Oxygen depletion is widespread in modern and ancient benthic environments. Most modern examples exhibit a significant cyclic fluctuation in O2 availability, ranging from tidal to seasonal cycles, and longer term periods (e.g. El Niño). In contrast, only long term changes have been generally recognized in ancient environments. Benthic organisms adapt along two pathways: (1) to low O2-values and free H2S (including chemosymbiosis); (2) synchronization of individual life cycles to the O2-fluctuation cycle by living as planktonic larvae in oxygenated surface waters, using oxygenated periods in bottom waters during the adult stage.
Examples of both adaptive pathways can be recognized in the benthic faunas of fossil oxygen-controlled environments, thus recording short and longer term fluctuations.