Puzzles about underwater stalactites

Calcite formations in a cenote in Mexico originated in a unique way

Unusual limestone features: underwater stalactites "Hells Bells" in the El Zapote Cave on Yucatan © EAN / IPA / INAH / MUDE / UNAM / Heidenlerg
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Unique natural phenomenon: In a water-filled cave on Yucatan, researchers have discovered extremely unusual stalactites. Because they are not grown in the air, but under water. The calcite formations, which are up to two meters long, are hollow on the inside and are trumpet-shaped at the lower end. Probably the result of a meeting of very special conditions in the water of the cave - and by the help of microbes.

Dripstones are fascinating works of art of nature. Typically, they form in caves within limestone formations. Rainwater seeps through the lime and absorbs calcium carbonate. At the cave ceiling, these saturated drops exit. As it drips, a portion of the calcium carbonate precipitates and remains attached to the ceiling of the cave - over the course of millennia, an ever-increasing lime cones - a stalactite.

Hollow and trumpet-shaped instead of massive cones

But apparently it is also quite different, as a discovery in the El Zapote Cave on the Mexican peninsula Yucatan demonstrates. In the water-filled sinkhole about 36 kilometers from Cancun divers have discovered strange bell-shaped stalactites. The formations baptized by Divers "Hell's Bells" are up to two meters long and strangely hollow inside.

Instead of becoming narrower like normal stalactites, these stalactites expand to their end. "They are similar to bells, lampshades, trumpets, trunks or tongue-shaped projections that reach up to 80 centimeters in diameter, " report Wolfgang Stinnesbeck from the University of Heidelberg and his colleagues. Frequently smaller daughter boulders grow in the larger formations.

The Hells Bells are up to two meters long - and thus the largest known underwater dripstones ever. © EAN / IPA / INAH / MUDE / UNAM / Heidelberg

Grown under water

Unusually also: The Hells Bells dripstones lie 29 to 35 meters under the water level of the Cenote. "The question was therefore whether these calcite structures are old and come from a time when the cave was dry down to these depths, " explain Stinnesbeck and his colleagues. "Or whether they belong to the rare Speleothermen, which grow under water and which are documented so far only from very few places of discovery." Anzeige

To solve this, the researchers salvaged samples of two dripstones and determined their age by means of uranium-thorium dating. The result: The Hells Bells are geologically still very young, they probably originated within the last 4, 500 years, as the dates showed. "So at least the copies we dated clearly grew in a time when the cave was already under water for a long time and the water level was about the present, " the scientists report.

Favorable water chemistry

But how is a dripstone growth under water possible at all? Evidence was provided by analyzes of water samples from the cenote. Because they revealed that the water in the cave is heavily stratified: over a deeper layer of sulfide-containing salt water lies a sap water layer reaching up to almost 30 meters depth and exactly in these are clearly the Hells Bells stalagmites grown.

The researchers suspect that the special chemical and physical conditions near the halocline of the boundary layer between salt and sine water have allowed the growth of "bells". Accordingly, dissolved calcium rises from the saltwater zone and leads to a supersaturation of the above-lying fresh water. Combined with the alternating low-oxygen and oxygen-rich layers, this allows the stalactites to grow there.

A favorable combination of chemical-physical conditions and microbial activity enabled the formation of the Hells Bells. EAN / IPA / INAH / MUDE / UNAM / Heidelberg

Help of microbes

In addition, however, other living actors could be involved in the growth of "bell bells". Because microbiological analyzes showed that the stalactites are inhabited by a very special community of bacteria. These include a noticeable number of microbes that process nitrogen compounds, increasing the pH of the surrounding water. This in turn favors the precipitation of the dissolved calcite

"It could be that the availability of organic carbon and nitrate, combined with the autotrophic activity of some bacteria, creates a microenvironment around the Hells Bells leading to a slow but massive precipitation of Calcite causes and ultimately the formation of formations, "the researchers speculate. This assumption is supported by some similarities of the underwater stalactites with stromatolites - pillow-shaped, layered rock formations, which also result from the use of microbes.

Remain puzzles

The origin details of the fascinating calcite formations in the Mexican underwater cave are still unclear. But it is already clear that the "hell bells" are quite unique: "The Hell Bells of the El Zapote Cave represent a mysterious ecosystem that allowed the formation of the largest known underwater speleotherms worldwide, " Stinnesbeck and his colleagues state. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2017; doi: 10.1016 / j.palaeo.2017.10.012)

(University of Heidelberg, 27.11.2017 - NPO)