Advances in Earth Science ›› 2002, Vol. 17 ›› Issue (1): 118-125. doi: 10.11867/j.issn.1001-8166.2002.01.0118

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Li Yu-mei   

  1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029,China
  • Received:2001-02-19 Revised:2001-06-04 Online:2002-12-20 Published:2002-02-01


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.

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