After a 4 km walk through a tropical rainforest, with so much humidity and heat, you finally begin to glimpse the Lachuá lake through the vegetation and start to speed up the pace to get to the moment when you can throw your backpack aside and jump into the water. At the edge of the pier, with a clear sky, you can easily see the forest surrounding the lake in an almost perfect circle.
Although the Lachuá lake is not (yet) listed among the “recommended cenotes” in the tourist guides (you can find it as a lake, but not in the list of cenotes), today I want to tell you why Lachuá is in fact a cenote, and how cenotes originate.
Brief geological history
Perhaps the most famous cenotes are those in the north of the Yucatan peninsula, formed in part by the impact of the Chicxulub meteorite 65 million years ago. But even before that impact, along with the Yucatan Peninsula, much of the lowlands of northern Central America were below sea level. On that submarine platform the remains of animals that use calcium carbonate in their bones, spicules or shells, layered over millions of years. Unlike organic parts that decompose, calcium carbonate accumulates.
These layers of accumulated calcium carbonate are what we now call limestone or karst. It is a very porous material and rich in calcium and magnesium – both of which are soluble in water. This porosity and solubility allows water to penetrate the pores, enlarging them as it dissolves the material, creating cracks and caves.
Mesoamerican Cenotes
Due to changes in sea level and the geological uplift of northern Central America, this platform emerged about 35 million years ago and has since been exposed. It is what today makes up the Yucatan Peninsula, part of Belize and Guatemala. Many of the cracks and caves formed before by seawater were left empty. Although many mineral processes give some strength or resistance to these structures, when the caves created by the water are too large, the land above is no longer supported and collapses, creating the cenotes.
All these fluctuations in sea level left us with caves, underground rivers, and dry cenotes or lake-cenotes. In fact, some 22,000 years ago during the last glacial maximum, the sea level dropped so low that many cenotes that we can swim in today were then dry. But it was not only those millions of years immersed in seawater that were responsible for the dynamics of the formation and hydrology of these sites. In addition to the sea and the distance to the water table, rainfall is another important factor in the dissolution of karst soils even today.
When rainwater comes into contact with carbon dioxide (CO2) – either in the atmosphere or in the soil through contact with products of decomposition reactions – it acidifies (carbonic acid). This penetrates and dilutes the limestone more easily than freshwater.
So besides a fascinating geological history, depending on the water table and other local conditions, we can have dry cenotes, like Cimarrón (Guatemala), or water-filled cenotes, like Lachuá (Guatemala), and the network of underground rivers that we still have no accurate idea about! They are all connected in different dimensions through water. Water continues and will continue to shape and constantly alter the wetlands we know today – and on which we depend.
Did you learn something new here? Consider supporting me on Ko-fi and get a personalized doodle!
[kofi]
Recommended references:
Pérez-Ceballos, R. et al. 2012. Regionalization Based on Water Chemistry and Physicochemical Traits in the Ring of Cenotes, Yucatán, México. Journal of Cave and Karst Studies.
Granados-Dieseldorff, P. et al. 2012. Fishes from Lachua Lake, Upper Usumacinta Basin, Guatemala. Check List 8(1):095-101