FEATURE ARTICLE
Climate and the Collapse of Maya Civilization
A series of multi-year droughts helped to doom an ancient culture
Larry Peterson, Gerald Haug
A First-millennium Rain Gauge
The measurement of elemental concentrations in sediments by traditional methods is time consuming and has the further drawback that it destroys the material under study. But recently geologists have overcome these problems with a technique called x-ray fluorescence, which involves illuminating a sample with x rays and measuring the amount of light given off as a function of wavelength. Suitable analysis of this light spectrum (which can be fully automated) reveals the concentration of various elements in the sample. This approach allows for the rapid assessment of elemental abundances in sediment cores that have been split down the middle, producing records that are far more detailed than what could be expected from extracting and measuring individual samples.
We initially made measurements of x-ray fluorescence using a core scanner housed at Bremen University in Germany, where the Ocean Drilling Program maintains a repository of cores. We determined the titanium and iron concentration at 2-millimeter spacings over a sediment section of interest, one that had already been dated using radiocarbon, but after finding nearly identical variations in these two elements, we chose to track only titanium.
Within this interval, and at this measurement resolution, the most obvious feature is the generally low titanium level in layers deposited between about 500 and 200 years ago, a period that corresponds to what some climatologists call the Little Ice Age. These results presumably reflect dry conditions and indicate that the intertropical convergence zone and its associated rainfall must not have reached as far north as they do now. We also found several other broad intervals of low titanium, including one in sediments deposited between about 800 and 1000 A.D., which corresponds to the period of severe drought that Hodell and his colleagues had inferred from their Yucatán lake cores.
Hodell's work had led to the impression that an extended "megadrought" plagued the Maya homeland for a century or two, with devastating consequences for the indigenous population. But this interpretation troubled some Mayanists. They pointed out archaeological evidence for considerable variability in the timing and regional pattern of collapse. A "one drought fits all" model seems too simplistic, given that the collapse apparently happened at different places at different times, while affecting some population centers hardly at all.
Although the Cariaco Basin is quite distant from the Yucatán, its unique sediments offered the possibility of obtaining an immensely detailed chronology of ancient climate swings, and we wanted to push the record as far as it would go so as to provide further insight into the climate during the Maya collapse. Unfortunately, we had reached the maximum analytical resolution of the Bremen core scanner. But with the help of Detlef Günther and Beat Aeschlimann at the Swiss Federal Institute of Technology in Zurich, we did much better using a special "micro" x-ray fluorescence system they had set up in their lab. This instrument was designed for small samples, not long stretches of deep-sea sediment, but it could accommodate short slabs of material cut from our cores. This device allowed us to make elemental analyses with a 50-micrometer measurement spacing, which in the Cariaco cores corresponds to about two months of time—an incredibly fine resolution for marine sediments, which more typically encompass hundreds to thousands of years of geologic history in a single sample.
Using Günther and Aeschlimann's wonderful instrument, we measured two slabs of sediment that together cover the time interval from about 200 to 1000 A.D., focusing on those layers deposited during the terminal Classic collapse. This interval revealed a series of four distinct titanium minima—likely multi-year droughts, which took place during a period that was already drier than normal. When exactly did these intense dry spells settle over the Maya heartland? Although the counting of sediment couplets gives precise information on the duration of these droughts (which range from three to nine years) and the spacing between them (around 40 to 50 years), the absolute dating of these events remains a little vague. Radiocarbon measurements for the core we used in combination with counting couplets would indicate that the four droughts struck around 760, 810, 860 and 910 A.D., but quoting such precise dates is somewhat misleading, given that the radiocarbon technique has an uncertainty of about ±30 years for samples of this age.
» Post Comment