Why does isotope fractionation occur?
In every chemical or physical reaction, e.g. oxidation or evaporation, the light isotopes react at a slightly higher rate than heavy ones, because the bonds in a molecule or crystal between lighter atoms vibrate with a higher frequency and therefore they split easier than those between heavier atoms. In the products of a chemical reaction are therefore, at least at the beginning, more light isotopes than in the reactants. The consequence is that some substances in nature contain more heavy isotopes (e.g. are enriched in heavy isotopes) than the others, depending on their formation pathway.
A great example of isotope fractionation is photosynthesis: plants preferable absorb light carbon (12C) than the heavy one (13C), therefore they are enriched in 12C (or depleted in 13C) compared to the atmospheric CO2. Isotope fractionation occurs also during phase transitions such as evaporation or condensation, or through exchange of isotopes (for instance between the lake water and air humidity).