CHANGES IN ORIGINAL CARBOBATE MINERALOGY ACROSS THE PERMIAN - TRIASSIC BOUNDARY AND ITS IMPLICATAIONS FOR THE MASS EXTINCTION PROCESS

It has been documented that the dominant mineralogy of marine carbonates changed from aragonite during icehouse periods (Mississippian – Triassic, Neogene – Present) to calcite during greenhouse episodes (Ordovician – Devonian, Jurassic – Paleogene). The transition is attributed to either a change in Mg/Ca ratio of seawater or to a variation in pCO₂ composition of the atmosphere.

Neither discussed nor debated adequately is a short term variation in the original carbonate mineralogy associated with the Permian – Triassic boundary (PTB) mass extinction. Data suggest that the mineralogy of precipitating marine carbonates changed from calcite in the uppermost Permian to aragonite in the lowermost Triassic in several Tethyan sections in Iran. This mineralogical change is supported by petrographic characteristics (fabric retention) and geochemical criteria (Sr concentration) of carbonates. The change from the calcite sea of the uppermost Permian to the aragonite sea of the lowermost Triassic included a transition period of carbonate infertile sea when the ocean was incapable of producing carbonates; it in fact dissolved previously deposited carbonate sediments on the seafloor.

The association of a change in marine carbonate mineralogy with the biggest mass extinction of the Phanerozoic has several major implications for the PTB mass extinction process. When present, the change in carbonate mineralogy indicates that the negative shift in δ¹³C values from Permian to Triassic is larger than it is observed on bulk carbonate analysis. The δ¹³C composition of the lowermost Triassic strata should be lowered by an additional 1.6‰ PDB to adjust for the fractionation between calcite and aragonite. Such an adjustment alone will constrain the viable kill mechanism to processes capable of producing a large negative δ¹³C shift.

Furthermore, the change in original carbonate mineralogy also points to changes in seawater chemistry. The possible causes include a rapid shift in the Mg/Ca ratio of seawater or ocean acidification due to increases in dissolved pCO₂ of seawater. The second mechanism is a more likely scenario because of the observed carbonate dissolution or precipitation of non-carbonate sediments at the Permian – Triassic boundary.