Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs

Abstract

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.

Coastal uplift rate at Matanzas (Cuba) inferred from MIS5e phreatic overgrowths on speleothems

“…Externally, the POS is granular with a cerebroid morphology, while the inner portion is characterised by a porous texture with irregular, elongated fenestrae (Figure a). In thin section, sparry‐calcite aggregates (Figure b) are connected by calcite branches, creating a very porous structure, with fenestrae mainly filled by needle‐like calcite crystals (Figure c; Bontognali et al., ). Microstructures with an elemental composition richer in C, Mg and Si with respect to the surrounding Ca‐carbonate occur in the external component of the POS and have been interpreted as fossilised extracellular polymeric substances and microbial biomass (Bontognali et al., ).…”

“…δ 13 C and δ 18 O stable isotope values measured from the mushroom cap core (SC2.4) are reported in Figure and Table (for methods, see Bontognali et al. ()). Locations where the powders from the POS sample were drilled are shown in Figure a.…”

“…All these speleothems occur together in the upper level of the cave and are no longer active, except for some vadose recent stalactites and stalagmites. Albeit many of these speleothems typically indicate a thermal origin, detailed studies indicate that these types of concretions can form by degassing and/or evaporation in non‐thermal shallow phreatic conditions (Bontognali et al., ; D'Angeli, De Waele et al., ; Ginés et al., ; Pomar, Ginés & Ginés, ). The fact that typical phreatic speleothems, such as mammillary calcite, appear in close relationship with vadose ones (i.e.…”

“…Similar POS, sometimes resembling Santa Catalina's mushroom‐shaped speleothems (commonly called “mushrooms”), have been found in caves in the Caguanes National Park, 250 km east of Matanzas on the north coast of central Cuba (Núñez Jiménez, ). Earlier investigations of the POS in Santa Catalina Cave have suggested that microbial activity and a fluctuating water level are the key factors necessary for their formation (Bontognali et al., ). POS occur in a limited altitudinal range of around 40 cm at 16 m asl in the uppermost level of the cave, and form the mushroom caps, reaching diameters of over 1 metre, and also bench‐like encrustations similar to cave shelfstone (here called balconies) along the walls of the same rooms (Figure ).…”

Speleothems in a north Cuban cave register sea‐level changes and Pleistocene uplift rates

“…The 'mushrooms' represent a composite structure and can be considered as the most significant speleothem of Santa Catalina Cave, being the result of an assemblage of both continental and rare exotic speleothems. A detailed analysis allowed us to recognize several stages of formation: an inner stalagmite is surrounded by a raft cone structure, subsequently covered by mammillary calcite formations, on which calcite rafts and POS deposited forming the mushroom caps (Bontognali et al, 2016), having the lower surface characterized by folia (D'Angeli et al, 2015) (Figures 1 and 2). Seven samples of POS (16 m apsl) were dated demonstrating the presence of standing water pools during MIS 5e.…”