REVISITING DEGREE OF PYRITIZATION AS A PALEOREDOX PROXY

Degree of pyritization (DOP), long used to constrain ancient marine anoxia, has refined value when viewed in the context of recent models for iron cycling in modern euxinic (anoxic-sulfidic) basins. Originally defined as a measure of the extent to which the detrital "reactive" Fe reservoir has been sulfidized to form pyrite, high DOP values were assumed to represent Fe limitation caused by persistent exposure to the ubiquitous sulfide of euxinic basins. Recent work has shown, however, that DOP values measured using the traditional HCl extraction can be low in modern coastal oxic settings despite prolonged exposure to high levels of pore water sulfide. More importantly, DOP can also be low in coastal euxinic sites, such as in the Black Sea, despite highly sulfidic conditions in the water column and sediment. Based on Black Sea trends, high DOP values, which are commonly assumed to represent all euxinic deposition, should only be observed in basin centers where siliciclastic accumulation rates are comparatively low. These observations have led to a recent model for Fe cycling that invokes scavenging of dissolved Fe within sulfidic water columns during syngenetic pyrite formation. Critically, this scavenged Fe is a reactive fraction that is decoupled from the local siliciclastic flux. Under conditions of low siliciclastic sedimentation, Fe augmentation via syngenetic pyrite formation results in elevated ratios of reactive to total Fe and total Fe to Al, as well as the classically high euxinic DOP values. On the continental margin, high siliciclastic-Fe inputs swamp the scavenged component, perpetuating continental Fe/Al ratios and low DOP values--despite the euxinic conditions.

Data supporting this model will be presented for the Black Sea, Cariaco Basin, Orca Basin and Effingham Inlet (an anoxic fjord on Vancouver Island). Collectively, these modern euxinic settings provide a gradient in siliciclastic sedimentation across which DOP variations can be assessed. These results have been extrapolated to Pennsylvanian and Devonian black shale sequences. Degree of pyritization, while linked to paleoredox, ultimately reflects the balance between water-column pyrite formation and Fe delivery through detrital sedimentation. When viewed carefully, DOP can provide information about both depositional parameters.