『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 lichen, 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 electron analysis confirmed the
bio-clay-mineralization with Al, Si, S, and Fe, around bacterial
cells. The results presented here will improve our understanding
of mm-scale bio-mineralization and bio-erosion in lateritic rocks.
Keywords: Lichen; Bacteria; Laterite; Rock erosion; Pisolite;
Microscopy; Bio-mineralization』
Introduction
Geological setting
Materials and methods
Results
Discussion
Conclusions
Acknowledgments
References