A drought lasting 13 years and several others that each lasted over three years may have contributed to the collapse of the Classic Maya civilisation, chemical fingerprints from a stalagmite in a Mexican cave have revealed.
A detailed analysis of oxygen isotopes in the stalagmite allowed a team of researchers, led by the University of Cambridge, to determine rainfall levels for individual wet and dry seasons between 871 and 1021 CE, which overlaps with the Terminal Classic period of Maya civilisation. This is the first time it has been possible to isolate rainfall conditions for individual wet and dry seasons during the Terminal Classic, the time of societal decline historically referred to as the Maya collapse.
During the Terminal Classic, limestone Maya cities in the south were abandoned and dynasties were ended, as one of the ancient world's great civilisations shifted north and lost much of its political and economic power.
The data contained within the stalagmite, from a cave in the Yucatán, showed that there were eight wet season droughts that lasted for at least three years during this period, with the longest drought lasting for 13 consecutive years.
This climate data aligns with existing historical and archaeological evidence: construction of monuments and political activity at several major northern Maya sites, including the famous city of Chichén Itzá, stopped at different times during this period of climate stress.
The accurately and precisely dated droughts provide a new framework for fine-grained analysis of the timing and dynamics of human-climate interactions in the region. The results are reported in the journal Science Advances.
"This period in Maya history has been a cause of fascination for centuries," said lead author Dr Daniel H. James, who conducted the research while a PhD student at Cambridge's Department of Earth Sciences. "There have been multiple theories as to what caused the collapse, such as changing trade routes, war or severe drought, based on the archaeological evidence the Maya left behind. But in the past few decades, we've started to learn quite a lot about what happened to the Maya and why, by combining the archaeological data with quantifiable climate evidence."
Starting in the 1990s, researchers began to piece together climate records with those left by the Maya, such as the dates they recorded on key monuments, to show that a series of droughts during the Terminal Classic was a likely contributor to the massive socio-political upheaval in Maya society.
Now, James and his co-authors from the UK, US and Mexico have used the chemical fingerprints contained in stalagmites from a cave in the northern Yucatán to bring those droughts into much sharper focus.
Stalagmites form when water drips from the ceiling of a cave, and the minerals contained in the water grow into large deposits on the cave floor. By dating and analysing the layers of oxygen isotopes within the stalagmite, the researchers extracted highly detailed information about the climate in the Terminal Classic period. Earlier research has measured the oxygen isotopes contained in lake sediment to determine the severity of drought, but lake sediment does not contain enough detail to pinpoint climate conditions in any given year at a particular site.
"It hasn't been possible to directly compare the history of individual Maya sites with what we previously knew about the climate record," said James, who is now a postdoctoral researcher at University College London (UCL). "Lake sediment is great when you want to look at the big picture, but stalagmites allow us to access the fine-grained detail that we've been missing."
Earlier research on stalagmites has determined annual average rainfall amounts during the Terminal Classic, but the Cambridge-led team have now been able to go further, and isolate information from individual wet and dry seasons, thanks to relatively thick (about 1mm) annual layers in the stalagmite used in this study. The specific oxygen isotopes in each layer are a proxy for wet season drought.
"Knowing the annual average rainfall doesn't tell you as much as knowing what each individual wet season was like," said James. "Being able to isolate the wet season allows us to accurately track the duration of wet season drought, which is what determines if crops succeed or fail."
According to the information contained in the stalagmite, there were eight wet season droughts lasting for at least three years between 871 and 1021 CE. The longest drought of the period lasted for 13 years. Even with the water management techniques that the Maya had, a drought that long would have had major impacts on society.
The climate information contained in the stalagmite lines up with the dates inscribed by the Maya on their monuments. In the periods of prolonged and severe drought, date inscription at sites such as Chichén Itzá stopped entirely.
"This doesn't necessarily mean that the Maya abandoned Chichén Itzá during these periods of severe drought, but it's likely that they had more immediate things to worry about than constructing monuments, such as whether the crops they relied on would succeed or not," said James.
The researchers say that stalagmites from this and other caves in the region could be vital in putting the puzzle of the Terminal Classic period together.
"In addition to what stalagmites can tell us about this period in Maya history, they might also be able to tell us about the frequency and severity of tropical storms, for instance," said James. "As a case study for fine-grained comparisons between climate and historical data, it's exciting being able to apply methods usually associated with the deeper past to relatively recent history."
The research was supported in part by the National Geographic Society and the Leverhulme Trust.
