Distribution_...n_South_China_Qian Deng.pdf

ORIGINAL ARTICLEDistribution and geochemical significance of trace elementsin kerogens from EdiacaranLower Cambrian strata in SouthChinaQian Deng1,2Guanyu Zhao1,2,3Haozhe Wang1,2,3Bin Cheng1,2Zewen Liao1,2Received:2 July 2022/Revised:12 September 2022/Accepted:16 September 2022/Published online:17 October 2022 The Author(s),under exclusive licence to Science Press and Institute of Geochemistry,CAS and Springer-Verlag GmbH Germany,part ofSpringer Nature 2022Abstract Some trace elements(TEs),both bio-essentialand redox-sensitive,are promising tracers of Earths sur-face processes.As an essential sedimentary organic matter,kerogen may have inherited and retained the key TEs fromorganisms.In this study,the distribution of TEs in kero-gens from two sections(Dongkanshang and Fengtan)inSouth China was analyzed to explore its biogeochemicalsignificance during EdiacaranLower Cambrian.Com-pared with V and Zn,the relative concentrations of Co,Ni,Cu,and Mo were generally higher in kerogens than in thewhole rocks.Enrichment of TEs in the kerogen wasinvolved in the selective utilization of TEs by differentorganic precursors and the competition of sulfide in thedepositional environment.The significant content of TEs inthe kerogens from deep-water facies corresponded to amore negative d13Ckeras a result of the dominant taxa ofchemoautotrophswithtremendousbioaccumulationpotential for trace metals and the reduction conditionsfavorable for organic matter preservation.Furthermore,thecontent variations of trace elements in the kerogens coin-cided with the rise and fall of the Ediacaran biota,implyingthat the trace elements might have played an important rolein early life evolution.Keywords Trace elements Kerogen EdiacaranEarlyCambrian Biogeochemical significance South China1 IntroductionTrace elements(TEs)can exist in sedimentary rockscombined with organic matter in addition to occurring in oradsorbed on minerals(Wu and Chu 2001;Han et al 2011).Some transition trace metals,such as nickel(Ni),copper(Cu),zinc(Zn),and molybdenum(Mo),are essential forthe growth of organisms and are sensitive to the redoxconditions of the sedimentary environment.These twofeatures make them critical components of biogeochemicalcycles in modern and past environments.At the molecularlevel,such elements function as active centers or structuralfactors in enzymes and are accumulated and stored byorganisms through biochemical mechanisms(Morel et al.2003).A portion of the assimilated TEs are usually dis-charged into the seawater during bacterial degradationprocesses,and the other parts are incorporated into thesediments during the deposition of the organic biomass(Morel and Price 2003;Sunda 2012).Besides the bioticprocesses,TEs in seawater could be adsorbed or com-plexed with organic matter through abiotic processes(Tribovillard et al.2006),such as being cheated by COOH,OH,SH,NH,and other ligands of organicmatter(Zhang et al.2019).It has been widely recognizedthat organic matter plays a vital role in the formation ofsome metal deposits(Orberger et al.2003;Coveney andPas ava 2004;Wen et al.2006;Kr bek et al.2007;Shi et al.2013),which also proves a close relationship betweenorganic matter and TEs.Zewen L1State Key Laboratory of Organic Geochemistry,GuangzhouInstitute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China2CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,China3University of Chinese Academy of Sciences,Beijing 100049,China Acta Geochim(2023)42(1):5062https:/doi.org/10.1007/s11631-022-00571-wKerogen is an insoluble sedimentary organic matter withstable properties and complex structures.It has excellentpotential in recording past biogeochemical cycles.A partof bio-essential trace elements,required explicitly byenzymes and proteins in organisms(Anbar 2008),will bedeposited along with the non-degraded organic matter andeventually conserved in the kerogen.TEs occur in kerogenmainly in the three states:organic-bonded,occluded,oradsorbed.For instance,V and Ni occur typically in kerogenin the form of metalloporphyrin(Van Berkel et al.1989).Se in kerogen may be organic-bonded or present as ultra-micro inclusions(Wen et al.2003).A variety of analytical methods have been conducted todetermine the content of TEs in kerogen,including directspectral quantitative determination(Yang 1982),instru-mental neutron activation analysis(INAA,Mercer et al.1993;Felitsyn and Morad 2002),atomic absorption spec-trometry(AAS,Akinlua et al.2007;Jegede et al.2018),X-ray fluorescence spectrometry(XRF,Akinlua et al.2010),and the most commonly used inductively coupledplasma mass spectrometry(ICP-MS,Wen and Qiu 1999;Piet al.2013;Adegoke et al.2014;Deng et al.2015;Guoet al.2016;Xu et al.2018).One of the difficulties inkerogen analysis derives primarily from the isolation pro-cedure,in which some acid-resistant minerals,e.g.,pyrite,rutile,and complex fluorides precipitate from the mixtureof HClHF acids may persist.In addition,some submicron-nano mineral p