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| Highlights |
Jocelynn
Johnson
Research Training Program, 2003
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A Microchemical Investigation of Fossilized Wood: Biological Preservation and the Influence of Mineralization Introduction The
degree of structural change in fossilized wood may be directly linked
to the minerals that replace the cellular structure. With improved knowledge
of the amount of biological structure that can be preserved in the process
of petrifaction of easily recognized organisms, the chances of identifying
microorganisms in other geological samples increase. As the project
deals with biological organisms, and minerals, four things were taken
into consideration: species of plant, degree of decay, minerals present,
as well as the most suitable imaging technique(s) for each specimen.
The process of petrifaction has several criteria that must be met before wood can be preserved.
Petrifaction occurs as dissolved minerals react with the carbonic acid, lignin and carbohydrates that are released as wood decays, and crystallize to replace the structures and tissues (permineralization). Alternatively, the minerals will crystallize in pore spaces, without preserving the tissues (replacement).
Determination of the amount of structure preserved, and by which minerals, was accomplished through the use of seven imaging techniques. One representative area was selected from each sample and each of the eight techniques was applied to this area. Light-based techniques include reflected and transmitted light microscopy, polarized light microscopy. Electron-based techniques include: back scattered electron imaging and x-ray mapping using the scanning electron microscope, charge-contrast imaging using the environmental scanning electron microscope, and catholuminescence imaging. Samples A and D were omitted from the charge-contrast imaging as the specimens did not produce usable information.
Nine minerals were identified in the six specimens. One specimen was monomineralic, with opal as the dominant mineral, and the remaining five specimens had one dominant mineral, as well as up to four additional accessory minerals. The dominant minerals were chalcedony (SiO2), opal (SiO2•nH2O), hematite (FeO2), dolomite (CaMg(CO3)2), limonite (FeOH• nH2O), and apatite (Ca5(PO4)3(OH,F,Cl), and may also occur as accessory minerals. The remaining three minerals occurring only as accessory minerals were pyrite (FeS2), calcite (CaCO3), and barite (BaSO4). The presence of barite in four samples is an oddity as it is a relatively rare mineral, and it’s occurrence may indicate the presence of limestone weathering or hydrothermal veins in the vicinity of the petrified wood deposit. Metallic aluminum found throughout the sections is thought to be an artifact of sample preparation. Biological Structures After
being preserved for millions of years, two of the six samples can be
definitely identified to their species; however, the overall preservation
of biological structures within the remaining specimens is very good.
Several notable features can be identified throughout these samples
in the photographs to the left, such as: 1.
Trachetic cells It is worth noting that some structures which are clearly visible in one imaging technique may be completely obscured in another, this validates the need for multiple imaging methods when dealing with complex specimens, such as petrified woods.
When examining petrified woods, four things must be taken into consideration: the amount of decay in a specimen, the species or group, and the type of petrifaction as well as the minerals present. Best
preservation: Sample F and A Acknowledgements: Thanks to Dr. Harry Alden of The Smithsonian Centre for Materials Research and Education, the staff of the Smithsonian National Museum of Natural History; Dr. Amelia Logan, Mr. Tim Rose, and Mr. Tim Gooding of the Department of Mineral Sciences; Dr. Peter Heaney of Penn State University, and the Smithsonian Women’s Committee for their endowment, which made this project possible. Want to ask a question? Visit the Message Board Virtual
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