葉云濤，王華建，翟儷娜，周文喜,4，王曉梅，張水昌，吳朝東

1.北京大學地球與空間科學學院，造山帶與地殼演化教育部重點實驗室，北京 100871 2.北京大學石油與天然氣研究中心，北京 100871 3.中國石油勘探開發(fā)研究院油氣地球化學重點實驗室，北京 100083 4.貴州大學資源與環(huán)境工程學院，貴陽 550025

新元古代重大地質(zhì)事件及其與生物演化的耦合關(guān)系

葉云濤1,2,3，王華建3，翟儷娜1,2,3，周文喜3,4，王曉梅3，張水昌3，吳朝東1,2

1.北京大學地球與空間科學學院，造山帶與地殼演化教育部重點實驗室，北京 100871 2.北京大學石油與天然氣研究中心，北京 100871 3.中國石油勘探開發(fā)研究院油氣地球化學重點實驗室，北京 100083 4.貴州大學資源與環(huán)境工程學院，貴陽 550025

新元古代的地球表層系統(tǒng)經(jīng)歷了超大陸裂解與重組、大規(guī)模冰期、古海洋氧化、埃迪卡拉生物群輻射與滅絕、后生動物興起等一系列重大變革，這些地質(zhì)事件與生物演化在時空上的耦合關(guān)系長期受多學科交叉研究領(lǐng)域的廣泛關(guān)注。Rodinia超大陸的裂解伴隨有超級地幔柱活動、古地磁真極移等復雜響應，裂解過程影響了大氣圈和水圈中氧氣和二氧化碳的循環(huán)，并可能直接導致了新元古代極端的氣候條件。構(gòu)造格局的變動對生物的影響主要體現(xiàn)在物質(zhì)來源和生存環(huán)境的改變上，強上升洋流和強地表徑流區(qū)域的富營養(yǎng)化促使生物大量繁盛?！把┣虻厍颉逼陂g巨大的選擇壓力為生物的多樣化演變提供了可能，而其后冰川的快速消融則促進了生產(chǎn)力的爆發(fā)式增長及多種沉積礦產(chǎn)的形成。與此同時，大氣—海洋氧氣含量的增加和海水化學結(jié)構(gòu)的改變使得多項元素及同位素指標發(fā)生了地質(zhì)歷史上最大幅度的波動，這種特殊的地質(zhì)背景可能最終對生物演化產(chǎn)生了極為深刻的影響。

新元古代；超大陸事件；冰期事件；大氣氧含量；生物演化

0 引言

新元古代氧化事件前后一直被認為是地質(zhì)歷史中的關(guān)鍵轉(zhuǎn)折時期，是地表環(huán)境從低氧到富氧、生物種群從原核類到疑源類再到真核類的進化輻射期，也是幾次全球性冰川的形成—消融期和超大陸的裂解—重組期，多項地球化學指標隨之發(fā)生了顯著波動[1-5]。其實，地球的演化進程包括了若干次超大陸裂解—重組和冰期—間冰期旋回[6]，這些地質(zhì)事件往往與大氣和海洋的氧化程度，以及生物演化極具耦合性(圖1)。超大陸和冰期旋回控制了海平面升降和生物生存空間，而生物與地表環(huán)境的相互作用又進一步影響了大氣圈、水圈的物質(zhì)循環(huán)。對于在地球演化過程中表現(xiàn)最為特征的生物相，真核生物出現(xiàn)、真核藻類停滯性發(fā)育、后生動物出現(xiàn)、埃迪卡拉生物出現(xiàn)并滅絕、寒武紀生物大爆發(fā)及進入顯生宙后的數(shù)次生物滅絕和復蘇，均被認為與古海洋化學條件的改變、冰期及超大陸事件密切相關(guān)[11-14]。在我國華南、塔里木等地，新元古代地層包含多套黑色巖系，它們不僅是優(yōu)質(zhì)烴源巖層，而且伴生有多種金屬、非金屬礦產(chǎn)，具重要經(jīng)濟價值[15-16]。

前人對這一時期的古構(gòu)造、古海洋及古生物等方面已開展了大量有益的工作[6,17-19]，但由于涉及內(nèi)容的廣泛性及多學科交叉研究的復雜性，這些地質(zhì)事件之間的相互關(guān)系及其控制因素等許多問題仍未得到圓滿的解決。為此，本文力圖從地球系統(tǒng)科學的角度出發(fā)，對新元古代重大地質(zhì)事件和生物演化進程做一綜述性評論，探討各要素之間的耦合機制，為地球科學綜合性研究的開展起到拋磚引玉的作用。

1 地質(zhì)事件

地球的表層系統(tǒng)在新元古代發(fā)生過劇烈變化，包括超大陸裂解與重組、極端氣候條件、古海洋氧化還原環(huán)境的改變等。這些重大地質(zhì)事件之間存在著一定程度的聯(lián)系，同時又可能影響了生物演化的進程。

圖1 地質(zhì)歷史時期的大氣氧含量[7]、超大陸[4]、冰期[6]、古海洋化學[8-9]及初級生產(chǎn)者[10]Fig.1 Temporal trends of atmospheric O2 content[7], supercontinent formation[4], glaciations[6], deep ocean chemistry[8-9] and primary producers[10]

1.1 超大陸事件

1 300～900 Ma期間，地球上曾存在一個包括了當時幾乎所有陸塊的超級聯(lián)合古陸，稱之為Rodinia超大陸，其范圍從赤道一直延伸到極地地區(qū)(圖2)。印度東高止山脈帶990～900 Ma的高級變質(zhì)巖區(qū)[21]及我國華南920～880 Ma的弧火山巖和蛇綠巖仰沖侵位[20]均記錄了Rodinia超大陸的匯聚過程，即格林維爾造山運動。其后，870 Ma和845 Ma的雙峰式侵入巖體代表了Rodinia裂解作用的開始[22-24]。廣泛的地幔柱活動主要發(fā)生在825 Ma和780～750 Ma兩個階段，證據(jù)包括：基性巖漿群[24-27]、高溫科馬提質(zhì)玄武巖[28]、區(qū)域性穹窿[25]、以及大陸裂谷[29]等。大約780 Ma之后，Rodinia的主體已基本位于中低緯度地區(qū)，Lietal.[30]用真極移理論解釋了這種板塊位置的快速變化，并認為非赤道地區(qū)超級地幔柱的形成使得地球核幔邊界以上的硅酸鹽巖殼圍繞格陵蘭附近的旋轉(zhuǎn)軸發(fā)生了近90°旋轉(zhuǎn)。該理論隨后在斯瓦爾巴特群島東部[31]及澳大利亞[32]等地相繼得到證實。

Rodinia超大陸解體后，在南半球地體重組形成了影響整個古生代的Gondwana超大陸(圖2)。其中，西Gondwana在650～600 Ma時已初具規(guī)模，而東Gondwana的匯聚主要發(fā)生在750～620 Ma和570～500 Ma[33-35]。隨著Mozambique洋的閉合，Gondwana超大陸得以最終形成，東西Gondwana相互聚合形成的一系列巨大山鏈可能代表了地球有史以來最大的一次陸陸碰撞[36]。

1.2 冰期事件

新元古代中晚期，地表氣候變化劇烈，以幾次大規(guī)模冰川的形成和消融為特征(圖3)。成冰紀Sturtian冰期(720 Ma)[38,42]和Marinoan冰期(635 Ma)[44-45]均表現(xiàn)出全球性低緯度冰川的分布特點，而發(fā)生于約750 Ma的Kaigas冰期和580 Ma的Gaskiers冰期由于沉積物厚度不穩(wěn)定且側(cè)向連續(xù)性差，因此只代表區(qū)域性冰川事件[43,46-48]。

“雪球地球”假說最早由Kirschvink[49]提出，Rodinia超大陸在低緯度的裂解被認為是導致其形成的關(guān)鍵因素，尤其720 Ma勞倫古陸北部Franklin大火成巖省的噴發(fā)使大量鐵鎂質(zhì)巖石在赤道地區(qū)遭受強烈風化[50]，其對CO2的消耗可能是觸發(fā)“雪球地球”的扳機[51]。Shenetal.[52]認為，新元古代中晚期之后臼齒碳酸鹽巖的消失說明溫室氣體甲烷的釋放量也發(fā)生明顯下降。海洋初級生產(chǎn)力的增加對應著冰期前δ13Ccarb的正漂，生物對碳的固定同樣利于大氣中的CO2的消耗[53]。此外，由于冰蓋相對于陸地和海水具有更高的反射率，兩極冰蓋擴張過程中，反射率變化所產(chǎn)生的正反饋效應很可能使地球在短時間內(nèi)進入全球性大冰期[54-55]。然而，鑒于一些冰期沉積物遠距離搬運的特征，Hydeetal.[56]提出了“半融雪球”概念，認為當時地表并未完全被冰覆蓋，赤道附近仍然能吸收足量太陽光能而防止冰蓋的形成?？紤]到地熱等因素，Ashkenazyetal.[57]也指出冰期強烈的海水混合及赤道翻轉(zhuǎn)環(huán)流會在大陸邊緣形成無冰水域。這種無冰水域的存在保證了海洋與大氣、陸地間的物質(zhì)能量交換，為生物在冰期的繁衍和冰期后的快速復蘇提供了保障。

圖2 Rodinia超大陸和Gondwana超大陸復原圖[20]Fig.2 Reconstruction of Rodinia and Gondwana[20]

圖3 新元古代無機碳同位素記錄[37-42]及冰期沉積物分布范圍[43]Fig. 3 Composite δ13Ccarb profile[37-42] and global distributions of Neoproterozoic glacial deposits[43]

成冰紀的冰川沉積物往往被一層碳酸鹽巖所覆蓋，其極負的δ13Ccarb值及與現(xiàn)代冷泉區(qū)相似的沉積構(gòu)造指示了當時大規(guī)模的甲烷滲漏[58-59]?！把┣虻厍颉逼陂g，在永久凍土帶和陸緣海區(qū)域可能形成巨量甲烷水合物；冰川消融初期水合物失穩(wěn)分解產(chǎn)生的甲烷將進一步加快冰蓋的融化，冰期積累的高濃度碳酸根離子與甲烷的氧化作用共同引發(fā)了蓋帽碳酸鹽巖的沉積[60]。BIF型鐵礦在新元古代的出現(xiàn)是這一時期極端氣候條件的另一重要體現(xiàn)[61-62]，其形成受控于海水中H2S與Fe2+的相對比例[63]。經(jīng)PAAS標準化后輕稀土虧損、重稀土富集的配分模式，高Y/Ho比及弱的Eu正異常說明BIF來源于火山熱液和海水的混合溶液[64-67]。冰期陸源輸入硫酸鹽含量的降低和洋中脊上覆靜水壓力的減小均會導致海底熱液流體具更高的Fe/S比[68]，缺氧停滯的海洋使Fe2+得以累積并在間冰期氧化形成全球性的BIF鐵礦[69-71]。這一結(jié)論與新元古代深海由硫化向鐵化環(huán)境的轉(zhuǎn)變相一致[72]。

1.3 氧化事件

海洋氧化還原條件的重建是古海洋研究的核心，對于解釋水圈和大氣圈、生物圈之間的相互作用至關(guān)重要。元古代海洋的水化學結(jié)構(gòu)一直備受爭論，其核心問題是硫化水體的形成機制與分布范圍?！癈anfield海洋”模型認為中元古代—新元古代中期深部海水廣泛發(fā)育硫化環(huán)境[73]，并以此解釋了真核生物在中元古代停滯演化的現(xiàn)象[74]，1.8 Ga首現(xiàn)的大型熱水噴流沉積礦床似乎支持硫化海洋的假設(shè)[75]。然而，Lietal.[76]根據(jù)我國華南新元古代陡山沱組Fe-S-C化學系統(tǒng)的研究，提出了具有三維結(jié)構(gòu)和動態(tài)變化的“硫化楔”模型(圖4)。隨后，該模型被證明普遍適用于元古代到寒武紀早期的海洋環(huán)境[77-79]。對中元古代海洋的模擬計算也顯示，其硫化面積可能不到總面積的1%～10%[80]。事實上，由于陸源物質(zhì)風化產(chǎn)生的硫酸鹽是海洋中硫的主要來源，受早期海水硫酸鹽儲庫和有機碳制造能力的限制，硫化水體主要發(fā)育在陸緣海區(qū)域，難以大范圍擴張，也很難長期穩(wěn)定維持。

最近，Zhangetal.[9]在我國華北下馬嶺組識別出了中元古代海洋“最小含氧帶”(圖4)?！白钚『鯉А焙Ｑ蠡瘜W結(jié)構(gòu)的存在表明當時大氣氧含量已經(jīng)足以維持水體下沉過程中氧氣的消耗。雖然中元古代可能曾出現(xiàn)過弱氧化的底水環(huán)境，但深海的普遍氧化主要發(fā)生在新元古代晚期之后。阿曼、澳大利亞及華南等地報道的埃迪卡拉紀地層中強烈的δ13Ccarb負漂移被認為是深海氧化的重要證據(jù)[81-82]。加拿大紐芬蘭地區(qū)Conception群的鐵組分數(shù)據(jù)也說明Gaskiers冰期之后深部水體普遍充氧[83]，這一時間與阿瓦隆底棲生物群的出現(xiàn)(579～565 Ma)大致對應[84-85]。

圖4 元古代海洋化學模型“Canfield海洋”模型[73]；“硫化楔”模型[76-77]；“最小含氧帶”模型[9]Fig.4 Conceptual models for the redox structure of Proterozoic ocean

2 地質(zhì)事件的地球化學記錄

晚新元古代的另一顯著特征即地層中碳、硫、鍶等穩(wěn)定同位素及鉬、鈾等氧化還原敏感元素的大幅波動(圖5)，這些地球化學記錄不僅反映了長時間尺度下的生物地球化學循環(huán)，還可能與許多全球性的地質(zhì)事件密切相關(guān)。

2.1 碳同位素

碳是生命和埋藏有機質(zhì)中最重要的組成元素。在有機質(zhì)制造和降解過程中，均會產(chǎn)生一定量的碳同位素漂移，而有機質(zhì)的制造和降解速率又往往與其地質(zhì)背景有關(guān)。如冰期時，光合生物的有機質(zhì)制造能力極低，伴隨著δ13Ccarb的負漂；而冰期結(jié)束后，海洋中初級生產(chǎn)力的增長使大量富輕碳的有機質(zhì)被埋藏，δ13Ccarb出現(xiàn)正漂[92]。有機、無機碳同位素在地質(zhì)歷史中總體表現(xiàn)出一致的變化趨勢，其中δ13Corg在元古代早期的幾次強烈負漂均被認為與微生物對甲烷的利用有關(guān)，暗示了當時大氣中極低的氧含量[93-95]。

新元古代超大陸裂解、全球性冰期等事件的集中發(fā)生對碳同位素產(chǎn)生了明顯影響。Gaskiers冰期前，δ13Ccarb以正值為主，僅在Sturtian、Marinoan兩次冰期前后存在短暫負漂，冰期結(jié)束后迅速恢復至正值區(qū)間，平均值約為+5‰[42,96]；而Gaskiers冰期之后，δ13Ccarb發(fā)生了地質(zhì)演化過程中最顯著的一次負漂移(圖3)。盡管不能排除成巖改造的影響[97-99]，但大部分學者仍認為這種階梯性特征與古海洋化學條件和有機質(zhì)產(chǎn)率的改變有關(guān)[81,100-101]。新元古代氧化事件之前缺氧分層的海洋十分有利于生物有機質(zhì)的制造和保存，δ13Ccarb與δ13Corg的解耦說明其溶解有機碳庫的規(guī)模可能10倍于同時期的無機碳庫[102-104]。Gaskiers冰期后，大氣氧含量的增加改變了海水的化學組成，深水有機碳被礦化從而參與到海洋表層的碳循環(huán)中，δ13Ccarb由+5‰快速下降至-12‰，隨后δ13Corg發(fā)生了相應負漂[82]。這種變化在全球許多地區(qū)的埃迪卡拉紀地層中均可進行對比，代表了這一時期深部水體的廣泛氧化[39,81,105]。

圖5 地質(zhì)演化過程中的地球化學記錄無機碳、鍶同位素[4]；黃鐵礦、硫酸鹽硫同位素[86]；鉬[87-88]、鉻同位素[89-90]；黑色頁巖中的鉬[91]、鈾含量[88]；總有機碳[91]Fig.5 Compilations of geochemical proxies through time

2.2 硫同位素

海洋中的硫循環(huán)與碳循環(huán)十分相似，黃鐵礦的埋藏同有機碳埋藏一樣，有利于大氣中氧氣的累積，而硫酸鹽和黃鐵礦間的硫同位素分餾則可用于解譯海水硫酸鹽濃度的改變[106-108]。δ34SPy和δ34SSO4曲線在地質(zhì)歷史時期大致相互耦合，但δ34SPy變化更為頻繁，這主要是由于硫的氧化、還原和歧化反應容易受局部沉積環(huán)境的影響。太古代δ34SPy平均值在0‰附近，說明當時還原性的海水中極度匱乏硫酸鹽[109]。新元古代晚期硫同位素分餾明顯增加，Δ34S由成冰紀末期的0‰左右增加至埃迪卡拉紀中期超過46‰(圖5)，這種顯著的同位素分餾被歸因于大氣氧含量升高引起的硫的歧化代謝作用[73,81,110]。另外，硫同位素波動還可能與冰期或其他生物地球化學擾動有關(guān)[111-112]。例如，納米比亞Rasthof組蓋帽碳酸鹽中δ34SPy的異常高值(>60‰)就反映了冰期后海水中極低的硫酸鹽濃度[112-113]。

2.3 鍶同位素

新元古代初期87Sr/86Sr介于0.705 2～0.705 5[114]。Rodinia超大陸的聚合使得古老陸塊被孤立于缺乏水分的內(nèi)陸，而陸緣地區(qū)遭受風化剝蝕的主要是具87Sr/86Sr低值的新生地殼，類似的現(xiàn)象在Gonwana超大陸和Pangea超大陸聚合時同樣存在[96]。晚新元古代到早寒武世期間，87Sr/86Sr由<0.706升高至>0.709[114-115]。Shields[115]對87Sr/86Sr的升高給出了3種可能的解釋：1)87Sr/86Sr值更高的巖石遭受風化；2)伴隨洋中脊擴張速率的降低，由洋中脊熱液交代及玄武巖熱液蝕變輸入的Sr相對減少；3)地表風化作用增強。盡管不能排除假設(shè)1)發(fā)生的可能，但目前Sr、Nd同位素數(shù)據(jù)均未顯示出放射性含量更高的巖石遭受了風化剝蝕[116]；而新元古代頻繁的構(gòu)造活動和海平面變化明顯與假設(shè)2)相悖。因此，大陸風化作用在新元古代晚期至早寒武世的顯著增強應該是87Sr/86Sr升高的主要原因，且這一推測與冰期后大氣中極高的CO2濃度相吻合[117]。

2.4 鉬、鉻同位素

富鐵化學沉積巖(如BIF、富鐵硅質(zhì)巖)中鉻同位素的變化也可用于示蹤大氣—海洋系統(tǒng)的氧化情況。新元古代之前的BIF與高溫巖漿巖的δ53Cr沒有明顯差異，只在2.8～2.45 Ga和1.88 Ga存在兩次小幅上升，而對沉積于Sturtian冰期的Rapitan組、Gaskiers冰期前后的Yerbal組和Cerro Espuelitas組的研究表明，其δ53Cr發(fā)生強烈正漂，最高達4.9‰[89]。另外，由于黑色頁巖中富含大量自生鉻，因此也可被用于鉻同位素測試，加拿大Wynniatt組頁巖(0.8～0.75 Ga)中高達2‰的δ53Cr正值可能揭示了新元古代氧化事件的序幕[90,125]。

2.5 氧化還原敏感元素

鉬、鈾等氧化還原敏感元素在沉積物中的富集程度除了與其自身的地球化學性質(zhì)有關(guān)外，還受控于海洋中該元素儲庫的大小。由于在缺氧硫化環(huán)境下，海水中的鉬、鈾幾乎被定量的扣留在沉積物中，因此硫化沉積物中的鉬、鈾含量能作為反映該元素在海水中可得性的指標[120,126]。太古代沉積物具有極低的鉬、鈾值，2 200～2 000 Ma前后鉬、鈾第一次明顯富集，這次大氣氧含量的增加同時對應了δ13Ccarb正漂移所指示的有機碳大量埋藏[91,127-129]。中元古代硫化水體的發(fā)育對海水鉬、鈾儲庫影響顯著，沉積物中鉬、鈾含量普遍較低。到新元古代中晚期之后，海洋的鉬、鈾儲庫再次擴大(圖5)，沉積物中鉬的含量甚至在Marinoan冰期后不久就曾短暫地接近現(xiàn)代水平[130-131]。

3 地質(zhì)事件與生物演化的耦合關(guān)系

在漫長的地球歷史中，生命完成了從以原核細菌為主的荒蕪狀態(tài)向顯生宙大型化、復雜化和軀體骨骼化的后生動物的轉(zhuǎn)變。生物的生存和輻射并不是隨意安排的，而是需要相當匹配的外周環(huán)境，包括溫度、水質(zhì)、氧氣及物質(zhì)能量等。真核藻類和后生動物在晚新元古代的集中演化與當時的地質(zhì)背景可能存在極大聯(lián)系。

3.1 超大陸事件與生物演化

超大陸裂解—重組對生物演化的影響主要體現(xiàn)在物質(zhì)來源和生存環(huán)境方面。新元古代Rodinia超大陸的裂解導致全球性海侵，并形成了大范圍的陸架盆地[132]。這些陸架盆地不僅具有豐富的陸源營養(yǎng)輸入，并且可能存在區(qū)域性上升洋流的貢獻[4,133]。為保證足夠的光能進行光合作用，元古代海洋中大部分生物的演化仍是在表層水中進行的，但持續(xù)的有機質(zhì)沉降會造成水體中營養(yǎng)物質(zhì)缺失，若得不到有效補充，將極大程度上影響生物繁育的可持續(xù)性[134-135]。只有當營養(yǎng)物質(zhì)通過上升洋流或陸源輸入重新供應到表層時，生物的繁育才能持續(xù)存在。同時，海岸帶水體的垂向混合為生物生存空間向海洋深部的擴展提供了可能。因此，超大陸裂解期常對應著富有機質(zhì)黑色頁巖的發(fā)育期，我國華南大塘坡組、陡山沱組等均沉積于Rodinia裂解時期，白堊紀時北大西洋開裂也使其兩岸發(fā)育了多套優(yōu)質(zhì)烴源巖。

在為生物提供必要的生存環(huán)境和物質(zhì)來源的前提下，超大陸事件還一定程度上影響著生物進化的方向。Petersetal.[136]提出世界范圍內(nèi)的寒武紀地層與其基底之間存在著一個穩(wěn)定的大不整合面，說明當時強烈的風化作用可能將大量無機離子帶入海洋中，使早寒武世海水的化學組成發(fā)生了巨大變化，以小殼動物群為代表的生物礦化機制在這一時期的產(chǎn)生可能就是對這種變化的應答[137-138]。

3.2 冰期事件與生物演化

冰期旋回的特征表現(xiàn)為溫室—冰室環(huán)境的交替。溫室條件下，海平面上升、淺海陸棚大面積形成。溫暖濕潤的氣候使地表化學風化作用大大加強，隨著陸源碎屑和淡水的注入，淺海將在較短的時間內(nèi)變?yōu)楦粻I養(yǎng)環(huán)境，十分利于浮游藻類的生長[139]。光合藻類產(chǎn)生的氧氣可能使大氣和淺海中的氧含量有所升高，為后生動物出現(xiàn)和演化提供基礎(chǔ)[140]。我國大塘坡組底部錳礦和陡山沱組磷礦與黑色頁巖的伴生關(guān)系即表明冰期后大量營養(yǎng)物質(zhì)在海洋中的富集促進了生物的繁盛[141-143]。相比之下，冰室環(huán)境中生物的生存面臨巨大的選擇壓力。一些生物的數(shù)量和種類在極冷事件中顯著降低，而另外一些類群的遺傳物質(zhì)可能在此期間發(fā)生了明顯變化。廣泛分布的冰川使得海水變得停滯、連通性降低，之前溫暖淺海中發(fā)育的微生物群落被隔離、封閉，由此產(chǎn)生了多樣化的生存環(huán)境，這些都與冰期后真核生物的多樣性演化關(guān)系密切[139,144]。此外，對成冰紀BIF的P/Fe比研究顯示，當時海水的磷含量較元古代早期發(fā)生了極大幅度的增長[145]。冰川對大陸巖石的研磨作用會在冰退時將大量磷元素釋放到海洋中[146]，從而為藻類的興盛提供養(yǎng)料。

新元古代冰期見證了生物進化的重要革新。我國華南大塘坡組、陡山沱組二段和四段發(fā)育的黑色頁巖分別記錄了3次冰期后生物的勃發(fā)。Sturtian冰期后，大塘坡組黑色頁巖中甾烷分布的C29優(yōu)勢和大量甲藻甾烷的發(fā)現(xiàn)，說明綠藻和溝鞭藻取代疑源類和菌藻類，成為沉積有機質(zhì)的主體[147-148]；Marinoan冰期后，褐藻等底棲藻類和動物胚胎化石開始出現(xiàn)，藍田生物群和甕安生物群是其中的典型實例[149-151]；Gaskiers冰期后，以廟河生物群為代表的藻類多細胞化、大型化和多樣性趨勢明顯[152]。三次冰期事件使得沉積有機質(zhì)的母質(zhì)來源由疑源類和菌藻類迅速演化為浮游藻類、底棲藻類和后生動物。由此可見，早期真核生物在冰期后較短的地質(zhì)時限內(nèi)就快速實現(xiàn)了多細胞化、組織分化、兩性分化和形態(tài)多樣化的轉(zhuǎn)變。

3.3 氧化事件與生物演化

氧氣含量的變化可能是與生物演化關(guān)系最為密切的限制性因素。作為真核生物專屬生標的甾烷，其前體四環(huán)膽甾烷的形成需要分子氧的參與，而后生動物的呼吸和膠原蛋白的形成同樣需要分子氧，因此氧氣被認為是真核生物和后生動物出現(xiàn)必要的物質(zhì)基礎(chǔ)[153-155]。Payneetal.[156]對地質(zhì)演化過程中生物類型的統(tǒng)計結(jié)果表明，古元古代和新元古代兩次大氧化事件分別對應了原核生物向真核生物的演化及單細胞生物向多細胞生物的演化。多細胞藻類和動物化石記錄在埃迪卡拉紀的突然增加不僅反映了后生動物數(shù)量和種類的變化，同時也說明了生命由無氧代謝向有氧代謝演化的一大進步。

對于氧化事件與生物演化之間的因果關(guān)系，有學者曾提出不同看法，認為新元古代末期浮游動物的牧食行為是導致深海氧化的主要原因[157-158]。然而，在埃迪卡拉紀地層中缺少以懸浮藻類為食的浮游動物的化石記錄，但這一時期的深海至少已發(fā)生了幕式氧化[131,159]。事實上，已知最早的以藻類為食的浮游動物化石發(fā)現(xiàn)于加拿大西北部Mount Cap組(515～510 Ma)[160]，濾食性海綿出現(xiàn)的時間雖然可能相對較早，但由于其主要依靠自由有機碳和細菌為食[161]，因此對海洋表層的生態(tài)系統(tǒng)不會產(chǎn)生明顯壓力。

另外，作為呼吸耗氧生物，海綿等早期后生動物生存所需的最小含氧量大約為0.5% PAL[162]。Planavskyetal.[90]根據(jù)鉻同位素數(shù)據(jù)推測，中元古代極低的大氣氧含量(<0.1% PAL)似乎是限制后生動物早期演化的關(guān)鍵因素。然而，Zhangetal.[9]對我國中元古代下馬嶺組沉積環(huán)境的模擬計算結(jié)果顯示，當時的大氣氧含量已高達4% PAL，這一發(fā)現(xiàn)可能需要研究者們重新評估氧氣含量對生物演化的限制作用。

4 結(jié)語

新元古代的地球表層系統(tǒng)經(jīng)歷了一系列重大地質(zhì)事件，這些地質(zhì)事件與生物革新的同時發(fā)生，表明早期地球環(huán)境的變化與生物演化之間存在著密切的耦合關(guān)系。當環(huán)境條件突破某些關(guān)鍵性約束后，生物類群的豐度和分異度就可能出現(xiàn)爆發(fā)式的增長。值得一提的是，地質(zhì)歷史中類似的關(guān)鍵時段均伴隨有大量黑色頁巖及金屬、非金屬礦產(chǎn)的形成。因此，以某一時期各種地質(zhì)事件為對象，開展古構(gòu)造、古氣候、古海洋、古生物等交叉學科的研究，不僅有利于我們了解地球系統(tǒng)的整體演化及各圈層間的相互作用，同時可以為多種沉積礦產(chǎn)及烴源巖發(fā)育機制的探討提供獨特價值。

致謝 感謝兩位審稿專家對論文提出的寶貴修改意見。

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Geological Events and Their Biological Responses During the Neoproterozoic Era

YE YunTao1,2,3，WANG HuaJian3，ZHAI LiNa1,2,3，ZHOU WenXi3,4，WANG XiaoMei3， ZHANG ShuiChang3，WU ChaoDong1,2

1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China 2. Institute of Oil & Gas, Peking University, Beijing 100871, China 3. Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, Beijing 100083, China 4. College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China

The Neoproterozoic Era witnessed a series of geological and biological events which may have significantly changed the Earth’s surface environment. These events are suspected to be linked and their temporal relationships have long been a focus of multidisciplinary studies. Superplume activity and true polar wander through the early Neoproterozoic led to the break-up of Rodinia supercontinent. Indeed, such a large perturbation of deep mantle dynamics exerted a crucial impact on the global cycles of O2and CO2, thus further inducing the extraordinarily dramatic climate. Biological consequences of tectonic re-configuration are mainly reflected in nutrient availability and living conditions. The elevated upwelling and surface runoff could sustain persistent blooms of marine organisms. A Snowball Earth hypothesis has been proposed to explain the tropical glaciation. During times of widespread ice, there must be an intense environmental filter on the evolution of early life. Moreover, the subsequent rapid melting of glaciers may result in the explosion of productivity as well as the formation of major sedimentary minerals. Besides these geological and biological events, this period is also characterized by prominent fluctuations of geochemical proxies, which indicate great changes of atmosphere and ocean at this critical interval.

Neoproterozoic; supercontinent; glaciation; atmospheric oxygen level; biological events

1000-0550(2017)02-0203-14

10.14027/j.cnki.cjxb.2017.02.001

2016-04-08； 收修改稿日期： 2016-04-28

高等學校博士學科點專項科研基金資助課題(20120001110052)[Foundation: Specialized Research Fund for the Doctoral Program of Higher Education, No. 20120001110052]

葉云濤，男，1991年出生，博士，地質(zhì)學(石油地質(zhì)學)，E-mail: yeyuntao@pku.edu.cn

吳朝東，男，教授，E-mail: cdwu@pku.edu.cn

P534.3 P597

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