Ancient Maya - Summer 2018

Drought and the Ancient Maya Civilization - National Oceanic and Atmospheric Adminstration

 

Originating in the Yucatan Peninsula, the ancient Maya civilization occupied a vast area of Mesoamerica between 2600 BC and 1200 AD. Constructing thousands of architectural structures and developing sophisticated concepts in astronomy and mathematics, the Maya civilization rose to a cultural florescence between 600 and 800 AD. Then, between 800 and 950 AD, many southern cities were abandoned and most cultural activities ceased. This period is known by archaeologists as the collapse of the Classic Maya civilization. The Maya, never able to regain their cultural or geographical prominence, were assimilated into other Mesoamerican civilizations until the time of the Spanish Conquest in 1530 AD. 

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The cause of the collapse of the Classic Maya civilization is one of the great archaeological mysteries of our time, and scholars have debated it for nearly a century. Some scientists suggest that a period of intense drought occurred in conjunction with the Classic Maya collapse and could have contributed to the Mayans’ misfortune.

 

Scientists reconstructed changes in the balance between precipitation and evaporation using the percent of sulfur in sediments and the oxygen isotopes of shells of gastropods and ostracods from Lake Chichancanab on the Yucatan Peninsula (Hodell et al. 1995 (link is external)).

Scientists have reconstructed climate at the time of the Mayan civilization by studying lake sediment cores from the Yucatan Peninsula (Hodell et al. 1995 (link is external); Curtis et al. 1996 (link is external); Hodell et al. 2005 (link is external)). It is possible to reconstruct changes in the balance between precipitation and evaporation (P−E), a common indicator of drought, by measuring oxygen isotope data from the shells of gastropods and ostracods. Lake H2O molecules containing the isotope 18O evaporate less easily than H2O molecules with 16O. Thus, during periods of strong evaporation, the lake water becomes enriched in 18O (values of δ18O are high). These isotopic values are incorporated into the growing shells of gastropods and ostracods that live in the lake.

Another proxy for P−E is the percent of sulfur in the lake sediments. Evaporation concentrates sulfur in the lake water. If the sulfur concentration becomes high enough, salts such as gypsum (CaSO4) will start to precipitate from the lake water and add sulfur to the lake sediments. The variations of sulfur percentage match the variations in oxygen isotopes closely. Corroborating one paleoclimate proxy with another is an important check on proxy records and gives us more confidence in them.
 

Distinct peaks in these two proxies reflect times of aridity on the Yucatan Peninsula. The most arid time of the last 2,000 years occurred between 800 and 1000 AD, coincident with the collapse of the Classic Maya civilization. A newer high-resolution analysis of rainfall proxies from cave deposits in the Yucatan and in Belize indicates that multiple, decadal-scale severe droughts occurred during this interval (Medina-Elizalde et al. 2010 (link is external); Kennett et al. 2012 (link is external)). Similar, though not necessarily synchronous, droughts appear to have happened in central Mexico as well (Stahle et al. 2011 (link is external); Lachniet et al. 2012 (link is external)). These findings support a strong correlation between times of drought and a major cultural discontinuity in Classic Maya civilization. It is also important to remember that other factors such as overpopulation, deforestation, soil erosion, and disease could have contributed to the demise of the Mayans.

Some important datasets related to drought and the collapse of the Mayan civilization:

Lake holds secrets to Mayan collapse, study finds

Severe drought may have led to the fall of the ancient civilization

Kate Furby, The Washington Post

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MEXICO

The sediment under a lake in Mexico contains some of the long-sought answers to the mystery of the Mayan demise.

Ancient Mayans, primarily concentrated in what is now the Yucatán Peninsula, were among the most advanced civilizations of their time. Mayans were some of the first to build cities. They used astronomy to advance agricultural production, and they created calendars and used advanced mathematics.

But despite all of their progress, the Mayan empire, built over thousands of years, may have crumbled in just a few hundred.

Scientists have several theories about why the collapse happened, including deforestation, overpopulation and extreme drought. New research, published in Science on Thursday, focuses on the drought and suggests, for the first time, how extreme it actually was.

While analyzing sediment under Lake Chichancanab on the Yucatán Peninsula, scientists found a 50 percent decrease in annual precipitation over more than 100 years, from 800 to 1,000 A.D. At times, the study shows, the decrease was as much as 70 percent.

While the drought was previously known, this study is the first to quantify the rainfall, relative humidity and evaporation at that time. It’s also the first to combine multiple elemental analyses and modeling to determine the climate record during the Mayan civilization demise.

Climate scientists commonly use sediment cores to determine the conditions of the past, like geological time capsules.

Each layer of sediment buried deep underground contains evidence of rainfall, temperature and even air pollution. Via chemical processes and interactions, the climate conditions are “recorded” in the surface soil at the time, and eventually buried. Scientists can bore a deep core of dirt, and carefully analyze it layer by layer, year by year to reconstruct a timeline.

For this study, scientists examined the layers of mud and clay in the cores from under Lake Chichancanab. During dry periods, the lake volume would have shrunk, said Nick Evans, a graduate student studying paleoclimatology at Cambridge University and first author of the study. As the water evaporated, lighter particles would have evaporated first, leaving behind heavier elements.

If the drought was intense and long-lasting, gypsum crystals formed and incorporated existing lake water directly into their structure. The “fossil water” inside the crystals allowed Evans and his co-authors to analyze the properties of the lake water during each period.

“It’s as close as you’ll ever get to sampling water in the past,” Evans said.

Evans and his team hope their research will help archaeologists understand how the ancient drought may have impacted Mayan agriculture at a critical time in their history.

Kate Furby, The Washington Post