Hells Bells

Deep down El Zapote cenote, a 50-meter-deep water-filled sinkhole in Quintana Roo, Mexico, on the Yucatan Peninsula, stalactites take a different form. Instead of the usual elongated, pointed shape hanging from the roof of caves, the stalactites in El Zapote are conical and hollow resembling bells or lampshades. Divers call them “Hells Bells”, after the song by the Australian hard rock band AC/DC.

Photo: Eugenio Acevez/ Jerónimo Avilés

Hells Bells are found at approximately the halfway mark as one descends down the hour-glass-shaped cavern. They occur within a narrow band about six meters across, but within this region they cover almost the entire surface area of the cave.

Hells Bells are usually circular or elliptical, but they do not form a complete ring. Most of them maintain an open segment of about one quarter to one third of the ring wall, that gives them a horseshoe-like appearance. The open portion of the cone always faces towards the cave walls. These speleothems, or cave formations, can grow exceedingly large, in excess of 2 meters in length and almost a meter in width. Their walls are up to 3 centimeters thick.

Carbonate deposits, and thus the growth of speleothems in caves, usually occur when evaporation or carbon dioxide escaping from water cause calcite to become saturated, which then precipitates and forms deposits. Underwater carbonate deposits are also known to form through biological and physical-chemical processes, and it is possible that Hells Bells appear to belong to this underwater type, as there is little evidence of exposure to air on them, and water levels in the cave appear to have always exceeded the depths at which the Hells Bells developed.

Image: Wolfgang Stinnesbeck

The cave systems of Yucatan are flooded by salty seawater seeping in from the bottom as well as fresh groundwater from precipitation that accumulate at the top. These two layers do not mix and are separated by a partially mixed layer called the halocline. Hells Bells are found at the margin between the halocline and the freshwater layer above. The Bells do not cross into the saline layer because calcite dissolves there.

Researchers have found that the oxygen content in the halocline layer is almost nil, while the freshwater layer contains oxygen. Professor Dr. Wolfgang Stinnesbeck from the Institute of Geosciences of the University of Heidelberg, whose team had explored the fantastic underworld and published a paper, believes that the growth of
these structures is regulated by specific physical and biogeochemical conditions above and in the halocline. The Bells are also exceedingly old. Radiometric dating indicates that they started growing some 5,000 years and the process continues to this day.

“El Zapote Hells Bells represent an enigmatic ecosystem providing the conditions for the formation of the biggest underwater speleothems worldwide”, concludes the paper. “They are morphologically unique and may have been produced under the hydrochemical mediation of polyspecific microbe colonies that still appear to be active today.”

Photo: Eugenio Acevez/ Jerónimo Avilés

References:
# Wolfgang Stinnesbeck et al, Hells Bells – unique speleothems from the Yucatán Peninsula, Mexico, generated under highly specific subaquatic conditions, Palaeogeography, Palaeoclimatology, Palaeoecology
# Wikipedia