easy way to see how silicates fit into the carbon cycle is to follow
the carbon around as it is exchanged along the red lines in the
surficial reservoir, and then between the surficial and carbonate
- We start with carbon in the atmosphere.
This is going to enter the soil dissolved in rainwater, where
it may have an abiotic weathering effect, or it will be fixed by
plants, driving a variety of weathering processes.
- Some rock types (like limestone) are already in the
carbonate reservoir. Silicates are NOT part of the carbonate reservoir.
They contain no
carbon, but any calcium they contain can be combined with CO2
the surficial reservoir to make limestone. (Magnesium leads to the same
result by a more complicated process.)
- The rocks are weathered, and the weathering products
are transported out
to sea. The carbon is found in bicarbonate ions; two per calcium ion.
- At sea, our two bicarbonate ions will react
with our calcium to form a new carbonate molecule containing ONE
carbon. The other carbon will be freed; it remains in the
surficial reservoir. So: no net change in surficial carbon if
we remove carbon from the carbonate reservoir and then put it back.
But if the rock contained no carbon, we have a net LOSS of
one carbon from the surficial reservoir.
- The new carbonate rocks may remain in the
reservoir for millions of years before they are either
uplifted and weathered, or subjected to metamorphosis.
In either case, they will be destroyed and return their one
carbon to the surficial reservoir.