This EAGER grant supports work on seismic reflection profiling in Lake Chalco, at the southern end of the Basin of Mexico, with the ultimate goal of developing a proposal to the International Continental Drilling Program. The Chalco basin, which was drained for agriculture in the 19th and early 20th Centuries, contains up to 500m of lacustrine sediments that have recorded several hundred thousand years (perhaps as much as million years) of subtropical American climate history. The Chalco records provide useful counterpoints to records of Middle American climate from terrestrial sites (e.g. spethelothems from Cave of the Bells, Arizona or lake sediments from Peten Itza, Guatemala or from Valles Caldera, New Mexico) as well as coastal marine settings (e.g. sediments of the Cariaco and Santa Barbara Basins). The climate history of Chalco may be relevant to evolution of the axolotl, an aquatic salamander endemic to Chalco and its neighboring basin that has been used as a 'model organism' in thousands of physiological and biomedical studies. Finally, the seismic survey and sedimentary record provides information relevant to the tectonic and volcanic history of the Basin of Mexico. Paleoclimate records from lacustrine deposits in terrestrial settings typically span the Holocene and only rarely have extended much deeper in time than the Last Glacial Maximum. Recent interest in lake drilling has begun to change this, with successful programs in Bear Lake (US), Valles Caldera (US), Peten Itza (Guatemala), Titicaca (Bolivia/Peru), Malawi (East Africa) and Bosumtwi (Ghana), and El'gygytgyn (Siberia) among others. The climate record from Chalco should provide insights on regional responses to past changes in coupled climate systems. For example, the processes that induce and propagate millennial-scale climate shifts (e.g the Dansgaard-Oeschger and Heinrich events) remain controversial; settling these controversies requires continuous high-resolution climate records from many areas of the planet, including tropical North America, where such information is lacking. In addition, Middle to Late Pleistocene interglacial conditions are of particular interest as possible analogs for the Holocene. Evidence from Antarctic ice cores document significant differences in atmospheric pCO2 and climate change over the Middle Pleistocene, but high-resolution continental records spanning this interval are rare globally and absent in North America. The full Chalco record has the potential to provide unique information that will fill these critical gaps, improving our understanding of our present interglacial and its future course.
Broader impacts This project provides opportunities for graduate and undergraduate student training and takes advantage of ongoing collaborations with Mexican colleagues, fostering collaboration among scientific communities in the U.S. and Mexico. This work obtains records that provide insights on climate and hydrological balance that have implications for resource management and regional planning. The sedimentary archive in Chalco contains information that is directly relevant to one of the most densely populated urban centers on our planet. Mexico City, home to over 17 million inhabitants, is built primarily on lacustrine sediments of the Basin of Mexico that were exposed when waters that fed the lakes were diverted for agriculture after the European conquest. Continued withdrawal of groundwater from aquifers associated with these lacustrine sediments is a major cause of subsidence. Knowledge of past change in hydrological balance will inform our understanding of regional water resource availability.
This grant provided funds for geophysical surveys necessary to develop strategies for drilling and recovering sediments from Lake Chalco, just south of Mexico City. The broader goals of the research program are to use chemical and biological records from the sediments build a 500,000 year climate history for this part of the world, with a particularly emphasis on changes in rainfall. The drilling operation is also of interest for a range of studies including volcanic history and hazards, seismic monitoring and earthquake hazard modeling, as well as hydrological monitoring. All of these areas of research have real relevance to the inhabitants of Mexico City. This grant provided funds for fieldwork to determine optimal sites for sediment record recovery and for evaluating potential risks associated with the drilling operation. Seismic reflection surveys and passive seismic surveys were undertaken in January 2011. Results of these geophysical surveys were presented at a planning workshop (March 2012 in Mexico City) and were an integral part of developing plans for drilling operations. A expanded set of passive seismic measurements was undertaken in June 2012 to enhance our dataset. The grant provided opportunities for students (undergraduate and graduate) from the University of Minnesota Duluth to participate in geophysical fieldwork in Mexico. Similarly, students from the National Autonomous University of Mexico (UNAM) participated in fieldwork and also had opportunities to work at laboratories at UMD to evaluate field results.
