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Unusual speleothems resembling giant mushrooms occur in Cueva Grande de Santa
Catalina, Cuba. Although these mineral buildups are considered a natural heritage, their
composition and formation mechanism remain poorly understood. Here we characterize
their morphology and mineralogy and present a model for their genesis. We propose that
the mushrooms, which are mainly comprised of calcite and aragonite, formed during four
different phases within an evolving cave environment. The stipe of the mushroom is an
assemblage of three well-known speleothems: a stalagmite surrounded by calcite rafts
that were subsequently encrusted by cave clouds (mammillaries). More peculiar is the
cap of the mushroom, which is morphologically similar to cerebroid stromatolites and
thrombolites of microbial origin occurring in marine environments. Scanning electron
microscopy (SEM) investigations of this last unit revealed the presence of fossilized
extracellular polymeric substances (EPS)—the constituents of biofilms and microbial
mats. These organic microstructures are mineralized with Ca-carbonate, suggesting that
the mushroom cap formed through a microbially-influenced mineralization process. The
existence of cerebroid Ca-carbonate buildups forming in dark caves (i.e., in the absence
of phototrophs) has interesting implications for the study of fossil microbialites preserved
in ancient rocks, which are today considered as one of the earliest evidence for life on
Earth.
Stromatolite are lithified, laminated, organosedimentary deposits. Preliminary examination of eight cenote lakes near Mt. Gambier has revealed the presence of tens of thousands of actively - forming stromatolites. Based on the external morphology, 14 different types of stromatolites have been identified, columnar growth forms are most common. Three genus of Diatom and three genus of Cyanobacteria are the most likely responsible for stromatolite development.
The cenotes near Mt Gambier are circular, cliffed, collapse dolines containing water-table lakes up to 125 m deep, floored by large rubble cones. They lie in a flat, coastal plain composed of mid-Tertiary limestone. Most of the deepest cenotes are concentrated in two small areas located along trends sub-parallel to the main joint direction in the limestone. The cenotes do not connect to underwater phreatic passages, and water chemistry data confirm that they are not part of an interconnected karst network. They formed by collapse into large chambers (up to > 1 million m3) that extended 125 m or more below the land surface. Several cenotes have actively growing stromatolites on the sub-vertical walls that started growing at 8000 years BP.
The caves that collapsed to form the deep Mt Gambier cenotes are much larger than shallow and deep phreatic caves in the area, and do not connect into deep phreatic systems. They were not formed by freshwater/seawater mixing, responsible for many of the well-known Yucatan cenotes, because they are not associated with locations of the mixing zone during previous high sea levels, and are much larger than caves presently forming along the mixing zone near Mt Gambier. Instead dissolution was most likely due to a process whereby acidified groundwater containing large amounts of volcanogenic CO2 ascended up fractures from the magma chambers that fed the Pleistocene–Holocene volcanic eruptions in the area; deep reservoirs of volcanogenic CO2 occur nearby.
Cave dissolution could have been due to release of CO2 during the Mt Gambier eruption 28,000 years ago, followed by collapse to form cenotes during the low sea levels of the Last Glacial Maximum 20,000 years ago. The cenotes then flooded 8000 years ago as sea level rose, and stromatolites began to grow on the walls.