Origin of the Red Earth sequence on the northeastern Tibetan Plateau and its implications for regional aridity since the middle Miocene

摘要

The Red Earth deposit around Xining, northeastern Tibetan Plateau, has been studied in terms of soil micro-structure, grain-size distribution, major- and trace-element chemistry and magne-tostratigraphy. Field investigations indicate that the Red Earth sequence consists of 25 visually definable yellow-brown and weakly-developed soils interbedded with reddish strongly-developed soils, and has a similar structure to the aeolian Red Clay deposit on the Chinese Loess Plateau. Grain-size analysis shows that the Red Earth deposit is very fine-grained with a negligible sand fraction ( > 63 μm). Thin-section observations under light microscopy show that the deposit has a clayey texture and the coarse fraction ( > 10 μm) mainly consists of quartz, feldspar and micas. Pyroxene and hornblende were also observed. This mineralogical composition of the coarse fraction is similar to that of the Quaternary loess. In addition, all of the mineral grains are semi angular semi-angular and are generally finer than 63 μm. There is a good agreement between the major and minor trace element chemistry of loess-soil units and the Red Earth deposit. The REE distributions of the loess-soil and the Red Earth deposit are similar in shape, with enriched LREE and fairly flat HREE profiles and a clear negative Eu anomaly. The geochemical characteristics of the Red Earth deposit are also identical to those of upper continental crust, thus indicating a wind-blown origin. Magnetostratigraphic investigation shows that onset of the Red Earth deposition predates 11.4 Ma BP (13.6 Ma at a nearby site). The distribution of the reported Miocene aeolian loess at Qin'an of the Loess Plateau is still unknown. Our results indicate that this aeolian deposit had extended to the northeastern Tibetan Plateau by at least the middle Miocene. The similarity of the element geochemistry between the Red Earth deposit and the overlying loess shows that they may have similar sources and dynamic transport system, and may indicate that the aridification of the interior of the Tibetan Plateau and central Asia began by at least the middle Miocene. Compared with the last intergla- cial-glacial loess, the grain-size of the Red Earth deposit is finer. This may indicate a lower energy transport agent and/or aridity in the dust source region. However, changes in grain-size and other proxies indicate many climatic fluctuations, with two important shifts at 9.61 —9.91 Ma and 7—8 Ma. In addition, the grain-size record shows a shift from a high-frequency and high-amplitude pattern to a low-frequency and low-amplitude pattern at around 10.4 Ma and may indicate a significant environmental event at this time. Previous research has shown that the intensity of the southwest Asian monsoon increased at this time, coincident with heavier foraminiferal oxygen isotope values and a sharp fall in sea-level. Thus the environmental event in the Northeast Tibetan Plateau at 10.4 Ma may have global implications.