Abstract:

During the early Toarcian multiple environmental perturbations led to the genesis of the Toarcian Oceanic Anoxic Event (T-OAE), expressed by the widespread occurrence of black shales that were formed under oxygen-deficient conditions. Factors promoting organic matter preservation also drove benthic extinction due to low aqueous O₂ levels and habitats poisoned by H₂S. However, the development of benthic barren intervals in oxygenated shelf areas, lacking black shales, indicates that even in these areas benthic communities experienced a crisis. To understand environmental changes and to determine factors driving benthic extinction we applied paleontological and geochemical techniques to the organic matter-lean Ratnek El Kahla section (Saharan Atlas, Algeria). During the Early Jurassic the study site was located at the northern Gondwana paleomargin, belonging to the southern West Tethys Shelf. The benthic barren interval coincides with the climax of the T-OAE that correlates with a major carbon cycle perturbation, expressed by a negative carbon isotope excursion. Compared to background values, sediments of the benthic barren interval are enriched in organic matter and redox-sensitive metals. In combination with the enhanced preservation of intact bacterial lipids and increased rates of organic matter sulfurization, data attest to a decline in pore water O₂ and increased H₂S concentration. It is thus reasonable to propose that, similar to northern shelf areas, benthic communities suffered from prolonged oxygen-deficient conditions. Absence of benthic activity and less intensive sediment reworking in combination with enhanced organic matter sulfurization favored organic matter preservation. Productivity proxies and δ¹⁵N data indicate moderate primary productivity rates. Lowered aqueous O₂ levels did not result from excessive aerobic organic matter breakdown. Seafloor anoxia was rather promoted by rising seawater temperatures and sluggish ocean circulation during a transition from icehouse into a greenhouse climate. These conditions may have been a pre-requirement for genesis of the T-OAE.