Challenging persistent water-column anoxia in deep-time epicontinental seaways: the proof is in the pudding (mud)

Mudstones encompass more than two-thirds of the sedimentary record and serve as a critical archive of Earth’s surface processes, climatic conditions, and long-term environmental change. Over recent decades, sedimentary geology has undergone repeated paradigm shifts that have reshaped interpretations of sediment transport, depositional processes, geochemical cycling, biological influences, and diagenetic pathways. Despite this progress, the potential role of water column anoxia/euxinia in the accumulation and preservation of organic matter-rich mudstones remains a matter of debate.

Persistent water column anoxia or euxinia requires highly restricted oceanographic conditions, such as long term water column stability, extremely sluggish circulation, and water-column stratification. These constraints greatly narrow the range of viable depositional processes in offshore environments. Using a global dataset spanning Devonian, Jurassic, Cretaceous, and modern organic-matter-rich intervals, this study supports a more dynamic scenario in which organic-matter preservation does not rely on sustained anoxic bottom waters. Instead, geochemical signals attributed to persistent anoxia originate via diffusion from overlying waters into reducing surface sediments during early diagenesis. Trace-metal enrichment hinges on the position of the redox boundary within the sediment and opens a broad array of environmental parameters for organic-matter enrichment in mud dominated offshore systems.

Sequence stratigraphic analysis suggests these systems developed on low gradient, mud dominated shelves with characteristic shifts in facies associations, stacking patterns, and stratal geometries. The strata record advective sediment transport over hundreds of kilometers with predictable shore-parallel and perpendicular trends. Petrographic examination shows muds dominated by water-rich aggregates with variable organic-matter enrichment. Recent ichnological studies document the presence of meiofaunal mixing, fecal pellets, and agrichnia traces in offshore settings.

Understanding the transition from aerobic–anaerobic–sulfidic metabolic pathways during early diagenesis is essential for determining how these deposits transform from fluid muds into anistropic mudstones. Integrating sedimentological, ichnological, and petrographic observations across scales enables a substantially refined perspective on the formation of organic-matter-rich mudstones.