may have contained are either unrecognisable or highly
questionable. Nevertheless, there are a few locations,
such as Pilbara (northwestern Australia) and Barberton
(eastern South Africa) dating from the period 3.5 to 3.2 Ga
containing exceptionally well-preserved rocks that can
be used for studying the primitive Earth. This means that
the record of early life is limited to these two ancient
terranes.
4.2.1 How Do Microorganisms Fossilise?
Microorganisms rapidly degrade in the natural environment
and, in order to be preserved as fossils, they need to be
very rapidly encrusted and replaced by a mineral (Westall
and Cavalazzi 2011). This happens where a particular mineral is supersaturated, the organic microorganisms acting as
nuclei for the precipitation of the mineral. Polymerisation
and dehydration of the mineral takes place, and, in anaerobic
environments, such as the early Earth or present-day anoxic
environments in the deep sea or lakes, the organic molecules
can become trapped in the mineral matrix. In oxidising
environments, for instance, in shallow water surface
sediments or hot springs, the organic matter is completely
oxidised and all that is left of the microorganism is the
mineral crust (and sometimes mould or cast). Further
processing is necessary to complete the preservation of the
encrusted microorganisms in the form of lithification
(cementation – again by another mineral) of the sedimentary
or rocky microbial hosts. The lithified rocks then need to
survive geological processing that includes plate tectonic
recycling, metamorphism and erosion.
4.2.2 Microorganisms Perform Isotopic
Discrimination
Extant living organisms preferentially use lighter isotopes:
12 C >
13 C,
1 H >
2 H,
14 N >
15 N,
32 S >
34 S,
54 Fe >
56 Fe
(Box 4.1). Thus, a change in the ratio of the ‘heavy’ isotope
to the ‘light’ isotope of an element suggests the presence of
different metabolic activities in ancient sediments. For
example, autotrophic activity will result in an enrichment
of
12 C atoms compared to
13 C atoms; an enrichment of
32 S is
evidence of sulphate-reducing activity, while
54 Fe enrichment indicates a capacity to respire iron. Note, however, that
some caution is needed in the use of isotopic ratios because it
has been shown that abiotic processes can also produce
isotopic ratios that overlap with those generated by
biological fractionation.
Box 4.1: Stable Isotopes and Isotopic Fractionation
Vincent Grossi
A chemical element X is characterised by its atomic
number Z (equal to the number of protons in the
nucleus) and its atomic weight A (equal to the number
of protons and neutrons). It is represented as
A
Z X (e.g.
12
6 C). Isotopes are atoms of the same element with
differing atomic masses (e.g. carbon-12, carbon-13
and carbon-14). That is, they have the same number
of protons but different numbers of neutrons. Of the 92
existing natural chemical elements, 71 have multiple
isotopes. Some isotopes are stable (e.g. carbon-13
or
13 C), while others are unstable and radioactive
(e.g. carbon-14 or
14 C) and disintegrate over time
into radiogenic isotopes (radionuclides).
Among the stable isotopes of an element, the
lightest is the most abundant (e.g.
12 C ¼ 98.9 %,
13 C ¼ 1.1 %;
16 O ¼ 99.76 %,
17 O ¼ 0.04 %,
18 O ¼ 0.20 %) and tends to be ‘used’ (or reacts)
more easily (or faster) than the heavier isotopes.
So-called isotopic fractionation is the discrimination
between the light and a heavy isotopes of an element
that occurs during (bio)chemical reactions and
physico-chemical processes. This fractionation leads
to a very small (on the order of ‰) but significant
change in the ratio of heavy to light isotopes (e.g.
13 C/
12 C,
18 O/
16 O,
2 H/
1 H or D/H) between a reagent
(e.g. CO 2 fixed by phototrophic organisms) and its
transformation products (e.g. organic compounds
formed from the CO 2 fixed by phototrophic
organisms).
The heavy/light isotopic ratio of an element in a
sample is determined using an isotope ratio mass
spectrometer (IRMS), which measures this ratio in
a purified gas (e.g. CO 2 , H 2 , N 2 ) produced from the
sample (originally solid, liquid or gas), and compares
it with that of a reference gas produced from an international (or universal) standard. These standards
exist for each element (C, H, O, N, S) commonly
measured by IRMS; for example, the standard for
carbon is a belemnite (calcite) from the Pee Dee
Formation (so-called PDB) with a
13 C/
12 C ratio
¼ 0.01124 and that for nitrogen is atmospheric
V. Grossi
Laboratoire de Ge ´ologie de Lyon: Terre, Plane `tes,
Environnement, UMR CNRS 5276, Universite ´ Claude Bernard
Lyon 1, 69622 Villeurbanne Cedex, France
(continued)
86
J.-C. Bertrand et al.
questionable. Nevertheless, there are a few locations,
such as Pilbara (northwestern Australia) and Barberton
(eastern South Africa) dating from the period 3.5 to 3.2 Ga
containing exceptionally well-preserved rocks that can
be used for studying the primitive Earth. This means that
the record of early life is limited to these two ancient
terranes.
4.2.1 How Do Microorganisms Fossilise?
Microorganisms rapidly degrade in the natural environment
and, in order to be preserved as fossils, they need to be
very rapidly encrusted and replaced by a mineral (Westall
and Cavalazzi 2011). This happens where a particular mineral is supersaturated, the organic microorganisms acting as
nuclei for the precipitation of the mineral. Polymerisation
and dehydration of the mineral takes place, and, in anaerobic
environments, such as the early Earth or present-day anoxic
environments in the deep sea or lakes, the organic molecules
can become trapped in the mineral matrix. In oxidising
environments, for instance, in shallow water surface
sediments or hot springs, the organic matter is completely
oxidised and all that is left of the microorganism is the
mineral crust (and sometimes mould or cast). Further
processing is necessary to complete the preservation of the
encrusted microorganisms in the form of lithification
(cementation – again by another mineral) of the sedimentary
or rocky microbial hosts. The lithified rocks then need to
survive geological processing that includes plate tectonic
recycling, metamorphism and erosion.
4.2.2 Microorganisms Perform Isotopic
Discrimination
Extant living organisms preferentially use lighter isotopes:
12 C >
13 C,
1 H >
2 H,
14 N >
15 N,
32 S >
34 S,
54 Fe >
56 Fe
(Box 4.1). Thus, a change in the ratio of the ‘heavy’ isotope
to the ‘light’ isotope of an element suggests the presence of
different metabolic activities in ancient sediments. For
example, autotrophic activity will result in an enrichment
of
12 C atoms compared to
13 C atoms; an enrichment of
32 S is
evidence of sulphate-reducing activity, while
54 Fe enrichment indicates a capacity to respire iron. Note, however, that
some caution is needed in the use of isotopic ratios because it
has been shown that abiotic processes can also produce
isotopic ratios that overlap with those generated by
biological fractionation.
Box 4.1: Stable Isotopes and Isotopic Fractionation
Vincent Grossi
A chemical element X is characterised by its atomic
number Z (equal to the number of protons in the
nucleus) and its atomic weight A (equal to the number
of protons and neutrons). It is represented as
A
Z X (e.g.
12
6 C). Isotopes are atoms of the same element with
differing atomic masses (e.g. carbon-12, carbon-13
and carbon-14). That is, they have the same number
of protons but different numbers of neutrons. Of the 92
existing natural chemical elements, 71 have multiple
isotopes. Some isotopes are stable (e.g. carbon-13
or
13 C), while others are unstable and radioactive
(e.g. carbon-14 or
14 C) and disintegrate over time
into radiogenic isotopes (radionuclides).
Among the stable isotopes of an element, the
lightest is the most abundant (e.g.
12 C ¼ 98.9 %,
13 C ¼ 1.1 %;
16 O ¼ 99.76 %,
17 O ¼ 0.04 %,
18 O ¼ 0.20 %) and tends to be ‘used’ (or reacts)
more easily (or faster) than the heavier isotopes.
So-called isotopic fractionation is the discrimination
between the light and a heavy isotopes of an element
that occurs during (bio)chemical reactions and
physico-chemical processes. This fractionation leads
to a very small (on the order of ‰) but significant
change in the ratio of heavy to light isotopes (e.g.
13 C/
12 C,
18 O/
16 O,
2 H/
1 H or D/H) between a reagent
(e.g. CO 2 fixed by phototrophic organisms) and its
transformation products (e.g. organic compounds
formed from the CO 2 fixed by phototrophic
organisms).
The heavy/light isotopic ratio of an element in a
sample is determined using an isotope ratio mass
spectrometer (IRMS), which measures this ratio in
a purified gas (e.g. CO 2 , H 2 , N 2 ) produced from the
sample (originally solid, liquid or gas), and compares
it with that of a reference gas produced from an international (or universal) standard. These standards
exist for each element (C, H, O, N, S) commonly
measured by IRMS; for example, the standard for
carbon is a belemnite (calcite) from the Pee Dee
Formation (so-called PDB) with a
13 C/
12 C ratio
¼ 0.01124 and that for nitrogen is atmospheric
V. Grossi
Laboratoire de Ge ´ologie de Lyon: Terre, Plane `tes,
Environnement, UMR CNRS 5276, Universite ´ Claude Bernard
Lyon 1, 69622 Villeurbanne Cedex, France
(continued)
86
J.-C. Bertrand et al.