North America, Mexico, Tamaulipas, southern part of the state, 12 km north-west from Aldama town
Cenotes of Aldama, Cenotes de Aldama
The area of the extensive El Rancho Azufrosa contains numerous very unusual natural landmarks - some of the deepest sinkholes of the world filled with warm, slightly acidic water. Some of these sinkholes have sealed themselves with plain travertine lids, others have floating travertine islands. This is the unique Sistema Zacatón - area where karst processes are influenced by volcanic heat.
History of exploration
Most likely these unusual sinkholes and also caves were well known to indigenous people. Félix María Calleja, viceroy of New Spain, in 1795 wrote: "There is a large cave lit by natural skylight (most likely Las Quarteles) and 200 varas (some 170 m) from this cave there is a deep cavity that has a lake with an island (most likely El Zacatón).".
This area earlier has not been too popular among explorers - it is hard to access. The local thickets are called "Tamaulipas Thorn Forest": here numerous species of plants have thorns and are irritating to the skin, here live lots of ticks and other parasites. Water filled sinkholes (called cenotes - Spanish version of Mayan d'zonot - "a hole in the ground") are even harder to access - these basically are wells with steep cliffsides around. Nonetheless Sistema Zacatón offers many great sights.
This area received more attention in late 1980ies and early 1990ies, as cave divers tried to reach the bottom of the deepest sinkhole - El Zacatón. It turned into historically important dive site with several world records set here.
Women's depth record in diving was reached here in 1993 by Dr. Ann Kristovich: she reached depth of 169 m.
In April 6 1994 here happened both a tragedy and amazing achievement. Two very experienced cave divers - Sheck Exley and Jim Bowden simultaneously dived in the lake. Jim Bowden reached the world record - 282 m but Sheck Exley died due to unexplained reasons at the depth of 268 - 276 m. Both divers were not too far from each other but in the darkness could see only the lights of other partner and at such record depth none of them was able to go and see how the other diver feels. True depth of El Zacatón was not reached and still there was no idea how deep it is.
These tragic events changed the strategy of further research. Researchers decided to used robotic equipment.
El Zacatón was selected as a suitable place for scientific experiments by NASA and several other institutions. This unusual place served as a testbed for semi-autonomous robot called Deep Phreatic Thermal Explorer (DEPTHX). This robot with success reached the bottom of the sinkhole in late May 2007 and mapped it, it brought also samples of new species of microorganisms.
One of the most active researchers of Sistema Zacatón is Marcus Gary, Jackson School of Geosciences at the University of Texas, with numerous other different specialists joining during the expeditions. Since the middle of 1990ies there have been made numerous field trips, very often using state of the art scientific equipment.
This research has given surprising results. One of the most unusual discoveries was the fact that some of the sinkholes have sealed themselves with travertine caps. It is very important to investigate the somewhat mysterious waterbodies below these caps without interfering into these isolated biotopes. To some extent this has been achieved by the use of electrical resistivity imaging (ERI) method.
Many potentially interesting features in Sistema Zacatón still need to be researched. Among other features, several locations (some even without names) look like unexplored sinkholes with travertine caps over them.
Volcano, groundwater and limestone
Sistema Zacatón is experiencing incredible geological processes which might be unique in the whole world.
The limestone where all the structures are located, was deposited in the late Cretaceous some 70 million years ago. Some 40 million years ago the area was rised above the water level and there formed contemporary Sierra de Tamaulipas. Some 25 million years ago, in late Oligocenic here started to form volcanic intrusions. In Late Pliocene (some 2.5 million years ago) volcanic eruptions took place here and continued throughout the Pleistocene, when in several locations of this area formed basaltic lava flows.
Groundwater and volcanic heat
As in any other area in the world, water here seeps inside the ground. But unlike most areas of the world it reaches volcanic heat here. And even more rare is that this heated and chemically altered water then reaches the base of very thick limestone layer. The result of this is exciting:
When groundwater reaches volcanic heat, it (obviously) is heated;
Warm water dissolves more than cold - groundwater becomes acidic due to the dissolved carbon dioxide and hydrogen sulfide;
Such hot and acidic water easily dissolves the limestone, forming voids at the base of limestone layer;
These voids become larger and larger and the limestone layer above them becomes weakened due to hot, acidic water. Rather soon the ceiling collapses, opening access of groundwater to a higher limestone layer;
When such collapse structure goes upwards through multiple layers of limestone, it takes the most rational and simple form - circle. Thus all sinkholes of Sistema Zacatón are rounded.
It is estimated that this phreatic process (geological process below the level of groundwater) continues here over the last 1.5 million years.
This is not the full story yet. The dissolved limestone - calcium carbonate - has to precipitate somewhere else. This happens when the groundwater becomes colder and less acidic. One such place is the surface of the sinkhole - and here comes the story about floating islands and travertine lids over the sinkholes.
Floating islands and travertine lids
First very unusual thing noticed about the Sistema Zacatón was a group of round, floating islands in El Zacatón. It is not common to see islands floating around!
Another unusual discovery was the hollow sound of the unusually plain bottom of Poza Seca, reached only in 2000. Yes, bottom of at least 8 sinkholes in Sistema Zacatón is plain like a pan - what contradicts to common sight in sinkholes elsewhere in the world. Often the bottom of sinkholes has a conical heap of collapse material - but not in Sistema Zacatón.
Electrical resistivity imaging (ERI) research affirmed the suspicions - these plain surfaces are not true bottoms, they are 2 - 4 m thick travartine lids with large, water filled voids underneath.
Earlier these were "usual" water filled sinkholes (even such formations are not usual!). But as the volcanic activity calmed down, the temperature of water gradually decreased and on the surface of these weird lakes appeared mats of precipitated calcium carbonate. Formation of these travertine lids was facilitated by the lush subtropical vegetation of Mexico.
Travertine lids of Sistema Zacatón have formed mainly at the level of present day water level but there are also some which have formed in times when water level was lower. Thus Poza Verde has a lid 45 m below the water level. But in rainy season also the surface level travertine caps have shallow lakes over them.
Waterbodies below the travertine caps may contain unique species of microorganisms able to survive in anoxic conditions and without the light. Contemporary science though has not solved a problem - how to research these possible life forms without endangering them. Any penetration through the travertine lid would open a path for invasion of other microorganisms potentially dangerous to this isolated biotope.
These phreatic sinkholes contain unique microbial communities, each adjusted to specific geochemical conditions of each sinkhole. Most diverse communities of microorganisms are in the deeper sinkholes - El Zacatón, La Pilita and Poza Caracol. Travertine walls of the upper part of these sinkholes are entirely covered with purple, red and green biomats. These organisms are utilizing the volcanically originated sulphur. This unusual biotope includes also phototrophic microbes - content of sulphur in water fluctuates during the day and night. Most likely microbial activity in these unique sinkholes is actively influencing their formation and many of the peculiarities observed here.
Green colored mats are more common closer to the surface of water. At the depth of some 3 m appear purple biomats. Below 100 m these mats disappear totally - thus it shows that these microbial communities are dependant on light.
Microbial mats seem to have high influence on sulphur content in the water. Certain species of bacteria turn H2S into H2SO4 - sulphuric acid. Thus there is no wonder that the cliff under these mats is turned into kind of carbonate "soup". In this process there is consumed oxygen in the water and created sulphuric acid.
Shortly after the discovery of unique qualities of Sistema Zacatón it is endangered by hordes of tourists. Many tourists do not respect the natural beauty, fragile environment of thorn forest and cenotes themselves, they don't respect the private property too. This top priority site has nearly no protection regime and no tourism policy elaborated and implemented.
Before such policy and protection is in place, please - better refrain from visits to Sistema Zacatón.