Marcasite in Black Shales—a Mineral Proxy for Oxygenated Bottom Waters and Intermittent Oxidation of Carbonaceous Muds

Black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions. Although bottom-water anoxia favors deposition of black shales, certain biological indicators, such as benthic fossils and bioturbation, suggest that bottom-water conditions may not have been as persistently anoxic as commonly presumed. Examination of twelve marine carbonaceous shale-bearing units, ranging in age from Middle Proterozoic to Cretaceous, suggests that in addition to paleontological clues, early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen-bearing bottom waters. Marcasite requires low pH values to form, and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition. In the studied examples, degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments, and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low pH values required for marcasite formation. Thus, observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts. Pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed. The latter consideration may, for example, prove important for those instances where a black shale is associated with an oceanic anoxic event (OAE), because these are commonly presumed to have been deposited under fully anoxic conditions.