My quick thought would be that there are offsets between all the pools.
Atmospheric CO2 <> dissolved CO2 <> oceanic bicarbonate <> oceanic
carbonate <> C3 plants <> C4 plants (or coals). Modern carbonates are
slightly different on average from ancient carbonates. Etc. Therefore
it seems to me that any variation in the amount of carbon implies an
exchange between pools of different compositions, and possibly also
differences in biological productivity and erosion.
Therefore while the changes may end up as too small to be meaningful
(largely because the atmospheric carbon pool is so much smaller than the
oceanic one), I would say that there are always fractionating processes.
Hilary Stuart-Williams PhD
Research Officer - Stable Isotopes
Research School of Biology
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The Australian National University
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