by Erik Thorson Brown, Maria del Socorro Lozano Garcia, Antje Schwalb, Victoria C. Smith, Victoria C. Smith, Blas Lorenzo Valero-Garcés, Josef Peter Werne, Frank Preusser, Margarita Caballero Miranda, Enrique Cabral Cano, Peter J. Fawcett, Jens Kallmeyer, Eric Morales Casique, Anders J. Noren, Beatriz Ortega Guerrero, Sebastian Watt, Liseth Carolina Pérez Alvarado, Amy Elanor Myrbo, Byron A. Steinman

We propose scientific drilling to recover a lacustrine sedimentary sequence contained in the Lake Chalco basin on the southern outskirts of Mexico City in the Valley of Mexico. These sediments have the potential to provide a >500,000 year record of North American climate—a unique climate archive that could develop into the “type sequence” for paleoclimate studies in the Americas. Chalco is well suited for reconstruction and investigation of interannual through orbital-scale variations in the North American monsoon and hydrologic variations of the neotropics. Indeed, ongoing work indicates that the system records environmental responses to both Milankovitch- and millennial-scale climate forcing. In addition to their potential to yield a unique and remarkable record of climate history, sediments from the Chalco Basin will provide histories of volcanic and seismic activity and the recent subsidence induced by groundwater extraction directly relevant to millions of people. Volcanic (recent Popocatépetl eruptions) and seismic (1985 magnitude 8.0 earthquake) hazards pose an ongoing threat to > 20 million people of the city of Mexico. The physical properties of these sediments are relevant to models of seismic wave propagation in the basin (and analogous basins worldwide) as well as understanding its intense subsidence and regional groundwater resources. Drilling for paleoenvironmental studies can be leveraged and complemented by continuous subsurface geophysical monitoring through instrumented boreholes that will provide additional insights into regional geologic hazards and better groundwater management. We have assembled an international team to oversee the drilling of Lake Chalco, led by PIs from the USA (Brown, Werne, Fawcett, Brudzinski, Noren), Mexico (Lozano, Ortega, Caballero, Cabral, Morales), Germany (Schwalb, Kallmeyer), Spain (Valero-Garces) and the UK (Smith, Watt). ICDP research themes addressed by the MEXIDRILL Program include: • The study of earthquakes and volcanoes and mitigation of their adverse effects. • The evaluation of the relative importance of anthropogenic versus natural forces in controlling climatic and environmental change. • The wise use of water resources. • The nature of critical interaction between the biosphere and the Earth's crust. • The nature of the changing parameters that control rates of ecosystem change and hence the evolution and extinction of species.

• 1. Obtaining a >500 kyr sedimentary record from the North American Neotropics that will be relevant to climate history as well as the regional history of eruptive volcanism. Chemical and physical analyses of these sediments will lead to enhanced understanding of climate and environmental change, volcanic history, and earthquake engineering. a. Paleoenvironmental areas of focus will include coupling of temperature and hydroclimate, atmospheric drivers of climate, and climate change velocity and biodiversity variability. b. The history of volcanic activity from the Chalco sediment can provide new information on how continental volcanic fields develop as well as ecosystem response to volcanic events. c. The Chalco basin is an ideal test-bed for the understanding and simulation of nonlinear 3D basin effects for earthquake engineering.
• 2. Utilizing boreholes for installation of seismic motion sensors – currently there are no such sensors installed at depth in the Basin of Mexico, in spite of the significant seismic risks. a. Areas of seismic interest include detailed geodetic and seismic studies of the subduction earthquake cycle and related natural hazards.
• 3. Utilizing boreholes for installation of hydrological sampling equipment for continued monitoring of the aquifer and aquitard beneath the Chalco Basin. a. The extreme hydrologic extraction rates and subsequent high subsidence rates pose a major challenge for a long-term, sustainable water supply of Mexico City. b. Instrumented boreholes will provide an educational resource for students in hydrogeology.

CHALCO, Hydrology, Lake Chalco, MEXIDRILL, Paleoclimate, Volcanic And Seismic Hazards

Latitude: 19.25718, Longitude: -98.97549