Holocene incisions and flood activities of the Keriya River, NW margin of the Tibetan plateau

Journal Pre-proofs Holocene Incisions and Flood Activities of the Keriya River, NW Margin of the Tibetan Plateau Ping An, LuPeng Yu, YiXuan Wang, XiaoDong Miao, ChangSheng Wang, ZhongPing Lai, Hongyuan Shen PII: DOI: Reference:

S1367-9120(19)30576-0 https://doi.org/10.1016/j.jseaes.2019.104224 JAES 104224

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Journal of Asian Earth Sciences

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25 December 2019 29 December 2019

Please cite this article as: An, P., Yu, L., Wang, Y., Miao, X., Wang, C., Lai, Z., Shen, H., Holocene Incisions and Flood Activities of the Keriya River, NW Margin of the Tibetan Plateau, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104224

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Holocene Incisions and Flood Activities of the Keriya River, NW Margin of the Tibetan Plateau

Ping An1, LuPeng Yu1*, YiXuan Wang2, XiaoDong Miao1, ChangSheng Wang1, ZhongPing Lai3, Hongyuan Shen1 1 Research Center of the Tibetan Environmental Change, Shandong Provincial Key Laboratory of Water and Soil Conservation & Environmental Protection, School of Resource and Environmental Sciences, Linyi University, Linyi 276000, China 2 Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810012, China 3 Institute of Marine Science, Shantou University, Shantou 515063, China

Abstract Fluvial terraces are common in the tectonically active western Kunlun Mountains (WKLM) region, northwestern margin of the Tibetan Plateau, and incision of rivers since 5 ka was attributed to the accelerated uplift of the WKLM. However, it is difficult to evaluate their tectonic or climatic origin without detailed chronology. In this study, Optically Stimulated Luminescence (OSL) dating was applied to fluvial/flood and aeolian sediments on five lowest fluvial terraces (49 m in total) along the Keriya River, and the 21 OSL ages (from 11 samples) revealed the incision processes since the mid-Holocene, especially the fast incision (3.5-0 ka, 10.9 mm/a) during the late Holocene. This was supported by the climatic background, i.e., increased precipitation from the westerlies and meltwater from frequent glacial advance-retreat events. Corresponding author at: Linyi University, Shuangling RD., Lanshan District, Linyi City 276000, Shandong Province, China. E-mails: [email protected], [email protected]. *

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As a response to the climatic changes, large-scale floods were frequent as well, e.g., at 3.5, 2.6, 0.87, and 0.25 ka, which were crucial for the fast incisions on the filling gravels. Additionally, the flood also caused the river’s further extension into the Taklamakan Desert and controlled the evolution of oases in the desert, including ancient cities and cultures. Consequently, we suggest that climatic change was important for the incision and formation of cut-in-fill terraces along the Keriya River during the Holocene, and influences from other factors, including surface uplift, should not be ignored, but were difficult to be evaluated.

Keywords: Fluvial terraces; OSL dating; Holocene climatic change; Palaeoflood; Tarim Basin; Knickpoint.

1. Introduction Climatic or tectonic origins are often two competing hypotheses for river incision and terrace formations, and controversies have been long existed, especially at the tectonically active regions. As the northwestern margin of the Tibetan Plateau (TP) and a region with numerous glaciers (Fig. 1A and B), the Western Kunlun Mountains (WKLM) region is both climatically sensitive and tectonically active in the Quaternary, including Holocene. River terraces in such region may help to reveal the roles of the climate or tectonics on river incision (Fig. 1C). The source region of the Keriya River locates at the conjunction of the two main faults (Fig. 1A and B) and is tectonically active as demonstrated by the Quaternary volcanic rocks, modern volcanic eruption (Liu et al., 1990), and frequent and strong modern earthquakes. The two layers of basalt (ca. 150 m above river-bed level, a.r.l.) erupted during

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1.05±0.11 Ma and 1.43±0.03 Ma (Liu et al., 1990; Otofuji et al., 1995; Zhao et al., 2008) suggested a relatively low long-term incision rate (ca. 0.1 mm/a) in the Keriya region. However, recent Optically Stimulated Luminescence (OSL) and 14C dating of the loess deposit on the T5 in Yangchang region (YC, in Fig. 1C, 49 m a.r.l.) (Han et al., 2014, 2019) suggested a higher incision rate since 8.5 ka. The accelerated incisions were also displayed with T3 (17.29±1.41 ka, 38 m a.r.l.) and T2 (4.71±0.40 ka, 23 m a.r.l.) of the Keriya River near Pulu village, several kilometers to the south of Yangchang region (Fig. 1C, Wang et al., 2004). Wang et al. (2009a) proposed that these terraces were mainly caused by tectonic uplift of the WKLM, which accelerated since 5 ka, after combining terraces of other rivers along the northern piedmont of the WKLM (Fig. 1B), e.g., Kalakash River in the Saitula region (T2, 6 m a.r.l., 5.18±0.47 ka) and the Yurungkash River (T2 of 5.53±0.06 ka). It seems plausible to attribute terrace formation to surface uplift in the tectonically active region. However, climatic fluctuations were common in the TP and westerlies controlled regions during the Holocene (e.g., Cheng et al., 2012; Yu and Lai, 2012, 2014; Yu et al., 2015; Liu et al., 2014; Chen et al., 2016, 2019; Han et al., 2019), which can affect incisions as well. Based on detailed chronology, more recent studies attribute the Holocene fluvial incisions to climatic fluctuations in tectonically active regions, e.g., the Eastern Kunlun Mountain (EKLM) regions, TP (e.g., Chang et al., 2017; An et al., 2018) and the Tian Shan region to the north of the Tarim Basin (TB) (Fig.1B, Wu et al., 2018; Lu et al., 2018). This implies that, with the similar tectonic and climatic background, the contribution of climatic change to fluvial incision processes in the WKLM region should not be ignored. However, the chronologies in former studies (Wang et al., 2004, 2009a) are too limited to demonstrate the origin of terrace sequences during the Holocene.

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OSL dating is suitable for dating the fluvial sediments, e.g., in the WKLM (Wang et al., 2004, 2009a) and ELKM regions (Wang et al., 2009b; Chen et al., 2011; Yu and Lai, 2014; Yu et al., 2015; Chang et al., 2017; An et al., 2018). In this study, it was applied to reveal the incision process of the Keriya River and to discuss its possible forcing mechanisms.

2. Geological setting, sections and OSL sampling The WKLM, with an average elevation of 5,500-6,000 m above sea level (a.s.l.), are part of the northwestern margin of the TP. To its north, locate the TB at 800-1,300 m a.s.l. (Fig. 1A and B). Many rivers originate from the WKLM, transporting water and sediments into the TB. The Keriya River, with a total length of ca. 650 km, originates from the Guliya Ice Cap on the WKLM. After the convergence of the two main tributaries, the Wugeyeke River and the Kulapu River at the YC region, the Keriya River flows northward and vanishes in the Taklamakan Desert (TD, Figs. 1B, 1C, and 3B). However, according to the sedimentary and historic records, with adequate flows the Keriya River can extend north further and support oases and ancient cities in the TD, or even drain into the Tarim River (Fig. 1B, e.g., Yang, 2001; Zhang et al., 2011). The catchment area above the hydrometric station (Fig. 1C, 1967 m a.s.l.) of the Keriya River is 7,358 km2, and its average catchment elevation is 4,832 m. This high elevation makes the river mainly fed by meltwater from glacier and snow (Chu et al., 2002; Lan, 2010). The mean annual flow is 4.92×108 m3, with 11.9% in spring, 66.6% in summer, 14.3% in autumn, and 7.2% in winter, respectively (Lan, 2010). A series of terraces develop along the Kulapu River (Figs. 1D and 2A). Besides the basalt covered highest terrace, there are at least nine lower terraces (T9-T1) in the YC region (Figs.

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1D, 2A, and 5A). The right bank is very steep, where the terraces were not well preserved. A gully flows across the terraces on the left bank allowed the stratigraphy investigation and sampling (Figs. 1D and 2) on T1-T5, except T6-T9. The relative altitudes to the river (a.r.l.) of gravel layers on T0 (floodplain, 2,366 m a.s.l.)-T5 terraces on the left bank are 3 m, 8 m, 25 m, 35 m, 41 m, and 49 m, respectively. The similar composition of thick gravel layers on T5-T1 suggests they are cut-in-fill terraces, and bedrock can be only observed beneath T1 and T0 (Figs. 2C and 3A). These terraces are overlaid by flood sediments and loess (Figs. 2 and 3), and the thicknesses of loess deposits varies from 0.5 to 18 m on different terraces. A hearth was found within the loess on T5 (Han et al., 2014, 2019) of this study, which is the earliest well-dated (7.6-7.0 ka by OSL and 14C) archaeological site in the TB. The Pulu terraces (Wang et al. 2004) locates in another sub-basin several kilometers to the south of the YC site (Fig. 1C), which make it difficult to compare the terraces directly without robust chronology. Sandy lenses were not found within the underlying filling gravel layers, and OSL samples were only taken from the overlying flood sediments on T1-T5 (location of T5: 36.21845° N, 81.52319° E) and loess deposits on T1, T4, and T5 (Fig. 3A) to constrain the time of the river’s incision processes. OSL ages from flood sediments can represent time of the flood events, and if more than one samples were taken from the flood sediments on one terrace (Tn), the lower and upper sample was used to represent the time of Tn’s incision (latest time, from T(n+1) to Tn) and abandonment (earliest time), respectively. The oldest age of overlying loess was regarded as the latest abandonment time when the terrace was no longer affected by floods. Eleven OSL samples were taken by hammering steel tubes (~25 cm long cylinder with a diameter of ~5 cm) into freshly cleaned sections, and bulk samples were taken for dose rate

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and water content analysis as well.

3. OSL dating 3.1 OSL sample preparation and measurement techniques The unexposed samples in the middle part of the tube were treated with 10% HCl and 30% H2O2 to remove carbonates and organic matter, and then followed with wet sieving (38-63 and 63-90 μm) and heavy liquid separation (SPT, 2.62-2.70 g/cm3 for quartz). The 63-90 μm fraction was etched by HF for 40 min to remove feldspar and the α-irradiation affected outer layer of quartz, and then with 10% HCl to remove fluoride precipitates. The etched quartz grains were re-sieved with 63 μm sieves. The mixed samples of 38-63 μm were etched with 35% H2SiF6 for two weeks to dissolve feldspars (Lai et al., 2007; Roberts, 2007) and then with 10% HCl to remove fluoride precipitates. The purity of quartz grains was checked by infrared (IR, λ=830 nm) stimulation. The quartz grains were mounted on the central part of stainless-steel discs using silicone oil. For 38-63 and 63-90 μm quartz, medium (6 mm) and small (2 mm) aliquots were used, respectively. The 38-63 μm samples were measured in Luminescence Dating Laboratory of Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, and 63-90 μm samples were measured in the Luminescence Research Laboratory of Linyi University, China. The luminescence was stimulated by blue LEDs (λ = 470±20 nm) at 130 oC for 40 s using a Risø TL/OSL-DA-20 reader with 90% diode power and was detected using a 7.5 mm thick U-340 filter (detection window 275-390 nm) in front of the photomultiplier tube. Irradiations were carried out using a

90Sr/90Y

beta source in the reader. Preheat plateau test, dose

recovery test, recuperation ratio and recycling ratio were analyzed on sample T3-2 (Fig. 4) to 6

choose a suitable preheat temperature and to check the suitability of Single Aliquot Regenerative (SAR) protocol (Murray and Wintle, 2000, 2003). The preheat plateau test was conducted with four discs under each preheat temperatures of 220, 240, 260, 280 and 300 ℃, respectively. The result (Fig. 4A) shows an equivalent dose (De) plateau at 220-280 ℃, except a little lower De of 240 ℃. The discs for dose recovery tests were bleached by 100 s blue light stimulation under room temperature twice and a pause of 10,000 s in-between (Roberts, 2006), then irradiated by beta source for 200 s as given dose. Recover doses were measured with four discs under each different preheat temperature. The ratios of recovery and given doses (Fig. 4B) display that there was no big difference for different preheating temperatures. Recuperation ratios (Fig. 4C) slightly increase with the temperature. Recycling ratios (Fig. 4D) are relatively stable between 260 and 300 ℃. With these criteria, the SAR protocol is proved to be suitable for the De determination of the fluvial sediments in this region, and 260 ℃ is a suitable preheat temperature. Signals of the first 0.64 s stimulation were integrated for growth curve construction after background (last 10 seconds) subtraction.

3.2 Equivalent dose determination IRSL contamination may cause many nondeterminacy to the quartz OSL ages, e.g., overestimation (difficult to be bleached), underestimation (fading), and affect the shape of growth curve (Lai and Brückner, 2008). Because the contamination of ISRL signals varies among aliquots, it is not adequate to evaluate IRSL contamination by checking a few aliquots before measurement, and each aliquot should be checked. For the single aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000, 2003), both IRSL contamination and

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recuperation were checked for each aliquot by R1’’ (Duller, 2003) and R0 cycle, respectively. However, they could not be checked for aliquots calculated with Standard Growth Curve (SGC) method (Roberts and Duller, 2004; Lai, 2006; Yu and Lai, 2012), which affects the accuracy of SGC method. In this study, we modified the SGC method to increase its reliability and accuracy by adding the criterion of IRSL and recuperation for each aliquot (Tab. 1), which is termed the “IRSL and recuperation checked SAR/SGC (IRC-SAR/SGC)”. In the IRC-SAR/SGC method, standardize natural OSL signals (LN/TN, step 2-6) were first measured as used in SAR protocol (Murray and Wintle, 2000, 2003), and then another 4 steps were added for IRSL check, i.e., an repeated test dose was given (step 7, Dt’=100 s, the same dose with the first Dt), preheated (step 8), and stimulated by IRSL (step 9, T’IRSL) and OSL (step 10, T’pIR-OSL), successively. The ratios of T’IRSL/T’pIR-OSL and Tn/T’pIR-OSL were used to evaluate the contamination of feldspar, and we use the former less than 0.05. Additionally, shapes of decay curves were referred as well, and only aliquots dominated by fast component were selected, e.g., OSL signals can decay to background within 1.5 seconds in this study (Fig. 5A). Aliquots passing the criterion of IRSL were then checked with recuperation, by running a R0 cycle and evaluate the value of L0/T0 and its ratio with Ln/Tn (Murray and Wintle, 2000). The aliquots passing the criterion of both IRSL and recuperation were selected to finish the following SAR cycles (R1-Rn, R0, R1’) or be used to calculate De with SGC. With this method, Des obtained by the IRC-SAR/SGC should be more reliable and accurate, and a lot of time could be saved by rejecting unqualified aliquots beforehand. For each sample, more than 30 aliquots were measured and at least 20 aliquots were selected. 4-10 of them were further used to build growth curves (Fig. 5B) and obtain SAR Des. The 4-10 growth curves were then

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averaged to build an SGC for each sample (red curve in Fig. 5B). Then Ln/Tn values of the other valid aliquots were fitted to their SGC to obtain SGC Des. During the calculation of the De, both the SAR Des and SGC Des were used equally. The mean De was used for the 38-63 μm quartz, while central age model (CAM) and minimal age model (MAM, Galbraith et al., 1999) were used for the 63-90 μm quartz. By the comparison of results from different grain size fractions and age models, the bleaching of OSL signals could be evaluated. The concentrations of U, Th and K (Tab. S1) were measured by neutron activation analysis in the Institute of Atomic Energy, China. For the 38-63 µm quartz grains, the alpha efficiency value was taken as 0.035±0.003 (Lai et al., 2008). The measured water content is often affected by the sampling weather and season, especially when the sediments are exposed in a natural section. To avoid this influence, the average water content is estimated as 5±2% for all samples based on the measured values to cover the possible range in such an arid environment. The dose rates are calculated by Dose Rate and Age Calculator (Durcan et al., 2015) and are shown in Table 2. The OSL ages are shown in Table 2 and S1 and Figure 3A.

4. Dating Results 4.1 Bleaching of OSL signals in flood sediments and age models The main concern for the application of OSL dating on fluvial/flood sediments is whether the sediments were well bleached. OSL ages of 38-63μm quartz (6 mm aliquots) were all older than those of the 63-90 μm (2 mm aliquots) by a few hundred to a few thousand years (Fig. 5C), suggesting the insufficient bleaching of flood sediments. The bleaching of finer

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components (38-63 μm in this study) was even worse, because they may experience shorter transportation time (Sanderson et al., 2007; Gray and Mahan, 2015; Thompson et al., 2018), from older source (Zhang et al., 2010) and more likely to be transported as aggregates (Rittenour, 2008; Gray and Mahan, 2015). All these factors were possible for the flood sediments in this study. For the coarse grains, partially bleaching existed as displayed by the scatter of Des (Fig. 5D-M), including the loess sample (Fig. 5D, E and L). The MAM De was applied to calculate the reliable age. The σb values of 0.05, 0.1, and 0.2 were compared (Tab. 2), and by the σb value of 0.05, the minimal De groups could be selected (Fig. 5D-M), and all the OSL ages are in stratigraphic order, e.g., the loess and flood ages on T1 and T4, and the flood event on T2-T4 during 2.6-2.8 ka. The choice of MAM (σb=0.05) was also proved by the loess sample T5-2, which was dated to 8.3±0.4 ka by CAM of both 38-63 and 63-90 μm fractions, but was calculated to 7.3±0.4 ka by using MAM (σb=0.05), the same with its corresponding

14C

age

(7.28±0.06 ka, mean value of four ages) in the hearth (Han et al., 2014). The age comparisons between flood events and other palaeoenvironmental records (Part 4.2 and Fig.6) further support the application of MAM. Usually, OSL signal of quartz in the aeolian sediments are well bleached, however, the deposition rate was too high in this region, e.g., the mean deposition rates were 2.1 mm/a since 8.5 ka on T5 (Han et al., 2014, 2019) and 4 mm/a since 2.5 ka on T4 (by this study). Consequently, it is possible that the strong dust storms re-transported the proximal insufficient-bleached flood sediments along the valley to the lower terraces occasionally, the poor bleaching environment (darkness, short distance, and short time) resulted in insufficient bleaching with residual ages of a few to several hundred years.

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The bleaching of fluvial sediments should be mainly attributed to the source of deposits (e.g., Zhang et al., 2010; Shen et al., 2015). Flood sediments of the Keriya River should mainly originate from the glacial clasts in the source region and eroded fluvial/flood sediment along the upper stream. The quick transportation by floods with high sediment concentration makes the OSL signals difficult to be sufficiently bleached. We suggest caution should be taken when using OSL to date glacial-sourced flood sediments in the WKLM region, and the application of coarser grains, smaller aliquots and MAM can help to solve the problem. For the selection of σb, the smaller values were suggested for the younger sediments of a few hundred to a few thousand years.

4.2 Chronology of incision processes and flood events All the ages from flood sediments can represent flood events, e.g., at 3.5 ka, 2.6 ka, 0.87 ka, and 0.25 ka. According to the ages of flood and loess, the processes of incision and terrace formation could be estimated as well. The abandonment of T5 happened during 13.6-8.5 ka, however, taking into account that the age of the flood sediments (38-63 μm, 6 mm aliquot) was very possible to have been overestimated, the overlying loess age (8.5±0.6 ka, Han et al., 2014) should be closer to the abandonment event. The river incised to the elevation of T4 before 3.5 ka (T5-T4, 8.5-3.5 ka, 1.6 mm/a) as indicated by the lower flood sediments. After affected by the large-scale flood at 2.6 ka, T4 was abandoned and started to receive loess deposits. The scale of flood at ca. 2.6 ka was very large as demonstrated by the sediments distributed on T2 (2.6±0.1 ka), T3 (2.6±0.2 and 2.8±0.2 ka) and T4 (2.7±0.1 ka), with a height

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difference of over 20 m. There are many interpretations for the sedimentary records of this flood, e.g., (1) the river has incised to T2, and the depth of the flood exceeded 20 m and left deposits on different terraces; (2) the valley was filled from T2 to T4 by the hyper-concentrated flood; and (3) T3 and T2 was incised during the long-lasting or frequent flood event/period. In any case, this flood was very large and had crucial implications for the geomorphologic evolution. By 2.6 ka, the river had incised to the level of T2 (T4-T2, 3.5-2.6 ka, 17.8 mm/a), and there is no estimation for the time of T3’s incision. T2 was influenced by another large-scale flood at ca. 0.87 ka. The ages of flood and loess on T1 constrained the abandonment of T1 to ca. 0.2 ka, however, it might have severely underestimated the incision time of T1. Alternatively, we suggest combining the T2 and T1 together, i.e., the Keriya River incised from the level of T2 to T0 during ca. 2.6-0 ka (8.5 mm/a). According to this incision rate, the river should have incised for 15.6 m by 0.87 ka, consequently, the depth of this flood might be over 17 m. By 0.25 ka, the river should have incised 21 m, close to the modern flood plain, which suggests the depth of this flood of ca. 4 meters. These are just rough estimation to the scale of floods, and more detailed chronology and sedimentology studies are needed in future studies to reconstruct the frequency and magnitude of floods during the late Holocene.

5. Discussions 5. 1 Knickpoint recession and river incision Crustal uplift and climatic changes are regarded as the most common origins for terrace formation, and disputes exist about which is the main factor. More studies emphasized the

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contribution of tectonic uplift on and around the tectonically active TP (e.g., Li, 1991; Wang et al., 2004, 2009a；Miao et al., 2008; Pan et al., 2009; Hu et al., 2010; Chen et al., 2011; Li et al., 2014). Actually, both terrace formation and headward erosion are the results of the knickpoint recession, and the preconditions for river incision are the existence of knickpoints (or knickzone) and adequate erosional ability. The knickpoint is point of abrupt change in the longitudinal profile of streams due to a change in base level (Harris, 1968), surface uplift (Cook et al., 2013; Goren et al., 2014; Wang et al., 2014; Wang et al., 2017), damming and/or filling of the valley (Korup and Montgomery, 2008), lithology change (Haviv et al., 2010), etc. The knickpoint also originates from the natural height difference before the formation/connection of river. In other words, surface uplift is only one way to generate or enlarge knickpoint. Consequently, for rivers not reaching the equilibrium status, with adequate erosional ability, the river will incise until it reaches the equilibrium (Howard, 1965). If the incision is paused, e.g., by climatic change, lithological change, or damming event, the river will erode laterally. Climatic change is the most common and regular factor to cause regional responses to rivers. Though Wang et al. (2004, 2009) concluded the accelerated uplift of the WKLM since ca. 5 ka based on the incisions of the Keriya River and other rivers, the fact is that the young Keriya River is experiencing strong headward erosion, and there are large numbers of knickpoints in the channel to cut through until it reaches equilibrium (Fig. 3B). These knickpoints mainly originate from the high relief between the WKLM and the TB, which accumulated during uplifts of the TP before the Holocene. In contrast, surface uplift amount and its contribution to the increase of the river’s erosional ability should be limited during the Holocene. Additionally, these studied terraces are mainly cut-in-fill terraces, which are usually

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regarded as climatic driven terraces, as in the EKLM region (Wang et al., 2009b; Chen et al., 2011; Chang et al., 2017; An et al., 2018) and Tian Shan region (Wu et al., 2018; Lu et al., 2018).

5.2 Climatic backgrounds for the incision of Keriya River 5.2.1 Increased precipitation and meltwater The Keriya River is mainly fed by the meltwater from the glaciers and precipitation in the source and upper stream regions on the TP (Fig. 1C), so both temperature (insolation) and precipitation (from Asian Summer Monsoon (ASM) and the westerlies) changes in these regions can affect the flow and erosional ability of the river. The incision of T4 happened during 8.5-3.5 ka, mainly during the mid-Holocene, when the ASM was strong and stable (Fig. 6H, Wang et al., 2005) as was recorded in the stalagmites records in the Kesang Cave in the Tianshan region (in Fig. 1B, Cheng et al., 2012), and the westerlies strengthened in Arid Central Asia (ACA) regions (Fig. 6G, Chen et al., 2016). The increased precipitation should have dominated the accelerated incision of the Keriya River during the mid-Holocene. With the retreat of ASM (Wang et al., 2005; Cheng et al., 2012), the Keriya region was mainly controlled by the westerlies during the late Holocene. The characteristic of regions controlled by the westerlies is the increase of precipitation during the late Holocene, as demonstrated by paleosol records in Tian Shan region (Figs.1B and 6C, Chen et al., 2016, 2019), flood records in the South Apes region (Wirth et al., 2013), and modeling results of winter precipitation in ACA region (Chen et al, 2016; Zhang et al., 2017; Li et al., 2018). Consequently, there should be adequate precipitation during the incision of the

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Keriya River from T5 to T0. The increased precipitation during winter to spring (Chen et al, 2016; Zhang et al., 2017; Li et al., 2018) and fluctuant temperature in response to orbital factor (Berger, 1991) and solar variability (Steinhilber et al., 2012) caused frequent expansion-retreat of glaciers on the northern TP (Seong et al., 2009; Liu et al., 2014). The four flood events in this study correspond to the periods of glacial melting recorded in the Kalakuli Lake, Pamir Mts., to the west of the WKLM (Figs.1B and 6E, Liu et al., 2014), suggesting that the glacier expansion-retreat cycles should have happened in the source region of the Keriya River as well, and contributed to the incisions.

5.2.2 Frequent flood and its palaeoenvoronmental implications Compared to the normal flow, floods can cause faster incision and knickpoint recession (Lamb et al., 2014; Baynes et al., 2015), due to the increased bedload abrasion (Cook et al., 2013). Cook et al. (2018) proposed that glacial lake outburst flood (GLOF) may dominate fluvial erosion, and the long-term valley evolution may be driven by GLOF frequency and magnitude, rather than by precipitation on the TP. The frequent modern floods in the Keriya River are mainly caused by meltwater from glacier and snow, rainstorm, and outburst of glacial lakes (Lan, 2010). In geological history, the flood should be frequent in the Keriya River as well. During the late Holocene, increased winter precipitation (Zhang et al., 2017) in the source region of the Keriya River could induce larger-scale snow-melting floods in summer on interannual scale. The combination of increased snow-melting water and rainstorms might have caused frequent floods in summer (Lan, 2010). Additionally, the large-scale flood

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event/period should be more efficient in removing the filling gravels in the channel and causing recession of the knickpoints, e.g., the floods at 3.5 ka, 2.6 ka, 0.87 ka, and 0.25 ka, especially the flood at 2.6 ka (over-20-meter-deep). These four floods correspond to the glacier retreat periods on the TP (Fig. 6E, Liu et al., 2014) and the termination of Holocene ice-rafting events (Bond Events 3-0) in the North Atlantic (Fig. 6F, Bond et al., 2001). Consequently, these floods should be directly related to both global and regional climatic changes. The large-scale flood at 2.6 ka with high volume of runoff and sediment concentration was very possible to belong to GLOF during the retreat of glaciers. The large-scale floods transported large amount of clasts to the valleys in the mid-stream and fans in the downstream. The grain size, Chemical Index of Alternation (CIA), and Rare Earth Elements (REE) records (Fig. 6B, C and D, Han et al., 2019) from the 18-m-thick loess deposits on the T5 of this study displayed abrupt changes following the floods at 3.5 ka and 2.6 ka. This might suggest the increase of new proximal dust sources offered by the flood sediments. The abnormal changes of LREE/HREE ratios, with the maximum at 3.5 ka and minimum at 2.6 ka (Fig. 6D, data from Han et al., 2019), further suggested the changes of sedimentary provenance. Besides the strengthening of wind (Han et al., 2019), the increase of proximal dust source (flood sediments) offer another interpretation for the increased dust deposition rate during the late Holocene, especially the 10-meter-thick loess on T4 since 2.5 ka (4 mm/a). The flood is the most effect way for the river’s extension into the dunefield by resisting the seepage and breaching the dune-dams. All the four flood events in this study correspond to the periods when the Keriya River extended further into the TD or even drainage into the Tarim

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River (e.g., Yang, 2001; Zhang et al., 2011). For example, the Yuansha ancient city developed since ca. 2.6 ka in the further downstream of the Keriya River in the central TD (Zhang et al., 2011), and another famous archaeological site, the Xiaohe Graveyard, at the Lop Nur region, in the eastern TD, developed during 3.7-3.5 ka (Idriss et al., 2007). These are two of the most important periods for the culture development in the TB, and for cultural and ethnic exchange between east and central Asia before 2 ka (Gao et al., 2008). Consequently, this consistency confirmed the reliability of large-scale floods in this study and displayed their implications for the evolution of aeolian and fluvial geomorphology, and the development of oases and human activities on them. In all, incisions of the Keriya River should mainly response to climatic fluctuations, i.e., high precipitation from maximal ASM and strengthened westerlies in mid-Holocene (T5-T4), and increased precipitation from westerlies with frequent glacial melting events and floods during the late Holocene (T4-T0), respectively.

5. 3 Other possible factors affecting incision of the Keriya River Headward erosion of the Keriya River is intensive in the source and upper stream regions on the northwestern TP due to the erosion from glacial movement. The catchment size, which controls the amount of flows, will change abruptly during the reorganization of river systems, e.g., drainage divides migration (Willett et al., 2014) and river/glacier piracy (Yang et al., 2015, 2019; Shugar et al., 2017; Fan et al., 2018). Satellite images display two nearby basins in the source regions of the Keriya River will be captured by headward erosion or spillover breaching in future (Fig. 1C). This suggests that the river system integration should be frequent and

17

associated with fluvial incisions in history, e.g., with frequent glacial expansion-retreat during late Holocene. The incision processes of the Kulapu River in this study were also affected by the evolution of the Wugeyeke River as well, which is the main tributary of the Keriya River and the erosional base for the Kulapu River (Fig. 3B). In addition, other factors like possible surface uplift, lithological changes of bedrocks in the channel (Howard, 1965) and damming (Korup and Montgomery, 2008) of the channel may affect the incision and lateral erosion processes as well. All these factors make the incision processes and terraces formation more complex, however, their influences are difficult to evaluate.

6. Conclusions Processes and mechanisms of river incision and terrace formation since the deglaciation are investigated based on the OSL chronology of terraces of the Keriya River. The flood sediments in this study were not well bleached, and we suggest coarser grain size, small aliquot, and MAM are more appropriate for OSL dating of fluvial/flood sediments. The detailed OSL chronology revealed the fast incision since the late Holocene, with increased precipitation from the westerlies and adequate meltwater from frequent glacial expansion-retreat cycles. Additionally, the increased flood frequency and magnitude were important for the incision. Four flood events were identified at ca. 3.5 ka, 2.6 ka, 0.87 ka, and 0.25 ka, matching well with the previously identified climatic events of the North Atlantic and TP, which further emphasized the importance of climatic changes. We suggest that climatic fluctuation is the most common factor to cause regional periodical incision, especially for the cut-in-fill terrace during the

18

Holocene, and more factors should be taken into account, instead of attributing incisions to tectonic uplift simply. Additionally, flood is an important surficial process based on climatic change, not only for the evolution of the valley, but also for the evolution of oases in the downstream in/around the desert, where human activities, cultural and ethnic exchanges rely on.

Acknowledgments We thank the two anonymous reviewers and the editors for their valuable comments and suggestions on the manuscript, and Prof. Han WenXia and Prof. Li GuoQiang for discussion. This study was supported by NSFC (41761144073, 41672167 and 41462006).

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Figure and table captions Fig. 1 A, Locations of the Tibetan Plateau, Yangchang (YC) site, and the dominant wind systems (ISM: Indian Summer Monsoon, EASM: East Asian Summer Monsoon). B, Locations of the Keriya River and other rivers, sites mentioned in this study (e.g., Yuansha ancient city (Zhang et al., 2011), Xiaohe Graveyard (Idriss et al., 2007), Kalakuli Lake (Liu et al., 2014), LJW10 paleosol (Chen et al., 2016), and Kesang Cave (Cheng et al., 2012)), cities/towns (white dots), and topography in surrounding regions. C, Source region of the Keriya River, with red lines display the rivers (the right longer tributary is the Wugeyeke River, while the left shorter one is the Kulapu River) and blue lines shows the area of the catchments; D, The terraces along the left bank of the Keriya (Kulapu) River in the YC region.

Fig. 2 Field photos of terraces (A and B), sections (C), and samplings (D and E) in the YC region.

Fig. 3 OSL ages of the sections (A) and longitudinal profile of the Keriya River (B). The ages marked with * are cited from Han et al., (2014), and the underlined one is a

14C

age.

Fig. 4 Preheat plateau (A), recycling ratio (B), recuperation ratio (C), and dose recovery ratio (D) of a flood sediment sample (T3-2) under different preheat temperatures.

Fig. 5 Decay curves (A) and growth curves (red for SGC) of sample T4-3. C displays the comparisons of coarse-grain quartz (2 mm aliquot, MAM/CAM) OSL ages with their

30

corresponding medium-grain quartz (6 mm aliquot, mean De) OSL ages or AMS

14C

age (only

for sample T5-2, from Han et al., 2019). D-M show De distribution (Abanico Plot, Dietze and Kreutzer, 2019) of all the samples dated with coarse-grain quartz, in which the lighter and darker bars display results of MAM (σ=0.05) and CAM, respectively. In D, the red arrow on the right mark the De suggested by AMS

14C

age of a big charcoal from the hearth located 5 cm

beneath this sample, confirming the choice of MAM (σ=0.05).

Fig. 6 Estimated incision rates during different periods (A) and comparison with grain size, CIA and REE records of the YC loess (B, C and D, Han et al., 2019), glacial extend/retreat record in the Kalakuli Lake (E, Liu et al., 2014), Bone events in North Atlantic (F, Bond et al., 2001), paleosol records of LJW10 section (G, Chen et al., 2016), and stalagmites δ18O records from Dongge Cave (H, Wang et al., 2005). The red circles display the ages of flood in YC region, while the red squares show the time of archaeological site in YC site (Han et al., 2014), and in the Taklamakan Desert (Zhang et al., 2001, Yidriss et al., 2007).

Tab.1 IRSL and Recuperation Checked SAR/SGC (IRC-SAR/SGC) procedure.

Tab. 2 Details of OSL dating results. The symbols ^ and # mark the results of 38-63 μm and 63-90 μm quartz, respectively.

Tab. S1 Supplementary information for OSL samples and ages.

31

Tab.1 IRSL and Recuperation Checked SAR/SGC (IRC-SAR/SGC) procedure. Step

Treatment

Observed

1

Give dose Di

-

2

Preheat , at 260℃ for10 s

-

3

OSL, at 125°C for 40 s

Li

4

Give test dose, Dt=100 s

-

5

Cut-heat, at 220℃ for 10 s

-

6

OSL, at 125°C for 40 s

Ti

Step 7-10 added for IRSL check

-

7

Give test dose, D’t=100 s

-

8

Cut-heat, at 220℃ for 10 s

-

9

IRSL, at 50℃ for 40 s

T’IRSL

10

OSL, at 125°C for 40 s

T’pIR-OSL

11

R0 cycle (step2-6) for recuperation check If passed the IRSL and recuperation check, finish the rest SAR cycles or use SGC.

32

L0/T0

Tab. 2 Details of OSL dating results. Sample Dose rate^

De^

Age^

Dose rate^

OD#

CAM De#

MAM De# (Gy)

(Gy/ka)

(Gy)

(ka)

(Gy/ka)

(%)

(Gy)

σb=0.2

T1-1

3.54±0.09

1.52±0.06

0.44±0.02

3.35±0.09

29

1.79±0.28

1.47±0.44

T1-2

3.61±0.09

2.47±0.07

0.68±0.03

3.41±0.09

41

1.31±0.12

T2-2

3.42±0.09 13.73±0.31

4.2±0.2

3.23±0.08

14

T2-3

3.68±0.09

6.04±0.18

1.6±0.1

3.47±0.08

T3-1

3.47±0.09 14.51±0.75

4.1±0.2

T3-2

3.56±0.09 15.25±0.37

T4-1 T4-2

σb=0.05

(σb=0.05, ka)

0.45±0.09

0.44±0.09

0.13±0.03

0.89±0.17

0.85±0.14

0.84±0.13

0.25±0.04

11.13±0.34

11.12±0.34

10.64±0.69

8.40±0.46

2.6±0.1

22

4.06±0.23

3.70±0.44

3.15±0.33

3.02±0.26

0.87±0.08

3.27±0.08

25

13.15±0.70

11.92±1.26

9.63±0.82

8.98±0.53

2.8±0.2

4.2±0.2

3.40±0.09

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12.50±1.01

11.65±1.13

10.13±1.07

8.71±0.74

2.6±0.2

3.52±0.09 18.46±0.40

5.2±0.3

3.33±0.09

20

11.89±0.55

11.73±0.85

9.53±0.80

9.14±0.67

2.7±0.2

3.68±0.09 15.85±0.41

4.3±0.2

3.45±0.09

22

11.99±0.55

10.84±1.14

9.15±0.72

8.72±0.58

2.5±0.2

T4-3

3.24±0.09 25.11±0.68

7.6±0.4

3.06±0.08

22

15.33±0.80

14.66±1.52

12.02±1.18

10.62±0.94

3.5±0.3

T5-1

3.86±0.10 52.52±2.04

13.6±0.7

-

-

-

-

-

-

-

T5-2

3.17±0.08 26.45±0.64

8.3±0.4

2.98±0.08

15

24.63±1.04

24.92±1.70

22.62±1.33

21.66±0.90

7.3±0.4

ID

σb=0.1

MAM Age#

The symbols ^ and # mark the results of 38-63 μm and 63-90 μm quartz, respectively.

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GA

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Highlights >OSL dating was used to date Holocene flood sediments on terraces of the Keriya River. >The bleaching was not good, and MAM, coarse grains, and small aliquots were used. >Climatic factors of precipitation and glacial meltwater controlled the incisions. >Flood is an important climatic factor for the fast incision during the late Holocene. >Flood affected river’s extension and development of oases in the Taklamakan Desert.

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CRediT author statement

Ping An: Investigation, Data Curation, Writing - Original Draft, Visualization. LuPeng Yu: Supervision, Conceptualization, Methodology, Resources, Writing - Original

Draft, Project administration. YiXuan Wang: Resources. XiaoDong Miao: Writing - Review & Editing. ChangSheng Wang: Software, Data Curation ZhongPing Lai: Resources. Hongyuan Shen: Writing - Review & Editing.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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We the undersigned declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the Corresponding Author is the sole contact for the Editorial process. He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. Signed by the corresponding author:

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