Geoscientists discover 500,000 years of climate history in central Mexico

The effects of climate change on the tropics are still poorly understood. However, the tropics are among the most densely populated areas in the world. Researchers at the Leibniz Institute of Applied Geophysics (LIAG) have now created an age-depth model and moisture distribution over the past 500,000 years of one of the oldest lakes in central Mexico, Lake Chalco.

The results are clear: Central Mexico experienced repeated dry spells related to the Earth’s natural oscillation. The researchers published their work in the journal Quaternary Science Reviews.

Central Mexico, due to its mild climate and fertile soil, has been continuously inhabited by people since the colonization of primitive civilizations and remains an area with one of the highest concentrations of population in the world. The combination of rapid population growth, expected future increases in air temperature, and the potential for drought in central Mexico suggest that this will remain an area strongly affected by climate change.

An improved understanding of both the mechanisms contributing to current climate change and its consequences for the biosphere, including human society, will not only provide the knowledge needed to address its impacts, but may also shed light on the forces that have driven them. similar events in The past.

In 2016, downhole measurements were taken in a borehole about 500 meters deep in Lake Chalco in an area on the outskirts of Mexico City. The research team used borehole geophysics, which measures the physical properties of sediments, to extract paleoclimate signals from lake deposits in the upper 300 meters to determine past climate conditions.

This is the first time that borehole geophysical data have been used to reveal the history of moisture content in lake sediments, providing insight into 500,000 years of climate history in central Mexico. In addition, the research team also dated the Chalco Lake sediments using astrochronology, a technique for calibrating sediments using Earth’s orbital cycles, the regular oscillating motion of Earth’s orbit around the sun.

The results show that central Mexico regularly experienced a dry spell over the past 500,000 years, when Earth’s orbit was at its most circular.

The specific geomorphology of central Mexico, due to the appearance of a large series of volcanic arcs due to the subduction of the Pacific Ocean plate under the North American continental plate, allowed the formation of an extensive internal drainage basin almost a million years ago. Today, this geological formation is called the Valley of Mexico. Since its formation, water has remained in this basin and covered nearly 1,500 square kilometers of the valley floor.

The water level in the lake fluctuated in response to alternating warm and cool periods in Earth’s paleoclimate. Earth has experienced cold (glacial) and warm (interglacial) periods in cycles of about 100,000 years for at least the last 1 million years. During the warm seasons, the highest rainfall in central Mexico raised the water level of the lake to 100 meters, and during the cold seasons, the water level dropped to a few meters due to drought.

“Lake sediments trace the history of the planet and preserve clues about past climate and environmental conditions. Through this study, we can determine how variable climate changes have been in the past and how the environment has responded,” explains Dr. Mehrdad Sardar Abadi, MexiDrill project coordinator at LIAG. “The successful implementation of the methodology and the results also help future paleoclimate studies that can build on it.”

Before the arrival of the Spanish, the Basin of Mexico was occupied by the Aztecs, who built a great city called Tenochtitlan on and around the lake system. In the early 1600s AD, the Spanish drained most of the lake system in an attempt to control flooding. Today’s Lake Chalco is a shallow marsh, which occupies an area less than 6 square kilometers south of Mexico City.

An important source of water in Mexico City comes from underground aquifers formed among the ancient lake sediment that is draining at an irreplaceable rate. As a result, Mexico City is rapidly sinking downward at about half a meter per year. Mexico City’s water crisis has become an ongoing problem with underground aquifers depleting and sinking the city.