How did Jurassic black shales form in European marine basins?

During the Jurassic Period, black shales formed in European marine basins primarily due to restricted water circulation, which led to oxygen-deficient conditions at the seafloor. These environments prevented the survival of burrowing organisms and hindered the efficient decomposition of organic matter, allowing for the accumulation of organic-rich sediments.123

The development of these anoxic conditions was often driven by a combination of restricted circulation and high levels of biological productivity. In some instances, such as during the Pliensbachian-Toarcian boundary, these low-oxygen intervals were associated with significant extinction events among bottom-dwelling invertebrates. Because the decomposition process was significantly decelerated by the lack of oxygen, organic material was preserved, eventually forming the black shales observed in the geological record today.134