We do, however, know that under certain anaerobic conditions and in loosely packed "Gaseous" marine sediment beds, there can be a fractionation effect due to bacterial activity as part of methane production.
In such a setting, C14 at deeper depths will be preferentially metabolized by the bacteria during the methane conversion process.
This methane can sometimes bubble up through the sediment.
The obvious effect of this is that deep sediment (in these specific sediments) will be depleted in C14 and if it were to be dated by the radiocarbon method, it would read artificially old.
An additional effect of this scenario has been suggested. At shallow depth, aerobic bacteria will be able to metabolise some of the carbon from the methane bubbles and fix them back into the sediment.
The result of this is that shallow sediment (in these specific sediments) will be artificially enriched in C14 and will date unrealistically young.
While these mechanisms do, indeed exist and have been documented, it must be noted that the specific sediments in which these conditions are prevalent, are known to be unsuitable candidates for Carbon 14 dating methods.
Sediments such as this exist in many places around the world. The
cited case study in the post by JohnFolton related to the sediments of Long Island Sound and the coast of germany.
Other such sites include the edge of the continental shelf along the eastern coast of Florida where landslides regularly release vast amounts of methane gas.
It has to be noted that this kind of fractionation will only occur in "Gaseous" loosely packed sediments with large particles and will not be found in lake varves such as Suigetsu, these sediments are tightly packed with very small particulate. They form a clay-like Consistency that would in no way be liable to allow "bubbling" of any kind.
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