Geoscience Reference
In-Depth Information
Don't worry, though, there's more to the story. Let's go back to our
jar experiment. The oxygen accumulates in the jar as long as the plant
is alive and growing. But, what happens when the plant dies? Like the
compost heap in our back yard, all sorts of bacteria and fungi will de-
compose the dead plant material, using oxygen to do so. Overall,
whether on the land or in the sea, it is estimated that some 99.9% of the
primary production on Earth is decomposed. The remaining tiny bit is
buried as unreactive organic matter in marine and freshwater sediments,
which may later solidify into rock. Indeed, only the organic matter that
is buried into sediments and transformed ultimately into rocks escapes
reaction with oxygen. Therefore, the burial of this organic matter repre-
sents a net oxygen source to the atmosphere. So, plants and cyanobac-
teria produce the oxygen, but it accumulates only because some of the
original photosynthetically produced organic matter is buried and pre-
served in sediments.
4
A lump of coal represents an oxygen source to
the atmosphere, as does a barrel of crude oil, organic fossils, and all of
the finely dispersed organic matter that gives the beautiful black color
to the ancient shales I so like to study.
5
There's another oxygen source that we need to consider. As we learned
in
chapter 2,
an anaerobic microbial process called sulfate reduction
respires organic matter using sulfate, and produces sulfide. This process
is quite common in nature. We can find sulfate-reducing bacteria in rot-
ten eggs, in our guts, and even on our teeth. They are most prominent,
though, in relatively isolated basins like the Black Sea and the Cariaco
Basin of the coast of Venezuela, in many Scandinavian fjords where
water circulation is restricted, and in most marine sediments at depths
where oxygen has been consumed by respiration. These sediment depths
range from only a millimeter or two in environments near the continen-
tal margins to centimeters or more, if we collect mud far offshore and
where the organic contents are relatively low. If there is iron around,
and there usually is, the sulfide reacts with the iron, forming a mineral
called pyrite (chemical formula FeS
2
), which we also encountered in
chapter 2.
In modern sediments, this “fool's gold” is typically found
as beautiful microscopic raspberry-like clusters (called framboids, from
framboise
, the French word for raspberry) some 5 to 50 microns in diam-
eter (
ig. 5.1)
. In ancient rocks, pyrite is often found as glistening golden
cubes, but in either case, the sulfide (and the iron) bound in pyrite is
