Abstract
The rock record provides unequivocal evidence for multiple times in Earth history during which the entire global ocean or parts of it were characterized by severe oxygen-deficiency. Evidence includes geological and paleontological observations but also diverse geochemical fingerprints such as trace element abundances, organic geochemical markers or various isotope records, all of which are diagnostic for water column anoxia. The duration of such episodes of oxygen-deficiency ranges from a few thousand to millions of years. In fact, considering the proposed temporal sequence of oxygenation of the atmosphere-ocean system during Earth's early history with the ventilation of the deep ocean not earlier than 1 Ga ago, it appears that the modern oxic world represents an exceptional state of the atmosphere-ocean system on our planet during its 4.5 Ga years history.
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