Poza Seca, Sistema Zacatón
|Coordinates:||22.9980 N -98.1615 W|
|No:||114 (list of all attractions)|
|Category:||Sinkholes, Biotopes, Lakes, bays and streams|
|Values:||Geology, Biology, Unexplained|
|Address:||North America, Mexico, Tamaulipas, southern part of the state, 12 km north-west from Aldama town|
The first sinkhole with travertine lid discovered in Sistema Zacatón is Poza Seca. This incredible water filled sinkhole has sealed itself with several metres thick travertine lid!
Sistema Zacatón is area with incredible characteristics unparalleled in the world. In total this area contains some 20 unusual features - sinkholes, caves, springs created by deep groundwater heated by volcanic processes. Karstification processes here continue today as well - acidic groundwater continues to strip away the limestone and deposits it in unusual travertine formations.
Poza Seca was discovered in January 2000 by Karen Hohle. Soon after the discovery scientists noticed hollow sound when walking on the dry, unusually plain bottom.
Later geophysical research revealed that the bottom is not a true bottom - it is 2 - 4 metres thick cap with at least 25 m deep waterbody below it. Most likely this mysterious waterbody is significantly deeper.
Soon afterwards it was discovered that several more sinkholes of Sistema Zacatón have similar travertine lids with waterbodies below - for example, Cenote Azufrosa, Poza Tule, Poza Verde, La Pilita and most likely Poza Garapata and several more structures, some without known names. El Zacatón certainly is in a process of forming such lid.
Such lids may be recognised by plain surface - in common sinkholes bottom never is plain. Sometimes these lids are located deeper, below layer of water. In rainy season even the dry lids may be covered with shallow water.
Heat and carbon dioxide
The water in sinkholes of Sistema Zacatón is warm - most likely due to the heat of volcanic processes below the thick layer of Cretaceous limestone. These same volcanic processes cause acidification of groundwater. And this warm, acidic water of course easily dissolves the limestone, gradually creating an underground void. When such void is large enough, ceiling collapses and thus it gradually rises up. At some moment the last collapse takes place, water has reached daylight and sinkhole has appeared!
In this moment the chemical processes become more complex. Water becomes colder, dissolved carbon dioxide - CO2 - outgasses in the air and particles of lime stay at the surface of water. Gradually they stick together, form mats and become covered with vegetation. Finally this material - travertine - forms a lid covering whole sinkhole!
Such process is very rare in nature.
Unknown life below
We can be sure that at the moment of closure Poza Seca contained several species of microorganisms. These microorganisms were encapsulated and had to face life without light. It is though possible that with some adaptation they have survived and evolved into unique species.
Research of such waterbody is complex engineering problem - it seems to be impossible without imminent threat to this whole ecosystem.
The fact that travertine lid of Poza Seca is located at the level of present day water level means that it has formed comparatively recently. This is attested by valuable find - next to Poza Seca there have been found fossils of mammoth - molars and tusks. These fossils were lithified in dense travertine. As mammoths lived here some 10,000 years age, this discovery might mean that active formation of lid over Poza Seca took place in this time period.