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
The Venezuelan Connection
Our own contribution to the understanding of climatic conditions during the time of the terminal Classic collapse comes from a distant location, one not inhabited by the Maya at all. Offshore of the northern coast of Venezuela sits a remarkable depression in the continental shelf known as the Cariaco Basin. Reaching depths of about a kilometer but surrounded by the shallow shelf and banks, the Cariaco Basin acts as a natural sediment trap. What is more, the shallow lip of the basin prevents its deeper waters from mixing readily with the open ocean to the north. As a result, deep Cariaco waters are devoid of dissolved oxygen (and have been since near the end of the last glacial period, some 14,500 years ago). The lack of oxygen means that the muddy floor of the basin cannot support bottom-dwelling marine organisms, which in other places churn up the sediment in their search for food. This lack of a deep-sea fauna preserves the integrity of the sediments, which here are made up of paired light and dark layers, each less than a millimeter thick.
The origin of these layers is easy enough to understand: During Northern Hemisphere winter and spring, the intertropical convergence zone sits at its southernmost position near the equator, which means that little rain falls over the Cariaco Basin. At this time of year, strong trade winds blow along the northern coast of Venezuela, causing cool, nutrient-rich waters to rise, which in turn allows plankton living near the surface to proliferate. When these organisms die, their shelly remains fall to the bottom, where they form a light-colored layer. During the summer, as the northern hemisphere warms, the intertropical convergence zone moves steadily northward until it takes up a position near the northern coast of South America. The trade winds diminish, and the rainy season begins, increasing the flow of local rivers, which then deliver a considerable load of suspended sediment to the sea. These land-derived materials eventually settle out of the water, leaving on the ocean floor a dark-colored layer of mineral grains on top of the earlier accumulation of light-colored microfossil shells.
Although burrowing organisms mix up such seasonal deposits elsewhere, the anoxic Cariaco Basin preserves these distinct light-and-dark couplets. This dramatic alternation in composition provides a built-in clock that geologists can use to determine with yearly resolution just when the sediments were laid down. And fortunately, at least for people interested in the history of Maya civilization, both the Yucatán and northern Venezuela experience the same general pattern of seasonal rainfall, with both areas today near the northern limit of the intertropical convergence zone. Hence marine sediments from the Cariaco Basin hold considerable information about the shifts in climate that the Maya experienced.
Our efforts to read that archive began in 1996, when the scientific drillship JOIDES Resolution, operated by an international research collaboration called the Ocean Drilling Program, sailed to the center of the Cariaco Basin. Once there, technicians obtained a 170-meter-long sequence of sediment cores expressly for the purpose of probing tropical climate change. The study of those sediments, which had accumulated at an enormous rate and had remained completely undisturbed since the time of deposition, offered us and other geologists a rare, high-resolution glimpse into the distant past. An important aspect of our work on these sediments has been to use the concentration of mineral grains eroded from land to gauge the amount of rain that fell on adjacent parts of the South American continent.
One could, of course, gain such an understanding by examining these sediments directly under a microscope, but characterizing vast numbers of sediment couplets in this way would have been extraordinarily tedious. So we sought out a more efficient approach. Of the several methods we explored, the most useful proved to be the measurement of titanium and iron, elements that are abundant in most continental rocks but not in the shelly remains of marine organisms. High levels of titanium and iron thus indicate that large amounts of silt and clay were washed off the adjacent land and swept into the basin. That is, finding lots of titanium and iron at a particular level in these sediments means that rainfall in this region—and by inference over the Yucatán—must have been high at the time of deposition. Low titanium and iron, by contrast, means that rain was sparse.
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