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.
Note that fractionation should not affect radiocarbon dates if they are done properly. This is because fractionation effects can be measured by measuring the delta-13 value as mentioned earlier in this thread; the delta-13 data is then used to correct the 14-C fractionation, yielding the correct date.
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