Life inhabits rocks: clues to rock erosion from electron microscopy of pisolite at a UNESCO heritage site in Brazil

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.

Degradação de gnaisse e granito em fachadas de edifícios históricos no centro do Rio de Janeiro

“…Sterfl inger e Pinar (2013) sugeriram que os fungos poderiam ser os microrganismos mais importantes envolvidos na degradação da pedra, mas também é sabido que as cianobactérias podem penetrar e crescer nas rochas (Gaylarde et al, 2012), obviamente causando danos � sicos apenas pela presença deles. No entanto, a capacidade das cianobactérias de solubilizar metabolicamente e depois permi� r ou causar a� vamente a redefi nição e realocação de minerais está bem documentada em crostas de sal (Canfora et al, 2016), bem como em edi� cios de arenito e itacuru, um � po de rocha rica em ferro usada nas missões jesuítas na América do Sul (Tazaki et al, 2009;Barrionuevo et al, 2016). As bainhas gela� nosas de cianobactérias fi lamentosas e esféricas são consideradas locais de nucleação de depósitos de cálcio (Gerbersdorf e Wieprecht, 2015), e as cianobactérias detectadas nas amostras deste estudo também poderiam ser, pelo menos em parte, responsáveis pelos abundantes cristais de calcita encontrados, assim como os fungos fi lamentosos (Bindschedler et al, 2016).…”

Atomic spectrometry update. X-Ray fluorescence spectrometry

“…XRF was one of a range of techniques applied and results showed that although a few trace elements were relatively immobile, many major and trace elements (particularly Ca and Na) were mobile from the beginning of granite weathering, effects that were associated with the breakdown of plagioclase and dissolution of biotite. A related form of degradation that leads to rock erosion is the effect of lichens, fungi, molds and bacteria on rock surfaces, a phenomenon that was studied by Tazaki et al 193 at the UNESCO World Heritage Site of Saint John Baptist Church, Rio Grande do Sul, Brazil. XRF was again one of the techniques (the others were TEM and STEM) used to study samples from a collapsed wall in the ancient church and showed that the pistolitic laterite surface was inhabited by lichens and fungi that effectively formed a bio-claymineralised surface dominated by the minerals goethite and kaolinite.…”

Iron-Coated Particles from Condensed Aalenian-Bajocian Deposits: Evolutionary Model (Iberian Basin, Spain)

“…Given the lack of evidence for nearby lateritic or plinthitic soils during this period, coeval Iberian volcanism is the most plausible source of iron. (Jeans et al 2000); 3 5 Indonesian iron ooids (Sturesson et al 2000); 4 5 Ejpovice iron ooids and volcanic rocks (Sturesson 2003); 5 5 Oxfordian Betic plinthitic iron ooids type A (Reolid et al 2008); 6 5 Sta Augusta laterites (Morey and Setterholm 1997); 7 5 Brazilian pisoids (Tazaki et al 2009); 8 5 Chondrite (Taylor and McLennan 1985); 9 5 PAAS (Taylor and McLennan 1985).…”