Mercury Spikes Suggest Volcanic Driver Of The Ordovician-Silurian Mass Extinction In South China

The Ordovician-Silurian?O-S?mass extinction was the first extinction and the second largest of the“Big Five”extinctions in Phanerozoic.In terms of species loss the O-S extinction represented about 86%of marine life,including graptolites,brachiopods,trilobite and conodont and so on.The O-S extinction was marked by two extinction phases.The first episode occurred at Late Katian?D.mirus Zone?to Early Hirnantian?N.extraordinarius?Zone,coincident with the beginning of Hirnantian glaciation.The second pulse occurred at the Late Hirnantian?from N.extraordinarius to N.persculptus?with the Gondwana ice sheet starting to melt.However,unlike the other major mass extinction events,the driver for the O-S extinction remains uncertain.According to this,various hypotheses have been proposed,including the climate change,Hirnantian glaciation,sea level change,redox conditions change,gamma ray burst,bolide impact and so on,and also including the large igneous provinces?LIPs?volcanisms in this study.Mercury?Hg?is a proxy for volcanic activity.In marine environments,organic matter?OM?has strong affinity to dissolved Hg.OM scavenges dissolved Hg within the water column through sorption,acting as a conveyor belt that transports Hg to the sea floor through OM deposition.In general,Hg data along with TOC?total organic carbon?values have parallel trends,and the Hg/TOC ratio remained relatively stable.However,intense LIPs volcanisms would inject large amount of mercury into the atmosphere.Mercury depositing from atmosphere to marine condition might exceed the maximum amount that the organic matter could absorbed in the ocean and broke this balance between mercury and TOC,resulting in relatively excessive mercury deposition for organic matter as the positive excursion of the ratio of Hg/TOC in sediments.Therefore,recently the positive excursions of Hg/TOC ratios have been used as a key proxy of intense LIPs volcanisms in geological time.The study of Hg stable isotopes provides a new means for further understanding of the global biochemical cycle of mercury and the source of mercury anomalies.According to previous studies,Hg emitted by volcanoes has?²02 Hg values between approximately-2.0‰and 0‰and no MIF(?¹⁹⁹Hg⁰‰).While MDF?denoted here by?²02 Hg?is ubiquitous and occurs during many physical,chemical,or biological transformations of Hg,MIF?denoted here by?¹99 Hg?arises mostly through photochemical.The two deposition pathways,terrestrial runoff and atmospheric Hg²⁺deposition,of volcanic Hg to marine sediments have very different Hg isotopic signatures.Overall,mercury deposited through terrestrial runoff tend to have negative?¹99 Hg and?²02 Hg;and mercury deposited through atmospheric Hg²⁺deposition tend to have positive?¹99 Hg values and less negative?²02 Hg.Therefore,Hg isotopes can be used effectively to trace whether the Hg anomalies in sediments derived from volcanics and the main pathways for mercury deposited into marine sediments.The major elements and trace elements can be used to indicate the environmental characteristics,such as chemical weathering,redox state,terrigenous clastic input and primary productivity.It is helpful to indicate the relationship between climate change and marine environment through Ordovician-Silurian boundary,and whether the mercury spikes are related to other environmental factors.In this study,we investigated the geochemistry of Hg,TOC,Hg/TOC,Hg stable isotopes,major elements and trace elements of Ordovician and early Silurian strata in Yichang?Huanghuachang,Chenjiahe,Wangjiawan and Dingjiapo sections?and Zunyi?Xishui section?in South China,to determine the association between Late Ordovician volcanism and the Ordovician-Silurian mass extinction event.The results of research show that during Early and Middle Ordovician,the Yangtze platform was shallow-marine environment covered by epeiric sea.Hg and TOC values range within low levels and the Hg/TOC ratios remain stable.This indicates that in general,due to the organic matter has strong affinity to mercury,Hg and TOC values have a covariant correlation and the Hg/TOC ratios basically keep stable.However,during Late Ordovician to Early Silurian,the Yangtze sea changed to a anoxic deep-water environment.Both of Hg and TOC values increase sharply,and Hg/TOC ratios show two significant positive shifts through the O-S boundary.The first one is in the Late Katian D.mirus Zone,coincident with the first?Late Katian?extinction episode.The second Hg spike occurs in the Middle Hirnantian Stage,upper N.extraordinarius–N.ojsuensis Zone,simultaneous with the second?Late Hirnantian?extinction pulse.These results suggest intense volcanism occurred during the Late Ordovician likely played an important role in the O-S event.Through the Late Ordovician to Early Silurian boundary,?¹99 Hg values in Wangjiawan and Dingjiapo sections have limited variability and always positive values,suggesting that the main source of mercury in sediments was volcanic Hg²⁺deposition from the atmosphere,rather than the deposition of terrestrial mercury.The?¹⁹⁹ Hg values shift positively at both of the two extinction pulses,this is because the intense volcanic activities released large amount of Hg that changed to Hg²⁺through oxidation.Positive ?¹⁹⁹ Hg values occurred during the MIF of Hg²⁺,resulting in the slight positive shift in?¹⁹⁹ Hg values.The major and trace elements can indicate the changes of environment.By comparing the Hg contents with the various indicators of depositional environment,we can determine terrigenous clastic input,redox condition,chemical weathering degree,which could possibly have different influences on mercury concentrations.Therefore,it provides an effective method to study the coupling relationship between mercury geochemistry and geological event.In our study,the main source of mercury spikes is the larger Hg flux related to the LIPs volcanisms.