Rixiang Zhu

Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China

The initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era. But the dating of this transition is uncertain, partly because desert sediments are usually scattered, discontinuous and difficult to date. Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago, on the basis of palaeomagnetic measurements and fossil evidence. A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils. This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch, at least 14 million years earlier than previously thought. Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia.

New evidence on the earliest human presence at high northern latitudes in northeast Asia

The timing of early human dispersal to Asia is a central issue in the study of human evolution. Excavations in predominantly lacustrine sediments at Majuangou, Nihewan basin, north China, uncovered four layers of indisputable hominin stone tools. Here we report magnetostratigraphic results that constrain the age of the four artefact layers to an interval of nearly 340,000 yr between the Olduvai subchron and the Cobb Mountain event. The lowest layer, about 1.66 million years old (Myr), provides the oldest record of stone-tool processing of animal tissues in east Asia. The highest layer, at about 1.32 Myr, correlates with the stone tool layer at Xiaochangliang, previously considered the oldest archaeological site in this region. The findings at Majuangou indicate that the oldest known human presence in northeast Asia at 40° N is only slightly younger than that in western Asia. This result implies that a long yet rapid migration from Africa, possibly initiated during a phase of warm climate, enabled early human populations to inhabit northern latitudes of east Asia over a prolonged period.

Tectonic evolution of the Tancheng‐Lujiang (Tan‐Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data

The north-striking Tancheng-Lujiang (Tan-Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2-E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan-Lu during the North China Block (NCB)-SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3-E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North-South China collision and account for Tan-Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling-Dabieshan and Sulu UHP belts while Tan-Lu acted as a transform fault that connected the two subduction zones, or (2) Tan-Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan-Lu. After North and South China impacted near Dabieshan, the Tan-Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan-Lu, with both right lateral strike-slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India-Asia collision.

Destruction of the North China Craton

A National Science Foundation of China (NSFC) major research project, Destruction of the North China Craton (NCC), has been carried out in the past few years by Chinese scientists through an in-depth and systematic observations, experiments and theoretical analyses, with an emphasis on the spatio-temporal distribution of the NCC destruction, the structure of deep earth and shallow geological records of the craton evolution, the mechanism and dynamics of the craton destruction. From this work the following conclusions can be drawn: (1) Significant spatial heterogeneity exists in the NCC lithospheric thickness and crustal structure, which constrains the scope of the NCC destruction. (2) The nature of the Paleozoic, Mesozoic and Cenozoic sub-continental lithospheric mantle (CLM) underneath the NCC is characterized in detail. In terms of water content, the late Mesozoic CLM was rich in water, but Cenozoic CLM was highly water deficient. (3) The correlation between magmatism and surface geological response confirms that the geological and tectonic evolution is governed by cratonic destruction processes. (4) Pacific subduction is the main dynamic factor that triggered the destruction of the NCC, which highlights the role of cratonic destruction in plate tectonics.

Wind-blown origin of the Pliocene red clay formation in the central Loess Plateau, China

In order to determine the depositional processes of the Pliocene red clay formation deposited directly beneath the Plio–Pleistocene loess in the Chinese Loess Plateau, four red clay sections spanning over 400 km are studied. Grain size analysis of closely spaced samples in the sections shows that the particles of the red clay are very fine with the sand fraction (>63 μm) being negligible, and that all the curves indicating changes in different grain size parameters have a similar pattern. The grain size records in the upper part of the four sections are almost identical with the median grain size centered at 4–8 μm. The REE patterns of 28 samples from the Jiaxian red clay section are all characterized by LREE enrichments, relatively flat HREE and slight negative Eu anomaly, being similar to those of eolian loess and the average upper continental crust. These lines of evidence point to a wind-blown origin of the red clay deposits. Accumulation of the loess–soil sequences in the Chinese Loess Plateau during the past 2.6 Ma can be therefore regarded as the continuation of the Pliocene atmospheric dust deposition. Observations of spatial grain size changes in the Loess Plateau suggest that the eolian red clay might be transported mainly by the westerlies, differing significantly from the overlying loess that was transported essentially by the East-Asia winter monsoonal winds.

Timing of the Jiufotang Formation (Jehol Group) in Liaoning, northeastern China, and its implications

The timing of the Jiufotang Formation remains speculative despite recent progress in the study of the Jehol Biota. In this paper we contribute to this topic with Ar-40/Ar-39 dating on K-feldspar (sanidine and orthoclase) from tuffs interbedded within the fossil-bearing shales of the Jiufotang Formation, from the upper part of the Jehol Group in Chaoyang, Liaoning, northeastern China. Ar-40/(39) Ar step heating analyses of K-feldspar and the SHRIMP U-Pb zircon data indicate that tuffs at the Shangheshou section erupted at 120.3 +/- 0.7 million years ago. This result confirms an Aptian age for the Jiufotang Formation that was mainly based on biostratigraphic evidence. It also places stringent controls on the age of the fossils from the formation, providing a minimum age (120 Ma) for the four-winged dinosaur, Microraptor, and the seed-eating bird, Jeholornis.

Earliest presence of humans in northeast Asia

The timing of the earliest habitation and oldest stone technologies in different regions of the world remains a contentious topic in the study of human evolution. Here we contribute to this debate with detailed magnetostratigraphic results on two exposed parallel sections of lacustrine sediments at Xiaochangliang in the Nihewan Basin, north China; these results place stringent controls on the age of Palaeolithic stone artifacts that were originally reported over two decades ago. Our palaeomagnetic findings indicate that the artifact layer resides in a reverse polarity magnetozone bounded by the Olduvai and Jaramillo subchrons. Coupled with an estimated rate of sedimentation, these findings constrain the layers age to roughly 1.36 million years ago. This result represents the age of the oldest known stone assemblage comprising recognizable types of Palaeolithic tool in east Asia, and the earliest definite occupation in this region as far north as 40° N.

Timing of the Tianshan Mountains uplift constrained by magnetostratigraphic analysis of molasse deposits

Abstract The Tianshan Mountains lie in the actively deforming part of the India–Asia collision zone. The thrust faulting and folding of the Neogene and Quaternary sediments, forming linear, roughly east–west anticlines at the northern and southern borders of the orogenic belts, indicate crustal shortening and thickening in late Cenozoic time. Detailed magnetostratigraphic studies of the molasse deposits of the folded Neogene and Quaternary strata in the northern edge of the Tianshan Mountains indicate that the onset of uplift occurred at ∼7 Ma. Deposition of the thick conglomerates between 7 and 2.58 Ma in the studied region was mainly in response to the tectonic uplift, while the accumulation of the early Pleistocene conglomerates (Xiyu Formation) was mostly controlled by both tectonics and climatic cooling. Our results indicate that the present high relief of the Tianshan Mountains is the result of two dominant phases of uplift occurring at about 7–2.58 Ma and the early Pleistocene.

Mineral magnetic properties of loess/paleosol couplets of the central loess plateau of China over the last 1.2 Myr

[1] We have conducted a multiparameter investigation of 15 loess-paleosol couplets (S0/L1 to S14/L15) from the Jiaodao section in the central loess plateau of China using environmental magnetic approaches coupled with soil science techniques. The magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis. High-temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the Lishi Formation to the Holocene black loam, possibly indicating a decrease in weathering intensity. This may reflect a long-term increase in aridity and/or a general long-term cooling trend of the interior of the Asian continent from 1.2 Ma to the present. Several samples display wasp-waisted hysteresis loops. These are most pronounced in moderately enhanced paleosols, less pronounced in the practically unaltered loess, and subdued in the well-developed paleosols, but wasp waistedness reappears in the most developed paleosols. This wasp-waistedness sequence suggests that the composition, concentration, and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess-paleosol sequence, but each factor has a different effect at different stages of pedogenesis.

Quantifying grain size distribution of pedogenic magnetic particles in Chinese loess and its significance for pedogenesis

Quaternary glacial/interglacial cycles have been imprinted on the Chinese loess/paleosol sequences through pedogenesis. In order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution (GSD). To date, the GSD has not been accurately determined because of the dearth of available means for analyzing extremely fine grained (nanometer-scale) pedogenic magnetic particles. Using low-temperature techniques, we systematically investigated the temperature dependency of χ fd (defined as χ1Hz − χ10Hz, where χ1Hz and χ10Hz are AC magnetic susceptibility measured at 1 and 10 Hz, respectively) from two characteristic loess profiles, one located at the western Chinese Loess Plateau and the other in the central plateau. On the basis of Neel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures, a quantitative GSD for pedogenic particles in Chinese loess/paleosols was constructed. We found that the dominant magnetic grain size lies just above the superparamagnetic/single-domain threshold (∼20–25 nm) and that the GSD is almost independent of the degree of pedogenesis. This observation agrees well with other constraints from previous studies. This new GSD model improves our understanding of the pedogenic processes in Chinese loess, enabling further explicit linkage of environmental magnetism to paleoclimate changes.