Large Perturbed C-N-S Biogeochemical Cycles And Their Driving Mechanism During The Early Triassic In South China

The Permian-Triassic boundary（～252 Ma）witnessed the most serious mass extinction over Phanerozoic time.In the aftermath of the extinction,the Early Triassic went through a prolonged process for rebuilding the marine ecosystem,which is known as a"delayed recovery".The delayed Early Triassic recovery has been closely related to the harsh marine environment（high temperature,anoxia,etc.）,but the specific linkage and mechanism are still unresolved.The marine carbon,nitrogen and sulfur cycles are significant for linkage between inorganic environments and organisms,which are key research areas for exploring the co-evolution of organisms and environments.Previous studies have revealed anomalous carbon,nitrogen and sulfur isotope fluctuations in seawater during the Early Triassic under prolonged high temperature and marine anoxia,indicating large perturbated carbon,nitrogen and sulfur biogeochemical cycles during this period.However,the details of the carbon,nitrogen,and sulfur isotope variations（i.e.,spatial and temporal characteristics）have not been well studied,limiting the understanding of the mechanisms underlying the anomalous carbon,nitrogen,and sulfur cycles and their interactions with biotic recovery.In this dissertation,we carried out high-precision carbon,nitrogen and sulfur isotope analysis on Lower-Middle Triassic successions in South China based on improved carbon and nitrogen stable isotope analysis techniques.On this basis,global comparisons are conducted to further explore the driving mechanisms of the abnormal carbon,nitrogen,and sulfur cycles during the same period.The main progress and innovative understanding are as follows.1.Improvements of isotope measurement technology.（1）A selective extraction method was developed for mixed calcite-dolomite minerals,which enables robust determination ofδ¹³C_carb of single mineral components in complex geological samples.（2）The structure of the reaction tube,chromatographic column,and split interface of the elemental analyzer-isotope mass spectrometer were modified and the analytical conditions were optimized to enhance detection sensitivity and improve accuracy.These modifications reduced the sample size requirements forδ¹⁵N analysis,enabling theδ¹⁵N analysis of sediment samples with low nitrogen content in the relevant work of this study.2.The Guandao section in Guizhou Province,the Shanggang section in Guangxi Province,and the Yashan and Chaohu sections in Anihui Province were selected to investigate the evolution and impact factors of the marine C-N-S cycles after the Permian-Triassic extinction event.Carbonate carbon isotopes(δ¹³C_carb),organic carbon isotopes(δ¹³C_org),bulk nitrogen isotopes(δ¹⁵N_bulk),carbonate-associated sulfur isotopes(δ³⁴S_CAS),and organic-,major-and trace-elements were analyzed to reconstruct high-precision carbon,nitrogen,and sulfur isotope evolution curves of the Early-Middle Triassic.Then,the study investigated the evolution processes and controls of the C-N-S cycles in the aftermath of the Permian-Triassic mass extinction.A biogeochemical model was used to explore the drivers and control mechanism of the anomalous C-N-S cycles.The main innovative insights obtained are as follows.（1）In terms of the carbon cycle,theδ¹³C_org profiles record 3-4 negative and positive excursions in the Lower Triassic limestone of the Chaohu and Guandao sections.Thereδ¹³C_org fluctuations correspond temporally to the variations inδ¹³C_carb,allowing for effective global comparisons.However,there are decoupling stages and disparities in the magnitude of change between pairedδ¹³C_org andδ¹³C_carb records,which was related to changes in organic matter sources.Theδ¹³C_carb records showed 3-4 negative and positive excursions in the Chaohu and Guandao sections,which can be effectively correlated on a global scale.The changes inδ¹³C_org of Chaohu were decoupled fromδ¹³C_carb fluctuations during the Induan and synchronized with theδ¹³C_carb fluctuations during the Olenekian.Theδ¹³C_org in the Guandao section was synchronous with theδ¹³C_carb changes during the Induan-to-Smithian stage and decoupled from the negativeδ¹³C_carb during the Spathian substage.The net carbon isotope fractionation values(Δ¹³C_carb-org)of the Chaohu sediments gradually increased from+20‰to+29‰associated with an increase in the C/N values from<4 to 20 during the Early Triassic,representing a shift of organic sources from anaerobic phototrophic bacteria to eukaryotic algae or a mixture of eukaryotic algae and chemoautotrophic bacteria.The significant increase inΔ¹³C_carb-org and C/N values at the Induan-Olenekian and Smithian-Spathian transitions indicates two brief eukaryotic algae blooms.The significant increase inΔ¹³C_carb-org at the Smithian-Spathian transition in several sections,including Chaohu,Yashan,Shanggang,and Hot Springs,indicates a comprehensive decline of anaerobic phototrophic bacteria and the flourishing of eukaryotic algae,leading to more effective removal of CO₂ from the surface environment and environmental amelioration.（2）Regarding the nitrogen cycle,this work recovered continuous marineδ¹⁵N record spanning～20 Myr from the end-Permian to the Late Triassic.Lowδ¹⁵N（～0‰）prevailed the Griesbachian-to-Smithian substages（i.e.,first～2 Myr of Early Triassic）after the sharp decrease around the Permian-Triassic boundary,reflecting nutrient-N limitation and enhanced nitrogen fixation.A large rise inδ¹⁵N（to+8‰）followed by a decline（to-2‰）reflects an aborted recovery of the marine N cycle during the Spathian Substage of the Early Triassic.During the Middle Triassic,δ¹⁵N fluctuations between+1‰and+4‰during the Anisian Stage,followed by stabilization around+4‰in the Ladinian Stage,suggest a slow stepwise re-establishment of the aerobic marine N cycle.Overall,the nitrogen cycle in South China experienced five stages:aerobic nitrogen cycle based on NO₃^-at the end of the Permian,anaerobic nitrogen cycle based on NH₄⁺during the Induan-Smithian,oscillation between anaerobic and aerobic nitrogen cycles in the Spathian,gradual stabilization in the Anisian,and complete restoration of the aerobic nitrogen cycle in the Ladinian-Carnian.Although both South China and northwestern Pangea experienced a transition to anaerobic N cycling during the Early Triassic,an earlier and more rapid onset of this event as well as larger N-cycle fluctuations occurred in South China during the recovery interval.The reconstruction of the aerobic nitrogen cycle based on NO₃^-was not completed until the late Anisian,lagging behind the amelioration of environments and increase in productivity that occurred at the boundary of the Early and Middle Triassic,which may have exerted pressure on biotic recovery.（3）In terms of the sulfur cycle,synchronous positive excursion ofδ¹³C_carb andδ³⁴S_CAS were recognized at the Induan-Olenekian and Smithian-Spathian transitions in Chaohu,which represent two global organic carbon and pyrite burial events.Theδ³⁴S_CASof Chaohu increased from+10‰～+20‰in the Induan to+30‰～+40‰in the Smithian,and then increased from～+30‰to near+40‰at the Smithian-Spathian transition.These two synchronous positive excursions ofδ¹³C_carb andδ³⁴S_CAS indicate low seawater sulfate concentrations in the Early Triassic as significant global features of the sulfur cycle.Rapid burial of organic carbon and pyrite caused synchronous positive excursions ofδ¹³C andδ³⁴S in the oceanic carbon and sulfur reservoirs through removing ¹²C and ³²S preferentially.The differences ofδ³⁴S_CAS fluctuations in various regions and water depths may be due to local redox conditions,organic carbon supply,and biological disturbance.（4）The results of COPSE modeling suggested that CO₂ degassing from the mantle can result in changes inδ¹³C_carb andδ³⁴S similar to those observed records in the Smithian-Spathian transition.The output of the model also matched the geological indicators of weathering intensity and temperature changes.The simulation used the average positive deviation amplitudes ofδ¹³C_carb andδ³⁴S_CAS during the Smithian-Spathian transition（+3.2±1.6‰and+7.3±2.3‰）for further quantitation,indicating that about 78,000（±13,000）Gt carbon with an averageδ¹³C of-5.5‰was slowly released into the atmosphere-ocean system at a rate of 0.26 Gt C/yr during the 300 Kyr of the early-to-middle Smithian period.Based on C-N-S isotope data and model simulations above,carbon release during the Early Triassic can explain the sustained high pCO₂,long-term greenhouse climate,enhanced continental weathering,ocean stratification and anoxia,and led to an abnormal coupling mechanism of the C-N-S cycle.Ocean anoxia and low sulfate concentration can cause the methane-sulfate transition zone moving upward,resulting in more release of CH₄.CH₄ can intensify NO₃^-consumption and lead to nitrogen deficiency with contribution from ocean stratification.Release of CH₄ and N₂O can contribute the maintenance of greenhouse climate.Nutrient nitrogen stress can enhance nitrogen fixation and cause ammonium marine conditions.Such nutrient supply and expansion of anoxia are beneficial to the proliferation of prokaryotes such as cyanobacteria and green sulfur bacteria.Organic matter particles from prokaryotic are more prone to be degraded during the sedimentation process,having positive feedbacks on stronger oceanic anoxia and stratification.Waning of volcanic activities,increase of ocean sulfate concentration,proliferation of eukaryotic algae,and thus improved C-N-S cycle dominated since the Spathian period,which played important roles in promoting biotic recovery.