An Indicator of Anoxicity (IA), defined as the ratio (Pyrite Fe + Oxide Fe)/Total Fe, can be used to recognize sediments deposited under an anoxic water column. Modern marine sediments deposited under oxygenated bottom waters mainly have IA values < 0.4 whereas sediments deposited under anoxic, iron-rich or sulfidic bottom waters mainly have IA > 0.5. IA values in the central Kimmeridge Clay (Upper Jurassic, UK) for fissile, organic-rich shales indicate that deposition occurred principally from anoxic bottom waters with infrequent oxygenation events.

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