Tungsheng Liu

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.

Towards an orbital time scale for chinese loess deposits

Abstract In this study, we analyzed the grain size of the Baoji loess-soil sequence at 10 cm intervals. Results show that grain size variations are very sensitive to loess-soil alternations, with loess units being much coarser than soils. Such a change in loess-soil grains is interpreted as the result of the glacial-interglacial cyclic variations in the intensity of the winter monsoon winds out of Siberia. The grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by Berger and Loutre (1991) under the control of major magnetic reversals. The tuning is independent of any correlation with δ18O signals in the deep-sea sediments. The resulting grain size time scale is tightly constrained, as suggested by the following facts: (1) the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records; (2) ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the K Ar -dated ages; (3) there is close coherence between the Baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0–2.5 Ma period; and (4) the grain size record on the orbital time scale shows a close similarity to the orbitally-tuned DSDP Site 607 and ODP Site 677 δ18O records. Examination of time-dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long-term monsoonal cycles, one occurring at about 0.8 Ma BP and the other around 1.7 Ma BP.

Pedostratigraphy and paleomagnetism of a ∼7.0 Ma eolian loess–red clay sequence at Lingtai, Loess Plateau, north-central China and the implications for paleomonsoon evolution

Abstract A 305-m-thick loess–red clay sequence was discovered recently at Lingtai, which is located in the middle part of the Chinese Loess Plateau. It consists of a complete Pleistocene loess–soil sequence with a thickness of about 175 m and 130 m of Tertiary red clay deposits. The red clay sequence at Lingtai is the thickest one presently known in the Loess Plateau. Field observations show that the Lingtai red clay contains over 110 couplets of pedogenic B horizons and horizontal carbonate nodule horizons, and that it can be regarded as an extremely thick soil complex. Paleomagnetic studies of 680 samples suggest that the basal age of the Lingtai loess–red clay sequence is about 7.05 Ma. Grain size analysis of samples taken at 3.3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess, thus suggesting a wind-blown origin of the Tertiary red clay and continuous atmospheric dust deposition in the Loess Plateau during the last 7.05 Ma. The pedogenic characteristics of the paleosols within the Pleistocene loess and the B horizons in the red clay suggest that the East-Asia summer monsoon in the latest Miocene may have already been slightly stronger than that during the Holocene. This implies that at about 7.5 Ma BP, the Tibetan Plateau could have been uplifted to a critical height in maintaining the East-Asia summer monsoon system. Observations of the loess–red clay sequence also suggest that the long-term changes in the East-Asia summer monsoon strength have been nonlinear since the latest Miocene.

Climate extremes in Loess of China coupled with the strength of deep-water formation in the North Atlantic

The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly 18 . developed S4, S5-1 and S5-3 soils corresponding to the marine d O stages 11, 13, and 15, respectively as evidence of 18 three episodes of great warmth and two coarse-grained loess units L9 and L15, corresponding to the marine d O stages 22, . 23, 24 and 38, respectively which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle. . Paleopedological, geochemical and magnetic susceptibility data from three loess sections Xifeng, Changwu and Weinan suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated . . mean annual temperature MAT of at least 4-68C higher and a mean annual precipitation MAP of 200-300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt the contact of the monsoonal . northward warm-humid air mass with the dry-cold southward one might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5-38C and 150-250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region. Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatestrsmallest Atlantic-Pacific 13 . d C gradients, respectively, indicating their relationships with the strength of Deep Water NADW production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 10 4 years. q 1998 Elsevier Science B.V. All rights reserved.

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.

Geochemistry of the Pliocene red clay formation in the Chinese Loess Plateau and implications for its origin, source provenance and paleoclimate change

Abstract Recent field observations and sedimentological studies suggest the eolian origin of the late Tertiary red clay formation underlying the Pleistocene loess-soil sequence in the Loess Plateau, thereby extending the wind-blown record from ∼2.6 Ma down to ∼7.0 Ma in north-central China. To address the source provenance of the red clay, major and trace element concentrations were analyzed on the samples from a north-south loess transect of the Loess Plateau and a red clay section at Jiaxian. Results show that there is a good agreement of both major and trace element compositions between the last glacial-interglacial loess-soil units and the Jiaxian red clay record. The REE distribution patterns of the loess-soil and red clay samples are remarkably similar in shape, with enriched LREE and fairly flat HREE profiles and clear negative Eu anomaly. The geochemical features of the red clay are also identical to those of upper continental crust. These chemical characteristics provide further evidence for eolian sedimentation of the red clay. From the relations both between Na2O/Al2O3 and K2O/Al2O3 ratios and between U/Pb and Th/Pb ratios, the eolian loess and red clay materials must have been subject to thorough sedimentary differentiation and moderate chemical weathering in the source area. The chemical similarity between soil unit S1 and the red clay implies that such an environment as occurred in the last interglacial period may have already formed in northwestern China during the late Tertiary. Comparison of down-section variations in elemental ratios with field observations of pedogenic development of the red clay suggests that CaO/Al2O3, Sr/Rb and Sr/Ba are more sensitive to climate changes than magnetic susceptibility, and can be used to reconstruct the climate history of the late Cenozoic.

Quantitative estimates of palaeoprecipitation at Xifeng, in the Loess Plateau of China

Abstract Using10Be, recent studies (Heller et al., 1993; Beer et al., 1993) have tried to reconstruct palaeoclimatic records by separating the secondary (pedogenic) and primary (aeolian; detrital) contributions to the magnetic susceptibility of Chinese loess. Here we use a technique, which monitors the temperature dependence of magnetic remanence, to estimate the magnetisation and relative contribution of the superparamagnetic (SP), single-domain (SD) and multidomain (MD) ferrimagnetic components in the Xifeng loess section. On the assumption that the SP ferrimagnetic component is dominantly of pedogenic origin, the magnetisation of this component has also been determined for samples taken from the topmost palaeosol (S0), at six sites over the Loess Plateau, and directly correlated with present day precipitation at these sites. This correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the Xifeng section, which covers about the last 140,000 years. The data indicate a considerably reduced precipitation during the glacial, loess-forming periods and a significantly higher level of precipitation than today during the last interglacial period.

Summer monsoon variations over the last 1.2 Ma from the weathering of loess-soil sequences in China

The loess-soil sequence in northern China contains a near continuous record of Quaternary paleoclimate. Magnetic susceptibility and grain size have so far been the only proxies available to address the long-term changes of the East-Asian paleomonsoon extending back to more than one million years. In this study, the ratio of CBD (citrate-bicarbonate-dithionite)-extractable free Fe2O3 (FeD), a measure of iron liberated by chemical weathering, versus the total Fe2O3 available (FeT) was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1.2 Ma. Variations of this index are highly consistent with other pedological indicators, but in addition provide a quantitative measurement of the degree of pedogenesis in the Loess Plateau. Since chemical weathering in the region mainly depends upon summer precipitation and temperature, weathering intensity primarily reflects changes in the East-Asian summer monsoon. The new proxy has been used to document a series of summer monsoon changes of global significance, which are not necessarily recorded by magnetic susceptibility.

Iron geochemistry of loess and red clay deposits in the Chinese Loess Plateau and implications for long-term Asian monsoon evolution in the last 7.0 Ma

Abstract Recently, some groups of authors have demonstrated that the Tertiary red clay deposits underlying the Pleistocene loess–paleosol sequence in the Chinese Loess Plateau are wind-blown in origin, thus extending the eolian record in the Plateau from 2.6 Ma back to about 7.0 Ma, and providing a good opportunity to reconstruct long-term monsoon changes. As magnetic susceptibility, the widely used paleomonsoon proxy of loess deposits, is problematic in reconstructing the monsoon history recorded in the red clay, the development of other proxies is urgently needed. In this study, we analyzed the ratios of citrate–bicarbonate–dithionite (CBD)-extractable Fe2O3 to total Fe2O3 concentrations in four loess sections along a north–south transect of the Loess Plateau, and in the Lingtai loess–red clay sequence of ∼305 m thick. The Fe2O3 ratio in the loess sections clearly shows higher values in soil units than in loess horizons, and a southward increase in this ratio is also remarkable along the transect. These are in good agreement with the temporal–spatial variations in the East Asia summer monsoon intensity identified by other monsoon proxies and soil development observations. The Fe2O3 ratio thus attests to being a good proxy indicator of the summer monsoon evolution. The Lingtai Fe2O3 ratio record shows high values over three time intervals: ∼4.8–4.1 Ma, ∼3.4–2.6 Ma, and during the interglacial periods of the past 0.8 Ma. The increase in summer monsoon intensity over the three intervals also coincides with the well-developed soil characteristics. It is therefore concluded that the East Asia summer monsoon has experienced a non-linear evolution since the late Miocene. The occurrence of a monsoon prime at about 4.8–4.1 Ma implies that the Tibetan Plateau, one of the most important boundary conditions in maintaining the monsoon circulation, must have been uplifted to a critical height at that time.

Preliminary magnetostratigraphy of a thick eolian red clay‐loess sequence at Lingtai, the Chinese Loess Plateau

Magnetic remanence and bulk susceptibility measurements are reported for a 305-m eolian loess-red clay sequence at Lingtai, the Chinese loess Plateau. Polarity zonation defined by about 680 remanence data yields a basal age of about 7.05 Ma for the section, which represents the oldest wind-blown dust deposited continuously in north-central China during the late Cenozoic. The resultant magnetostratigraphic scheme implies a nearly linear accumulation rate (about 2.98 cm/ka) of atmospheric dust on the Loess Plateau during the period of 7.05 to 2.58 Ma. Comparison of the bulk susceptibility record with the pedogenic characteristics of the red clay observed in the field suggests that magnetic susceptibility in the red clay is not as useful as in the loess-soil sequence to indicate summer monsoon variations.