Earth and Planetary Science Letters 515 (2019) 79–89

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Earth and Planetary Science Letters

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Evidence for early (≥12.7 Ma) eolian dust impact on river chemistry in the northeastern Tibetan Plateau ∗ ∗ Xiaobai Ruan a,c,d, Yibo Yang a,b, , Albert Galy d, Xiaomin Fang a,b,c, , Zhangdong Jin e, Fei Zhang e, Rongsheng Yang a,c,d, Li Deng e, Qingquan Meng f, Chengcheng Ye a, Weilin Zhang a,b a Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, b CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), Beijing 100101, China c University of Chinese Academy of Sciences, Beijing 100049, China d Centre de Recherches Pétrographiques et Géochimiques, UMR7358, CNRS, Université de Lorraine, 54500 Vandoeuvre les Nancy, France e State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China f School of Earth Sciences & Key Laboratory of Western China’s Mineral Resources of Province, University, Lanzhou 730000, China a r t i c l e i n f o a b s t r a c t

Article history: As one of the largest dust sources on the Earth’s surface, dryland in Central Asia gives rise to thick eolian Received 12 October 2018 deposits over East Asia (e.g., the Chinese Loess Plateau, CLP) and significantly influences the regional Received in revised form 9 March 2019 hydrochemistry in the downwind drainage areas. However, the formation of thick eolian dust deposits Accepted 13 March 2019 requires not only climatic prerequisites for dust emission and transport but also climatic and topographic Available online 27 March 2019 conditions favourable for deposition and accumulation. The scarcity of widespread eolian deposition Editor: D. Vance around the CLP before 7-8 Ma hinders a full understanding of the processes and mechanisms of Central Keywords: Asian aridification. The deposition of eolian dust also impacts the hydrogeochemistry of fluvial systems eolian dust and the precipitation of authigenic phases in continental sedimentary systems could be an archive for Sr isotopes studying eolian dust dynamics when pure eolian deposits are scarce. Here, we present the Ca-Mg-Sr carbonate concentrations and 87Sr/86Sr isotope compositions of bulk carbonates in a new fluvial sequence (12.7-4.8 Linxia Basin Ma) of the Basin. The Mg/Ca and Sr/Ca ratios of the carbonate describe a power law relationship Xining Basin with a power coefficient of ∼0.8, lower than the coefficient characteristic of prior calcite precipitation (PCP). An input of eolian dust with the dissolution of Mg-rich carbonate is likely responsible for the deviation from a pure PCP process. The bulk carbonates also show a general decrease of 87Sr/86Sr ratios from 12.7 to 4.8 Ma, with a transition around 8.6 Ma revealed by lower Sr/Mg ratios. The comparison of these proxies to a previously reported fluvial section (12.2-5.1 Ma) in the Linxia Basin, ∼200 km to the southeast, shows that the 87Sr/86Sr ratios of the bulk carbonates and water-soluble salts in the Linxia Basin are around 0.7098, which is 0.0018 lower than those in the Xining Basin before 8.6 Ma, but shows a significant rise between 8.6 and 7.0 Ma. The two basins share the same range of carbonate 87Sr/86Sr ratios when sediments are younger than 7 Ma. For the last 7 Myrs, the evolution of the 87Sr/86Sr ratios in bulk carbonates of fluvial sediments and Pliocene-Quaternary eolian deposits found in the Xining Basin are similar to those in typical eolian red clays/loess-palaeosol sequences on the CLP. These results suggest a transition of the hydrochemical regime at 8.6 Ma in the Linxia Basin from a catchment only influenced by the weathering of its bedrock to one significantly impacted by eolian dust input. In the Xining Basin, the carbonate elemental and 87Sr/86Sr ratios are consistent with a hydrochemistry more impacted by the presence of the eolian dust. There, the dust input occurred earlier, at ≥12.7 Ma, though it has strengthened since 8.6 Ma. The eolian dust impact on fluvial systems in the Xining Basin was much earlier than in the Linxia Basin and also preceded the initial accumulation of widespread eolian red clays on the CLP (7-8 Ma), suggesting a temporally propagating and spatially stepwise expansion of eolian dust delivery across the Asian inland during the late Cenozoic. © 2019 Elsevier B.V. All rights reserved.

1. Introduction

* Corresponding authors at: CAS Center for Excellence in Tibetan Plateau Earth Dust can influence both the regional and global climate (Kok et Sciences, Chinese Academy of Sciences (CAS), Beijing 100101, China. al., 2018)throughits direct impacts on incoming solar and terres- E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Fang). https://doi.org/10.1016/j.epsl.2019.03.022 0012-821X/© 2019 Elsevier B.V. All rights reserved. 80 X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 trial radiation (Tegen et al., 1996), its indirect impacts on cloud 2. Geological settings and stratigraphy formation (Sassen et al., 2003), and iron fertilisation for ocean phytoplankton production (Martin and Fitzwater, 1988). Since the The Xining Basin lies on the northeastern margin of the TP, Messinian (7.2-5.3 Ma), the arid part of Central Asia has been at an average elevation of 2100 m above sea level. The Xining one of the largest dust sources (An et al., 2001; Engelbrecht and Basin is dominated by an arid/semi-arid continental climate with Derbyshire, 2010), delivering dust to East Asia (Tanaka and Chiba, occasional dust storms during spring. The basin is confined by three NW-SE-trending dextral transpressional faults, is surrounded 2006), the Pacific Ocean (Rea et al., 1998), and even North Amer- by the Laji Shan, Riyue Shan and Daban Shan mountains to the ica and Greenland (Jaffe et al., 1999;Bory et al., 2002). Dust south, west and north, respectively (Dai et al., 2006), and opens emissions from Asian desert- and loess-covered regions and allu- to the Minhe-Lanzhou Basin to the east (Fig. 1). The Laji Shan vial piedmonts with poor vegetation (Nie et al., 2015), give rise is mainly composed of Palaeozoic basic to intermediate volcanic, to thick eolian deposition on the relatively flat and stable high- volcaniclastic, and clastic rocks, while the Daban Shan is com- lands in East Asia (Guo et al., 2002) and significantly influence posed of early Palaeozoic marine clastic, volcanic and volcaniclastic the regional hydrochemistry in the downwind drainage area (Jin rocks (Wang et al., 2015). A more than 2000 m-thick Cenozoic et al., 2011). Such dust related deposits are therefore highly sen- sedimentary succession lies unconformably on Jurassic-Cretaceous sitive indicators of the regional evolution of the landscape and terrestrial clastic rocks (Wang et al., 2015; Zhang et al., 2016), in- climate. However, the accumulation of thick and continuous eo- cluding the Palaeogene Xining Group and Neogene Guide Group. lian deposits during the late Cenozoic (e.g. on the CLP) requires The Xining Group is characterised by reddish siltstones and mud- not only climatic prerequisites for dust emission and transport, stones intercalated with gypsum beds, and the Guide Group is but also relatively flat, stable topography and relatively dry condi- characterised by siltstones and mudstones intercalated with sand- tions to minimise erosion and generate significant deposition (Pye, stone or conglomerate beds, with the grain size gradually becom- 1995; Guo, 2017). The existing pure eolian red clays widely spread ing coarser upward along the sequence (Dai et al., 2006; Zhang et around the CLP are not older than 7-8 Ma (Ding et al., 2001; al., 2016). The Cenozoic sediments have now been cut through by the Huangshui River and its tributaries, which have formed river Qiang et al., 2001), which prevents a full understanding of the on- terraces mantled by loess and eolian red clays (Lu et al., 2004; set and evolution conditions of dust emission and deposition prior Zhang et al., 2017). to that time. The Mojiazhuang (MJZ) section is located in the northeastern The fluvial basins in the northern Tibetan Plateau (TP) are char- part of the Xining Basin, 25 km northeast of Xining City (Fig. 1; acterised by distinct river water cation compositions during the Yang et al., 2017b). This 336 m-thick section includes the late spring through the dissolution of eolian dust carbonates and salts Miocene Xianshuihe Formation and the Pliocene Linxia Formation. along with secondary calcite precipitation (Jin et al., 2011). There- The section was dated by high-resolution magnetostratigraphy, fur- fore, detailed spatial and temporal investigations of the eolian dust ther constrained by the occurrence of late Miocene mammal fos- impacts on hydrological conditions in arid drainage areas could sils (Chilotherium wimani, Hipparion dongxiangense and Parelasmoth- be a useful tool in understanding regional dust emissions dynam- erium sp), and is considered to be deposited between 12.8 Ma ics linked to the late Cenozoic climate change and tectonic uplift and 4.8 Ma (Fig. 2; Yang et al., 2017b). Three sedimentary facies of the northern TP. Lacustrine or fluvial carbonates and salt min- are identified in the section (Fig. 2). The lower part of the sec- erals (e.g., gypsum) are suitable hosts for tracing hydrochemical tion (0-137 m, 12.8-8.6 Ma) is dominated by distal sedimentation changes induced by eolian dust, especially when their 87Sr/86Sr ra- of yellow-brownish mudstone with laminated marl, indicating a tios (e.g., Naiman et al., 2000;Van der Hoven and Quade, 2002; floodplain with shallow lakes. The interval from 0-58 m includes greyish-green laminated marl and palaeosol complexes with clearly Jacobson, 2004) or Nd isotopes (Jacobson and Holmden, 2006)are identified Bt and Bk horizons (Fig. 2). The middle part of the sec- combined with other geochemical proxies. Carbonates and gypsum tion (137-252 m, 8.6-6.3 Ma) records a braided river environment are widespread in semi-arid and arid fluvial settings, and fluvial dominated by massive brownish mudstone and siltstone (Fig. 2) in- and palaeosol carbonate compositions in the Linxia Basin on the tercalated with conglomerate beds (1-3 m in thickness) or lenses northeastern TP have already demonstrated the importance of the (Fig. 2). The upper part of the section (252-336 m, 6.3-4.8 Ma) regional eolian dust impact in the long-term evolution of basin flu- corresponds to an alluvial fan facies, characterised by massive and vial chemistry (Yang et al., 2017a). poorly sorted conglomerate beds (more than 10 m in thickness, To increase the spatial and temporal knowledge of the late Fig. 2). The top conglomerate is overlain by a 12 m-thick reddish Cenozoic eolian dust influence on fluvial basins, we document a mudstone layer (Fig. 2), wedging out horizontally and covered by new sedimentary record of the eolian impact on fluvial hydro- loess, which is thought to represent eolian red clays or fluvial sed- chemistry, in the Xining Basin on the rim of the northeastern TP, iments incorporating a large portion of eolian material. Combined and compare this record with the existing archive in the Linxia with the dominant southerly palaeocurrents (Fig. 2), the sedimen- Basin. Westerlies and the East Asian winter monsoon are inferred tation rate (Fig. 2) indicates that the Daban Shan has experienced ∼ to be the dominant atmospheric circulation systems transporting rapid uplift since 8.1 Ma (Fig. 2b; Yang et al., 2017b). Asian dust to the North Pacific during the late Cenozoic (Nie et al., 3. Materials and methods 2014, 2018). The new late Cenozoic fluvial section in the Xining Basin and the previously reported section in the Linxia Basin are To determine the mineralogical composition of the sediments, both located along these circulation pathways (Fig. 1). The stud- representative samples of loess, red clays and MJZ-section sed- ied section in the Xining Basin was precisely dated from 12.7-4.8 iments were selected for XRD analyses at the Micro Structure Ma by high-resolution magnetostratigraphy and two mammal fos- Analytical Laboratory, Peking University, on the Rigaku D/max-rA ◦ ◦ sils (Yang et al., 2017b), allowing a comparative study with the diffractometer (Cu, Ka1, 1.5406 Å, 40 kV, 40 mA, 3-35 C at 0.01 ◦ fluvial section (12.2-5.1 Ma) in the Linxia Basin. Such comparison steps, 10 /min). Carbonate carbon and oxygen isotopic composi- of the late Cenozoic eolian impact history aims to unravel the spa- tions were measured to provide auxiliary indicators of the origins tial evolution of the eolian dust impact on fluvial chemistry in the of the MJZ sedimentary carbonates. 20 samples with distinct car- northeastern TP. bonate Mg/Ca ratios were selected and analysed on a Finnigan X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 81

Fig. 1. (a) The locations of the Xining Basin, the Linxia Basin, the Lingtai eolian deposition sequence, and the inferred prevalent westerlies and East Asian winter monsoon paths during the late Miocene and Pliocene (modified after Chen and Li, 2013 and Nie et al., 2014). (b) Geological map of the Xining Basin (modified after Dai et al., 2006). (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)

MAT-252 stable isotope mass spectrometer at the Northwest Insti- search, Chinese Academy of Sciences (ITP-CAS), Beijing. The results tute of Eco-Environment and Resources, Chinese Academy of Sci- were normalised to the weight of the bulk oven-dried sample. ence, Lanzhou. The analytical precision was better than 0.02h for Replicate analyses show the relative standard deviation for all ele- δ18O and δ13C. ments of less than 2%. 271 mudstones and siltstone/fine sandstones were collected at 23 samples that span most of the MJZ section and have differ- 1-2 m intervals in the MJZ section. To compare the MJZ sec- ent Mg/Ca-Sr/Ca ratio patterns were chosen, as were eolian dust tion fluvial sediments with typical eolian deposits from the Xining samples including 2 MJZ loess, 2 PZS loess and 3 HW red clays, Basin, Pliocene-Quaternary eolian samples from various locations for 87Sr/86Sr ratio analyses of the HOAc and water leachates. Pre- and ages were included in this research. The samples consisted treatment for Sr isotope analysis was performed in the Class 10000 of 5 loess samples mantling the MJZ section, 11 loess samples clean lab at the Institute of Earth Environment, Chinese Academy from the Panzishan (PZS) borehole (Lu et al., 2004), and 32 red of Science (IEE-CAS) in Xi’an, China. The Sr separation includes clay samples from the Houwan (HW) section (Zhang et al., 2017) the use of a Sr-spec ion-exchange column (Eichrom Technologies) (Fig. 1b). Identical analytical investigations of the MJZ sediments and follows the standard procedure described in Jin et al. (2011). were used for those Pliocene-Quaternary eolian samples. All sam- The Sr isotopic ratios were measured using multi-collector ICP- ples were treated by two-step water/1 M acetic acid (HOAc) leach- MS (Thermo Finnigan Neptune Plus) at the IEE-CAS. The measured ing to obtain the water-soluble and carbonate fractions following 87Sr/86Sr ratios were normalised to 86Sr/88Sr = 0.1194. The Sr the method described by Yang et al. (2015). The concentrations of standard SRM 987 yielded a mean value of 0.710300 ± 0.000030 Ca, Mg and Sr in the HOAc leachates were determined using in- (n = 10) during duplicate and periodic checks. ductively coupled plasma optical emission spectrometry (ICP-OES) The carbonate content of 27 MJZ samples with different Ca con- (Leeman Labs Prodigy-H) at the Institute of Tibetan Plateau Re- centrations in the HOAc leachate was also estimated by directly 82 X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89

Fig. 2. The stratigraphy of the MJZ section. (a) Lithology and sedimentary facies; (b) geomagnetic polarity (Yang et al., 2017b); (c) sedimentation rates; (d) palaeocurrents and gravel statistics; (e) loess and mudstone layers covering the top conglomerate layer; (f) thick conglomerate with mudstone lens at 71-81 m; (g) thick uniform mudstone/silt- stone at 123-133 m; (h) gravel layer at 198 m; (i) palaeosols intercalated with grey greenish marl layers at 284-297 m; (j) well-developed palaeosols at 314-317 m; and (k) marl layers and calciferous mudstone (strong palaeosols) at 324-327 m. measuring the carbon content of carbonate minerals with an auto- low-carbonate samples. The result of the comparative test indicates matic carbon analyser. The inorganic carbon content was measured that the diluted HOAc leachates of the samples with low carbonate by a Shimadzu TOC-VCPH carbon analyser in ITP-CAS, with 50% contents may not fully represent the carbonate compositions, and phosphoric acid applied to dissolve the carbonates during the mea- thus, in the following, we used only samples containing >3% car- surement. bonate samples (based on the [Ca] and [Mg] in the HOAc leachate) to evaluate the carbonate elemental and Sr-isotope compositions 4. Results in the MJZ section. The carbonate Ca concentration of the MJZ section range from Clay minerals, quartz, microcline, albite, and mica were de- 10525 to 213770 μg/g, Ca/Mg, Ca/Sr, and Sr/Mg ratios range tected as major silicate minerals in all samples of the MJZ sec- from 4.7 to 71.4 mol/mol, 0.5 to 5.2 mol/mmol and 4.9 to 19.6 tion, with no significant content changes along the whole section mmol/mol, respectively (Fig. 5). The Ca concentration (a good (Fig. 3). Eolian (loess and red clays) samples exhibit silicate mineral proxy for the carbonate content of the sediment), Ca/Mg and Ca/Sr compositions similar to those of the MJZ section samples. Among ratios exhibit in-phase variations (Fig. 5), and the Ca/Mg and Ca/Sr the carbonate minerals, calcite is abundant in the eolian samples and the pre-8.6 Ma section samples (0-137 m), and less abundant ratios are positively correlated (Fig. 6). Eolian samples from the in the post-8.6 Ma section samples (137-336 m). Dolomite appears Xining Basin, MJZ loess and PZS loess have uniform elemental con- only in the eolian samples, and red clays contain relatively more centrations, and the HW red clays have compositions similar to dolomite than does loess (Fig. 3). The carbonate content can be es- those of the loess except for a very scattered, but on average, lower timated based on the data from the total carbon analyser, or from Ca/Mg ratio (Fig. 6). the [Ca] and [Mg] in the HOAc leachate. These two measurements The most striking feature of the carbonate records of the MJZ (Fig. 4) show excellent consistency when the carbonate contents section is the clear transition period at ∼8.6 Ma. This transition are greater than 5%, but the consistency becomes poor for very can be identified based on the Ca concentration and Ca/Mg and low-carbonate samples (Fig. 4), suggesting that significant amounts Ca/Sr ratios. The carbonate content is 16.7 ± 6.3% before 8.6 Ma of non-carbonate Mg and Ca are leached by the 1 M acetic acid in and 10.6 ± 6.3% after. The Ca/Mg molar ratio is 27.6 ± 17.0 before X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 83

Fig. 3. XRD diffractograms of representative samples from the MJZ section sediments and eolian deposits in the Xining Basin.

Fig. 4. Correlation of the CO2 contents determined by two methods. The vertical axis represents the CO2 content calculated from the Ca and Mg concentrations in HOAc leachates, assuming that the measured Ca and Mg were from CaCO3 and MgCO3, respectively. The horizontal axis represents the CO2 content measured by an automatic carbon analyser through the gas volume method. The five samples plotted on the vertical axis near 0.001 mmol/g have CO2 contents below the detection limit of the carbon analyser. and 16.4 ± 10.3 after while the Ca/Sr ratio dropped from 2.0 ± and Ca/Sr ratios, which are close to the corresponding values of 1.2 mol/mmol to 1.6 ± 0.9 mol/mmol after the transition (Fig. 5). the loess mantling the MJZ section (Fig. 5). These characteristics In contrast, the Sr/Mg ratios increase from the bottom to ∼11 Ma imply that post-8.6 Ma samples have a carbonate composition that and then decrease slowly towards low values at ∼8.6 Ma (13.8 overlaps the composition of almost all the local eolian carbonate mmol/mol on average) and continue to oscillate around an average materials, except ∼1/3 of the red clays samples from HW with of 10.6 mmol/mol upward (Fig. 5). Mg/Ca ratios co-vary with Sr/Ca Mg/Ca molar ratios above 0.2 (Fig. 6). ratios and follow a power law with samples older than 8.6 Ma Among the 20 samples analysed for their carbon and oxy- and younger than 8.6 Ma characterised by an exponent (slope in a gen isotopic composition of carbonate, only 9 samples (covering log-log plot) of 0.828 and 0.784, respectively (Fig. 6). The transition both pre- and post-8.6 Ma samples) yielded reliable results, since at ∼8.6 Ma also corresponds to a sedimentological boundary with the other 11 samples have carbonate contents that are too low. the lower section dominated by a floodplain environment replaced The δ18O ranges from −8.4h to −4.2h, and the δ13C ranges by a braided river environment above. The reddish mudstone layer from −6.2h to −4.4h on the VPDB scale (Fig. 7). The corre- above the top conglomerate bed (deposited from 5.2 to 4.8 Ma) is lation between δ18O and δ13Cis significant (p = 0.025). All the characterised by a carbonate content of 11.8 ± 3.8% but low Ca/Mg HOAc leachate yielded reliable 87Sr/86Sr ratios, while only 44% 84 X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89

Fig. 5. Time series of the carbonate Ca, concentration and Mg/Ca, Sr/Ca and Sr/Mg ratios of the MJZ section. Star symbols above each data series correspond to loess samples on the top of the MJZ section. The boundary at 8.6 Ma, marked by a dashed line, also corresponds to a sedimentological boundary with the lower section dominated by a floodplain environment replaced by a braided river environment above. The top reddish mudstone layer is marked by red shading.

Fig. 6. (a) Carbonate Mg/Ca versus Sr/Ca ratios on a logarithmic scale, with the MJZ section sediments and different types of eolian deposits from the Xining Basin. Grey cross represents carbonate bedrock data from the NE Tibetan Plateau (Yang et al., 2015). The frame outlined with dashed lines delimits the extent of Fig. 6b. The beige diamond represents the carbonate modern dust endmember (Mg/Ca ratio, 124 mmol/mol and Sr/Ca ratios, 1.08 mmol/mol), representing the average of modern surface sediment carbonate compositions from arid soils and loess from north China (Yang et al., 2017a). The black curve corresponds to the mixing of the average composition of local low-Mg loess with the average composition of the dolomite. Nearly half of the red clay samples from the HW section can be explained by such mixing. (b) Zoom of part of (a), distinguishing the pre-8.6 Ma and post-8.6 Ma samples from the MJZ section of the Xining basin. The carbonate compositions of the HLD section in the Linxia Basin (Yang et al., 2017a)are reported as triangles and separated into younger (open triangle) and older than ∼8.6 Ma. The green and blue straight lines represent the best fit by power laws of the pre- and post-8.6 Ma sub-datasets, respectively. The black straight line represents the PCP evolutionary path, with a slope of 0.97 (corresponding to DSr = 0.06 and DMg = 0.03) and is the best fit of the pre-8.6 Ma data of the HLD section. The numbers noted along the PCP evolution correspond to the fractions of the initial Ca remaining in the solution. The three beige curves represent the mixing of the average composition of modern dust with a carbonate composition affected by three different (0, 50 and 80%) amounts of Ca precipitation as calcite, with a mix-proportion interval of 20% marked by the beige crosses. and 57% of the MJZ section and eolian samples, respectively, re- The 87Sr/86Sr ratios of HOAc leachate for the loess are nearly con- leased enough Sr to return reliable water-soluble 87Sr/86Sr data. stant with values of 0.7107 and 0.7105 for MJZ and PZS, respec- The 87Sr/86Sr ratios of the water leachate and HOAc leachate of the tively; and those of water leachate for MJZ loess are from 0.7115 MJZ section samples vary between 0.7113 and 0.7127 and between to 0.7116. The 87Sr/86Sr ratios in HOAc and water leachate of the 0.7111 and 0.7131, respectively. The 87Sr/86Sr ratio curves of both HW red clays vary from 0.7111 to 0.7112, and around 0.7113, re- HOAc and water leachate show similar decreasing trends (Fig. 8). spectively. X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 85

5. Discussion

5.1. Eolian dust identification in the MJZ section fluvial sediments

In fluvial environments, the precipitation of secondary carbon- ate (calcite) will change the chemistry of the river water. Such evolution can be geochemically described by a Rayleigh distillation equation that generates a positive correlation between the Mg/Ca and Sr/Ca ratios, and is often described as a prior calcite precip- itation (PCP) process (Fairchild and Treble, 2009;Sinclair, 2011; Yang et al., 2015). The range of compositions of the calcite formed during PCP processes describes a linear evolution on a log-log plot of Mg/Ca versus Sr/Ca with a slope of (DSr − 1)/(DMg − 1), where DSr and DMg are the partition coefficients of Sr and Mg in calcite. If DMg and DSr are known, the log-log plot of Mg/Ca versus Sr/Ca can be used to discriminate the carbonate composition formed by the PCP process from other processes (Fig. 6). Of course, PCP has no effect on 87Sr/86Sr ratios, and the com- bination of Mg/Ca, Sr/Ca, and 87Sr/86Sr ratios allow to distinguish the impact of eolian input on sedimentary carbonates (Yang et al., 2017a). In particular, an apparent lower (DSr − 1)/(DMg − 1) ra- tio (hereafter called apparent PCP slope) associated with higher Fig. 7. Isotopic compositions of the carbon and oxygen of the carbonate on the VPDB 87Sr/86Sr ratios since ∼8 Ma in the Linxia basin, has been at- scale of the MJZ section samples compared with those of bedrock carbonates on the NE Tibetan Plateau (Han et al., 2014), lacustrine sediments under different hydro- tributed to the addition of eolian dust from outside the basin logical conditions (Talbot, 1990), and dolomites (Li et al., 2007). (Fig. 9). For the Xining basin, a mixing model using the Mg/Ca and Sr/Ca ratios and considering PCP and an eolian carbonate contribu- tion shows that the authigenic carbonate composition are always influenced by the eolian carbonate contribution (Fig. 6). The low- ering of the Sr/Mg ratios after 8.6 Ma (Fig. 5), corresponding to an

Fig. 8. The temporal evolution of carbonate and water-soluble salt 87Sr/86Sr ratios in the Xining and Linxia Basins from ∼12 to ∼5 Ma, compared with the carbonate 87Sr/86Sr ratios of the loess and eolian red clays in the Lingtai Section on the CLP (Rao et al., 2008)and the marine benthic oxygen isotope curve (Zachos et al., 2008). The ages of the Xining eolian samples are from Zhang et al. (2017)for HW red clays and Lu et al. (2004)for PZS loess. The loess collected on the top of the MJZ section is plotted with an age of zero. 86 X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89

Fig. 9. Carbonate 87Sr/86Sr versus Mg/Ca (a); Sr/Ca (b); and Sr/Mg (c). The curves in (a) and (b) correspond to the mixing of the eolian dust end-member (beige diamond) with individual carbonate samples older than 8.6 Ma from the HLD section of the Linxia Basin (Yang et al., 2017a). Such model can explain the elemental and isotopic composition of the post-8.6 Ma carbonates samples from the Linxia Basin. The chemistry of the authigenic carbonates from the Xining Basin (MJZ) also converges towards the chemistry of the carbonate of the eolian dust end-member but requires another end-member besides the record of the weathering without eolian contribution in the Linxia Basin (pre-8.6 Ma HLD samples). This is likely a combination of different lithology, different weathering regime in the Xining Basin and might also witness a greater contribution of dolomite in the eolian dust delivered to the catchment of the Xining basin, as recorded by the late Pliocene-Pleistocene HW red clays from the Xining Basin (Figs. 3 and 6a). apparent lower apparent PCP slope (Fig. 6), would suggest a rise in ∼0.06 (Li and Li, 2014; Yang et al., 2015) and is higher than the the eolian influence at the transition period at ∼8.6 Ma. A similar apparent PCP slope defined by the carbonate compositions of the interpretation can be reached considering mixing models using el- MJZ section (Fig. 6). Since carbonates in local eolian deposits of the emental ratio (Mg/Ca or Sr/Ca) and 87Sr/86Sr ratio since the eolian basin generally show high Mg/Ca ratios, especially for the HW red dust endmember is characterised by a high 87Sr/86Sr ratio (Fig. 9). clays (Fig. 6), the eolian carbonate composition can be well con- However, such interpretation requires that carbonates in the MJZ strained by a mixing model between the dolomite endmember and section are mostly authigenic. the relatively low-Mg loess endmember (Fig. 6). Mixing between a series of PCP process-formed carbonates and a Mg-enriched eolian 5.1.1. Authigenic carbonate in the Neogene sediments of the Xining dust can yield the observed lower apparent PCP slope of the MJZ basin section (Fig. 6). The input of eolian dust with Mg-rich carbonate Two lines of evidence suggest that carbonates are mostly au- could therefore be the main cause for the decrease in the appar- thigenic. First, dolomite is absent throughout the section (Fig. 3). ent PCP slope from 0.828 at pre-8.6 Ma to 0.784 at post-8.6 Ma Because pedogenic dolomite hardly appears in soils, dolomite is an (Fig. 6). It is worth noting that the value of the apparent PCP slope indicator of detrital carbonate input (Meng et al., 2015). Marine of <0.95 for sediments older than 8.6 Ma indicates a significant strata are exposed in the Xining Basin catchments and dolomite eolian dust input since at least 12.7 Ma in the Xining Basin. is detected in the adjacent sand desert on the northern TP (Li et Furthermore, the decline of the apparent PCP slope around 8.6 al., 2007). The disappearance of dolomite in the MJZ section sed- Ma might indicate a strengthened dust input around that transition iments suggests that detrital carbonate is completely dissolved by period, which is also consistent with the aridification inferred from the weathering on the slopes surrounding the basin and/or during the lower authigenic carbonate content and the δ13C and δ18O co- the riverine transport. Second, the stable isotopic composition of variation. This interpretation would be consistent with the increase MJZ carbonate is clearly offset (mainly in δ13C) from the isotopic in the dust deposition rate recorded in the North Pacific sediments composition of typical detrital dolomite (Li et al., 2007) and re- (Rea et al., 1998), the onset of the red clay deposition on the east- gional bedrock carbonate on the northeastern TP (Han et al., 2014) ern CLP (Ding et al., 2001; Qiang et al., 2001) and in the nearby (Fig. 7). This observation suggests that detrital carbonate from nei- Linxia Basin (Yang et al., 2017a)at ∼8 Ma. The input of more Mg- ther eolian dust nor catchment erosion significantly influences the rich bedrock carbonate due to tectonic uplift-driven denudation MJZ carbonate composition. The positive correlation between the and weathering in the catchment could also explain such changes. δ13C and the δ18Oof the MJZ section carbonates might suggest However, this factor seems to be less important since dolomite has that they formed under hydrological conditions involving signifi- not been detected in clastic sediments (Fig. 3) or in gravel layers cant evaporation (Talbot, 1990), such as a flood plain or a braided since 8-9 Ma (Fig. 2). riverbed in arid or semi-arid climates. Carbonate Ca-Mg-Sr compositions are thus likely to be a reli- 5.1.2. Fingerprinting the carbonate of the eolian dust with 87Sr/86Sr able record of authigenic carbonate content and composition. In ratio that case, the general decrease in the Ca concentration at approxi- The 87Sr/86Sr isotope composition is a powerful tool for dis- mately 8-9 Ma (Fig. 5) could reflect less authigenic carbonate pre- criminating carbonate provenance. Carbonate 87Sr/86Sr ratios in cipitation and/or more siliclastic detrital input to the basin. This northern China eolian dust are generally higher than those in is more likely the result of a decrease in the flux of weathering- marine carbonates. For example, the carbonate 87Sr/86Sr ratios delivered cations due to the regional aridification, as recorded in are 0.7108-0.7110 for Quaternary loess (Yang et al., 2000), and the Linxia Basin (Yang et al., 2016), and also supported by the δ13C 0.7114-0.7117 for eolian red clays (Rao et al., 2008), whereas and δ18Ocovariation, related to significant evaporation. The vari- those of Phanerozoic marine carbonates range from 0.7067-0.7091 ations in the Mg/Ca and Sr/Ca ratios exclude a pure PCP process (e.g. Burke et al., 1982). Furthermore, the river water chemistry of as the only factor regulating the carbonate composition (Fig. 6). the endoreic catchment of the Lake , close to the west of Indeed, the apparent PCP slope of a pure PCP process in north- the Xining Basin, exhibits higher 87Sr/86Sr ratios in spring when ern China is uniformly >0.95 based on a DMg of ∼0.03 and DSr of dust storms are frequent (Jin et al., 2011). Therefore, if an iso- X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 87 lated basin receives more eolian dust, the 87Sr/86Sr ratios of fluvial higher value than that of the marine carbonate. This implies that water strongly buffered by the rapid chemical weathering of ma- variable 87Sr/86Sr ratios in the labile fraction of eolian dust can re- rine carbonate bedrock (Jin et al., 2011)will move towards higher sult from the climate-controlled silicate weathering intensity in the 87Sr/86Sr ratio by the dissolution of the carbonate of the eolian source area of eolian dust. The 87Sr/86Sr ratios of the carbonates dust endmember. This scenario corresponds to the temporal evo- of the eolian samples collected in the Xining Basin in combina- lution of the Linxia Basin, where eolian dust started to significantly tion with those of the MJZ section display a general decrease that influence the fluvial chemistry at ∼8 Ma, as suggested by the rapid roughly corresponds to global cooling since ∼12.7 Ma (Fig. 8). Such increase in the 87Sr/86Sr ratios in the water and HOAc leachates decrease might be related to changes in the average condition of (Fig. 8). The 87Sr/86Sr ratios in the water and HOAc leachates in erosion and weathering in the dust source area caused by the late the Xining Basin overlap not only the range of the post-8 Ma wa- Cenozoic global cooling since the mid-Miocene climatic transition ter and HOAc leachates in the Linxia Basin, but also that of the at ∼14 Ma. red clay carbonate 87Sr/86Sr ratios in the Lingtai section (Fig. 8), a typical red clay section of the CLP (Rao et al., 2008). These data 5.2. Asynchronous eolian dust impact on the Xining and Linxia Basins provide solid evidence for an eolian dust impact hypothesis for the Xining and the Linxia Basins. The Linxia Basin lies approximately 200 km east of the Xin- During the pre-8.6 Ma period, the 87Sr/86Sr ratios in water and ing Basin, and both basins are located beneath the westerly/East HOAc leachates of the Xining Basin remain within the same range Asian winter monsoon pathway (Fig. 1). The distinct patterns of of or even slightly higher than the 87Sr/86Sr ratio of the carbonate the eolian dust input in both basins could characterise late Ceno- of the eolian dust, and exhibit a different pattern from the same zoic dust transport dynamics on the northeastern TP and monsoon proxy in the Linxia Basin (Fig. 8). Such 87Sr/86Sr ratios of the au- boundary fluctuations linked to expansion and contraction of the thigenic carbonate of the Xining Basin could be the signature of a East Asian monsoon humid region. Differences in the sedimenta- heavy influence from eolian dust or a change in the lithology of tion flux of eolian dust caused by transport distance might explain the exposed bedrocks and/or weathering conditions. The relatively the contrasted dust input between the two basins. Dust with sizes low values of the 87Sr/86Sr ratios (Fig. 8), without any overlap with of 5-50 μm is normally transported to distances of less than 100 typical 87Sr/86Sr ratios of silicate rocks, suggest a paleo-riverine km, whereas dust with sizes of 2-10 μm can be transported long- chemistry always poorly influenced by the weathering of silicate distance (thousands of kms) through the troposphere (Péwé, 1981). lithology (Yang et al., 2017a). The 87Sr/86Sr ratio of 0.7119 in the In the Linxia Basin, the grain size of the late Miocene eolian dust dissolved load of the modern Huangshui River draining the whole ranges from 10 to 70 μm (Fan et al., 2006), suggesting a nearby lo- Xining Basin (Wu et al., 2005)also suggested a minor weather- cation of the source area of the eolian dust. Therefore, the ∼200 ing input of high 87Sr/86Sr ratios from metamorphic carbonate or km distance between the Xining and the Linxia Basins is large silicate. In contrast, this 87Sr/86Sr ratio is near the upper limit of enough to see a component in the dust source present in the Xin- the 87Sr/86Sr ratios ranging from 0.7105-0.7116 in the water and ing Basin but not in the Linxia Basin. Moreover, if the grain size HOAc leachates of the local eolian deposits (Fig. 8), suggesting a of the Miocene eolian dust in the source area is coarse enough, dominant eolian dust impact in the modern arid Xining Basin. the Xining Basin could have been subjected to significant eolian Furthermore, before 8.6 Ma, the climate in the Linxia Basin is dust input, while the Linxia Basin couldn’t. The second factor that suggested to be warmer and more humid than that in the Xin- should be considered is the moisture gradient on the northeast- ing Basin, as shown by the fossil mammal and pollen evidences ern TP. Under the modern climate, both basins are located at the (Ma et al., 1998; Fang et al., 2016; Yang et al., 2017b). A more hu- margin of the influence of the East Asian monsoon, with more mid climate in the Linxia basin before 8.6 Ma is also supported by than 80% of the annual precipitation falling in summer. The pre- many layers of well-developed palaeosols in fluvial environments cipitation gradient between the two basins is quite steep, and the near the basin margin (Fig. S1) and the occurrence of a large lake precipitation difference along the moisture pathway is remarkable, in its centre (Fang et al., 2016); whereas, only some palaeosols even on such a short distance. Since the Linxia Basin has been and a shallow lake developed in the Xining Basin (Fig. S1). Since more humid than the Xining Basin since 12.7 Ma (see section none of these basins buried detrital carbonate, suggesting the total 5.1.2), the contribution of the eolian dust to chemical weather- dissolution of bedrock carbonates, enhanced chemical weathering ing recorded in the authigenic phases of the basin is more diluted related to a more humid climate should result in a higher silicate in the Linxia Basin by the chemical weathering of the bedrock. weathering flux. In that case, a higher 87Sr/86Sr ratio would be ex- Even considering a constant eolian dust flux for the two basins, pected for the authigenic carbonate of the Linxia Basin than those the more humid climate in Linxia will generate higher weathering from the Xining Basin. However, the observed 87Sr/86Sr ratios of fluxes and thus the contribution of the eolian dust to the riverine the water and HOAc leachates in the Linxia Basin are lower than chemistry (recorded by authigenic carbonate) will be lower than those in the Xining Basin before 8.6 Ma, and therefore a tempo- that in the Xining basin. Therefore, the asynchronous eolian dust ral change in the two basins related to climatically variable silicate impact on the river chemistry in the Xining and Linxia Basins be- weathering has to be excluded. tween >12.7 Ma and 8.6 Ma could indicate that an arid/humid Thus, if these pre-8.6 Ma carbonate and water-soluble salt climatic boundary was located between the two basins or that a 87Sr/86Sr ratios were mostly due to eolian dust impact, then the spatial difference in the extent of the dust deposition existed. Con- decreasing 87Sr/86Sr ratios in MJZ carbonate and water-soluble sidering the increased eolian dust accumulation in the northern salts would indicate a long-term decrease in the 87Sr/86Sr ratios of Pacific (Rea et al., 1998) and the Sea of Japan (Shen et al., 2017)at the labile fraction of the eolian dust. The dust in the Asian inland approximately 8-9 Ma, the initiation of the dust-influenced hydro- is a mixture of weathered and unweathered material of the Earth’s logical chemistry in the Linxia Basin around the same time could surface, and authigenic phases (Engelbrecht and Derbyshire, 2010). suggest that the arid/humid climatic boundary has already moved The Sr isotope ratios in the labile fraction of eolian dust is likely to the southeast of the Linxia Basin at ∼8 Ma. to be significantly influenced by authigenic phases (carbonate and sulfate precipitated under arid conditions), and can reflect the av- 6. Conclusion erage weathering conditions (including silicate weathering) of an area (Naiman et al., 2000). Thus, the 87Sr/86Sr ratio of the authi- Our results show that the carbonates in the MZJ section of the genic phases of the dust in the Asian inland is likely to yield a Xining basin are mainly authigenic, when samples with >3% car- 88 X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 bonate content are considered. The Mg/Ca and Sr/Ca ratios of the Dai, S., Fang, X., Dupont-Nivet, G., Song, C., Gao, J., Krijgsman, W., Langereis, C., carbonate describe a power law relationship with a power coef- Zhang, W., 2006. Magnetostratigraphy of Cenozoic sediments from the Xining ficient of ∼0.8, lower than the coefficient characteristic of a pure Basin: tectonic implications for the northeastern Tibetan Plateau. J. Geophys. Res., Solid Earth 111, B1102. PCP process. 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