Researchers from the University of Cambridge, along with an international team of colleagues, have discovered a unique set of inscriptions on the walls of a cave in central China that tells how drought affected the population living there over the span of five centuries.
As reported in the journal Scientific Reports, the Cambridge-led team used inscriptions from the walls of Dayu Cave in the Qinling Mountains to detail the impact of seven drought events which took place over a 400-year span from 1520 to 1920. That information, combined with an in-depth chemical analysis of stalagmites in the cave, painted a detailed picture of those events.
Co-author Dr. Sebastian Breitenbach of Cambridge’s Department of Earth Sciences explained to redOrbit that people would enter the cave in groups of 100 or more to pray for rain, and to give a written account of their activities on the cave walls. Much to his surprise, his team found that the geochemical reconstruction closely followed the record left behind by the population.
Dr. Breitenbach called the discovery “amazing” and said that it marked “the first time we see such a clear cut link between our geochemical evidence and historical information right in very same place.” The link was made possible in part due to “the high chronological precision” of the dating process used by the research team as they analyzed stalagmite composition.
Combining historical accounts and chemical factors
“What I take as two of the of the most important implications from our study,” he said in an email, “is that we have now for the first time a direct link between historic information (the inscriptions) and our palaeoclimatic reconstruction from the same place,” which he added, “makes the interpretation of the geochemical evidence much more robust and less ambiguous.”
Previous research conducted in caves and lakes in China have found a possible link between global warming and the downfall of several Chinese dynasties, including the Tang, Yuan, and Ming Dynasties, the researchers said in a statement. Some of the inscriptions found in Dayu Cave show that residents prayed for rain during droughts in 1528 and May 1891.
Both of those dates coincide with drought events in China, the study authors said. The lack of rainfall in the 1890s led to severe starvation and local social instability, culminating in a conflict between the citizens and the government in 1900. They also noted that the drought of 1528 led to a lack of food, starvation, and even some reports of cannibalism. Evidence of such drought events were found in the form of specific elemental concentrations in the cave itself.
According to Dr. Breitenbach, his team looked at several different chemical factors, including the ratios of heavy-to-light stable oxygen isotopes (d18O), carbon isotopes (d13C) and strontium to calcium ratios in the stalagmites. The oxygen and carbon isotopes in stalagmites reflect the oxygen isotope composition of the parent drip water from which they formed, he explained, and this drip water itself originated from the rainfall coming in from the top of the cave.
What the elemental content tells about the cave’s formation
Drip water has a certain isotopic composition which depends on a combination of factors known as its “moisture history,” he explained, which itself depends upon the water’s place of origin, the length of its transport path, the amount of rainfall and other factors. Also, the d18O signal is only understandable within the context of modern climatic conditions, he noted.
Likewise, the carbon isotope signal in stalagmites is complex, but less dependent upon the cave’s temperature, and changes along with variations in the vegetation cover above the cave, as well as the CO2 composition within the soil. In this particular region of China, however, d13C is altered in correlation with changes in drip rate within the cave. In short, droughts show up as intervals in which there are less negative (heavy) d13C values, which is what the research team found.
During dry periods, calcium is removed from the small amounts of water that enters the cave, Dr. Breitenbach said, which is why strontium to calcium ratios can be associated with drought. When calcium is removed before it reaches the rock, strontium becomes enriched in the water, and then moves on to the cave, eventually making its way into the stalagmites.
“The inscriptions are amazing in that they give us direct information on the climate conditions above the cave at the time they were written. Because multiple factors can influence our geochemical proxies, their interpretation in terms of drought or flooding or other environmental changes might be questioned without extra information,” he said. “Here we have a completely independent account on what happened at the time – people tell us about their misery.”
Dr. Breitenbach added that the discovery “is a stark reminder of the influence climate has on us as society, and the vulnerability of civilization to even relatively small changes in climate. That our highly industrialized lifestyle is quite different from pre-industrial society in China is clear, but bearing the drought in California in mind, it is evident that sustained shifts in hydrological pattern can very severely impact large populations – especially so in the developing world.”
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Feature Image: This is an inscription from 1891 found in Dayu Cave. It reads: On May 24th, 17th year of the Emperor Guangxu period (June 30th, 1891 CE), Qing Dynasty, the local mayor, Huaizong Zhu led more than 200 people into the cave to get water. A fortuneteller named Zhenrong Ran prayed for rain during a ceremony. (Credit: L. Tan)
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