Loess-red Clay Sequences Research Articles

Iron oxide characteristics of the Chinese loess-red clay sequences and their implications for the evolution of the East Asian summer monsoon since the Late Oligocene

The most significant climatic change in East Asia involved the formation and evolution of the coupled monsoon-arid environmental system during the Cenozoic, which was dynamically linked to global cooling and the growth of the Qinghai-Tibet Plateau. However, our understanding of monsoon evolution since the Late Oligocene remains controversial due to differences in the interpretation of various environmental proxies from loess archives. In this study, the composition and relative abundance of pedogenic iron oxides of two eolian sequences from the Lingtai section and the Zhuanglang drill core were investigated using rock magnetic, optical spectroscopy, and geochemical analyses. The results indicate that while the composition of iron oxides in the Mio-Pliocene red clay and Quaternary loess-paleosol sequences is similar, the concentrations of magnetite, maghemite, hematite and goethite are quite different between loess, paleosol and red clay. The combination of magnetic proxies (magnetic susceptibility, Fed/Fet, and color a* values) reveals that precipitation and temperature played varying roles in driving pedogenesis and chemical weathering since the Late Oligocene. During the Late Oligocene to Early Miocene, the occurrence of relatively strong pedogenesis and chemical weathering was likely the result of high temperatures rather than high precipitation. The East Asian summer monsoon intensified during the Middle Miocene Climate Optimum (16.5–14.5 Ma), in response to global climatic warming; subsequently, during the Middle to Late Miocene, the monsoon intensity gradually weakened due to global cooling. After ~8 Ma, the summer monsoon strengthened substantially and reached a maximum during 4.5–2.6 Ma, possibly as a result of the uplift of the northeastern margin of the Qinghai-Tibetan Plateau during the Late Miocene. The monsoon intensity then commenced a decreasing trend in the Early Pleistocene, but then strengthened again after ~1.0 Ma.

Grain-size analysis of Upper Pliocene red clay deposits from Linxia Basin: Implications for Asian monsoon evolution on the NE margin of the Tibetan Plateau

Little is known about the evolution of the East Asian winter monsoon (EAWM) on the NE margin of the Tibetan Plateau (TP) during the late Pliocene-early Pleistocene, due to the poor exposure of pre-Quaternary eolian deposits and ambiguities in the interpretation of paleoclimatic proxies. In this study, we performed detailed grain-size analyses of eolian sequences (spanning 3.7–2.4 Ma) from two new boreholes in Linxia Basin, in the NE TP. We compared our results with contemporaneous loess-red clay sequences across the Chinese Loess Plateau (CLP) and identified a substantial decrease in the thickness and grain size of upper Pliocene-lower Pleistocene loess and red clay sediments from the western to central parts of the CLP. This observation supports the view that the grain size of upper Pliocene eolian sequences from Linxia Basin can be used to reconstruct the evolution of the EAWM. In addition, it demonstrates that the modern spatial pattern of the EAWM regime in the western CLP has been maintained since at least the late Pliocene. Records of both median grain-size (Md) and sand content from the boreholes exhibit an overall upward-increasing trend during the interval from ~3.7–2.4 Ma, indicating a long-term stepwise intensification of the EAWM and aridity on the NE margin of the TP. In addition, a dramatic coarsening of the grain size in loess unit L34 documents a large climatic shift at ~2.75 Ma, which may be closely linked with the major intensification of Northern Hemisphere glaciation during the late Pliocene.

A new record of late Pliocene-early Pleistocene aeolian loess–red clay deposits from the western Chinese Loess Plateau and its palaeoenvironmental implications

The loess–red clay sequences in northern China provide high-resolution terrestrial records of Asian monsoon evolution and aridification of the Asian interior. To date, however, aeolian deposits of late Pliocene-early Pleistocene age (3.5–2.4 Ma) have only rarely been reported from the western Chinese Loess Plateau (CLP), which significantly hinders our understanding of the distribution of aeolian deposits and the palaeoenvironmental evolution of the region. Here, we present magnetostratigraphic, lithologic and magnetic susceptibility results for two recently-drilled boreholes from the north bank of Baxie River, central Linxia Basin, which are highly correlative with those of the loess–red clay deposits spanning the interval from 3.6 to 2.4 Ma in the eastern CLP. Our results provide the first direct evidence for the occurrence of late Pliocene-early Pleistocene aeolian deposits in the western CLP and provide new insights into the distribution of aeolian deposits in northern China. The spatial coherence of the magnetic susceptibility fluctuations further indicates that magnetic susceptibility is a powerful tool for stratigraphic correlation of late Pliocene aeolian deposits in the western CLP. In addition, our results demonstrate that erosional events may have occurred in the early or middle Pleistocene, and they may provide new insights into the reasons for the absence of loess–red clay deposits from 3.5 to 2.4 Ma in most parts of the western CLP.

Unmixing hysteresis loops of the late Miocene-early Pleistocene loess-red clay sequence.

Magnetic paleoclimatic records often represent mixed environmental signals. Unmixing these signals may improve our understanding of the paleoenvironmental information contained within these records, but such a task is challenging. Here we report an example of numerical unmixing of magnetic hysteresis data obtained from Chinese loess and red clay sequences. We find that the mixed magnetic assemblages of the loess and red clay sediments both contain a component characterized by a narrow hysteresis loop, the abundance of which is positively correlated with magnetic susceptibility. This component has grain sizes close to the superparamagnetic/stable single domain boundary and is attributed to pedogenic activity. Furthermore, a wasp-waisted component is found in both the loess and red clay, however, the wasp-waisted form is more constricted in the red clay. We attribute this component to a mixture of detrital ferrimagnetic grains with pedogenic hematite. The abundance of this component decreases from the base to the top of the red clay, a pattern we attribute to decreased hematite production over the Chinese Loess Plateau (CLP) due to long-term climate cooling. This work demonstrates the potential of hysteresis loop unmixing to recover quantitative paleoclimatic information carried by both low and high coercivity magnetic minerals.

A long-term increasing aridification and cooling trend at the Chinese Loess Plateau during the Pliocene

The development of Asian aridification and cooling is a research hotspot nowadays. Loess-Red Clay sequence in the Chinese Loss Plateau provides an excellent terrestrial archive of such paleoclimatic evolution. This paper applied the magnetic “unmixing” technique to isolate the magnetic components of the Red Clay sequence for the first time. The Jiaxian section in the north edge of Chinese Loess Plateau is sensitive to paleoclimate changes because it is in the transitional zone of the monsoon and non-monsoon area. Systematic magnetic measurements and XRD analysis consistently indicate that the magnetic components of Red Clay are predominantly magnetite, maghemite and hematite. The strongly developed paleosols exhibit high content of soft magnetic minerals, higher Ms and Mrs, and lower Bc and Bcr. While the weakly developed paleosols and loess samples display the opposite features. With increasing alternating field, the contributions of soft components to SIRMx mT are cleaned gradually (SIRM is the saturation isothermal remanent magnetization; SIRMx mT represents the residual SIRM after alternating field demagnetization at x mT). The significant contributor to SIRM100 mT is hematite, an indication of the intensity of chemical weathering. Variations in the ratios of SIRM100 mT/SIRM, SIRM100 mT/SIRM60 mT, and SIRM100 mT/SIRM30 mT document a long-term up-section decreasing trend during the last 5–2.6 Ma, with similar trends to the long-term up-section increase in δ18O values of marine records. Decreased hematite content indicates weakened chemical weathering, which further implies an increasing aridification and cooling and a gradually weakened summer paleomonsoon mainly in the depositional area (Chinese Loess Plateau). In some loess layers, where hematite is mainly of aeolian origin, the mineral magnetic signals may indicate aridification and cooling in the Asian interior (source region). This long-term aridification and cooling trend is probably attributed to the comprehensive activity of ongoing global cooling, progressive uplift of the Tibetan Plateau and changes in land–sea distribution.

Luminescence sensitivities of quartz grains from eolian deposits in northern China and their implications for provenance

The thermoluminescence (TL) and optically stimulated luminescence (OSL) sensitivities of quartz grains from deserts and loess–red clay sequences are used to trace eolian provenances in northern China. Our results indicate that the 110°C TL peak and OSL sensitivities of quartz grains show differences among Chinese deserts, which can be subdivided into four groups according to the spatial variations of luminescence sensitivities. Such differences are related mostly to the regional difference in rock types of mountains surrounding or adjacent to the deserts. We also examine the possible provenance changes between the Quaternary loess and the Tertiary eolian red clay, and the results indicate that the luminescence sensitivity of Tertiary red clay is higher than that of Quaternary loess (L1, L15, and L33), implying source materials of the eolian deposits changed relative to those of the Quaternary.

New eolian red clay sequence on the western Chinese Loess Plateau linked to onset of Asian desertification about 25 Ma ago

The expansion of inland Asia deserts has considerably influenced the environmental, social and economic activities in Asia. Aridification of inland Asia, especially timing of the initiation of Asian desertification, is a contentious topic in paleoclimatology. Late Cenozoic eolian loess-red clay sequences on the Chinese Loess Plateau, which possess abundant paleoclimatic and paleo-environmental information, can be regarded as an indicator of inland Asia desertification. Here we present a detailed magnetostratigraphic investigation of a new red clay sequence about 654 m in Zhuanglang located at the western Chinese Loess Plateau. Sedimentological, geochemical, mineralogical, and quartz morphological lines of evidence show that the red clay is of eolian origin. Magnetostratigraphic correlations indicate that this core sequence spans from 25.6 to 4.8 Ma, and typical eolian red clay appears as early as 25 Ma. This extends the lower limit of the red clay on the Chinese Loess Plateau from the previously thought early Miocene back into the late Oligocene. This new red clay record further implies that the inland Asia desertification was initiated at least by the late Oligocene. This sequence provides a unique high-resolution geological record for understanding the inland Asia desertification process since the late Oligocene.

A ∼6 Ma chemical weathering history, the grain size dependence of chemical weathering intensity, and its implications for provenance change of the Chinese loess–red clay deposit

Major element chemical analyses of the Baishui loess–red clay sequence (35° 24′ 10″N, 106° 54′ 43″E, Gansu Province, China) reveal that, the chemical weathering intensities inferred from CIA (chemical index of alteration) values have progressively weakened and increased in amplitude with decreasing age. The general trend of the CIA can be correlated well with the oxygen isotope variations of the deep sea sediments. Analyses of multiple size fractions (<5 μm, 5–20 μm and 20–63 μm) indicate that the CIA and the major element abundances are strongly grain size dependent. The chemical alternation of all three size fractions is relatively weak, and the long-term variation in chemical composition of the loess–red clay is mainly controlled by the mixing of fresh detritus from different source areas. The CIA variability of the coarse fractions (5–20 μm and 20–63 μm) within the section is apparently different from that of the fine fraction (<5 μm), implying a different erosion–transportation–deposition–weathering history. From 1 Ma onward, CIA values for all size fractions decrease, indicating an increasing input of fresh or poorly weathered detritus in the desert source regions of loess deposits. Sequential variations in the K 2O/Al 2O 3, Na 2O/Al 2O 3, CaO/Al 2O 3 and MgO/Al 2O 3 ratios from 4.5 Ma to about 0.6 Ma in bulk and size fractioned samples may reflect provenance change as well as changing chemical weathering processes.

An 8.1Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau

Carbonates in loess-red clay sequences consist mainly of calcite and dolomite. The EDTA analysis of carbonates in different size fractions and magnetic susceptibility reveal that calcite is a sensitive index of summer monsoon. The chemical analysis of carbonates and calcite from an 8.1 Ma loess-red clay sequence at Chaona on the Chinese central Loess Plateau shows that the evolution of the Asian summer monsoon experienced four stages, namely 8.1―5.5 Ma, 5.5―2.8 Ma, 2.8―1.5 Ma and 1.5―0 Ma, with increasing intensification and fluctuation, suggesting a possible combining impacts of uplift of the Tibetan Plateau and global changes on the Asian summer monsoon.

Red clay sediment in the central Chinese Loess Plateau and its implication for the uplift of the Tibetan Plateau

The widely distributed red clay sediment underlying the Chinese Loess Plateau truly records the Neogene environmental evolution, and its genesis and development are intrinsically related to the uplift processes of the Tibetan Plateau and the evolution of East Asia monsoon system. In this paper, a detailed magnetostratigraphy of a loess-red clay section (107°13′E, 35°02′N) from the central Loess Plateau is reported. The loess-red clay sequence is composed of 175 m Quaternary loess-paleosol sequence and 128 m Neogene red clay sediments. Based on the correlation with the standard geomagnetic polarity time scale, the paleomagnetic results indicate that the age of Chaona red clay sequence extends to 8.1 Ma, which is the older red clay deposition in the central Chinese Loess Plateau. The commencement of red clay at ∼8.1 Ma may imply that the Ordos planation surface was broken by the movement of the Haiyuan-Liupanshan Faults, which was related to the uplift of the Tibetan Plateau induced by the collision of India Plate and Eurasian Plate. And the western part adjacent to the Tibetan Plateau was uplifted to form the embryo of the Liupan Shan (Mts.) and the eastern part was down-faulted to receive red clay deposition. We link this faulting to an initial uplift of the Tibetan Plateau. The undulating nature of the broken Ordos planation surface may explain the chronological differences and depth discrepancies among various cross-sections of red clay.

The evolution of coupling of Asian winter monsoon and high latitude climate of Northern Hemisphere

The evolution and driving mechanism of the Asian winter monsoon system are of great importance to understanding the present-day climate. Through high-resolution particle size analysis of the oldest loess-red clay sequence known so far (with a basal age of about 8 Ma) and comparison of the results with oxygen isotope curves from North Atlantic marine sediments, 4 stages of the evolution of the Asian winter monsoon were clearly demonstrated. During the first stage, between about 8.1 and 4.3 Ma, there was no relation between Asian winter monsoon and Northern Hemisphere ice volume and high latitude climate inferred from marine sediments. A weak relation developed during the second stage, about 4.3 to 3.5 Ma. During the third stage (3.5 to 2.6 Ma) an Asian winter monsoon system similar to the present formed, initiating a stronger relation between the winter monsoon and Northern Hemisphere ice volume and high latitude climate. In the final stage (2.6 to 0 Ma) the present Asian winter monsoon system was fortified and stabilized and changes in the winter monsoon system were almost in phase with Northern Hemisphere ice volume and climate. The staggered uplift of Tibetan Plateau at ≈8, 3.6, 2.6 Ma and later might be the driving force for the evolution of the Asian winter monsoon.

The evolution of coupling of Asian winter monsoon and high latitude climate of Northern Hemisphere ——Grain evidence from 8.1 Ma loess-red clay sequence on the Chinese central Loess Plateau

The evolution and driving mechanism of the Asian winter monsoon system are of great importance to understanding the present-day climate. Through high-resolution particle size analysis of the oldest loess-red clay sequence known so far (with a basal age of about 8 Ma) and comparison of the results with oxygen isotope curves from North Atlantic marine sediments, 4 stages of the evolution of the Asian winter monsoon were clearly demonstrated. During the first stage, between about 8.1 and 4.3 Ma, there was no relation between Asian winter monsoon and Northern Hemisphere ice volume and high latitude climate inferred from marine sediments. A weak relation developed during the second stage, about 4.3 to 3.5 Ma. During the third stage (3.5 to 2.6 Ma) an Asian winter monsoon system similar to the present formed, initiating a stronger relation between the winter monsoon and Northern Hemisphere ice volume and high latitude climate. In the final stage (2.6 to 0 Ma) the present Asian winter monsoon system was fortified and stabilized and changes in the winter monsoon system were almost in phase with Northern Hemisphere ice volume and climate. The staggered uplift of Tibetan Plateau at ~8, 3.6, 2.6 Ma and later might be the driving force for the evolution of the Asian winter monsoon.

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.

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

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.

Eolian evidence from the Chinese Loess Plateau: the onset of the Late Cenozoic Great Glaciation in the Northern Hemisphere and Qinghai-Xizang Plateau uplift forcing

On the basis of a newly-constructed record of magnetic susceptibility (SUS) and the depositional rate change of eolian loess-red clay sequences in the last 7.2 Ma BP from the hea Plateau, together with a cornperison of a record of °18O values from the equatorial East Pacific Ocean and eolian Quartz flux variations fmm the North Pacific Ocean, the evolutiomuy process of the Late Cenozoic Great Glaciation in the Northern Hemisphere can be divided into three stages: the arrival stage around 7.2–3.4 Ma BP, the initial stage at about 3.4—2.6 Ma BP, and the Great Ice Age since 2.6 Ma BP. The evolution of the East Asian monsoon is characterized by paid winter and summer monsoons, and it is basically composed of the initial stage of weak winter and summer monsoons, the transitional stage of simultaneous increase in intensity of winter and summer monsoons, and the prevailing stage of strong winter and week summer monsoons, or weak winter and strong summer monsoons. The Late Cenowic global tectonic uplift, paaicdarly the Qinghai-Xizang Plateau uplift and the associated CO2 concentration variation, controls the dng processes of the onset of Great Glaciation and the long-term changes of East Asian monsoom climate in the Northern Hemisphere to a large extent. The accelerating uplift of the Qinghai-Xizang Plateau between 3.4 and 2.6 Ma BP provided an important driving force to global climiatic change.

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.