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Archive for July 2013

Pollen may help solve the mystery of why a pre-Columbian city in Mexico was abandoned

My research focuses on the Holocene geological time period, the last 10,000 years of Earth history. An accurate picture of past climate can help us understand the relationship between past environmental change and ancient societies. In this post, I describe how my summer fieldwork and my broader dissertation project link summer rainfall, microscopic grains of pollen, and an ancient city in Mexico.

The pre-Columbian city of Cantona is an impressive sight, even today. Located near the border of the Mexican states of Puebla and Veracruz, in the Oriental Basin, it covers 12.6 km2 and features a complex network of streets, ball courts, and pyramids. At its peak, between 1,750 and 1,000 calendar years before present, the site was an important Mesoamerican center of trade in obsidian. Archaeological investigations suggest that the city was abandoned abruptly at approximately 1,000 years ago. While the cause is unknown, many have invoked climate change as a possible contributing factor, partially because of the lack of surface water in the region today. This, however, requires an accurate picture of climate change at least 900 years before we had reliable, instrumental measurements of temperature and rainfall.

Cantona ball court

A ball court at the pre-Columbian city of Cantona. Photo by Tripti Bhattacharya.

All is not lost, however. It turns out that past climates leave microscopic traces in lake sediments. A wet climate, for instance, will leave behind different clay minerals than a dry climate. The pollen preserved in lake sediments can also tell us about the regional vegetation at the time the sediments were deposited, which in turn reflects regional climate. Various techniques like radiocarbon dating can be used to establish the age of the sediments. Detailed analysis of pollen composition or sediment geochemistry can therefore provide a long-term perspective on climate change. My research focuses on lake sediments from a volcanic crater lake called Aljojuca that is approximately 30 km south of Cantona. In 2007, a team from the GFZ German Research Centre for Geosciences and Mexico’s Universidad Nacional Autónoma de México took a 12 m sediment core from the lake. Radiocarbon dating suggests that this core contains a continuous, 6,000-year sediment record.

This summer, I collected pollen samples from contemporary vegetation to improve our interpretation of our pollen results from the Aljojuca core. Understanding how vegetation changes along natural gradients of temperature and precipitation (for instance, up the slope of a mountain) can help us interpret the results we see in fossil pollen assemblages. We can even characterize the composition of the pollen created by local vegetation by analyzing the modern pollen preserved in surface soils, small ponds, or even cushions (called polsters) of moss!