Evolution of permafrost in Northeast Chinasince the Late Pleistocene

In Northeast China, permafrost advanced and retreated several times under the influences of fluctuating paleo-climatesand paleo-environments since the Late Pleistocene. During the last 60 years, many new data were obtained and studies wereconducted on the evolution of permafrost in Northeast China, but so far no systematic summary and review have been made.Based on sedimentary sequences, remains of past permafrost, paleo-flora and -fauna records, and dating data, permafrostevolution since the Late Pleistocene has been analyzed and reconstructed in this paper. Paleo-temperatures reconstructedfrom the remains of past permafrost and those from paleo-flora and -fauna are compared, and thus the southern limitof permafrost （SLP） in each climate period is inferred by the relationship of the permafrost distribution and the meanannual air/ground temperatures （MAAT/MAGT）. Thus, the evolutionary history of permafrost is here divided into fivestages： （1） the Late Pleistocene （Last Glaciation, or LG） （65 to 10–8.5 ka）, the Last Glaciation Maximum （LGM, 21–13 ka）in particular, the coldest period in the latest history with a cooling of about 6~10 °C, characterized by extensive occurrencesof glaciation, flourishing Mammathas-Coelodonta Faunal Complex （MCFC）, widespread aeolian deposits, and significantsea level lowering, and permafrost greatly expanded southwards almost to the coastal plains （37°N–41°N）; （2） the HoloceneMegathermal Period （HMP, 8.5–7.0 to 4.0–3.0 ka）, 3~5 °C warmer than today, permafrost retreated to about 52°N; （3） theLate Holocene Cold Period （Neoglaciation） （4.0–3.0 to 1.0–0.5 ka）, a cooling of 1~3 °C, some earlier thawed permafrost wasrefrozen or attached, and the SLP invaded southwards to 46°N; （4） the Little Ice Age （LIA, 500 to 100–150 a）, the latestcold period with significant permafrost expansion; and （5） climate warming since the last century, during which NortheastChina has undergone extensive permafrost degradation. The frequent and substantial expansions and retreats of permafrosthave greatly impacted cold-region environments in Northeast China. North of the SLP during the HMP, or in the presentcontinuous permafrost zone, the existing permafrost was largely formed during the LG and was later overlapped by thepermafrost formed in the Neoglaciation. To the south, it was formed in the Neoglaciation. However, many aspects ofpermafrost evolution still await further investigations, such as data integration, numerical reconstruction, and merging ofChinese permafrost history with those of bordering regions as well as collaboration with related disciplines. Of these, studies on the evolution and degradation of permafrost during the past 150 years and its hydrological, ecological, and environmentalimpacts should be prioritized.

Tessellons, topography, and glaciations on the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau has developed into a vast fortress-like structure that has recently presented a barrier limiting the egress of moisture-bearing air masses. Lower sea levels also affected the climate. This paper examines their effects on the current evidence for the timing of past glaciations, and the development and evolution of permafrost. There are two theories regarding glaciation on the Qinghai-Tibet Plateau （QTP）. Kuhle suggested that there was a major, unified ice-cap during the Last Glacial Maximum （LGM）, whereas major Chinese glaciologists and others have not found or verified reliable evidence for this per se. There have been limited glaciations during the last 1.1 Ma B.P. but with increasing dominance of permafrost including both primary and secondary tessellons infilled with rock, sand or loess. The East Asia Monsoon was absent in this area during the main LGM, starting at 〉30 ka B.P. on the plateau, with sufficient precipitation reappearing about 19 ka B.P. to produce ice-wedges. A weak Megathermal event took place between 8.5 and 6.0 ka B.P., followed by Neoglacial events exhibiting peak cold at 5.3-4.7 ka, 3.1-1.5 ka, and the Little Ice Age （LIA） after 0.7 ka. Subsequently, mean annual air temperature has increased by 4 ℃.

Climatic change in Western North America during the last 15,000 years:The role of changes in the relative strengths of air masses in producing the changing climates

In the Cordillera of western North America, the influence of the Pacific Interdecadal Oscillation only affects coastal areas west of the Coast Range and the lowlands of western and southern Alaska. The rest of the area is subject to a climate controlled by the relative strengths of three distinct air masses, viz., the cold cA/cP air that is dominant in winter, the mP air bringing cool moist air over the mountains throughout the year, and the dry hot cT air from the deserts of the southwestern United States. The Arctic Front marks the boundary between the cA/cP air mass and the other two. Changes in the relative strengths of these air masses appear to explain the climatic changes documented throughout the region. Thus, in the last 30 years, the average position of the Arctic Front has moved north from about 53°N to 58°N, causing the warming in northern British Columbia and cooling south of Calgary, Alberta. This concept of changing positions of the air masses also appears to explain the mechanism behind the past climatic changes in this region. During the last Neoglacial event （c.1400-1900 A.D.）, it appears that the cA/cP air mass had strengthened enough to push the Arctic Front south of the 49th parallel. Incursions of mP air increased with localized areas of short-term heavy snowfalls resulting in small-scale advances of glaciers in these regions. This accounts for the variability in timing and extent of these glacial advances, while the resulting increased Chinook activity produced the development of a sand sea between Medicine Hat and Regina on the southern Prairies. The cT air mass was relatively weak, permitting these changes. During the maximum of the Altithermal/Hysithermal warm event （6,000 years B.P.）, the Arctic Front had retreated into the southern Yukon Territory as the cT air mass became stronger. The mP air could not move inland as easily, resulting in drier climates across the region. Prairie plants mi- grated into the southern Yukon Territory, and land snails from the eastern United States were able to migrate up the Saskatchewan River system as far as Lake Louise, Alberta. On the southern Prairies, the many small sloughs and lakes dried up. During the maximum of the Late Wisconsin Glacial event （15,000 years B.P.）, the Arctic Front had moved south to the vicinity of 30°N, while there had been a southward movement of the Zone of Intertropical Convergence from the equator to about 10°S. The mP air was also very strong and dumped enormous quantities of snow in the glaciated Canadian Cordillera, but it does not appear to have moved south any distance into the northern United States, witness the limited glaciation and widespread permafrost that developed there. Instead, there is evidence for buffering of the climatic changes in the closed basins in the northern Cordillera of the contiguous United States. The source of the cT air mass had moved south into the northern part of South America, permitting an exchange of savannah biota between the two continents. An extensive area of white dune sands inundated both savannah and forest along the inland hills in Guyana. This parallels the massive changes in African climatology during the last Ice Age （Fairbridge, 1964）. If these changes occurred each time there was a major glaciation in the Northern Hemisphere, this would explain the movement of biota from all terrestrial environments between the two American continents in the last 2 million years. A similar northward movement of climatic belts occurred in South America, with the cA air from Antarctica expanding northwards into southern Argentina and Chili. However paucity of data and the potential effects of El Ni o and the Southern Oscillation make it difficult toprovide details of the changes there in the present state of knowledge. This technique of studying the mechanisms of present-day climatic changes and applying the results to past climatic events has considerable potential for elucidating past climatic changes elsewhere in continental regions. This may prove particularly valuable in studying the Siberian anticyclone that is the main cause of the distribution of permafrost, but this will need international cooperation to be successful.