.

Palynological Evidence of Late Pleistocene Soft‐sediment Deformation Event in the Northeastern Margin of the Tibetan Plateau

“…These characteristics are prerequisites for cryoturbation. Furthermore, according to previous studies, cryoturbation mainly occurs in the following situations: (1) its depth is 3-5 m below the surface; (2) the climate transitions from cold to warm, and the permafrost tends to degenerate, but it has not completely melted (Cui, 1980;He et al, 2021a). In the study area, the burial and climatic conditions of the softsediment deformation met the requirements of cryoturbation.…”

“…Freezethaw action (cryoturbation) is a fold, curl, or wrapped shape formed by the displacement caused by the frost heaving force in the upper and lower directions and the creeping action of the saturated thawed layer, with a symmetric wave, tongue, anchor, or pan shape. Cryoturbations are uniformly and continuously distributed in the horizontal direction, and the upper and lower interfaces of cryoturbations are visible (Wang and Bian, 1993;Harris et al, 2017;He et al, 2021a). The fold structure triggered by earthquakes generally had a smallscale and irregular interlayer deformation, symbiosis with other liquefaction deformation structures, poor orientation, overlying nearly horizontal strata, and underlying deformation layers (Du, 2011;Du and Yu, 2017).…”

Hugh French memorial for Permafrost and Periglacial Processes

“…(2021, this issue) 23 synthesizes existing knowledge and presents new paleoenvironmental data about the occurrence and dating of relict sand wedges, suspected ice-wedge pseudomorphs, and cryoturbations. The coldest periods with well-developed permafrost during the last 50,000 years occurred during the early local Last Glaciation (50 ka BP) and the local Last Permafrost Maximum (25,000-19,000 years ago).…”

“…Another important region of past permafrost is the Ordos Plateau of northern China, a largely desert area of ~130,000 km 2 at elevations of ~1,000–1,300 m above sea level and surrounded to the west, north, and east by the Yellow River. The paper by He et al (2021, this issue) 23 synthesizes existing knowledge and presents new paleoenvironmental data about the occurrence and dating of relict sand wedges, suspected ice‐wedge pseudomorphs, and cryoturbations. The coldest periods with well‐developed permafrost during the last 50,000 years occurred during the early local Last Glaciation (~50 ka BP) and the local Last Permafrost Maximum (~25,000–19,000 years ago).…”

“…While he particularly focused on ground ice 2–4 and cryostratigraphy, 5–7 he made substantial contributions to topics such as thermokarst processes and landforms (Figure 2), 8,9 frost mounds, 10,11 slope processes and pediments, 12–15 periglacial involutions, 16 and weathering processes and related landforms 17,18 including in hot deserts such as in Australia (Figure 3). 19 His interest in Quaternary environments started during his early research in the UK 12,20 and continued throughout his life, with analyses of deposits and relict landforms in many parts of the world, including the eastern USA, 21 Europe, 22 China 23 and Antarctica 24–27 . Hugh also worked on engineering problems in permafrost areas, especially in the Canadian Arctic, including those related to oil exploratory drilling and disposal of drilling fluids 28,29 .…”