Differential Mechanism Of Terrestrial Environmental And Floral Changes Driven By Large Igneous Provinces

The Quaternary is facing the change in the global environment and plant diversity related to the sixth biological crisis caused by rapid global warming,the study of deep-time can provide a reference for the prediction of environmental and plant change.With the continuous amalgamation of Pangaea,two significant changes in the terrestrial environment and flora occurred in the late Late Permian（Permian-Triassic mass extinction,PTME,～252 Ma）and the early Late Triassic（Carnian Pluvial Episode,CPE;～234-～232 Ma）,which was accompanied by the emplacement of the Siberian（at high latitude land in the Northern hemisphere）and the Wrangellia（at equatorial latitudes in eastern Panthalassa in the Northern hemisphere）large igneous provinces（LIPs）.The former represents the largest mass extinction event in Earth‘s history,resulting in substantial loss of plants,and accompanied by abnormal phenomena including lack of global coal seams,dry climate conditions,the collapse of soil system,and so on.The latter is considered to be a major critical period for the long-term slow and difficult recovery of terrestrial ecosystems after PTME and is also known as the dawn and dawn of the modern ecosystem.The CPE event caused an increase in the global carbon cycle,climate wetting,and plant radiation,and accompanied by the reappearance of the global coal seam,the emergence of modern coniferous forests,and the rise of dinosaurs.The existing evidence shows that the continuous input of huge amounts of CO₂ into the atmospheric system driven by the emplacement of the Siberian and the Wrangellia LIPs was the main reason for the changes in the terrestrial environment and plant during the PTME and CPE interval.However,the differential driving effect and mechanisms of these two LIPs on the terrestrial environmental and floral changes are still poorly understood.To reveal the differential driving effect of the LIPs on the terrestrial environment and flora,we take the typical continental strata of the Sunjiagou Formation to the lower part of the Liujiagou Formation in the ZK21-1 borehole of the Yuzhou Coalfield and the Dayulin section of the Yiyang Coalfield and the upper part of the Tanzhuang Formation to the lower part of the Anyao Formation in the ZJ-1 borehole of the Jiyuan Basin in the southern North China Plate（NCP）as the target strata.Based on the theories and methods of chronostratigraphy,petrology,geochemistry,mineralogy,and paleobotany,we conducted the studies on the integrated chronostratigraphy(inferred from zircon U-Pb dating,spore-pollen fossils,organic carbon isotopic compositions(δ¹³C_org),chemical index of alteration（CIA）,and Ni enrichment anomaly),the large igneous province records（inferred from Ni and Hg enrichment anomalies）,paleoenvironmental（including the carbon cycle,terrestrial weathering,wildfires,paleo-productivity,and redox conditions）and paleobotanical（inferred from spore-pollen fossils）recoveries,and the differential driving effects and driving mechanisms of the large igneous province on terrestrial environments and plants.This thesis is important for the in-depth understanding of the evolution of the surface Earth system driven by volcanism and can provide a scientific basis for the prediction of current and future environmental,climatic,and floral changes in the surface Earth system.（1）A high-resolution comprehensive chronostratigraphic framework of the target strata was established based on the zircon U-Pb dating,biostratigraphy,and chemostratigraphy.Based on previous research results,new Zircon U-Pb dating,and biostratigraphic（including spore-pollen fossils）and chemostratigraphic(includingδ¹³C_org and CIA)data,we determined that the lower part of the Sunjiagou Formation belong to the late Permian and the middle and upper parts of the Sunjiagou Formation belong to the Permian-Triassic transition in the Yuzhou Coalfield and the Yiyang Coalfield,and the upper part of the Tanzhuang Formation to the lower part of the Anyao Formation belong to the Late Triassic Carnian in the Jiyuan Basin from the Southern NCP.（2）The records of the LIPs of Siberian and Wrangellia were recovered by the Hg,Cu,and Ni enrichment anomalies in the target strata.The former shows two Hg enrichment anomalies in the Sunjiagou Formation,accompanied by Cu（Cu/Al ratios）and Ni（Ni/Al ratios）enrichment anomalies respectively,identifying two eruption pulses of the Siberian LIP;the latter shows four Hg（Hg/TOC ratios）enrichment anomalies from the upper part of the Tanzhuang Formation to the lower part of the Anyao Formation,identifying four eruption pulses of the Wrangellia LIP.（3）The records of the terrestrial environment（including the carbon cycle,terrestrial weathering,climate,wildfire,and water environment）in the target strata are restored by various alternative indexes such asδ¹³C_org,CIA,mineralogical index of alteration（MIA）,kaolinite content,and organic matter composition.The former recorded one significant negativeδ¹³C_org excursion associated with the of global negativeδ¹³C_org excursion,two periods of terrestrial weathering enhanced（inferred by the CIA,MIA,and kaolinite values）with relatively-short humid climates,frequent wildfires during the Late Permian（inferred by the fusinite and/or charcoal）that the arid climate promoted the development of terrestrial wildfires,and an increase of productivity（inferred by the P/Al ratios）and a stable water condition（inferred by the Th/U ratios）;the latter recorded four significant negativeδ¹³C_org excursion associated with the global negativeδ¹³C_org excursions,four periods of terrestrial weathering enhanced（inferred by the CIA and kaolinite）with the prevailing warm and wet climates,the low frequency of wildfires（inferred by the fusinite）that the humid climate hindered the development of terrestrial wildfires,and four times of the water productivity improvement（inferred by the P/Al ratios）and poor oxygen-anoxic environmental conditions appeared（inferred by the Th/U ratios）.（4）The records of floral changes（including large-scale disappearance/extinction and radiation）in the target strata are restored by spore-pollen fossils.The former identified 18 genera of spores and 25 genera of pollen in the Sunjiagou Formation,and recorded a rapid decline in the land plant‘s diversity,the latter identified 5 genera of algae,26 genera of spores,and 28 genera of pollen from the upper part of the Tanzhuang Formation to the Anyao Formation,and recorded the rapid increase of algae content and hygrophytes during CPE interval.According to the change of spore-pollen fossil content and the genetic relationship of its parent plants,the former recorded a hot arid and semi-arid climatic environment and the plant diversity decreased rapidly,the latter recorded a warmer and more humid climatic environment during the CPE interval（inferred by the results of hygrophytes,hygrophyte/xerophyte ratios,and principal component analysis）.（5）The driving effect and mechanism of LIP on the terrestrial environment and flora changes were summarized and suggested.The former releases a large amount of poor-¹³C CO₂,which causes global warming and leads to frequent wildfires during the PTME interval,driving the rapid decline of terrestrial plant content and diversity and the collapse of the soil system;the latter releases a large amount of poor-¹³C CO₂,which caused global warming and the increase of terrestrial water cycle and rainfall during the CPE interval,driving the terrestrial hygrophytes increase,lake expansion,and an algae bloom.The difference in global rainfall and terrestrial water cycle caused by the Siberian and the Wrangellia LIPs is the main reason for the difference in terrestrial environment and flora during the PTME and CPE intervals respectively.According to the location and nature of the LIPs,it is considered that the eruption of the Siberian LIP at high latitude in the northern hemisphere belongs to a land?dry‘volcano,while the eruption of the Wrangellia LIP at equatorial latitudes in eastern Panthalassa in the Northern hemisphere belongs to shallow sea?wet‘volcano.In the context of similar paleogeographic background（global plate tectonic location）and atmospheric circulation,the?dry‘volcanoes of Siberia triggered a limited（and relatively short）water cycle and increased rainfall through global warming,while the?wet‘volcanoes of Wrangellia triggered a global water cycle and increased rainfall through global warming.This is because the?wet‘volcanoes of are Wrangellia constantly pump water vapour into the atmosphere by evaporating large amounts of water around them,and this is the main reason for the difference between the terrestrial environment and plant changes during PTME（including the loss/extinction of plant,the loss of global coal seams,and climate aridity）and CPE（including the radiation of plant,the reappear of global coal seams,climate wetting,and the diversification of dinosaurs and modern conifer groups）intervals.