Life inhabits rocks: clues to rock erosion from electron microscopy of pisolite at a UNESCO heritage site in Brazil |
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Authors: | Kazue Tazaki Ryuji Asada Zara Gerhardt Lindenmayer Tatsuya Shirotori Juliana Missiaggia Vargas Carlos Henrique Nowatzki Osmar Wöhl Coelho |
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Institution: | (1) Department of Earth Sciences, Kanazawa University, Kanazawa Ishikawa, 920-1192, Japan;(2) Programa de Pos-Graduacao em Geologia, Ciencias Exatas e Tecnologicas, Universidade do Vale do Rio dos Sinos-UNISINOS, Av. Unisinos, 950, CEP 93022-000 Sao Leopoldo, Rio Grande do Sul, Brazil;(3) Curso de Graduacao em Geologia. CNPq-PIBIC. Ciencias Exatas e Tecnologicas, Universidade do Vale do Rio dos Sinos-UNISINOS, Av. Unisinos, 950, CEP 93022-000 Sao Leopoldo, Rio Grande do Sul, Brazil;(4) Nucleo de Estudos e Pesquisas em Geologia Arqueologica—NEPGEA, Rua Dr. Armando Petrelli, 1113, CEP 88390-000 Barra Velha, Santa Catarina, Brazil |
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Abstract: | Rock erosion is attracting increasing attention from scientists worldwide. The area encompassing the Saint John Baptist Church,
Saint John Village, XVII century ruins in Rio Grande do Sul at the UNESCO World Heritage Site is considered a Brazilian treasure.
However, the risk of damage to this site from rock erosion has recently increased tremendously. Generally, the rocky construction
such as fence, wall and tomb stone, seems strong but is actually extremely sensitive to erosion caused by lichens, fungi,
molds and bacteria. Because of biological erosion and massive exposure, the fresh rock is dominated by clays and microorganisms.
Water-adsorbing clays and microorganisms influence the mechanisms of the rock erosion. In this study, the formation of bio-clay-minerals
in porous structure of pisolite was demonstrated using electron microscopy. Bacterial clay mineralization can deform the rock
structure and even produce organic materials. Biological activity could easily corrode rocky constructions around the Saint
John Baptist Church site. The rocks are pisolitic laterites possibly formed in Tertiary over the Kretaceous Parana flood Basalts.
Samples inhabited by lichens and fungi were collected from a collapsed wall in the ancient church. The zonal reddish-brown
pisolites are 4 mm in diameter in a matrix of clays associated with porous and empty spaces. Elemental distribution maps from
X-ray fluorescence microscopy show iron-rich spherules of pisolite, whereas the matrix is composed of Al, Si, Mn, and Sr;
thus producing goethite and kaolinite. Transmission electron microscopic observation showed that various types of bacteria
inhabit the spherule and are associated with clay minerals and graphite. STEM elemental analysis confirmed the bio-clay-mineralization
with Al, Si, S, and Fe, around bacterial cells. The results presented here will improve our understanding of nm-scale bio-mineralization
and bio-erosion in lateritic rocks.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Lichen Bacteria Laterite Rock erosion Pisolite Microscopy Bio-mineralization |
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