Intermittent euxinia:: Reconciliation of a Jurassic black shale with its biofacies

Isorenieratane, a molecular fossil derived from the brown strain of the green sulfur bacteria Chlorobiaceae, is ubiquitous in the organic carbon-rich, argillaceous, fossiliferous Peterborough Member (Callovian, UK) of the Oxford Clay Formation, indicating a temporal overlap of the euphotic and sulfidic zone in the water column. The presence of euxinic conditions in the water column is inconsistent with the abundant benthic fauna in that formation, which indicates bottom-water oxygenation. These conflicting geochemical and paleontological data suggest intermittent euxinia during deposition. The duration of oxic-dysoxic events can be estimated by considering the life mode, life span, and colonization time of the benthic fauna incorporated in these sedimentary rocks. As previously proposed, the preservation of sedimentary organic matter, based on Rock-Eval parameters, is directly related to the frequency and/or duration of oxic bottom-water conditions.