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.