Geoscience Reference
In-Depth Information
water, carbon dioxide, and methane are easily represented. The remains of vascular plants
have high H/C and O/C ratios; those of algae, high H/C and low O/C ratios. Removal of
H
2
O and CO
2
by heating drives organic material into the field of kerogen and then coal.
Subsequent removal of liquid hydrocarbons and methane drives the residue into the field
of low-H coal.
It is a classically held view that major coal deposits formed in periods following low-
ering of sea levels, which were the most prone to deposition in landlocked basins. In
contrast, deposition of the largest oil source rocks coincided with transgressive periods
and increased biological productivity. The largest fraction of mature organic matter in the
crust is actually stored, not in coal, the second largest reservoir, and not in oil either, but
in methane hydrates: at pressures exceeding 100 atmospheres and temperatures lower than
18
◦
C, methane forms liquid hydrates or clathrates which are found in very large amounts
beneath the permafrost of continents at high latitudes and in the sediments of the marine
continental slopes.
8.2 Some specifics of biological activity
In geological cycles, biological compounds are rarely considered as a source or a sink of
elements simply because biomass is only a small mass fraction of geological reservoirs,
such as the ocean or even soils. The relationship of life to geochemical pathways is a bit
like that of the beaver to the river: it is there to mine available energy that would otherwise
have been dissipated with no profit to any living thing. There is clearly more to life than
building a lodge to protect offspring against predators, but the principle is still valid: life
is opportunist and gets a free ride on untapped energy flows. And just as the beaver has a
strong impact on the river and associated habitats, life affects the ocean, the atmosphere,
and leaves fairly strong footprints in the chemistry of sediments. A plankton-free ocean
would certainly have a chemistry very different from our familiar seawater. We will see
that life may even be held responsible for some remarkable geodynamic features such as
no less than the stabilization of granitic continents.
Two characteristics of organic material are fairly striking. First, it rises incredibly high
on the redox scale. Trading electrons between very different redox potentials is the hall-
mark of biological activity. Life is about pushing C and N towards the most reduced
state possible and then using a multistage transfer of high-energy electrons between a
reduced primary electron donor (PED) and a terminal electron acceptor (TEA) to pro-
duce mechanical work and build structural and functional molecules. The cell builds up
the only energy form it can use, adenosine-triphosphate or ATP, using external inputs, so
that new proteins can be synthesized for maintenance and mechanical work performed.
For autotrophs (plants), the primary source of energy is solar electromagnetic radiation in
the visible part of the spectrum. Pigments, most notably chlorophyll, capture this energy
in a process called photosynthesis, which may be either oxygenic (cyanobacteria, algae,
plants release oxygen) or anoxygenic. In most biological systems, energy is stored as
