The sediment source characteristics of cascading hazards are critical to understanding their formation mechanisms, evolutionary processes, and implications for regional risk assessment. On 18 December 2023, a shallow-focus Ms6.2 earthquake struck Jishishan County, Gansu Province, triggering a large-scalemodern secondary landslide-mudflow disaster within the eastern Guanting Basin, located in the Lajishan Fault Zone on the NE Tibetan Plateau. A multiproxy approach was applied, incorporating optically stimulated luminescence (OSL) dating, magnetic susceptibility, grain-size, geochemical elements, and soil micromorphology. The results indicate that: (1) The paleo-flashflood hazard deposits formed during 12.01 ± 0.78–9.26 ± 0.68 ka. During the late Pleistocene-early Holocene transition period, unstable climate and frequent intense rainstorms, coupled with fragmented terrain and significant relief, concurrently triggered this paleo-flashflood event. (2) The paleo-mudflow hazard deposits formed during 5.09 ± 0.33–4.68 ± 0.28 ka. Paleo-earthquake activity was a primary trigger for the large-scale paleo-mudflow event in this period. (3) The Malan loess, interbedded with paleo-flashflood and paleo-mudflow hazard deposits, collectively constitutes the source material for the modern secondary disaster. Paleo-hazard deposits within the eastern Guanting Basin provided abundant material sources for the modern secondary disaster, indicating a close linkage between paleo-hazard and modern disaster occurrences in this region. These findings are of significant scientific importance for understanding the temporal patterns and genetic mechanisms of cascading hazards in the transitional zone between the NE Tibetan Plateau and the Loess Plateau, as well as for regional hazard risk assessment, disaster prevention and mitigation.

The loess-paleosol sequence developed in the Zoige Basin preserves a comprehensive archive of post-paleolake surface processes following the retreat and eventual disappearance of the paleolake. This study focuses on the Ouqiangcun (OQC) section, which is developed on the second terrace of the Yellow River within the Maqu region. A comprehensive analysis of its geochemical properties and micromorphological features was carried out, while the depositional chronology was constrained through optically stimulated luminescence (OSL) dating. The results reveal that: (1) The continuous aeolian deposition began on the second terrace of the Yellow River in the Maqu reach around 10 ka BP, producing an uninterrupted loess-paleosol sequence: fluvial sediment (T2-al, >10.0 ka BP) - loess (L-1, 10.0-8.5 ka BP) - paleosol (S-0, 8.5-3.0 ka BP) - topsoil (L-0 + MS, 3.0-0 ka BP). (2) Geochemical, A-CN-K, and micromorphological analyses show that weathering and pedogenesis in the OQC section are generally weak, dominated by limited plagioclase alteration, with paleosol exhibiting slightly enhanced but still low weathering. (3) The variations in weathering intensity within the OQC section effectively record the paleoclimate change history since the formation of the second terrace of the Yellow River in the Maqu reach: early Holocene (10.0-8.5 ka BP) was cold and dry with active aeolian deposition forming loess (L-1); mid-Holocene (8.5-3.0 ka BP) was warm and humid with reduced aeolian activity and enhanced weathering forming dark gray paleosol (S-0); late Holocene (3.0-0 ka BP) returned to cold and dry conditions, leading to loess (L-0) deposition and development of modern soil.

The Yellow River is well known for frequent levee breaches and channel migrations, yet its flood history and hydroclimatic-geomorphic drivers remain poorly understood. The Sanhuajian reach (Sanmenxia–Huayuankou) is critical for assessing how extreme flooding responds to channel migration in the mid-lower Yellow River. Herein, well-preserved overbank flood deposits (OFDs) consist of well-sorted silt with massive-blocky structure, dominate by coarse silt (55.9–63.9%). OSL dating constrains these OFDs to 2.01–1.96 ka and 0.48–0.23 ka. Palaeoflood and historical records identified five flood clusters (∼2250, ∼2000, ∼1750, ∼500 and ∼ 250 a BP) during the Late Holocene. These palaeofloods show a strong correlation with downstream river breach and channel evolution, as also indicated by evidences from paleolake retreat, episodic deposition and lake infilling in the lower Yellow River. Furthermore, we attribute the palaeofloods to a multi-genetic mechanism, involving abnormal monsoon circulation, typhoons and orographic blocking, supported by the “2021.7”, “1975.8” and “1958.7” extreme floods. These results are significantly important in understanding the geographically highly interconnected flood-channel and flood-climate relationships across the mid-lower Yellow River basin.

Ancient agricultural development has been affected by climate change. The upper Yinghe River draining into the Huaihe River represents an important region for tracing the origins of primitive agriculture in China. To clarify the relationship between primitive agriculture and climate change in this region, optically stimulated luminescence (OSL) dating and physicochemical analyses were performed on a Holocene loess-paleosol PLG profile. The results indicated that paleosol (S0) developed during the mid-Holocene climatic optimum (8500–3100 a BP). The OSL ages of 7220±700 and 6800±400 a were obtained from the lower and middle parts of S0, respectively, and the cultural remains at 70 cm yielded an OSL age of 6890±400 a. These chronological results were consistent with the age of the Peiligang Culture (7800–7300 a BP) during the Neolithic period. In the S0 layer, the fine silt and clay contents and magnetic susceptibility reached their highest values, whereas the higher total organic carbon (TOC) content and lower pH values indicated a warmer and more humid climate. Such favorable hydrothermal conditions promoted weathering and pedogenesis, facilitated S0 formation, and created suitable soil conditions for agricultural development during the Peiligang cultural period. These findings suggest that climate exerted key control on Holocene loess pedogenesis and that Neolithic primitive agriculture was closely associated with the favorable climate and well-developed soils of the mid-Holocene climatic optimum.

The Yangguanzhai Ruins within the Guanzhong Basin in the middle Yellow River, represent one of the most important archaeological sites for understanding social organization during the Miaodigou phase of the Yangshao culture (5.6–4.9 ka BP). It has twice been selected as one of China’s “Top Ten Archaeological Discoveries.” Through extensive and detailed field investigations at the ruins, two units of palaeoflood slack-water deposits were identified in the YGZ-C profile. These deposits were examined using field observations and laboratory analyses, including grain-size distribution, magnetic susceptibility, geochemical elements, micromorphological features, optically stimulated luminescence (OSL) and AMS 14C dating. The results reveal two extreme Holocene palaeoflood episodes in the Jinghe River, which are crucial for understanding regional hydrological responses to monsoonal climate change. Combined OSL and AMS 14C dating results, together with stratigraphic relationships, and archaeological ages, indicate that these two palaeoflood episodes broadly correspond to intervals of global climatic deterioration at 6.0–5.0 ka and 1.8–1.7 ka, respectively. Notably, multiple Holocene geological records in the middle Yellow River further indicate the occurrence of frequent extreme floods and droughts during 6.0–5.0 ka. In response to such climatic instability, the inhabitants during the Miaodigou phase of the Yangshao culture at the ruins likely constructed a large ditch and pond to cope with increasing environmental stress. This multifunctional hydraulic system, which combined flood control and water storage, represents a key proactive adaptation strategy to environmental change. These findings not only enrich the geological record of Holocene palaeoflood episodes in the Jinghe River, but also provide new insights into how prehistoric societies responded to abrupt climatic and environmental changes.

Marine Isotope Stage 3 (MIS3) was a relatively warm and humid, although variable, time in the last glacial period in the northern hemisphere. This climate phase is well-recorded in various archives (e.g., ice cores, speleothems, and loess) from many parts of the world, including China, although its expression (e.g., warm-humid vs. cool-dry) differed between the hemispheres. However, despite its location in a critical climatic boundary zone and high sensitivity to climate changes, few studies have investigated the MIS3 climate based on the loess records in the Hanzhong Basin (China). In this study, we performed systematic sampling of a loess-paleosol sediment sequence and performed physicochemical analyses and optically stimulated luminescence dating after detailed field observations and descriptions. The results indicated that the time of formation of the lower part of the Malan loess (L-1(-)2) occurred between 55.0-25.6 ka BP, corresponding to MIS3. The magnetic susceptibility, clay content (<5 mu m), a*, a*/b*, and contents of Fe2O3, Al2O3, Chemical Index of Alteration, and eluvial coefficient were all significantly higher, while the sand content (>50 mu m), L*, ba, a, and the contents of Na2O, CaO, and Na/K were lower than those in the loess L-1(-)1 (corresponding to MIS2). These findings indicate that the degree of weathering of the loess in MIS3 was higher than that in MIS2, suggesting that the change to a warm-humid climate during MIS3 is reflected in the sediment of the Hanzhong Basin. However, the climate was not as warm or as humid as that during the mid-Holocene, as indicated by the weaker degree of weathering of the paleosol S-0. The climate fluctuated obviously during MIS3, including three warm-humid intervals and three dry-cold intervals. Although the relatively warm-humid and unstable characteristics of the late MIS3 in the Hanzhong Basin are similar to the climate records in other parts of the world, they are considered to be regionally specific responses of the Hanzhong Basin to global climate change.

The causes of the prehistoric catastrophe recorded at the Lajia ruins-located in the Guanting Basin, upper reaches of the Yellow River-have sparked intense academic debate. However, whether the Lajia ruins were buried and destroyed by outburst floods from the Yellow River, landslide dam outburst floods from Jishi Gorge or earthquake-induced flashfloods and mudflows in tributary valleys is still inconclusive. To reveal the direct cause of the prehistoric catastrophe, here we performed sedimentary provenance analysis (including heavy mineral assemblages and zircon typomorphism) of the overburden layer at the Lajia ruins. Our results indicate that the flashflood deposits covering the western and southwestern Lajia ruins are mixtures of old metamorphic bedrock from Laji Mountain at the source of the L & uuml;jiagou gully and sand and clay materials from the Tertiary red layer mountain and gully region. The red mudflow deposits covering the eastern and southeastern Lajia ruins were sourced mainly from the large ancient landslide zone of the Tertiary red layer mountain region on the northern slope of the Guanting Basin. Therefore, the deposits covering the Lajia ruins are typical flashflood and mudflow deposits. Combined with the densely distributed ground fissures and severely damaged ruins, these lines of evidence indicate that a strong earthquake occurred along the Lajishan fault zone at 3850a BP, and in conjunction with the torrential rain, it triggered large-scale flashfloods and mudflows from the northern valley slope of the central Guanting Basin, which inundated the Lajia ruins. The landslide-mudflow disaster triggered by the 2023 Jishishan earthquake in the same fault zone can serve as an important analogue for comparative research on the Lajia ruins. Overall, our results provide direct evidence and a new research perspective for accurately identifying the cause of the Lajia ruins destruction, and further support ongoing regional studies concerning this complex disaster chain.

Rainfall erosivity constitutes a crucial driving factor in soil erosion. Based on daily rainfall data collected from 38 meteorological stations in the Huaihe River Basin (HRB) of China during 1960–2019, using Sen’s slope estimation and the Mann-Kendall trend test methods, we employed a daily rainfall erosivity model to analyze the dynamic variations of rainfall erosivity and its correlation with the El Niño-Southern Oscillation (ENSO), which plays a key role in the monitoring, evaluation, prediction, and prevention of regional soil and water loss. The results indicate that the annual mean rainfall erosivity in the HRB between 1960 and 2019 is 5522.14 (MJ·mm)/(ha·h) and exhibits a decline rate of 46.32 (MJ·mm)/(ha·h·10yr). Spatially, the annual mean rainfall, erosive rainfall, and rainfall erosivity display a decreasing trend from the south to the north, as well as from the southeast to the northwest of the basin. The variation trend of rainfall erosivity differs markedly across regions. Specifically, the central plain area of the basin exhibits an increasing trend, while both the western and eastern areas present an opposite trend. Furthermore, the rainfall erosivity varies significantly across various sub-watersheds with the order: upper-HRB > lower-HRB > Yishusi River watershed > middle-HRB. In addition, ENSO is regarded as one of the main factors that lead to regional rainfall erosivity changes. During the El Niño period, a relatively lower rainfall erosivity compared to that of the La Niña period is observed, while its correlation with ENSO indices is more significant than that in La Niña periods (P < 0.05).

Paleochannel sections can provide abundant information on environmental changes at a basin scale. The interplay among meander cutoff dynamics, extreme overbank flooding and climate transitions remains poorly constrained in major river systems on the northeast (NE) Tibetan Plateau. Through systematic and detailed field investigations within the Zoige Basin on the plateau, a typical paleochannel (paleo-oxbow lake type) section in the Yellow River was selected to study its evolutionary history. Integrated field sedimentological observations and laboratory physico-chemical analyses reveal that the stratigraphic succession comprises, in ascending order: riverbed deposits, river-lake transition deposits, paleo-oxbow lake deposits (A), overbank flood deposits (OFD), paleo-oxbow lake deposits (B) and modern soil. Optically stimulated luminescence (OSL) dating results indicate that the paleochannel in the Yellow River underwent a neck cutoff at 4290 +/- 490 a, leading to the formation of the paleo-oxbow lake. This marked the gradual transition from a fluvial environment to an oxbow lake environment. An episode of extreme overbank flooding in the Yellow River, documented by the OFD, occurred at between 3280 +/- 260 and 3010 +/- 450 a. This period corresponds to the climatic transition from the midHolocene Climatic Optimum to the late Holocene, when anomalous atmospheric circulation patterns and increased precipitation variability triggered extreme overbank flooding within the basin. These results are significant for unraveling the evolutionary process of river channels and the climatic background of extreme floods on the Tibetan Plateau.

Large herbivores of grazing activities are the most important production factor to maintain the permanent settlement of human beings in the high-altitude area of the Tibetan Plateau, China, and their population changes are the products of the interaction between climate change and human activities since the Holocene. Using pollen and fungal spore fossil records from a profile since mid-Holocene in the Zoige Basin, the history and controlling factors of vegetation change and grazing activities were revealed. The results showed that the history of vegetation change and grazing activities in the Zoige Basin could be divided into three stages: (i) During the period of 8.5-5.8 ka, alpine meadow dominated by Cyperaceae developed in the Zoige Basin with the highest tree pollen content and the best hydrothermal conditions. The low concentration of coprophilous fungal spore at this stage was probably related to wild herbivores. (ii) During the period of 5.8-3.6 ka, the alpine meadow dominated by Cyperaceae developed further, the tree pollen content decreased slightly, and the climate began to change to drought, but the overall climate was still warm and humid. At this stage, coprophilous fungal spores occurred continuously and their concentration gradually increased, suggesting that there were human activities in the basin and grazing activities began at about 5.8ka. (iii) During about 3.6-0 ka, the vegetation type changed to the desert steppe dominated by Chenopodiaceae and Asteraceae, and after 0.5 ka, the alpine meadow redeveloped. The pollen content of trees decreased significantly, and the climate was mainly cool and dry. Grazing activities, as represented by the concentration of coprophilous fungi spores, began to increase gradually after about 3.6 ka. There was a reverse correlation between grazing activities and the dry and cool climate in this stage. The dry and cool climate was one of the key factors to promote the development of grazing in the Zoige Basin. The research results further complement the evolution of human activities and environmental background in the eastern Tibetan Plateau, and have certain reference significance for understanding the impact of climate change on human survival strategies in the plateau environment.

Understanding paleoenvironment changes and their driving mechanisms are crucial for predicting and responding to future climate change. Through extensive field investigations, a complete Holocene sedimentary profile has been discovered in the Zoige Basin of the eastern Tibetan Plateau. The history of Holocene wildfire activities and climate change in this region was reconstructed using various paleoclimate proxies, including charcoal, magnetic susceptibility, and total organic carbon. This study also elucidated the relationship between Holocene paleoenvironmental conditions and wildfire activities. The results indicated that during the Holocene, the wildfire activities in the Zoige Basin were primarily regional, with local fires mainly involving woody plants and regional fires dominated by herbaceous plant burning. In the early Holocene, before 8.5 ka, despite a temperature increase, the climate in the Zoige Basin remained dry and cold. During this period, an aeolian sand layer was deposited, and the limited plant biomass restricted wildfire occurrence, resulting in the low frequencies of both local and regional fires. During the middle Holocene (8.5–3.1 ka), the warm and moist climate driven by the East Asian summer monsoon led to the development of a paleosol layer in the Zoige Basin. The increased temperature and precipitation enhanced the plant biomass, and frequent regional and local wildfire activities were observed. In the Late-Holocene, after 3.1 ka, as the climate cooled and dried, a modern soil layer formed and wildfire activities decreased. The reduction in plant biomass likely contributed to the decline in regional wildfires, whereas the intensification of local fires may have been influenced by human activities. The results enhanced our understanding of wildfire history and climate evolution in the Zoige Basin.

The Zoige Basin, located in the eastern region of the Tibetan Plateau of China, is characterized by its climate sensitivity and complex surface environment. It is vital to understand the response of the surface processes to environmental changes in the Zoige Basin since the last deglaciation, as well as the response of environmental changes and surface processes on the Tibetan Plateau relate to global changes. In August 2020, a field investigation was conducted in the Zoige Basin. A complete set of stratigraphic profile from the high platform at the front of the glacial-diluvial fan in the Maqu reach of the western basin was selected as the research subject. Optically stimulated luminescence (OSL) dating samples and sedimentary samples were collected from key layers and brought back to the laboratory for experimental analysis, and the surface processes and environment changes since the last deglaciation in the Zoige Basin were investigated through particle size analysis and OSL dating. The results showed that during the last glacial period before 14.5 kyr, a substantial glacial-alluvial fan composed of sand and gravel layers was formed by flash flood processes triggered by glacial meltwater and heavy precipitation. In the Bølling-Allerød warm period of 14.5–11.7 kyr, a warm and humid climate prevailed, with the formation of silty bog environments in the shallow depressions at the glacial-alluvial fan’s forefront and the development of gray-green bog soil deposits. However, during the Younger Dryas period, an abrupt climate deterioration occurred, with the upper section of the gray-green bog soil layer in the shallow depression to experience folding and deformation due to surface freeze-thaw actions. During the early Holocene period from 11.7 kyr to 8.5 kyr, the climate was relatively dry, resulting in prevalent aeolian sand activitities. Coarse silt accumulated in the shallow depression, and sedimentary facies alternating between aeolian sand and bog soil deposits developed owing to strong wind patterns on the plateau surface. In the warm and humid period from 8.5 kyr to 3.1 kyr, increased weathering and pedogenesis enhanced clay content in sediments, which developed into the paleosol. In the late Holocene, starting from 3.1 kyr, the climate became relatively dry once more, with aeolian sand activity prevalent. The coarse silt that accumulated during the late Holocene transformed into subalpine meadow black soil because of rising temperature and humidity levels. These findings indicated that the developmental process of the high platform in the Maqu reach of the Zoige Basin, which was significant for understanding the environmental changes and surface processes in the source region of the Yellow River since the last deglaciation.

Aeolian deposits in the Zoige Basin provide direct evidence for understanding the evolution of the surface environment after the disappearance of the paleolake. The complete aeolian sequence was deposited on the paleo-lakeshore terraces of the Tangke ancient city area. This study focuses on the TKGC section, a representative aeolian deposit on the paleo-lakeshore terrace, and conducts a systematic investigation of its sedimentological, micromorphological, geochemical, and chronological characteristics. The results show that: (1) The chronostratigraphic framework of the TKGC section is as follows: aeolian sand (15.5-13.6 ka BP)- aeolian loess (13.6-8.5 ka BP)-paleosol (8.5-3.0 ka BP)-recent loess (3.0-1.3 ka BP)-topsoil (1.3-0 ka BP). (2) Consistent with previous understandings of global climate change, the loess and paleosol sequences in the alpine Zoige basin also reflect varying intensities of weathering and pedogenesis during different periods. Sedimentological, geochemical, and micromorphological evidence consistently indicate strong pedogenesis during the paleosol development, while pedogenic processes were weak during periods of aeolian sand and loess accumulation. (3) Following the disappearance of the paleolake, from 15.5 to 13.6 ka BP, the region experienced harsh climatic conditions and intense aeolian activity, leading to the deposition of coarse-grained aeolian sand. Around 13.6 ka BP, aeolian activity declined, resulting in the accumulation of finer-grained aeolian loess. By 8.5 ka BP, the regional climate shifted from cold and arid to warm and humid, initiating the development of the paleosol. Around 3.0 ka BP, climatic conditions reverted to colder and drier states, enhancing aeolian processes and facilitating the formation of the recent loess layer. Subsequently, around 1.3 ka BP, the topsoil began to form atop the recent loess deposits.

Palaeoflood natural archives provide vital insights for reconstructing extraordinary flood magnitudes and understanding climate change. Palaeohydrological investigations were conducted in the upper Huai River, North China Plain. A Holocene loess-paleosol sedimentary profile containing six palaeoflood slackwater deposits (SWD, SWD6-SWD1) was identified in the Luzhuang reach of the upper Huai River, where the Maoji River (a tributary of the Huai River) enters the Huai River. Sediment samples were collected for physicochemical analysis and optically stimulated luminescence (OSL) dating, respectively. The results revealed that the sedimentary accumulation consists of modern soil (MS), paleosol (S-0), aeolian loess (L-0), and transitional loess (L-t) sediments were predominantly silt (>50 %), whereas SWDs were mainly sand (>60 %). Geochemically, the elemental composition of SWDs was similar to that of MS, S-0, and L-0 sediments, but distinct from L-t. The MS, L-0, S-0, and L-t sediments demonstrated moderate weathering that was more pronounced than that of SWDs. Using OSL dating and stratigraphic chronological frameworks, six extraordinary palaeoflood events have been identified in the Luzhuang reach of the upper Huai River since approximately 8.5 ka in the Holocene. The palaeoflood magnitudes were reconstructed using the HEC-RAS hydraulic model, with six reconstructed palaeoflood peak discharges ranging from 9260 to 17,810 m(3)/s. Moreover, the sensitivity of the peak discharge calculated using the HEC-RAS model to the roughness coefficient was low, with a relative error of only -8.7 %-2.9 %. These extraordinary palaeoflood events corresponded to periods of climate change and instability, closely related to the El Ni & ntilde;o-Southern Oscillation (ENSO). This study provides valuable insights into the effects of global change on regional hydrological systems.

Through field investigation in the Zoige Basin of the eastern Tibetan Plateau, a natural and complete sedimentary profile was found on the steep cliff of the first terrace of the right bank of the Yellow River in the Maqu reach. Careful field observation, analysis of particle size characteristics, and optically stimulated luminescence dating were conducted. The results showed that the paleo-riverbed pebble layer and the floodplain sandy silt layer at the bottom of the sedimentary profile constituted a typical binary sedimentary structure layer, in which the shallow depression bog mud layer that formed during 5.0–4.2 ka was lenticularly sandwiched in the floodplain sandy silt layer, indicating that the first terrace of the Yellow River in this reach began to form at 5.0 ka. The climate was dry and cold during 4.2–4.0 ka, and strong wind on the plateau surface transformed the floodplain sediments into aeolian sand. The paleoflood slackwater deposits sandwiched in the aeolian sand layer indicated that the source of the Yellow River has experienced several catastrophic flood events during this period, inundating the first terrace ground. During 4.0–2.8 ka, mud bogs formed in shallow depressions of the terrace under the relatively humid climate. After 2.8 ka, sandstorms drove the accumulation of coarse silt over the terrace ground, forming aeolian loess. The upper part developed into modern subalpine meadow black soil with granular structure through biological weathering since 2.0 ka. The study results revealed the formation age and its surface development process of the first terrace of the Yellow River in the Zoige Basin, which has important scientific significance for in-depth understanding of the water system evolution and environmental changes in the source area of the Yellow River.

Paleoflood slackwater deposits have recently been investigated in small to moderately sized rivers. However, our understanding of the characteristics of paleoflood slackwater deposits in large rivers remains limited, which poses a notable obstacle to accurately predicting the magnitude, frequency and force of extraordinary floods. To address this research gap, this study compares the characteristics of paleoflood slackwater deposits in the middle Yarlung Zangbo River (YZR) and the upper Hanjiang River in China. Paleoflood slackwater deposits and aeolian deposits (e.g. loess, paleosol S0 and aeolian sand) were investigated and sampled from these locations. For the paleoflood slackwater deposits and aeolian deposits, sedimentology (e.g. grain size distribution, magnetic susceptibility and geochemical elements) was analyzed in the laboratory. The macroscopic features of the paleoflood slackwater deposits in the middle YZR valley are very similar to those of the upper Hanjiang River valley. The paleoflood slackwater deposits were dominated by sand and silt in the middle YZR and by silt and sand in the upper Hanjiang River; this implied that the paleoflood slackwater deposits were suspended sediments of floodwater from different source regions. Compared with the paleoflood slackwater deposits in the upper Hanjiang River and other large rivers in China (e.g. the Yellow and Yangtze Rivers), the paleoflood slackwater deposits contained more coarse particles in the middle YZR; this result may be closely related to the particular hydrogeologic and geomorphic conditions (e.g. rich sandy sediment, high riverbed slope and narrow valley) in the southeastern Tibetan Plateau. Compared with the paleoflood slackwater deposits in the upper Hanjiang River, the paleoflood slackwater deposits presented considerably worse sorting in the middle YZR; this may be closely related to the short transport distance in the middle YZR. The magnetic susceptibility values of the paleoflood slackwater deposits were relatively higher in the middle YZR and the upper Hanjiang River, suggesting that these slackwater deposits with minimal pedogenesis may contain a relatively high ferromagnetic mineral content because of the formation process of slackwater deposits. The sediment provenance of the paleoflood slackwater deposits may be closely related to that of the loess in the middle YZR valley, which implies that the sediment provenance of deposits of different genetic types may be greatly affected by the particular hydrologic-climatic and geomorphic conditions in the southeastern Tibetan Plateau.

A typical Holocene aeolian loess-paleosol section was identified in the Zoige Basin on the northeastern (NE) Tibetan Plateau. Detailed micromorphological observations and systematic studies were conducted using a Leica DMRX polarizing microscope, and microstructure identification and quantitative analysis were performed using Leica Qwin3.2 software. The results revealed the following findings. (1) In the Zoige Basin of the alpine-cold region, the micromorphological characteristics include coarse grain sizes, mainly ranging from 100 to 200 mu m, with quartz and plagioclase as the main mineral types. The secondary minerals consist mainly of small amounts of secondary calcites and secondary clay minerals. The organic matters includ humus, plant residues, and charcoal fragments. The main void type is stacked voids with rough walls. (2) Compared to loess, the coarse grain parameters (area, perimeter, length/width ratio, roundness, and equivalent circle diameter) in the paleosol decrease significantly, and the unstable minerals exhibit obvious signs of weathering. The porosity and humus content increase. In modern soil, coarse grain parameters increase, unstable mineral boundaries become evident, and the porosity decreases. These characteristics suggest a variation in the intensity of weathering and pedogenesis following a weak-strong-weak pattern, indicating the climatic evolution in the Zoige Basin from dry-cold to warm-wet and back to dry-cold since the Holocene. (3) Compared with the Loess Plateau, the loess-paleosol micromorphology of the Zoige Basin in the alpine-cold region is characterized by significantly higher unstable mineral content, coarser grain size, less secondary mineral content, and mainly vesicular voids with rougher walls. These characteristics indicate significant differences in weathering and pedogenesis between the two regions owing to variations in geographical environments. The hydrothermal conditions of the Zoige Basin in the alpine-cold region were lower than those in the Loess Plateau, resulting in less intense weathering and pedogenesis. These results could provide the micromorphological evidence for reconstructing the Holocene paleoclimate evolution in the NE Tibetan Plateau.

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Paleochannel sedimentary sequences can provide abundant information on regional environmental changes. A typical paleochannel (paleo-oxbow lake type) section of the Yellow River was identified within the Zoige Basin on the NE Tibetan Plateau. A multi-index approach was used to accurately identify sediments of different genetic types, such as riverbed deposits of the Yellow River, paleo-oxbow lake deposits, and overbank flood deposits (OFD) in the section. Based on optically stimulated luminescence (OSL) and AMS 14C dates, we examined the environmental evolution recorded by the section. The results show that: (1) The section is a record of environmental change since 4.17 ± 0.49 ka. During 4.17 ± 0.49 to 3.24 ± 0.26 ka, the ancient Yellow River occupied the channel. At 3.24 ± 0.26 ka, the paleochannel experienced a neck cutoff, and the fluvial environment began to change into the oxbow lake environment. After 2.45 ± 0.11 ka, the paleo-oxbow lake gradually disappeared. Subalpine meadow soil has developed at this site since 1.31 ± 0.05 ka. (2) Paleoenvironmental proxies indicate that the Zoige Basin was warmer and wetter before ~3.00 ka, and became drier and colder after ~3.00 ka, which may be mainly related to the weakening of the East Asian summer monsoon (EASM) and the strengthening of the Westerlies. (3) Two episodes of extreme overbank flooding occurred at 2.96 ± 0.24 to 2.87 ± 0.27 ka and 1.84 ± 0.20 to 1.70 ± 0.16 ka, correlated with climate shift period from the mid-Holocene climatic optimum to the late Holocene and the Dark Age Cold Period (DACP), respectively. Due to the relatively cold and dry climate in these periods, glaciers generally advanced on the Tibetan Plateau, and the contribution of snow and ice meltwater weakened. Therefore, the strong rainfall caused by the abnormal atmospheric circulation may be the main cause of these extreme overbank flooding.