“On June 8th, 46th year of the Emperor Kangxi period, Qing Dynasty, the governor of Ningqiang district came to the cave to pray for rain.”
The above entry is just one out of 70 inscriptions found in Dayu Cave in central China, written on the wall during a time of severe drought in the summer of 1707. In a study published 13 August in Scientific Reports, scientists translated the cave inscriptions and found evidence in seven of them of major droughts—in 1528, 1596, 1707, 1756, 1839, 1891, and 1894—five of which were previously unknown to historians.
The researchers then used geochemical analysis of formations within the cave to confirm this record of droughts. With their findings, they were also able to model the likelihood of major future droughts in the region.
The Summer Monsoon
Agriculture in China is dependent on the summer monsoon, when roughly 70% of the year’s rain falls over a few months. Historical records during dry periods recount political strife, widespread starvation, and even cannibalism in the worst of these events, according to the study. This suffering is reflected in the cave’s graffiti, where record keepers wrote that the “mountains are crying due to drought” and that people came to pray for rain.
The inscriptions, which included dates from the Chinese calendar, also add validity to the notion that drought may have caused the demise of several Chinese dynasties, the researchers said.
“We find in our record a stark reminder of the influence climate has on society and the vulnerability of civilization to even relatively small changes in climate,” said Liangchen Tan, lead author of the study and an associate professor at the Institute of Earth Environment at the Chinese Academy of Sciences in Beijing.
Building Records of Past Droughts
To find evidence of past droughts within the geochemical records of the cave, the scientists turned to stalagmites—cones of limestone that grow upward from the cave floor over thousands of years. Stalagmites form when water percolating into the cave from above drips from the cave’s ceiling to the floor below.
As the fallen water evaporates from the cave floor, it leaves behind a thin coating of solid minerals. Those deposits build, layer by layer, into a cone that holds a chemical record of the surrounding environment. Scientists analyze the layers of these cave formations like tree rings; each layer contains a wealth of information, including temperature and rainfall abundance at the time the layer was added.
To conduct their analyses, the team looked at the ratios of heavy carbon and oxygen isotopes, specifically carbon-13 (13C) and oxygen-18 (18O), to lighter isotopes within each mineral layer.
When water drips into a cave, it immediately degasses, releasing absorbed carbon dioxide. The carbon dioxide molecules with the lighter carbon isotope 12C will degas first, leaving relatively more 13C behind.
If conditions are drier, there’s more time between drips for lighter isotopes to leave, so the solid mineral left behind becomes especially enriched with 13C. Dry conditions also increase the abundance of 18O compared with its lighter counterpart, 16O.
Importance of Cave Inscriptions
The researchers pieced together a timeline of wet and dry periods using radiometric dating techniques along with the measurements of carbon and oxygen isotope ratios. They found that times of drought recorded in the geochemical record aligned closely with the inscriptions that reflected a drought, Tan said.
“Our study also suggests the importance of cave inscriptions in climate and [historical] study, which were ignored before,” Tan added.
“This study is unique as the authors capitalize on an opportunity to use in situ historical records of drought to test the ability of [stalagmite] isotopic and geochemical compositions to serve as a proxy of past drought,” said Corinne Wong, an associate professor in the Department of Earth and Environmental Sciences at Boston College, who wasn’t involved in the study.
The researchers also used patterns of 18O/16O ratios within the stalagmite layers to develop a mathematical model to investigate future precipitation changes and the likelihood of drought.
Although it’s impossible for a model to predict precisely when a drought might hit, a model that recreates past events known to have occurred gives scientists more confidence in the model’s predictions of the future, said Sebastian Breitenbach, a coauthor of the paper and a researcher at the University of Cambridge in the United Kingdom.
In this case, the team’s model, calibrated to the pre-1900 drought record obtained from Dayu Cave, did indeed reflect a major drought that occurred in China in the 1990s. By extrapolating to the year 2042, the researchers found that another drought could occur in the 2030s.
Further, their model shows that conditions could be drier between now and 2042 relative to the past 500 years.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2015), Chinese cave inscriptions tell woeful tale of drought, Eos, 96, doi:10.1029/2015EO034521. Published on 21 August 2015.