On World Environment Day, we will present an extraordinary phenomenon which is currently being researched jointly by German and Mexican researchers in the flooded cave systems of the Yucatán Peninsula.

(17.06.19) World Environment Day, established by the United Nations (UN) in 1972, is celebrated each year on 5 June with the aim of increasing awareness in society for environmental issues, the importance of environmental protection and the protection of our natural resources.

In Mexico, efforts are being made to conserve what are known as the cenotes or collapse dolines, which start out as subterranean groundwater reservoirs and become accessible when the limestone cave ceiling breaks away from the Earth's surface. On the Yucatán Peninsula, one of the few regions in Mexico with an abundance of freshwater, there are more than 10,000 cenotes, which were a source of water for human settlements for centuries.

The Mayan people used them as a water source and gave them spiritual meaning, regarding them as entrances to the underworld. Countless archaeological discoveries have been made in these flooded cave systems, including artefacts used for religious rituals and the Eve of Naharon, one of the oldest human skeletons found on the American continent.

However, population growth, environmental pollution and inadequate management of the cenotes not only pose a risk to their ecosystems and natural resources, they also threaten the extensive archaeological and palaeontological heritage that can be found there. From a scientific perspective, their preservation is therefore of vital importance; the cenotes are natural and mysterious laboratories harbouring valuable knowledge about the past.

For some years now, research has been focussing on an extraordinary geological formation that was discovered in the flooded cave systems of the Yucatán Peninsula. This phenomenon is also known locally as "Hells Bells". These are hanging speleothems, similar to stalactites, which stand out with to their unusual cone shape. Some are up to a metre in length and have a diameter of 80 centimetres.

These formations, discovered in underwater caves of the famous Cenote Zapote and in other flooded caves west of the city of Puerto Morelos, have been puzzling the researchers.

In general, hanging dripstones are formed as a result of the physicochemical processes involved in the depositing and hardening of carbonates, and hanging dripstones or stalactites normally have a tip that points downwards. But the shape and size of the speleothems in El Zapote are unique. They expand outwards at the bottom in the shape of a cone, are hollow inside and have a round, elliptical cross section – like a bell.

Researchers from Germany and Mexico are studying this phenomenon in a joint project to uncover the secrets of this mystery. The initiative has been funded by the DFG and its Mexican partner organisation CONACYT since 2007, with researchers from the University of Heidelberg, the State Museum of Natural History Karlsruhe and Museo del Desierto in Saltillo participating in the project.

The research is based on the hypothesis that this unique feature is down to the depositing of calcium carbonate caused by microbiological activity. The hanging structures grow to a depth of 36 meters in a dark environment. A freshwater reservoir is located nearby directly above a poisonous, low-oxygen salt water area with high levels of hydrogen sulphide.

Samples were taken and sent to the laboratories of the University of Heidelberg by Prof. Dr. Wolfgang Stinnesbeck, the German project coordinator. Dating revealed that the oldest layer of the speleothems is 4,500 years old and the youngest is 300 years old, indicating that these formations must have developed in the last 10,000 years.

10,000 years ago, sea levels were 100 meters below current levels and the caves and cenotes on the Yucatán Peninsula had not yet been flooded with water. But, following the global temperature rise around 4,500 years ago, the ice melted in the cold regions of the Earth, resulting in the elevated water levels that we have today and the flooding of the deep cave areas. This indicates that the "Hells Bells" formed entirely underwater, which is unusual for speleothems.

The researchers participating in the project believe that there is a direct connection between the development of these formations and the unusual hydrological and biochemical conditions of the environment. It is a transitional zone between water layers with varying salt content, referred to as the halocline.

Analyses showed that the bacteria in this area absorb the carbon dioxide from the water, thereby reducing its acidity, in order to perform their vital functions. As a result, fewer dissolved minerals are retained and a micro-environment is created, resulting in the precipitation of the dissolved minerals and ultimately the formation of the bells, layer by layer.

"If our interpretation is correct and the formation of the bells is shaped by the special water zone, the halocline, we might have a new tool on our hands for determining how the depth of this layer develops over time. This could give us an insight into the amount of precipitation on the Yucatán Peninsula," says Stinnesbeck.

"This would allow us to gain a better understanding of climatic changes during the ice age when the ecosystems in North America underwent drastic changes," added Dr. Arturo González, cooperation partner in Mexico. He explains that the project is part of a larger study that began in 1999 with the aim of finding out more about the arrival of the first humans in this region and the role they played in the extinction of more than 70% of the mammals native to the area, including elephants, sabre-toothed tigers, camels and sloths.

Stinnesbeck stresses that bilateral cooperation in the research project is vital: "Our Mexican partners showed us the location and helped us make contacts with local divers. They also provided the complex infrastructure required for taking samples in an underwater cave that is more than 50 meters deep, part of which is a poisonous, sulphide-rich environment. In turn, the team from Heidelberg brought along laboratory equipment and specialist expertise in hydrogeochemistry and petrography. Genome sequencing is carried out by the Karlsruhe Institute of Technology."