R. Raiswell, R. Newton, P.B. Wignall; An Indicator of Water-Column Anoxia: Resolution of Biofacies Variations in the Kimmeridge Clay (Upper Jurassic, U.K.). Journal of Sedimentary Research ; 71 (2): 286–294. doi: https://doi.org/10.1306/070300710286
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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.