Fluvial discharge and sea-level changes controlling black shale deposition during the Paleocene-Eocene Thermal Maximum in the Dababiya Quarry section, Egypt

摘要

The formation of black shales along the Tethyan margins at the Paleocene-Eocene Thermal Maximum ("PETM"; similar to 55 Ma) may have been an important feedback mechanism to reduce the warming by excess carbon burial. However, the detailed evolution of sediment redox conditions during this event is still poorly constrained. We address this issue by a high-resolution mineralogical and geochemical investigation of the outer neritic Dababiya Quarry PETM section in Central Egypt, which serves as the Global boundary Stratotype Section and Point for the base of the Eocene. There, the base of the PETM beds corresponds to the onset of sediment lamination indicative of oxygen-deficiency at the seafloor. The absence of calcium carbonate and a major increase in phyllosilicate abundance and change in detritus-sensitive trace elements is indicative of severe carbonate dissolution in addition to enhanced fluvial input, erosion of coastal low lands, and deposition during low or slightly rising sea level. Subsequently, fully anoxic conditions were established for a short period during the peak phase of the PETM as recorded by the strong relative enrichment of redox-sensitive trace elements and organic carbon compared to background sediments. The decoupling of the Fe and Mo contents from the detrital fraction may even indicate that euxinic conditions existed with free hydrogen sulfide in the water column. Concurrent to the onset of anoxia, high carbonate and quartz contents as well as high elemental ratios of Si/Al suggest a major drop of siliciclastic sediment supply (sediment starvation), most likely caused by a rapid sea level rise. The final recovery phase of the PETM is associated with a progressive restoration of the pre-PETM siliciclastic sedimentation, temporary (seasonal) anoxic to dysoxic conditions, and high phosphorus enrichment, followed by carbonate-dominated sedimentation and the return to well-oxygenated conditions.