Paleoceanographic Characteristics Of Key Environmental And Climatic Events During The Triassic In The Paleo-Tethys And Panthalassic Oceans

Permian-Triassic mass extinction is the biggest biotic extinction,extremely environmental and climatic disaster in the Phanerozoic.It is one of the hot problems of the Earth Science.However,comparing to the detail studies of the Permian-Triassic mass extinction,relationships between environmental changes and biotic evolution in the ⁵0 Ma long Triassic period,which is characterized by a series of significant environmental and climatic events as well as biological recovery and radiation,are not well known so far.After the mass extinction,the extremely environmental and climatic events have been repeated several times in the Early Triassic,leading to the delayed recovery of marine organisms and ecosystems.Of which,the Smithian-Spathian boundary event marks by significant biological,environmental and climatic changes.Extensively biological radiation and stable prototype of modern ecological system were built at the Middle Triassic until 1 Ma long-lasting mid-Carnian torrential rainfall event in the early Late Triassic.The mid-Carnian torrential rains not only seriously affected biotic groups that recovery and radiated from the Early and Middle Triassic,but also changed the orientation of biological evolution.The Smithian-Spathian boundary event and mid-Carnian torrential rainfall event are both global events that have been emphasized in the recent years,but little is known about the processes of the environmental and climatic changes,their relationships and manifestation in different regions around the world.Here we measured elements and isotopes of whole rocks and conodont bioapatite at several sections in the Paleo-Tethys and Panthalassic Ocean.Sea surface temperature?SST?,land weathering,marine strontium isotope composition,seawater redox state and marine carbon-sulfur cycles were discussed.We also compared and explained the differences of the events in both Paleo-Tethys and Panthalassic Ocean.We built a precise time framework and conducted detail studies on the sedimentary history at the Kamura section in the Panthalassic atoll which lay the foundation for the further studies of the environmental and climatic events.A total of 14 conodont zones are recognized from the uppermost Changhsingian to the Upper Norian.The Lower Triassic contains the Hindeodus parvus,Isarcicella isarcica,and H.postparvus zones?Griesbachian?,the Neospathodus dieneri Zone?Dienerian?,and the Novispathodus ex gr.waageni-Parachirognathus Zone?Smithian?,while the upper Lower Triassic?Spathian?is completely missing at Kamura.The overlying Middle Triassic section consists of the Chiosella ex gr.timorensis-Cratognathus,?Paragondolella‘ ex gr.alpina-P.excelsa,Budurovignathus hungaricus,and B.mungoensis zones,followed in the Upper Triassic by the Quadralella tadpoleGladigondolella malayensis,Q.lobata-Q.carpathica,Primatella cf.orchardi-P.permica,Epigondolella rigoi-Pa.hallstattensis,and E.ex gr.bidentata-Norigondolella steinbergensis zones.The exact stratigraphic intervals present at Kamura were refined through intercalibration of the conodont biozonation data with its ?¹³Ccarb profile,which shows a characteristic series of negative and positive excursions that correspond to well-established global reference profiles.The Permian–Triassic boundary is placed 0.20 m above the Mitai-Kamura formation contact?a reference datum corresponding to the end-Permian extinction horizon?based on the first appearance of H.parvus,and the Olenekian–Anisian boundary is placed 17.60 m above the reference datum based on the first appearance of C.ex gr.timorensis.Kamura carbonates were assigned to six fabric types and 17 subtypes?= microfacies?.The microfacies succession records a deepening immediately following the end-Permian mass extinction,a strong shallowing during the mid to late Smithian,development of an unconformity spanning the entire Spathian,and minor sea-level fluctuations during the Middle and Late Triassic.The Spathian unconformity is probably attributed to a eustatic fall related to a sharp cooling event?and,thus,waxing of continental ice mass?at the Smithian–Spathian boundary that exposed the Kamura atoll.In the present study,elements and isotopes of conodonts were used as a proxies for ancient seawater chemistry and paleomarine environmental reconstruction.So we also studied the changes of elements and isotopes in conodonts during diagenesis.Our new findings on the conodont diagenesis are listed as below.By comparing REEs in conodonts and whole rocks,we found that REEs in the conodont samples were acquired mainly from clay minerals in the host sediment during burial diagenesis,based on strong positive correlation of REEs in conodonts to Th,strong positive correlation of REE and trace-element enrichment in conodonts and whole-rock samples,similar patterns in Triassic conodonts and whole-rock samples,and lower Y/Ho ratios in conodonts?mean ³3?.Conodonts show pronounced middle rare earth element?MREE?enrichment,a pattern that is unambiguously of diagenetic origin owing to its association with lower Y/Ho ratios.With increasing MREE enrichment of conodont samples,U concentrations and La N/YbN ratios shift from high to low,and Mn concentrations from low to high.These patterns suggest that conodont diagenesis was initiated at shallow burial depths under suboxic conditions?i.e.,in the zone of Mn?IV?and Fe?III?reduction?but continued at greater burial depths,with most acquisition of secondary REEs at later diagenetic stages.Our findings indicate that?1?conodont apatite frequently does not preserve a recognizable hydrogenous REE signal,and?2?the results of many earlier studies in which REEs in bioapatite were used as a paleoseawater proxy may need re-evaluation.By comparing structure and components in whole conodonts?i.e.,reflecting the composition of its outer layer?and split conodonts?i.e.,reflecting the composition of its interior?,we found that the outer surfaces consist of hydroxyfluorapatite and the interiors of strontian hydroxyfluorapatite.Ionic substitutions resulted in characteristic Raman spectral shifts in the position?SS1?and width?SS2?of the ?1-PO3-4 stretching band.Sr uptake was the dominant influence on diagenetic redshifts of SS1.Most specimens consist mainly of amorphous or poorly crystalline apatite,which is inferred to represent the original microstructure of conodonts.Oxygen isotopes show substantial variation within the conodont study specimens.Albid crown is on average 0.28-0.32 ‰ more depleted in 18O?equivalent to 1.2-1.4 °C higher temperatures?than hyaline crown and basal body,and the interiors of conodont elements are 1.08±0.37 ‰ more depleted in 18O?equivalent to 3.0-6.4 °C higher temperatures?relative to their outer layers.Although albid crown is widely regarded as better preserved than other conodont tissue types,its 18O-depleted composition and greater development of secondary crystallinity suggest that,in fact,it may be the most strongly altered tissue type.We first reviewed the Smithian-Spathian boundary?SSB?because it has not been formally defined,then proposed our definition,before a detail studies on the Smithian-Spathian boundary event.We reviewed biostratigraphy and carbon isotope stratigraphy around the SSB at the western Boreal,eastern Boreal,eastern Panthalassa,western Panthalassa,eastern Paleotethys,southwestern Tethys and western Tethys.The roles of key genus and species of both ammonoids and conodonts on the boundary definition were discussed.Based on the relationships between ammonoids,conodonts and shifts of carbon isotope profiles,we pointed out the incorrectly definition of the SSB in several sections.Based on our proposal,the Smithian thermal events probably start the early time of the late Smithian?between N3 and m?N3-P3??and the SSB was marked by the global cooling.We also proposed to define the SSB based on the first ocurrance of conodont Nv.pingdingshanensis at the West Pingdingshan section in Chaohu,South China.Carbon isotope shift from N3 or m?N3-P3?to P3 is also a good proxy for the SSB expecially in sections where fossil scarce.We studied three SSB sections?Shitouzhai,West Pingdingshan and Jiarong?in South China at the eastern Paleo-Tethys and Kamura section in Southwest Japan at Panthalassic Ocean.We found?1?SST of the Panthalassic Ocean is 8-12? lower than Paleo-Tethys in the Smithian,so the Smithian thermal event is not significant in the Panthalassic Ocean.?2?In the Paleo-Tethys,terrestrial inputs is little in the Dienerian-Smithian transition,raising in the early to middle Smithian,high inputs in the late Smithian and decline in the early Spathian.While in the Panthalassic Ocean,terrestrial inputs is none in the Dienerian-Smithian transition,raising but little in the early to middle Smithian,decline in the late Smithian.?3?Shallow water in the Paleo-Tethys is anoxic in the early Smithian,oxygen in the middle Smithian,hypoxia in the late Smithian and oxygen in the early Spathian.Deep water in the Paleo-Tethys is oxygen in the middle Smithian,hypoxia in the late Smithian,euxinic at the SSB transition and oxygen in the early Spathian.Shallow water in the Panthalassic Ocean is hypoxia in the early Smithian,oxygen in the middle Smithian,hypoxia in the late Smithian.?4?Duing the Smithian,?¹³Ccarb and ?³⁴SCAS show negative correlations at the shallow water in both the Paleo-Tethys and Panthalassic Ocean,while positive correlations at the deep water in the Paleo-Tethys.?³⁴SCAS values in the Panthalassic Ocean is lower than that in the Paleo-Tethys.Lower sulfate concentration and seawater retention time in the Panthalassic ocean maybe the reason for the difference.We studied Yaojiawan section?Upper Triassic?in Southwest China at the eastern Paleo-Tethys and Kamura section in Southwest Japan at Panthalassic Ocean.We found during mid-Carnian torrential rainfall event,?1?SST raised ⁵-7? in both Paleo-Tethys Panthalassic Ocean.?2?Terrestrial inputs is significant in the Paleo-Tethys than in the Panthalassic Ocean.?3?Seawater 87Sr/86 Sr values raise persistently during the event.?4?Anoxic to euxinic seawater conditions occurred at shallow water in both the Paleo-Tethys and Panthalassic Ocean,in which the anoxic is much serious and longer in the Paleo-Tethys than in the Panthalassic Ocean.?5??¹³Ccarb and ?³⁴SCAS show positive correlation in the shallow water at the Panthalassic Ocean.Generally,land weathering and anoxia enhanced with carbon-sulfur cycle anomaly happened at the beginning of the temperature raise suggested the control of climate on the environment changes both on the land and in the ocean.