Field Report: Paleoenvironments of Mexico
Paleoenvironments of Mexico
|Setting up piston corer at Laguna de Los Palau, San Luis Potosi, Mexico|
Mexico contains a rich history of human settlement and is a fascinating setting in which to study human-environment interactions. Mexico’s importance is highlighted by the fact that it was a major site of crop domestication, and the fact that it played host to key Mesoamerican civilizations, including the Aztec and the Maya. Despite decades of study, important uncertainties remain about how prehistoric environmental change influenced the development of these agricultural societies and, later, the abandonment of many sites. Answering these questions requires improving our understanding of the timing and spatial patterns of paleoenvironmental change across Mexico. My summer research sought to use lake sediments as archives of environmental change: My analysis of sediments from Aljojuca, a volcanic lake in Central Mexico’s Trans-Mexican Volcanic Belt, seeks to establish a chronology of environmental change and human settlement at the site of Cantona, while analysis of sediments from Laguna de Los Palau in San Luis Potosi seeks to clarify how past environmental change might have shifted the northern border between agricultural and hunter-gatherer societies (1).
Cantona lies in the modern Mexican state of Puebla. Despite its role as a center of obsidian production and a regional power following the decline of Teotihuacan, little is known about the chronology of occupation and abandonment at this site (2). My research will analyze a sediment core from nearby volcanic lake to reconstruct late Holocene (5000 yr BP to present) to develop a record of climate change and anthropogenic land use near this site. Volcanic crater, or maar, lakes’ unique geology often creates laminated sediments that preserve high resolution records of past environmental change. A team of researchers from have already obtained a sediment core from the lake Aljojuca to the southeast of Cantona.
|Taking pollen samples from sediment core from Aljojuca lake.|
My research focuses on analyzing this core, using multiple proxies of environmental change: sediment chemistry and analysis of stable isotopes of oxygen in carbonates precipitated in lake sediments will provide a record of regional aridity. Pollen and charcoal analysis will shed light on the extent and timing of human influences on the landscape, as the presence of agricultural indicators like maize, increased regional burning, and the rise of disturbance taxa like grasses, amaranths, and asters indicate human settlement and agriculture.
|Laguna de Los Palau is a dry maar lake to the east of the city of San Luis Potosi, near the town of Armadillo. While the lake contained water into the historical period, it was drained in the 1970s to reclaim the land for agriculture. The region is subtended by carbonate rocks, which have weathered to thick clays in the maar crater. Surrounding vegetation is generally dry, and common trees include oak and juniper. If this record stretches through the late Holocene, there may be evidence for human impacts on the environment, and potentially evidence for cultivation.|
Research supported by the Stahl Endowment helped establish a well-dated chronology for the sediment core from Aljojuca, and the extraction of a sediment core from Laguna de Los Palau.
A grant from the Stahl Endowment helped us obtain three radiocarbon dates from pollen samples and macrobotanical fragments in the Aljojuca core. One date appeared to be contaminated by the ‘old carbon effect’, whereby aquatic plant samples appear artificially depleted in radiocarbon because they incorporate old, soluble carbonate into their tissues. However, the two reliable dates, combined with three other previously obtained dates, suggest that this sediment core is over 2000 years old. We are still waiting on a date from the base of this 12 m core, but tentatively estimate that the record may be 3000 - 4000 years old. This means that this sediment core may provide a record of environmental change stretching back to the Pre-Classic period, and may record changes associated with both the settlement and abandonment of nearby sites like Cantona.
The Stahl Endowment grant also supported travel to Mexico and fieldwork to obtain a sediment core from Laguna de Los Palau. We used a narrow gauge piston corer to obtain 6 m of sediment in total, although recovery was incomplete for most of the sediment sequence due to the density of the clay sediments. Initial analyses of the sediments suggest a dramatic transition two meters down in the sediment core from dark, organic sediments to white carbonate sediment. This suggest a past, dramatic environmental change. Unfortunately we have been unable to find any material for radiocarbon dating in the core, so elucidating environmental history from this core will be difficult. It is possible that wetlands to the east of this maar may result in more promising results, and we may return next summer.
In the future, I plan to continue analyzing the pollen record from Aljojuca, and am currently developing a procedure to maximize the amount of pollen we can extract from the core’s sediment matrix. I also plan to examine the elemental composition of core sediments, as well as the oxygen isotope ratios of carbonate minerals preserved in the core, which will provide a record of aridity.
While our results from Laguna de Los Palau are disappointing, I hope to continue looking at other lake sites in northern Mexico that may yield datable material and well-preserved pollen that can then be used to elucidate the region’s environmental history.
IMG_3340: Setting up piston corer at Laguna de Los Palau, San Luis Potosi, Mexico
IMG_3349: Sampling the wall of a soil pit at Laguna de Los Palau
IMG_3081: Taking pollen samples from sediment core from Aljojuca lake
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