[Aztlan] GOOD MAYA COLLAPSE RESEARCH

[michael ruggeri]

http://www.americanscientist.org/template/AssetDetail/assetid/44510

Climate and the Collapse of Maya Civilization

A series of multi-year droughts helped to doom an ancient culture

Larry C. Peterson, Gerald H. Haug

With their magnificent architecture and sophisticated knowledge of  
astronomy and mathematics, the Maya boasted one of the great cultures  
of the ancient world. Although they had not discovered the wheel and  
were without metal tools, the Maya constructed massive pyramids,  
temples and monuments of hewn stone both in large cities and in  
smaller ceremonial centers throughout the lowlands of the Yucatán  
Peninsula, which covers parts of what are now southern Mexico and  
Guatemala and essentially all of Belize. From celestial  
observatories, such as the one at Chichén Itzá, they tracked the  
progress of Venus and developed a calendar based on a solar year of  
365 days. They created their own system of mathematics, using a base  
number of 20 with a concept of zero. And they developed a  
hieroglyphic scheme for writing, one that used hundreds of elaborate  
signs.

During its Classic period (250–950 A.D.), Maya civilization reached a  
zenith. At its peak, around 750 A.D., the population may have topped  
13 million. Then, between about 750 and 950 A.D., their society  
imploded. The Maya abandoned what had been densely populated urban  
centers, leaving their impressive stone edifices to fall into ruin.  
The demise of Maya civilization (which archaeologists call "the  
terminal Classic collapse") has been one of the great anthropological  
mysteries of modern times. What could have happened?

Scholars have advanced a variety of theories over the years, pinning  
the fault on everything from internal warfare to foreign intrusion,  
from widespread outbreaks of disease to a dangerous dependence on  
monocropping, from environmental degradation to climate change. Some  
combination of these and other factors may well be where the truth  
lies. However, in recent years, evidence has mounted that unusual  
shifts in atmospheric patterns took place near the end of the Classic  
Maya period, lending credence to the notion that climate, and  
specifically drought, indeed played a hand in the decline of this  
ancient civilization.

Rainforest Crunch

Given the common image of lost Maya cities buried beneath tangles of  
jungle vegetation, it may come as a surprise to discover that the  
Yucatán is, in fact, a seasonal desert. The lush landscape depends  
heavily on summer rains for nourishment, rains that vary considerably  
across the peninsula. Annual precipitation ranges from as little as  
500 millimeters along the northern coast to as high as 4,000  
millimeters in parts of the south. As much as 90 percent of this  
moisture falls between June and September, and a pronounced winter  
dry season runs from January to May.

This wet-dry contrast results from the seasonal migration of moisture  
associated with the intertropical convergence zone, an atmospheric  
feature that is sometimes known as the "meteorological equator." In  
this zone, the easterly trade winds of the northern and southern  
tropics converge, forcing air to rise and bringing on cloudiness and  
abundant rainfall. During the winter months, the intertropical  
convergence zone shifts far to the south, and dry conditions prevail  
over both the Yucatán Peninsula and northern South America. Then,  
with the coming of summer, this zone migrates north again, bringing  
life-giving rain to the Yucatán and southern Caribbean region.

The Maya had to deal with this seasonal contrast and, in particular,  
had to cope with a long dry season each year. This feature of their  
environment had special significance, because surface waters tend to  
dissolve the limestone bedrock of the Yucatán, forming caves and  
underground rivers but leaving little opportunity for water to flow  
over land. So the Maya could not simply locate their settlements  
along major watercourses. Even important regional centers—such as  
Tikal, Caracol and Calakmul—developed in places that were without  
permanent rivers or lakes. The lack of surface water for four or five  
months of the year in such areas spurred the construction of large- 
scale water-collection systems.

Many cities were designed to catch rainfall and channel it into  
quarries, excavations and natural depressions that had been specially  
prepared to retain the captured water without letting it seep into  
the ground. Tikal, for example, had numerous reservoirs, which  
together were capable of holding enough water to meet the drinking  
needs of  roughly 10,000 people for about 18 months. The Maya also  
built reservoirs on the tops of hills, using gravity to distribute  
the water through canals into complex irrigation systems. Despite the  
sophistication of their hydrological engineering, the Maya ultimately  
depended on the seasonal rains to replenish their water supplies,  
natural groundwater being inaccessible over a considerable portion of  
their realm.

In his fascinating book, The Great Maya Droughts, independent  
archaeologist Richardson B. Gill persuasively argues that a lack of  
water was a major factor in the terminal Classic collapse. Gill pulls  
together an enormous amount of information on modern weather and  
climate, draws on the record of historical droughts and famines, and  
heaps on evidence from archaeology and from geological studies of  
ancient climates. To demonstrate the importance of the porous  
limestone bedrock, for example, he quotes Diego de Landa, Bishop of  
Yucatán, who in 1566 wrote: "Nature worked so differently in this  
country in the matter of rivers and springs, which in all the rest of  
the world run on top of the land, that here in this country all run  
and flow through secret passages under it."

Gill builds an impressive case. When his work was first published  
(five years ago), the most compelling evidence for drought came from  
sediment cores that David A. Hodell, Jason H. Curtis, Mark Brenner  
and other geologists at the University of Florida had collected from  
a number of Yucatán lakes. Their measurements of these ancient  
deposits indicate that the driest interval of the last 7,000 years  
fell between 800 and 1000 A.D.—coincident with the collapse of  
Classic Maya civilization. Later work by these same investigators  
found evidence for a recurrent pattern of drought, which seems also  
to explain other, less dramatic breaks in Maya cultural evolution.

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.

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.

All in the Timing

Scholars generally agree that the terminal Classic collapse occurred  
first in the southern and central Yucatán lowlands and that many  
areas of the northern lowlands underwent their own decline a century  
or more later. This pattern of abandonment is opposite to what one  
might expect based on the modern pattern of rainfall, which  
diminishes markedly from south to north. Some Mayanists have pointed  
to this incongruity as evidence against drought having played a  
significant role. However, an additional factor that must be  
considered is the availability and access to natural water sources,  
which could have sustained the population during extended periods of  
drought.


During the peak of Maya civilization, as now, an important source of  
fresh water for human activities was from the natural underground  
aquifer. This aquifer is generally more accessible in the northern  
end of the peninsula, where the Maya were able to reach the water  
table at various sinkholes (places where the roof of an underground  
cavern had collapsed) or by digging wells. However, as one moves to  
the south, the landscape rises in elevation, and the depth to the  
water table increases, making direct access to groundwater  
unfeasible, at least for the Classic Maya with the technology of  
their time. Thus the more southern settlements, which were totally  
dependent on rainfall and reservoirs for their water needs, were more  
likely to be susceptible to the effects of prolonged drought than  
were cities with direct access to subsurface sources. This critical  
difference helps explain why drought could have caused greater  
problems in the normally wetter south.

Although there is general agreement that the abandonment of major  
population centers began first in the south and then spread to the  
north, Gill proposed a more controversial tripartite pattern of  
collapse. Based on an analysis of the last recorded dates carved into  
stone monuments known as stelae at major Maya sites. Gill argued that  
there were, in fact, three phases of drought-related collapse between  
about  760 and 910 A.D., with a distinct regional progression.

The first phase, according to Gill, occurred between 760 and 810. The  
second phase was largely over by about 860. The third and final phase  
terminated around 910. Noting a similarity between the end dates of  
these three phases and the timing of especially severe cold spells in  
Europe (as evidenced in Swedish tree-ring records), Gill speculated  
that the abandonments occurred rather abruptly at the end of each  
phase, that they were primarily the result of droughts and that these  
droughts were linked to the cold conditions at higher latitudes.


Gill's model of three phases of collapse, and especially the  
archaeological basis for their proposed timing, has been the subject  
of much debate. There is considerable disagreement, for example, over  
the interpretation of the last dated inscriptions on stelae as  
accurate records of city abandonment. Furthermore, Gill considered  
only the largest Maya sites in his original analysis. So there is  
certainly some room for doubt. Nevertheless, the drought events we  
inferred from the Cariaco Basin record match Gill's three phases of  
abandonment remarkably well.

The onset of Gill's first phase at about 760 A.D. is clearly marked  
in the Cariaco record by an abrupt decrease in inferred rainfall.  
Over the subsequent 40 years or so, there appears to have been a  
slight long-term drying trend. This period then culminated in roughly  
a decade or more of severe drought, which, within the limits of our  
chronology, agrees well with the end of Gill's first phase. Societal  
collapse at this time was limited to the western lowlands, a region  
with little accessible groundwater and where the inhabitants depended  
almost entirely on rainfall to satisfy their needs.

The end of Gill's second phase of collapse is also marked in the  
Cariaco Basin record by a distinct interval of low titanium  
concentrations, suggesting an unusually severe drought that lasted  
for three or four years. City abandonment during this phase of  
collapse was largely restricted to the southeastern portion of the  
lowlands, a region where freshwater lagoons may have provided a  
source of water up to that point.

According to Gill, the third and final phase of collapse occurred at  
about 910 A.D., affecting population centers in the central and  
northern lowlands. And low titanium values in the Cariaco Basin  
sediments indicate yet another coincident period of drought, one that  
lasted for five or six years.

Although the match between Gill's drought model and our findings is  
quite good, we accept that no single cause is likely to explain a  
phenomenon as complex as the Maya decline. In his recent book  
Collapse: How Societies Choose to Fail or Succeed, Jared Diamond  
argues that a confluence of factors may have combined to doom the  
Maya. These include an expanding population that was operating at or  
near the limits of available resources, environmental degradation in  
the form of deforestation and hillside erosion, increased internal  
warfare and a leadership focused on short-term concerns. (Sound  
familiar?) Nevertheless, Diamond posits that climate change, in the  
form of droughts, may have helped bring things to a head, triggering  
a series of events that destabilized Maya society.

Some archaeologists have pointed out that the control of water  
reserves provided a centralized source of political authority for the  
ruling Maya elites. Periods of drought might then have undermined the  
institution of Maya rulership when existing technologies and rituals  
failed to provide sufficient water. Large population centers  
dependent on this control were abandoned and people moved  
sequentially eastward and then northward during the successive  
droughts to find more stable sources of water. However, unlike what  
transpired during previous intervals of too little rainfall, which  
the Maya must certainly have weathered before, the landscape during  
the final stages of collapse was at carrying capacity (because of the  
growth of Maya population during wetter times), and migration to  
areas less affected by drought was no longer possible. In short, they  
ran out of options.

Climate in Human History

The ability to combine geological archives with traditional  
archaeological and historical information provides a powerful means  
to examine the societal response to climate shifts of the distant  
past. Although the socioeconomic impacts of recent El Niño events or  
of the infamous Dust Bowl drought of the 1930s are easy enough to  
study, climatologists still know relatively little about the  
consequences of older and longer-period changes in climate. In recent  
years, however, high-resolution records from ice cores, tree rings,  
corals and certain deep-sea and lake sediments have begun to provide  
an increasingly precise record of climate change for the past few  
millennia.

The coincidence of drought and collapse within the Maya civilization  
is just one example. In the American Southwest, tree-ring evidence  
for a prolonged drying of climate between about 1275 and 1300 has  
long been thought to play a role in the disappearance of the cliff- 
dwelling Anasazi people. And there are indications that similar  
changes in climate may have been responsible for other major events  
in human history as well. The collapse of the Akkadian Empire in  
Mesopotamia about 4,200 years ago, the decline of the Mochica culture  
in coastal Peru about 1,500 years ago and the end of the Tiwanaku  
culture on the Bolivian-Peruvian altiplano some 1,000 years ago have  
all now been linked to persistent long-term drought in those regions.  
Before the geological evidence for these ancient droughts became  
available, each of these cultural collapses, like that of the Maya,  
had been interpreted solely in terms of human factors—warfare,  
overpopulation, resource depletion.

The rise and fall of the Classic Maya provides a textbook example of  
human social evolution. It is therefore significant to discover that  
the history of the Maya was so closely tied to environmental  
constraints. If Maya civilization could collapse under the weight of  
natural climate events, it is of more than academic interest to  
ponder how modern society will fare in the face of an uncertain  
climate in the years ahead. An understanding of how ancient cultures  
responded to climatic changes in the past may thus provide important  
lessons for humanity in the future.


Bibliography

Carr, R. F., and J. E. Hazard. 1961. Tikal Report No. 11: Map of the  
Ruins of Tikal, El Peten, Guatemala. Philadelphia: University Museum,  
University of Pennsylvania.
deMenocal, P. B. 2001. Cultural responses to climate change during  
the Late Holocene. Science 292:667-673.
Diamond, J. 2005. Collapse: How Societies Choose to Fail or Succeed.  
New York: Viking.
Gill, R. B. 2000. The Great Maya Droughts: Water, Life, and Death.  
Albuquerque: University of New Mexico Press.
Haug, G. H., D. Günther, L. C. Peterson, D. M. Sigman, K. A. Hughen  
and B. Aeschlimann. 2003. Climate and the collapse of Maya  
civilization. Science 299:1731-1735.
Hodell, D. A., J. H. Curtis and M. Brenner. 1995. Possible role of  
climate in the collapse of Classic Maya civilization. Nature  
375:391-394.
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Related Internet Resources
Original research by Peterson and Haug
Cariaco Basin Climate Record
Related article from BBC News
Tikal Digital Access Project
Related article from planeta.com

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