Studying Past Environments to Understand Our Global Environmental Future
BY SHERYL LUZZADDER-BEACH
At first glance, ancient Mesoamerica and the modern world have major differences: diverse environments, different human histories, and different technological advances among many others. But closer examination through the lens of geoarchaeology provides clues to environmental change, and human impact on and adaptations to changing environments that span the globe. It offers valuable lessons from the past for present and future generations on global environmental change. An ongoing study in Belize, Central America, reveals large scale environmental changes, some caused by human and some by natural agency, and remarkable human adaptations. The research also reveals the complex, two-way connections between the environment and society regarding human impacts and human response that still apply today.
BELIZE, CENTRAL AMERICA
Our project in Belize, Central America, began as assessments of water and land resources, and the study of patterns of water and land use of the ancient Maya. Our team pursues this line of research to answer the following questions of sustainability: How did the ancient Maya feed their large populations? What resources were available to the Ancient Maya? Where and how did they conduct their farming? How did they conserve their water and land resources? Despite their resource conservation techniques, why did Maya society experience a substantial collapse eleven centuries ago? And, what can we learn from these examples in our modern struggles to sustainably feed ourselves?
Our research in the Maya world is located, in part, in the northwest corner of modern Belize, where it borders with Mexico and Guatemala, in the heart of the Maya Lowlands region of Mesoamerica. This tropical region has a distinctive geology of limestone, and it is split by several normal faults running generally north-south through the region. These faults create a stair-step topography leading from the flat, costal plain of Belize on the western shore of the Caribbean Sea, to higher elevations in Guatemala to the west. The very first stair step or escarpment runs through the La Milpa and Blue Creek Archaeological sites in northwestern Belize. This topography influences the water resource distribution of the region—also known as hydrology—and impacts the quality of the waters as well. Most of the monumental architecture and the majority of the settlement features discovered thus far sit atop the escarpment, to catch the breeze, and to take strategic advantage of a commanding view of the surrounding landscape. Below the escarpment are many smaller settlements, farming communities, and a massive complex of what appear to be ditched fields, where it is hypothesized that the Maya grew crops to feed their large populations, perhaps engaged in aquaculture, and, according to some of our archaeobotanical evidence, also cultivated tree crops. Our research team discovered that a thick layer of sediment buried the prime, ancient farm soils. These buried soils are also known as paleosols, and in this region they date to about 2,500 to 5,000 years ago. How did the sediment get there? Was it from human over-use of the upslope lands such as tree clearing, urbanization, and farming? Or from eroding away the soils and covering the farmlands below? More background is required before answering this question.
Besides the ditched fields on the coastal plain below, evidence of intensive agriculture appears in the upland in the form of terrace walls, human made reservoirs, so-called aguadas, weirs, and diversions on streams to deliver water to fields below. What is remarkable about the terrace walls is that they function even to this day to help slow soil erosion. Trapped behind the walls is evidence of agriculture and massive movement of soils and sediments from upslope; yet the walls have held for centuries, perhaps averting an earlier or more catastrophic environmental demise. In fact, rates of erosion were higher in earlier times of lower Maya populations who were pioneering the region than in the height of the Maya Classic around 700 AD, demonstrating that the conservation structures were effective. There is evidence all over the Guatemalan and Belizean portion of this escarpment environment that soil erosion occurred in the region on a massive scale in the form of a “Maya Clay” sediment layer found in lakes throughout the region. Is there other, overlooked evidence that needed to be considered to answer the question of the buried paleosols in the lowlands, or can we safely conclude it is the same process?
Besides physical proof of the movement and deposition of sediments we can turn to chemical evidence to trace soils and waters from their source regions and geologic parent materials. Although there is a strong literature on the use of soil chemistry in geoarchaeology, one of the long neglected areas of geoarchaeological evidence has been water chemistry, which, from a human settlement perspective, is an important variable to consider. Water supply is an important factor in locating cities and in conducting agriculture, while the quality of that water is critical to human health, agricultural success, and healthy ecosystems—whether we are studying ancient or modern landscapes. Water can be a more ephemeral resource; however, because its residence time in a landscape is far less than soils or geologic deposits, it offers direct evidence of environmental conditions. Building on our studies of modern water quality and agriculture, we have engaged in a fifteen-year project monitoring the quality of the waters of this region, and made significant discoveries of regional differences and limitations on its use. Assuming there have been no major changes in the geologic materials that impart to water its chemical characteristics, we can also make assumptions about past conditions.
First, the surface waters, including rivers and streams, as well as ground waters, such as underground aquifers and springs, are of significantly better quality in the upland environments than in the low lying coastal plain environments. This makes sense in light of settlement patterns favoring the uplands and the pattern of intensive agriculture in which the Maya engaged in the uplands. Furthermore, the escarpment and coastal plain waters’ geochemistry chart—which tracks the relative proportions of dissolved ions in the waters—as if from completely different sources. This is a surprising finding because the region is underlain by limestone, which is assumed to be a fairly uniform aquifer material made from ancient ocean floor sediments. The limestone is highly fractured and jointed where horizontal layers join. It has numerous water-formed caves and connections running through it, which suggests that any water falling on the surface will easily flow in and mix with other water, creating a fairly uniform chemistry from the geologic materials in which the water resides. We have found that high-flow, high-ion springs of groundwater emerge from the base of the escarpment, filling lakes at the base of this fault, and contributing surface and ground water of a far different and far more mineralized content than the upland waters. This water dominates the region of the ditched fields, which are now wetlands, and suggests a deep, unrelated groundwater source that does not reach the top of the escarpment. This creates a mystery: why would the ancient Maya invest so much human labor capital in a massive engineering effort to build ditched fields if the water was not suitable for farming let alone for drinking due to high mineral or ionic content? And, did the ancient Maya cause the demise of their agricultural resource through upland erosion that buried their prime farming soils under up to 2 meters of sediment? The answers lie in global environmental changes at work in this region and in the geochemistry of the materials.
The evidence for the ditched structures and the ancient prime farm soils now resides below a rising ground water table. We have discovered this evidence through more than thirty excavations of these field complexes with trenches approximately 15 meters long, 2 meters wide, and more than 2 meters deep. The ground water table has been driven up over the last 1,000 years by sea level rise, as documented by researchers studying other wetland or ditched agriculture sites in northern Belize and in documented coastal Maya settlement sites that are now under sea level. Furthermore, we have compared the chemical signatures of the soil sediments from the upland sediments with the sediments choking the ditched fields below, and they are unrelated. For example, the upland sediments have very low percentages of CaSO4·H2O or gypsum. The coastal plain sediments are dominated by gypsum above the buried soil and are not related to the upland sediments. Using micromorphology, we have also been able to microscopically examine thin sections of the soil to study the mineral content and structure of these sediments.
Our final key to the question of the massive sediment build up, or aggradation, on top of the paleosol comes from our water chemistry studies. It turns out that the high-ion, high mineral content water that runs though the streams and wetlands of this coastal plain region is also saturated with gypsum, matching the soil evidence in the same region. The groundwater has been pushed up over time by sea level rise in the coastal plain region. This site is the last site to experience the rise, being the farthest inland. Other researchers also observed the gypsum content in their soil samples. The mineral-laden groundwater rise is the mechanism for the aggradation, as gypsum precipitates out naturally either through being saturated or via evaporation during the dry season, leaving behind the minerals in the same way that minerals form in a teakettle boiled dry.
We can conclude that the aggradation, then, was not caused by human agency, but by a natural process. As ground water was driven up by sea level rise, farmers ditched their field to reclaim them from the rising waters. This occurred after the paleosol was buried, because the ditching rarely reaches to this ancient soil level. There is also evidence of a large flood event across the region, adding to the sedimentation over the paleosol. Interestingly, one of the largest ditched field complexes in our study region dates to about 900-1,000 years ago, which postdates the Maya collapse, suggesting that there were groups of people remaining in the region who were able to continue large scale agriculture in support of their people. These studies of an ancient society give us hope, based on the resilience and methods of these and other ancient peoples, that we can face modern natural disasters, and global environmental challenges, as well as counteract some of the effects of human alterations of the landscape.
Sheryl Luzzadder-Beach (slbeach@gmu.edu) is Associate Professor and Associate Chair in the Department of Geography and Geoinformation Science at George Mason University (GMU) (http://esgs.gmu.edu). This research was funded, in part, by grants from the GMU Center for Global Studies, Mason’s Office of the Provost, the National Geographic Society, and the National Science Foundation. This article was first published in print and citations have been removed due to space limitations, but are available from the author.
