Entwicklung einer neuen Präparationsmethode und Untersuchung verkieselter Mikrofossilien des Präkambriums mit Hilfe der Rasterkraft- und Elektronenmikroskopie
In this thesis nanoscopic structure analysis is applied to precambrian microscopic fossils permineralised and bodily conserved in finegrained chert rock. For the first time atomic force microscopy (AFM) is used to image and analyse the three dimensional fine structure of the walls of single fossil unicells. Structural AFM data is complemented and backed by transmission electron microscopy yielding information on the crystalline nature and chemical composition of the quartz and the kerogen components of the fossil cell walls. The data is supplemented by raman spectra of the same single fossils, providing information on the molecular constitution of the kerogen material, as well as by quantitative electron microprobe data on the carbon content of the fossil cell walls. For AFM experiments a new preparation method was devised, using hydrofluoric acid to expose single fossils buried in the rock matrix. The method is applied to intact conventional thin sections of chert without disturbing the fossil cells in their environment, thus maintaining the petrographic context of the sedimentary structure. As AFM conventionally images surfaces that are smooth on a micrometer scale, the etching process has to be precisely adjusted in order to achive a balance between exposing cell walls well enough for structural examination and retaining surfaces smooth enough for the atomic force microscope. The cell walls produced by the etch method introduced here protrude from the surrounding rock matrix by approximately two microns. A detailed study of the etching behaviour by macroscopic and AFM analysis provides information on the dissolution speed of whole samples and of specific sites within the fossilmatrix compound. It is shown that etch resistivity depends on the crystal size and the carbon concentration around and within the fossil. Subject to examination were 850 million year old cyanobacteria from the Bitter Springs Formation of the Northern Territoriy, Australia, and 650 million year old acritarchs from the Chichkan Formation of Kasachstan. A classical analysis of size distributions within single populations of the microorganisms by light microscopy classified the cyanobacteria as Myxococcoides minor Schopf 1968, and the acritarchs are probably single celled planctonic algae, belonging to the classes of Chlorophyceae and/or Rhodophyceae. However, as the true nature of precambrian microorganisms cannot be proven by light microscopy alone, the desire to gather more information on the fine structure of the cells arises. This task was adressed here. The analysis of the carbon concentration in the fossil cells shows that the main body of the cell wall in the acritarchs studied here is composed of quartz, and only about one percent of the total volume of the cell wall is made up of carbon. In two specimens the spacial distribution of kerogen within the cell walls was charted. In one case the fossil wall was composed of crystalline quartz lamellae, enveloped by a 30 nanometer thick carbonaceous membrane. In contrast, the other specimen showed totally non crystalline quartz in the fossil wall, with the carbon content distributed homogeneously throughout the cell wall, increasing along a concentration gradient towards the wall center. This difference between the two cells hints at the concept that the fossilisation process strongly influences the wall structure. However, in both cells the quartz and the kerogen is arranged in a regular tile structure that may be influenced, if not controlled, by the biological structure of the original organic cell. This was also observed in other cells. A detailed three dimensional analysis of the size and orientation of platelets composing the tile structure disclosed by etching forms the basis of this concept: In all analysed cells the platelets are oriented parallel to the radius of the cell. The average width of the platelets ranges from 260 to 330 nm, with a maximum error of 25%. A second set of smaller platelets was detected in two cells, showing widths ranging from 10 to 30 nm and occurring in quartz as well as in the carbon membrane. AFM images show that the smallest platelets have a polygonal shape and sit on or possibly comprise the bigger platelets. LaserRaman spectra showed that the kerogen in the fossils consists of polycyclic aromatic hydrocarbons forming a network of interlinked planar carbonaceous molecules. Taking the polygonal shape of the platelets into account, it seems plausible that the original organic cell substance recrystallised during fossilisation to build up this molecular network structure comprised in the platelet components of the fossils. If this happened in a way that conserved biological structures, these may be found in the fossil. A clue to this concept was found in the carbonaceous membrane mentioned earlier: A carbon structure was visualised in a cross section of the amorphous membrane which could possibly have been a basis for the fomation of the 10 to 30nm platelets, as both are of approximately the same dimension, the same orientation, and are adjacent to each other. It is conceivable that these carbon structures represent genuine nonrecrystallised biostructures. A fossilisation model is proposed that is based on the structure of the biological membrane (or cell wall) and the flux of silica solution during silicification.
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