Subaqueous speleothems (Hells Bells) formed by the interplay of pelagic redoxcline biogeochemistry and specific hydraulic conditions in the El Zapote sinkhole, Yucatán Peninsula, Mexico

Abstract

<p><strong>Abstract.</strong> Unique bell-shaped underwater speleothems were recently reported from the deep (~ 55 m) meromictic El Zapote sinkhole (cenote) on the Yucatán Peninsula, Mexico. The local diving community has termed these speleothems as Hells Bells because of their shape and appearance in a lightless environment in ~ 28–38 m water depth above a sulfidic halocline. It was also suggested that Hells Bells form under water, yet the mystery of their formation remained unresolved. Therefore, we conducted detailed hydrogeochemical and geochemical analyses of the water column and Hells Bells speleothems including stable carbon isotopes. Based on the comprehensive results presented in this study we deduce that both, biogeochemical processes in the pelagic redoxcline and a dynamic halocline elevation of El Zapote cenote, are essential for Hells Bells formation. Hells Bells most likely form in the redoxcline, a narrow 1–2 m thick water layer immediately above the halocline where a pelagic chemolithoautotrophic microbial community thrives from the upward diffusion of reduced carbon, nitrogen and sulfur species released from organic matter degradation in organic-rich debris. We hypothesize that chemolithoautotrophy, in particular the proton consuming nitrate-driven anaerobic sulfide oxidation, favors calcite precipitation in the redoxcline and hence Hells Bells formation. A dynamic elevation of the halocline as a hydraulic response to recharge events, e.g. hurricanes, is further discussed, which might explain the shape of Hells Bells as well as their occurrence over a range of 10 m water depth. Finally, we infer apparent prerequisites for Hells Bells formation considering the exclusivity of these underwater speleothems to only a few cenotes of a restricted area of the northeastern Yucatán Peninsula.</p>

Microbially promoted calcite precipitation in the pelagic redoxcline: Elucidating the formation of the turbid layer

“…Previous studies proposed the redoxcline, which overlaps with the turbid layer, as the water layer in which calcite is precipitated (Ritter, 2020; Ritter et al, 2019). Here, we conducted a specific study that targeted the hydrogeochemical profile of the turbid layer at the time of microbiome sampling.…”

“…However, this hypothesis is solely based on the observation of structures in the cave ceiling that were interpreted as bubble trails. Prior to that, Ritter et al (2019) reported detailed hydrogeochemical profiles, chemical and optical analyses on Hells Bells carbonate speleothems and SEM analyses on filtrates of the turbid layer of El Zapote. They showed that Hells Bells most likely grow in the lowermost freshwater body as indicated by uranium isotopes of Hells Bells carbonate samples and the comparison of Mg/Ca ratios of the water body and Hells Bells carbonates.…”

“…They termed these extraordinary structures as Hells Bells due to their appearance in darkness and the vicinity to a sulfidic halocline. Hells Bells are characterized by their bell‐ or cone‐like shape and downward extension with a hollow interior reaching sizes of up to ~4 m. Hells Bells are built up by alternating layers of micritic and sparry or dog‐tooth‐like calcites with crystal sizes of up to several mm, which indicate inorganic precipitation (López‐Martínez et al, 2020; Ritter et al, 2019; Stinnesbeck et al, 2018). They apparently grow underwater (Stinnesbeck et al, 2018) in an otherwise carbonate dissolving karstic aquifer environment, where the water masses are generally only saturated or even undersaturated with respect to calcite.…”

“…The redoxcline overlaps with a distinct milky, white horizontal cloud, the turbid layer, into which some Hells Bells partly reach or hang completely inside (Figure 1b–e). Based on these results, a biologically induced authigenic calcite precipitation within the turbid layer was hypothesized (Ritter et al, 2019). This would imply a chemolithoautotrophic community that promotes basification via CO 2 fixation and the catalysis of a nitrate‐dependent incomplete sulfide oxidation to zero‐valent sulfur.…”

“…This would imply a chemolithoautotrophic community that promotes basification via CO 2 fixation and the catalysis of a nitrate‐dependent incomplete sulfide oxidation to zero‐valent sulfur. The permanent fixation of this calcite in Hells Bells over a wide vertical range of ~10 m water depth might be the result of a variable elevation of the halocline and with it the redoxcline and thus the zone of calcite precipitation (Ritter, 2020; Ritter et al, 2019). Also, such a repeated elevation of the calcite dissolving halocline could explain the observed internal layered structure of Hells Bells speleothems, which most likely represents repeated phases of calcite growth and dissolution.…”