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Palaeoflood records have provided the extended perspective required to understand linkages between extreme floods and climate variability. During our palaeo‐hydrological investigations, two units of palaeoflood slackwater deposits (SWD) were found on the river banks in the Maqu‐Oula reach along the mainstream of the Yellow River on the northeast (NE) Tibetan Plateau. A series of samples were taken from the sediment profiles at Taiwuruo (TWR) and Daerqiong (DEQ) sites for laboratory analysis. Grain‐size distribution and micro‐morphological characteristics of quartz grains indicate that these well‐sorted SWD units were derived from the suspended sediment load in floodwater. They have recorded two episodes of overbank flooding in the Yellow River source area. Using the optically stimulated luminescence (OSL) dating technique, the first episode of overbank flooding was dated to 13.6–13.0 ka and correlated with the Bølling–Allerød warm interval of the last deglaciation. The second episode occurred at 11.0–10.0 ka, that is, the early Holocene. In a warm‐humid climate, these overbank flooding episodes may have resulted from the accelerated melting of mountain glaciers and snow over the headwaters of the alpine and subalpine regions on the NE Tibetan Plateau. These results are important in facilitating an in‐depth understanding of the hydro‐climatic response of the Tibetan Plateau to global climate warming occurring presently. [ABSTRACT FROM AUTHOR]

Palaeo-earthquake event recorded by loess rapture fissures (N30°–40°W and N40°–50°E trending) and palaeo-mudflow event recorded by red clay deposits were identified at the Machangyuan Ruins in the Huangshui River valley, at the foot of the northeastern Tibetan Plateau. Comparative analysis of grain-size distributions and geochemical elements of local outcrops indicates that the red mudflows were sourced from the gullies on the valley side where unconsolidated Neogene red clay formation was exposed The palaeo-earthquake was associated with regional tectonic structures including a NNW-trending left-lateral strike-slip fault (Lajishan fault) and a NE-trending local fault (Bazhougou fault). Analysis combining optically stimulated luminescence ages and radiocarbon ages of archaeological record dates the palaeo-earthquake and palaeo-mudflow events to ca. 4.80 ka BP. During that stage, many Neolithic settlements distributed around the Machangyuan Ruins. Enhanced human activities of the Majiayao Culture disturbed the landscape of the Minhe Basin causing widespread soil erosion. Contemporaneous storm rain and earthquake mobilized the loosened sediments in the upper stream gullies forming mudflows. The hollow ground around the Machangyuan Ruins was covered by mudflow and the earthquake fissures were filled in by the mudflow at the same time. This study provides important insights into early human impact during climatic and tectonic events in the environmentally vulnerable zones over the world. [ABSTRACT FROM AUTHOR]

Global change induced regional environmental variation and related catastrophic surface processes have greatly affected human life and social development during the last decades. These occurrences have also been highlighted by pre-historical catastrophes exposed by archaeological excavation in the Lajia Ruins at the northeast margin of the Tibetan Plateau, which has attracted considerable focus from both geologists and the world media. The cause of the disasters responsible for the destruction of the Lajia Settlement (14C age 4.15–3.85 ka BP) of the Qijia Culture remains controversial due to insufficient investigation of the Holocene sedimentary succession needed to establish a complete pedo-stratigraphic sequence and chronology, and thus to precisely constrain environment variation and related surface processes at that time. During our field investigations, a complete and continuous Holocene soil and sediment profile was identified in the northern part of the Lajia Ruins uniquely. Soil and sediment samples were taken systematically for physical and chemical analysis and optically stimulated luminescence (OSL) dating after detailed field observations and descriptions. The results demonstrate that the pedo-stratigraphic sequence includes Malan loess (L 1-1 , >11.50 ka BP) and transitional loess (L t , 11.50–8.50 ka BP) in the bottom, a composite palaeosol (S 0 , 8.50–3.10 ka BP) in the middle, and recent loess (L 0 , 3.10–1.50 ka BP) and modern soil (MS, 1.50–0 ka BP) in the top. This pedo-stratigraphic sequence is comparable to those identified in the central part of the Loess Plateau. However, the mid-Holocene palaeosol (S 0) is inserted by three layers of red clay (RC 1 , RC 2 , and RC 3) and flashflood deposit (FFD), and thus subdivided into four sub-layers (S 0-upper , S 0-upper middle , S 0-lower middle , and S 0-lower). The red clays and the flashflood deposit are inferred to represent abrupt events of flashflood and mudflow that were dated to 3.96–3.65 ka by OSL and to 3.85–3.60 ka BP by AMS 14C methods. Indeed, the entire settlement, dwellings, pottery artifacts, and human skeletons in the Lajia Ruins are directly packed by these deposits. The Lajia Ruins is thus considered to be destroyed by enormous flashfloods and mudflows coming from the Ganggou and Lüjiagou gullies, which submerged the settlement at 14C 3.85 ka BP or OSL 3.96 ka. These disastrous events are related to climatic deterioration since 4.20 ka BP that has resulted in severe convective weather and strong rainstorms affecting the northeast margin of the Tibetan Plateau. With consideration of the badly deformed ground and dwelling floors in the Lajia Ruins by crisscrossing earthquake fissures, that are also infilled and packed by flashflood and mudflow deposits, it is concluded that great earthquakes occurred as the first hitting to the settlement at Lajia. The enormous flashfloods and mudflows may have been induced by multiple earthquakes and rainstorms simultaneously. These results bear important implications for a better understanding of pre-historic environmental change and human-land relationship evolution at the northeast margin of the Tibetan Plateau. • Establishing a complete stratigraphic sequence and chronological framework in Lajia Ruins. • Delineating the position of prehistorical settlement in the pedo-stratigraphy. • Identifying the causations that ruined the Lajia settlement of the Qijia Culture. • Revealing prehistoric environmental change and human impact. [ABSTRACT FROM AUTHOR]