Geoscience Reference
In-Depth Information
1.083). Chemically, the two carbon isotopes are nearly identical, but
not completely so, and as we shall see, these slight differences in chemi-
cal reactivity give rise to signals we can detect and interpret.
We now come back to the question of life. As explored in
chapters 2
rived from the atmosphere or dissolved in water (for example, plants do
this through photosynthesis). The organisms doing this must convert the
CO
2
to organic matter, and this is accomplished by a variety of different
types of biochemical reactions involving enzymes. Plants, as well as
cyanobacteria, use the enzyme Rubisco. We met Rubisco in
chapter 3,
but what we didn't say was that Rubisco preferentially uses the carbon-
12 in CO
2
over the carbon-13. This means that plants and cyanobacteria
are depleted in carbon-13 (and at the same time enriched in carbon-12)
compared to the distribution of carbon-12 and carbon-13 atoms in the
CO
2
from which the organic matter was formed. Now, we need a lan-
guage to express these ratios. Isotope ratios are typically measured on
instruments known as mass spectrometers. The isotope ratio we get for
our sample is reported relative to the same ratio measured on a standard
with a known ratio of C-13 to C-12 atoms. We end up with small num-
bers when we do this, so we multiply by 1000 to give us numbers we can
easily discuss. For the carbon system, we report the isotopic composi-
tion of our sample as δ
13
C.
9
The more positive the δ
13
C value, the more
enriched the sample is in C-13. Turning back to Rubisco, the enzyme
preferentially selects C-12 by about 2.5% over the CO
2
in the environ-
ment. If we use the nomenclature we just learned, this means that the
organic matter formed by Rubisco is 25 per mil (‰) depleted in C-13
relative to the CO
2
.
10
Minik measured the δ
13
C of graphite from Isua and found that the
graphite was depleted in C-13 by about 17 per mil (the isotopic differ-
ence between graphite and the inorganic carbon found in other sedi-
mentary rocks at Isua), which are amounts consistent with production
by cyanobacteria (
ig. 6.3)
. Do we, then, have solid evidence for cyano-
bacteria as far back as 3.8 billion years ago? Sadly, no. The problem is
that many types of organisms other than cyanobacteria also use Rubisco,
producing a similar carbon isotope signal. So, we cannot conclude that
cyanobacteria were present at Isua. But, we have pretty good evidence
that life was there. The Isua organic carbon isotope signal is consistent
