【doc】底栖藻对扬子地台西缘晚奥陶世生态危机的改善作用

【doc】底栖藻对扬子地台西缘晚奥陶世生态危机的改善作用 底栖藻对扬子地台西缘晚奥陶世生态危机 的改善作用 古生物,41L2:2I1.218t2002年4月) ActaPMac~-mtologica$1nica,41(2):2112I8(Apt,2002) 底栖藻对扬子地台西缘晚奥陶世 生态危机的改善作用 李越冯洪真李军 1】中国科学院南京地质生物研究所南京210008 2)南京^学地球科学系南京210093 提要晚盟陶世的H乩纳太阽冰川导致垒球性海坦和两幕生物灭绝事件,第一幕,也是高潮幕,发生于 Rawtheyan期末;第二幕发生于Hirnamian期末在经历第一幕事件的过程巾.扬子地台卣缘的四川汉源地区太渡 河组浅水相灰岩和硅质灰岩中出现了极其繁盛的底栖藻类.局部层位中町达到造礁的程度大量底栖藻通过光合 作改善缓冲了这一棱海地区的低温,敏氧的灾变环境.并成为生态系统重建的先驱群落局部浅海环境的正常 化为后生动物腕足类,一叶虫和海百台在Hirnantian期南郑组的短暂复苏创造了生存条件:当后生动物再度成为 生态系统的主体时,啃食力的增大使底栖藻的居群数量骤然减低始于Hirnantian后期冰川消融的海进造成第 二幕14i?nlia动物群的灭绝 关键词灾变环境底栖藻晚奥陶纪世扬子地台缘 1前言 地史长河中历次短暂的转折时期,地球化学异 常值和生物多样性的骤然下降存在着惊人的一致, 基于此,古生物学家们提出了显生宙数次生态系统 恶化,生物集群灭绝事件假说.目前这一建立在突 变-仑基础上的假说逐渐得以证实(Alvarez,1980; Benton,1995;Jablonski,1986;Droseretn, 2000).研究大灭绝期生物环境的相关性,是这一 领域的重要内容之一,灾变事件之后不同门类的复 苏,生态系统的重建过程(Kauffmaneta1.,l995; 戎嘉余等.1996),生态系统崩溃和重建的动力机制, 以及它们之间的相关性等研究,引出的一一系列额的 假说(KauffmanandHarrles,1996).奥陶纪末生物 集群灭绝事件之起因,冰盖说最为广泛接受 (Brenchley,1984;BrenchleyandNewall,1984; RongandHarper,l988),大量的陆水冰盖凝聚 的方式盖覆于当时南半球板区,造成全球海退及温 度梯度增强,海水盐度变化,缺氧,水体分层等多重 灾变环境,而Hi~nantian后期冰川迅速消融,全球性 海进,导致了Rswtheyan末和Hirnantian末两幕起 因不同的生物灭绝事件华南的生物地层对此事件 的记录尤为显着(陈旭等,1990).第一幕,即高潮 幕,是以冰盖的逐渐凝聚,海平面持续下降而导致浅 水相至较深水相笔石的不断衰减,深水相的Foli en动物群也可能在此事件中灭绝;第二幕以第一 幕后广布的浅水相Hir*2antia动物群灭绝为终结 (RongandHarper,l999).从第一幕的灭绝事件后 接踵而束的Hirnantia动物群在AshgiI1末期,即 Hirnantian期的广泛分布,可推知两幕事件之间的 陆表海环境曾经一度趋于缓和本文选择扬子地台 西缘临近康滇古陆的四川汉源晚奥陶世的地层宴 例,分析底栖藻类在局部地区对改善第一幕生物灭 绝事件持续期问的近岸浅海环境的作用 收稿日期:200l10l2 '同家自然科学基盘(No49972007,.中国科学院古生物学与古^类学特别支待费.知 识创新工程经费资助课题 2l2古生物 2实例 2.1地层序列 扬子区西缘汉源地区(插图1)的晚奥陶世地层 以黑色岩系与碳酸盐岩交替发育为特征,笔石序列 因而并不连续,与同期中下扬子区广泛分布的所谓 宜昌型五峰组类型黑色笔石页岩不同,冯洪真等 (1993)称之为汉源型五峰期沉积.由上至下为龙马 溪组底部,南郑组,大渡河组,以川西汉源轿顶IJJ剖 面(插图2)和大瓦山剖面为代表龙马溪组底部为 碳质,白云质硅质岩,属Normalograptuspersculp— tUS带,现归于奥陶系之顶;南郑组为深灰色中厚层 细砂岩,含微晶灰岩角砾和生物骨屑灰岩透镜体,具 有典型的"Hirnantia"动物群分子,如腕足类Dol— mariettatestudinaria.nantiasagittt)'era.Hin— deliacr(23512incipiens,Kinnellakeilanae,Parom? loren&cfpolonica,Plectothyrellaplatys— trooides;大渡河组下部为菱锰矿,上部渐变为泥 岩和钙藻硅质灰岩(冯洪真等,1991).大渡河组泥 岩中含丰富的笔石,根据Chen等(2000)对扬子区 奥陶志留纪界线上下的地层划分,这一笔石组合可 对比到DecelZograptusm坍卢"带.大渡河组与 南郑组之间为假整合接触,这与Rawtheyan期冈瓦 纳大陆冰盖扩增造成全球性海平面跌落是相关的, 特别是在汉源这样一个位于康滇古陆的近岸带,基 底沉降速率通常很低,因而当全球海平面下降时,就 第41卷 更易于露出海面虽然在非连续笔石相地层中无法 精确拟定Rawtbeyan的顶界,但全球性海平面的持 续跌落具有一致性,它正是发生于大渡河组上部相 当于Decellograptusc0mpleJus带的时限第一幕 灭绝事件的过程中,大渡河组上部出现的浅海相碳 酸盐岩,包括灰岩和硅质灰岩沉积,可能是 Rawtheyan期全球海平面持续下降的结果,冯洪真 等(1993)分析的该地区五峰期海水盐度异常期也恰 好发生在这一时限内.之后的海平面已趋于相对稳 定,基底的沉降能造成Hirnantlan期的沉积空间. 冰盖消融导致的海进开始于Hirnantian期后期,相 当于Normalograptusperscullxus带的时限,龙马溪组 底部的含笔石硅质岩正是全球海平面上升的产物. 2.2第一幕生物灭绝事件期的底栖藻 这一时期海水盐度和温度变化主要受控于南极 冰盖凝聚,导致海洋水层上下交换不畅,底层水的盐 度高,密度大.轿顶山剖面底层水古盐度变化曲线 的结果说明,盐度的增高段与第一幕生物(主要是后 生动物)灭绝事件相吻合(插图2):在经历 Rawthevan期的海洋生物事件持续过程中,康滇古 陆之近岸区出现了大量的底栖藻,轿顶山剖面的藻 席富集层厚054m,与轿顶山临近的大瓦山剖面可 见由底栖藻建造厚可达15m的点礁(曲红军, 1988).底栖藻的类型按形态划分有两种:叶状藻和 丛状藻,叶状藻起障积作用,丛状藻起粘结作用,并 能聚积成核型石(插图3A). 插图1奥陶纪末期扬子地台和邻近地区的地理模式(据Chen".2000)以盈研究区位 置 MapshowingtheI…tOrdovicianpa】啷o.graphlcpattemofYangtzea 【formanditsneibouring—H (fromChPnet.2000】andthel~ca[itLcofthes~Iicminprintsludy 第2期李越等:底栖藻对扬子地台缘晚奥陶世生态危机的改善作用2】3 时代蛆岩性古盐厦(据冯洪翼等,事件生物群 (Age)(Fm(Lithology)1993)(Paleosalinity, Ferll{etal1993)(Events)(Biota) 言 耋—高('cs.,\u一.^ cs. 蠹 :雹i鐾lofmetazo~n) 苫 害\底栖藻繁盛 丧I\首幕生物【B.曲理 J伸直l灭绝事件g?thriving)l:馨 』n岬H?带的笔石 , ///? (Siliecarous shale) 圆 钙质粉砂岩 (Catca~ous siItstone] 圈 硅质蒹灰岩 (Silicerous algallimestone) 圈 泥质蕞褒岩 (Argillacerous algallimestone) 圜 锰矿层 (Manganese LMCFm: 龙马澳组 (LtmgmncahiFm 插图2川西授源轿顶山奥陶纪末地层序列以及盐度变化事件和生物娄型 StratigraphLcscquen~'es,pal~salinitych~ge.eventsandbk~licsuccessionoftI1esfOrdov[ ci~[九_I?Jiaodit~simnSeclion,Hanvuan.wcSichuan 3底栖藻对重建正常浅海环境的作用 突发性的生物集群灭绝事件(massextinotion) 波及整个生物圈的所有物种,而晚奥陶世 Rawtheyan期的这一次由冰盖凝聚导致的生物灭绝 事件,相对要延续一定的时间(Sheehanetal, J996),因此生态群落有一定的时间发生演替.最 近,Copper(2001)通过对北美乃至全球范围奥陶一志 留纪间的生物礁时空分布和造礁群落的分析,认为 冈瓦纳的冰盖和海退事件对生物礁的影响主要表现 在低温海水背景下碳酸盐岩生产量的降低和海平面 跌落对造礁空间的限制两方面.H[rnantian期间的 生物礁不但在数量和规模上较Rawtheyan期有所 扩增,而且礁群落的生物多样性明显超过了 Rawtheyan期和志留纪最早期.由此而推知,Hir nantian期的海洋环境曾经一度趋于缓和.这一时 期的海洋生态系统重建的动力机制很值得探讨,或 许,在不同的古地理背景下存在不同的重建方式 来自汉源的例证表明浅海底栖藻类大量繁盛并造礁 正是处于冰盖凝聚至消融过程中.藻类本身属于自 养型生物,是海洋生态系统中营养物质和碳酸盐岩 的初级生产者,它们具有更强的抗灾变能力.并且繁 殖快,适应性广,能通过光合作用提高海水含氧量, 特别是在后生动物属种大量灭绝,居群规模急剧减 小后,后生动物繁盛期对藻类的啃食压力大为减小, 因而得以大量繁衍.生物事件后底栖藻类作为一种 灾后泛滥单元(disasterlaxa)并不鲜见(Kerr,19tM; Schubert,1992),晚泥盆世的Frasinan\Famennian 事件(钟铿等,1992),古生代末多幕式的P\T事件 (Grolzingere/al,1995)后都有这种现象如果我 们仅仅是将Rawlheyan第一幕灾变时期的底栖藻 当作灾后泛滥单元似乎稍逊不足,汉源地区的底牺 藻繁盛几乎是伴随着后生动物灭绝过程同步发生 的,即它们不但是在后生动物贫乏,啃食压力减小这 样一种特殊灾变环境中的泛滥单元,还是修复第一 幕灭绝事件发生过程中日趋崩溃的海洋生态系统的 先驱生物.也正是由于底栖藻建造的碳酸盐岩台地 之后,以至海洋底层水的温度,盐度,含氧量恢复到 正常状态,才使Hirnantia动物群得出现和生存 海百合是奥陶纪正常浅海相地层中最常见的后生动 物化石之一,它在第一幕事件后和Iqirnantfa动物 群同时出现,正是浅海环境一度缓和的标志当然, 藻类对局部近岸区浅海生态环境的改善能力毕竟有 限,终究难以抗拒冈瓦纳冰盖消融,全球性的快速海 进导致的"Hirnantio"动物群灭绝事件=但是底栖 藻的繁盛和灾难环境的修复对后生动物在灭绝事件 过程中的新生起了重大的作用. 2】4占生物第41卷 插图3川西汉源轿顶山第一幕生物灭绝事件期间的底栖藻和其后复苏期的海百合 &nthi?tgaeffirstextinctionep~deandcJinoide~of~Dveuyep1s0.JiaodingshanSection.Hany~n, WestSichuan A:叶状藻(lg).x25.JrT:B:叶状藻(al舭e).×25,Jr7;c:丛状藻(algae).×40,Jr7;D:丛状藻(gae).X160.JrT; E:毒百台茎碎屑砂岩(crinoidaldebrissandstone),x25.J .?:lII 第2期李越等:底栖藻对扬于地台西缘晚奥陶世生志危机的改善作用 4结论 藻类是地球牛物圈最早的造礁生物之一.通过 20多亿年的漫长岁月,彻底改观了早期地球的大气 圈和水圈的物理化学性质.其本身也成为高等动植 物的祖先.晚奥陶世灾变期间,它们又在汉源这样 的近岸浅海区成为生态系统修复的主要动力.底栖 藻在Rawtheyan期的一度繁盛取代了灭绝事件之 前晚奥啕世后生动物为主的海洋生态系统主体构建 者地位.说明生物圈自身能通过群落替代的方式在 局部地区能有效地缓冲地史灾变期的外界压力.来 自扬子区西缘汉源的实例可为探讨第一幕生物灭绝 事件后局部浅海区正常海洋环境的再造和后生动物 复苏的动力机制提供线索: 本文承蒙薛耀松.欧阳舒研究员的不吝赐教,陈 旭研究员阅改文稿,均此深表感谢 参考文献 Alvarczl_W.v…W,As~roFl980Extra【cmLT1日1CRUSOfar1 ('retaeeou~Tertiaryetinction:F.xperimentalresu[tsandth~>re!i alinterpretation?.208lO951098 BentoilMJ.I995Diver,(ficalionandextinclioninthehistor>oflife Science26852—58 BmnchleyPJ,t984LateOrdovicianExtinctionsand【heirRelation ahip.the(;ondwanaOlaeiatfim?:BrenchleyPJ(edJFn11 and(?,《0^fl\Vi~eysLtd291—3f BrertchleyP』.Newall(;,1984LateOrdovieiancnv1~DnlCn[change~ andtheireffect0nfaunsj:Bru~mDLLedJA~pectsOf()|_ dovldanSystem,】i(ntUniv0slo295:6580 ChenXu,Rong『1ayu,1990Conce~sanda~mlyMsof…extincFion w…thexampler,』[.areOr&lvicianc…【hi:RongJiayu.? (eds)Selectedhp?)fTh~3reticalPal~ntalogyNanjing: Nanjingun1verMtyPress91120finChinese) ('henXu,RonRJia-yu,MitchellCet2?OLateOldovlclantO earlierSilu~angraptollteandbrachiop~x{bioT~nationh.mYangtze region,SomhChina.with,global(~~rrelatJ0nGmlMag,137 (6):623—650 C~~pperP,20[1lReefduringthemultiplecris~协qstheOrdovi, 咖—【u…boundary:Anti&~Fi[Mandast…Canadaand 生物学撮第41卷 BENTHICALGAEINIMPR0VEMENT0FEC0L0GICCRISIS0FTHELATE 0RDoVICIANINTHEWESTMARGINOFTHEYANGTZEPLATF0RM LIYHe 1%anjinglmlitute.,Gecdog~,andPala~nt,dory,CAme~eAcademyof&ience.~,Nan; 2ng210008, FENGHongZhen 1EarthScience1)ep~ert~1)ellfrq,nnjingU….Na)oing210093 IIJtin Nanjinglnst!tuteit帅{('andPalaeorttolog.,.fhweAcademyqSciencesNanfing210008) KeywordsDisasterenvironment,Improvement,Benthicalgae,LateOrdovicianWestmargi nofYangtzePlatform bummary Itisimportantinevolutionarypaebioogyt() proposethehypothesisofseveraldeteriorationsofthe ecosystemandthetaxonomicn]assextinctionduring thePhanerozoicbasedontheevidenceofgeochem— istryanomalouscoincidingwithrapidlydropofbiodi versitywithinashorttirilejntervaI(A]varezetal,, 1980;Benton,1995;Jablonski,1986;Droseret ,2000)Consequently,thehypothesisof"catas— trophisnf'hasbeenverifiednowadaysAtthesame lime.aseriesofllewhypothesesonthebiologicalre— coveryafterdisasterevent,thedynamicmechanism ofthecollapseandreconstructionoftheecosystem, andtherelevaneebetweenthemisbeingtested (KauffmanandErwin.1995:KauffmanandHar ties,1996;Rongeta1.,I996)Amongthecauses ofthetWOepi.-~x:lesofextinctionduringtheAshgillian (LateOrdovician),theglacioeustatica—level changestheoryisthemostwidelyaccepted(Brench— ley.1984;BrenchleyandNewa]l,1984;Rongand Harper,1988).Duringthefirstepisode (Rawtheyan.1%hgillian).plentyoffreshwaterveas condensedontheGondwanalandicebergspreadingon the~>uthernHemisphere.whichcausedregression andeffectedseveralrelevantdisasterenvironments suchastheclimaticgradientsteepened,changesof palcosalinity,anoxic,lowertemperature.andthe lavcredseawaterAsaresultofthat,theshallowto deepermarinegraptolitesanddeeperaFolimena Faunabeearneextinctstepwise]yHowever,thesec— ondepisode(Hirnantian,Ashgil1)event,indicated bvtheextinctionofshallowwaterHirnantiaFauna, wascausedbytherapidlytransgressionoficeberg mehingI'hedrasticaldecreaseofAshgiIlbiodiversity WaScorrespondingtoit,whichiswellprovedby"the biostratigraphicrecordfromSouthChina(Chenand Rong,1990;RongandHarper,1999)Itconbede— ducedfr0mthewidedistributionoftheHirnantia FaunaduringtheHirnantianthatsoonafterthefirst episodeextinction.Theepicontinenta[seaenviron mentbetweenthetwoepic~destendedtobemitigat— edInthispaper.theauthorstrytoemployanexam— pleofLateOrdovicianfromHanyuan,Sichuan, whichislocatedonthewestmarginoftheYangtze pieLtformandneartheDianqianOIdland,andtoana一 ]yzethebenthiealgae,whichmighthaveimproved shaUowri/arineenvironmentwithinthefirstepisode ofcrisisduration StratigraphicSequenceThe[ithologyoftheLate OrdovicianinthewestmarginoftheYangtzePlatformis calledthe'HanyuanType,whichischaracterizedby graptolitieblackshalesintercalatedwithcarbonatesand differentfromthe"YichangType",whichischaracter izedbytypicalgraptol[ticblackshalesandwidelydis— tt~hutedinMiddJe—LowYangtzePlatformatthesameage fFengelal,1993)(Fig1)TheUpperOrdovician stratafromHanyuanincludethebaseoftheLungmaehi Formation,theNanchengFormation,andtheDaduhe Formationindescendingorder(Fengelal,1991) (Fig.2).TheHirnantiaFaunaoccurringinthe NanchengFormationiscomposedofDamanellatestu— i~arla,Hirnantiasagittifera.HindellaffYassaincipi一 ?.KinneHal~el'&nae,~"o~tla[ortlenaCipolonica, Plectothyellaplatystrophoides(brachiopodj.Thebot tomoftheLungmachiFormationbelongstotheNor— rnalograptuspersculptusBiozonethatiscorrelatedwith theuppermostOrdovician(Chenela1.,2000j.The 第2期李越等:底栖藻对扬子地台西缘晚奥陶世生态危机的改善作用 JiaodiugshansectionandtheDawashansectionsymbolize thesestratigraph[csequencesBetweentheDaduheFor mationandtheNanchengFormationisapamcomfornfily, whichcolnl2ideswiththeglobalsea【eveIdec?netothe IO\VSt8ndcausedbvtheGundwanalandicesheetThear easnearseashore&renlorelikelytoappearabovethesea leve【thanotherareasinthelatestRawtheyanwhenthe sea—leveldropratedoesnotcorrespondtosubsidenceand accunmlationAhhoughthetopofRawtheyancarlnotbe confirmedw-ithindisconnectedgrapto]iticstratase— quences,thegLobaLfallofthesealeverisun~nlnlOUS, therefore.thetopoftheDaduheFormationcanbere gardedasthesummitofthefirstepisodeextinction.Car— bonatesappearedontheupperpertoftheDaduheForma tlon,inc[udingcalcareousalgaem[criteandcalcareousaL— gaesiltynficrite,istheresultoftheconsistentgLoba[re- gressioninRawtheyan.Atthistime,thesearevelisrda— tivelystabteandthebasa[subsidencecanvacaterooDlfor Hirnantiansediments Benthicalgaeduringthefirstepisodeextinction Duringthisperiod.thepa[eosa[inityandthetemperature ofthemarinewaterweremainlyinfluencedbytheglacier' scondensationofthefreshwaterTheglaciationcon strainedthesmoothexchangeofseawaterbetweentheup— perpartandthelowerpart,therefore,thepaLec6aLinityof the&cahottonlwashigherandthedensity,waslarger throughtheanalysisdiagramofthatfromtheJiaoding shahsection.Thepaleosalinityrosecoincidedwiththe firstepisodeofmassextinctionthatappearedmainlyin rnetazoanaccordingtothepresentresearches(Fig2)1t happenedtoheduringtheprocessofthemarineorganisnl eventofRawtheyantI~atappearedplentyofbenthiccal— careousalgaenearthe.seashoreoftheDianqianOtdland. Thethicknessofthebenthicalgaethrivin~gbedofthe Jiaodiugshansectionunderstudyis054rnwhilethatof Dawashansection,notfarfromJiaodingshan.section,is l5I71thickStromato[ites(Qu.1988)Accordingtothe morphologyofbenthicalgae,theycanbeclassifiedinto twocategories=phylold—shapeandcoLoniaLshapeThe phy[old-shapedalgaeformedbafflestone.whilethecoLo— nia[一shapedalgaeformedbindstoneTheblndstonecanaL一 ?accumulatetoforraoncolites(Fig.3) Someinlpactdisasterevents.suchasthatoneatthe Cretaceous/Tertiaryboundary(Alvareza1.,l980), almostbroughttotalinfluenceonthewh.lebiotaofbio sphere.However,theregressionbecauseoftheglacier condensationrannmchslowerthanthetransgressionbe— causeofthemeltingoftheglacier,isaprogressmadestep bystep(Sheehanelal,l996)Thefollowingmassex tinctionduringtheRawtheyanalsoadvancedstepwise[y; thus.theecotogicalconmmnitiescouldreplaceBased0n theworld—widedatabaseofthereeftentpora[andspatia】 distributionbetween0rdovicianandSi【uriahboundarv. Copper(2001)suggeststhatcoolersea—surfacetenlpera tureslhnitthecarbonateproductivtyofreelecosystem andrestricttheaccornnlodationspaceforreefgrowth Furthermore,inthepa[eotropics,theHirnanlianinter— glacialsshowedhigherbiodiversitythaneitherthepreced ingRawtheyanorthefollowingRhuddanlan(earliestSil uriah)warmintervalsTheevidencefromHanyuanl【]us. tratesthatit,,gasJustatthesunlmitofthepa[eosa[inityin theglacialepochthatthebenthicalgaebecantethriving andevenconstructedalgalreefsThealgae,withthe photosynthesis,aretheprmaryproductiveforcesofthe nutrimentandcarbonaresinthemarineecosystemThey havegreatabilitiesofresistancetodisaslersandstrongre productivecapacityandincreasethequantityofoxygenin themarinethroughphotosynthesisThealgaenauLtip[ied intoalargenumbersincethepressureofgrazingxa/asde creased.especiallyafterextinctionofthemetazoanand thesharpdectineoftheirpopu[ation.Asthedisastertaxa, thebenthicalgaewerenotseldomseenaftermath(Kerr. [994:SchubertandBottjert.1992).whichhada】?oc— curmdaftertheFrasinan\FamennianeventintheLate Devonian(Zhongela1.1992)andthePermian\Trias siceventbytheendofPateozoic(GrotzingerandKnott. 1995)ltisaLiedenotenokLghtoviewthebenthicaLgae inthefirstcrisisepisodeoftheRawtheyanasthedi~ster taxabecausethebethicalgaeintheHany/tanthrivedaL mostattheganletimeastheprocessofthefatlnaextinc— tionThatis.theatgaewerenotonlythedisastertaxain theperticu[arenvironmentofthelackofcompetitorand thedecreaseofthepr~qureofgrazing.buta】s.thepio neerintheimprovenlentofecosystemwhentheformer marineecosystemcoltapsedgraduallyIt\~-asjustafterthe 【imitedcarbonateplatfonnthatwasformedbythebenthic algaethattheglrnantiafaunaappearedOfCOUrSe,the abilityofairaeimprovedtheenvironmentinso~leshaltow marine~verellmitedTheg[oba【massextinctionkvasirre— sistibleTthrivingofthebenthicalgaeandtherestora tionofthedisastrousenvlromnentlaidahintforeshadow ingforthenewbornoftheHirnantiaFaunaandtheCOnl— munitysuccessionThus,assoonasthedisastrousenvi— romnentrecoveredandtheH/rnantmFaunaoccupiedthe shallowmarine,thepressureofgrazingincreasedagain. CrlnoidesisoneofthecommonestmetazoanfossiIsinthe 218占生物 strataoftheshallowmarinefaciesinthe0rdovicianand whoseappearanceisthesymbolofthenormalizationofthe sha]lowmarineonce A[gaearetheearliestreefbuddersoftheearthbio. sphereFhroughi]torothan2billionF~arsevolution,the algaechangedcompletelytheatmosphereandthehydro- sphereoftheearthduringtheearlyperiodandthus?