Scientists found evidence of droughts in ancient human history by looking at the layers in a stalagmite from Mexico.
Introduction
Can you imagine almost no rain falling for 13 years? Even with modern technology, life is difficult without rain. Farmers can’t grow crops, and there isn’t enough drinking water.
People living in the ancient Maya civilization may have faced severe droughts. The Maya civilization peaked in Mesoamerica more than a thousand years ago. They built towering temples, used advanced math, and wrote in hieroglyphics. They lived in city-states, each with its own warrior-king and a council of advisors. They had a calendar system called the Long Count and carved important dates on monuments and buildings.
Maya civilization changed a lot between the years 800 and 1000 CE. We call this period the Maya Terminal Classic. Historians and archaeologists noticed that there were fewer Long Count dates on monuments during this time. They also found that people stopped building and making things. This was based on radiocarbon dating. We think that the Maya people abandoned their cities to move further north, where they built new places to live.
Why did this happen? Changes in the climate, such as long droughts, may have caused crops to fail and people to go hungry. But it’s hard to know if climate change caused these cities to collapse. We don’t have a written record of the climate from the period.
Luckily, we have a natural record! In caves, there are mineral columns called stalagmites. When it rains, water seeps through the cave roof and drips into the cave. This water is called drip water. Stalagmites grow taller as drip water leaves behind layers of calcite. This is like a tree growing a new ring every year. Each new layer in the stalagmites holds information about the climate in the year it formed.
We wondered if we could see evidence of droughts in stalagmites. We wanted to compare this record with the human history of the Classic Maya civilization. This could help tell us more about the effects of drought on the Maya people.
Methods
The stalagmite we studied (Figure 1A) came from a cave in the Yucatán Peninsula in Mexico. This would have been near two major ancient Mayan cities.
First, we had to work out the age and date of the stalagmite.
We ground up small sections into a powder. We measured chemical isotopes of the elements uranium and thorium. These isotopes change over time. By studying how much they have changed, we can figure out the age of the rock or object.
We cut the stalagmite in half and used a video microscope to count all the calcite layers inside.
Next, we looked for evidence of drought in the stalagmite layers.
We used chemical analysis to measure oxygen isotopes in the layers. This tells us how rainy or dry it was when each calcite layer formed.
Every month for a year, we measured the amount of rainwater and collected drip water (Figure 1B). We analyzed the oxygen isotopes in the drip water. This helped us check that changes in oxygen isotopes matched changes in the amount of rainfall.
Finally, we compared our climate data to Long Count dates on carved monuments in the region.
Results
We found that part of the stalagmite spans the years 871-1021.
This means it was forming during the period when Mayan civilization was changing.
There were around 180 layers in the stalagmite. Each layer represents a year.
We found evidence of drought in the stalagmite layers.
Our oxygen isotope measurements of the calcite layers showed eight extreme droughts of three or more years in a row. This included a 4-year drought starting in 894 CE and a 13-year drought from 929 to 942 CE (Figure 2).
The drip water we collected showed that our oxygen isotope measurements matched the rainy and dry seasons.
We know from Long Count dates that the Maya people did not build many monuments after 900 CE. They had stopped building in this region by 925 CE. This lines up with the droughts in the late 800s and early 900s.
Discussion
We learned that there were many long droughts in the region in the late 800s and early 900s. Not long after this, the Maya stopped building temples and monuments. Historians and archaeologists know from studying Long Count dates that the Classic Maya political system had collapsed by 950. This was 8 years after the end of the 13-year drought we found.
The Maya had many strategies to deal with drought, like water wells and canals. But we think repeated long droughts were serious enough to cause crop failure and hunger. The droughts may also have led to conflict and disease, bringing political instability.
Our stalagmite gave us a lot of information. We collected more details than previous studies on stalagmites in the region. We could see what the climate was like every year from 871-921 and 1070-1100 CE. Natural records can be a powerful tool to help us understand what the world was like long ago.
Conclusion
Studying the stalagmite gave us a natural record of droughts that happened in the 800s and 900s. Major political changes in the Maya civilization happened around the same time. This shows that our scientific evidence supports what historians already believed.
In school, science and history are often taught as separate subjects. But when we use them together, they can help us learn even more. Scientists can study rocks, tree rings, ice, or even old bones to understand what life was like long ago. What part of history interests you most? How could you use science to learn more about it?
