Research ArticleGEOLOGY

Pre-Quaternary decoupling between Asian aridification and high dust accumulation rates

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Science Advances  14 Feb 2018:
Vol. 4, no. 2, eaao6977
DOI: 10.1126/sciadv.aao6977


Theories of late Cenozoic climate cooling assume that central Asian aridification and high dust accumulation rates in the Chinese Loess Plateau and the North Pacific Ocean are genetically related. On the basis of detailed sediment provenance analysis, we show that high dust accumulation rates in the Chinese Loess Plateau and the North Pacific Ocean during the late Miocene-Pliocene were mainly caused by increased erosion in the Qilian Mountains and low-elevation eastern Asia areas, driven by the effects of East Asian summer monsoon intensification. We conclude that precipitation-driven erosion increased dust input to the North Pacific Ocean and may have played a pivotal role in late Cenozoic climate cooling.


The Chinese Loess Plateau (CLP) preserves a near continuous record of wind-derived dust accumulation since the Miocene (15). Understanding the provenance of the dust on the CLP is not only key to accurately interpret causation of dust accumulation rate records on the CLP and reveal Asian aridification processes, but it also helps shed light on dust sources downwind from the CLP in the North Pacific Ocean (6, 7). However, the provenance history of the CLP dust is not clear, especially for the late Miocene-Pliocene Red Clay (fine-grained loess with reddish color) Formation (8, 9) east of the Liupan Shan (Shan means mountain in Mandarin Chinese), which was deposited before the ~2.7-Ma (million years ago) amplification of Northern Hemisphere glaciations. Understanding the source of lithogenic dust during this period is critical because of the potential role of dust-borne Fe in phytoplankton production in Fe-limited portions of the oceans (10). This period corresponds with an approximate 10-fold increase in dust mass accumulation in the North Pacific Ocean (6), which today is Fe-limited.

Progress in zircon U-Pb dating techniques allows the use of single-grain zircon U-Pb ages to more accurately and precisely pinpoint CLP dust provenance, which may come from geographically distinct locations (1116). Many traditional provenance tools, such as whole-rock geochemistry (for example, Nd, Sr, and Pb isotopes), provide less precise geochemical provenance information than the single-grain detrital mineral approach (17, 18). Zircon U-Pb geochronology generates a series of U-Pb ages and thus has the potential to differentiate between multiple and mixed sources. However, most previous zircon U-Pb geochronology studies reported <150 zircon U-Pb ages per sample (11, 19), which prevents a statistically robust comparison of relative proportions of different age groups, where sources share similar zircon U-Pb age peaks (16, 20, 21). Here, we apply the new method of large observation (that is, Large-n), detrital zircon geochronology, to the Red Clay Formation of the CLP to characterize the relative contribution of detritus from different lithogenic dust sources in central-eastern Asia (16).

Red Clay detrital zircon geochronology

Twelve Red Clay samples ranging in age from ~8.1 to 2.9 Ma were collected from the Chaona section, central CLP (Fig. 1 and data file S1). The ages of individual Red Clay samples are based on previous paleomagnetic dating (8). To compare with the Red Clay results, we group-published zircon U-Pb ages of Quaternary loess units L9, L15, and L33 from Luochuan to represent old loess, and we lump-published zircon U-Pb ages of loess L5 and younger loess from different locations (excluding the northeastern CLP Jingbian site in both cases) to represent young loess (table S1) (11, 2225). Previous research demonstrates that the northeastern CLP Jingbian site has more North China Craton-derived dust contribution, different from the other CLP sites that came mainly from northern Tibet-derived dust (22, 26). To that end, we separate Jingbian loess samples from the rest of the loess samples to compare with the Red Clay samples.

Fig. 1 Map of the Chinese Loess Plateau and potential dust source regions, with inset map showing the North Pacific Ocean.

The main map corresponds to the dashed black rectangle in the inset map. Loess sites mentioned in this study are labeled using white dots, and some source region sample sites were labeled using white squares. The filled white oval and the unfilled red oval correspond to low-elevation eastern Asia (LEEA) and the Qaidam + Gobi (Qilian Shan) source region, respectively. H, Siberian high pressure system; EASM, East Asian summer monsoon; EAWM, East Asian winter monsoon. Revised from Pullen et al. (11). Pink color in main map indicates Cenozoic volcanics.

Zircon age populations from the Red Clay samples mainly fall into four age populations: ~1500 to 2750, 700 to 1200, 400 to 500, and 200 to 300 Ma, with the most prominent age peaks at ~450 and 250 Ma (Fig. 2). The ratio of these prominent age clusters varies between samples and can be potentially used as an indication of provenance. The n200–300Ma/n400–500Ma ratio (the number of ages falling within 200 to 300 Ma versus 400 to 500 Ma) increases between 8.1 and 7.7 Ma, decreases between 6.2 and 5.8 Ma, and increases from 4.4 to 3.8 Ma (Figs. 2 and 3). The n200–300Ma/n400–500Ma ratio for the Quaternary loess samples is lower than those from the Red Clay Formation between 4 and 2.7 Ma (Figs. 2 and 3). The n1500–2750Ma/n400–500Ma ratio (the number of ages falling within the 1500- to 2750-Ma age range versus the 400- to 500-Ma age range) and n200–300Ma/n400–500Ma generally covary with each other (Fig. 3, D and E).

Fig. 2 Chaona Red Clay zircon U-Pb ages and comparison with loess zircon U-Pb ages.

Black and blue lines are normalized probability density function and kernel density estimation plots, respectively, and open rectangles are age histograms. Pink arrows highlight increases and decreases of the 200- to 300-Ma age population. The shades highlight samples with similar zircon U-Pb age distribution. CN-xMa indicates the depositional age of the Chaona Red Clay samples.

Fig. 3 A comparison of North Pacific, Chinese Loess Plateau, and global paleoenvironmental records.

(A) Dust accumulation rate on the CLP (8). (B) North Pacific dust mass accumulation rate (6). (C) North Pacific dust 206+207+208Pb/204Pb ratio (7). (D) Ratio of number of zircon ages within the 1500- to 2750-Ma range over that within the 400- to 500-Ma range [n(1500–2750Ma)/n(400–500Ma)] on the CLP (this study). (E) Ratio of number of zircon ages within the 200- to 300-Ma range over that within the 400- to 500-Ma range [n(200–300Ma)/n(400–500Ma)] on the CLP (this study). (F) Magnetic susceptibility record on the CLP (1, 8). The lower and upper part of the record is from Qinan (1) and Chaona (8), respectively (see fig. S3 for further justification of using these data). (G) Benthic oxygen isotope record (35). CLP data are all from the Chaona section. The two yellow shades correspond to those in Fig. 2. The average dust accumulation rate of the Chaona section during 8 to 6, 6 to 3.8, and 3.8 to 2.7 Ma is ~24, ~21, and ~28 m My−1, respectively (8).

Comparison with detrital zircon geochronology of potential source regions

We compare detrital zircon U-Pb ages of the Red Clay and loess on the CLP with potential source regions (table S1 and fig. S1) (11, 12, 19, 23, 2630), based on n1500–2750Ma/n400–500Ma and n200–300Ma/n400–500Ma (Fig. 4), and visualize differences using multidimensional scaling (MDS) (fig. S2) (21). Results based on the two methods are similar (Fig. 4 and fig. S2). The Red Clay and the early Quaternary loess (Old loess) detrital zircon age signal can be explained by a mixture between the middle reaches of the Yellow River (MYR)/eastern Mu Us desert (EMU) source and the Qaidam Basin/Gobi (Qilian Shan) source. MYR and EMU dust is derived from an underlying basement (North China Craton and overlying Mesozoic sandstone; hereafter referred to as low elevation eastern Asia or LEEA) and has zircon U-Pb age peaks high in the relative proportions of 200- to 300-Ma and 1500- to 2750-Ma ages (Fig. 4 and fig. S1) (26, 28). In contrast, both the Qaidam Basin and Gobi (Qilian Shan) are sourced mainly from the high Qilian Shan, which has lower proportions of zircon ages in the ranges of 200 to 300 and 1500 to 2750 Ma but higher proportions of 400- to 500-Ma ages (Fig. 4 and fig. S1) (12, 26).

Fig. 4 Biplot of n(200–300Ma)/n(400–500Ma) and n(1500–2750Ma)/n(400–500Ma) from Chaona Red Clay and potential source regions (black dots).

The ratios for Quaternary loess from the northeastern Jingbian site (brown crosses) and those excluding the Jinbian site on the CLP (brown dots) are also shown for comparison. See table S1 for data references. L3, third loess layer counting from the top of sequence. The tilted black line near the middle of the graph separates high (to the upper right; plus and triangle) from low (to the lower left; dot and square) dust accumulation rate Red Clay samples. The solid and dashed double arrows indicate the schematic mixing pattern for the young loess and for the old loess and Red Clay, respectively.

The late Quaternary loess provenance signal can be explained by a mixture between the Qaidam Basin/Gobi (Qilian Shan) source and the upper reaches of the Yellow River (UYR)/proximal CLP deserts, including the west Mu Us, Tengger, and Badan Jaran deserts (solid double arrow in Fig. 4 and fig. S2) (12, 26). It is also clear that central Asian mountains and basins that are further from the CLP, including the Tianshan, Gobi Altai, Pamir, Tarim Basin (Taklamakan Desert), West Kunlun, and Altun Shan, are unlikely to be the main source regions for detrital zircon crystals in the Red Clay and loess deposits of any age (Fig. 4 and fig. S2). We also exclude the Songpan-Ganzi complex (31, 32) as a significant source for the CLP because prevailing southwesterly paleo-winds would have been necessary. These winds are in conflict with southward decreasing bulk sediment and quartz grain size in the Red Clay sequence (33), which requires the dominantly dust-carrying wind in the Red Clay sequence to have a strong northerly component. Furthermore, the predominately Middle–Upper Triassic strata of the Songpan-Ganzi complex have an overabundance of ~780- and ~1850-Ma zircons relative to the 200 to 300 and 400 to 500 Ma observed in the Red Clay Formation reported here, which we use as key provenance indicators (31). To that end, significantly more ~780- and ~1850-Ma age detrital zircons would be present in the Red Clay Formation if the Songpan-Ganzi complex was a significant source.

Comparison with North Pacific dust provenance and accumulation rate records

Dust accumulation rates in the central CLP and in the North Pacific Ocean generally covary: Relatively high dust accumulation rates are observable in the ranges of 8 to 6 and 3.8 to 2.7 Ma, and dust accumulation rates reached sustained high values after ~2.7 Ma (Fig. 3, A and B). However, at finer scales, differences exist between the two records. We attribute the finer–time scale differences in the dust records between these two areas to the dominant dust-transporting winds: East Asian winter monsoon for the CLP (2) and westerlies for the North Pacific Ocean (6). For example, the North Pacific record shows high dust accumulation rates during 8 to 7.5 Ma, whereas the CLP record does not show a similar variation (Fig. 3, A and B). This difference might be due to weaker East Asian winter monsoon (2, 34) and by inference less effective wind transport capacity during earlier time intervals of warm climate (Fig. 3, A, B, and G) (35). In contrast, strengthening of westerly winds is closely related to the uplift of the Tibetan Plateau, and northeastward growth of the Tibetan Plateau at ~8 Ma might have strengthened the westerly wind speeds so that dust can be effectively transported to the downwind North Pacific Ocean (3638).

The two intervals (8 to 6 and 3.8 to 2.7 Ma) of high dust accumulation during the late Miocene-Pliocene generally yield higher n200–300Ma/n400–500Ma and n1500–2750Ma/n400–500Ma ratios than other periods, with an exception at ~6.2 Ma, where the n1500–2750Ma/n400–500Ma ratio was low for unclear reason (Figs. 2 and 3, A, B, D, and E, and table S1). The zircon age patterns for the Quaternary loess and period of low accumulation rates during 6 to 4 Ma are similar and have lower n200–300Ma/n400–500Ma and n1500–2750Ma/n400–500Ma ratios.

We compared the loess and Red Clay detrital zircon U-Pb geochronology results with the ratio of radiogenic lead (206Pb, 207Pb, 208Pb) versus nonradiogenic 204Pb (206+207+208Pb/204Pb) from the North Pacific Ocean (Fig. 3C) (7). Pb isotopes from the North Pacific mostly represent Asian dust, with higher radiogenic Pb ratios generally reflecting older source regions (7). Noticeably, the 206+207+208Pb/204Pb ratio in the North Pacific Ocean (7) also displays similar variations to the n200–300Ma/n400–500Ma and n1500–2750Ma/n400–500Ma ratios and the CLP and North Pacific dust accumulation rates (Fig. 3, A to E). Considering the long distance between the CLP (Fig. 1) and the North Pacific dust site and different dust transport winds, this similarity is striking.


Dust accumulation rate variations in the North Pacific Ocean and the CLP are often used to infer central Asian aridification history (2, 6). However, pollen and stable isotope records from central Asia (3941) reveal increasingly arid conditions throughout the Miocene-Pliocene time, in contrast with the dust accumulation rate variations (Fig. 3). To explain this apparent discrepancy about central Asian aridification history, below, we explore an alternative forcing for high dust accumulation rates in the CLP and the North Pacific Ocean.

Intervals of higher n1500–2750Ma/n400–500Ma and n200–300Ma/n400–500Ma for the Red Clay sequence and higher 206+207+208Pb/204Pb ratio in the North Pacific Ocean generally correspond to higher dust accumulation rates (Fig. 3). This pre-Quaternary pattern seems to suggest that intervals with higher dust accumulation rates in the CLP and the North Pacific Ocean are associated with an increase in LEEA dust contribution instead of the Qaidam Basin and Gobi (Qilian Shan) dust source. However, the Red Clay n1500–2750Ma/n400–500Ma and n200–300Ma/n400–500Ma values are much closer to those from the Qaidam Basin and Gobi (Qilian Shan) source than LEEA, suggesting that the contribution of LEEA was lower. For example, the n200–300Ma/n400–500Ma ratios are higher during the higher dust accumulation rate interval at 3.8 to 2.7 Ma as compared to the lower dust accumulation rate interval at 6 to 4 Ma. Assuming that the Qaidam Basin and Gobi (Qilian Shan) source and LEEA are the two major sources for the CLP dust during the late Miocene-Pliocene, as suggested by the provenance data (Fig. 4 and fig. S2), one can calculate that LEEA contributed ~11 and ~21% dust to the CLP for 6- to 4-Ma and 3.8- to 2.7-Ma time periods, respectively, equal to an approximately 10% increase. The average dust accumulation rate in the CLP during 6 to 4 and 3.8 to 2.7 Ma is ~21 and ~28 m My−1 (million years), respectively, equal to an approximately 30% increase. Thus, the LEEA dust contribution increase can only explain one-third of the dust accumulation rate increase in the CLP, not to mention the observed >10-fold dust accumulation rate increase in the North Pacific Ocean (Fig. 3B).

Given that the LEEA dust contribution increase alone is insufficient to explain the observed dust accumulation rate increase, we infer that the dust contribution from the Qaidam Basin and Gobi (Qilian Shan) source must have also increased during the late Miocene-Pliocene. However, the dust contribution from these sources increased proportionally less than that from LEEA, resulting in the pattern that higher dust accumulation rates generally correspond to higher n1500–2750Ma/n400–500Ma and n200–300Ma/n400–500Ma ratios in the CLP and to higher 206+207+208Pb/204Pb ratio in the North Pacific Ocean. This interpretation is consistent with the East Asian monsoon precipitation pattern, whereby LEEA receives more precipitation, and likely more erosion, than the Qaidam Basin and Gobi (Qilian Shan) source, which is further inland and away from the monsoon moisture source (Fig. 1).

If this inference is correct, then the high dust accumulation rate intervals must correspond to intensified monsoon precipitation. Previous studies (26, 42) have proposed that precipitation increase can potentially increase river erosion, releasing sediment from parent rocks for wind to pick up. Strikingly, each of the three North Pacific high dust accumulation rate intervals (8 to 7.5, 6.5 to 6, and 3.8 to 2.7 Ma) during the Miocene-Pliocene corresponds to a phase of intensified monsoon precipitation, as is shown by the magnetic susceptibility record on the CLP (Fig. 3F) (1, 8), and is thus consistent with the connection between precipitation and dust provenance.

This improved knowledge of North Pacific dust provenance resolves the aridification record discrepancies between central Asian regions (3941) and the North Pacific Ocean (6). We conclude that intensified central-eastern Asia erosion driven by East Asian summer monsoon precipitation increased dust input to the North Pacific Ocean. The late Pliocene North Pacific Ocean dust accumulation rate increase was accompanied by a climate cooling trend associated with the onset of Northern Hemisphere glaciations (Fig. 3G) (35). In contrast, no synoptic climate cooling trend was observed during ~6 to 3.8 Ma, when North Pacific dust accumulation rate was lower and the East Asian summer monsoon was less intense (Fig. 3G). This observation is consistent with the hypothesis that dust-promoted marine phytoplankton production might cause climate cooling, thus supporting the iron hypothesis (10, 43).

Although Quaternary dust provenance and accumulation rate variations are not the main focus of this study, we explore the causes for their variations in the CLP and the North Pacific Ocean with knowledge gained from the study of the Miocene-Pliocene. We attribute the early Quaternary loess and North Pacific dust accumulation rate increase mainly to increased dust contribution from the Qaidam Basin and Gobi (Qilian Shan) source of the northern Tibetan Plateau relative to LEEA based on the mixing pattern in Fig. 4. Intensified Quaternary glaciation in the Qilian Shan and enhanced aridification of the Qaidam Basin would have potentially generated more friable fine-grained sediment in this area (39, 44), which can result in more dust transport to the CLP and downwind to the North Pacific Ocean. In addition, Red Clay–loess magnetic susceptibility and heavy mineral composition consistently show a major change in precipitation gradient on the CLP at the Pliocene-Quaternary boundary; precipitation was more meridional during the late Miocene-Pliocene than the Quaternary (45). As a result, LEEA received less precipitation in the Quaternary than before, which should have also caused an LEEA dust contribution decrease to the central CLP and the downwind North Pacific Ocean during the Quaternary. We note that, in comparison with the early Quaternary loess from the rest of the CLP (labeled as Old loess in Figs. 2 and 4 and fig. S2), the zircon age populations from the Jingbian early Quaternary loess (labeled as Jingbian Old loess) are more similar to the LEEA source (Fig. 4), indicating more dust input from LEEA (22). This is likely due to proximity of this site to LEEA.

Previous provenance studies show that the UYR likely already existed during the early Quaternary (26), although its drainage area was smaller than the late Quaternary (46). Therefore, some dust contribution from reworked UYR sediments is also possible. Considering the smaller drainage area of the UYR during the early Quaternary (46) and its well-established relationship between catchment size and sediment load, and with all other conditions remaining the same (47), the UYR contribution would be expected to be smaller than during the late Quaternary.

The paleo-MYR and its tributaries in LEEA are located north to northeast of the CLP and, based on modern wind directions, are not directly upwind of the CLP (Fig. 1). However, magnetic fabrics of late Quaternary loess demonstrate a northeasterly paleo-dust transport direction (48), which supports our inference that the paleo-MYR and its tributary rivers could be important source regions for the CLP. This southwestward dust transport direction might be a result of regional topographic modification (the northeast-southwest–striking Lüliang Shan) to the winter wind pattern (Fig. 1) (49).

We attribute the late Quaternary loess and North Pacific dust provenance signal mainly to a mixture between the Qaidam Basin/Gobi (Qilian Shan) source and the UYR/proximal CLP deserts, consistent with many previous studies (12, 26). Drill cores in the proximal deserts to the CLP reveal that they did not form until the late Quaternary and their formation is closely linked to the formation and drainage area expansion of the UYR (50, 51). This observation suggests that the deserts proximal to the CLP are important loess sources but only since the late Quaternary.

In summary, we demonstrate that dust accumulation increase can be achieved via erosion increase driven by monsoon precipitation, as opposed to only enhanced continental aridity. Although aridity is a logical deduction when interpreting increased dust accumulation rates in stratigraphic records, aridity is not always the primary cause when there is sediment available to be transported by the wind.


Red Clay samples were disaggregated using ultrasonic disruption to remove clay minerals and break apart polymineralic particles. Detrital zircon grains were separated by heavy liquids. A scanning electron microscope (SEM) was used to generate high-resolution backscattered electron (BSE) images of the grain mounts. Dating of detrital zircon crystals from fine-grained eolian strata was accomplished using U-Th-Pb Large-n (that is, large observation data sets) and small diameter beam (that is, 12 μm) during laser ablation inductively coupled plasma mass spectrometer analysis (16). This approach allows analysis across a wide range of fine grain sizes that are typical of loessic deposits. Large-n allows for robust intersample comparisons of relative proportions of ages (16).

U-Th-Pb isotope ratios were collected at the Arizona LaserChron Center ( at the University of Arizona. Measurements were made using a Teledyne Photon Machines G2 solid state NeF excimer laser ablation system coupled to a Thermo Fisher Scientific ELEMENT 2 single collector inductively coupled plasma mass spectrometer.

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Before analysis, grains were imaged to provide a guide for locating analysis pits in optimal sites and to assist in interpreting results. Images were made with a Hitachi 3400N SEM ( The BSE images used in this study were masked to identify crystals <20 and >20 μm; however, all grains >12 μm were analyzed during this investigation—the analytical limitation of the laser defined by the precision of U-Pb and Pb-Pb isotope ratios. This grain-size differentiation was created for future investigations but was not applied here.

Isotope fractionation was corrected for using a suite of zircon reference materials, including: FC-1 (53), R33 (54), and SL (55). Signal intensities were measured with a secondary electron multiplier that operates in pulse-counting mode for signals less 4 million counts per second (cps) and in analog mode above 4 million cps. The calibration between pulse-counting and analog signals was determined line by line for signals between 50,000 and 4 million cps and was applied to >5 million cps signals. Because of the nonlinearity reported for secondary electron multipliers in isotope mass spectrometry (56, 57) and the desire for the highest accuracy, 206Pb/238U ratios with 238U >5 million cps were calculated from 235U*137.88. Thus, isotope ratios were calculated from measurements made within one detection mode of the secondary electron multiplier (that is, discrete pulse counting) rather than reporting ratios measured in mixed modes (for example, 206 measured in pulse-counting mode and 238 measured in analog mode).

Data were reduced using an in-house Excel spreadsheet (that is, E2ageclac). E2agecalc calculates the average intensities for each isotope. Mass 204 was corrected for 204Hg using the mass 202 and the natural 202Hg/204Hg ratio of 4.3. This Hg correction was typically not significant for most analyses because most Hg backgrounds were low. Initial Pb was corrected on the basis of the measured 206Pb/204Pb and the assumed composition of Pb based on Stacey and Kramers (58). Fractionation of 206Pb/238U, 206Pb/207Pb, and 208Pb/232Th was corrected by a sliding-window average of eight reference material analyses. The ratio between sample and reference analyses for this study was 5:1. This approach accounted for instrumental drift, typically <1%/hour for 206Pb/238U. Measurement uncertainties for 206Pb/238U and 208Pb/232Th were based on scatter about a regression of the measured values. Uncertainties for 206Pb/207Pb and 206Pb/204Pb were based on the SD of measured values because these ratios did not provide estimates of time-dependent fractionation during laser ablation. The sum of these uncertainties and overdispersion factor (determined by the analysis of reference materials) was reported as the internal (that is, measurement) uncertainty for each analysis. These uncertainties are reported at the 1σ level. U and Th concentrations were estimated from published concentrations for FC-1. External (that is, systematic) uncertainties for 206Pb/238U, 206Pb/207Pb, and 208Pb/232Th included scatter of the reference material analyses, age uncertainties for reference materials, uncertainties in common Pb composition, and decay constants uncertainties for 235U and 238U. However, following the convention for detrital mineral U-Th-Pb dating, external uncertainties were not reported here with the ages. The “best age” for each analysis used in plotting and interpretations was from 206Pb/238U for <1000 Ma and from 206Pb/207Pb for >1000 Ma ages. Analyses with >10% uncertainty (1σ) in 206Pb/238U age were not included. Analyses with >10% uncertainty (1σ) in 206Pb/207Pb age were not included, unless the 206Pb/238U age was <400 Ma. For ages younger than 1000 Ma, the discordance was defined as (207Pb/235U-206Pb/238U)/207Pb/235U*100; for ages older than 1000 Ma, the discordance was defined as (207Pb/206Pb-206Pb/238U)/207Pb/206Pb*100. Analyses with >15% discordance and with >10% reverse discordance were not included.


Supplementary material for this article is available at

fig. S1. Zircon U-Pb age distribution of the potential source regions for the CLP.

fig. S2. MDS plot of Red Clay/loess and potential source region sample zircon U-Pb geochronology data.

fig. S3. Magnetic susceptibility record for the CLP (1, 8).

table S1. Description of samples shown in Fig. 4.

data file S1. Chaona Red Clay zircon U-Pb geochronologic data.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.


Acknowledgments: Constructive comments by three reviewers significantly improved the manuscript. Funding: This work was funded by the National Key Research and Development Program of China (grant no. 2016YFE0109500), the (973) National Basic Research Program of China (grant no. 2013CB956400), the National Natural Science Foundation of China (grant nos. 41422204, 41672157, and 41761144063), and the NSF (grant nos. 1348005 and 1545859). Author contributions: J.N., A.P., and C.N.G. designed the experiments. A.P., W.P., and Z.W. performed the experiments. All authors analyzed the data. J.N. and A.P. wrote the manuscript with help from the other authors. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
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