Eastern Chinese Loess Plateau Research Articles

Obliquity-paced summer monsoon from the Shilou red clay section on the eastern Chinese Loess Plateau

Abstract The red clay of the Chinese Loess Plateau (CLP) is an important geological archive for understanding the variability in the late Neogene East Asian monsoon. The periodicity of the summer monsoon of the red clay on the eastern CLP is dominated by eccentricity cycles within the constraints of the palaeomagnetic chronological framework, whereas global climate change characteristics represented by the deep-sea oxygen isotope record at that time show a dominating obliquity cycle. Here, we analyzed the East Asian summer monsoon proxies from the Shilou red clay section with the cyclostratigraphy method. The results show that the summer monsoon variation was dominated by obliquity, the optimum deposition rate was 4.451 cm/kyr, and the floating age of the Shilou red clay section was ca. 1.7 Ma. The late Neogene East Asian summer monsoon inferred from the eastern CLP was thus paced by the obliquity cycle, which is consistent with global change.

Chronology and paleoclimatic context of hominin occupations in the Fenhe River Basin of northern China during the middle to Late Pleistocene

The relationship between climate change and hominin evolution over the Pleistocene in East Asia is still poorly understood, in particular due to the limited number of archaeological finds and fossils with reliable chronologies. Paleolithic sites preserved in loess-paleosol sequences in eastern Chinese Loess Plateau provide prospective evidence for understanding hominin evolution and dispersion across the Middle to Late Pleistocene in northern China. The Fenhe River Basin has been known as a key area for paleolithic sites and fossil hominins. Here, quartz Optically stimulated luminescence (OSL) and K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating was employed on 61 samples from 13 paleolithic sites in the Fenhe River Basin. The reliability of the ages are checked by analysis of luminescence characteristics and comparison between quartz OSL and K-feldspar pIRIR ages. Paleoclimatic records were established using alternating loess-paleosol sequences, and multi-proxy indices of grain size, magnetic susceptibility, CaCO3 and color from the dated sites. These results provide the reliable chronology for hominin presence in the Fenhe River Basin spanning the past three glacial-interglacial cycles. Our results show that this region experienced predominantly dusty, dry and cold conditions during glacials and comparatively moist and warm interglacial conditions, that are sensitive to variations in East Asian Summer and Winter Monsoon (EASM and EAWM respectively) intensities over glacial-interglacial timescales. The Fenhe River Basin is a key geographic refuge for hominins during glacial-interglacial climatic changes in northern China. It was occupied over glacial and interglacial periods spanning at least the last 370 ky, and the evidence for hominin presence is especially clear from ∼250 to 60 ka and since ∼35 ka. The frequency of hominin activity, as indicated by the number of archaeological sites and density of stone artifacts, increased during warm and humid periods of MIS 7 and MIS 5, in contrast to the cold, windy, and dry period of MIS 6. Site numbers increased in MIS 3 and the Last Glacial Maximum, in association with microblade technology. The dating results indicate that hominins with ‘modern’ features may have occurred in the Fenhe River Basin ∼140 ka ago, however, the exact timing of modern human occupation in the region needs a more combined approach consisting of morphological, ancient DNA and geochronological studies.

Mineral magnetic variation of the minle loess/palaeosol sequence of the late glacial to holocene period in the northeastern Tibetan Plateau

SUMMARY Located at the modern junction of the Asian Summer Monsoon and the Westerlies, the northeastern Tibetan Plateau and western Chinese Loess Plateau are in distinct climatic setting from the middle and eastern Chinese Loess Plateau. Besides, Holocene climate variations near the Asian Summer Monsoon margin are highly debated with different models proposed. We carried out environmental magnetic analyses on the Minle loess section, with continuous deposition between 13.3–0.5 ka, located near this margin. Results show that the magnetic susceptibilities of palaeosols are dominantly contributed by superparamagnetic (SP) and single-domain (SD) maghemite and/or magnetite produced by pedogenesis. Limited changes in the content of haematite indicate that dust origin hardly influenced the magnetic mineral variations. The section is divided into four substages according to the changes in magnetic indices and provides a full scene of Holocene climatic evolutions. During the coldest and driest Late Glacial (13.3–11.7 ka), it displays the weakest pedogenesis, as suggested by the lowest SP and SD concentration. The strongest pedogenesis in the Minle section produced higher magnetic susceptibility than the loess records in the Chinese Loess Plateau during the early Holocene Optimum (11.7–8.1 ka). The subsequently declined summer insolation, equatorward migrated Westerlies and the weakened Asian Summer Monsoon led to weak pedogenesis during the middle Holocene (8.1–5.3 ka). Stronger pedogenesis during the late Holocene (5.3–0.5 ka) than the middle Holocene might be due to lower evapotranspiration from weaker insolation and/or to the enhanced winter Westerlies. We attribute the spatiotemporal differences in climatic records of the Chinese loess to precipitation, effective humidity and aeolian geomorphological processes.

Influence of monsoon anomalies on intra-annual density fluctuations of Chinese pine in the Loess Plateau

In the past few decades, the East Asian summer monsoon (EASM) has experienced an unprecedented weakening, exacerbating drought in northern China, especially in the monsoon margin area. Improving our understanding of monsoon variability will benefit agricultural production, ecological construction, and disaster management. Tree-ring is widely used as proxy data for extending the monsoon history. However, in the East Asian monsoon margin, the tree-ring width were mostly formed before the rainy season, thus may have limited ability to indicate the monsoon variability. Intra-annual density fluctuations (IADFs) can provide higher resolution information on tree growth as well as evidence of short-term climate events. Here, we used Chinese pine (Pinus tabuliformis Carr.) samples from the eastern edge of the Chinese Loess Plateau (CLP), where the climate is deeply affected by monsoon, to investigate the response of tree growth and IADFs frequency to climate variation. We show that tree-ring width and IADFs record significantly different climatic signals. The former was mainly affected by moisture conditions at the end of the previous growing season and the current spring. While the latter was common in years when severe droughts occurred in June and July, especially in June. This period coincides with the onset of the EASM, so we further analyzed the relationship between IADFs frequency and the rainy season. Both correlation analysis and the GAM model suggest that the frequent occurrence of IADFs may be related to the late start of the monsoon rainy season, meaning that we have found a new indicator in tree-ring records that can capture monsoon anomalies. Our results provide further insight into drought variation in the eastern CLP, which also implicates the Asian summer monsoon dynamic.

Proximal Supply in Loess Deposition of the Chinese Loess Plateau

AbstractPrevious studies suggest that the northeastern Tibetan Plateau has been the major source area for the Chinese Loess Plateau (CLP); this contribution was used to prove that the Yellow River played a dominant role in transporting the materials from the northeastern Tibetan Plateau, making them available for the CLP. Accordingly, the proximal North China Craton is considered to have increased supply potential of dust to the CLP further eastward. However, the origin of the eastern CLP and the relationship between loess and proximal river sediments have never been systematically studied, which prevents us from working out the surface process between source and sink. In this study, we conducted U–Pb dating and analyses of the grain morphologies of detrital zircons and heavy mineral assemblages on the upper Quaternary loess and riverbed sediments on the eastern and central CLP. We also built a data set (n = 48,161) of the zircon U–Pb ages of the Quaternary loess, deserts, and potential source areas. Our results show that most loess samples on the eastern CLP were strongly affected by the North China Craton. The dust supply ability of rivers to loess is very limited. Along the dust transport path, the proximal mountains, rivers and loess can all provide dust for loess deposition. Our findings emphasize the important role proximal supply has played in the evolution of the CLP since at least the late Neogene, for example, the northeastern Tibetan Plateau for the western and central CLP, and the North China Craton for the eastern CLP.

Tectonic Forcing of the Extreme Aridification of the East Asian Interior at Around 900 ka–Insights From the Spatially Inconsistent Magnetization of Chinese Loess

AbstractThe upper sandy loess unit L9 on the Chinese Loess Plateau (CLP) corresponds to marine isotope stages 22–24, and it represents aeolian deposition under conditions of extreme aridification. However, the forcing mechanism remains controversial. Numerous paleomagnetic studies in the eastern CLP show that the coarsest part of L9 is remagnetized and has a normal geomagnetic polarity. However, our results show that in loess sections in the western CLP the coarsest part of L9 records a primary reverse polarity. This spatially inconsistent magnetization pattern originates mainly from the different magnetic carriers of the characteristic remanent magnetization (hematite in the western CLP and magnetite in the eastern CLP), which suggests a different dust provenance between the western and eastern CLP. We ascribe this spatial contrast in dust provenance to the episodic uplift of the northeastern Tibetan Plateau, which also led to the extreme aridification of the East Asian interior at ∼900 ka.

Insight into spatial-temporal patterns of hydroclimate change on the Chinese Loess Plateau over the past 250 years, using new evidence from tree rings

The climate aridity since the mid-20th century has raised concerns about water resources on the Chinese Loess Plateau (CLP). A lack of extended observation-like precipitation records for the eastern CLP (ECLP) means that it remains unclear whether or not the current arid state of the CLP is unprecedented, and the spatial-temporal characteristics of hydroclimatic variability across the CLP over past centuries are not well understood. Here we present a regional hydrological-year precipitation reconstruction for the Heichashan Mountains, which successfully captures hydroclimate changes on the ECLP since 1773 CE. The reconstruction explains 48.72 % of the observed variance for 1957–2019 CE and reveals a wetting trend since the early 2000s and shows 2014–2020 CE to have been the second wettest period over the past 248 years. 1910–1932 CE was the longest and driest period over the past centuries. Furthermore, the 19th century was relatively wet, whereas the 20th century was dry. We demonstrate that droughts tend to occur in warm periods. Combining our new reconstruction with previously published hydroclimatic reconstructions, we find that hydroclimate has changed synchronously on the ECLP and the western CLP (WCLP) for most of the past two centuries. Some regional differences do exist, for example in the 1890s–1920s, when aridity gradually intensified across the ECLP, no similar drying is evident in records for the WCLP, although the 1920s megadrought occurred in both the ECLP and WCLP. Another difference is in the onset of the 20th-century aridity, which began in the 1950s on the ECLP, around 20 years later than it began on the WCLP. In addition to the known influences of the Asian Summer Monsoon and related large-scale circulations, this work highlights a major finding that the 1920s megadrought may be related to a regime shift in Northern Hemisphere temperature.

Quartz luminescence sensitivity variation in the Chinese loess deposits: the potential role of wildfires

AbstractThe sensitivity of optically stimulated luminescence (OSL) of quartz grains has been applied to trace the provenances of sediments in various environments. Here, we studied the OSL sensitivity of quartz grains in four sections (Jingyuan, Huanxian, Xifeng and Luochuan) along a west–east transect on the Chinese Loess Plateau (CLP). The quartz OSL sensitivity in the palaeosol units is higher overall than that in the loess units, and it shows an increasing trend from the west CLP to the east CLP. In palaeosol units, it increases by a factor of ~20 along the west–east transect; while in loess units it increases by a smaller degree of 3–5. The change of provenances and the change of distances between the provenances and the loess sections cannot fully account for the large degree of sensitivity variation between the loess and palaeosol units in the central and eastern CLP. We propose that wildfires may have played an important role in enhancing the quartz OSL sensitivity in the palaeosol units in the central and eastern CLP. Quartz OSL sensitivity has the potential to be used as a proxy for palaeowildfires.

Groundwater Quality and Associated Human Health Risk in a Typical Basin of the Eastern Chinese Loess Plateau

Groundwater is an important source for drinking, agricultural, and industrial purposes in the Linfen basin of the Eastern Chinese Loess Plateau (ECLP). To ensure the safety of drinking water, this study was carried out to assess the quality using the water quality index (WQI) and potential health risks of groundwater using the human health risk assessment model (HHRA). The WQI approach showed that 90% of the samples were suitable for drinking, and Pb, TH, F−, SO42−, and TDS were the most significant parameters affecting groundwater quality. The non-carcinogenic health risk results indicated that 20% and 80% of the samples surpassed the permissible limit for adult females and children. Additionally, all groundwater samples could present a carcinogenic health risk to males, females, and children. The pollution from F−, Pb, and Cr6+ was the most serious for non-carcinogenic health risk. Cd contributed more than Cr6+ and As to carcinogenic health risks. Residents living in the central of the study area faced higher health risks than humans in other areas. The research results can provide a decision-making basis for the scientific management of the regional groundwater environment and the protection of drinking water safety and public health.

A half-millennium perspective on recent drying in the eastern Chinese Loess Plateau

Northern China has experienced dramatic drying since the 1960s CE. However, the scarcity of long-term observations hampers the evaluation of recent drought variations from a long-term perspective. Here, we present a tree-ring-width chronology of Chinese pine (Pinus tabuliformis Carr.) from the eastern edge of the Chinese Loess Plateau (ECLP), which is the longest tree-ring chronology on the ECLP (1535–2018 CE). This chronology was used to reconstruct the average April-June self-calibrated Palmer Drought Severity Index (scPDSI). The reconstruction explains 64.3% of the variations in the observed scPDSI during the calibration period (1954–2000 CE). The driest and wettest years over the full reconstruction were 1759 CE and 1649 CE, respectively. The two most prolonged drought intervals occurred in 1892–1943 CE and 1737–1775 CE, while the two most prolonged pluvial intervals occurred in 1950–1992 CE and 1645–1682 CE. The reconstruction captures the recent drying since the 1960s. However, we found that the recent notable drying is not unprecedented over the last half-millennium, as more dramatic drought fluctuations occurred around the 1650s-1690s. The spatial correlation with sea surface temperature (SST) indicates that the drought variations on the ECLP might be modulated by the North Atlantic SST tripolar pattern. This study provides a reference for long-term drought evaluation, which may have implications for drought prediction and adaptation on the ECLP.

The 173‐kyr Obliquity Cycle Pacing the Asian Monsoon in the Eastern Chinese Loess Plateau From Late Miocene to Pliocene

AbstractWe conduct magnetostratigraphy and cyclostratigraphy on an eolian red clay sequence spanning 7–3.1 Ma in the eastern Chinese Loess Plateau. We pay more attention to the unconventional 173‐kyr obliquity modulated cycle which is associated with the secular frequency interference of orbital inclinations between Earth and Saturn. Similar to the 405‐kyr eccentricity cycles, the 173‐kyr obliquity band proved to be an important orbital driving component by the latest astronomical findings, and can serve as a chronometer of terrestrial air circulation reconstruction back to 7 Ma. Comparison with magneto‐cyclo‐bio‐stratigraphic timescales of another noted eolian sequence from the adjacent area, the 173‐kyr cycle is exceptionally presented as a robust recognition of the cyclical sedimentary patterns for the East Asia monsoon deposits.

Hydrochemistry and source apportionment of boron, sulfate, and nitrate in the Fen River, a typical loess covered area in the eastern Chinese Loess Plateau

Fen River Basin (FRB) is water-deficient and strongly influenced by human activities in the eastern Chinese Loess Plateau. The spatio-temporal variation and controlling factors of hyrochemistry and quality, sources of high boron, sulfate, and nitrate of surface waters in FRB were unclear. Major ions, δ11B, δ15N, and δ18O in surface waters in dry season and wet season of FRB were analyzed and correlation analysis (CA), principal component analysis (PCA), self-organizing map (SOM), forward model, and Bayesian isotope mixing model (MixSIAR) were used to solve above problems. Results showed that average riverine δ11B, δ15N, and δ18O of FRB was 7.8‰, 11.2‰, and 1.3‰ (1SD), respectively. Dissolved solutes ranked midstream > downstream > upstream with water type of Na +-Cl-, Ca2+-Mg2+-Cl-, and Ca2+-HCO3-, respectively. Low dissolved solutes were in forest areas while high values were in cropland and city areas. SOM analysis indicated that hydrochemistry was both influenced by natural (upstream) and pollutional input (midstream and downstream) and variation between dry season and wet season was minor. The abnormally high boron concentrations were mainly from silicate weathering (43%) and evaporites dissolution of loess (32%), urban and industrial input contributed 15% of riverine boron. High SO42− (207 ± 267 mg/L, 1SD) was mainly from sulfates. δ15N and δ18O analysis indicated that nitrification was the primary N cycling process. Further, MixSIAR showed that NO3− was mainly from municipal sewage (∼67%) and the total contribution of chemical fertilizer and soil nitrogen was ∼30% with slightly higher values in upstream and wet season. Influenced by land-use types, evaporite dissolution, and anthropogenic input, water quality below midstream was worse and strict sewage reduction policies must be developed. This study highlights the significant influence of evaporite dissolution of loess and anthropogenic input (urban and industrial input for B and sewage for NO3−) on hydrochemistry and water quality.

The 173-kyr obliquity cycle pacing the Asian monsoon in the eastern Chinese Loess Plateau from Late Miocene to Pliocene

The 173-kyr obliquity cycle pacing the Asian monsoon in the eastern Chinese Loess Plateau from Late Miocene to Pliocene

Chronology and dust mass accumulation history of the Wenchuan loess on eastern Tibetan Plateau since the last glacial

Loess deposits are spread widely over the eastern Tibetan Plateau (ETP), and constitute key terrestrial archives for reconstructing the paleoenvironments of the late Quaternary, which are still poorly understood. This study creates a detailed chronology of the Wenchuan loess sequence in the ETP through the quartz optically stimulated luminescence dating and radiocarbon dating. The results show that loess has accumulated in the area since at least ∼57.7 ka. We show that the variation in the history of the mass accumulation rate (MAR) spanned 4.8–108.0 g cm−2 ka−1, with a mean value of 43.9 g cm−2 ka−1 since the last glacial. The MAR of MIS 3 was the highest over the last glacial, with two peaks at 48–44 ka and 35–32 ka; whereas the MAR of MIS 2 was slightly lower and its peak appears at 21–18 ka. The MAR during the Holocene was generally low. Moreover, four enhanced dust events were superimposed on changes in the long-term MAR and correlated with the corresponding Heinrich events. The variation in the MAR of Wenchuan loess since the last glacial period has been similar to that in the eastern Chinese Loess Plateau (CLP) but differing from those of the western CLP. Our results reveal the spatial difference of MAR since last glacial, and can contribute to a better understanding of the link between the evolution of dust deposition and environmental changes in the ETP.

East-west asymmetry in the distribution of rainfall in the Chinese Loess Plateau during the Holocene

As the cradle of Chinese civilization the Chinese Loess Plateau (CLP) and its climatic and environmental history have attracted much research attention; however, the spatial characteristics of Holocene climatic conditions across the CLP remains unclear. We investigated five loess sections from the Ganjia Basin in the western CLP, and used paleoclimatic indexes from one well-dated loess-paleosol sequence to reconstruct precipitation variation during the past ~15,000 years. The chronology of the loess section is well established, based on 12 AMS 14C dates from bulk organic matter. Loess deposited before 8 ka (1 ka = 1000 years before 1950 CE) indicates that precipitation was low from ~15 to 8 ka. Subsequently, a paleosol, characterized by higher magnetic susceptibility and more negative organic carbon isotopic values, developed under the moist conditions of the middle to late Holocene. Comparison with paleoclimatic records from the CLP revealed that precipitation across the CLP was low during the early Holocene and reached its highest level during 8–3 ka, against the background of an enhanced East Asian summer monsoon (EASM). In the eastern CLP (the region east of the Liupan Mountains), the climate became dry after 3 ka because of a weakening of the EASM; however, precipitation in the western CLP was relatively high at this time. This observation of an east–west asymmetry in precipitation in the CLP during the Holocene based on geological data is supported by numerical modeling results from PMIP3, which indicate that it could have been caused by the westward extension of the western Pacific subtropical high. Our findings provide an analog for the study of interglacials on a longer time-scale and they demonstrate that the pattern of moisture conditions between the eastern and western CLP may differ on sub-orbital timescales.

Effects of vegetation cover and slope on soil erosion in the Eastern Chinese Loess Plateau under different rainfall regimes.

Soil erosion is a critical environmental problem of the Chinese Loess Plateau (CLP). The effects of vegetation cover on soil erosion reduction under different rainfall types are not well understood especially in the eastern Chinese Loess Plateau (ECLP). In this study, we monitored runoff and sediment yield at the Fengjiagou water and soil conservation station with five types of vegetation cover (arbor trees (ARC), shrubs (SHC), arable (ABC), natural vegetation (NVC), and artificial grass (APC)) and three slope gradients (10°, 15°, and 20°) in the ECLP. Based on long-term monitoring data, five rainfall types were classified by the maximum 30 min rainfall intensity (I30). We also quantitatively revealed the interactive effects of different types precipitation, vegetation cover and slope gradients on regional soil erosion. The results showed that (1) The RII (13 times) and RIII (eight times) type are the most threatening erosive rainfall in this region. (2) The ARC and SHC type were most beneficial for soil and water conservation in the ECLP; The APC and ABC are not conductive to the prevention of regional soil erosion. (3) Runoff and sediment yields increased with the slope gradient. The farmland is vulnerable to soil erosion when the slope gradient exceeds 10°. The results of this study can improve the understanding of regional soil erosion processes on the ECLP and provide useful information for managing regional water and land resources.

Paleowind directions from the magnetic fabric of loess deposits in the western Chinese Loess Plateau and implications for dust provenance

AbstractThe aeolian loess-paleosol sequences in the Chinese Loess Plateau (CLP) are an excellent archive of variations in atmospheric circulation in the geological past. However, there is no consensus regarding the roles of the East Asian winter monsoon and westerly winds in transporting the dust responsible for loess deposition during glacial and interstadial periods. We conducted detailed measurements of the anisotropy of magnetic susceptibility (AMS) on two parallel loess profiles covering the most recent 130 ka in the western CLP to determine paleowind directions. Results show that the magnetic lineations of the loess and paleosol units in both sections are significantly clustered along the northwest to southeast direction. These observations demonstrate that the prevailing wind system responsible for dust transport in the western CLP was the northwesterly winter monsoon, rather than the westerly winds. The AMS-derived dust-bearing wind direction was relatively stable during the last glacial and interglacial cycle in the western CLP, consistent with sedimentary and AMS evidence from the eastern CLP. Accordingly, it is reasonable to conclude that large areas of deserts and Gobi deserts areas located in the upwind direction were the dominant sources for the aeolian deposits of the Loess Plateau.

Groundwater hydrochemistry, source identification and pollution assessment in intensive industrial areas, eastern Chinese loess plateau.

Groundwater is essential for regional ecological-economic system and is an important resource of drinking water, especially in the Chinese Loess Plateau (CLP), where is a typical water-limited ecosystem. Groundwater quality deterioration will affect water security and exacerbate the water shortages. Groundwater hydrochemistry, pollution source apportionment, quality and health risks were evaluated based on analysis of major ions and selected trace elements in seasonal samples of the Fen River Basin (FRB) in the eastern CLP. Groundwaters in the FRB were mainly HCO3--Ca2+-Na+ water type with low dissolved solutes in upstream samples, high values in midstream samples and medium values in downstream samples. Solutes in upstream samples were mainly derived from carbonate weathering, while those in midstream and downstream samples came from silicate weathering, evaporites dissolution and anthropogenic sources. Self-organizing map (SOM) showed the hydrochemistry remained unchanged from dry to wet season for most sampling points. The seasonal variations of Ag, Cd, Ni, Pb, and Tl were significant due to anthropogenic input. High NO3- in upstream and downstream samples resulted primarily from sewage discharge, and high SO42- in midstream and downstream samples was from gypsum- and coal-related industries. In addition, anthropogenic input related to coal industries significantly aggravates pollution of As, Ni, Ag, Fe, and Mn. Influenced by evaporites and anthropogenic input, midstream samples had high salinity, total hardness and water quality indices (WQIs) and were unsuitable for irrigation or drinking purposes. Seasonal variation of WQI in the FRB was unsignificant except Jiaokou River sub-basin, where groundwater quality was worse in the wet season than the dry season due to coal mining. Great attention should be paid to the high non-carcinogenic risks of exposure to F, V, Mn, and Cr via dermal absorption, particularly for children. Overall, groundwater quality in the FRB was best in upstream, medium in midstream and worst in midstream based on different index. Groundwater quality is deteriorated by anthropogenic input and the sewage discharge in the FRB should be strictly controlled. Our report provides a reference for groundwater pollution evaluation and source identification in similar areas.

Early Pleistocene pollen record from the western Chinese Loess Plateau and its implications for the evolution of the East Asian Summer Monsoon

The East Asian Summer Monsoon (EASM) is the most important and active atmospheric circulation system in Asia. However, its evolution during the Early Pleistocene, and that evolution's associated drivers, remain controversial. Here, for the first time, a high resolution pollen record was obtained from a loess-paleosol sequence located in the Lanzhou Basin, western Chinese Loess Plateau (CLP), and used to reconstruct the evolution of EASM intensity during the Early Pleistocene (2.2–1.7 Ma). The type of vegetation indicated by analysis of the pollen assemblage is comparable to the forest-steppe vegetation type currently distributed on the eastern CLP. This area's present-day mean annual temperature (MAT) and mean annual precipitation (MAP) are ~6–10 °C and ~400–550 mm, respectively, implying the existence of a stronger EASM intensity during the 2.2–1.7 Ma period. When the auxiliary function of the diversity index was applied, the Early Pleistocene EASM, as recorded by the vegetation successions identified within the loess-paleosol sequence, showed a generally stronger intensity during the 2.2–2.06 Ma period, a relatively weak intensity between 2.06 Ma and 1.97 Ma, and then a gradual intensification from 1.97 Ma until 1.7 Ma. Further comparisons demonstrated that the zonal thermal gradient in the Equatorial Pacific may have played an important role in driving the evolution of the EASM over a tectonic timescale during the Early Pleistocene. In contrast, over a sub-orbital or orbital timescale, the EASM intensity appears to have been closely correlated to global temperature variations, given that the MATs of the Early Pleistocene were higher than today's. This would imply that both low-latitude forcing and global temperature change were important drivers influencing variations in precipitation in Northern China, against a background of ongoing global warming.

Causation and mechanism of magnetic susceptibility trend in Upper Miocene–Pliocene red clay deposits of the eastern Chinese Loess Plateau

The Upper Miocene–Pliocene red clay deposits of the Chinese Loess Plateau (CLP), like the overlying Quaternary loess, have recorded abundant paleoclimatic evolution information. In this study, we found that the variation in the magnetic susceptibility of seven sections across a north–south climate gradient in the eastern CLP exhibited an oscillatory upward trend. Prior to this research, much less was known about the causation and mechanism of this trend. Among the seven sections, the Jiaxian and Duanjiapo sections are located in the northernmost and southernmost of the climatic gradient, respectively. These representative sections were therefore selected to investigate the causation and mechanism of the aforementioned trend using magnetic measurements. The results were as follows. (1) The absolute and relative content of fine-grained ferrimagnetic minerals displayed an oscillatory upward increasing trend in both of the red clay sequences, which was the primary reason for the susceptibility increase in fluctuations. (2) The absolute content of antiferrimagnetic minerals was nearly equivalent in each red clay sequence. (3) Based on the combination of the results of present study and previous study findings, we found that the weathering and pedogenesis intensity of the red clay sequence showed an oscillatory upward trend during 7–2.6 Ma, which was closely linked with the strengthened East Asian summer monsoon in oscillations. These findings are probably the main cause for the oscillatory upward increasing fine-grained ferrimagnetic minerals in Upper Miocene–Pliocene red clay sequence. Further, this is likely the mechanism for the oscillatory upward increase of χ in the red clay sequence.