Global oolite deposits across the Permian–Triassic boundary: A synthesis and implications for palaeoceanography immediately after the end-Permian biocrisis

The Permian–Triassic mass extinction (PTME) is not only a dramatic loss in biodiversity and major change in ecosystem structures, but also coincided with the formation of abundant unusual sedimentary structures. Of these, ooids were widespread in shallow marine carbonate settings during the Permian–Triassic (P–Tr) transition, and giant ooids occurred more frequently at this critical period than any time during the Phanerozoic. Global review of 43 oolite-bearing Permian–Triassic boundary (PTB) sections with reliable biostratigraphic controls indicates that ooids occurred mostly in coincidence with the latest Permian extinction (LPE) and its immediate aftermath. Ooids became widespread over extensive regions just after the LPE during the interval corresponding to the conodont Hindeodus changxingensis Zone. They persisted into the earliest Triassic until the conodont Isarcicella isarcica Zone. In addition, oolites are often found in association with microbialites in low-latitude shallow-marine settings. Proliferation of ooids over the P–Tr transition indicates an extensive range of warm waters with high level of carbonate saturation state that prevailed in the oceans during that time. The latest Permian ooids were usually small (0.3 to 0.7 mm in diameter), aragonitic, poorly preserved and recrystallized, while moderately to well-preserved morphology, bimineralic, and oversized forms usually occurred in the I. isarcica Zone of the earliest Triassic and afterwards. Widespread aragonitic ooids in the end of the Permian reinforce the scenario that an “aragonite sea” period may have resulted in the dramatic losses of skeletal organisms that precipitated low-Mg calcite and hampered their recovery in the aftermath. The anomalous primary mineralogy of Lower Triassic ooids implies that previously assumed stable seawater composition during the Early Triassic needs to be revaluated.