Search

The exhumation history and tectonic evolution of the Qilian Shan at the north-eastern margin of the Tibetan Plateau has been widely debated. Here, we present apatite fission-track (AFT) data for 12 Ordovician granodiorite samples along a vertical transect in the eastern Qilian Shan. These thermochronometry data indicate that the eastern Qilian Shan experienced a three-stage cooling history, including: (i) rapid initial cooling in the late Cretaceous; (ii) a stage of quasi isothermal quiescence from similar to 80 to 24Ma; and (iii) rapid subsequent cooling beginning in the early Miocene. The inferred cooling rates for the three stages are 6.8 +/- 4.9 degrees CMa(-1), 0.6 +/- 0.2 degrees CMa(-1) and 2.7 +/- 0.9 degrees CMa(-1) respectively (+/- 1 sigma). Assuming a geothermal gradient of 25 degrees Ckm(-1), the exhumation rates for the three stages are 0.27 +/- 0.20mma(-1), 0.017 +/- 0.007mma(-1) and 0.11 +/- 0.04mma(-1) respectively (+/- 1 sigma). We suggest that the late Cretaceous cooling records collision of the Lhasa block with the Eurasian continent and that the Miocene cooling represents uplift/exhumation of the Qilian Shan.

The modern southern border of the Tengger Desert protrudes southward at the east and west ends and recedes northward in the central part. However, no satisfactory explanation can be given for the shape of this border according to geomorphological elements. From the west to the east of the south of the Tengger Desert, there is a gradual change in precipitation, humidity, temperature, and mean wind speed, which differs from the abrupt modeled change in the desert’s southern border distribution. The indication is that these meteorological elements are not the main factors controlling the migration of the southern border of the Tengger Desert. The extreme and maximum wind speeds at the east and west ends of the Tengger Desert are stronger than those of the central part. Consequently, the sand dunes at both ends move faster than those of the central part, causing the southern border of the Tengger Desert to protrude southward at both ends and recede northward in the central part. As a result, winds occurring in winter and spring, especially the strong northwest winds in spring, are the main controlling factor for the migration of the southern border of the Tengger Desert.

The Tengger Desert and Badain Jaran Desert are two typical deserts in the Northwest Arid Area of China (NAAC), located near the margin of the monsoonal region. Through chronological research of the oldest loess sections formed from synchronous deposition of the desert dusts, we suggest that the initial present day Tengger and Badain Jaran deserts appeared no later than about 0.85 Ma. Because this timing of present day desert initiation in the eastern part of the NAAC is very similar to the other two deserts in the west, we suggest that the NAAC formed at approximately 0.85 Ma. The uplifting of the Tibetan Plateau accelerated at around 0.85 Ma, an event that caused the Plateau to enter the cryosphere. This resulted in remarkable enhancement of the winter monsoon as well as bifurcation of the mid-lower Westerlies. This is the most fundamental factor that affected the formation of the NAAC.

An abrupt climatic change during the MIS 5a/4 transition is evident in the loess records of China (S1/L1). Proxies including geochemical elements, grain size, soil color, magnetic susceptibility and carbonate (CaCO 3 ) content indicate a warming interval, which lasted approximately 3 ka, during the MIS 5a/4 transition in both the Wangguan and Shagou loess sections, located in Sanmenxia (Henan Province) and Wuwei (Gansu Province), respectively. Both the winter and summer monsoon proxies demonstrate that this warming interval occurred at the same time in both sections (nearly 70.5–73.6 ka BP), with maximum warming from 71.4 to 72.0 ka BP. This study suggests a universal abrupt warming interval in the East Asia monsoon region at this time. Comparisons with marine, terrestrial and ice-core records indicate this event was very likely an abrupt global warming interval during the last glacial–interglacial transition.

The Shagou section contains rich information about the formation and evolution of sand storms in the windward side of of the Tengger Desert (southern Tengger Desert, east of the Hexi Corridor). By investigating the grain size classes and the standard deviation of three sample groups in the Shagou section we discover that every group has four sensitive grain size components, and each sensitive grain size component has a tendency to be finest for the palaeosol stratum, coarsest for the sand stratum, and the loess intermediate between the two. The three sample groups have differences in their grain size distributions. Both S1 (average) and L1 (average) have two peaks. L1 (sand stratum) has three peaks and an additional coarse peak (275.4-550 µm) which is consistent with the grain size distribution pattern of the modern sand storm samples in the Tengger Desert. Moreover the additional peak of the palaeo-sand stratum correspondingly the interval of the grain size is exactly consistent with one of the four sensitive grain size components. Because the sand stratum in L1 of the Shagou section was the direct result of sand storm process of the Tengger Desert, the sand group percent of 275.4-550 µm in the Shagou section could be used as a sensitive indicator of sand storm in the south of the Tengger Desert (east of Hexi Corridor).

Based on the Aeolian loess sequence-stratigraphic division and paleomagnetic dating on terraces, we found that an undiscovered terrace with the age of 1.0 Ma BP lies between the Dunwashan terrace and Wuyishan terrace. This recorded an intensive yellow River incision event during that period. Results of paeomagnetic dating and optically stimulated luminescence (OSL) dating of the Wuyishan loess section and Zaoshugou loess section on the fourth Yellow River terrace (T4) show that the age of the fourth Yellow river terrace in Lanzhou area is 0.86 Ma terrace in Lanzhou area. The discovery of the 1.0 Ma Yellow River terrace and redating of the age of 0.6 Ma terrace in Lanzhou area provide new insights into further research on the evolution of Yellow River. *Supported by National Natural Science Foundation of China (Grant Nos.40421101 and 40471016), and the Major State Basic Research Development Program of China (Grant Nos. 2005CB422001)

Here we report our recent magnetic fabric investigation of loess deposition in Shagou section, located at the northeastern Qilian Mountains, the northeastern rim of the Tibetan Plateau. On the basis of environmental magnetism data, we indicate that the variation of anisotropy of magnetic susceptibility (AMS) parameters, especially the foliation (F) and degree of anisotropy (P), might be more sensitive to the environmental change in the arid and semiarid regions than the magnetic susceptibility fluctuation. During the investigated interval, from 0.83 to 0.128 Ma, most of the middle to late Pleistocene significant climate change can be unraveled by the AMS parameters, such as the strengthening of cold/dry climate, the step drying event occurred nearly 250 ka, and the severe environmental change in MIS16. Our results also suggest that there is strong correlation between median diameter (Md) of grain size, F, and P. We propose that the AMS parameters can act as an important paleoenvironmental change indicator in the arid and semiarid regions.

Analysis of morphological properties in the Sanmen Gorges region of the Yellow River shows that four river terraces have been developed at the entrance to the gorges. Commonly, thick aeolian loess-paleosol sequence was deposited on the terraces, which makes it easy to date the terraces. The dating of paleomagnetic, loess-paleosol sequence matching and thermoluminescence show that terraces T2, T3 and T4 were formed 0.129 Ma, 0.625 Ma and 0.865 Ma ago, respectively. The formation of these terraces in this region indicates that the Yellow River has experienced four intensive downcutting events during the last 0.9 Ma. The Yellow River cut through the Sanmen Gorges and inpoured into the East China Sea no later than 0.865 Ma. ∗ Supported by National Natural Science Foundation of China (Grant Nos. 40471016 and 40401007), Innovation Team Project of National Natural Science Foundation of China (Grant No. 40421101), and the Doctoral Foundation of Ministry of Education, China (Grant No.20030730017)

The eastern part of Qilian Mountains experienced strong tectonic uplift during the late Quaternary, and climate record there was influenced by Tibetan Plateau to some extent. Based on studies on the fluvial terrace series and eolian loess deposition, we find that the tectonic uplifts of the Tibetan Plateau had coupled with climatic changes in our studied region and others since the mid-Pleistocene. The uplift that occurred at 0.83 Ma corresponded to significant desert expansion in L6 and periodic variation since MIS16, while the 0.14Ma one to the further drying in northwest China. Those coupled events may indicate that tectonic uplift drove climatic changes, and the Tibetan Plateau has important impacts on East Asian Monsoon system.

The 28 m high-resolution Shajinping loess section in Lanzhou on the western Chinese Loess Plateau records a 60 ka, millennial summer monsoon variation. The record shows that Asian summer monsoons have rapid episodic pulse enhancements spanning only ca. 1–2 ka in high-frequency domain and having sub-Milankovitch cycles of progressive weakening in low frequency domain in the last glaciation. Soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements, resulting in weakly or moderately developed paleosol sequences. Both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm (Dansgaard–Oeschger) episodes and long-term cooling (Bond) cycles of the North Atlantic climatic records, indicating a possible teleconnection between tropic oceanic air masses and the North Atlantic climatic system. But differences exist for the transition of MIS 2/3 and the Holocene, where extraordinarily heavy dust-input events and fairly variable climatic fluctuations occur for the former and latter, respectively. A westerlies-swing model is proposed to interpret this link.

Lanzhou Univ, Minist Educ, Key Lab Western Chinas Environm Syst, Lanzhou 730000, Peoples R China

Lanzhou Univ, Minist Educ, Key Lab Western Chinas Environm Syst, Lanzhou 730000, Peoples R China