最近2.5Ma黄土高原环境变化研究进展——来自洛川黄土地层的证据

洛川黄土—古土壤序列记录了最近2.5Ma黄土高原自然环境的演化和变迁。国内外研究人员运用地层学、古生物学、土壤学、磁学、沉积学、岩石学、矿物学、地球化学等多学科的研究方法，统一地层划分，明确成因机制，建立时间标尺，分析气候指标，提取环境信息，逐步揭示了自早更新世以来该区气候由暖湿而冷干、频繁剧烈波动的历史以及古生物的演替过程，为全球变化研究提供了重要资料。

LUOCHUAN LOESS-PALEOSOL SEQUENCE AND PALEO-ENVIRONMENTAL IMPLICATIONS: A BRIEF REVIEW

Luochuan section is located in Shaanxi province, at the central of Chinese Loess Plateau. Sandy shale and sand stone at Triassic is the bottom of gully. Red clay of Late Pliocene about 15m thick overlies the bedrock. The loess layer is 130m thick, and overlies red clay. 
Some reliable polarity columns for the Luochuan section has been derived in order to determine the important polarity boundaries and the age of loess layer exactly. M/G boundary lies at the Wucheng Loess /Red Clay boundary, Olduvai subchron is recorded at the middle-upper portion of Wucheng, Jaramillo subchron and B/M boundary lies in Lower Lishi loess. Other methods such as ¹⁴C and thermoluminescent can date Malan and S₀, and time scales established respectively by Liu, Ding, Lu and Kukla have showed the age of each loess layer. 
Loess-paleosol sequences in Luochuan China provide a detailed record of environment changes during the last 2.5million years. The histories of flora, fauna and climate could be observed by means of some proxies such as susceptibility, granularity, snail species, sporopollen, phytolith, CaCO₃, stable isotope, trace element, amino acid, and so forth. Investigation of them display that the long-term changes represented by Luochuan Loess-paleosol sequences have borne a multi-cycled feature with repeated climatic fluctuations from warm-humid to dry-cold. 
Before Olduvai subchron, some kinds of large mammals included Eucienoceros、Ursus sp.、Lynxshansius inhabited Luochuan basin; but after this, most of them disappeared except lagomorph and rodent. In this sequence from the bottom upwards, the content of Ca、Sr and Fe²⁺ tends to increase; on the contrary, Al、K、Ti、Mn and Fe³⁺  decreases; acidic amino acids are present in relatively greater amounts than alkaline amino acids; and CaCO₃ becomes more and more abundant. All of the facts imply that the depositional environment of Luochuan section has a bias to an alkaline and reductive thing. On the whole, the climate of Luochuan is becoming drier and colder.
The grain size distribution, magnetic susceptibility, the ratios of Rb/Sr, Mg/Al,Mn²⁺ and δ¹³C in Luochuan sequence exhibits multi-cycled climatic changes as regular fluctuation. These are also evidenced by alternations of different types of loess and paleosol. The climate curve of Luochuan can also be correlated with the results of Pacific Ocean deep-sea cores' studies. S₀ is comparable to oxygen isotope stage 1, Loess L₁ correlates with stage 2,3,and 4, and paleosol S₁ correlates with stage 5. Loess and paleosol means the term for glaciation and interglaciation respectively.
The grain size distribution represents a proxy climate index on winter monsoon of Asia; and then, magnetic susceptibility, the ratios of Rb/Sr, Mg/Al and Mn²⁺ are indexes related to past changes of precipitation and vegatation, and thus to summer monsoon. Luochuan profile has offered many data about winter or summer monsoon at central Loess Plateau. Soils were forming during intervals of strong summer monsoon, whereas loess units were deposited at times of reduced monsoon intensity.In fact, the Chinese loess-paleosol can be view as a proxy record of Asian monsoon variability extending over the last 2.5 Ma. It is pretty helpful at the study on monsoon in East Asia.
Mollusk and phytoliths in Malan loess and S₀ record the paleovegetation since the last interglaciation. Spore and pollen reveals the history of flora. All of the data about biota indicate that an adverse steppe with a dry-cold climate and a more dense forest and steppe vegetation with a temperate and moist climate existed alternately.δ¹³C in organic matter demonstrates such conclusion, too.
Comparison of mollusk records from the Luochuan loess sequence with those of grain-size and aridity index from RC 27-61 core in the India ocean reveals obvious similarities in several time intervals, suggesting that climate variations in the Loess Plateau may have been also subjected to the Indian monsoon to a certain extent in the geological past. The presence of some specific cold-humid and cold-dry species represents deteriorated climate conditions, which might be the cooling events similar to the Heinrich events recorded in the North Atlantic.  
    The major clay minerals in the Luochuan loess section are illite, kaolinite, montmorillonite, chlorite, vermiculite and a small amount of disordered or ordered mixed-layer ones. In the study of clay minerals, a method for showing the “crystallinity” of illite was introduced to interpret the geological environment. The “crystallinity” of illite in Brunhes chron reflects a strong fluctuation than ever, and it indicates some intense climatic changes. 
Some quantitative results about paleoenvironment have been obtained. The 0.31% change of δ¹⁸O of meteoric water reflects the change of 1℃ in formation temperature. Base on above, the temperature difference between the last glaciation and the interglaciation is not smaller than 6℃ in Luochuan. The formation temperature of S₅ is 14.5 ℃. At the same time, the formation precipitation of most paleosols in Luochuan is 600～800 mm every year.  
 In the Luochuan district, loess began to be deposited 2.5Ma BP, and loess layers formed in the period of dry-cold climate, and the paleosols in the period of mild-humid climate, thus forming the uninterrupted loess-paleosol sequence. Base on the proxy climatic indexes in Luochuan, we have obtained a curve of paleoclimatic change 2.5 Ma. This curve may be compared with that of δ¹⁸O stages from deep-sea deposits. It can be divided into many climatic cycles. These cycles may have a direct relationship with that of the change in the earth orbit.