End Permian To Middle Triassic Plant Evolution Pattern And The Environmental Impact

The terrestrial plant is the key to the‘Carbon Neutral’project on the land,while the interaction between plants and the physical environment remains unclear.This study focuses on the most intense environment and land ecosystem stress through the whole Phanerozoic—Permian-Triassic period to quantify the plant power and explore the co-evolution between the paleo-flora and the environment,thus providing deep-time examples for future earth system prediction.About 250 million years ago（Ma）,the earth entered extremely warm house conditions after the end-Permian Mass Extinction event.The high temperature during Early Triassic is regarded as the consequence of the Siberian Trap degassing.However,former earth system simulation failed in catching the～5 Myrs（Million years）hotness in the Early Triassic and all the results suggested the atmospheric CO₂ dropped back to pre-extinction level within a few hundred thousand years.The terrestrial plant ecosystem absence in the recent paleo-Earth System model might be the potential reason.The～7 Myrs‘Coal Gap’in the Early Triassic indicated the decline of land plants and the absence of a plant biomass preservation environment.The increase of terrestrial debris in the marine strata saw the unstable soil system resulting from plant crises during this period.Studying the Permian-Triassic land plant biomass fraction in the global carbon cycle and the plant-related bio-weathering process is vital to the land module in the Earth System model.However,whether the plant has extinction or not is disputed,let alone the plant evolution pattern and the land biomass quantification.This research proposed three scientific questions based on the above puzzles:（1）whether global land plants were extinct or not in the End Permian and what the plant extinction and recovery patterns were during Late Permian to Middle Triassic;（2）Late Permian–Middle Triassic global vegetation and plant productivity reconstruction;（3）How land plant influenced the environment and climate in the Earth system model and explore the environmental-climatic driving mechanism of land plant and tetrapods extinction.For the first question,this study first carries out fieldwork in the Middle Triassic Badong Formation which contains the key transition stage during the plant recovery in the Triassic.The sedimentary facies,plant fossil fragments preservation condition and the morphology study of those plants in the Badong Formation are analyzed to reconstruct the local land ecology and landscape.Subsequently,those methods are applied to the South China Late Permian–Middle Triassic floras,to unravel the evolution pattern of the land plants and ecosystems.Take the South China study as a scale,the global terrestrial chronostratigraphy and plant data were collected for exploring the global plant evolution pattern.The squares and interpolated diversity analysis of our plant database indicates the sampling density influence is small and our database is reliable.We normalized the plant data by deleting the morphospecies with no taxonomic reconstruction potential.Then the normalized data is used in studying the global flora character by the family level clustering and the species diversity of each plant morphological group for the global scale plant evolution.For the second question,all the flora paleo-coordinates are reconstructed by Gplates using the same rotation files.Each flora type is expanded in its hospitable paleo-climate zone indicated by the climatic sedimentary records.Those Late Permian–Middle Triassic floras are matched with recent floras sharing similar character and function to reconstruct the paleo-plant productivity.For the last question,this study hires the advanced deep-time Earth system model SCION.The land plant productivity based on the plant fossil,paleo topography and lithology distribution are combined for the land 3D reconstruction and loaded into SCION to simulate the Late Permian–Middle Triassic global Carbon,Nitrogen,Oxygen,Sulphur and Phosphorus circulation processes for evaluating the plant power.The spatial temperature and runoff maps produced by SCION are taken for exploring the driven mechanism of the land plant and tetrapods evolution during this period.All the above is for the plant and environment co-evolution study,assessing the threshold value in the Earth system.One key point of this study is to find the biota and their host that is sensitive to the environment and climate change,to monitor the recent ones for alarming the Six Mass Extinction.Another one is to find the essential biota and environment for the Earth System stabilization that need to be protected.By carrying out the above methods in solving the three scientific questions,we get the following six conclusions:（1）The Middle Triassic Badong Formation in the upper and middle Yangtze area of South China contains seven sedimentary facies,from the land to the ocean including River floodplain facies（RFF）,Coastal plain and delta facies（CDF）,Sabkha facies（SBF）,Terrigenous clastic tidal flat facies（TTF）,Intermittent lagoon facies（ILF）,Longshore bar facies（LBF）,Restricted platform subtidal facies（RSF）.The landscape in Middle Triassic South China was composed of the upland conifer Voltzia and other seed plants,lowland ferns Todites,Anomopteris,Cladophlebis,sphenophyte Neocalamites,and unknown gymnosperm Pelourdea（Yuccites）,marsh and delta single Neocalamites group,coastal and tidal“mangrove-like”lycopod Lepacyclotes（Annalepis）and Pleuromeia,Peltaspermum might also live in this region.（2）The South China Cathaysian flora reached its peak in Late Permian Wuchiapingian and declined in Changhsingian,which all extinct in the End Permian and preceded the marine animal extinction a few ten thousand years.Till Late Triassic,the land plant diversity is recovered to pre-extinction level.The plant evolution pattern from Late Permian to Middle Triassic in South China could be classified into four stages.The first stage is the decline of the Cathaysian flora and extinction at the end of Changhsingian,which is characterized by the plant extinction rate significantly exceeding the origination rate and the absence of long-ranging taxa and coal deposition.The second stage is the survival of a few Cathaysian elements which lasted soon,and the dominance of the opportunistic herbaceous lycopod Tomiostrobus in the Permian Triassic transition at the bottom of the Kayitou Formation.The plant extinction rate is still higher than the origination rate in this period.The third stage is the plant diversity recovery recorded in the middle Early Triassic Feixianguan flora,highlighted by the plant origination rate exceeding the extinction rate for the first time.This period flora is typical of Mesophytic conifer,fern and seed fern,while the biomarker data indicated a high fraction of lycopod.The rare record of plant fossils,low C/N ratio and the absence of coal records of this period showed the plant biomass or abundance hadn’t happened yet.The low ecological niches were still monotonic.The fourth stage is the abundance and biomass recovery in the Lingwen and Badong formations.The plant extinction and origination rate equilibrates since the Olenekian Smithian,indicating the fast flora succession under high environmental pressure.Lingwen and Badong floras are the further development of the Feixianguan flora.The coal re-deposition in the lower Badong Formation and the appearance of land tetrapod Lotosaurus supported the recovery of the land ecosystem.The lethal hotness in the Early Triassic and cooling down in the Middle Triassic are coincident with the extinction of the peat Cathaysian flora and the ecosystem recovery.（3）We identified two different plant evolution events in the Permian to Triassic.The first one is the transition from Paleophyte to Mesophyte,which was the selective extinction of the hydrophytic spore plant and the fast evolution of the xeric gymnosperm.This event happened on a global scale while the time and pattern varied,and was driven by the forming of the Pangea supercontinent.Second one,the End Permian plant extinction only happened in the low to middle latitude（45°N–45°S）and is distinguished by the rapid decrease of the plant abundance and diversity with a species-level extinction rate of up to 86%,and the absence of coal deposition.This event reversed the global plant diversity gradient in the Early Triassic with the diversity centre transferred to high latitude.The evolution pattern of the land plant and tetrapod was similar.There was a refuge in high latitude and altitude areas.This extinction event happened at the same time in the low to middle latitude area,preceding the marine extinction event of 110–700 kyrs（Kilo years）.（4）The reconstruction of the land vegetation and Net Plant Productivity（NPP）result shows the plant diversity and biomass centre is in the low latitude area along the Tethys Ocean in the End Permian,and the inner continent area was low in NPP due to the aridity.The End Permian global plant productivity is 57.3–59.6 Pt C/yr,which dropped 65–79%after the low to middle latitude plant extinction.The global NPP dropped to 12.3–20.3 Pt C/yr in the Early Triassic Induan and the productivity centre transferred to high latitudes such as Siberia and Argentina.The high latitude floras further developed in the Olenekian and the global NPP increased to 25.4–34.5 Pt C/yr.The low to middle latitude tropical biome along the Tethys Ocean recovered and the land NPP centre returned to the low latitude,increasing back to 55.2–67.5 Pt C/yr in the Middle Triassic,(Pt=10¹⁵t).（5）The spatial plant fossil-based land NPP map,paleotopography map and lithology distribution are loaded into the biogeochemistry-climate model SCION.The simulation result indicated the Siberian Trap degassing triggered the End Permian’s rapid warming and mass extinction.It is worth noting that the absence of land plants in the Early Triassic,especially the extinction of the tropical rainforest along the Tethys Ocean,lead to the significant decrease of terrestrial plant carbon fixation through photosynthesis and the silicate weathering,causing the～5 Myrs hotness in the Early Triassic.The global average land surface temperature,reconstructed by the land plant vegetation based on the plant fossil and the air temperature produced by SCION,increased by～13.3℃,which amplified the local hydro cycle and caused the～64%increase of the global land physical erosion,fitting the geological record.The sensitivity analysis of those uncertain factors in this study including basal and granite exposed area,high CO₂ fertilization effect of Early Triassic on plant NPP,and plant-related weathering factor all confirmed that when CO₂ concentration exceeded the tipping point,the positive influence of CO₂ fertilization on the plant is far less than the negative impact of climate change,and the plant is the key of environment and climate stabilization.（6）The SCION model environment and climate simulation results indicate the Pangea supercontinent convergence resulted in the drought between north and south15°–45°latitude,which is the main cause of the Paleophytic dominant spore plant dying,in other words,the transition from the Paleophyte to the Mesophyte.The over 40℃land temperature killing zone expanded to the north and south 30°,together with the no runoff zone in the north and south 15°–45°,driving the extinction of land plants and tetrapods in the low to middle latitude（45°N–45°S）.This evidence shows the aridity driven by the Pangea supercontinent and the extreme hothouse driven by the Siberian Trap degassing were sufficient enough for the End of Permian land Mass Extinction.The SCION-produced global latitudinal surface temperature curve indicated the high latitude local species especially plant is very sensitive to climate change and is suitable for taking as the alarm biota and area.Furthermore,the land surface temperature in the low latitude area more easily exceeds the life temperature tipping point in the warming event due to its high baseline temperature.The low-latitude biota,especially the tropical rainforest system is the key to the Earth’s stabilization and hospital environment and requires as much protection as possible.