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The red clay and loess‐paleosol sediments of the Xiaoshuizi and Halagu planation surfaces of the northeastern Tibetan Plateau (TP) provide insights into the response of paleo‐atmospheric circulation and surface processes to TP uplift and global climate changes. We present silicate Sr‐Nd isotope compositions and bulk sediment grain‐size parameters of the red clay and loess‐paleosol sequences, with the aim of reconstructing changes in the transporting wind systems and sediment source regions. The results indicate that the red clay on the planation surface was transported mainly by the westerlies from the Tarim Basin and the Qilian Mountains, while the loess‐paleosol sediments were transported mainly by the East Asian winter monsoon (EAWM) from the Badain Jaran Desert, Tengger Desert, and Qaidam Basin. These shifts in transport mechanism and source were likely related to major changes in the topography of the TP and the expansion of Northern Hemisphere glaciation since the Middle Pliocene. In addition, we reconstructed the evolution of the paleo‐atmospheric circulation from the Late Miocene to the Middle Pliocene. During the Late Miocene, both the paleo‐EAWM and East Asian summer monsoon (EASM) were weak, and westerly winds dominated the climate of the study area. During the Early Pliocene, both the paleo‐EAWM and EASM exerted an increasing influence on the study area. We ascribe the slight enhancement of the paleo‐EAWM to the gradual growth of the northern TP and the significant enhancement of the paleo‐EASM to global climate changes. Key Points: A major transition in the sediment source and transport dynamic of eolian sediments on the NE TP occurred between the red clay and the overlying loess depositsFrom the Pliocene onward, the paleo‐EAWM and EASM were intensified and exerted an increasing influence on the XSZ planation surfaceThe contribution of the CAO to the XSZ red clay showed an increasing trend from the Late Miocene to the Middle Pliocene [ABSTRACT FROM AUTHOR]

Spatiotemporal variations in East Asian Monsoon (EAM) precipitation during the Quaternary have been intensively studied. However, spatial variations in pre-Quaternary EAM precipitation remain largely uninvestigated, preventing a clear understanding of monsoon dynamics during a warmer climatic period. Here we compare the spatial differences in heavy mineral assemblages between Quaternary loess and pre-Quaternary Red Clay on the Chinese Loess Plateau (CLP) to analyze spatial patterns in weathering. Prior studies have revealed that unstable hornblende is the dominant (∼50%) heavy mineral in Chinese loess deposited over the past 500 ka, whereas hornblende content decreases to <10% in strata older than ∼1 Ma in the central CLP because of diagenesis. In the present study we found that hornblende is the dominant heavy mineral in 2–2.7 Ma loess on the northeastern CLP (at Jiaxian), which today receives little precipitation. Conversely, hornblende content in the upper Miocene-Pliocene Red Clay at Jiaxian is <10%, as in the central CLP. The early Quaternary abundance of hornblende at Jiaxian indicates that the current northwestward-decreasing precipitation pattern and consequent dry climate at Jiaxian must have been initiated since ∼2.7 Ma, preventing hornblende dissolution to amounts <10% as observed in the central CLP. By contrast, the 7 Ma and 3 Ma Jiaxian Red Clay hornblende content is significantly less than that of the Xifeng samples, despite the fact that today Xifeng receives more precipitation than Jiaxian, with expected enhanced hornblende weathering. This suggests that the northeastern CLP received more precipitation during the Late Miocene-Pliocene than at Xifeng, indicating that the precipitation gradient on the CLP was more east–west during the Late Miocene-Pliocene rather than northwest-southeast as it was in the Quaternary. A comparison of magnetic susceptibility records for these sections confirms this inference. We attribute this major change in climatic patterns at ∼2.7 Ma to decreased northward moisture transportation associated with Northern Hemisphere glaciation and cooling in the Quaternary. This study therefore demonstrates the potential usefulness of employing heavy mineral analysis in both paleoclimatic and paleooceanographic reconstructions. [ABSTRACT FROM AUTHOR]

The Pliocene epoch (5.3–2.6 Ma) is the most recent geological interval in which atmospheric CO 2 levels were similar to those of the present day (∼400 ppmv). This epoch is therefore considered to be the best ancient analog for predicting a future anthropogenic greenhouse world. In order to determine the response of the East Asian Summer Monsoon (EASM) rainbelt during Pliocene warmth, a 71.9 m-thick aeolian ‘red clay’ sequence at Pianguan was investigated. Rock magnetic experiments suggest that magnetite of pseudo-single domain size is the dominant remanence carrier in the ‘red clay’ sequence. Magnetostratigraphic data, constrained by lithostratigraphy, show that the polarity zones of the ‘red clay’ section correlate with those between subchrons C2An.2r and C3An.2n of the geomagnetic polarity time scale (GPTS), yielding an age range of 6.9–2.9 Ma. The ‘red clay’ deposits exhibit enhanced weathering intensity over two time intervals, namely 5.23–4.3 Ma and 3.7–2.9 Ma, as evidenced by their well-developed pedogenic characteristics, as well as their high free to total Fe 2 O 3 ratios and high redness (a ∗ ) values, which in turn indicate an increased summer monsoon intensity during most of the Pliocene. Furthermore, the pedogenic characteristics of the well-weathered Pliocene soils were compared with those of paleosol unit S 5 (one of the best-developed soil units found in Pleistocene loess) from the Yulin, Luochuan and Lantian sections, which constitute a north-south transect across the Chinese Loess Plateau (CLP). The Pliocene soils at Pianguan show a pedogenic development similar to the S 5 (∼0.5 Ma) at Luochuan in the central Plateau, which is located some 3.7° latitude south of Pianguan, but this development is much stronger than that observed at Yulin in the north, and weaker than that seen at Lantian in the south. This may imply a more northerly penetration (∼400 km) of the monsoon rainbelt during Pliocene warmth compared with the Pleistocene interglacial period (∼0.5 Ma ago), supporting the prediction that the EASM rainbelt shifts northward in a warmer world. In addition, our results show that Pliocene warmth was unlikely to have been characterized by a permanent El Niño-like state, but was possibly influenced by a La Niña-like state. [ABSTRACT FROM AUTHOR]

The red-clay sequence on the Chinese Loess Plateau (CLP) was deposited during the late Miocene-Pliocene and is encoded with important information of past climate changes. However, it has received much less study in comparison to the overlying Pleistocene loess-paleosol sequence. In this paper, we review recent progress in characterizing the environmental magnetic parameter-based paleoclimate history recorded by the red-clay sequence. Several key conclusions are as follows. 1) the red clay and the loess-paleosol sequences have similar magnetic enhancement mechanisms but magnetic minerals in the red clay sequence have experienced a higher degree of oxidation than in the loess-paleosol sequence. 2) The CLP experienced a cooling and wetting trend from 4.5 to 2.7 Ma, caused by ice sheet expansion and East Asian summer monsoon intensification, respectively. 3) The above conclusions benefit from backfield remanence curve unmixing and comparison of magnetic grain size/concentration records, which are particularly useful in separating the temperature from the precipitation signal. A clear need in future studies is to explore the concentration and the grain size variations of hematite and goethite in the red-clay sequence and their formation mechanisms. The payback would be a clear understanding of climate history during the late Miocene-Pliocene, a possible analog for future warmer climate.

Geochemical characteristics of sediments are responses to physical and chemical alteration in landslides. However, consequences of in situ interactions associated with landslides are difficult to distinguish from those related to long-term weathering in young soft sediments such as loess. In this study, geochemical characteristics of the Kulcs landslide in Hungary are studied to identify the provenance of the loess-paleosol-red clay sequence and geochemical signatures that can potentially be attributed to the effects of landsliding. Results indicate that sliding is largely initiated by the lithological changes within the landslide body. Sediments above the sliding zone closely resemble the non-slipped Pleistocene old loess deposits from Hungary. It is also confirmed that the sliding zone develops in old paleosols in the loess sequence and red clays at its base which are all characterized by the enrichment of Al, K, Na, HO and considerable depletion in Ca and Mg associated with carbonates. Altogether, these geochemical characteristics indicate that chemical weathering trend of unconsolidated landslide sediments is slightly modified by the redistribution of carbonates and decomposition of plagioclase. It is assumed that the distribution of Mn and Ba is modified by the water-sediment interaction in the landslide. [ABSTRACT FROM AUTHOR]

QEMSCAN-based (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) heavy mineral analysis has recently been demonstrated an efficient way to allow a rapid extraction of provenance information from sediments. However, one key issue to correctly obtain a provenance signal using this technique is to clearly separate effects of diagenetic alteration on heavy minerals in sediments, especially in fine-grained loess. Here we compare heavy mineral assemblages of bottom Quaternary loess (L33) and upper Pliocene Red Clay of three sites on the Chinese Loess Plateau (CLP). Two sites (Chaona and Luochuan) with similar modern climate conditions show similar heavy mineral assemblages but contain much less of the unstable heavy mineral amphibole than the drier Xifeng site. This result provides strong evidence supporting that climate-caused diagenesis is an important factor controlling heavy mineral assemblages of fine-grained loess. However, heavy mineral assemblages are similar for loess and paleosol layers deposited after 0.5 Ma on the Chinese Loess Plateau regardless of climate differences, suggesting that time is also a factor controlling heavy mineral assemblages of loess and Red Clay. Our high resolution sampling of the upper Miocene-Pliocene Chaona Red Clay sequence reveals similar heavy mineral compositions with a minor amphibole content, different from the drier Xifeng site results of the same age. This result indicates that the monsoonal climate pattern might have been maintained since the late Miocene. Furthermore, it indicates that the heavy mineral method is promising in tracing provenance for sites northwest of the Xifeng site on the Loess Plateau. [ABSTRACT FROM AUTHOR]

A clear understanding of the provenance of late Cenozoic Chinese loess and the underlying Red Clay deposits will shed light on the history and mechanisms of Asian aridification. Although much progress has been made in understanding the source of Quaternary loess on the Chinese Loess Plateau (CLP), the provenance of the underlying upper Miocene–Pliocene Red Clay sequence is largely unknown. Here we present the first provenance history of the Red Clay sequence based on zircon U–Pb ages from the central CLP. Visual and statistical analyses of the U–Pb age populations and comparison with results from potential source regions reveals that (1) the lowermost Red Clay of the late Miocene (depositional age of ∼8 Ma) is likely sourced from the nearby Liupan Mountains and the Qaidam Basin; (2) the middle Red Clay (5.5–4 Ma) of the early-mid Pliocene is sourced mainly from the Taklamakan desert, transported via lower-level westerly winds; (3) the upper Red Clay of the late Pliocene (∼3 Ma) is sourced from mixed areas, although western source materials from middle-northern Tibetan plateau (including Qaidam Desert sediments and materials eroded from the Qilian Mountains) sediments appear to dominate; and (4) the Quaternary loess is also sourced from mixed source regions, albeit with dominant northern CLP proximal desert sediments transported via winter monsoon winds, which in turn may be transported from mountain source regions of the northeastern Tibet and Gobi Altai via major river systems. This long term shift in sources suggests a progressive eastward aridification during the Pliocene in Asia with the specific timing of provenance shifts synchronous with large-scale climatic transitions and Tibetan uplift, demonstrating that Asian desertification is controlled by both factors. [ABSTRACT FROM AUTHOR]

Abstract: Heavy-mineral analysis is a sensitive technique in constraining provenance of sandstone, but has rarely been applied to loess. Here we report a heavy-mineral study of selected samples from the Luochuan, Xifeng and Caoxian loess-Red Clay sections on the Chinese Loess Plateau, based on the novel QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) technique. We found that heavy mineral assemblages of loess deposited through the past 500kyr are similar and unchanged by post-depositional chemical dissolution. In contrast, in samples deposited from 900ka to 3Ma, the relative proportion of stable minerals tends to increase down section. In addition, the Xifeng samples consistently display higher contents of unstable ferromagnesian minerals than the Luochuan samples. Detailed analysis of surface textures displayed by different minerals by optical methods indicates that such a compositional difference can be ascribed to more effective diagenetic dissolution for the Luochuan section, explained by more extensive percolation of interstitial waters in wetter climatic conditions. Interestingly, heavy-mineral assemblages in the underlying upper Miocene Red Clay from Xifeng (deposition age~7Ma) are similar to those of recent loess deposited since 500ka. This similarity indicates that climate and/or local preservation conditions hampered dissolution reactions, thus helping to preserve an original provenance signal that remained largely unchanged throughout the considered time period. Our study demonstrates that climatically- and time-controlled diagenesis plays a key role in determining the composition of heavy-mineral assemblages contained in loess deposited several hundreds of thousands years ago. We also show that by using both QEMSCAN and traditional optical techniques on the same samples we can obtain fundamental complementary information for a correct interpretation of the heavy-mineral assemblage. [Copyright &y& Elsevier]

Geochemical and mineralogical studies of palaeosols provide essential information for palaeoclimatic and palaeoenvironmental interpretation of continental deposits and can present a proxy for palaeoclimate. Red clays in the central Carpathian Basin (Hungary) (Tengelic Red Clay Formation; Kerecsend Red Clay Formation), overlain by loess-palaeosol sequences, were studied. Results from geochemical climofunctions applied to Upper Pliocene-Lower Pleistocene red clays and palaeosols located in the Carpathian Basin, and clay mineralogy, indicate that the palaeoclimate was considerably more humid and warmer during the Late Pliocene-Early Pleistocene in comparison to modern values. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic grain-size variations have been used as sensitive paleoclimate proxies to investigate the evolution of the East Asian summer monsoon, but their relationship with temperature and precipitation is not entirely clear. Here we find that two magnetic grain-size proxy records (χARM/χLF and χARM/SIRM, where χARM, χLF and SIRM are anhysteretic remanent magnetization susceptibility, magnetic susceptibility measured at 470Hz and saturation isothermal remanent magnetization, respectively) of Chinese loess and red-clay sediments co-vary during the last 6Ma, except between ~4.5 and 2.6Ma, when these two records had opposite trends. We attribute this disparate behavior to the different responses of χARM/χLF and χARM/SIRM to temperature and precipitation during ~4.5–2.6Ma, when temperature and precipitation on the Chinese Loess Plateau were decoupled. A comparison of the loess and red-clay χARM/χLF and χARM/SIRM records with the global ice-volume proxy records reveals that χARM/χLF is more sensitive to temperature variations than χARM/SIRM. The results suggest that temperature on the Chinese Loess Plateau had a cooling trend from ~4.5 to ~2.6Ma, whereas rainfall tended to increase. Our studies demonstrate that joint analysis of loess χARM/χLF and χARM/SIRM records can reveal paleoclimatic information that cannot be revealed by a single parameter. [Copyright &y& Elsevier]