The trace metal content of recent organic carbon-rich sediments: Implications for Cretaceous black shale formation

Organic carbon-rich sediments are enriched in several redox-sensitive and stable sulfide-forming trace metals (TM) and form an important sink in global TM cycles. In today's ocean such sediments are deposited in coastal upwelling areas like those in the Gulf of California, off Peru or off the Namibian coast and euxinic basins like the Black Sea. In an attempt to distinguish both environments by their specific TM patterns, some important conclusions may be drawn: 1) The TM enrichment seen in both environments broadly is rather similar. 2) Upwelling sediments are often enriched in Cd and P and depleted in Co and Mn. 3) Sapropels have a tendency towards higher enrichments in Ba and Mo, S, Re, As, Cu, Ni, Sb, and Fe, reflecting the strong sulfidation in an anoxic water column. 4) Upwelling systems cover a broad range of environmental settings where the steepness of the slope, the proximity of H2S to the sediment seawater interface and the intensity of bio-accumulation and regeneration play an important role for TM accumulation. 5) The TM content of anoxic basins is mainly controlled by TM availability in the water column and sedimentation rate. 6) Enhanced bio-productivity is the main switch turning an oxygenated into an anoxic environment.Cretaceous black shales (OAE 2) exhibit TM signatures of both, coastal upwelling areas and sapropels. The strong enrichments in Cd, Mo, Tl, V, Cu, Ni, (Bi), and Sb demonstrate that bio-accumulation and presence of H2S in the water column and associated sulfidation processes persisted during this time interval in large parts of the Proto-Atlantic. The usefulness of Ba as a paleoproductivity-proxy is obscured by diagenesis. The extreme accumulation in Ag and Zn requires either an additional TM source, for instance hydrothermal input, or a different TM seawater composition during the Cretaceous.The geochemical data suggest, that the major driving force for the widespread occurrence of C/T black shales seems to be the increase in volcanic activity and associated CO2-input throughout the Cretaceous. The ocean-atmosphere system operated in a mode different from today during this time interval. After conversion of the “nutrient” CO2 into organic matter the system switched back into the “normal” operational mode.