Geoscience Reference
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a
b
10 μm
10 μm
c
d
e
f
g
h
3D Raman
1.5 μm deep
2.25 μm deep
3.0 μm deep
3.75 μm deep
4.25 μm deep
10 μm
10 μm
10 μm
10 μm
10 μm
10 μm
Figure 6.5. One of the Apex Chert fossils shown as an optical microscopic image on the
upper left (a). Panel (c) shows a 3-D raman image of the area in panel (a) within the rect-
angular box. Panels (d-h) show 2-D raman images in different fields of focus. the upper
right image (b) shows the fossils out of the focal plane but highlights that the fossils are not
formed around quartz grain as shown by the arrows. From schopf and kudryavtsev (2012).
reproduced with permission.
chapter 2, there is also evidence for specific microbial metabolisms such
as sulfate reduction and methanogenesis at that time.
So, another potentially promising insight into the antiquity of cyano-
bacteria bites the dust. hat now? Is there anything else we might be
able to look at? There are, in fact, other kinds of fossils that might help.
hen we think of fossils, we normally think of bones or shells, or in the
case of microbes, the cells themselves. However, the cells, and particu-
larly the cell membranes, also contain compounds that can be preserved
in rocks. The cell structures may be gone, but some of their chemical
constituents may persist in chemical forms known as biomarkers. Some
of these biomarkers can be distinctive of certain types of organisms.
This sounds pretty promising, but there are some significant issues to
consider. One problem is that in really old rocks, as we are considering
now, one can only expect to find miniscule amounts of biomarker mol-
ecules. Even in the best cases, these rocks have been heated sufficiently
to transform most recognizable biomarkers into unrecognizable forms.
Another problem, which is made doubly important because of the low
amounts of biomarkers in the rock, is contamination by younger mate-
rial, and this contamination can come from nearly everywhere.
The best rocks for this kind of study are obtained by drilling for fresh
material buried under the surface of Earth's surface. However, normal
drilling usually involves petroleum lubricants. Talk about contamina-
 
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