Marine Redox Variability And Its Coupling With Oceanic C-N Biogeochemical Cycling Across The Permian-triassic Boundary In South China

The largest mass extinction event in the Phanerozoic has occurred during the Permian-Triassic boundary?PTB?.Although oceanic anoxia is regarded as a leading cause of the PTB mass extinction,its timing,extent,spatial-temporal distribution and underlying causes remain unclear.Carbon and nitrogen are essential bio-organic elements,which are the bridge between environment and life recorded by ?¹³C and ?¹⁵N in geological time.Although significant excurison in marine C-N isotope during the PTB crisis have been examined,highly spatial heterogeneity in C-N isotope are supported by some studies for different depositional environment,which may be affected by water column redox variations during the PTB.Iron speciation and redox-sensitive trace elements are sensitive and efficient paleoredox indicators widely applied to paleocean systems.Here,we conducted and integrated,high resolution Fe-S-C-trace element study at Liangfengya?LFY;shallow carbonate platform?,Ganxi?GX;slope?,and Chaotian?CT;basin?sections,integrating these results with previsouly published data from Shangsi and Xiakou,in order to explore the oceanic redox variations and their environmental controls during the PTB,South China.Based on those findings,we further explore the oceanic C-N cycles processes and their relationships with contemporaneous redox changes,in order to better understand the co-evolution between environment and life.The main results of this thesis are shown as following:1.The integrated Fe-S-C-trace element data suggest that:?1?a highly spatial heterogeneity of bottom and phothic waters in South China,which behaving as the dynamically coexistence of the mid-depth euxinic water and suboxic-oxic water in surface and deep water,?2?the euxinic water extended to deep-water setting at the early Changhsingian age but turn into ferruginous conditions gradually at the mid-Changhsingian age,ferruginous conditions punctuated by both euxinic and oxic episodes spanning the latest Permian mass extinction,?3?a large spatial heterogeneity in pyrite sulfur isotopic compositions(?³⁴Spy)were co-existed with dynamic redox variations: heavy ?³⁴Spy in oxic water,lighter ?³⁴Spy in low sulfate concentration of ferruginous water,and heavy ?³⁴Spy in relative ample sulfate concentration of euxinic water.A large spatial gradient in pyrite sulfur isotopic compositions during PTB is consistent with a limited oceanic sulfate concentration during this period.2.Redox vaiations were controlled by comtemporaneous oceanographic changes in the upper oceanic thermocline region:?1?euxinia in deep water of early Changhsingian may record elevated marine productivity that possibly related to upwelling?2?the euxinic-to-ferruginous transition from the late Changhsingian to the early Triassic can be attributed to a combination of reduced sulate availability and rising terrigenous iron inputs?3?transient intervals of euxinia during PTB were facilitated by enhanced sulfate fluxes related to episodic volcanism durig this interval.3.There are a highly spatial heterogeneity in ?¹³C_carb and ?¹⁵N in the end Permian,South China.12 C and 14 N are more enriched in deep-water sections than shallow-water sections around WCB.vertical ?¹³C_carb gradient and ?¹⁵N gradient?shallow to deep?reached to +7.1 ‰ and +4.9 ‰ respectively,but those gradients of ?¹³C_carb and ?¹⁵N gradually reduced to the level of modern ocean at the mid-Changhsingian age and disappeared at the late Changhsingian age.The large gradients in C-N isotope around WCB document the upward of OMZ in deep water and intensified stratification.The decrase in C-N isotope gradients in the midChanghsingian age are consistent with the downward of OMZ.The homogenization of C-N isotopic gradients during the mass extinction correspond to the widely distribution OMZ.The resboud of C-N isotopic gradient post-extinction is attributed to the temporary recovery of oceanic environment.4.Intergrated ocean chemistry variation,?¹³C_carb and ?¹⁵N in Ganxi,the finding shows that redox variation,marine productivity,volcanic activity played an important factor in the changes of C-N cycle process of Ganxi.In particular,carbon and nitrogen proxies show pronounced patterns of covariation as well as an alternation between two distinct modes of behavior.One mode is characterized by low ?¹⁵N?<0.5 ‰?,high ?¹³C_org?>-26 ‰?,low and nearly invariant C/N ratios?⁴-8?,.The second mode is characterized by high ?¹⁵N?>0.5 ‰?,low ?¹³C_org?<-26 ‰?,higher and variable C/N ratios?²0-50?.The two boundaries?WCB and PTB?exhibit the second?ferruginous-euxinic and nitrate-limited?mode,as does an extended interval of the lower Changhsingian,whereas the upper Changhsingian and lowermost Triassic exhibit the first mode?suboxic and nitrate-unlimited?.Three different patterns between C/N ratios and ?¹⁵N indicate a change in between nitrogen fixation rates and denitrification rates.5.Based on published fossil record and our paleoceanographic chemistry and C-N-S isotope results,we found that euxinic conditions in the early Changhsingian age correspond to rare in quantity and diversification of fossil records,and dominant in anoxic morphology,implying an ecological response of marine biology to environmental change.The contratidiction between euxinic waters and rare fossil records may be attributed to the short-lived intervals of dysoxic-oxic conditions allowed benthic fanua to infiltrate the sediment or euxinic water not to infiltrate the sediment owing to the dynamic OMZ.The downward of OMZ in the mid-Changhsingian age with increasing quantity and biodiversity in basin?Chao tian section?also supported the reconstructed dynamically OMZ model.During the mass extinction,the widespread ocanic anoxia and nitrate limited with large loss of biodiversity demostatedthe important role of turbulent oceanic anoxic during the mass extinction.After the main episode of mass extinction,the ocean water gradually oxidized,providing space for remaining species,but the recovery still remain slow,which may be controlled by the other factors except anoxia.Our research shows the spatial distribution and evolution of marine biology was controlled by the spatial and temporal fluctuation of oceanic chemistry during P-Tr interval.