Desert migrations during the past Ma

The long-term evolution of the deserts of northern China remains poorly understood because of the sparseness of directly extractable geological evidence of suitable type and quality to be found within them. Given that the loess deposits making up the Loess Plateau of China lie immediately south and southeast of these deserts, and so are largely a product of winds from these dryland sources, it is considered that well-dated, semi-continuous loess records may provide valuable insights into the recent history of the deserts and their margins.

In this study, a thick eolian loess-red clay sequence at Jingbian near the Mu Us desert were sampled and analyzed, The aim was to use spatial changes in loess particle size to establish a semi-quantitative relation between particle size and desert margin location, and to reconstruct shifts in desert margins since the late Pliocene.

The Jingbian section (37°40'N, 108°31'E), at 1370 m above sea level, lies on the summit of the Baiyu Mountains. It is located only 12 km south of the present margin of the Mu Us Desert. No local sources of sand, such as river channels, are present in this area. The section is composed of a 252 m thick Pleistocene loess-soil sequence resting on a 30 m Pliocene red clay deposit, making a total thickness of 282 m. A previous magnetic polarity study showed that this sequence has a basal age of 3.5 Ma. Field observations have demonstrated that the development of the paleosols within both the Pleistocene and Pliocene eolian deposits is much weaker at Jingbian than in the main body of the Loess Plateau series, and that the Pleistocene loess-soil stratigraphy correlates well with the classic loess sections. To the best of our knowledge, the Jingbian section is the only desert margin eolian sequence known to cover the whole Pleistocene and the late Pliocene. Its proximity to the dust source region makes it ideal for the study of long-term desert changes.

In the field, samples were taken at 5-10 cm intervals, making up a total of 3440 samples. The magnetic susceptibility values at Jingbian are 2-4 times higher in paleosols than in loess horizons, and grain sizes are much coarser in loess than in soils. Both the susceptibility and grain-size records further support our field observation that this eolian sequence is a well preserved and almost complete record, making it possible to develop a time scale for this sequence by correlating its loess-soil unit succession to the Chinese Loess Particle Time Scale (Chiloparts). The Chiloparts record was developed by stacking five individual loess grain-size records that were tuned to the obliquity and precession records of the Earth's orbits. The Jingbian magnetic susceptibility and grain-size records, plotted on the Chiloparts time scale, are shown in Fig. 5.3, together with a composite marine oxygen isotope record.

The > 63 and > 125 ^m (%) records at Jingbian show four stepped increases in sand-sized particle content. The late Pliocene red clay (below L33) contains few sand particles, indicating that the dust was transported in suspension, mainly from a remote source. From 2.6 to 1.2 Ma, sand content in interglacial soils remains low, whereas it varies generally between 18% and 25% in glacial loess except for the case of L15 and L16. This suggests that, during glacial periods, the desert environment advanced to a location no more than 200 km from the present northern margin of the Loess Plateau. In the part of the section deposited between 1.2 and 0.7 Ma,

Fig. 5.3 Changes in magnetic susceptibility (A) and grain-size data (B, C, D) at Jingbian, and correlation with a stacked marine 5 18O record (E). The time scale of the Jingbian section was developed by correlating the loess-soil units with the Chiloparts record (modified after Ding et al., 2005).

sand content increases to 12% in soils and to 43% in loess with a substantial increase in > 125 pm particles, implying a large-scale advance of the desert margin during both glacial and interglacial times. Throughout material deposited in the interval 0.7-0.2 Ma, > 63 pm particles range from 30% in soils and 55% in loess units, with the > 125 pm particles exceeding 8%. This suggests that the distance between the Loess Plateau and the present desert margin was less than 100 km. During the last two glacial periods, eolian sand was directly deposited at Jingbian, indicating a further southward desert shift.

The Jingbian sand-sized particle record clearly demonstrates that, superimposed on the glacial-interglacial oscillations, the deserts in northern China experienced significant expansion at 2.6, 1.2, 0.7 and 0.2 Ma, directly implying a stepwise southward retreat of the monsoon rainfall belt, associated with a complementary reduction in summer monsoon strength, in the past 3.5 Ma.

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