LATE TERTIARY DEVELOPMENT OF ARIDIFICATION IN NORTHWESTERN CHINA: LINE WITH THE ARCTIC ICE-SHEET FORMATION AND TIBETAN UPLIFTS
晚第三纪中国西北干旱化的发展及其与北极冰盖形成演化和青藏高原隆升的关系

The Xifeng red-earth sequence, a type section for the eastern loess region, isstudied based on thin section analyses, sedimentological, and geochemical to addressits origin and climatic implication. The result shows that this formation consists ofthree parts. The lower Part (> 6.2 Ma B. P.) is a fluvial-alluvial deposit; themiddle part (from 6.2 to 3.4 Ma B. P.) was derived from eolian dust deposition, asstated in earlier study, but significanhy affected by weathering processes andgroundwater oscillations. The upper part (from 3.4 to 2.6 Ma B. P.) is a typicaleolian formations, similar to the Quaternary loess-soil sequence. Since eolian dustdeposits at the middle Reaches of the Huanghe River was mainly transported from thedesert in northwestem China, we interpret the eolian origin of the red-earthformation started at 6.2 Ma B. P. as an indication of the strengthening of thearidification on the Asian continent.Variations of the eolian grain-size in the middle and upper part of the sequenceindicate that continental aridity were stronger from 6.2 to 5.4 Ma B. P., weaker from 5.2to 3.4 Ma B. P. and then significantly sdtrengthened at 3.4 Ma B. P. and 2.6 Ma B. P.,respectively. These changes are highly consistent with the North Pacific eolian record. amarine record of Asian continental climate, and also coupled with the Arctic ice-sheetdevelopment. These results suggest that the Asian aridification were strongly forced bythe Arctic ice-sheet development, probably through the strengthening of the Siberian High.The aridification occurred at 3.4 Ma B. P. and 2.6 Ma B. P. were also synchronouswith intense-uplift of the Qinghai-Xizang Plateau. suggesting a causal link.