The Spatial And Temporal Distributions Of Ooids And Their Petrological And Geochemical Compositions:Implications For Paleoceanographic Conditions In The Permian-Triassic Transition

The Permian-Triassic mass extinction(-252 Ma)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 the characteristics were more distinctive at this critical period than any time during the Phanerozoic.The Early Triassic was an unusual period biotically accompanied by large-scale environmental disturbances,including extreme high-temperature,very low marine sulfate concentrations,greatly enhanced atmospheric CO2 partial pressure,hothouse,severe perturbations of stable carbon isotopes,elevated weathering rates,and so on.Global review of over forty 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 earliest Triassic until the conodont Isarcicella isarcica Zone,and more concentrated in the conodontClarkina carinata Zone,Neospathodus dieneri Zone,and Pachycladina-Parachirognathus assemblage zone during the Early Triassic.In addition,oolites are often found in association with microbialites in low-latitude shallow marine settings near the PTB.Proliferation of ooids over the P-Tr transition indicates an extensive range of warm waters with high level of carbonate saturation state prevailed in the oceans during that time.Widespread erosion horizon and following oolite close to the PTB may indicate the scenario of oceanic acidification and rebounding to carbonate supersaturation condition.Signals indicative of microbial activity and paleo-seawater chemistry were identified in marine ooids from the Lower Triassic of South China using in situ laser ablation-inductively coupled plasma-mass spectroscopy(LA-ICP-MS).Quantitative trace element profiles were made across successive growth laminae in the cortices of selected ooids.Petrographic observation revealed layers with intense fluorescence having structures that resemble microbial filaments or extracellular polymeric substances.These layers possess light rare earth element(LREE)depletion patterns similar to modern seawater,negative Ce anomalies indicative of a well-oxygenated environment of formation,and relatively high concentrations of nutrient-related metallic elements(e.g.,Zn and Ba).These findings imply that(1)ooids may be robust recorders of paleo-seawater chemical signals such as REE+Y distributions,and(2)mineralization of surficial biofilms produced by photosynthetic microbes was an important factor in the formation of Lower Triassic marine ooids.The present study may provide a new perspective on the genesis and geochemical composition of ooids generally.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 Isarcicella isarcica Zone of 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.Traditional view believed,the Permian-Triassic transition was just situated at the period of the "aragonite sea",and the primary mineralogy of ooid was supposed to be aragonite and possibly high-Mg calcite specially.Butthisstudy showed large quantities of originally low-Mg calcitic and bimineralic ooids episodically developing in the Lower Triassic strata.Both petrographic and cathodoluminescence examinations were used to identify the compositions of ooid primary mineralogy.Meanwhile,geochemical analyses have been also carried out by electron probe microanalysis and in situ analysis of laser ablation inductively coupled plasma-mass spectrometry(LA-ICP-MS)in order to estimate trace element concentrations of preserved ooids.Lines of evidences from morphologies,manners of preservation of cortical layers,as well as geochemical data exhibit these ooids are greatly different with the original mineralogy of aragonite ooids in the latest Permian.The anomaly in primary ooid mineralogy may indicate the variations of seawater major-ion compositions during the Early Triassic.The work could offer a new perspective to understand the paleoceanographic environments after the end-Permian mass extinction event and other periods with extensive oolite development.