Productivity versus Preservation: Black shale deposition of the Early Toarcian (Jurassic) ocean anoxic event in the Northern German Basin

Abstract

The Early Toarcian Ocean Anoxic Event (OAE) and the corresponding North European black shales reflect a period of enhanced organic matter preservation. Although this event has seen an extensive amount of study the environmental conditions, mechanisms, and processes that resulted in the deposition of the organic-matter-rich sediments are still a matter of discussion and not yet fully understood. In this study, we present a new interdisciplinary approach to the Toarcian black shales from the Northern German Basin. Making use of petrographic, geochemical and palynological analysis, dynamic and previously unnoticed, subtle changes were observed. Two different episodes of black shale depositions are recognized. The first period, in the exaratum subzone, aligns with a global perturbation of the carbon cycle. We argue that this perturbation lead to salinity driven stratification and a stressed upper water column. With a low sea-level at the time of this episode basin restriction formed the perfect catalyst enhancing the anoxic conditions in the basin leading to a preservation driven black shale deposition. Furthermore, the clustered debated sphearicals present in this episode are interpreted to be juvenile Tasmanites and indicate environmental stress. During the second episode, in the falciferum subzone, further developing sea-level rise and the inflow of colder, nutrient-rich artic waters led to a shift in the deposition of organic matter. Weakening of the salinity stratification and improvement of life in the upper water column led to enhanced productivity. With dysoxia only prevailing in the deeper parts of the basin the black shale deposition became more productivity driven than the first episode. Within this second period of black shale deposition levels of faecal pellets and the prasinophyte genus Tasmanites in the rock record are significantly high.