Volcanic Eruption Intensity And Time Limit During The Late Permian To Early Triassic And Its Linking With The Mass Extinction And Subsequent Recovery

The Permian-Triassic?252 million years ago?mass extinction constitutes the greatest biocrisis experienced by life in the Phanerozoic,during which climate and environment underwent remarkable changes.The event markedly changed the aspect of life and is responsible for much of the structure of marine and terrestrial ecosystems today.The cause of this catastrophic event has remained enigmatic,but growing evidence shows that intense volcanic activity was coincident with the mass extinctin,such as the Siberian Traps Large Igneous Province?STLIP?and widespread volcanic ash besd in South China.Meanwhile,marine ecosystems devastated by the mass extinction had a highly prolonged recovery,and did not substantially recover until after the Smithian–Spathian substage boundary?SSB?of the Lower Triassic.This interval was marked by another severe loss of marine biodiversity coupled with global extreme climate change as well as carbon isotopic shift.Although renewed STLIP volcanism has been suggested,there remains controversy as to the cause of the protracted Early Triassic recovery as no direct geochemical evidence linking the two.Therefore,a detailed study of the Late Permian-Early Triassic volcanism can not only provide a clear understanding of the intensity,duration and frequency of volcanism during that time,but also further analyze the causal relationship between volcanism,the mass extinction and following recovery.Multiple claystone beds are prominent near the Permian-Triassic boundary sections in South China.The mass extinction horizon is always characterized by ash bed.Understanding the nature and source of the volcanic ash beds can help know the tectonic setting across the Permian-Triassic boundary and shed new light on the causal relationship between the volcanism and mass extinction.In this study,in total 21volcanic ash layers from across the Permian-Triassic boundary in South China were analyzed at the Shangsi,Jianshi and Meishan sections.Understandings are as follows:?1?Volcanic ash geochemistry indicates dacite and rhyolite compositions and that REE pattern and trace element spider plots are similar to subduction-related rocks.Zircons from the ash layers yield comparatively low Nb/Hf and high Th/Nb ratios,falling in the range of arc/orogenic-related settings.The relatively high U/Yb values and prominently negative anomalies in Eu*values accord with a continental crust origin,which is different from the STLIP rocks that are mainly derived from mantle materials.?2?Zircon Hf-isotope compositions show that?_Hf?t?values vary from-11.7 to 1.8,indicating involvement of both juvenile and ancient crustal components.Compared to the ash beds in Clarkina?C.?yini and Isarcicella?I.?isarcica conodont zones,the ash beds near the biotic extinction horizon display a large variation in?_Hf?t?and relatively positive average values,indicating input of juvenile mantle or crust,and implying rapid transit through the crust.Spatial and temporal distribution of ash beds from thirty one PTB sections worldwide reveal that volcanic ash beds or occur mainly in the Tethys region and were sourced locally.Integration of Hf-isotope and trace-element compositions from magmatic zircons suggests that the volcanism occurred along the convergent continent margin in,or near,southwestern South China as a result of the closure of the Paleo-Tethys Ocean.Coincidence of the mass extinction with flood basalt eruptions in Siberia is well established,but the exact causal connection between the eruptions and extinction processes is uncertain due to the absence of direct geochemical evidence linking the two.Furthermore,whether the STLIP had a global impact on the ecosystem is unkown.Recently,it is argued that increases in Hg preserved in sedimentary records can be used as a proxy for increased volcanic inputs.Volcanic Hg is largely released as gaseous Hg⁰and removed from the atmosphere either by direct uptake and absorption,or by oxidation to Hg²⁺species,then accumulates in ocean and terrestrial environments through rainfall or adsorption.In relatively normal environment,there is often a linear relationship between the content of Hg and total organic carbon content?TOC?in sediments.However,when the Hg is excessively input,the linear relationship will be broken,showing an Hg spike in sediments.Hg isotopes can undergo both large mass-dependent fractionations?MDF?and mass-independent fractionations?MIF?in nature.Hg-MDF(?²⁰²Hg)can result from many pathways,including physical,chemical and biological reactions,whereas Hg-MIF(?¹⁹⁹Hg)is controlled by more limited pathways?mostly photochemical?and is unlikely to be altered in the post-depositional processes.Hence,Hg-MIF(?¹⁹⁹Hg)is generally a more conservative tracer of Hg spike and can be used to identify the STLIP records in sediments.This paper selected six Permian-Triassic boundary marine sections in South China and North India,and systematically measured Hg contents and Hg isotopic compositions,and discussed the relationship between Hg and sedimentary environment with the help of major and trace elements,and obtained the following main understandings:?1?The six marine sections all record high Hg concentrations and Hg/TOC ratios at the mass extinction horizons.This enrichment does not show any correlation with lithology,redox and sedimentation rate variations during that time,indicating enhanced Hg import.The?¹⁹⁹Hg values at the mass extinction horizons in Shangsi,Daxiakou,Chaohu and Guryul Ravine sections all show sustained positive values,indicating a predominant atmospheric-derived signature of volcanic Hg.In contrast,the nearshore environment of the Meishan section displays a negative ¹⁹⁹Hg and?²⁰²Hg signature,interpreted to be related to terrestrial Hg sources.This indicates that sediments can receive variable proportions of their Hg from direct atmospheric and terrestrial inputs.For Meishan,a significant terrestrial Hg swamped out the atmospheric-derived signature.?2?Almost all the marine sections record high Hg/TOC values from the C.meishanensis to I.isarcica conodont zones,consistent with massive STLIP sill intrusion,indicate high rate of Hg emission during that time and that the enhanced Hg release had impacted global Hg cycle.In order to explore the spatio-temporal impact of Hg on the end Permian to Early Triassic ecosystems,and its possible link to the STLIP,this paper selected sections from South China and northern India,and determined Hg/TOC values and Hg isotopic composition to trace oceanic Hg levels.Previous studies have focused on the Hg record from the Boreal realm across the end Permian to Early Triassic.The two regions reported on here were located significantly further away from the STLIP,which was located in the northern hemisphere at that time.Occupying paleogeographic positions that were equatorial?South China?and high southern?India?paleo-latitudes.These records provide important new insight into the extent of global Hg loading to the environment.Main understandings are as follows:?1?Hg/TOC values are dramatically elevated approaching the end Permian mass extinction horizon and maintain high values until the base of I.isarcica conodont zone.In the stratigraphically overlying beds,Hg/TOC generally displays lower values with slight fluctuations through the two sections,indicating a normal Hg deposition.The slight fluctuations are likely related to the increased terrestrial Hg influx associated with strong chemical weathering in the Early Triassic,as shown by a positive correlation between the contents of Hg and Al,and by less positive?¹⁹⁹Hg values in Early Triassic samples.?2?Hg chemostratigraphy in the Early Triassic from the Boreal realm show a minor shift in Hg/TOC values in Late Smithian.However,the Chaohu and Guryul Ravine sections show overall background Hg levels in the Early Triassic.One possible explanation could be that eruption rates of potentially renewed STLIP volcanism were lower,such that Hg with a relative short atmospheric residence time does not increase significantly as compared to the longer residence time of greenhouse gases such as volcanic CO₂.In this case,volcanic CO₂ emissions may drive global warming but not leave a significant Hg spike in the rock record.The mass extinction process in terrestrial environment along with its correlation with marine sections has always been a difficult problem due to the absence of index fossil on land.Furthermore,the STLIP records and the geochemical behavior of Hg in terrestrial environment have not been explored.In order to solve these issues,this paper selected terrestrial Chahe section and transitional Jinzhong section in South China,and systematacially measured organic carbon isotopes and Hg contents,and obtained the following main understandings:?1?The organic carbon isotopes at the two sections show two obvious negative shifts in the lower and upper parts of the Kayitou Formation.The two shifts can be well correlated with the carbon isotopic shifts in the C.meishenensis and I.staeschei conodont zones at the marine sections.Intergated with fossil data,this paper proposes that the Kayitou Formation witnessed terrestrial extinction process.Corresponding to the two negative shifts in organic carbon isotopes,two Hg enrichments can be observed in the Kayitou Formation,indicating enhaced import of Hg and carbon which had impacted Hg and carbon cycle in terrestrial environments during that time.?2?Hg and TOC always show strong positive correlation in marine sedments.However,in terrestrial section,an insignificant relationship between Hg and TOC is observed,may suggest that a significant proportion of Hg is associated with clay minerals in terrestrial environments.Thus,Hg/TOC ratios can not be used to recognize abnormal Hg deposition in terrestrial environment.