Many ancient inscriptions were disclosed in Dayu Cave, which indicate that local ancient people visited the cave frequently, at least 70 times during 1520–1920 CE. According to the inscriptions, seven major drought events were clearly described , occurring in 1528 CE, 1596 CE, 1707 CE, 1756 CE, 1839CE, 1891 CE and 1894 CE (Fig. 2), respectively. These inscriptions described many details of the droughts (Table 1). For example, one of them (Fig. 2A) stated: “On May 24th, 17th year of the Emperor Guangxu period, Qing Dynasty [the traditional Chinese calendar, equivalent to June 30th, 1891 CE], the local mayor, Huaizong Zhu led more than 200 people into the cave to retrieve water. A fortuneteller named Zhenrong Ran prayed for rain during a ceremony”. Three years later in 1894 CE (June 12th, 20th year of the Emperor Guangxu period, Qing Dynasty), another drought event occurred. The same mayor and fortuneteller again led more than 120 people into the cave to collect water (Fig. 2B). Another inscription indicated that “On June 8th, 46th year of the Emperor Kangxi period, Qing Dynasty [July 7th, 1707 CE], the governor of Ningqiang district came to the cave to pray for rain”.

Table 1 Seven drought events recorded in the ancient inscriptions inside Dayu Cave during the period of 1500–1920 CE. Full size table

Figure 2 Photos of ancient inscriptions inside Dayu Cave, which recorded seven drought events. The yellow and red panels mark dates and the descriptions of drought events, respectively. All photos were taken in Dayu Cave by L Tan. Full size image

The seven drought events described in the inscriptions are notably reflected in the stable isotopic and trace elemental records of a stalagmite DY1 from the Dayu cave (Fig. 3). DY1 was collected about 1 km from the cave entrance, covering the period from ca. 1265 to 1982 CE continuously (Fig. S4, Table S1). The initial low-resolution δ18O results from DY1 stalagmite were reported in 200919. Here we built a more solid age model with additional six 230Th dates and a higher resolved (~1.3 yrs) stable isotopic and trace elemental profiles capturing annual δ18O, δ13C (Table S2) and Sr/Ca ratio variations during the last 500 years.

Figure 3 Comparison of drought events recorded in the inscriptions with speleothem δ18O, δ13C and Sr/Ca records in Dayu Cave during the last 500 years. The black triangles indicate 70 visits recorded in the cave, with some occurred in the same year. The orange squares indicate seven historical drought events occurred in 1528 CE, 1596 CE, 1707 CE, 1756 CE, 1839CE, 1891 CE and 1894 CE, respectively. (A) δ18O record of DY1 (dark green). The red line represents annual rainfall amount record from the Ningqiang meteorological station, 38 km south of Dayu Cave, during the period 1957–2000 CE, with a 3-year moving average. (B) Detrended Sr/Ca record of DY1(light blue); (C) δ13C record of DY1 (purple). Black vertical bars show locations of 230Th dates, with errors of ±0.4 to ±4 years. The straight lines in panel A and C indicate the average δ18O (−7.19‰) and δ13C (−2.54‰) values of the entire series, respectively. Full size image

“Hendy test”20 results show that both the δ18O and δ13C remain constant along growth layers of DY1 (Fig. S5). Some limitations of “Hendy test” were reported. For example, the isotopic equilibrium could theoretically occur in the center of the speleothem at the same time that kinetic fractionation occurs at the flanks21. However, the stalagmite was most likely deposited at isotopic equilibrium conditions, if the isotopic values remain constant along growth layers. In addition, the DY1 δ18O record is similar to a calcite stalagmite SF1 (r = 0.21, N = 393, p < 0.001) from the Buddha Cave in the southern Qinling Mountains22, 300 km northeast of the Dayu Cave, on decadal scale, with different mineral compositions and amplitudes of δ18O variations (Fig. S6). The correlation coefficient is not very high, mainly because of the uncertainties of the chronology of SF1, which was based on two average growth rates during the last 500 years. The “Hendy test”20 and “Replication test”21 indicate that DY1 deposited under conditions close to isotopic equilibrium and its δ18O and δ13C variations can be interpreted as proxies primarily reflecting climate and environment variations.

There is a significant negative correlation between the DY1 δ18O and the annual rainfall during the period between ~1957 and 1982 CE (r = −0.44, N = 24, p < 0.05). The correlation coefficient is not very high which may be ascribed to the “Smoothing effect” of the δ18O in drip water. The intra/inter- annual mixture of “fresh water” and “old water” may occurred in the karst aquifer23 of Dayu Cave, because of the thick epikarst zone (~80m). The age uncertainties may also play a role. The age model between the section of 1950–1982 CE of DY1 was built by an average growth rate of 0.197 mm/yr based on two 230Th dates at 1970 ± 1 and 1894 ± 0.4 CE. Because of possible variations in growth rate, the use of the average value may cause age uncertainties in given subsamples. Nevertheless, the δ18O sequence agrees with the observed annual and monsoon rainfall amount on long-term trend, with higher δ18O values corresponding to reduced precipitation and vice versa (Fig. S7). During the last 500 years, the notable positive excursions in δ18O coincided with the droughts documented in the inscriptions, corroborating a generally inverse relationship between rainfall amount (mainly from summer monsoon) and speleothem δ18O in this region.

The δ13C values24 are relatively higher during each drought event in the last 500 years too (Fig. 3). In fact, there is a significant positive correlation (r = 0.37, P < 0.01, N = 393) between the δ13C and the δ18O records of DY-1 during 1500–1982 CE (Fig. S8). Speleothem δ13C values have bedrock, atmospheric and soil gas sources25. As a result, many factors may affect the speleothem δ13C variations, including: (1) the fracture of epikarst zone and difference of lattice work in vadose zone (open/closed systems), (2) the extent of dissolution of the host rock, (3) the overlying vegetation types and density, (4) the microbial activity in the overlying soil, (5) prior precipitation of calcite (PCP) in the epikarst zone, (6) and the evaporation and degassing of drip water25,26,27. The constant temperature inside Dayu Cave suggests it is a closed system. Factors (3) and (4) are related to the vegetation change and climatic conditions. On decadal- to annual- time scales, cold and dry climate could reduce the vegetation cover and microbial activity and result in higher δ13C values in speleothems. Factors (2) and (5) are related to hydrogeochemical processes in the epikarst zone and affected by climatic conditions. The increased residence time of the seepage water during drier conditions may allow more bedrock to be dissolved, favor PCP in the unsaturated zone, resulting in higher δ13C values in speleothem25,26,27. In addition, dry condition may enhance the evaporation and CO 2 degassing of drip water28 and cause higher δ13C of speleothems in Dayu Cave.

As shown in Fig. 3, the droughts in 1596 CE, 1707 CE, 1756 CE and 1839CE corresponded well with elevated Sr/Ca ratios. The other three droughts in 1528 CE , 1891 CE and 1894 CE are also comparable with increased Sr/Ca ratios, considering age differences caused by different sampling intervals and paths. As discussed before, drier conditions could promote longer water residence times in the epikarst, decreased drip rates and enhanced CO 2 degasing into air voids within the unwetted epikarst. These conditions lead to Sr/Ca ratios higher than congruent bedrock dissolution due to preferential removal of Ca during PCP and increase the Sr/Ca ratio in speleothem29,30,31. The positive correlation between δ18O and Sr/Ca records of DY1 (r = 0.22, P < 0.01, N = 393) further confirm the observed inverse relationship between speleothem δ18O and rainfall amount in this region.

In summary, Dayu Cave provides for the first time an in situ comparison between historical drought events and speleothem records from the same cave. The in-phase variations in speleothem δ18O, δ13C and Sr/Ca during droughts in the last 500 years demonstrate a convincing anti-correlation between rainfall amount and speleothem δ18O in this region32.