The Analytical Method For Specie-specific Isotopic Composition Of Mercury In Sedimentary Rocks And Its Application In Paleoenvironment

Mercury（Hg）and its stable isotope characteristics in sedimentary rocks are emerging proxies of geological processes and environmental changes in the past.Hg and its isotopes in ancient sedimentary rocks are often used to indicate the volcanic activities and the relative redox changes closely related to life evolution.However,all current studies on Hg and its stable isotopes in sedimentary rocks are based on bulk analysis（measurement of the whole sample）,which could obscure important information on the speciation of Hg and the relative environmental processes that led to its speciation changes.In marine environments,Hg tends to bond to organic matter,sulfide,clay minerals and iron-manganese oxides.Moreover,the speciation of Hg in authigenic sedimentary rocks are affected by the redox condition and chemistry of ocean waters at the time of sedimentation.Thus,the different forms of Hg and their isotope compositions in sedimentary rocks are potentially better proxies of Hg sources and environmental processes compared to the bulk analysis.However,the studies on Hg speciation in sediments are rarely performed and no data are reported on the isotope compositions of different Hg forms in sedimentary rocks.In this study,we first developed a novel method for the separation and isotope analysis of different Hg forms in sedimentary rocks based on the thermo-desorption technique,and used this method to test the isotope compositions of different Hg forms,in order to better understand the source and process of Hg in sediments.Then we apply this method to study the environmental changes during major mass extinction events in the late Devonian.The method development includes three steps:（1）organic matter and major minerals（sulfide and clay mineral）that have high affinities with Hg were mixed with Hg standard solution(Hg²⁺)to produce corresponding Hg-mineral compounds,and then the thermo-desorption curves of various Hg species were recorded and the temperatures for their desorption were optimized;（2）the above-mentioned single-mineral Hg compounds were physically mixed to test their separation by stepwise thermo-desorption.The gaseous elemental Hg evolved at different temperatures were collected by using strong oxidizing solution（0.2%KMn O₄+5%H₂SO₄）for isotope analysis;（3）the Hg speciation and their isotope compositions of various reference materials（yellow red soil,stream sediment,marine sedimentary rocks）were analyzed using the method developed in step（1）and（2）,in order to validate the applicability of this method in actual samples.Our results show that:（1）The peak pyrolysis temperatures（T_P）for organic-,sulfide-and kaolinite-bonded Hg are 220±20℃、292±21℃、356±23℃,respectively.More than 90%of Hg can be released by keeping the temperature at T_P for more than or equal to 40 minutes;（2）There is a small difference for Hg isotope when the pyrolysis recovery is greater than 48%.Based on these results,the optimal values for the heating rate,temperature range and time of collection were selected for the separation of different Hg forms in mixed Hg compounds.The average difference inδ²⁰²Hg andΔ¹⁹⁹Hg between each Hg form and their origin reference values was less than 0.20‰and 0.03‰,respectively,validating that this method can separate different Hg forms without causing significant isotope fractionation.The method established above was applied to study the Pho Han Formation from Vietnam,South China,across the Devonian-Carboniferous boundary（DCB,359 Ma）.DCB is marked by one of the largest mass extinction events on Earth history（also called Hangenberg event）,but the cause of this event is still under debate.Both large-scale volcanism and ocean anoxia have been proposed to cause this extinction.We found that Hg concentration was abnormally high during the Hangenberg event,but there was no significant Hg enrichment when normalized to total organic carbon（TOC）or total sulfur（TS）.Hg concentrations show much stronger correlation with TS than with TOC,suggesting that the elevated Hg across DCB was likely caused by enhanced scavenging of Hg by sulfide minerals.Furthermore,the odd mass independent fractionation of Hg isotopes（MIF）shows consistently positive signals across the entire formation with no systematic trend,which indicates that Hg was dominantly from atmospheric deposition with no major changes in the source of Hg across the DCB.Combined with previous evidence on redox conditions,our new Hg isotope data argues against a global scale volcanism as the trigger of the Hangenberg Crisis and extinctions at the DCB,but instead suggests that enhanced ocean anoxia/euxinia as a result of increased bio-productivity was likely the major local trigger in the carbonate basins of South China.