EARLY TRIASSIC MARINE ANOXIA

The extent and duration of marine anoxia in the Early Triassic was unique for the Phanerozoic. Following their rapid onset of anoxia, coincident with the Permo-Triassic mass extinction, such conditions became established in diverse marine locations ranging from shallow-water, storm-influenced shelves all the way to the abyssal depths. Such extent cannot be explained using traditional black shale models, that either favor silled basin or upwelling locations, and the Early Triassic conditions have so far defied all attempts to model them. There was something exceptional and unprecedented about anoxia at this time. Two key factors may have been at play: extreme warmth and primary production dominated by cyanobacteria.

Organic matter remineralization increases rapidly with temperature, and yet this increase of oxygen demand competes with decreasing dissolved oxygen levels found in warmer waters. Sea-surface temperature records shown a phase of extreme warmth in the Early Triassic and this may have lead to a strong temperature control on organic matter distribution. Thus, black shales are best developed either in the highest latitude shelf seas (e.g. Australia) or in the cooler, abyssal settings of Panthalassa.

Water column oxygen demand is also dependent on the transit time of organic particles from surface waters to sediment. It is likely that grazing zooplankton were severely affected by mass extinction (radiolarians were nearly wiped out) and nitrogen isotope studies suggest cyanobacteria-dominated plankton populations in the immediate aftermath of this crisis. A severely weakened biological pump (no fecal pellets, tiny cyanobacterial particles) in the Early Triassic would have greatly exacerbated water column anoxia.