Introduction

The mid-latitude arid Asian interior—including Central Asia and Xinjiang province of western China—is Earth’s largest extra-tropical arid region1. The precipitation here is influenced primarily by the intensity and path of the westerlies2. This crosses the endoheric Tarim Basin (TB), bordered by the Tian Shan in the north, Pamir Mountains to the west, and the Kunlun Mountains on the south (Fig. 1). The TB currently receives an average of <50 mm annual precipitation, concentrated in the summer months (Figs. 1, S1). The hyper-arid region, like many subtropical arid regions, is predicted to become drier in response to global warming3,4. Yet it appears to be getting warmer and wetter5,6, possibly from a Clausis-Clapeyron effect7 and/or a northward shift of the thermal equator8,9. Additionally, the TB experiences multifaceted interactions between tropical and subtropical climatic influences10,11, including the Rossby wave train influence on the middle-latitude westerly jet and a super-regional summer monsoon circulation12. These interactions significantly contribute to increased rainfall, particularly during extreme precipitation events over arid Asia13. Consequently, the existing mismatch between climate projections and observations impedes our understanding of how the climate in the arid Asian interior, especially the hyper-arid TB, will change in a world that is projected to become warmer in the coming centuries.

Fig. 1: Regional setting of the Tarim Basin (TB), location of AQ16 and YM22 loess sections in the Kunlun Mountains and precipitation characteristics near the study site.
Fig. 1: Regional setting of the Tarim Basin (TB), location of AQ16 and YM22 loess sections in the Kunlun Mountains and precipitation characteristics near the study site.
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a The horizontal winds field (arrows, m/s) at 850 hPa isobaric in summer averaged from 1979 to 2008 (ERA5 Reanalysis Derived data) and annual mean precipitation (shaded, mm/day) from 1979 to 2008. White line marks 1500 m elevation. b The area of the TB is ~0.4 million km2 between 36–42°N and 74–95°E, and hosts the largest mid-latitude sand sea, known as the Taklamakan Desert97. Study site locations (AQ16 and YM22) are marked with yellow stars. The arrows depict the dominant pattern of Tarim Basin mid-tropospheric (about 600 hPa) circulation (red arrows) and near-surface winds (white arrows) in the Tarim Basin. The direction of the calculated maximum sand drift potential is nearly the same as the wind direction ①28. c Annual cycle of climatologically-averaged precipitation (mm/month) at the Yutian weather station. d Year-to-year variation of summer-mean monthly precipitation anomaly at Yutian from 1956 to 2019. The dashed-line indicates the linear trend. The meteorological data is from http://data.cma.cn.

The relatively warm mid-Holocene (8.2–4.2 ka BP, ka BP = thousand years before the present) serves as a geological baseline, offering insights into the atmospheric and hydrological dynamics that might be anticipated under future climate forcing14,15. Two primary hypotheses have been proposed to explain hydroclimate changes over the TB during this period. 1. The “westerlies hypothesis”2 argues that the westerlies play the key role in modulating hydroclimate changes over the TB16, leading to drier conditions during the mid-Holocene warm period17 when weakened westerlies decreased local precipitation potential and overall water availability18. 2. The “monsoon hypothesis”19, on the other hand, suggests that enhanced monsoon circulation during the early- to mid-Holocene extended the Asian summer monsoon (ASM) rainbands as far north as the Xinjiang region, thereby increasing water availability via increased monsoonal atmospheric precipitation20. For example, lake hydrodynamics in Lake Bosten21 were significantly enhanced, and freshwater lake environments in Lake Balikun22,23. Meanwhile, a piece of evidence indicates increased runoff into the Tarim Basin during the late Holocene24,25,26, suggesting a colder and wetter pattern attributed to the southward displacement of the westerly jet25. These studies indicate that the hydroclimate change of certain areas in the arid Asian interior, including the TB, experience influences from both the monsoon and westerly2,23,27,28 since the mid-Holocene. However, the TB is among the least studied regions in the Asian interior, with regard to Holocene paleoclimatology. There are limited continuous, absolutely dated, high-resolution records available to examine the respective impacts of the summer monsoon and westerly jet on hydrological cycling. Addressing this research gap is crucial, highlighting the need for new, robust records from the TB.

Along the southern margin of the TB, the northern slopes of the Kunlun Mountains have the world’s thickest loess record1,29, and are exceptionally well-suited for reconstructing the region’s Holocene climate history30. Recent meteorological analyses indicate that precipitation variations in the high-elevation Kunlun Mountains are primarily influenced by changes in 500 hPa westerly airflow, while the low-elevation closed basin margins experience heavier rainfall due to monsoonal moisture intrusion during the summer10. To achieve a coherent interpretation of summer monsoonal and westerly moisture changes, we targeted two high-resolution loess records from distinct locations and altitudes: YM22 at a high elevation (3500 m) and AQ16 at a lower elevation (2600 m) (Fig. 1). Hydroclimate proxies include the stable carbon isotope ratios and 14C ages derived from loess sediments. Rainstorm proxies are established utilizing loess provenance proxies, including rare-earth element (Ce and Nd) abundances, as well as 87Sr/86Sr and 143Nd/144Nd values. Our aim is to investigate the relative impacts of the summer monsoon and westerly jet on hydrological cycling across the TB during past warm periods, with implications for future hydroclimate changes and water availability.

Dust storms in the TB and loess records from the Kunlun Mountains

The TB experiences dust storms more than 140 days per year from spring to autumn, caused by westerly winds31. Concurrently, being located downwind of the Taklamakan Desert and the pre-hill floodplain fan, the process of dust storm accumulation results in loess deposition on the grasslands of the northern slopes of the Kunlun Mountains, ranging from 2500 m to 4000 m32,33 (Fig. 1). As a result, loess sequences in the Kunlun Mountains record linked processes of dust generation, airborne transport, dust deposition32,34, basin-scale post-depositional pedogenic weathering33 and global change1,29.

At 3500 m altitude in the Kunlun Mountains, we collected loess samples from a site named YM22 (36°03'50″ N, 81°05'46″ E), marked by a steep cliff in a north-south-oriented gully. This cliff was recently formed due to flash flooding. Within the gray-yellow loess, we identified at least 71 preserved charcoal-rich dark layers, typically 0.5–2 cm thick (Fig. S2). The charcoal particles range from 1 to 10 mm in diameter, with the largest nodules reaching up to 40 mm in diameter (Fig. S2).

At 2600 m, AQ16 (36°25′18″ N, 81°58′25″E) is located in a 15 m-deep valley about 5 km southwest of the Aqiang River (Fig. S3). It overlays the Lower Pleistocene Xiyu Conglomerate. AQ16 is predominantly composed of well-sorted gray-yellow sandy loess (Fig. S3). Along the southern margin of the TB translation to the Kunlun Mountains, field observations have identified coarser grain sizes from the AQ16 site compared to the YM22 site due to its proximity to the upwind direction of the dust storm sources, including the Taklamakan Desert and pre-hill floodplain fans.

Results

Geochronology

We present 26 accelerator mass spectrometry (AMS) 14C dates on charcoal from YM22 (“Methods”), derived primarily from short-lived plants (primarily grass) recovered from loess. These have an analytical uncertainty (2 SD) ranging from ±14 to ±20 years (Table S1, “Methods”). Due to the lack of charcoal fragments, the chronology of the loess sequence in the Kunlun low-altitude region is based on 5 optically stimulated luminescence (OSL) dates that were obtained from AQ16 (Table S2, “Methods”). The feasibility of OSL to determine accurate ages from these sediments was confirmed by tests using the single-aliquot regenerative-dose protocol (Fig. S4).

The 5 OSL dates in AQ16 and 26 14C dates in YM22 were incorporated into a Bayesian age-depth model in the Bchron package35 and geoChronR package36 using R software to produce the chronological framework (Figs. S5, S6). Results indicate that the upper 54 m in AQ16 spans a period from ~15 to 1 ka BP, and the upper 16 m in the YM22 spans a period from 8.2 ka BP to present. Based on the age-depth results, the sedimentation rates of the loess at the two sites tend to be highly consistent.

Environmental proxies

We analyzed particle size, organic carbon isotopes, and inorganic carbon isotopes at both sites to reconstruct environmental changes (Fig. S7). At YM22, the mean grain size (MGS) varies from 19 μm to 30 μm, indicating a two-stage change from an average value of 22 μm during the mid-Holocene to 25 μm in the late Holocene. We obtained 525 δ13CIC and 80 δ13CTOC measurements, with average temporal resolutions of ~16 years and ~105 years. YM22 δ13CIC have values ranging from −0.01‰ to −4.61‰ and exhibit a negative trend during the late Holocene compared to the middle Holocene. The δ13CTOC values within the YM22 loess sequence also exhibit two distinct phases, fluctuating between −22.4‰ and −25. 2‰ with a mean of −23.1‰ (Fig. S7).

At AQ16, our focus was solely on the overlapping time period spanning from 8 to 1 ka BP, despite the availability of environmental proxies in AQ16 dating back to the early Holocene before 8 ka BP. The MGS at AQ16 ranges from 40 μm to 65 μm. Unlike findings from YM22 in higher elevation loess records, AQ16’s MGS is coarser during the mid-Holocene compared to the late Holocene (Fig. S7). Additionally, in contrast to the substantial variation in δ13CIC values observed in YM22, the δ13CIC variability of AQ16 is very limited, with the δ13CIC values fluctuating between −0.8‰ and 0.4‰ with a mean of 0‰ (Fig. S7). Although the δ13CTOC values within the YM22 loess sequence exhibit similar patterns with YM22, they fluctuate between −22.2‰ and −23.8‰ (Fig. S7). Additionally, we provide 14C “reservoir offset ages” from 15 carbonates (F14R-atm-IC, see Ref. 37) and 15 bulk organic carbon (F14R-atm -BOC, see Ref. 37), with average temporal resolutions of ~500 yr, to aid in the interpretation of hydroclimate change in AQ16. Corrected for deposition age, the reservoir’s relative enrichment values calculated relative to the atmosphere varies between +0.02–0.12‰ for IC and +0.17–0.70‰ for BOC, corresponding to a range of approximately 16,710–29,790 and 2835–14,160 14C years, respectively (Tables S3 and S4).

Provenance proxies

We also reconstructed past changes in loess provenance using four groups of independent proxies (rare earth elements—Ce, Nd; and radioactive isotopes—87Sr/86Sr and 143Nd/144Nd) (See “Methods”) since the last deglacial, we observe coherent changes across all proxies (Fig. S7). To be consistent with the environmental proxies, these are focused on the period from 8–1 ka BP.

Climate model simulations

To gain a better understanding of hydroclimate changes and climate dynamics over the TB during the Holocene, we examined the results of an independent high-resolution transient model experiment38. The simulation was conducted using the Community Earth System Model (CESM) with a relatively high horizontal resolution for the atmospheric module (0.47° in latitude and 0.63° in longitude; details in Methods). The wind directions, strength, water vapor fluxes at different heights in the TB, and the position of the westerly jet have been calculated and are presented in Fig. S9 for comparison between proxies and the model in the discussion.

Discussion

Weakened westerly jet and drier Asian interior during the mid-Holocene

The mid-Holocene warm period represents a well-defined global mean surface temperature warm period14,39,40. Specifically, there is evidence of intense warming (∼2 °C) at Lake Balikun nearby the TB during the mid-Holocene, rather than in the late-Holocene22. We use grain size from YM22 to reflect wind strength, validated both conceptually and empirically as a reliable proxy for dust storm activity in nearby loess records1,30. δ13CIC and δ13CTOC serve as dependable indicators of pedogenic conditions and vegetation metabolic input to soils41,42,43, influenced by hydroclimate under dominant C3 vegetation44. Their validity is confirmed through a strong correlation observed between surface samples and modern hydroclimate data (Fig. S9), as well as through their consistent application in loess studies (see Supplementary Text 1).

At YM22, an increased MGS and a consistently enriched trend in both δ13CTOC and δ13CIC were observed during the mid-Holocene compared to the late-Holocene (Fig. 2), suggesting drier conditions under weakened westerly winds30. Moreover, consistent with the wind and hydroclimate variations reflected by MGS and stable isotopes in YM22, the modelling results show that the westerly jet stream wind speed at 200 hPa fluctuated between 13 m/s to 25 m/s, decreased from a mean of roughly 21 m/s to 18 m/s over the transition from the late to middle Holocene (Fig. 2). This decrease was accompanied by a significant decrease of approximately 40% in westerly water vapor fluxes at 600 hPa (Fig. 2). These findings indicate a weakened westerly and drier atmospheric moisture state in the mid-troposphere during the middle, compared to the late Holocene. AQ16 exhibits more enriched δ13CIC and δ13CTOC values from the mid- to late-Holocene, suggesting a drier mid-Holocene period, and potentially drier than YM22 (Fig. S7). However, it is difficult to attribute the modest δ13CIC changes in AQ16 to a single hydroclimate mechanism. Radiocarbon isotopes of OC and IC in loess are more sensitive to soil changes than stable isotopes45, and can trace small shifts in pre-existing organic carbon and detrital carbonates replaced by in situ vegetation metabolic and pedogenic activities, as seen in AQ16 (see Supplementary Text 2). 14C reservoir offset ages of IC and OC have a strong positive correlation with δ13CIC (R = 0.65, n = 15, p < 0.05) and δ13CTOC (R = 0.89, n = 7, p < 0.05) values since the mid-Holocene (Fig. S10), supporting the argument that stronger vegetation and pedogenic conditions with more vegetation input to the soils in wetter conditions caused stronger in situ 13C and 14C. The average reservoir 14C age offset of IC in the mid-Holocene is 23.6 thousand years (kyr, n = 6), compared to 18.6 kyr during the late-Holocene (n = 9) (Fig. 2), supporting a drier mid-Holocene. Our F14 R-atm-OC data also indicate an arid mid-Holocene climate state, with reservoir 14C age offsets in the biosphere scaling up from 5.9 kyr (n = 9) during the late-Holocene (Fig. 2) to 10 kyr (n = 6) in the mid-Holocene. All these observations corroborate a drier climate state of AQ16 during the mid-Holocene. We smoothed the variance using a bootstrap technique, running 1000 times using a local polynomial regression (LOESS) with 4000-yr windows in Acycle software46. The result shows that the westerly and hydroclimate conditions transitioned from weaker and drier in the mid-Holocene to stronger and moister in the late-Holocene (Fig. 2).

Fig. 2: Wind and hydroclimate pattern over the Asian Interior and comparisons with modeling results.
Fig. 2: Wind and hydroclimate pattern over the Asian Interior and comparisons with modeling results.
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a Wind strength changes are indicated by MGS in YM22 at 3500 m elevation. b, c Hydroclimate change is reflected by δ13CIC and δ13CTOC of YM22, within larger values representing drier hydroclimate conditions. We smoothed the variance by a bootstrap method with 1000 iterations using a local polynomial regression (LOESS) with 4000-yr windows in Acycle software46. 14C/OSL age offsets of organic carbon (d, 14CBOC) and inorganic carbon (e, 14CIC) at the AQ16 site at 2600 m elevation. Larger values representing drier hydroclimate conditions. f Simulated changes of summer westerly jet intensity at 200 hPa in the Tarim Basin. g Simulated changes of summer water vapor flux at 600 hPa in the Tarim Basin (this study). The “-” represents the direction of the wind, which is easterly. Changes in age offsets of inorganic carbon (h), organic carbon (i), westerly jet intensity (j) and water vapor flux at 600 hPa (k) from warmer to cooler climates.

The weakened westerly jet stream and drier state during the mid-Holocene is not a local phenomenon and is consistent with climate change observed in other areas. For example, hydroclimate records from the nearby Tianshan loess16, Yili loess47, loess of Kazakhstan48, and dune records in the Western China49 all point to a similar drier mid-Holocene pattern (Fig. S11). Also, our record is consistent with the hydroclimate patterns observed in high-altitude lake records in the Asian interior, such as Lake Karakul in Pamir Mountains50 and peat records in the Altai Mountains17 and Tianshan Mountains51, which show arid conditions over the mid-Holocene. Moreover, the westerly intensity and the hydroclimate proxy’s variation used in this work all matched well with other documented drier Eurasian hydroclimate regimes modulated by the weakened westerly at an orbital timescale52,53. At last, mean summer soil moisture is also selected for the model output (Fig. S11), which matched well with our records in the Kunlun Mountains and other widely distributed Chinese loess records, that exhibit a drier soil moisture state in the mid-Holocene than in the late-Holocene. The corroborative dataset provided by the TB proxy and model records here significantly expands the inferred range of mid-Holocene drying from a regional to a continental scale.

Co-variation of increased mountain sediment sources in AQ16 and African-Asian Monsoon intensification during the mid-Holocene

An evaluation of geochemical, isotopic, and grain size characteristics of modern samples indicates increased mountain sediment sources in the low-altitude AQ16 loess sequence during the mid-Holocene compared to the late-Holocene. Firstly, rock samples from the Kunlun Mountains exhibit higher Ce and Nd concentrations as compared to sediments from the basin (Fig. 3), representing an end-member component sourced from the mountains. The mean Ce and Nd values in the mid-Holocene sediments were 59 ppm and 23 ppm, respectively, resembling modern fluvial sediments from the mountains54,55,56 (Fig. 3). Conversely, the mean Ce and Nd values of late-Holocene sediments closely resemble those of desert dune samples and saline lake sediments over the Tarim Basin54,55,56(Fig. 3). The presence of elevated Ce and Nd contents at AQ16 during the mid-Holocene signifies a pronounced contribution from mountainous sediment sources. Secondly, higher 87Sr/86Sr ratios and lower 143Nd/144Nd ratios in mid-Holocene sediments suggest a shift towards contributions from mountain regions, as these isotopic signatures are characteristic of mountain rocks and minerals. The average 87Sr/86Sr value of mid-Holocene sediments is 0.004 higher than the average for late-Holocene sediments (Fig. 3e), and the difference of the pattern and amplitude are in agreement with that between the mean 87Sr/86Sr values from Taklamakan Desert silts and Eastern Kunlun Mountain sediments57 (Fig. 3f). The mean 143Nd/144Nd values of mid-Holocene sediments are also different from those of late Holocene sediments (Fig. 3). Thirdly, a disparity is observed between the MGS from the mid-Holocene and late-Holocene, with an inferred weakened westerly wind strength and finer MGS recorded in YM22 and a coarser MGS observed in AQ16. This discrepancy suggests that the coarser MGS in AQ16 during the mid-Holocene may be associated with sediment derived from mountain erosion processes58, rather than changes in wind intensity as indicated by YM22.

Fig. 3: Past changes in AQ16 loess provenance using four groups of independent proxies, and their potential dust source area.
Fig. 3: Past changes in AQ16 loess provenance using four groups of independent proxies, and their potential dust source area.
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The variation of values in Ce (a), Nd (c), 87Sr/86Sr (e) and 144Nd/143Nd (g) of middle Holocene sediments and late Holocene sediments. All middle Holocene sediment values are different than those of late Holocene sediments. The Ce and Nd composition of surface sediments from the Tarim Basin were higher in river samples (mean value of 55 ppm and 22 ppm, respectively) than in dune samples (mean value of 44 ppm and 19 ppm respectively), and both values were significantly higher than those for saline lake samples (mean value of 22 ppm and 10 ppm respectively) (b, d)55,56. UCC denotes the average composition of the upper continental crust. f The mean 87Sr/86Sr values from Taklamakan Desert silts and Eastern Kunlun Mountain sediments. h A scatterplot displaying the correlation between Nd and the 143Nd/144Nd. The red dots on the plot correspond to samples from the late Holocene, while the yellow dots represent samples from the middle Holocene. For convenience, the 143Nd/144Nd ratios are reported in the epsilon notation (εNd).

Unexpectedly, loess provenance proxies exhibit an inverse relationship to westerly wind intensity. However, they exhibit a remarkable consistency with changes in the African-Asian monsoon regime (including the North African Summer Monsoon = NASM, and ASM) proxy records (Fig. 4). The temporal change in higher mountain sediment sources during the mid-Holocene and lower mountain sediment sources during the late-Holocene correspond to stronger and weaker African-Asian monsoons. One striking feature is rainstorms, which is marked by an abrupt decrease in intensity around 4.5 ka BP (Fig. 4). The timing of this rainstorm transition in the TB is coeval with monsoon collapses seen over northern Africa59,60, Laos61, Tibet,62,63, central China64, northeast China65 and north-central China66. This suggests that a strong coupling might exist at the superregional scale between the intensified of the African-Asian summer monsoon and the higher mountain sediment sources in the Kunlun Mountains.

Fig. 4: Comparison of provenance-based rainstorm proxy (a, this study) with African-Asian monsoon regime proxy records.
Fig. 4: Comparison of provenance-based rainstorm proxy (a, this study) with African-Asian monsoon regime proxy records.
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a Provenance_based rainstorm shifts over Tarim Basin (this study), within larger values representing increased mountain sediment sources. North African summer monsoon record indicated by δDwax data from Lake Challa in Tanzania (b)60 and Gulf of Aden core P178-15P (c)59. Higher/lower δDwax value denotes lower/higher monsoon intensity. d–f ASM intensity indicated by stalagmite δ18O values in Laos (d)61, Tibet (e)62, and south-central China (f)64. Higher/lower δ18O value denotes lower/higher monsoon intensity. g The ASM intensity variation represented by the Lake Qinghai record63 at the northwestern margin of the modern summer monsoon limit. SMI is the Summer Monsoon Index defined for Lake Qinghai in reference. 63, where higher/lower value denotes higher/lower monsoon intensity. h Lake Xiaolongwan record65 at the northeastern part of the modern summer monsoon limit, and PCA (Principal Components Analysis) was conducted on the terrestrial pollen percentages in ref. 65. The higher/lower PCA1 value denotes lower/higher monsoon intensity. i The lake level reconstruction for the past 8 kyr in Lake Dali66 at the central part of the modern summer monsoon limit. The higher/lower lake level denotes lower/higher monsoon intensity. Black dashed bands indicate the collapse of the Asian and African monsoons at the middle-late Holocene transition.

Intensified summer rainstorm events during mid-Holocene

Enhanced sediment supply from mountain sources is primarily governed by increased mountain erosion rates and elevated sediment fluxes through fluvial systems and rainstorm58,67,68 (Fig. 4a, see Supplementary Text 3). Field experiments and lab modeling in arid regions of southwest Asia have demonstrated a strong correlation between erosion and sediment transport and the intensity of rainstorms on the Jordanian Plateau69. In our study area, annual runoff and upstream sediment supply rose by 18% and 63%, respectively, over the past 60 years68. Furthermore, there has been a notable increase in reported rainfall extremes near our study site13. For instance, in June 2018, a rainstorm in Aqiang town, near the AQ16 site, deposited 94.7 mm of precipitation13. Comparison of the intensity of Kunlun Mountain erosion recorded in Lake Heihai70 and extreme precipitation events identified in the Qaidam Basin71 supports the argument that intensified summer rainstorms and high erosion were a regional phenomena during the mid-Holocene (Fig. 5).

Fig. 5: Comparison of loess provenance changes with records of bedrock erosion and rainstorm events.
Fig. 5: Comparison of loess provenance changes with records of bedrock erosion and rainstorm events.
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a The Ce changes (this study) illustrating shifts in cerium concentrations over time. b The Nd changes (this study) showing variations in neodymium concentrations. c Extreme dry and wet annual events recorded in the Qaidam Basin71 over the past 6700 years. These events were identified in the precipitation reconstruction by surpassing the 10th and 90th percentiles of the entire period. d Rainstorm events record of Lake Heihai70 in the Kunlun Mountains. Cluster 2 denotes the association of lake surface samples to the mineralogical cluster. The change of this proxy record captures variations in runoff caused by precipitation linked to Summer Monsoon intensity.

Further evidence is presented to support the concept of intensified summer rainstorm events during the mid-Holocene. Firstly, the coherent change in isotopes in atmospheric precipitation shown in Fig. 6 indicate a northward movement of summer monsoonal water vapor across the Tibetan Plateau and Kunlun Mountains towards the Tianshan Mountains. The Guliya ice cores’ D-excess record in the Kunlun Mountains72, situated 150 km southwest of AQ16, the Bangong Lake δD record73 and the Delinha tree-ring δ18O record71 from the northwestern Tibetan Plateau, as well as δ18O records in the Tianshan stalagmite19, all exhibit more negative values likely due to an intensified and poleward shift in the summer monsoon regime during the mid-Holocene72 (Fig. 6), although other explanation still exist74,75. Secondly, the trend of a summer low-level flow moisture intensity index, which we define as the zonal water vapor flux at 850 hPa. This index reflects mid-Holocene summer monsoon penetration into the TB (see more in “Methods”). The summer low-level flow moisture intensity was chosen as the basis for analysis because current research reveals that the TB low-level flow is vital for rainstorm development in this desert, providing the moisture and energy needed to generate storm fronts76. The summer low-level flow moisture intensity varied between 1.0 and 2.5 g*s/kg - with much larger fluxes in the mid Holocene as compared to the late Holocene (Fig. 6). This index is consistent with changes in rainstorm patterns and atmospheric isotopes at orbital to millennial timescales. Moreover, in contrast to flow moisture at 200 hPa and 500 hPa, summer low-level flow moisture intensity indicates easterly moisture transport (Fig. S8), possibly from the Asian summer monsoon10. In addition, the 500 hPa vertical motion during the middle and late Holocene were calculated to specify the essential conditions for rainstorms, showing that the anomalous local ascending motion is stronger in the mid-Holocene than in the late-Holocene in the Kunlun Mountains (Fig. S12). Therefore, under the condition that an unusual easterly influx of moisture and anomalous local ascending motion occurred simultaneously over the TB, it is reasonable to expect enhanced erosion along with intensified rainstorm events during the mid-Holocene.

Fig. 6: Comparison between the provenance index in the AQ16 sequence and the summer moisture source, summer monsoon intensity, and water vapor flux at 850 hPa in the Tarim Basin.
Fig. 6: Comparison between the provenance index in the AQ16 sequence and the summer moisture source, summer monsoon intensity, and water vapor flux at 850 hPa in the Tarim Basin.
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a Provenance_based rainstorm shift over Tarim Basin (this study), within larger values representing increased mountain sediment sources. b Normalized D-excess of Guliya ice core in the Kunlun Mountains72, which is near our loess sites (no more than 100 km in a straight line from the YM22 profile and no more than 200 km in a straight line from the AQ16 profile). Higher/lower D-excess is determined by lower/higher monsoon contribution in precipitation. c δ18O in the Tianshan stalagmite records19 are similar to those in southern China cave records, which are characterized by more negative values during the middle Holocene. Higher/lower δ18O values denote lower/higher monsoon intensity and/or moisture source change. d Summer monsoon intensity indicated by the Delingha tree-ring record located in the Qaidam Basin71. The thin line represents the original data, while the thick line illustrates the data after applying a smoothing technique at 20 points. Higher/lower δ18O values denote lower/higher summer monsoon intensity. e Simulated changes of summer water vapor flux from east at 850 hPa in the Tarim Basin (this study). The “-” represent the direction of the wind, which is easterly. The thin line represents the original data, while the thick line illustrates the data after applying a smoothing technique at 5 points.

These observations all suggest intensified summer rainstorms, enhanced erosion, and significantly increased fluvial sediment fluxes in the Kunlun Mountains during the mid-Holocene, influenced by the strengthened African-Asian summer monsoon. While glacier activity may affect mountain erosion processes, its impact seems minimal during the mid-Holocene in this case, as evidenced by the lack of correlation between glacial history and erosion intensity77. As a caveat we acknowledge the relatively large uncertainty in mesoscale mechanisms driving increased rainfall extremes does not establish causal links with the African-Asian monsoon at present. Nevertheless, findings from Zhao et al. 78 suggest stronger southerly winds penetrating the TB during periods of enhanced summer monsoon activity, as indicated by exotic fossilized Tsuga pollen grains found in the region. Further validation of this mesoscale mechanism could be achieved through the use of high-resolution and comprehensive Earth system models in future research.

Possible climate dynamics

The position of the westerly jet depends largely on the meridional atmospheric temperature gradients between low and high latitudes79. Under the conditions of a decreased NH latitudinal insolation gradient and latitudinal temperature gradient (LTG) during the middle Holocene as compared to the late Holocene52,53 (Fig. 7), the peak NH summer westerly jet could migrate significantly northward80. Simulated results show that the summer westerly jet axis over the TB stayed north of 43° N for most of the middle Holocene and frequently tracked south of 43° N in the late Holocene (Fig. 7c). Consequently, the northward migration of the westerly jet and coherently weakened westerly wind strength at a higher-level over the TB during the mid-Holocene. This was associated with a decrease in water vapor fluxes at 600 hPa (Fig. 2), and decreased baroclinicity and cyclogenesis52, and led to reduced rainfall across the studied region.

Fig. 7: Insolation-/temperature-gradient driving the interplay of the monsoon and westerlies.
Fig. 7: Insolation-/temperature-gradient driving the interplay of the monsoon and westerlies.
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a Annual latitudinal insolation gradient for the boreal52. b Reconstructed slope of the boreal latitudinal temperature gradient (LTG)52 between the mid-latitudes and high-latitudes, shading indicates 1-sigma standard deviation and SD refers to standardized units. c Simulated changes of summer westerly jet position over the TB (this study). The thin line represents the original data, while the thick line illustrates the data after applying a smoothing technique at 5 points. d Hydroclimate changes are reflected by smoothed δ13CIC of YM22 in the Kunlun Mountains (this study), within smaller values representing drier hydroclimate conditions. e low-latitude (30°N-30°S) interhemispheric insolation gradients82. f temperature contrast39,83 calculated from zonal averages from tropical regions (30°N to 30°S) and the boreal extra-tropics (>30°N). g Monsoon intensity inferred from stalagmite δ18O values in Oman98. h Provenance shift over the TB (this study), within larger values representing increased mountain sediment sources.

By contrast, the boreal tropical region experienced a greater intensity of mid-summer insolation81,82 and heat energy39 compared to low-latitude regions of the Southern Hemisphere during the middle Holocene. This resulted in an increased interhemispheric insolation contrast and temperature gradients (Fig. 7), as discussed by Deininger83. Consequently, the strengthened interhemispheric temperature contrast8 intensified cross-equatorial atmospheric mass transport84 associated with enhanced African-Asian monsoon in the Eurasian continental zone. The synchronous change of lower-level water vapor flow, atmospheric isotopes, and summer monsoon intensity presents a compelling list of evidence to assess the extent to which monsoonal water vapor would have substantially penetrated into the arid TB during the mid-Holocene compared to the late-Holocene. This facilitated the transport of incident energy both aloft and towards higher latitudes of the Asian interior81,82,85, significantly amplifying the potential for convection and the formation of intense summer rainstorm events on the northern slopes of the Kunlun Mountains. This process might play a role in dictating the occurrence, or absence of rainstorm events across the Kunlun Mountains. In turn, these processes result in an increase in mountain erosion and an increase in the availability of fluvial sediment materials sourced from mountainous regions, changing the provenance of the AQ16 loess, as described above (Fig. S13).

Concluding remarks—implications for the future change of hydroclimate and water availability

This work provides a robust database and significant information and contribution to mid-Holocene climate change, aligning with established literature of the region, and indicating a drier westerly regime coexisted with strengthened rainfall extremes over the hyper-arid TB during the mid-Holocene warm period. Unlike previous studies that focused on the control of the Asian interior, climate either from westerly circulation16 or from monsoonal flows19, this paper presents a novel way to examine the different hydroclimate impacts of summer monsoons and westerly circulation in the hyper-arid Asian interior since the middle Holocene by studying geochemical proxies from two unique loess records.

More broadly, we observe nuanced climate variability modulated by the westerly jet position and westerly circulation, and by low-level monsoonal circulation in the Asian interior during the mid-Holocene. This, along with independent transient simulations and thermodynamic considerations offer a comprehensive view of the paleoclimatology of Asian interior. It suggests that a meridional shift of the westerly jet driven by mid- and high-latitude temperature gradient changes in the boreal summer and creates a fundamental control on average water availability in the Asian interior. These paleoclimatology findings and dynamics carry contextualized implications, suggesting a future warmer scenario where heightened aridity is coupled with amplified rainfall extremes in arid western China. Given that the water stored in mountain regions is the most important resource for determining available freshwater upon which humans and ecosystems depend, our study suggests reduced water availability (droughts) punctuated by extreme rainfall events (floods) in the future under global warming-induced changes in westerly winds.

Methods

Sampling

The AQ16 section consists of three trenches and a pit excavated from the floor of the valley, and measures approximately 83 m in length (Fig. S3). Trench 1 extends to 20 m below the ground surface. The stratigraphy of Trench 1 overlaps Trench 2, and the stratigraphy of Trench 2 overlaps Trench 3. The trenches and pit were sampled at 2 cm intervals. Section YM22 is about 1.5 km southwest of Ya Men Village in Cele County. Section YM22 was sampled every 1 cm from top to bottom.

OSL dating

Quartz OSL dating was conducted at the Institute of Earth Environment, Chinese Academy of Sciences (IEECAS). Sediment extraction was conducted in a subdued, red-light environment. A total of 2.5 cm of sediment was removed and discarded from both ends of the sample tube to avoid potential light exposure.

Fine-grained (4–11 μm) quartz was extracted using pretreatment procedures developed to study loess from the the Chinese Loess Plateau86 and Xinjiang loess47. The samples (~50 g) were first treated with 30% H2O2 and 30% HCl to remove organic materials and carbonates, respectively. Samples were washed with distilled water until neutral, and then 4–11 μm polymineral grains were separated following Stokes Law. These polymineral fine grains were immersed in 30% hydrofluorosilicic acid (H2SiF6) for 3–5 days, and then were washed in HCl and distilled water to extract the fine-grained quartz component. The purified quartz was deposited on 9.7 mm-diameter stainless steel discs using ethanol for measurement.

Sediment from the internal portions was used to measure the equivalent dose (De) and for radioisotope (U, Th, and 40K) concentrations. All OSL measurements were performed using an automated Daybreak 2200 OSL reader equipped with infrared (880 ± 60 nm) and blue (470 ± 5 nm) LED units and a 90Sr/90Y beta source for irradiation. The quartz grains were stimulated at 125 °C with blue LEDs (power ~45 mW cm−2) for 60 s, and the OSL signal was detected using an EMI 9235QA photomultiplier tube coupled in front with two 3-mm U-340 (290–370 nm) glass filters.

For dose-rate (Dr) calculation, U and Th concentrations were measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), while X-ray Fluorescence (XRF) was used to determine K concentrations. Dose-rate conversion factors were taken from Guérin, Mercier87. The α-value of fine-grained quartz was assumed to be 0.04 sensu88. The cosmic dose rates were calculated as given in Prescott and Hutton89.

Considering the possible variation in water content with paleoclimate change over geological time, measured and published water contents in loess sequences from the Chinese Loess Plateau86,90 and Central Asia47 were considered. We assumed a water content of 10 ± 5% for the samples based on the inferred hydroclimatic history of the sample sites. Dr was calculated using the Dose Rate and Age Calculator program91, and quartz OSL ages were obtained by dividing the measured De by Dr. The quartz OSL ages for all samples and their related parameters are presented in Table S1.

Radiocarbon dating

For bulk TOC samples, before chemical pretreatment, plant roots were physically removed. Samples were crushed gently with a clean mortar and pestle. Then samples were treated with boiling 2 M HCl for over 24 h to remove carbonates, and were then rinsed with distilled water until neutral. Pretreated samples and CuO powder were placed into 9 mm quartz tubes, evacuated to <10−4 Torr, and combusted at 850 °C. Sample CO2 gas was extracted under vacuum and cryogenically purified. For IC samples, oven dried (45 °C) samples were physically examined to remove plant roots, pebbles, and any detrital materials prior to analysis. Purified CO2 was converted to graphite with an Fe catalyst and measured at the Xi’an 3 MV HVE Tandetron or at the MIni CArbon DAting System (MICADAS) located at the Xi’an AMS Center, IEECAS.

Isotopic analyses and the calculation of the hydroclimate index

For δ13CIC, samples at 20-cm intervals for AQ16 and 3 to 5 cm intervals were analyzed for YM22. Oven dried (45 °C) samples were physically examined to remove plant roots, pebbles, and any detrital materials prior to carbonate isotope analysis. Then, ~2 g sediment samples were ground in an agate mortar and sieved through a 100 mesh (150 μm) screen. Analyses of carbonate samples were performed using an isotope ratio mass spectrometer (MAT-252) with an automated carbonate preparation device (Kiel II) at IEECAS. Repeated analyses of laboratory carbonate standards with known δ18O (−14.720 ± 0.094‰) and δ13C (−6.360 ± 0.075‰) values were performed daily to ensure instrumental precision. Typical standard deviation for the repeated analyses of these laboratory standards is less than ±0.1‰.

For δ13CTOC, crushed power samples were combusted for 4 h at 850 °C in evacuated sealed quartz tubes in the presence of 1 g of CuO, 1 g of Cu and Pt foil. The resulting CO2 was cryogenically purified, and the isotopic ratios of the purified CO2 were measured using a Finnigan MAT 251 gas source mass spectrometer at the IEECAS. The overall precision of the δ13CTOC analyses was better than 0.2‰. All stable isotope ratios are reported in parts per mill (‰) deviations against the Vienna Pee Dee Belemnite (VPDB) standard.

Reservoir 14C age offsets

The age offset values between 14CBOC and OSL in AQ16 loess were calculated by subtracting the age values obtained from a Bayesian model (on the basis of OSL age values) using the AMS 14C ages at corresponding locations. More details of the analytical procedures are provided in Ref. 37 and 92.

Elemental and grain size analyses

Elemental neodymium (Nd) and cerium (Ce) concentrations were measured at intervals of 10 cm. All samples were pulverized to <75 µm (200 mesh size) using a Shatter Box and pressed into 60 µm diameter discs for measurement. Samples were measured using a polarized energy-dispersive X-ray fluorescence spectrometer (Model Epsilon 5, PANalytical, The Netherlands) at the School of Geography, South China Normal University. Samples at 10-cm intervals were analyzed using 5 g compacted onto discs with 32-mm diameters. Elemental concentration is expressed in parts per million (ppm) following conversion of spectra into concentrations. We use Nd and Ce to create a non-dimensional hydroclimate index. We begin by interpolating these two unevenly spaced paleoclimate time series into evenly spaced time series. Each time series was interpolated to 100 years, then visually compared to its interpolated time series. Subsequently, the two uniformly spaced time series were normalized using the Acycle software. Lastly, the Acycle software was employed to merge the curves, resulting in a non-dimensional provenance index.

The grain size of the powdered samples was analyzed using a Malvern Mastersizer particle size analyzer. Before analysis, samples were pretreated with hot H2O2 to remove organic matter and with 2 M HCl to remove carbonates, following the standard procedure used at South China Normal University and IEECAS.

87Sr/86Sr and 143Nd/144Nd analyses

Samples were ground to 100 mesh (149 μm) and then extracted with a buffer solution at 25 °C. The chemical procedures are described in detail by Ma et al. 93. Two buffer solutions of ammonium acetate with a pH of ~7 and ~3 were used to extract elements mainly from the exchangeable fraction and carbonate phases, respectively93. A leaching time of more than 72 h was chosen to allow adequate exchange. The leachates and residues (silicate fraction) were separated by centrifugation at 4000 r/min for 10 min. Chemical procedures including sample digestion and Sr purification were performed in an ISO class 6 clean room in the Chinese Academy of Sciences (CAS) Key Laboratory of Crust-Mantle Materials and Environments at the University of Science and Technology of China (USTC), Hefei, China.

The chemical procedures of Sr and Nd isotope analyses were described in detail by Chen et al., 94 and Chen et al. 95, respectively. Approximately 50 mg for Sr analyses and 200 mg for Nd analyses of sample powder were digested in a mixture of HF-HNO3-HCl (Fisher Chemical, TraceMetalTM Grade purified by double sub-boiling distillation). Sr was isolated from the matrix elements (including Na, Fe, Ti, Mg, Mn, Al, K, Rb, and Ca) and some interfering elements (Lu, Yb, Er and Hf) by conventional ion-exchange chromatography with AG50W-X12 cation exchange resin (37 to 74 microns; Bio-Rad®, USA). The Sr recovery rates of the sample were ~99%. The total procedural blanks were lower than 0.1 ng, which is negligible compared with the amount of Sr (1.5–2 ug) in the samples. Nd was separated from other rare-earth elements on quartz columns using 1.7 mL Teflon powder as the cation exchange medium.

Sr and Nd isotopic composition was measured by MC-ICP-MS (Neptune Plus, Thermo Fisher Scientific®, Bremen, Germany) in the CAS Key Laboratory of Crust-Mantel Materials and Environments at USTC. Nd was loaded as phosphate on pre-conditioned Re filaments and measurements were performed in a Re double filament configuration. The mass bias produced by the instrument was normalized by 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. For convenience, the 143/144Nd ratios are reported in the epsilon notation (εNd), where εNd = (143Nd/144Ndsample / 143Nd/144Ndsample – 1) × 10,000, CHUR stands for Chondritic Uniform Reservoir with a 143Nd/144Nd ratio of 0.51263896.

The long-term external precision (2 standard deviations) was estimated based on in-house reference materials, USTC-Sr (better than 19 × 10−6, n = 57, for 87Sr/86Sr)94, GB-Sr (better than 22 × 10−6, n = 47, for 87Sr/86Sr) (Chen et al., 2022) and NBS-987 (better than 22 × 10−6, n = 98, for 143Nd/144Nd)95. To evaluate the whole chemical procedure, we measured two USGS reference materials BCR-2 and BHVO-2. The results (BCR-2: 87Sr/86Sr = 0.705112 ± 0.000004, 143Nd/144Nd = 0.512635 ± 0.000005; BHVO-2: 87Sr/86Sr = 0.703526 ± 0.000003, 143Nd/144Nd = 0.512976 ± 0.000005) are consistent with previous studies94,95. Total procedural blanks were <80 pg for Sr and <60 pg for Nd in this study, which is negligible compared with the amount of Sr and Nd (>1 ug) in the samples.

CESM model simulations

The numerical simulations we utilized are described in detail by Shi et al. 38. The model employed is the Community Earth System Model, a fully coupled earth system model developed by the National Center for Atmospheric Research (Boulder, USA). To better capture the regional features of TB precipitation, the regional topography, e.g., Tian Shan, has to be resolved in the model. Thus, a relatively high horizontal resolution for the atmospheric module, i.e., Community Atmosphere Model 4, is chosen to be 0.47°*0.63° in latitude and longitude. The atmospheric module is coupled with a slab ocean, in which only the thermodynamic feedback of surface ocean is included. An accelerated transient experiment covering the past 10 kyr was carried out, in which only changes in orbital parameters are included. Firstly, the orbital parameters in the model simulation are prescribed to 10 kyr and integrated for 50 years as a spin up time. After that, the orbital parameters at the end of each model year are changed to be the values of every realistic 50-year interval. Hence, a total of 200 model years are calculated for the whole of 10 kyr by accelerating the variations in the orbital cycles by a factor of 50. The mean time of the middle Holocene and late Holocene in our calculation is the period modeled between 8.2–4.2 ka BP and 4.2–0 ka BP, respectively. At each time interval of the 200 model years, the 200 hPa zonal wind between 80° and 90°E was averaged first, then the latitude where the maximum value appears between 30° and 50°N was specified. The summer (June, July and August) low-level flow moisture intensity over the TB is quantified as the easterly water vapor flux at 850 hPa averaged over the 75–95°E, 36–42°N domain. The meridional position of the Westerly jet over the TB was tracked through the last 8 kyr in our simulation.