The Maya, a Mesoamerican society considered the most advanced Native American civilization of its time, began a complex development around 2000 B.C. Divided into two main periods, Preclassic and Classic, the Mayans underwent a relatively abrupt collapse between 750-900 A.D., known as the Terminal Classic. The cause of rapid decline of such an intricately developed people has since been a topic of interest. One theory in particular is growing increasingly irrefutable: climate change. In 2007, James W. Webster et al., used stalagmite evidence from the Macal Chasm, a cave in Belize, to demonstrate that climate change, specifically drought, may have played an integral role in the Maya demise.
Stalagmites are formations resultant from the dripping of mineralized solutions into caves through the overlying soil and subsequent deposit of calcium carbonate. Because of the CO2 present in the atmosphere, rainwater is naturally acidic, and thus dissolves calcium carbonate when percolating through calcium carbonate rich soils. Upon entering a cave, water’s concentration of calcium carbonate is so high that some precipitates out when the water drips through the cave, forming stalagmites. Webster et al. used several proxy factors from stalagmites to indicate the climate in Belize during which the stalagmite sample was formed, including reflectance, color, and luminescence of the sample, as well as carbon and oxygen isotopic records.
Luminescence “is produced by organic acids and so is related to productivity in the soil and vegetation cover above the cave [ . . . ] as a proxy for availability of moisture” (Webster et al. 9). The particular sample from the Macal Chasm demonstrates long periods of higher luminescence—moisture—with interjecting periods of lower luminescence—drought. Color has a strong correlation with luminescence. Browner color indicates the accumulation of dust on the stalagmite, implying that there was not enough water in the cave to keep the formations free of dust (9). Dryer climates, as indicated by the color measurements, occurred during the same time periods as lower luminescence, indicating drought. Reflectance also correlates to the two aforementioned values, but is a less dependable indicator because calcium carbonate in general does not reflect much light. Combined, however, the three measurements consistently agree on times of more moisture and times of less moisture, and overall suggest several periods of drought that the Maya faced.
Another important variable measured from the stalagmites are the oxygen and carbon isotopic records, which point to different climate indicators. Oxygen isotopic records signify the amount of rainfall at the time of the stalagmite formation. Oxygen has two common isotopes, oxygen-16 and oxygen-18, the latter being heavier. The stalagmite sample used by Webster et al. was very near the entrance of the cave, meaning a higher likelihood of exposure to outside climate conditions. Because of its lower weight, oxygen-16 is more readily evaporated from the stalagmite than oxygen-18, which leaves behind more oxygen-18 and thus a higher value of the oxygen isotope ratio. More evaporation would occur from the stalagmite in drier climates, so a higher isotopic ratio value suggests less rainfall. The carbon isotopic record, on the other hand, indicates the amount of vegetation present over the cave. Three common isotopes of carbon are carbon-12, carbon-13, and carbon-14, with their weights increasing respectively. Vegetation prefers to use the lightest of the three isotopes because capturing it requires the least amount of energy; therefore, a cave covered with large amounts of vegetation, indicative of a generally wetter climate, would have a lower carbon isotopic ratio. Because both isotope ratios are ultimately suggestive of rainfall levels, they are strongly correlated. The Webster et al. data records lower values (wetter climates) and higher values (drier climates) of each isotope ratio during the same time periods. The matching records greatly increase the dependability of the data.
The Webster et al. data for all five of the proxies measured, shown above, demonstrates remarkable levels of agreement for periods of wet and dry climates. Webster et al. identifies 4 significant periods of drought from the data. The first, occurring around 141 A.D., corresponds to the Preclassic Abandonment, which is archaeologically recorded as a cessation of construction in several major Maya locations (2). The next evident drought comes around 517 A.D., which marks the beginning of a period described as the Maya Hiatus, archaeologically recognized as a period with a decrease in the amount of dedication of monuments. The third drought comes as a series at the peak of the Maya Classic Period, when it is thought that the Maya were at their highest population, and thus, extremely dependent on water for agriculture and consequently vulnerable to drought. The droughts ranged from 780-1139 A.D., with the Maya civilization thought to be completely collapsed around 910 A.D. The fourth and final significant drought identified by Webster et al. around 1472 A.D. comes after the Maya Terminal Classic, but is significant for another reason: it was recorded in Maya Books (14). The confirmation of this portion of the data amplifies the reliability of the rest of the data projected on the Maya Preclassic and Classic periods.
While no theory on the Maya collapse is unquestionably conclusive, strong evidence is building that climate change in the form of drought imposed a significant burden on the civilization, given their degree of dependence on rainfall. Though the collapse of the Mayan society was likely a combination of multiple factors, and perhaps a snowball effect of all the factors combined, it is becoming progressively clearer that climate change as a cause should not be dismissed.
Webster, James W., George A. Brook, L. Bruce Railsback, Hai Cheng, R. Lawrence Edwards, Clark Alexander, and Philip P. Reeder. “Stalagmite Evidence from Belize Indicating Significant Droughts at the Time of Preclassic Abandonment, the Maya Hiatus, and the Classic Maya Collapse.”Palaeogeography, Palaeoclimatology, Palaeoecology 250.1-4 (2007): 1-17. Print.
Sydney MacEwen, an LA native, is an upcoming Junior pursuing a BS in Environmental Studies and a minor in Geological Hazards. This is her first trip to Belize. She’s particularly interested in climate change and related policy. She hopes to pursue a Master of Arts in Environmental Studies following her undergraduate education.