RELATIONSHIP OF REDOX AND SALINITY IN PALEODEPOSITIONAL SYSTEMS (Invited Presentation)

Redox variation in paleodepositional systems is an exceedingly common phenomenon, sometimes being attributed to changes in phytoplankton productivity, watermass circulation, or water-column stratification, although it is often difficult to accurately identify the underlying mechanism. New elemental salinity proxies that can be applied to shale and mudstone formations have begun to yield surprising insights in this regard, demonstrating that redox variation is often tightly coupled to salinity variation. For example, in the Cryogenian Nanhua Basin of South China, the basal member of the Datangpo Formation exhibits fluctuations between Mn-carbonates deposited under ferruginous, fully-marine salinity conditions and shales deposited under euxinic, brackish conditions, probably due to intermittent influx of marine waters into a semi-restricted epicratonic basin. Such relationships can be helpful in constraining the mechanism of paleoenvironmental change. For example, in the Toarcian succession of the Cleveland Basin (U.K.), a shift to less-saline conditions during deposition of the Jet Rock (Toarcian OAE) demonstrates that basinal anoxia was triggered by sea-level fall, more humid climatic conditions, or a combination thereof, rather than by sea-level rise, as earlier postulated. Another important finding is that much of the redox-salinity variation in epicratonic settings is water-depth-related, reflecting differences between better-oxygenated, less-saline surface watermasses and oxygen-deficient, more-saline deep watermasses. Distinct vertical gradients in redox and salinity conditions can be identified in many settings, e.g., on the Late Ordovician Yangtze Platform of South China and the Late Devonian Illinois Basin of the U.S. midcontinent region. Future paleoenvironmental studies based on shale or marl formations will need to incorporate elemental salinity data in order to accurately characterize redox and other environmental changes and to understand their underlying relationships.