MINERALOGY AND GEOCHEMISTRY OF THE PERMIAN – TRIASSIC BOUNDARY (PTB) STRATA IN THE ABADEH SECTION OF IRAN AND THEIR IMPLICATIONS FOR THE MASS EXTINCTION

The Uppermost Permian of the Abadeh section of Iran consists of three distinct lithologic intervals. Unit 5 (56 m) occurs at the base and consists of skeletal packstone. It grades upward to gray, lime mudstones of Unit 6 (18 m) which in-turn transitions into red, lime mudstones of Unit 7 (18 m). These strata represent a deepening upward succession from shallow water to deep subtidal environments. The Triassic section begins with 1 m of synsedimenetary cement, 0.5 m of wackestone, and 1.5 m of packstone-grainstone followed by over 100 m of lime mudstone.

Permian δ¹³C compositions remain constant at about 4‰ PDB, but begin a nearly 5‰ PDB decrease at 3 m below the PTB reaching their lowest value at about 3m above the boundary; thereafter, δ¹³C compositions increase to 2.3 ‰ PDB in the lowermost Triassic. These variations suggest the addition of a large amount of carbon with low δ¹³C compositions into the Late Permian ocean. Sr concentrations are about 2000 ppm in Unit 5 and gradually decrease to 600 ppm in Unit 6, and eventually reach a constant value of 700 ppm in Unit 7 and remain at this composition through the PTB interval and the lowermost Triassic strata. Sr values indicate an aragonite ocean at the base of the section which gradually became calcitic upward but immediately became aragonitic again after the mass extinction. Total organic carbon (TOC) content decreases from a value of 0.2% at the base of the section to 0.05% in the uppermost Permian and the lowermost Triassic. The lowering of TOC values suggests either a decrease in organic productivity, or an increase in bottom water oxidation, or a low rate of sedimentation, or a combination of these factors.

Mineralogical characteristics of the Abadeh section is attributed to ocean acidification and associated changes in seawater composition. The lowering of seawater pH resulted in a change from an aragonite producing ocean in Unit 5 to a calcite generating one in Unit 7. Eventually, a threshold was reached and the ocean became highly acidic to the point of dissolving previously deposited platform carbonates as well as causing the marine mass mortality. Ocean acidification was most likely due to the release of methane from gas hydrates reservoirs by global warming and its injection into the ocean and aerobic oxidation there.