Biological processes that discriminate against heavier isotopes will tend to have approximately double the effect against C-14 that will show up vs.
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Another impact is due to water source and varying resevoir corrections. This is only species dependence in that it is an environmental dependence. For the Peruvian Holocene we observed that estuarine mollusks were in equilibrium with the atmosphere while open ocean mollusks required a large resevoir correction.