Avances de la Geomorfología en España 2012-2014

Susanne Schnabel y Álvaro Gómez Gutiérrez (editores)

1

Avances de la Geomorfología en España 2012-2014

Editores: Susanne Schnabel y Álvaro Gómez Gutiérrez

XIII Reunión Nacional de Geomorfología, Cáceres, 2014.

ISBN: 978-84-617-1123-9

XIII Reunión Nacional de Geomorfología, Cáceres 2014

Relieves Graníticos y Cársticos 498

SPELEOTHEMS IN MAGMATIC ROCK CAVES AND ASSOCIATED

MICROORGANISMS

Espeleotemas en cuevas de rocas magmáticas y microorganismos asociados. J. R. Vidal Romaní1,2, L. González López,1,2, M.J. López Galindo1, J. Sanjurjo Sánchez1,2 y

M. Vaqueiro Rodríguez1,2

1 Instituto Universitario de Geología. Campus de Elviña. Edificio SAI,-3. 15071 Coruña. juan.vidal.romani@udc.es

2 Club de Espeleoloxía “A Trapa”(CETRA).

Abstract: At first, these speleothems had been considered to be caused by the rock weathering s.l. originated by water, but later scanning electron microscopy showed their direct relation with the troglobiontic activity, hence the name of biospeleothems. They have three types of components: inorganic, due to physicochemical disaggregation of the rock by water; biological, formed by the troglobionts incorporated in the sediment; and biomineral, authigenic minerals related to the reaction of the mineral substratum with the metabolic products derived from the organic activity. These speleothems are micro ecosystems where bacteria, fungi, algae, testate amoebae, mites, collembolan and arachnids live, develop and die. They form a trophic net where bacteria, which disaggregate the minerals and the organic matter from the rest of microorganisms, prevail. Neither the cave’s lithology nor the climatic zone where the cave is located affects the troglobionts as to the dimensions or the chemical-mineralogical composition of the speleothems.

Palabras clave: bioespeleotemas, rocas magmáticas, granito, bacterias, ópalo-A. Key words: biospeleothems, magmatic rocks, granite, bacteria, opal-A. 1. INTRODUCTION

The study of speleothems associated with cavities developed in magmatic rocks (effusive or intrusive) poses great difficulties due to the dimensions of the deposits almost exclusively formed by organic remains and amorphous minerals, which requires to be mostly studied by scanning electron microscopy (S.E.M.). Other difficulties are due to the scarce knowledge of the relationship between the genesis of the deposits and the organic activity developed in the underground environment, which has shown to be essential in the final morphology of the speleothems. The conditioning factors in the formation of these deposits are the Si availability and its association with water flows that move very slowly by capillarity and/or superficial tension because the gravity (dripping) is little important in the sedimentary process. In these environments, the sedimentation is related to the reduction-detention of the water contribution to the underground system, which

will cause the water evaporation, the immediate deposition of the charge transported in dissolution or dragging, and the death, or the development of resistant forms of the microorganisms associated with these deposits. Obviously, the biological activity in the underground system will act during the wet stage, thus the main phase in the speleothem development is directly linked to this organic activity. When the water disappears from the underground environment, the organic activity stops and the microorganisms change from being direct elements in the speleothem development to elements (passive) or fabric of the sediment, or simply acting as physical support of the charge transported by the water in dissolution or dragging. The sedimentary, mineralogical, morphological features of these deposits as well as the organisms that live with them are uncommonly uniform in the deposits of these types described in all the climatic zones of the Earth (Fig. 1).

XIII Reunión Nacional de Geomorfología, Cáceres 2014

Relieves Graníticos y Cársticos 499

Fig. 1. Distribución mundial, en gris, de afloramientos de

rocas magmáticas con localización de las cavidades estudiadas y su relación con las zonas climáticas. Los

triángulos negros señalan los puntos de muestreo correspondientes a este trabajo.

Fig. 1. World distribution, in gray, of magmatic rocks outcrops where the studied cavities are located and their

relation with climatic zones. The black triangles mark the sampling points corresponding to this work.

1.1.Types of speleothems.

There are two main speleothems regardless their location at the ceiling, on the walls or bottom of the cavity. A simple classification has been made only based on the morphology: (1) planar and (2) cylindrical. The first is formed from the laminar water flow of little volume and low velocity, which causes the water film to be subdivided by the previous rock weathering. This distribution of clasts in the perimeter of the water drops originates a cell structure (microgour fields) of shuffle size, form and distribution (ceiling speleothems) or with preferential orientation, when there is some water flow that produces a preferential enlargement of the microgours (bottom or wall speleothems) along with the movement direction. In these types of deposits, the interaction with life is of minor importance, at most they are used as physical base by bacteria, algae, collembolan and mites for their activity and always showing scarce morphology. The second type of deposit, cylindrical speleothem, originates the development of lineal forms with different thicknesses which may grow individually or in groups and to any direction. In the literature, they are called, usually and wrongly, stalactites and stalagmites because of their morphological similarity to the speleothems of karstic systems s.s., though these terms must be disregarded as in the cavities of magmatic rocks the dripping is not

an important process and their forms are due to the interaction with microorganisms (bacteria). The cylindrical ones are microbial mats that receive the water contribution through capillary channels or by simple superficial condensation. Their internal texture allows assimilating their growth with a process similar to the one proposed for the stromatolites, though at smaller scale, hence the name of terrestrial micro stromatolites may be more adequate than the one normally used. Other microorganisms of greater size are frequently observed, apart from bacteria, associated with these deposits: algae (diatoms), testate amoebae, collembolan, mites, etc., which use the biomineral substratum as physical settlement base or for their occasional movement.

Fig. 2. Estructura de un microestromatolito terrestre. (A Trapa, Pontevedra, España)

Fig. 2. Internal structure of a terrestrial micro stromatolite. (A Trapa, Pontevedra, España)

1.2. Microorganisms associated with

speleothems

According to all these data, speleothems of granite cavities may be considered as a micro ecosystem where a varied association of microorganisms: bacteria, algae, fungi, testate amoebae, mites, collembolan, arachnids, etc. live, develop and die. The study of these types of ecosystems (Vidal Romaní and Vilaplana 1984; Kashima, Irie and Kinoshita 1987; Vidal Romaní et al. 2010) has allowed us to show the relationship between the troglobiontic activity and the formation of speleothems, justifying so the name of biospeleothems (Forti 2001; Vidal Romaní et al. 2010). In these deposits, three types of

XIII Reunión Nacional de Geomorfología, Cáceres 2014

Relieves Graníticos y Cársticos 500

components are differentiated in agreement with the relation with the microbiological activity: (1) inorganic due to the granular disaggregation of the rock (detritic fraction) caused by the physical or chemical action of the water, (2) biological corresponding to microorganisms which live in the underground environment and incorporate into the sediment totally or partially, and (3) biomineral (Westall and Cavalazzi, 2011) formed by the direct or indirect interaction between the inorganic mineral substratum and the metabolic products generated by troglobionts which increase the reaction capacity of the infiltrated water, accelerating the dissolution of pre-existing minerals, even the most stable ones like quartz, and forming new ones (authigenic minerals) (Fig. 2).

Fig. 2. Nanoesferas del biomineral Opal-A, Ávila, Spain.

Fig. 2. Nanospheres of Opal-A, Ávila, Spain.

In the magmatic rocks caves, Si is the essential element of some biominerals (e.g. the biogenic silicon which forms the frustules of the diatoms or the plates of the testate amoebae (Fig. 3), common organisms in these environments, but also there are other biominerals (gypsum, anhydrite) whose relation with the troglobios is less direct. Once the three components in dissolution, suspension or dragged have been incorporated into the infiltration water, the sedimentation is produced when the water contribution decreases or stops prevailing its evaporation.

Fig. 3. Placas de una ameba testácea Corythion sp.

Integrándose en el sustrato. Cueva “Castelo da Furna”, Portugal.

Fig. 3. Plates of testate amoeba Corythion sp. integrating into the substratum. “Castelo da Furna” Cave, Portugal.

2. DISCUSSION AND CONCLUSIONS

It is clear that the relationship between the morphology of the speleothems and the organic activity is different depending on the type of implied organism. Bacteria are undoubtedly the microorganisms with greater influence on the development of speleothems (Fig. 4), and especially of the cylindrical ones because their growth causes filament framework or even tree forms which, once the organisms die or remain latent, will be used as base for the sedimentation of the silicon dissolved in water that will cover the framework formed by bacteria.

Fig. 4. Recubrimiento por bacterias filamentosas de la

superficie de un espeleotema. Castelo da Furna, Norte de Portugal.

Fig. 4. Cover of filamentous bacteria of the surface of a speleothem. Castelo da Furna, Norte de Portugal.

XIII Reunión Nacional de Geomorfología, Cáceres 2014

Relieves Graníticos y Cársticos 501

In the planar speleothems, the detritic component prevails, and though it is usual that all the microorganisms existing in the environment use such component as physical base (diatoms, testate amoebae, collembolan) for their activities, only mites develop constructive activity such as the excavation of refuges and nests (Figs. 5 and 6).

Fig. 5. Ácaro excavando su refugio en la superficie de una colada. Tcharkulda, Western Australia.

Fig. 5. Mite excavating its cache on the surface of a flowstone. Tcharkulda, Western Australia.

Fig. 6. Colonia de nidos de ácaros excavadas en un micro gour de una colada . Las Jaras, Córdoba, Spain.

Fig. 6. Nesting colonies of mites on the surface of a flowstone microgour. Las Jaras, Córdoba, Spain.

In both types of speleothems, the death of microorganisms may produce fragmentation of their shells, which are incorporated as fabric element to the sediment though, due to their size and scarce abundance, they have a secondary role in the growth and final volume of the speleothem.

Acknowledgments

Ana Martelli translated the text into English and revised the layout of this paper. REFERENCES

Caldcleugh A. 1829. On the geology of Rio de

Janeiro. Transactions of the Geological Society, 2: 69-72.

Forti P. 2001. Biogenic speleothems: an overview. International Journal of Speleology, 30A(1/4), 39-56.

Kashima N., Irie T., Kinoshita N. 1987. Diatom contributions of coralloid speleothems, from Togawa-Sakaidani-do Cave in Miyazaki Prefecture, Central Kyushu, Japan. International Journal of Speleology, 1, 95-100.

González López L., Vidal Romaní J. R., López Galindo M.J., Vaqueiro Rodríguez M., Sanjurjo Sánchez J. 2013. First data on testate amoebae in speleothems of caves in igneous rocks. Cadernos do Laboratorio Xeolóxico de Laxe, 37, 37-56.

Vidal Romaní J. R., Vilaplana J. M. 1984. Datos preliminares para el estudio de espeleotemas en cavidades graníticas. Cadernos do Laboratorio Xeolóxico de Laxe, 7, 305-324.

Vidal Romaní J.R., Grajal M., Vilaplana J.M., Rodríguez R., Macias F., Fernández S., Hernández Pacheco E. 1979. Procesos actuales: micromodelado en el granito de Monte Louro, Galicia España (Proyecto Louro). Actas IV Reunión Grupo Español de Trabajo del Cuaternario, Banyoles (España), 246-266.

Vidal Romaní J.R., Sanjurjo J., Vaqueiro, M., Fernández Mosquera D. 2010. Speleothem development and biological activity in granite cavities. Geomorphologie: relief, processus, environment, 4, 337-346.

Westall, F., Cavalazzi, B. 2011. Biosignatures in Rocks (In Encyclopedia of Geobiology Ed. Reitner, Joachim and Thiel, Volker). pp.189-201. Springer Netherlands.