The Yellow River catchment in central China is one of the cradles of Chinese civilization. Since the mid-Holocene, the runoff and erosion in this area, which includes the Chinese Loess Plateau, have ...been affected by anthropogenic land cover changes (ALCC) and Asian monsoon climatic changes. We applied the reconstructions of Holocene climate (temperature, precipitation, insolation and CO2 concentration) and KK10 land cover change scenarios, to determine the evolution of discharge and sediment flux in the Beiluo River in the Chinese Loess Plateau using the Landlab landscape evolution model combined with a module that considers the impact of vegetation on water balance and soil erodibility. The modeling results show that climate change and ALCC resulted in large fluctuations of both discharge and sediment fluxes since the mid-Holocene. For the evolution of discharge, climate change is the dominant controlling factor, and land-use change has a secondary effect. Comparison of results of modeling scenario's with and without land-use change shows that it contributes less than 5% to changes of runoff during the time period before 1000 BCE. However, the impact of land-use change is significant for sediment load, especially since 5000 BCE. Thus, our results provide evidence for a much earlier start of the human impact on soil erosion and fluvial processes compared to earlier studies. Our modeling also shows that the incorporation of eco-hydrological processes in landscape evolution modeling is important for the accurate simulation of runoff and discharge in response to climate and land-use change, at least in a semi-arid environment.
•The effects of climate change and human activities since mid-Holocene were modeled.•Results show that discharge fluctuations in the river are dominated by climate change.•Results show that the sediment load is mainly controlled by land cover change.•The impacts of land cover change to sediment flux become important since 5000 BCE.•The simulations indicate an early start of human impact on soil erosion and landscape evolution in central China.
Although the development of Gobi Desert in central and eastern Asia has greatly affected the regional and even the global climate, its precise origin and evolution have yet to be determined. The ...three preconditions for the formation of Gobi Desert are: i) a dry climate, ii) basin landforms and iii) abundant sediment production. In this study, we present a synthesis of both new and published data on the formation and evolution of Gobi Desert in central and eastern Asia. We conclude that the combined effects of mountain building, the mid-latitude westerly circulation and changes in the Asian monsoon, accompanied by global cooling, were principally responsible for the formation of modern Gobi Desert landscapes in central and eastern Asia during the late Pliocene. The arid climate in central and parts of eastern Asia probably developed in the early Cenozoic, from ~50 Ma. Related events included the collision of the Indian and Asian plates, the closure and complete retreat of the Paratethys Ocean from central Asia, and the growth of the Himalayas and the Tibetan Plateau in the Eocene through late Miocene, which blocked the water vapor supply and intensified the aridification of the Asian interior. Superimposed on the topographic changes was the process of stepwise global cooling since the early Oligocene, and in particular since the late Miocene, which controlled the formation and evolution of the Gobi Desert landscape. Global cooling weakened the Asian monsoon circulation, strengthened the westerly circulation and enhanced physical weathering processes in mountain areas, which together promoted both the aridification of the Asian interior and sediment production. These processes finally resulted in the establishment of the modern Gobi Desert landscape in the late Pliocene. We estimate that the modern Gobi Desert landscape was formed at ~2.6 Ma and was the result of the stepwise evolution of Asian topography and climate during the Cenozoic, dominated by Asian tectonic deformation and uplift, and the evolution of Asian monsoon climate and the westerly circulation, forced by global temperature change.
The eastern margin of the Tibetan Plateau represents one of the morphologically most active regions on Earth, where the interplay of recent crustal deformation and subsequent fluvial landscape ...adjustment has affected the course of continental-scale rivers by river piracy events. Based solely on field observations, such an event has been hypothesised for two of the largest tributaries of the Yangtze River: the Jialing and Hanjiang Rivers. To test this hypothesis, we employ a novel combination of independent methods including a provenance study based on age distributions of detrital zircons from both modern riverbeds and river terraces and a morphometric analysis of river channels and drainage divides. We supported the morphometric analysis with a time-dependent numerical model describing the evolution of river channel long profiles and drainage divides in a succession of river capture events. Analysed zircon ages show clearly distinguishable distributions for the modern Jialing and Hanjiang Rivers, but similar distributions for the recent Hanjiang River up to its topmost terraces. This suggests that the capture of the Hanjiang headwaters by the Jialing River is unlikely to have taken place during the last 1.2 million years. However, several knickpoints in the main stem and the tributaries of the Jialing River cluster at an elevation of about 900 m and separate steeper (downstream) from less steep channel segments (upstream), which is consistent with the morphological expression of a major capture event. χ mapping indicates drainage divide asymmetry at catchment scale with on average steeper rivers on the Jialing side, whereas Gilbert metrics show a symmetric divide at hillslope scale. This numerical model explains this apparent contradiction by the travel time of capture-related knickpoints from the capture point towards the watershed, where χ mapping indicates divide asymmetry immediately after the river capture, while Gilbert metrics are only affected as soon as the knickpoints reach the channel heads and the divide effectively starts moving. Based on knickpoint travel distances and constraints on regional incision / uplift rates, we estimate the possible date of river capture to be the Pliocene. This is earlier than the formation of the terraces investigated in the provenance study but recent enough that most of the drainage divides are still unaffected and currently almost stable. Only the wind gap located in the almost dry valley connecting the two competing drainage systems is likely to have shifted towards the Hanjiang side. We suggest that this resulted in the capture of another important tributary of the Hanjiang River (the Heishui River) by the Jialing drainage system. Our results illustrate the complex evolution of drainage networks along the eastern margin of the Tibetan Plateau, and highlight the importance for combining provenance and morphometric analyses in regions of active landscape rejuvenation where river captures are likely to occur.
•Study of river capture between Yangtze River tributaries, Eastern Tibetan Plateau•Applies provenance, fluvial metrics and modelling to Jialing and Hanjiang Rivers•Modern and river terrace zircon ages suggest that capture is older than 1.2 Ma.•River profile and drainage divide metrics record morphological response of capture.•Modelling suggests a Pliocene capture as a regional response to plateau uplift.
The variability and dynamics of the Asian monsoon (AM, hereafter indicates both of the summer and winter monsoons.) at orbital and millennial timescales has attracted wide interest. Chinese loess ...deposits, covering an area of ~500 × 103 km2 and with a thickness of several tens to more than three hundred meters, is an ideal continental archive to reconstruct AM variations during the Quaternary epoch. Over the past thirty years, since the earliest paper published in an international journal that linked the Asian monsoon and Chinese loess deposits, many studies have been undertaken focusing on this research topic. These results have greatly deepened our understanding of the variations of the AM climate and their driving mechanisms during the past ~2.6 Myr. In this paper, we emphasize recent progress on the AM variability and dynamics revealed by Chinese loess records; in particular, we discuss the reliability and precision of the timescale and the monsoon proxy indicators for the loess-paleosol sequences, which are two fundamental aspects for understanding AM behavior. We analyze what we know and discuss what we do not know about the AM. We refine the timescale for the typical loess-paleosol sequence in the central Chinese Loess Plateau (CLP), which can be used as a new timescale to study palaeoclimate and palaeoenvironmental changes in the monsoonal Asia. We conclude that low-latitude insolation changes induced by precession, and global ice volume, temperature at high-latitudes, as well as sea-level changes forced variations of the AM at orbital time scales. High-latitude cooling events and low-latitude hydroclimate process at millennial time scales, such as climate changes associated with Heinrich events and Dansgaard-Oeschger cycles in Northern Atlantic and Greenland, have also modulated AM variability. We suggest that high-latitude forcing of AM variations occurs through ocean and atmospheric circulation linkages, although the roles of atmospheric CO2, ocean and vegetation feedbacks need further investigation. In future study, high-resolution independent dating, novel proxy indices and transient numerical simulations are still basic tools to understand the loess deposition and AM variations that require considerably more work. Issues such as reliable spatial comparison and regional linkages of records (dependent on precise and accurate numerical chronologies), and quantitative reconstruction of the AM variations, should be given priority in the study the past climate change in Asia, and low-latitude hydroclimate process.
Quantitatively reconstructing precipitation is an important component of paleoclimate research. The mean annual precipitation (MAP) of a closed-basin lake can be estimated by the energy–water balance ...model (EWBM). The EWBM can be divided into linear and nonlinear models, with determination of evaporation over water and land surface being the main components. However, the influence of vegetation changes on different models and the potential of the models to reconstruct vegetation remain unclear. Dali Lake, a closed-basin lake in the marginal region of the East Asian summer monsoon (EASM), is selected as the study region to test the feasibility of a new algorithm, i.e., the comparative method. This method is performed by comparing the reconstructed MAPs given by linear and nonlinear models for each stepwise reduced or increased forest fraction until both MAPs are approximately equal within the error range. The results indicate that the linear model is more sensitive to vegetation changes than the nonlinear model; however, both linear and nonlinear models can obtain the modern MAP correctly. Additionally, the comparative method can successfully determine the vegetation cover. Using the reported paleolake levels of Dali Lake, we calculate the Holocene MAP and vegetation of the Dali Lake basin. The results show that the early and middle Holocene MAP obtained by the comparative method is ∼10 mm higher than that obtained by the nonlinear model, while the difference in the late Holocene is not insignificant. The reconstructed vegetation is generally consistent with the pollen-indicated vegetation changes in the Dali Lake region and the monsoon marginal zone in North China. Our study suggests that both linear and nonlinear models can provide reliable MAP values for basins with known vegetation. For basins with unknown vegetation, the nonlinear model with variable parameters or the more advantageous comparative method is preferred. In cases where the basin mainly consists of two vegetation types, the comparative method enables to reconstruct the fraction of two vegetation types.
•Linear energy-water balance model is more sensitive to catchment vegetation.•New algorithm combines linear and nonlinear energy-water balance models.•Comparative method can reconstruct Holocene precipitation and forest fraction.
Climate factors, including precipitation and related vegetation cover, which are vital features of monsoon climates, differ across global/regional areas and are highly variable on a catchment scale. ...The correlation between fluvial processes and the phases of monsoon climate change are poorly understood in the north-eastern Tibetan Plateau (NETP), partially because direct monsoon climatic proxies from fluvial sediment are lacking. In this study, we compared the fluvial activity with monsoon climate changes reconstructed from pollen analysis, optically stimulated luminescence (OSL) dating, and sedimentary features of terrace sediments in the Datong catchment. The results show that pollen from upstream, for example, 20–30 km from the sample site and transported by water flow, makes the greatest contribution to the pollen assemblages of overbank sediments, which provides reliable regional climatic information. The overbank sediments of the fourth terrace (T4) are dated to around 57.1 ka; beneath these, the lower braided-river gravel was likely deposited during the early part of the Marine Isotope Stage (MIS) 4. The pollen-inferred vegetation is forest-steppe, reflecting a strong monsoon under warm and wet climate conditions during the deposition in the middle sediment unit, possibly during MIS 3. Afterwards, alluvium aggradation ended, and pronounced incision and abandonment was observed in the terrace surface followed by loess deposition which likely occurred during MIS 3–2, at the initiation of the decline of the monsoon. The basal sediment unit of the second terrace (T2) was dated to approximately 6.5–5.0 ka, indicating the start of alluvial aggradation during the Holocene Optimum. Relatively high arboreal pollen from the basal sediment unit of T2 shows a relatively strong monsoon with a warm and wet climate. This was followed by the deposition of the middle sediment unit when the climate was relatively drier with a weak monsoon, recorded by herbaceous vegetation dominated by Artemisia and Chenopodiaceae. It was deduced that during the following cold–warm transition, the river could incise deeply, and this terrace was formed under increasing monsoon conditions. The deep incision and formation of the two studied terraces correlate with different phases of climate change: a warm-to-cold climate transition (decreasing monsoon) for T4 and a cold-to-warm climate transition (increasing monsoon) for T2. We argue that the extreme climatic events at millennial–centennial scales during the Holocene, in addition to orbital-scale changes of monsoon variation, might have led to fluvial thresholds being crossed, causing intense incision and terrace formation. However, the correlation of fluvial processes in the NETP as a response to the East Asian monsoon at different temporal scales could be highly variable.
•Pollen and sediment record are used to study the fluvial response to monsoon climate.•River incision and terrace forming are correlated to transitions of monsoon climate.•Extreme climatic event may cause river aggradation and erosion in NE Tibetan Plateau.
This paper presents new field observations that assist in further defining the extent of Last Permafrost Maximum (LPM) permafrost and frozen ground in north China. In that respect, this region is ...specific by its arid environment and continental position. Bedrock exposures and sand and gravel pits were examined along a south-north transect from southern Ordos as far as the vicinity of Dongsheng District, Ordos City on the Ordos Plateau (North China). Attention focussed upon the recognition of sand-wedge remains and possible cryoturbations. Two types of wedge structures were recognized: type A wedges extend to depths in excess of 3.0 m and are thought to have formed under conditions of continuous permafrost. Type B wedges are restricted to the upper 1.5 m and are thought to reflect discontinuous permafrost and/or deep seasonal frost. OSL dating of wedge infills indicates that type A wedges formed between 23–19 ka and younger type B wedges formed until the early Holocene. It is now possible to delineate a more exact position of the boundary between continuous permafrost and discontinuous permafrost and/or deep seasonal frost on the Ordos Plateau during the LPM. This boundaries occur between Dongsheng and Jinbian at 37°-39°N.
•Precise location of southern limit of continuous permafrost during the Last Permafrost Maximum (c.19-23 ka) in north China.•Narrow zone of discontinuous/sporadic permafrost during the Last Permafrost Maximum (c.19-23 ka) in north China.•Permafrost extent in N China at LPM was specific by its aridity, continentality and proximity to the Siberian High Pressure.
One significant change of terrestrial landscapes in response to past climate change has been the transformation between activity and stability of extensively distributed wind-blown sand dunes. The ...relations between the dynamics of the aeolian landscape and its drivers are not yet completely understood, however. Evidence of aeolian sand deposition during the Last Glacial Maximum (LGM) is scarce in many mid-latitude dune fields, whereas abundant evidence exists for aeolian sand accumulation during the deglaciation, i.e. after about 15 ka. Whether this contrast actually reflects changes in dune activity is still unclear, making paleoclimatic interpretation uncertain. Comprehensive field investigation and luminescence dating in the Mu Us dune field, north-central China, demonstrates that aeolian sands deposited during the LGM are preserved as fills in periglacial sand wedges and beneath loess deposits near the downwind dune field margin. The scarcity of LGM dune sand elsewhere in the dune field is interpreted as the result of intensive aeolian activity without substantial net sand accumulation. Increasing sand accumulation after 15 ka, reflected by much more extensive preservation, signals a change in sand supply relative to sand transportation through the dune field. Reduced wind strength and other environmental changes including regional permafrost degradation after 15 ka transformed the dune field state from net erosion to net accumulation; the dunes, however, remained largely mobile as they were in the LGM. Similar diverging patterns of dune sand accumulation and preservation before and after 15 ka in many mid-latitude dune fields imply broad climatic controls linked to the changes in high-northern-latitude forcing.
•A similar pattern of sand age distribution since the LGM is found in many mid-latitude dune fields.•Scarcity of LGM dune deposits reflects intensive aeolian activity.•Weaken wind and/or greater sediment influx can enhance net sand accumulation.•High-northern-latitude forcing of mid-latitude dune fields via the atmospheric bridge is proposed.•Global warming will have a strong effect on northern mid-latitude dune fields.
River piracy has been reported worldwide and was regarded as one of the main drivers for landscape evolution. In this study, we introduce an example of river piracy in the northern Qilian Shan ...(northeastern Qinghai-Tibet Plateau), and further interpret its regional geomorphological implications. Chi-map analysis, erosion potentiality evaluation, and river profile inversion are used to verify the river piracy. The results confirm that the transverse river, the Hongshuiba River, has captured the longitudinal river, the Zhulongguan River at ~0.35 Ma, due to the headward-migrating erosion of the main stem of the Hongshuiba River. The drainage reorganization, with longitudinal rivers captured by transverse rivers, is a common and continuous landform process along with the outward growth of the northern Qilian Shan. Moreover, the present Qilian Shan watershed geomorphic system could be considered as a transient stage of drainage evolution during the mountain building.
•The Hongshuiba River has captured the Zhulongguan River at ~0.35 Ma•River piracy is common phenomenon in northern Qilian Shan•The present Qilian Shan topography is in a transient stage of drainage evolution during the mountain building.
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