Embed Size (px)
Citation preview
Earth and Planetary Science Letters 515 (2019) 79–89
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
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, Chinab CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), Beijing 100101, Chinac University of Chinese Academy of Sciences, Beijing 100049, Chinad Centre de Recherches Pétrographiques et Géochimiques, UMR7358, CNRS, Université de Lorraine, 54500 Vandoeuvre les Nancy, Francee State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, Chinaf School of Earth Sciences & Key Laboratory of Western China’s Mineral Resources of Gansu Province, Lanzhou 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:Received 12 October 2018Received in revised form 9 March 2019Accepted 13 March 2019Available online 27 March 2019Editor: D. Vance
Keywords:eolian dustSr isotopescarbonateLinxia BasinXining Basin
As one of the largest dust sources on the Earth’s surface, dryland in Central Asia gives rise to thick eolian deposits over East Asia (e.g., the Chinese Loess Plateau, CLP) and significantly influences the regional hydrochemistry in the downwind drainage areas. However, the formation of thick eolian dust deposits requires not only climatic prerequisites for dust emission and transport but also climatic and topographic conditions favourable for deposition and accumulation. The scarcity of widespread eolian deposition around the CLP before 7-8 Ma hinders a full understanding of the processes and mechanisms of Central Asian aridification. The deposition of eolian dust also impacts the hydrogeochemistry of fluvial systems and the precipitation of authigenic phases in continental sedimentary systems could be an archive for studying eolian dust dynamics when pure eolian deposits are scarce. Here, we present the Ca-Mg-Sr concentrations and 87Sr/86Sr isotope compositions of bulk carbonates in a new fluvial sequence (12.7-4.8 Ma) of the Xining Basin. The Mg/Ca and Sr/Ca ratios of the carbonate describe a power law relationship 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.
* Corresponding authors at: CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), Beijing 100101, China.
E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Fang).
https://doi.org/10.1016/j.epsl.2019.03.0220012-821X/© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Dust can influence both the regional and global climate (Kok et al., 2018) through its direct impacts on incoming solar and terres-
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 formation (Sassen et al., 2003), and iron fertilisation for ocean phytoplankton production (Martin and Fitzwater, 1988). Since the Messinian (7.2-5.3 Ma), the arid part of Central Asia has been one of the largest dust sources (An et al., 2001; Engelbrecht and Derbyshire, 2010), delivering dust to East Asia (Tanaka and Chiba, 2006), the Pacific Ocean (Rea et al., 1998), and even North Amer-ica and Greenland (Jaffe et al., 1999; Bory et al., 2002). Dust emissions from Asian desert- and loess-covered regions and allu-vial piedmonts with poor vegetation (Nie et al., 2015), give rise to thick eolian deposition on the relatively flat and stable high-lands in East Asia (Guo et al., 2002) and significantly influence the regional hydrochemistry in the downwind drainage area (Jin et al., 2011). Such dust related deposits are therefore highly sen-sitive indicators of the regional evolution of the landscape and climate. However, the accumulation of thick and continuous eo-lian deposits during the late Cenozoic (e.g. on the CLP) requires not only climatic prerequisites for dust emission and transport, but also relatively flat, stable topography and relatively dry condi-tions to minimise erosion and generate significant deposition (Pye, 1995; Guo, 2017). The existing pure eolian red clays widely spread around the CLP are not older than 7-8 Ma (Ding et al., 2001;Qiang et al., 2001), which prevents a full understanding of the on-set and evolution conditions of dust emission and deposition prior to that time.
The fluvial basins in the northern Tibetan Plateau (TP) are char-acterised by distinct river water cation compositions during the spring through the dissolution of eolian dust carbonates and salts along with secondary calcite precipitation (Jin et al., 2011). There-fore, detailed spatial and temporal investigations of the eolian dust impacts on hydrological conditions in arid drainage areas could be a useful tool in understanding regional dust emissions dynam-ics linked to the late Cenozoic climate change and tectonic uplift of the northern TP. Lacustrine or fluvial carbonates and salt min-erals (e.g., gypsum) are suitable hosts for tracing hydrochemical changes induced by eolian dust, especially when their 87Sr/86Sr ra-tios (e.g., Naiman et al., 2000; Van der Hoven and Quade, 2002;Jacobson, 2004) or Nd isotopes (Jacobson and Holmden, 2006) are combined with other geochemical proxies. Carbonates and gypsum are widespread in semi-arid and arid fluvial settings, and fluvial and palaeosol carbonate compositions in the Linxia Basin on the northeastern TP have already demonstrated the importance of the regional eolian dust impact in the long-term evolution of basin flu-vial chemistry (Yang et al., 2017a).
To increase the spatial and temporal knowledge of the late Cenozoic eolian dust influence on fluvial basins, we document a new sedimentary record of the eolian impact on fluvial hydro-chemistry, in the Xining Basin on the rim of the northeastern TP, and compare this record with the existing archive in the Linxia Basin. Westerlies and the East Asian winter monsoon are inferred to be the dominant atmospheric circulation systems transporting Asian dust to the North Pacific during the late Cenozoic (Nie et al., 2014, 2018). The new late Cenozoic fluvial section in the Xining Basin and the previously reported section in the Linxia Basin are both located along these circulation pathways (Fig. 1). The stud-ied section in the Xining Basin was precisely dated from 12.7-4.8 Ma by high-resolution magnetostratigraphy and two mammal fos-sils (Yang et al., 2017b), allowing a comparative study with the fluvial section (12.2-5.1 Ma) in the Linxia Basin. Such comparison of the late Cenozoic eolian impact history aims to unravel the spa-tial evolution of the eolian dust impact on fluvial chemistry in the northeastern TP.
2. Geological settings and stratigraphy
The Xining Basin lies on the northeastern margin of the TP, at an average elevation of 2100 m above sea level. The Xining Basin is dominated by an arid/semi-arid continental climate with occasional dust storms during spring. The basin is confined by three NW-SE-trending dextral transpressional faults, is surrounded by the Laji Shan, Riyue Shan and Daban Shan mountains to the south, west and north, respectively (Dai et al., 2006), and opens to the Minhe-Lanzhou Basin to the east (Fig. 1). The Laji Shan is mainly composed of Palaeozoic basic to intermediate volcanic, volcaniclastic, and clastic rocks, while the Daban Shan is com-posed of early Palaeozoic marine clastic, volcanic and volcaniclastic rocks (Wang et al., 2015). A more than 2000 m-thick Cenozoic sedimentary succession lies unconformably on Jurassic-Cretaceous terrestrial clastic rocks (Wang et al., 2015; Zhang et al., 2016), in-cluding the Palaeogene Xining Group and Neogene Guide Group. The Xining Group is characterised by reddish siltstones and mud-stones intercalated with gypsum beds, and the Guide Group is characterised by siltstones and mudstones intercalated with sand-stone or conglomerate beds, with the grain size gradually becom-ing coarser upward along the sequence (Dai et al., 2006; Zhang et al., 2016). The Cenozoic sediments have now been cut through by the Huangshui River and its tributaries, which have formed river terraces mantled by loess and eolian red clays (Lu et al., 2004;Zhang et al., 2017).
The Mojiazhuang (MJZ) section is located in the northeastern part of the Xining Basin, 25 km northeast of Xining City (Fig. 1; Yang et al., 2017b). This 336 m-thick section includes the late Miocene Xianshuihe Formation and the Pliocene Linxia Formation. The section was dated by high-resolution magnetostratigraphy, fur-ther constrained by the occurrence of late Miocene mammal fos-sils (Chilotherium wimani, Hipparion dongxiangense and Parelasmoth-erium sp), and is considered to be deposited between 12.8 Ma and 4.8 Ma (Fig. 2; Yang et al., 2017b). Three sedimentary facies are identified in the section (Fig. 2). The lower part of the sec-tion (0-137 m, 12.8-8.6 Ma) is dominated by distal sedimentation of yellow-brownish mudstone with laminated marl, indicating a floodplain with shallow lakes. The interval from 0-58 m includes greyish-green laminated marl and palaeosol complexes with clearly identified Bt and Bk horizons (Fig. 2). The middle part of the sec-tion (137-252 m, 8.6-6.3 Ma) records a braided river environment dominated by massive brownish mudstone and siltstone (Fig. 2) in-tercalated with conglomerate beds (1-3 m in thickness) or lenses (Fig. 2). The upper part of the section (252-336 m, 6.3-4.8 Ma) corresponds to an alluvial fan facies, characterised by massive and poorly sorted conglomerate beds (more than 10 m in thickness, Fig. 2). The top conglomerate is overlain by a 12 m-thick reddish mudstone layer (Fig. 2), wedging out horizontally and covered by loess, which is thought to represent eolian red clays or fluvial sed-iments incorporating a large portion of eolian material. Combined with the dominant southerly palaeocurrents (Fig. 2), the sedimen-tation rate (Fig. 2) indicates that the Daban Shan has experienced rapid uplift since ∼8.1 Ma (Fig. 2b; Yang et al., 2017b).
3. Materials and methods
To determine the mineralogical composition of the sediments, representative samples of loess, red clays and MJZ-section sed-iments were selected for XRD analyses at the Micro Structure Analytical Laboratory, Peking University, on the Rigaku D/max-rA diffractometer (Cu, Ka1, 1.5406 Å, 40 kV, 40 mA, 3-35 ◦C at 0.01◦steps, 10◦/min). Carbonate carbon and oxygen isotopic composi-tions were measured to provide auxiliary indicators of the origins of the MJZ sedimentary carbonates. 20 samples with distinct car-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-tute of Eco-Environment and Resources, Chinese Academy of Sci-ence, Lanzhou. The analytical precision was better than 0.02� for δ18O and δ13C.
271 mudstones and siltstone/fine sandstones were collected at 1-2 m intervals in the MJZ section. To compare the MJZ sec-tion fluvial sediments with typical eolian deposits from the Xining Basin, Pliocene-Quaternary eolian samples from various locations and ages were included in this research. The samples consisted of 5 loess samples mantling the MJZ section, 11 loess samples from the Panzishan (PZS) borehole (Lu et al., 2004), and 32 red clay samples from the Houwan (HW) section (Zhang et al., 2017)(Fig. 1b). Identical analytical investigations of the MJZ sediments were used for those Pliocene-Quaternary eolian samples. All sam-ples were treated by two-step water/1 M acetic acid (HOAc) leach-ing to obtain the water-soluble and carbonate fractions following the method described by Yang et al. (2015). The concentrations of Ca, Mg and Sr in the HOAc leachates were determined using in-ductively coupled plasma optical emission spectrometry (ICP-OES) (Leeman Labs Prodigy-H) at the Institute of Tibetan Plateau Re-
search, Chinese Academy of Sciences (ITP-CAS), Beijing. The results were normalised to the weight of the bulk oven-dried sample. Replicate analyses show the relative standard deviation for all ele-ments of less than 2%.
23 samples that span most of the MJZ section and have differ-ent Mg/Ca-Sr/Ca ratio patterns were chosen, as were eolian dust samples including 2 MJZ loess, 2 PZS loess and 3 HW red clays, for 87Sr/86Sr ratio analyses of the HOAc and water leachates. Pre-treatment for Sr isotope analysis was performed in the Class 10000 clean lab at the Institute of Earth Environment, Chinese Academy of Science (IEE-CAS) in Xi’an, China. The Sr separation includes the use of a Sr-spec ion-exchange column (Eichrom Technologies) and follows the standard procedure described in Jin et al. (2011). The Sr isotopic ratios were measured using multi-collector ICP-MS (Thermo Finnigan Neptune Plus) at the IEE-CAS. The measured 87Sr/86Sr ratios were normalised to 86Sr/88Sr = 0.1194. The Sr standard SRM 987 yielded a mean value of 0.710300 ± 0.000030 (n = 10) during duplicate and periodic checks.
The carbonate content of 27 MJZ samples with different Ca con-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-matic carbon analyser. The inorganic carbon content was measured by a Shimadzu TOC-VCPH carbon analyser in ITP-CAS, with 50% phosphoric acid applied to dissolve the carbonates during the mea-surement.
4. Results
Clay minerals, quartz, microcline, albite, and mica were de-tected as major silicate minerals in all samples of the MJZ sec-tion, with no significant content changes along the whole section (Fig. 3). Eolian (loess and red clays) samples exhibit silicate mineral compositions similar to those of the MJZ section samples. Among the carbonate minerals, calcite is abundant in the eolian samples and the pre-8.6 Ma section samples (0-137 m), and less abundant in the post-8.6 Ma section samples (137-336 m). Dolomite appears only in the eolian samples, and red clays contain relatively more dolomite than does loess (Fig. 3). The carbonate content can be es-timated based on the data from the total carbon analyser, or from the [Ca] and [Mg] in the HOAc leachate. These two measurements (Fig. 4) show excellent consistency when the carbonate contents are greater than 5%, but the consistency becomes poor for very low-carbonate samples (Fig. 4), suggesting that significant amounts of non-carbonate Mg and Ca are leached by the 1 M acetic acid in
low-carbonate samples. The result of the comparative test indicates that the diluted HOAc leachates of the samples with low carbonate contents may not fully represent the carbonate compositions, and thus, in the following, we used only samples containing >3% car-bonate samples (based on the [Ca] and [Mg] in the HOAc leachate) to evaluate the carbonate elemental and Sr-isotope compositions in the MJZ section.
The carbonate Ca concentration of the MJZ section range from 10525 to 213770 μg/g, Ca/Mg, Ca/Sr, and Sr/Mg ratios range from 4.7 to 71.4 mol/mol, 0.5 to 5.2 mol/mmol and 4.9 to 19.6 mmol/mol, respectively (Fig. 5). The Ca concentration (a good proxy for the carbonate content of the sediment), Ca/Mg and Ca/Sr ratios exhibit in-phase variations (Fig. 5), and the Ca/Mg and Ca/Sr ratios are positively correlated (Fig. 6). Eolian samples from the Xining Basin, MJZ loess and PZS loess have uniform elemental con-centrations, and the HW red clays have compositions similar to those of the loess except for a very scattered, but on average, lower Ca/Mg ratio (Fig. 6).
The most striking feature of the carbonate records of the MJZ section is the clear transition period at ∼8.6 Ma. This transition can be identified based on the Ca concentration and Ca/Mg and Ca/Sr ratios. The carbonate content is 16.7 ± 6.3% before 8.6 Ma 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 ±1.2 mol/mmol to 1.6 ± 0.9 mol/mmol after the transition (Fig. 5). In contrast, the Sr/Mg ratios increase from the bottom to ∼11 Ma and then decrease slowly towards low values at ∼8.6 Ma (13.8 mmol/mol on average) and continue to oscillate around an average of 10.6 mmol/mol upward (Fig. 5). Mg/Ca ratios co-vary with Sr/Ca ratios and follow a power law with samples older than 8.6 Ma and younger than 8.6 Ma characterised by an exponent (slope in a log-log plot) of 0.828 and 0.784, respectively (Fig. 6). The transition at ∼8.6 Ma also corresponds to a sedimentological boundary with the lower section dominated by a floodplain environment replaced by a braided river environment above. The reddish mudstone layer above the top conglomerate bed (deposited from 5.2 to 4.8 Ma) is characterised by a carbonate content of 11.8 ± 3.8% but low Ca/Mg
and Ca/Sr ratios, which are close to the corresponding values of the loess mantling the MJZ section (Fig. 5). These characteristics imply that post-8.6 Ma samples have a carbonate composition that overlaps the composition of almost all the local eolian carbonate materials, except ∼1/3 of the red clays samples from HW with Mg/Ca molar ratios above 0.2 (Fig. 6).
Among the 20 samples analysed for their carbon and oxy-gen isotopic composition of carbonate, only 9 samples (covering both pre- and post-8.6 Ma samples) yielded reliable results, since the other 11 samples have carbonate contents that are too low. The δ18O ranges from −8.4� to −4.2�, and the δ13C ranges from −6.2� to −4.4� on the VPDB scale (Fig. 7). The corre-lation between δ18O and δ13C is significant (p = 0.025). All the 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-leased enough Sr to return reliable water-soluble 87Sr/86Sr data. The 87Sr/86Sr ratios of the water leachate and HOAc leachate of the MJZ section samples vary between 0.7113 and 0.7127 and between 0.7111 and 0.7131, respectively. The 87Sr/86Sr ratio curves of both HOAc and water leachate show similar decreasing trends (Fig. 8).
The 87Sr/86Sr ratios of HOAc leachate for the loess are nearly con-stant with values of 0.7107 and 0.7105 for MJZ and PZS, respec-tively; and those of water leachate for MJZ loess are from 0.7115 to 0.7116. The 87Sr/86Sr ratios in HOAc and water leachate of the HW red clays vary from 0.7111 to 0.7112, and around 0.7113, re-spectively.
X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 85
Fig. 7. Isotopic compositions of the carbon and oxygen of the carbonate on the VPDB 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-logical conditions (Talbot, 1990), and dolomites (Li et al., 2007).
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 87Sr/86Sr ratios since ∼8 Ma in the Linxia basin, has been at-tributed to the addition of eolian dust from outside the basin (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 the eolian influence at the transition period at ∼8.6 Ma. A similar interpretation can be reached considering mixing models using el-emental ratio (Mg/Ca or Sr/Ca) and 87Sr/86Sr ratio since the eolian dust endmember is characterised by a high 87Sr/86Sr ratio (Fig. 9). However, such interpretation requires that carbonates in the MJZ section are mostly authigenic.
5.1.1. Authigenic carbonate in the Neogene sediments of the Xining basin
Two lines of evidence suggest that carbonates are mostly au-thigenic. First, dolomite is absent throughout the section (Fig. 3). Because pedogenic dolomite hardly appears in soils, dolomite is an indicator of detrital carbonate input (Meng et al., 2015). Marine strata are exposed in the Xining Basin catchments and dolomite is detected in the adjacent sand desert on the northern TP (Li et al., 2007). The disappearance of dolomite in the MJZ section sed-iments suggests that detrital carbonate is completely dissolved by the weathering on the slopes surrounding the basin and/or during the riverine transport. Second, the stable isotopic composition of MJZ carbonate is clearly offset (mainly in δ13C) from the isotopic composition of typical detrital dolomite (Li et al., 2007) and re-gional bedrock carbonate on the northeastern TP (Han et al., 2014)(Fig. 7). This observation suggests that detrital carbonate from nei-ther eolian dust nor catchment erosion significantly influences the MJZ carbonate composition. The positive correlation between the δ13C and the δ18O of the MJZ section carbonates might suggest that they formed under hydrological conditions involving signifi-cant evaporation (Talbot, 1990), such as a flood plain or a braided riverbed in arid or semi-arid climates.
Carbonate Ca-Mg-Sr compositions are thus likely to be a reli-able record of authigenic carbonate content and composition. In that case, the general decrease in the Ca concentration at approxi-mately 8-9 Ma (Fig. 5) could reflect less authigenic carbonate pre-cipitation and/or more siliclastic detrital input to the basin. This is more likely the result of a decrease in the flux of weathering-delivered cations due to the regional aridification, as recorded in the Linxia Basin (Yang et al., 2016), and also supported by the δ13Cand δ18O covariation, related to significant evaporation. The vari-ations in the Mg/Ca and Sr/Ca ratios exclude a pure PCP process as the only factor regulating the carbonate composition (Fig. 6). Indeed, the apparent PCP slope of a pure PCP process in north-ern China is uniformly >0.95 based on a DMg of ∼0.03 and DSr of
∼0.06 (Li and Li, 2014; Yang et al., 2015) and is higher than the apparent PCP slope defined by the carbonate compositions of the MJZ section (Fig. 6). Since carbonates in local eolian deposits of the basin generally show high Mg/Ca ratios, especially for the HW red clays (Fig. 6), the eolian carbonate composition can be well con-strained by a mixing model between the dolomite endmember and the relatively low-Mg loess endmember (Fig. 6). Mixing between a series of PCP process-formed carbonates and a Mg-enriched eolian dust can yield the observed lower apparent PCP slope of the MJZ section (Fig. 6). The input of eolian dust with Mg-rich carbonate could therefore be the main cause for the decrease in the appar-ent PCP slope from 0.828 at pre-8.6 Ma to 0.784 at post-8.6 Ma (Fig. 6). It is worth noting that the value of the apparent PCP slope of <0.95 for sediments older than 8.6 Ma indicates a significant eolian dust input since at least 12.7 Ma in the Xining Basin.
Furthermore, the decline of the apparent PCP slope around 8.6 Ma might indicate a strengthened dust input around that transition period, which is also consistent with the aridification inferred from the lower authigenic carbonate content and the δ13C and δ18O co-variation. This interpretation would be consistent with the increase in the dust deposition rate recorded in the North Pacific sediments (Rea et al., 1998), the onset of the red clay deposition on the east-ern CLP (Ding et al., 2001; Qiang et al., 2001) and in the nearby Linxia Basin (Yang et al., 2017a) at ∼8 Ma. The input of more Mg-rich bedrock carbonate due to tectonic uplift-driven denudation and weathering in the catchment could also explain such changes. However, this factor seems to be less important since dolomite has not been detected in clastic sediments (Fig. 3) or in gravel layers since 8-9 Ma (Fig. 2).
5.1.2. Fingerprinting the carbonate of the eolian dust with 87Sr/86Sr ratio
The 87Sr/86Sr isotope composition is a powerful tool for dis-criminating carbonate provenance. Carbonate 87Sr/86Sr ratios in northern China eolian dust are generally higher than those in marine carbonates. For example, the carbonate 87Sr/86Sr ratios are 0.7108-0.7110 for Quaternary loess (Yang et al., 2000), and 0.7114-0.7117 for eolian red clays (Rao et al., 2008), whereas those of Phanerozoic marine carbonates range from 0.7067-0.7091 (e.g. Burke et al., 1982). Furthermore, the river water chemistry of the endoreic catchment of the Lake Qinghai, close to the west of the Xining Basin, exhibits higher 87Sr/86Sr ratios in spring when 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 water strongly buffered by the rapid chemical weathering of ma-rine carbonate bedrock (Jin et al., 2011) will move towards higher 87Sr/86Sr ratio by the dissolution of the carbonate of the eolian dust endmember. This scenario corresponds to the temporal evo-lution of the Linxia Basin, where eolian dust started to significantly influence the fluvial chemistry at ∼8 Ma, as suggested by the rapid increase in the 87Sr/86Sr ratios in the water and HOAc leachates (Fig. 8). The 87Sr/86Sr ratios in the water and HOAc leachates in the Xining Basin overlap not only the range of the post-8 Ma wa-ter and HOAc leachates in the Linxia Basin, but also that of the 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 provide solid evidence for an eolian dust impact hypothesis for the Xining and the Linxia Basins.
During the pre-8.6 Ma period, the 87Sr/86Sr ratios in water and HOAc leachates of the Xining Basin remain within the same range of or even slightly higher than the 87Sr/86Sr ratio of the carbonate of the eolian dust, and exhibit a different pattern from the same proxy in the Linxia Basin (Fig. 8). Such 87Sr/86Sr ratios of the au-thigenic carbonate of the Xining Basin could be the signature of a heavy influence from eolian dust or a change in the lithology of the exposed bedrocks and/or weathering conditions. The relatively low values of the 87Sr/86Sr ratios (Fig. 8), without any overlap with typical 87Sr/86Sr ratios of silicate rocks, suggest a paleo-riverine chemistry always poorly influenced by the weathering of silicate lithology (Yang et al., 2017a). The 87Sr/86Sr ratio of 0.7119 in the dissolved load of the modern Huangshui River draining the whole Xining Basin (Wu et al., 2005) also suggested a minor weather-ing input of high 87Sr/86Sr ratios from metamorphic carbonate or silicate. In contrast, this 87Sr/86Sr ratio is near the upper limit of the 87Sr/86Sr ratios ranging from 0.7105-0.7116 in the water and HOAc leachates of the local eolian deposits (Fig. 8), suggesting a dominant eolian dust impact in the modern arid Xining Basin.
Furthermore, before 8.6 Ma, the climate in the Linxia Basin is suggested to be warmer and more humid than that in the Xin-ing Basin, as shown by the fossil mammal and pollen evidences (Ma et al., 1998; Fang et al., 2016; Yang et al., 2017b). A more hu-mid climate in the Linxia basin before 8.6 Ma is also supported by many layers of well-developed palaeosols in fluvial environments near the basin margin (Fig. S1) and the occurrence of a large lake in its centre (Fang et al., 2016); whereas, only some palaeosols and a shallow lake developed in the Xining Basin (Fig. S1). Since none of these basins buried detrital carbonate, suggesting the total dissolution of bedrock carbonates, enhanced chemical weathering related to a more humid climate should result in a higher silicate weathering flux. In that case, a higher 87Sr/86Sr ratio would be ex-pected for the authigenic carbonate of the Linxia Basin than those from the Xining Basin. However, the observed 87Sr/86Sr ratios of the water and HOAc leachates in the Linxia Basin are lower than those in the Xining Basin before 8.6 Ma, and therefore a tempo-ral change in the two basins related to climatically variable silicate weathering has to be excluded.
Thus, if these pre-8.6 Ma carbonate and water-soluble salt 87Sr/86Sr ratios were mostly due to eolian dust impact, then the decreasing 87Sr/86Sr ratios in MJZ carbonate and water-soluble salts would indicate a long-term decrease in the 87Sr/86Sr ratios of the labile fraction of the eolian dust. The dust in the Asian inland is a mixture of weathered and unweathered material of the Earth’s surface, and authigenic phases (Engelbrecht and Derbyshire, 2010). The Sr isotope ratios in the labile fraction of eolian dust is likely to be significantly influenced by authigenic phases (carbonate and sulfate precipitated under arid conditions), and can reflect the av-erage weathering conditions (including silicate weathering) of an area (Naiman et al., 2000). Thus, the 87Sr/86Sr ratio of the authi-genic phases of the dust in the Asian inland is likely to yield a
higher value than that of the marine carbonate. This implies that variable 87Sr/86Sr ratios in the labile fraction of eolian dust can re-sult from the climate-controlled silicate weathering intensity in the source area of eolian dust. The 87Sr/86Sr ratios of the carbonates of the eolian samples collected in the Xining Basin in combina-tion with those of the MJZ section display a general decrease that roughly corresponds to global cooling since ∼12.7 Ma (Fig. 8). Such decrease might be related to changes in the average condition of erosion and weathering in the dust source area caused by the late Cenozoic global cooling since the mid-Miocene climatic transition at ∼14 Ma.
5.2. Asynchronous eolian dust impact on the Xining and Linxia Basins
The Linxia Basin lies approximately 200 km east of the Xin-ing Basin, and both basins are located beneath the westerly/East Asian winter monsoon pathway (Fig. 1). The distinct patterns of the eolian dust input in both basins could characterise late Ceno-zoic dust transport dynamics on the northeastern TP and monsoon boundary fluctuations linked to expansion and contraction of the East Asian monsoon humid region. Differences in the sedimenta-tion flux of eolian dust caused by transport distance might explain the contrasted dust input between the two basins. Dust with sizes of 5-50 μm is normally transported to distances of less than 100 km, whereas dust with sizes of 2-10 μm can be transported long-distance (thousands of kms) through the troposphere (Péwé, 1981). In the Linxia Basin, the grain size of the late Miocene eolian dust ranges from 10 to 70 μm (Fan et al., 2006), suggesting a nearby lo-cation of the source area of the eolian dust. Therefore, the ∼200 km distance between the Xining and the Linxia Basins is large enough to see a component in the dust source present in the Xin-ing Basin but not in the Linxia Basin. Moreover, if the grain size of the Miocene eolian dust in the source area is coarse enough, the Xining Basin could have been subjected to significant eolian dust input, while the Linxia Basin couldn’t. The second factor that should be considered is the moisture gradient on the northeast-ern TP. Under the modern climate, both basins are located at the margin of the influence of the East Asian monsoon, with more than 80% of the annual precipitation falling in summer. The pre-cipitation gradient between the two basins is quite steep, and the precipitation difference along the moisture pathway is remarkable, even on such a short distance. Since the Linxia Basin has been more humid than the Xining Basin since 12.7 Ma (see section 5.1.2), the contribution of the eolian dust to chemical weather-ing recorded in the authigenic phases of the basin is more diluted in the Linxia Basin by the chemical weathering of the bedrock. Even considering a constant eolian dust flux for the two basins, the more humid climate in Linxia will generate higher weathering fluxes and thus the contribution of the eolian dust to the riverine chemistry (recorded by authigenic carbonate) will be lower than that in the Xining basin. Therefore, the asynchronous eolian dust impact on the river chemistry in the Xining and Linxia Basins be-tween >12.7 Ma and 8.6 Ma could indicate that an arid/humid climatic boundary was located between the two basins or that a spatial difference in the extent of the dust deposition existed. Con-sidering the increased eolian dust accumulation in the northern Pacific (Rea et al., 1998) and the Sea of Japan (Shen et al., 2017) at approximately 8-9 Ma, the initiation of the dust-influenced hydro-logical chemistry in the Linxia Basin around the same time could suggest that the arid/humid climatic boundary has already moved to the southeast of the Linxia Basin at ∼8 Ma.
6. Conclusion
Our results show that the carbonates in the MZJ section of the 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 carbonate describe a power law relationship with a power coef-ficient of ∼0.8, lower than the coefficient characteristic of a pure PCP process. The input of eolian dust with the dissolution of Mg-rich carbonate is likely responsible for the deviation from a pure PCP process. An eolian dust contribution to the chemical weath-ering recorded in the authigenic phases of the basin is confirmed by the observation of high 87Sr/86Sr ratios in the Xining Basin. In detail, a transition period around 8.6 Ma can be defined based on 1) a change in sediment facies (from flood-plain to braided river environments), 2) a lower power coefficient of the Mg-Ca-Sr systematic, and 3) a slightly lower carbonate 87Sr/86Sr ratios, consistent with a greater contribution of the eolian dust. The simil-itude in the carbonate 87Sr/86Sr ratios of the Xining (this study) and Linxia (Yang et al., 2017a) basins and of the red clays/loess-palaeosols on the CLP (Rao et al., 2008) suggests that eolian dust was widespread and dominant since 8-9 Ma. Our results indi-cate also that the eolian dust impact on river chemistry in the Xining Basin dated back to at least 12.7 Ma, earlier than the ini-tiation of eolian red clay accumulation on the CLP by 7-8 Ma or the significant influence of dust in the Linxia basin, highlighting the spatial difference between dust emission and deposition dy-namics caused by the late Cenozoic cooling and the uplift of the northern TP. Our study further implies that a large amount of eo-lian dust has been deposited in downwind fluvial-lake systems as “mixed” eolian and riverine sediments in the northeastern TP during the late Cenozoic rather than on relatively flat and sta-ble topography to form pure eolian sequences. This provides an explanation for the lack of continuous eolian red clay sequences in the western CLP (west of the Liupan mountain, Fig. 1) in con-trast with the continuous eolian red clay sequences that are com-mon in the eastern CLP since 7-8 Ma (e.g., Ding et al., 2001;Qiang et al., 2001).
Acknowledgements
This work is co-supported by the National Natural Science Foundation of China (41771236; 41620104002; 41872098), Strate-gic Priority Research Program (XDA20070201), External Coopera-tion Program (131C11KYSB20160072) and the Youth Innovation Promotion Association (2018095), of the Chinese Academy of Sci-ences, and opening fund of the State Key Laboratory of Loess and Quaternary Geology (SKLLQG1841). X. Ruan and R. Yang are sup-ported by the China Scholarship Council. We thank Prof. Chunhui Song and Mr. Xiaohui Fang, Shuang Lü, Haichao Guo, Yulong Xie, Ziqiang Mao, Shengcheng Lu for the assistances in the field, Cheng Wang for carbonate content measurements, the two anonymous reviewers and the Editor for their thorough and constructive com-ments.
Appendix A. Supplementary material
Supplementary material related to this article can be found on-line at https://doi .org /10 .1016 /j .epsl .2019 .03 .022.
References
An, Z., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times. Na-ture 411, 62–66.
Bory, A.M., Biscaye, P.E., Svensson, A., Grousset, F.E., 2002. Seasonal variability in the origin of recent atmospheric mineral dust at NorthGRIP, Greenland. Earth Planet. Sci. Lett. 196, 123–134.
Burke, W.H., Denison, R.E., Hetherington, E.A., Koepnick, R.B., Nelson, H.F., Otto, J.B., 1982. Variation of seawater 87Sr/86Sr throughout Phanerozoic time. Geology 10, 516–519.
Chen, Z., Li, G., 2013. Evolving sources of eolian detritus on the Chinese Loess Plateau since early Miocene: tectonic and climatic controls. Earth Planet. Sci. Lett. 371–372, 220–225.
Dai, S., Fang, X., Dupont-Nivet, G., Song, C., Gao, J., Krijgsman, W., Langereis, C., Zhang, W., 2006. Magnetostratigraphy of Cenozoic sediments from the Xining Basin: tectonic implications for the northeastern Tibetan Plateau. J. Geophys. Res., Solid Earth 111, B1102.
Ding, Z.L., Yang, S.L., Hou, S.S., Wang, X., Chen, Z., Liu, T.S., 2001. Magnetostratigra-phy and sedimentology of the Jingchuan red clay section and correlation of the Tertiary eolian red clay sediments of the Chinese Loess Plateau. J. Geophys. Res., Solid Earth 106, 6399–6407.
Engelbrecht, J.P., Derbyshire, E., 2010. Airborne mineral dust. Elements 6, 241–246.Fairchild, I.J., Treble, P.C., 2009. Trace elements in speleothems as recorders of envi-
ronmental change. Quat. Sci. Rev. 28, 449–468.Fan, M., Song, C., Dettman, D.L., Fang, X., Xu, X., 2006. Intensification of the Asian
winter monsoon after 7.4 Ma: grain-size evidence from the Linxia Basin, north-eastern Tibetan Plateau, 13.1 Ma to 4.3 Ma. Earth Planet. Sci. Lett. 248, 186–197.
Fang, X., Wang, J., Zhang, W., Zan, J., Song, C., Yan, M., Apple, E., Zhang, T., Wu, F., Yang, Y., Lu, Y., 2016. Tectonosedimentary evolution model of an intracontinen-tal flexural (foreland) basin for paleoclimatic research. Glob. Planet. Change 145, 78–97.
Guo, Z.T., Ruddiman, W.F., Hao, Q.Z., Wu, H.B., Qiao, Y.S., Zhu, R.X., Peng, S.Z., Wei, J.J., Yuan, B.Y., Liu, T.S., 2002. Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China. Nature 416, 159–163.
Guo, Z.T., 2017. Loess Plateau attests to the onsets of monsoon and deserts. Sci. Sin. Terrae 47, 421–437 (in Chinese).
Han, W., Fang, X., Ye, C., Teng, X., Zhang, T., 2014. Tibet forcing Quaternary stepwise enhancement of westerly jet and central Asian aridification: carbonate isotope records from deep drilling in the Qaidam salt playa, NE Tibet. Glob. Planet. Change 116, 68–75.
Van der Hoven, S.J., Quade, J., 2002. Tracing spatial and temporal variations in the sources of calcium in pedogenic carbonates in a semiarid environment. Geo-derma 108, 259–276.
Jacobson, A.D., 2004. Has the atmospheric supply of dissolved calcite dust to seawa-ter influenced the evolution of marine 87Sr/86Sr ratios over the past 2.5 million years? Geochem. Geophys. Geosyst. 5. https://doi .org /10 .1029 /2004GC000750.
Jacobson, A.D., Holmden, C., 2006. Calcite dust and the atmospheric supply of Nd to the Japan Sea. Earth Planet. Sci. Lett. 244, 418–430.
Jaffe, D., Anderson, T., Covert, D., Kotchenruther, R., Trost, B., Danielson, J., Simp-son, W., Berntsen, T., Karlsdottir, S., Blake, D., Harris, J., Carmichael, G., Uno, I., 1999. Transport of Asian air pollution to North America. Geophys. Res. Lett. 26, 711–714.
Jin, Z., You, C.F., Yu, J., Wu, L., Zhang, F., Liu, H.C., 2011. Seasonal contributions of catchment weathering and eolian dust to river water chemistry, northeast-ern Tibetan Plateau: chemical and Sr isotopic constraints. J. Geophys. Res. 116, F04006.
Kok, J.F., Ward, D.S., Mahowald, N.M., Evan, A.T., 2018. Global and regional impor-tance of the direct dust-climate feedback. Nat. Commun. 9, 241.
Li, G., Chen, J., Chen, Y., Yang, J., Ji, J., Liu, L., 2007. Dolomite as a tracer for the source regions of Asian dust. J. Geophys. Res., Atmos. 112, D17.
Li, T., Li, G., 2014. Incorporation of trace metals into microcodium as novel proxies for paleo-precipitation. Earth Planet. Sci. Lett. 386, 34–40.
Lu, H., Wang, X., An, Z., Miao, X., Zhu, R., Ma, H., Li, Z., Tan, H., Wang, X., 2004. Geo-morphologic evidence of phased uplift of the northeastern Qinghai-Tibet Plateau since 14 million years ago. Sci. China, Ser. D 47, 822–833.
Ma, Y.Z., Li, J.J., Fang, X.M., 1998. Pollen assemblage in 30.6-5.0 Ma redbeds of Linxia region and climate evolution. Chin. Sci. Bull. 43, 301–304.
Martin, J.H., Fitzwater, S.E., 1988. Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic. Nature 331, 341–343.
Meng, X., Liu, L., Balsam, W., Li, S., He, T., Chen, J., Ji, J., 2015. Dolomite abundance in Chinese loess deposits: a new proxy of monsoon precipitation intensity. Geo-phys. Res. Lett. 42, 10391–10398.
Naiman, Z., Quade, J., Patchett, P.J., 2000. Isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the southwest United States. Geochim. Cosmochim. Acta 64, 3099–3109.
Nie, J., Peng, W., Möller, A., Song, Y., Stockli, D.F., Stevens, T., Horton, B.K., Liu, S., Bird, A., Oalmann, J., Gong, H., Fang, X., 2014. Provenance of the upper Miocene-Pliocene red clay deposits of the Chinese Loess Plateau. Earth Planet. Sci. Lett. 407, 35–47.
Nie, J., Stevens, T., Rittner, M., Stockli, D., Garzanti, E., Limonta, M., Bird, A., Andò, S., Vermeesch, P., Saylor, J., Lu, H., Breecker, D., Hu, X., Liu, S., Resentini, A., Vezzoli, G., Peng, W., Carter, A., Ji, S., Pan, B., 2015. Loess Plateau storage of Northeastern Tibetan Plateau-derived Yellow River sediment. Nat. Commun. 6, 8511.
Nie, J., Pullen, A., Garzione, C.N., Peng, W., Wang, Z., 2018. Pre-Quaternary decou-pling between Asian aridification and high dust accumulation rates. Sci. Adv. 4, eaao6977.
Péwé, T.L., 1981. Desert dust: an overview. Spec. Pap., Geol. Soc. Am. 186, 1–10.Pye, K., 1995. The nature, origin and accumulation of loess. Quat. Sci. Rev. 14,
653–667.Qiang, X.K., Li, Z.X., Powell, C.M., Zheng, H.B., 2001. Magnetostratigraphic record of
the late Miocene onset of the East Asian monsoon and Pliocene uplift of north-ern Tibet. Earth Planet. Sci. Lett. 187, 83–93.
X. Ruan et al. / Earth and Planetary Science Letters 515 (2019) 79–89 89
Rao, W., Chen, J., Yang, J., Ji, J., Zhang, G., Chen, J., 2008. Strontium isotopic and elemental characteristics of calcites in the eolian dust profile of the Chinese Loess Plateau during the past 7 Ma. Geochem. J. 42, 493–506.
Rea, D.K., Snoeckx, H., Joseph, L.H., 1998. Late Cenozoic eolian deposition in the north pacific: Asian drying, Tibetan uplift, and cooling of the northern hemi-sphere. Paleoceanography 13, 215–224.
Sassen, K., DeMott, P.J., Prospero, J.M., Poellot, M.R., 2003. Saharan dust storms and indirect aerosol effects on clouds: CRYSTAL-FACE results. Geophys. Res. Lett. 30, 1633.
Shen, X., Wan, S., France-Lanord, C., Clift, P.D., Tada, R., Révillon, S., Shi, X., Zhao, D., Liu, Y., Yin, X., Song, Z., Li, A., 2017. History of Asian eolian input to the Sea of Japan since 15 Ma: links to Tibetan uplift or global cooling? Earth Planet. Sci. Lett. 474, 296–308.
Sinclair, D.J., 2011. Two mathematical models of Mg and Sr partitioning into so-lution during incongruent calcite dissolution: implications for dripwater and speleothem studies. Chem. Geol. 283, 119–133.
Talbot, M.R., 1990. A review of the palaeohydrological interpretation of carbon and oxygen isotopic ratios in primary lacustrine carbonates. Chem. Geol., Isot. Geosci. Sect. 80, 261–279.
Tanaka, T.Y., Chiba, M., 2006. A numerical study of the contributions of dust source regions to the global dust budget. Glob. Planet. Change 52, 88–104.
Tegen, I., Lacis, A.A., Fung, I., 1996. The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419–422.
Wang, Y., Zhang, J., Qi, W., Guo, S., 2015. Exhumation history of the Xining Basin since the Mesozoic and its tectonic significance. Acta Geol. Sin. (English Edi-tion) 89, 145–162.
Wu, L., Huh, Y., Qin, J., Du, G., van Der Lee, S., 2005. Chemical weathering in the Upper Huang He (Yellow River) draining the eastern Qinghai-Tibet Plateau. Geochim. Cosmochim. Acta 69, 5279–5294.
Yang, J., Chen, J., An, Z., Shields, G., Tao, X., Zhu, H., Ji, J., Chen, Y., 2000. Variations in 87Sr/86Sr ratios of calcites in Chinese loess: a proxy for chemical weathering associated with the East Asian summer monsoon. Palaeogeogr. Palaeoclimatol. Palaeoecol. 157, 151–159.
Yang, R., Fang, X., Meng, Q., Zan, J., Zhang, W., Deng, T., Li, B., 2017b. Paleomagnetic constraints on the middle Miocene-early Pliocene stratigraphy in the Xining basin, NE Tibetan plateau, and the geologic implications. Geochem. Geophys. Geosyst. 18, 3741–3757.
Yang, Y., Fang, X., Galy, A., Jin, Z., Wu, F., Yang, R., Zhang, W., Zan, J., Liu, X., Gao, S., 2016. Plateau uplift forcing climate change around 8.6 Ma on the north-eastern Tibetan Plateau: evidence from an integrated sedimentary Sr record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 461, 418–431.
Yang, Y., Fang, X., Galy, A., Zhang, G., Liu, S., Zan, J., Wu, F., Meng, Q., Ye, C., Yang, R., Liu, X., 2015. Carbonate composition and its impact on fluvial geochemistry in the NE Tibetan Plateau region. Chem. Geol. 410, 138–148.
Yang, Y., Galy, A., Fang, X., Yang, R., Zhang, W., Zan, J., 2017a. Eolian dust forcing of river chemistry on the northeastern Tibetan Plateau since 8 Ma. Earth Planet. Sci. Lett. 464, 200–210.
Zachos, J.C., Dickens, G.R., Zeebe, R.E., 2008. An early Cenozoic perspective on green-house warming and carbon-cycle dynamics. Nature 451, 279–283.
Zhang, J., Wang, Y., Zhang, B., Zhang, Y., 2016. Tectonics of the Xining Basin in NW China and its implications for the evolution of the NE Qinghai-Tibetan Plateau. Basin Res. 28, 159–182.
Zhang, W., Zhang, T., Song, C., Appel, E., Mao, Z., Fang, Y., Lu, Y., Meng, Q., Yang, R., Zhang, D., Li, B., Li, J., 2017. Termination of fluvial-alluvial sedimentation in the Xining basin, NE Tibetan Plateau, and its subsequent geomorphic evolution. Geomorphology 297, 86–99.
