Introduction

The Loess Plateau (LP) is one of the most important cradles of Chinese civilization1 and today more than 100 million people live in this region. Located on the northern limit of the East Asian summer monsoon (EASM), this environmentally sensitive region depends largely on summer monsoon precipitation to sustain its agriculture. In addition, as the main sediment source and water-catchment area of the Yellow River, the LP is the most important water source for northern China2. Precipitation variations on the LP are critical for the water and life securities over the whole Yellow River basin3,4. Widely distributed loess/paleosol sequences in the LP have revealed the East Asian monsoon variability over tectonic, orbital and millennial timescales5,6,7,8. However, the loess/paleosol sequences cannot record high-frequency climate variations because of their sedimentation and diagenetic processes. Despite some paleoprecipitation reconstructions9,10,11, the characteristic and mechanism of decadal scale precipitation variation on the LP, especially the western LP, during historical times are still poorly understood. Moreover, the relationship between extreme decadal scale rainfall/drought events on the LP and the water security of the Yellow River remain unclear.

Wuya Cave (33°49′14″ N, 105°25′35″ E, 1370 m a.s.l.) is located on the southwestern margin of LP, Gansu Province, China (Fig. 1). Regional climate is semiarid, with annual precipitation of 470 mm and annual temperature of 14.9°C (Supplementary Fig. S1). In summer, the summer monsoon brings warm humid air and causes substantial monsoon rainfall at the site, with ~80% of the annual rainfall falling between May and September (data from Wudu meteorological station, 60 km southwest of Wayu Cave). During winter, the Siberian–Mongolian High and westerly winds maintain cold and dry conditions12.

Figure 1
figure 1

Location of Wuya cave and moisture transport pathway.

(A) Rainy season (Jun.–Sep.) water vapor transport (kg m−1s−1) averaged for the period 1961–2012 at low level (700 hPa) based on the NCEP/NCAR reanalysis datasets59, using the Grid Analysis and Display System (GrADS). The shading shows absolute value of water vapor transport at each grid. The red star indicates Wuya Cave. The green solid circle indicates Sanmenxia hydrological station where the historical river runoff data of the Yellow River are monitored. The area enclosed in yellow line denotes the Loess Plateau; (B) An enlarged map showing the locations of Wuya Cave, Huangye Cave, Mt. Xinglong, Mt. Guiqing and the Wudu meteorological station. Topographic GTOPO30 data are from the USGS Earth Resources Observation and Science Center.

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Two columnar stalagmites (Supplementary Fig. S2), with lengths of 63 mm for WY27 and 84.5 mm for WY33, were collected inside the cave about 500 m from the entrance in March 2011. Both stalagmites were receiving drip water from the ceiling at the time of collection. The polished sections of the stalagmites show continuous, clear laminae with alternations from a dark, compact layer (DCL) to a white, porous layer (WPL). The visible white layers are opaque under transmission light microscope, but luminescent under mercury light source UV reflected light (Supplementary Fig. S3), suggesting they are mainly composed of organic substances13. As suggested by observations in other caves14,15, annual flush of organic substances from the soil during monsoon rain seasons may form these laminar divisions.

Results

Lamina counting results suggest that there are 370 ± 15 and 262 ± 10 (±4% error applied to the lamina counts) DCL-WPL couplets in WY27 and WY33, respectively. High detrital 232Th levels of 7–28 × 103 ppt result in large dating uncertainty of 113–874 years for 230Th dates (Supplementary Table S1) and hinder precise age models.

To further constrain the chronology, we use 210Pb dating. The 210Pb dating results show exponential decrease of excess 210Pb activities with growth depth in the two stalagmites (Supplementary Figs. S4, S5 and Table S2), indicating the upper sections of WY27 and WY33 are younger than ~130 years16,17. The average growth rate calculated from the depth profile of excess 210Pb for the top 23.5 mm of WY27 (0.171 mm/yr) is in good agreement with that estimated by lamina counting (0.156 mm/yr). The growth rate for the top 39 mm of WY33 determined by 210Pb (0.289 mm/yr) is slightly lower than that calculated by lamina counting (0.361 mm/yr), which may be caused by the high sampling width (3–5 mm) and the errors of 210Pb dating (<10%) and lamina counting (±4%). The comparison suggests the DCL-WPL couplets in both stalagmites are annual. According to the layer counting chronologies, WY27 grew from 1641 to 2010 CE and WY33 from 1749 to 2010 CE (Supplementary Fig. S6).

Stalagmites WY27 and WY33 show similar decadal scale δ18O variations during the contemporaneous period of 1749–2010 CE (Fig. 2), but WY33 has a higher average resolution (1.6 yrs) than WY27 (2.9 yrs, Supplementary Table S3). The replicated δ18O records lend further support to the laminal chronologies. A “Hendy test”18 on three growth layers of WY33 and two growth layers of WY27 indicates the δ18O remain constant along a single growth layer (Supplementary Fig. S7). The replicated δ18O records and “Hendy test” results suggest both stalagmites were deposited under oxygen isotopic equilibrium conditions18,19.

Figure 2
figure 2

The stalagmite δ18O series of (A) WY33 and (B) WY27.

The green, broken lines with arrows in both series indicate linear trends toward more enriched values since the end of the 19th century. A minor difference of 0.5‰ was observed for the contemporaneous δ18O segments of the WY27 and WY33 records, which may be attributable to slightly prior calcite precipitation in different water flow paths above the stalagmites11,60.

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Stalagmite δ18O variation can be affected by different factors, including temperature, rainfall amount, moisture recycling and circulation, change in the precipitation-to-evaporation ratio and changes in the moisture source and transport pathway20,21,22,23. Previous studies11,12 suggested there was an anti-correlation between speleothem δ18O and rainfall amount in this monsoonal front region during the last 1800 years on decadal to centennial scales. Recently, by combining simulation records and paleo-moisture records, Liu et al.24 suggest speleothem δ18O records in northern China can represent the EASM intensity and local monsoon rainfall, with negative δ18O records representing enhanced monsoon intensity and monsoon rainfall in northern China.

The comparison between our annually-laminated stalagmite δ18O and the annual rainfall (mainly contributed by monsoonal rainfall) records from the nearest meteorological station confirm a robust anti-phase relationship between our stalagmite δ18O and local rainfall amount (r = −0.44, p < 0.01) on both decadal scale and long term trend during the observed period of 1951–2009 CE (Supplementary Fig. S8). Good coherence of stalagmite δ18O and meteorological record supports the robustness of this laminal chronology and an isotopic equilibrium statement during deposition.

The δ13C records of the two stalagmites show similar variations with their δ18O records on annual- to decadal- scales (Supplementary Fig. S9), although their long term trends differ (Supplementary Figs. S10, S11). The correlation coefficient for the detrended δ13C (Δ13C) and δ18O records of WY27 (Δ18O) is r = 0.42 (p < 0.01) and is r = 0.52 (p < 0.01) for WY33. During wetter conditions, the decreased residence time of the seepage water may cause less bedrock to be dissolved, resulting in lower stalagmite δ13C values. In addition, a wet climate favors vegetation growth and biological productivity, which may also contribute to the relatively lower δ13C values on a decadal scale. In contrast, the increased residence time of the seepage water during drier condition may allow more dissolution of the bedrock, resulting in the heavier δ13C values25. Decreased precipitation may also reduce vegetation cover and favor prior carbonate precipitation in the unsaturated zone above the cave and enhanced degassing of CO2 within the cave26,27, also resulting in higher δ13C values. Thus, the co-variations of the Δ13C and Δ18O time series lead further support to the anti-phase relationship between our stalagmite δ18O and local rainfall amount on annual- to decadal- timescales.

Spatial analysis shows significant positive correlations between precipitation changes around Wuya Cave area and the LP, especially the western LP during the period 1950–2011 CE (Supplementary Fig. S12). Therefore, we interpret our stalagmite δ18O record as a reliable indicator of precipitation changes on the western LP and even the whole LP region, with higher stalagmite δ18O values representing lower precipitation and vice versa.

Our stalagmite δ18O records show good coherence with the tree-ring based annual rainfall reconstruction from Mt. Xinglong10 (Fig. 3) and drought reconstruction from Mt. Guiqing9 on the western LP. They are also consistent with a previously published relative low resolution stalagmite δ18O record from the Huangye Cave11 in this area (Fig. 3). Based on the reconstructions, a series of decadal-scale drought events on the western LP during the last 370 years can be identified: 1670s–1680s, 1760s–early 1770s, 1810s–1820s, 1860s–1870s, late 1910s–1920s, 1970s and late 1990s. A remarkable decreasing trend in the precipitation since the end of the 19th century is shown in both speleothem time series and the tree ring record (Fig. 3). In contrast, notable negative δ18O peaks, which indicate humid climate, can be observed in both series during the periods of 1690–1730 CE, 1780–1800 CE, 1830–1850 CE and 1880–1900 CE.

Figure 3
figure 3

Comparison of stalagmite δ18O records with tree-ring record on the western LP and river runoff record of the middle Yellow River during the past 370 years.

(A) WY27 δ18O record from Wuya Cave (dark blue) and HY3 δ18O record from Huangye Cave11 (Green); (B) WY33 δ18O record from Wuya Cave; (C) Tree-ring based annual rainfall reconstruction from Mt. Xinglong10 (the red line is smoothed with a 5-year running mean); (D) Historical river runoff reconstruction for the middle Yellow River from Sanmenxia hydrological station37 (the blue line is smoothed with a 5-year running mean). The shaded gray bars indicate correlations of various proxies to seven drought events on the western LP. The black, broken lines with arrows indicate a decreasing trend of precipitation on the western LP since the end of the 19th century. The stars in panel D indicate four large outburst events of the Yellow River in 1841–1843, 1855, 1887 and 1938 CE33,38.

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The identified drought intervals on the western LP during the last 370 years show a periodicity of about half a century (Fig. 3). Power spectrum analysis indicates that the most significant periodicity of the WY33 δ18O series is 52 years, which passes the 99% significant level (Supplementary Fig. S13), further supporting the quasi-50-year periodicity.

Discussion

Impacts

There were two large-scale land reclamations on the western LP during the past four centuries. The first one happened in the early 18th century during the Qing Dynasty28,29 and the other one happened in the late 1930s and 1940s30. Both land reclamations corresponded to humid periods in our reconstruction (Fig. 3), suggesting that humid climate, in addition to the orientation of policy28,29,30, provided favorable conditions for agricultural developments on the western LP during these periods.

Decadal scale droughts on the western LP during historical times caused serious damage to the society in this environmental sensitive region. For example, historical documents recorded the Weihe River on the western LP dried up in the summer of 1862 CE when a serious drought occurred31. The drought in the late half of the 1870s was much more serious and affected a vast region from LP to North China Plain and even from South and Southeast Asia to the Great Basin of North America32. Chinese local officers at that time regarded this drought as the severest one since the establishment of the Qing Dynasty (1644–1911 CE) and referred to it as “Dingwu Disaster”33. The drought had caused catastrophic collapse of the society in northern China. About half of the population (160–200 million) was affected and at least 10 million people were killed by famine and pestilence caused by the severe drought33,34. The affected areas in China of the drought in the 1920s were similar to that in the 1870s35. Descriptions like “not even a blade of grass grows”, “river dried up”, “big cannibalism”, “terrible holocaust” were reported in the newspapers at that time33,36.

The historical river runoff changes of the middle Yellow River from Sanmenxia hydrological station during the period of 1766–2004 CE37 match the stalagmite-inferred precipitation data (Fig. 3). The synchroneity indicates their close relationship. For instance, when droughts occurred in the 1860s–1870s, late 1920s, 1970s and the late 1990s on the LP, abnormally low river runoff of the middle Yellow River was observed (Fig. 3). In contrast, four megafloods of the Yellow River occurring in 1841–1843, 1855, 1887 and 1938 CE33,38 coincided with the humid intervals in this region (Fig. 3).

Driving forces

The positive teleconnection between summer precipitation over northern China and Indian monsoon have been reported according to modern instrumental data39. A recent study40 suggested during the monsoonal season of the last 50 years (1961–2012CE) water vapor over the LP was transported mainly from the tropical Indian and Pacific Oceans by southerly monsoon flow and that southerly water vapor transport controlled the interannual variability of monsoon precipitation on the LP. Our stalagmite δ18O records show broad similarities with the stalagmite δ18O record from the core monsoon region of India41 on decadal scale during the last 370 years (Fig. 4). The drought events recorded in our records were also observed in the Nepal Himalaya, an Indian monsoon front region, as inferred from a 223 years tree ring δ18O record42. The coincidence of the drought events in the Indian monsoon region and the western LP suggests the important influence of Indian monsoon on our study area. When the Indian summer monsoon declines, weakened southwesterly winds reduce the water vapor transport from the tropical ocean to the north and induce a drought on the western LP. Conversely, strengthened Indian summer monsoon can enhance the southwesterly winds and the water vapor transport, resulting in increased monsoon precipitation on the western LP43.

Figure 4
figure 4

Comparisons among stalagmite δ18O records from Wuya Cave, Indian monsoon proxy, PDO index and Westerly proxy records.

(A) WY27 (blue) and WY33 (yellow) δ18O records. The systematic difference of −0.5‰ between WY 33 and WY27 records was removed; (B) Stalagmite δ18O record from the core monsoon region of India41; (C) Tree-ring based PDO index44 (smoothed with a 5-year running mean); (D) Northern Hemisphere westerly index based on Ca2+ in GISP2 ice core51 (smoothed with a 5-year running mean). The shaded gray bars indicate drought events on the western LP and their correlation with various proxies.

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The stalagmite-inferred decadal-scale droughts on the Western LP coincide with positive values of Pacific Decadal Oscillation (PDO) index (Fig. 4), i.e. warm PDO phases44. The correspondence is especially significant during the severe droughts in the 1860s–1870s and late 1910s–1920s. The only exception is the drought in the 1810s–1820s, which seems to correspond to a cold PDO phase in Fig. 4. However, in another two records of PDO reconstructed from the historical rainfall proxy data over eastern China45 and the tree ring data over Asia46, the PDO was in its warm phase during this drought event. It has been suggest the East Asian monsoon was weak and the Western Pacific Subtropical High (WPSH) moved southward during warm PDO phases. These conditions could result in decreased precipitation in northern China, including the western LP39,47. In contrast, the strengthened East Asian monsoon and northward shifting of the WPSH in cold PDO phases could enhance the monsoon precipitation on the western LP39,47.

The significant 52-year cycle (Supplementary Fig. S13) observed in the stalagmite δ18O time series is generally consistent with a 55–60 years cycle in the observed Indian monsoon rainfall48 and the 50–70 years cycle in the PDO index records49, further supporting the important influences of Indian monsoon and PDO on precipitation variations on the western LP.

Previous studies39,43,50 reveal a stepwise shift of the EASM and its associated migrations of monsoon air masses and rain belt were closely related to seasonal changes in the westerly upper-level jet stream and the WPSH. As the western LP is located near the northern limit of the EASM, it has been suggested the westerly jet could be an important factor for the regional precipitation by affecting the northward shift of the monsoon rain belt. By comparing Wuya stalagmite records with the reconstructed strength of the Northern Hemisphere westerly in GISP2 ice core51, we find concomitant changes between decadal-scale droughts on the western LP in the 20th century and the strengthened westerly jet (Fig. 4). However, the opposite is true before the 20th century. Therefore, the relationship between the precipitation on the western LP and the Northern Hemisphere westerly on decadal scale is still uncertain.

Modern instrumental data show a declining trend of the EASM since the end of the 19th century52. Although the trend of the Indian summer monsoon during the past century remains debatable53, the observed seasonality of wind field at 850 hPa within the South Asian domain suggests a decreasing monsoon trend since 1948 CE54. The reduced southerly water vapor transport caused by declined Asian summer monsoon may contribute to the decreasing trend in precipitation variations on the western LP since the end of the 19th century.

Methods

We used a hand-held carbide dental drill, with a 0.9-mm diameter drill bit, to recover subsamples (50–100 mg powder) along the growth axes of WY27 and WY33 for 230Th dating. Seven subsamples were dated with U-series methods at the Minnesota Isotope Laboratory on an inductively coupled plasma mass spectrometer (Thermo-Finnigan ELEMENT) using procedures described in ref. 55 and ref. 56. Corrections for initial 230Th were made assuming an initial 230Th/232Th atomic ratio of 4.4 ± 2.2 × 10−6.

A total of 20 subsamples were drilled along the growth axes of the stalagmites and were dated with 210Pb methods at the Institute of Earth Sciences, Academia Sinica. About 400–700 mg powders were dissolved in HCl. 210Pb activities were determined via 210Po by alpha spectrometry using 209Po as a yield determinant57. The analytical error based on counting statistics was <3%.

About 300 powdered subsamples (~50–80 μg) were drilled out by using a hand-held carbide dental drill, with a 0.3-mm diameter drill bit, at an interval of 0.5 mm along the central growth axes of the stalagmites for stable isotope analyses. All isotopic compositions were measured on an IsoPrime100 gas source stable isotope ratio mass spectrometer equipped with a MultiPrep system at the Institute of Earth Environment, Chinese Academy of Sciences. The δ18O values reported here are relative to the Vienna PeeDee Belemnite (VPDB) standard. Repeated measurements of one internal laboratory standard TTB1 showed that the long-term reproducibility of the δ18O analyses was better than ±0.06‰ (1σ).

The spectral analysis was performed using Redfit35 with parameters of the software set at values as follows: nsim = 1000, mctest = T, rhopre = −99.0, ofac = 2, n50 = 4, iwin = 1 (see ref. 58 for details).