楊向榮，嚴德天，張利偉，張寶，徐翰文，劉文慧，鄖嘉琳

中國地質(zhì)大學(武漢)構(gòu)造與油氣資源教育部重點實驗室，武漢　430070

0　引言

奧陶紀—志留紀轉(zhuǎn)折期是一個重要的地質(zhì)歷史時期(圖1)，研究發(fā)現(xiàn)這個時期全球板塊構(gòu)造形態(tài)、古氣候條件、古海洋氧化還原條件、生物群類型等都發(fā)生了巨大的改變[1-5]，同時期在我國南方沉積了一套黑色頁巖，并且該套頁巖被作為我國頁巖氣勘探開采的重要層位[6-8]。因此人們逐漸認識到厘清赫南特冰期不僅對了解地球地質(zhì)歷史時期環(huán)境的演化及其與生物進化間的協(xié)同關(guān)系具有重要理論意義，更對華南頁巖氣開采具有重要指導(dǎo)意義。盡管學者對赫南特冰期的古海洋環(huán)境轉(zhuǎn)變與冰期成因機制做了大量工作，但由于涉及內(nèi)容的廣泛性及學科交叉研究的復(fù)雜性，對兩者之間的相互關(guān)系與及觸發(fā)機制等許多問題仍未得到較好的解決。為此本文力圖從地球科學的角度出發(fā)，綜合國內(nèi)外的研究成果，闡述奧陶紀—志留紀轉(zhuǎn)折期古海洋元素地球化學循環(huán)與古氣候轉(zhuǎn)變，并對兩者的協(xié)同演化關(guān)系做出簡要的綜述評論。

1　赫南特冰期

過去五億年以來極地地區(qū)有三個時期形成了廣泛分布的大陸冰川，其中一次就發(fā)生在晚奧陶世赫南特時期(約444百萬年前)。反映該時期氣溫顯著下降及冰蓋形成的證據(jù)很多，比如δ13C發(fā)生了正漂、海平面的下降、凱迪期冰川沉積物與沉積旋回的出現(xiàn)、含有燧石和磷酸鹽的碳酸鹽巖以及全球海水中的δ18O值比無冰雪覆蓋時期要高[1,9-14]。此外，全球的海洋水體循環(huán)程度與極地—赤道的溫度梯度密切相關(guān)，其也可以揭示當時的氣候變冷事件，因為溫室氣候條件不利于溫度梯度的形成，進而導(dǎo)致海洋循環(huán)的減弱，相反溫度下降會促進溫度梯度的形成，導(dǎo)致強烈的海洋循環(huán)。因此Pohl采用FOAM重建了奧陶紀時期海洋表層水體以及海洋—大氣的循環(huán)模式，發(fā)現(xiàn)赫南特冰期引發(fā)了11 ℃的氣溫變化，并導(dǎo)致了強烈的海洋水體循環(huán)[15]。通過古CO2含量計算模型[16]以及生物—非生物指標數(shù)據(jù)[17-19]顯示該時期CO2的含量高達14～22倍現(xiàn)今的水平，而冰期就發(fā)生在這樣的高CO2含量時期[20-21]。

2　赫南特冰期的影響

2.1　海洋碳循環(huán)

晚寒武世時期全球C循環(huán)被認為經(jīng)歷了強烈波動[22-23]，而這種波動是由不穩(wěn)定的古海洋氧化還原條件以及間歇性的硫化水體擴張活動引發(fā)的[22-25]。到了早—中奧陶世δ13Ccarb值變得相對穩(wěn)定[26-27]，反映了該時期較穩(wěn)定的全球C循環(huán)[28]。到了晚奧陶世時期全球C同位素出現(xiàn)了兩次波動即GICE和HICE事件，上奧陶統(tǒng)古登貝爾階碳同位素正漂幅度較小(約+2‰～+4‰)，而在上奧陶統(tǒng)赫南特階出現(xiàn)的δ13Ccarb正漂則更加強烈(約+3‰～+7‰)且持續(xù)時間更長(約1Myr)[23,29-30](圖2)。晚奧陶世赫南特時期全球有機碳同位素δ13Corg(約-1‰)數(shù)據(jù)顯示了較一致的趨勢，即晚奧陶世凱迪期δ13Corg值保持較穩(wěn)定的低值，隨后在赫南特早期逐漸發(fā)生正漂，并在早志留世的魯?shù)て讦?3Corg值重新恢復(fù)到凱迪期水平，但是δ13Corg漂移事件存在一定的空間差異性，即其開始漂移的位置可以在凱迪階—赫南特階界限之下(在Dicellograptusmirus生物帶)[31]，也可以在界限處[32]或者在界限之上(中M.extraordinarius-N.ojsuensis生物帶)[33]。HICE事件的發(fā)生被很多研究認為是由于大量有機碳的埋藏[27,34]，另一種假設(shè)則認為HICE事件與暴露地表的碳酸鹽巖風化作用有重要內(nèi)在聯(lián)系[1, 35]，但結(jié)合鈣同位素等信息指示風化作用并不足以引發(fā)全球尺度的碳同位素波動[30,36]。兩種模式的不同之處在于對赫南特階冰期空氣中CO2分壓變化時間節(jié)點的確定。在生產(chǎn)力假設(shè)中大氣CO2分壓的下降及冰期的到來是由于海洋生產(chǎn)力的增強，導(dǎo)致富含12C同位素有機碳的大量埋藏并離開了海洋—大氣系統(tǒng)，那么海相碳酸鹽巖中C同位素開始升高處反映了海平面的變化、海洋表面溫度的降低、冰川的擴張[2,37-38]。相反，碳酸鹽巖風化假設(shè)認為空氣中CO2分壓的下降發(fā)生在海相碳酸鹽巖中C同位素升高之前，這可能是由于強烈的硅酸鹽風化或者構(gòu)造作用引發(fā)的火成巖風化對CO2消耗量的增強[39]。最近對美國猶他州和內(nèi)華達州的五個剖面進行的高精度δ13Ccarb研究發(fā)現(xiàn)，HICE時期δ13Ccarb最大值展現(xiàn)了明顯的平面梯度，即從盆地中心(+7.5‰)向內(nèi)陸架地區(qū)逐漸減少(+2.5‰)。這種梯度性可能是由于該時期海平面較低，海盆周圍隆起地區(qū)被剝蝕得更嚴重，引發(fā)了區(qū)域性的不整合特征[35]。當然由于不同地質(zhì)年代以及不同地質(zhì)相帶生活的生物存在一定差異性，因此缺乏較獨立的生物地層單元同時能夠控制淺水碳酸鹽巖及深水泥頁巖區(qū)，導(dǎo)致有機碳同位素δ13Corg與無機碳同位素δ13Ccarb間存在不一致性，引發(fā)學者思考對HICE的使用與解釋的正確合理性[40]。

圖2　奧陶紀—志留紀轉(zhuǎn)折期全球碳同位素記錄Fig.2　The variation of carbon isotope during O-S transition

2.2　海洋水體硫循環(huán)

地球系統(tǒng)中S元素的地球化學循環(huán)可以解釋海相沉積物中硫同位素δ34S波動[41-46]，沉積物在硫化或鐵化還原水體中通常會發(fā)生微生物硫酸鹽還原作用(MSR)[47-49]，其可以引發(fā)海水硫酸鹽中70%的34S發(fā)生分餾[50-52]，MSR作用也被廣泛用于解釋奧陶紀—志留紀轉(zhuǎn)折期S同位素波動特征。同δ13Corg變化趨勢一樣，δ34Spyrite在凱迪期階保持穩(wěn)定，而在赫南特期發(fā)生顯著的正漂事件，隨后在早志留世魯?shù)て谥匦禄謴?fù)到冰期之前的水平[3,31,33-34](圖3)。S同位素與C同位素的耦合作用對解釋該時期古海洋—古氣候變化具有重要意義，Youngetal.[53]從阿巴拉契亞盆地記錄的中—晚奧陶世C-S同位素信息中發(fā)現(xiàn)了兩次重要波動，即δ34SCAS出現(xiàn)顯著(12‰)負偏的同時δ34Spyrite(+10‰)和δ13Ccarb(+2‰)發(fā)生了正漂，這種演化關(guān)系反映了赫南特時期全球黃鐵礦埋藏速率以及海水硫酸鹽與沉積黃鐵礦間的S同位素分異程度降低。引發(fā)這些變化的原因很多，一是海洋系統(tǒng)逐漸富氧化，削弱了微生物硫酸鹽還原作用(MSR)，進而降低了黃鐵礦的沉淀量[31,34,54]。此外富黃鐵礦沉積物風化作用的增強也是一個可能的因素，但目前還缺少相關(guān)的沉積學與地球化學證據(jù)能夠支持這一假設(shè)。

2.3　海洋水體Os、Sr循環(huán)

海水的Os同位素組成反映了兩種來源Os的相對強弱，一是地殼碎屑物質(zhì)中高放射性O(shè)s(187Os/188Os風塵=1.05，187Os/188Os大陸地殼或河流=1.05)，另一個是來源于大洋地幔圈或地核的低放射性O(shè)s(187Os/188Os地幔或地核=0.13)[55-58]。Os元素在海洋中一般存在約10～40個千年，相較海洋混合的時間更長，因此使用Re-Os體系以及初始Os同位素(Osi)組成可以分析古海洋Os循環(huán)[58-62]，進而反映海洋沉積物來源變化[63-64]。有學者對蘇格蘭Dob′s Linn剖面(GSSP)做了Os同位素測定，并用來分析赫南特期古氣候變化[65](圖3)，發(fā)現(xiàn)Complanatushe和anceps生物帶早期Osi值從0.28增高到1.08，這種具放射性來源的Osi(1.08)較整個顯生宙甚至是現(xiàn)今海洋來說都是偏高的(～1.06)[63,66-68]。初始Os同位素(Osi)的變化趨勢反映了晚奧陶世凱迪期風化作用具有增強的趨勢，這很可能會促使大氣CO2分壓降低，進而引發(fā)冰期的形成。而后在赫南特階底部的Extraordinarius帶開始降低為低放射性來源，這被解釋為赫南特階大陸冰蓋的形成降低了化學風化作用的強度[65-66]。

古海洋中Sr同位素組成受陸源風化物質(zhì)與地幔熱液流體的影響[69-70]，至今很多關(guān)于古生代87Sr/86Sr比值研究被用來分析古海洋環(huán)境、古構(gòu)造演化以及古陸風化作用[70-74]。其中上奧陶統(tǒng)沉積物中的87Sr/86Sr同位素曲線顯示出非常大的波動范圍[75](圖4)，早—中奧陶世海水87Sr/86Sr值開始下降(0.709 0～0.708 8)，在晚奧陶世早期驟降為0.707 8，這也被認為是顯生宙以來最大的一次波動[39]，隨后在晚奧陶世—早志留世時期開始逐漸上升[69,75-76]。如同對新生代87Sr/86Sr值波動的解釋一樣[77-78]，大多數(shù)人認為奧陶紀晚期87Sr/86Sr值的劇烈上升是由于海平面降低、構(gòu)造造山運動加強，進而引發(fā)的大陸風化作用增強[75,78-80]。

圖3　奧陶紀—志留紀轉(zhuǎn)折期地球化學元素記錄Fig.3　The variation of geochemical proxies during O-S transition

圖4　奧陶紀—志留紀轉(zhuǎn)折期全球Sr同位素記錄[75-76]Fig.4　The variation of strontium isotope during O-S transition[75-76]

2.4　海洋水體氧化還原環(huán)境

Mo元素的含量可以被用來判定全球古海洋氧化還原條件[81-85]，而如今使用Mo同位素進一步探討古海洋水體氧化還原條件的演化[86-89]。赫南特冰期Mo同位素出現(xiàn)了明顯的波動(圖3)，從中國南方的王家灣剖面來看，冰期前后具有較高的δ98Mo值，反映了缺氧的海洋環(huán)境，而在赫南特時期的一次突降指示了一次水體氧化事件[87,89,90-92]。U同位素同樣可以被用來評估古海洋水體的氧化還原特性[93-97]，晚奧陶世晚期與早志留世早期海水中的δ238U被認為大約在-0.60‰到-0.85‰之間，而該時期河流中的δ238U則與現(xiàn)今一致約為-0.3‰[98-100]。通過計算可以發(fā)現(xiàn)，缺氧水體中的U通量與其他來源的U通量分別占據(jù)該時期U含量的46%～63%和37%～54%，并且缺氧水體中的U通量是現(xiàn)今海洋中的4～9倍，反映冰期前后古海水的嚴重缺氧[92]。Fe組分分析是重建海洋水體環(huán)境的一種重要手段[101-103]，通常認為在晚奧陶世晚期和早志留早期全球海水的FeHR/FeT>0.38且Fepy/FeHR<0.8，指示了缺氧鐵化的水體環(huán)境，而赫南特期的FeHR/FeT<0.38反映了氧化的海洋環(huán)境[104](圖3)，不過同開闊海的強水體循環(huán)不同，在局限海下水體更易形成缺氧的硫化水體[105]。但與先前普遍認為的冰期海水逐漸富氧化不同，有研究發(fā)現(xiàn)Fe組分特征在整個晚奧陶世包括赫南特時期都指示了一種嚴重缺氧的環(huán)境[106]。

2.5　海洋生物滅絕

赫南特階被認定為奧陶紀最末期不到2 Myr的一段地層[107-108]，研究認為N.extraordinarius-N.ojsuensis帶為該階的下部筆石帶[109-110]，其底界與N.extraordinarius和N.ojsuensis的首現(xiàn)一致。而其后的研究表明這兩種筆石的首現(xiàn)并不完全相同，在王家灣北剖面(GSSP)N.ojsuensis的首現(xiàn)位于N.extraordinarius首現(xiàn)層位之下4 cm處，這種先后順序均可和美國內(nèi)華達[111]、西伯利亞科累馬[112-113]以及哈薩克斯坦南部[114-115]等剖面對比，故而該筆石帶改名為N.extraordinarius帶，并作為赫南特階的底界。赫南特階的頂界與志留系底界重合，因此赫南特階就包括下部的N.extraordinarius帶和上部的N.persculptus帶。

該時期生物滅絕事件消除了海洋無脊椎生物中49%～61%的屬以及約86%的種[116-118]，使其成為顯生宙以來僅次于P-T的第二大生物滅絕事件，在這次滅絕事件中受到強烈波及的生物包括三葉蟲、腕足類、筆石、牙形蟲、無脊椎珊瑚、幾丁蟲。并且本次事件可劃分為兩個階段，第一幕滅絕發(fā)生在赫南特冰期的起始階段，對應(yīng)于445 Ma前的N.extraordinarius-N.ojsuensis生物帶，并在全球具有有廣泛分布[119-122]，第一幕導(dǎo)致了N.extraordinarius帶底部的全球范圍內(nèi)的筆石消亡，僅在揚子區(qū)是個例外，因此Mitchelletal.[123]認為當時揚子區(qū)是筆石的避難所。盡管揚子地區(qū)剖面表明此帶的滅絕率也很高，但是不少筆石動物群的分子仍然穿越了大滅絕的主幕而延入赫南特階[124]，并且這一滅絕事件發(fā)生的時間是穿時的，在淺水區(qū)發(fā)生得早而深水區(qū)則晚[117,124-126]。第二幕則發(fā)生在約444 Ma前N.persculptus生物帶[4,119-121]。赫南特時期海洋水體以赫南特貝動物群的涼水腕足類為代表，它以命名屬赫南特貝Hirnantia為代表、伴以一些其他獨特分子和廣布型分子[119]，赫南特貝動物群在揚子區(qū)從近岸至遠岸的一系列產(chǎn)地，首現(xiàn)層位逐漸升高，動物群延續(xù)時限越來越短，而多樣性卻越來越高。縱向考察表明，近岸產(chǎn)地的赫南特貝動物群多樣性由高向低演變，而遠岸較深水產(chǎn)地的多樣性演變則由低變高。隨地點和層位而發(fā)生的環(huán)境因子變化(如水深、底質(zhì)等)，赫南特貝動物群對應(yīng)于不同的群落、亞群落或群集[127]。赫南特階時期，海底缺氧范圍逐漸收縮，進而削弱了反硝化作用，因此提供了足夠的生物氮給喜寒的真核生物，烴類生物標志化合物中的原核生物比例在該時期逐漸降低[128]以及微型浮游生物(疑源類)的增長[129]證明了這個觀點，然而這樣的環(huán)境轉(zhuǎn)變非常不利于喜好高營養(yǎng)成分、低氧含量的筆石生存，導(dǎo)致了它的大面積絕滅[129-131]。即使有學者提出了海底硫化缺氧的環(huán)境可能是導(dǎo)致生物滅絕的原因[131]，但是人們更愿意相信氣候的轉(zhuǎn)變是觸發(fā)這兩幕絕滅事件的誘因。值得注意的是氣溫的變冷是冰期的直接效果，而海底硫化缺氧環(huán)境位置的下移則與冰期海平面的下降有關(guān)，因此無論哪種機制都與赫南特冰期的出現(xiàn)息息相關(guān)。

3　赫南特冰期的成因機制

3.1　風化作用

地質(zhì)歷史時期大氣CO2濃度的緩慢下降可以解釋為構(gòu)造造山運動形成隆起，進而大大增強了硅酸鹽巖、玄武巖的風化作用[1]，依據(jù)海水中87Sr/86Sr的比值[39,75]以及CIA指數(shù)[132]，表明其風化速率在早—中奧陶世時期是較強的，但在晚奧陶世開始出現(xiàn)顯著的降低。值得注意的是，在冰期開始之前的凱迪期Osi值明顯增高[65]，這一趨勢表明該時期風化作用的增強，并且可以被解釋為構(gòu)造運動的加強引發(fā)的大陸硅酸鹽風化作用增強。研究發(fā)現(xiàn)由于硅酸鹽巖風化強度的階段變化，導(dǎo)致大氣中的CO2分壓在距今400～360 Ma以前的泥盆紀出現(xiàn)了明顯的下降，最后導(dǎo)致全球氣溫下降以及極地的冰期形成[16]，因此一些學者就認為晚奧陶世時期巖石的風化作用逐漸增強是觸發(fā)赫南特冰期形成的重要機制[65,133]。溫暖潮濕的溫室氣候條件可以促使硅酸鹽巖化學作用的加劇，另外當大陸板塊運動經(jīng)過赤道地帶時，充足的降水量也可以增強風化作用[27]。而近年來發(fā)現(xiàn)植物可能是風化作用增強的重要誘因，植物再生長過程中需要吸收礦物質(zhì)—包括磷酸鹽、鉀元素、鈣元素、鎂元素和鐵元素，而陸生植物的出現(xiàn)可以加強巖石中重要的營養(yǎng)物質(zhì)釋放，尤其是陸生維管植物的出現(xiàn)大大增強了對含有營養(yǎng)物質(zhì)的硅酸鹽礦物的風化作用[134]，比如對鈣元素的風化因子就達到2～10。Timothyetal.[135]分別觀察了苔蘚植物(即無維管植物)與非生物因素控制下的花崗巖和安山巖的風化強度，證實了無維管植物的確可以增強硅酸鹽巖的風化作用(圖5)，并且利用COPSE模型[17]對480 Ma以來的CO2水平以及全球氣溫變化進行了模擬，發(fā)現(xiàn)在陸生維管植物與無維管植物的影響下，在約458 Ma和444 Ma以前出現(xiàn)了兩個極低值，分別對應(yīng)了GICE以及HICE事件。

圖5 奧陶紀—志留紀轉(zhuǎn)折期風化作用強度(據(jù)Yan et al.[132]; Timothy et al.[135]改)其中紅實線、綠實線、橙虛線分別代表依據(jù)早期植物的磷酸鹽風化作用預(yù)測的全球溫度、大氣CO2分壓、碳同位素變化趨勢Fig.5　The intensity of weathering during O-S transition(based on Yan et al.[132]; Timothy et al.[135])Model results, predicted Ordovician variations in red solid line, global temperature, green solid line, atmospheric CO2, and orange dotted line, δ13C of marine carbonates, for: baseline model with Ordovician adding transient enhancement of phosphorus weathering by early plants

3.2　火山作用

研究表明通過火山活動或者變質(zhì)作用導(dǎo)致的大量的C輸送到海洋—空氣系統(tǒng)中是導(dǎo)致溫室氣候的一個重要觸發(fā)機制，而風化作用的加強在很多時代也被認定為劇烈的火山活動所引發(fā)，即火山噴發(fā)出的大量CO2氣體以及易風化的玄武巖都能促進大陸風化作用[136-138]。相較于火山系統(tǒng)，沿著大陸邊緣的巖漿弧俯沖帶由于將地殼中儲存的碳酸鹽礦物分解，因而被認為釋放了更多的CO2氣體到空氣中，盡管對于當時這種方式釋放的CO2量計算還存在困難，但現(xiàn)今的大陸火山弧釋放的CO2量大約為150 tg C/年，而大洋中脊大約釋放12～60 tg C/年，大洋板內(nèi)火山釋放1～30 tg C/年[139-141]，因此大陸火山弧可能在控制地球地質(zhì)歷史時期的氣候轉(zhuǎn)變過程中扮演了重要角色。羅迪尼亞超大陸在約900萬年以前開始形成，在約750萬年以前經(jīng)歷了延展隆升期，最終在約600萬年前開始崩解[142-144]。羅迪尼亞超大陸的存在就反映了大陸弧火山的減少，而隨后的崩解就帶來了大陸弧火山的增加，大氣中CO2含量在寒武紀達到頂峰，而后岡瓦納大陸在中奧陶世開始形成，大陸弧火山的量再次降低，導(dǎo)致大氣CO2含量出現(xiàn)降低[145]，并且McKenzieetal.[142]發(fā)現(xiàn)年輕碎屑鋯石含量的波動與冰期旋回具有一致性。與之前的CO2觸發(fā)機制不同，Buggischetal.[146]認為火山活動釋放的SO2對晚奧陶世赫南特冰期的形成具有重要作用，由于SO2在對流層極易被氧化固定，而在平流層形成的氣溶膠則會阻擋太陽熱能，引發(fā)地球表面的溫度急劇下降，這樣來看晚奧陶世凱迪期火山活動的減弱或許是觸發(fā)冰室氣候的關(guān)鍵條件。

3.3　有機碳埋藏

赫南特冰期與有機碳埋藏速率的加強存在一定的聯(lián)系，即冰期前海洋生物死亡促進了大量有機碳埋藏[31,147-148]，進而導(dǎo)致大氣CO2分壓下降[20,149]。Rosenauetal.[150]在古登貝爾階GICE時期發(fā)現(xiàn)了較高的δ18O值，認為該時期δ18O值增加了1.5‰，隨后降低為19.0‰，并在其附近波動，這些結(jié)果表明在GICE時期內(nèi)存在短期的冷卻事件，因此凱迪階前(赫南特階前)出現(xiàn)的古登貝爾C同位素正漂(GICE)就可能是赫南特冰期的一個前奏[11,27,148]，這個時期海平面明顯上升，碳酸鹽沉積從熱帶過渡到溫帶[148,151]，磷酸鹽和硅質(zhì)沉積增多[11,151]。桑迪階與凱迪階界限附近的GICE事件可能代表了全球范圍內(nèi)的有機碳埋藏的加強[12,27,39]，而晚凱迪期δ13Corg值的大幅度變化又反映了低于晚奧陶世冰蓋形成的臨界pCO2閾值[21,148-149]。同樣的，GICE期后有機碳持續(xù)埋藏，CO2分壓繼續(xù)降低，最終引發(fā)了時間更長、強度更大的赫南特階碳同位素波動HICE與冰期。在這樣的模式中，中奧陶世到晚奧陶世存在一個氣候逐漸降低的平臺，代表了CO2分壓在地質(zhì)時間尺度上的累積降低，直到其突破所需的臨界值后，觸發(fā)了赫南特期出現(xiàn)短暫的氣溫陡降[133]，這就排除了該時期火山活動引發(fā)的氣候突變[152]。

3.4　其他地質(zhì)事件

Herrmannetal.[153]認為晚奧陶世古地理和CO2濃度的協(xié)同作用，加上海平面的下降、極地海洋熱傳送的減少導(dǎo)致冰蓋的形成，進而觸發(fā)了全球氣溫降低。冰川海洋以及大陸沉積物在時空上的分布指示了冰期始于中奧陶世，同時沉積學和動物形態(tài)特征也表明了晚奧陶世溫暖水體向寒冷水體的轉(zhuǎn)變[154-157]，但是這種轉(zhuǎn)變模式存在一定問題，因為根據(jù)古地理重建和古氣候指標，當時的北美處于熱帶到亞熱帶的緯度范圍。而晚奧陶世岡瓦納大陸的向南移動和海平面的下降可能導(dǎo)致了南大洋的熱傳導(dǎo)減少，最終引發(fā)了岡瓦納冰期的形成或者加強[153]。同時，諸如早期較弱的太陽光照強度[158]、由宇宙射線引發(fā)的云層反射率高低[159]也被用來解釋氣溫驟降。值得注意的是全球的氮循環(huán)轉(zhuǎn)變可能是觸發(fā)冰期的重要機制之一，這是因為固氮作用速率以及相伴生的氧化氮的產(chǎn)量對氣候環(huán)境顯得尤為重要[160]，氧化氮(N2O)是一種強有力的溫室氣體，其引發(fā)溫室氣候的效力是同等量CO2的300倍，并且在大氣中的持續(xù)時間也超過了100年[161-162]。因此當N2O這一產(chǎn)物大量減少時，就有助于全球氣溫變冷甚至是冰期的到來。即使赫南特冰期的結(jié)束被歸結(jié)于大氣CO2含量的上升[1]，但是也有可能海相氮循環(huán)在這一事件中扮演了重要的角色[161]，因為在全世界多個剖面的赫南特冰期結(jié)束位置都出現(xiàn)了δ15N的漂移[30,129]，這一信號反映了該時期反硝化作用的增強，進而導(dǎo)致更多的N2O被釋放到大氣中增強了溫室效應(yīng)，導(dǎo)致冰期的結(jié)束。

4　不同地質(zhì)作用耦合關(guān)系

地球作為一個整體系統(tǒng)，地質(zhì)歷史時期發(fā)生的各種地質(zhì)過程會相互作用、相互影響、相互制約。晚奧陶世火山活動較為頻繁，火山噴發(fā)出大量的CO2氣體，這一過程能引發(fā)強烈的大陸鋁硅酸鹽化學風化作用[65,128]。那么火山活動之后風化作用的增強就可以解釋晚奧陶世時期大量陸殼放射性187Os、87Sr同位素向海水的運移，進而引發(fā)該時期187Os/188Os與87Sr/86Sr的升高[65,75-76]。但長時期強烈的風化作用便會大量消耗CO2，使得大氣CO2分壓持續(xù)降低，加上火山活動噴發(fā)的SO2在平流層形成的氣溶膠會阻擋太陽熱能[148]，可以引發(fā)全球氣溫降低。氣溫的降低能加劇海洋系統(tǒng)的循環(huán)，海洋表層與底層水體的循環(huán)會促進營養(yǎng)物質(zhì)的循環(huán)，進而誘發(fā)高的生物生產(chǎn)力與有機碳埋藏，這一過程不僅能進一步降低大氣CO2分壓水平，促進氣溫的降低，并且有機碳的下沉過程還會大量消耗海水中的氧氣，在陸棚處形成缺氧的底層水體[31,104]。赫南特時期生物的絕滅事件眾多學者有不同的解釋，比如有人認為該時期海洋硫化水體的偏移使得大量海洋生物不適應(yīng)新的生活環(huán)境發(fā)生了滅絕[5]，而更多的學者則將其歸咎于赫南特時期氣溫的驟降引發(fā)了顯生宙以來第一次生物滅絕事件。

5　存在問題

對于這一地質(zhì)轉(zhuǎn)折期古氣候條件的研究還存在很多爭論：

利用碳同位素[19,163]及顯生宙碳循環(huán)模型[163-166]等古氣候條件恢復(fù)都顯示晚奧陶世時期整體處于一個高CO2分壓條件(>8 PAL)，但有趣的是依據(jù)前人設(shè)計的復(fù)雜環(huán)境模型表明只有當CO2分壓下降到8 PAL時才會引發(fā)冰期的形成[20-21]，這種矛盾性的存在可能受限于計算得到的CO2分壓曲線的精度，具體原因還有待考證。

Os，Sr同位素數(shù)據(jù)顯示晚奧陶世時期放射性187Os、87Sr含量逐漸升高，這被很多人解釋為風化作用的升高[65,75-76]，但陸源風化指數(shù)CIA值[132]與Osi指數(shù)在凱迪期表現(xiàn)出明顯不同的變化趨勢，加之上奧陶統(tǒng)五峰組發(fā)育多層鉀質(zhì)斑脫巖，反映了大規(guī)模的火山噴發(fā)活動[167]，那么晚奧陶世凱迪晚期海洋水體中放射性187Os含量增高[65]或許指示了火山作用的減弱，而非風化作用的增強，總之目前對該時期大陸風化作用與火山作用的耦合關(guān)系，尤其是火山活動對海洋環(huán)境的影響還較為缺乏，其機制值得深思。

研究發(fā)現(xiàn)早凱迪期的氣溫驟降(GICE)后有氣溫的回暖[146]，這就為有機碳埋藏的時間尺度與冰期開始的時間節(jié)點提出了新的異議。另外諸如對冰期持續(xù)的時間長短存在差異性；志留紀之初的氣候是迅速變暖還是持續(xù)寒冷也有不同的解釋和結(jié)論；生物演化、海洋環(huán)境與氣候轉(zhuǎn)變?nèi)绾蜗嗷ビ绊懪c制約有待研究。而如此多的爭論存在的原因很多，比如缺乏獨立的生物地層單元同時能夠控制淺水碳酸鹽巖及深水泥頁巖區(qū)，缺少轉(zhuǎn)折界限處高精度的地球化學信息與全球等時地層格架下的對比，以及風化作用、成巖作用和構(gòu)造熱事件對古海洋與古氣候重建時的干擾，這也就需要我們今后更多的工作來克服這些局限性。

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