德雷克海峡打开与构造—古海洋—古气候演变

The interaction between the opening of the Drake Passage and global paleoceanographic-paleoclimatic change

摘要

德雷克海峡的打开受到南美南部和南极半岛北部板块运动与构造演化影响,并导致南极绕极流和现今大洋环流模式最终形成,是理解新生代全球变化的关键因素之一。前人通过古大陆重建与古海洋学相关方法研究德雷克海峡打开与南极绕极流形成过程,本文总结了前人研究成果,并综合南极半岛北部与南美南部新生代构造演化和全球大洋底栖有孔虫δ¹⁸O、δ¹³C、全球深海溶解氧含量、大气CO₂浓度、全球大洋生产力变化等数据,提出40~35、30~25和20~18 Ma 3期德雷克海峡打开与南极绕极流加强的关键阶段。这3期事件发生于南极半岛北部与南美南部的关键构造演化阶段,其中后两个阶段发生在大西洋经向翻转流形成之后,对应全球大洋底栖有孔虫δ¹³C低值、大气CO₂浓度低值、深海溶解氧低值、南大洋高生产力和赤道海区低生产力阶段。据此,我们提出南极半岛北部与南美南部相关构造事件导致南极绕极流增强,并主导了这些时期的古海洋与古气候变化,这一认识有助于厘清德雷克海峡演化的重要阶段及其对全球变化的影响。

关键词

南极半岛北部 / 南美南部 / 德雷克海峡 / 板块构造 / 古海洋 / 古气候

Key words

Northern Antarctic Peninsula / Southern South America / Drake Passage / plate tectonics / paleoceanography / paleoclimate

0 引言

新生代以来,全球经历了“温室地球”“热室地球”向“冰室地球”转变,板块重组,海峡打开与关闭,大洋环流演化,大气CO₂浓度变化和南北极冰盖逐渐形成等重大地质事件^[1-2],厘清这些事件之间的关系是理解新生代全球变化的核心之一。南极是位于地球最南端被大洋隔离的一个大陆,发育地球上最大的冰盖。作为全球最大冷源,南极冰盖强烈影响着新生代地球。前人曾提出南极绕极流(Antarctic Circumpolar Current,ACC)形成导致南极冰盖形成^[3],但数值模拟显示ACC形成并不能直接导致全球降温和南极冰盖形成,而全球大气CO₂浓度降低到一定程度后会导致全球降温直至南极冰盖形成^[4⇓-6],这些争议阻碍了我们对南极冰盖形成的理解。问题核心之一在于,不同研究给出的ACC形成时间不一致,这又阻碍了对ACC与南极冰盖形成、全球变冷之间关系的讨论。位于南极半岛与南美板块之间的德雷克海峡/斯科舍海,以及澳大利亚板块与东南极克拉通之间的塔斯马尼亚海峡打开导致ACC形成(图1a)。前人对塔斯马尼亚海峡打开过程进行了较好的制约,Bijl等^[7]对U1356站位沟鞭藻组合的变化研究提出塔斯马尼亚海峡浅部洋流贯通发生在49~48 Ma。海底磁异常条带与陆上古地磁重建显示澳大利亚在晚白垩世—中始新世海底扩张速度较慢(<5 mm·a^-1,半扩张速率),43~39 Ma期间速度为15~50 mm·a^-1,随后在33 Ma之前提高到24 mm·a^-1,在这之后提高到34 mm·a^{-1 [8-9]},在35~32 Ma期间澳大利亚板块与东南极在南塔斯曼高地彻底分离^[10-11],塔斯马尼亚海峡形成深部洋流。与塔斯马尼亚海峡不同的是,德雷克海峡内不同洋盆打开时间不确定,且有众多小陆块和火山弧可能阻碍深部洋流形成(图1b^[12]),导致德雷克海峡演化仍存在较大争议。为进一步厘清ACC形成与德雷克海峡打开之间的关系,本文总结了已发表的南极半岛北部与南美南部新生代关键地质事件,并综合分析全球大洋底栖有孔虫δ¹⁸O、δ¹³C、深海溶解氧含量、大气CO₂浓度、大洋生产力等数据,以期进一步理解南极半岛北部—南美南部地质演化重要阶段及其与德雷克海峡打开之间的关系,厘清德雷克海峡在新生代构造—古海洋—古气候演化中的地位。

1 南极半岛北部—南设得兰群岛新生代关键构造演化

德雷克海峡位于南极半岛与南美板块之间,其逐步打开主要发生在新生代,涉及南极半岛、南美、斯科舍和南桑威奇等多板块相互作用(图1)。南极半岛北部—南设得兰群岛作为德雷克海峡南边界,其构造—岩浆演化与海峡打开过程息息相关(图2a)。南极半岛自古生代以来持续受到太平洋板块俯冲影响,但由于研究程度较低,其构造属性仍存在争议。Vaughan和Storey^[13]、Vaughan等^[14]以南极半岛南部大规模脆性—脆韧性东帕默地剪切带为界,将南极半岛分为西部、中部和东部构造域(图2a)。西部构造域由南极半岛西南部亚历山大岛、阿德莱德岛和南极半岛北部南设得兰群岛组成,以亚历山大岛浊积岩、外来洋底玄武岩和沉积岩组成的侏罗纪—白垩纪勒梅群和南设得兰群岛斯科舍杂岩群为主^[14-15],可能为弧前增生楔或者外来地体^[13]。西部构造域向东至东帕默地剪切带为中部构造域,其中帕默地西北部出露的正片麻岩变质基底被侏罗纪以来的辉长岩、花岗岩等侵入,可能为外来地体在白垩纪拼贴到东部构造域^[13]。Ferraccioli等^[16]进一步根据地球物理特征和岩性特征将中部构造域划分为西部(长英质)和东部(镁铁质)区域。

东帕默地剪切带以东至大西洋沿岸为东部构造域,主要为格雷厄姆地晚石炭世—晚三叠世特里尼蒂半岛群浊积岩、中侏罗世钙碱性硅质大火成岩省、侏罗系普思特山组钙碱性火山岩和早侏罗世—早白垩世拉塔迪组海相至陆相砂岩/泥岩^[17-18]。Burton-Johnson和Riley^[19]通过总结南极半岛地层、岩浆岩、地球化学、地球物理和古地磁等资料,提出南极半岛中部与东部构造域拥有相同地质特征的变质基底,并且在基底上发育的地层可对比,据此提出南极半岛主体是在古太平洋板块(凤凰板块)持续俯冲下形成的以中—新生代岩浆弧为主的地体,支持Suárez^[20]的观点。南极半岛岩浆岩年龄分布范围为240~10 Ma^[21],主要发育187~182、171~167和157~153 Ma 3期与冈瓦纳大陆裂解有关的,以酸性岩浆岩为主的岩浆作用^[22]和晚侏罗世以来与凤凰板块俯冲有关的岩浆作用^[23]。为研究白垩纪岩浆迁移过程,Gao等^[24]总结了前人在南美南部和南极半岛发表的岩浆岩数据,发现白垩纪时南极半岛与南美南部在古太平洋板块俯冲作用下经历了类似的构造—岩浆过程,包括:140~130 Ma古太平洋板块向海沟方向回卷与岩浆作用向太平洋一侧迁移;130~120 Ma古太平洋板块平板俯冲与岩浆作用自海沟向内陆扩展;120~90 Ma期间古太平洋板块向海沟方向回卷与岩浆作用再次向太平洋一侧迁移,支持南极半岛主体是在古太平洋板块俯冲作用下,发育在冈瓦纳大陆边缘的岩浆弧为主的地体^[19-20]。凤凰板块回卷使晚白垩世之后岛弧岩浆作用主要集中在半岛西部,主要岩性为花岗闪长岩、闪长岩和辉绿玢岩等侵入岩,火山岩仅在当科海岸、阿德莱德岛和屈韦维尔岛等局部地区出露^[21,25-26]。其中新生代侵入岩具有低Sr/Y,轻稀土中等富集,重稀土平坦HREE和中等至弱的负Eu异常,在Sr/Y—Y图解中位于正常弧岩浆区,AMF和CMF值较低,Mg^#小于45和正ε_Nd(t)值,说明其母浆来源于新生下地壳和少量被卷入的沉积物以及少量幔源物质^[25,27]。杜梅岛广泛分布着闪长岩和闪长岩包体,其形成年龄(55.7~54.6 Ma)一致,富集大离子亲石元素、亏损高场强元素并具有正ε_Nd(t),支持两者同时、同源生成,岩浆来源于玄武质岩浆上升导致的新生变玄武质下地壳部分熔融^[26]。

南设得兰群岛位于南极半岛北西端,布兰斯菲尔德海峡在约4 Ma之后将其与南极半岛分开^[23](图2a,b^{[13,19,28⇓-30]}),主要出露石炭—三叠纪低级变质弧前盆地沉积岩或增生杂岩,侏罗纪—新生代火山岩、火山碎屑岩、火山碎屑沉积岩和沉积岩(图2c—e),仅在乔治王岛巴顿半岛等少数地区出露花岗闪长岩和闪长岩等侵入体^[31⇓-33]。古地磁重建显示南设得兰群岛在140 Ma时位于南美安第斯山脉西侧^[34],在140~120 Ma期间经历大规模南向运动到达南极半岛北部,120~100 Ma期间运动规模减弱,直到100~90 Ma南设得兰群岛与南美最南端西侧安第斯山脉经历了相同程度大规模逆时针旋转,而同期南极半岛内部却缺少类似构造旋转,这期逆时针旋转可能由南美板块整体南向运动挤压所致^[24]。群岛内火山岩主要岩性为岛弧玄武岩、安山岩和玄武—安山岩,自早白垩世至中新世呈现自南西向北东逐渐变年轻的趋势^{[23,29,32,35⇓⇓⇓⇓-40]},其中早白垩世火山岩主要分布在娄岛、雪岛和利文斯顿岛西部,晚白垩世火山岩主要分布在利文斯顿岛东部,格林尼治岛和罗伯特岛,古新世以来的火山岩主要分布在纳尔逊岛和乔治王岛,代表凤凰板块逐渐向北东方向俯冲消减^[23,32]。新生代火山—沉积地层主要分布在南极半岛北部与南设得兰群岛的纳尔逊岛和乔治王岛,火山岩主要岩性为岛弧玄武—安山岩,而沉积岩为研究南极半岛—南设得兰群岛古气候演化提供了宝贵材料^[41-42]。中国在菲尔德斯半岛建立了第一个南极科学考察站——长城站,建站初期我国科研人员刘小汉和郑祥身^[36]、李兆鼐等^[37]、李兆鼐和刘小汉^[43]、冯宁生等^[44]、沈炎彬^{[45⇓⇓-48]}对菲尔德斯半岛火山—沉积地层进行了系统研究,划分出半三角组、碧玉山组、玛瑙滩组、化石山组、岩块山组和长山组,出版了《南极乔治王岛菲尔德斯半岛和纳尔逊岛斯坦斯伯赖半岛地质图》^[49]。菲尔德斯半岛内火山岩低钾高铝,具有高铝玄武岩(钙碱性玄武岩)和低钾拉斑玄武岩(岛弧拉斑玄武岩)的双重特征,是典型的岛弧火山活动产物^[50]。考虑到前人对菲尔德斯半岛地层年代限制多用K-Ar和Rb-Sr法^[51],可能无法反映真实地层年代,因此Gao等^[35]在菲尔德斯半岛碧玉山组、玛瑙滩组、岩块山组和长山组中采集了火山岩样品,并开展了同位素年代学与古地磁学研究,通过⁴⁰Ar/³⁹Ar逐步加热法得到这些火山岩坪年龄为56~52 Ma,在系统总结乔治王岛地层年代学等资料的基础上,提出乔治王岛内火山—沉积地层从菲尔德斯半岛至梅尔维尔角(图2e)逐渐变年轻^[38]。朱铭等^[51]在菲尔德斯半岛获得46~44 Ma玄武岩全岩和斜长石⁴⁰Ar/³⁹Ar坪年龄,与Wang等^[52]在巴顿半岛和韦弗半岛获得的一次大规模岩浆活动时间(约45 Ma)一致,代表菲尔德斯半岛也受到了这次岩浆事件影响。

南设得兰群岛火山作用与凤凰板块俯冲息息相关,Willan和Kelley^[31]通过对利文斯顿岛赫德半岛与巴纳德角玄武—安山岩和英安岩岩脉年代学与产状分析,提出以北北东走向为主的51~45 Ma岩脉与凤凰板块在约52.3 Ma时向南极半岛俯冲速度突然减小以及俯冲角度变大^[53]引起的南极半岛岩石圈伸展有关。而31~29 Ma东南东和南东走向的岩脉与凤凰板块的俯冲方向垂直,考虑到这段时间凤凰板块较低的俯冲速率^[53],推断其形成更有可能与鲍威尔盆地打开导致的北东—南西向伸展有关^[31]。为进一步厘清南极半岛北部—南设得兰群岛晚白垩世以来岩浆活动与凤凰板块俯冲之间的关系,Gao等^[30]对采自罗伯特岛铜矿半岛和乔治王岛海狮呷的玄武岩开展了⁴⁰Ar/³⁹Ar年代学与古地磁学研究,将铜矿半岛铜矿组玄武岩年龄限制在89~88 Ma(图2d),并总结南极半岛—南设得兰群岛新生代火山岩和侵入岩年代(⁴⁰Ar/³⁹Ar与U/Pb年龄),发现90~63 Ma期间岩浆活动主要集中在南极半岛北部—南设得兰群岛西侧,没有发生明显迁移,但是自63~51 Ma,岩浆活动发生了自西向东小规模迁移,51~44 Ma变为自东向西迁移,之后又变为自西向东迁移(图2b),岛弧岩浆作用在南设得兰群岛延续到约20 Ma,在梅尔维尔角等地以岩脉状产出(图2e),而俯冲于南极半岛之下的凤凰板块还残存于南极半岛岩石圈之下^[54]。岩浆活动也表现出分期性,几次大规模岩浆事件分别发生在57~44、28~24和7~4 Ma,这些峰期事件对应凤凰板块—南极板块汇聚速率突然变化,即构造体制发生转换的阶段,而汇聚速率稳定时期仅有少量或者没有岩浆活动(图3b^[55],c^{[30,56⇓⇓⇓-60]}),说明板片俯冲速度改变是南极半岛峰期岩浆作用主控因素^[30]。为理解南极半岛板块运动与凤凰板块俯冲之间的关系,Gao等^[30]结合在铜矿半岛与海狮呷新获得的古地磁数据与已发表数据,计算了南极半岛北部—南设得兰群岛62(D=354.5,I=-71.0,α₉₅=4.2)、56(D=164.2°,I=77.1°,α₉₅=5.9)、54(D=337.2°,I=-78.3°,α₉₅=5.7)、51(D=341.3°,I=-71.5°,α₉₅=5.6)和47 Ma(D=337.7°,I=-76.2°,α₉₅=7.3)的平均古地磁方向,识别出南极半岛北部—南设得兰群岛62~55 Ma发生过一次快速顺时针旋转,对应凤凰板块—南极板块之间较高汇聚速率,55~47 Ma期间没有明显构造旋转,对应凤凰板块—南极板块之间较低汇聚速率,47 Ma之后转变为逆时针旋转,而凤凰板块—南极板块之间汇聚角度也从约60°减小到40°以下(图3a)。凤凰板块从对南极半岛接近正向俯冲逐渐变为斜向俯冲,导致两板块间作用力沿垂直与平行南极半岛轴向方向发生分解,而只有垂直于南极半岛方向的作用力才能导致南极半岛发生旋转,因此凤凰板块斜向俯冲减小了板块俯冲对南极半岛构造旋转造成的影响。减弱的凤凰板块俯冲作用削弱了外力对南极半岛运动的影响,导致南极半岛约47 Ma之后随东南极发生逆时针旋转,但凤凰板块在62~55 Ma期间与南极半岛汇聚角度较大,汇聚速度较快,使南极半岛北部—南设得兰群岛在这段时间的磁偏角偏离了从东南极视极移曲线推算的磁偏角(图3a^[61])。

凤凰板块俯冲还导致南极半岛—南设得兰群岛发生过快速地壳抬升和剥蚀,其中南极半岛北部格雷厄姆地和南设得兰群岛锆石与磷灰石裂变径迹研究揭示95~70、55~45、40~35和18~5 Ma几次快速抬升冷却事件^{[57⇓⇓-60]}(图3c),中新世磷灰石(U-Th)/He年龄显示从南西向北东自阿德莱德岛至昂维尔岛有逐渐变年轻的趋势,Guenthner等^[59]将其归因于洋中脊从南西向北东与海沟逐渐碰撞导致板片窗打开与软流圈物质上涌使地表快速抬升剥蚀,板片窗打开也导致南极半岛南部分布中新世火山岩^[62-63]。在南极半岛南部亚历山大岛,磷灰石裂变径迹揭示研究区在100 Ma至今持续抬升冷却,其中40~35 Ma期间经历了一次快速抬升事件,通过45 ℃/km的地温梯度计算,可得40 Ma以来剥蚀量为2.5~3.5 km,40~35 Ma期间剥蚀近1 km^[64]。

在汇聚板块边缘低角度俯冲构造挤压阶段,俯冲板片与上覆板块耦合度高,地热梯度较低,一般缺少弧岩浆作用^[65],对应南极半岛80~62 Ma期间较弱的弧岩浆作用。当板片俯冲速度开始减小时,俯冲角度变大,构造体制从压缩变为伸展,这会使板片与地幔楔之间发生解耦,地热梯度增高,引起俯冲板片脱水,进一步导致地幔楔下部地幔交代岩发生熔融,形成岛弧岩浆^[65]。板块汇聚速率在42~30 Ma降低并没有产生明显的岩浆作用,可能是降低的板块汇聚角度对南极半岛施加侧向挤压,不利于岩浆喷出地表^[30]。因为同样是板块汇聚速率降低,但是板块汇聚角度较大的55~48 Ma期间,南极半岛仍有岛弧岩浆作用发生。另外,通过与岩浆事件对比,我们发现新生代早期55~45和40~35 Ma的快速抬升事件发生在凤凰板块—南极板块汇聚速率较小的阶段^[30](图3b,c),凤凰板块运动速度降低会导致其俯冲于南极半岛之下的部分发生板片回卷,导致软流圈物质上涌,而软流圈物质上涌会使南极半岛靠近俯冲带的地区发生地表抬升^[66],这可以解释新生代早期的两阶段地表快速抬升剥蚀。

2 南美南部火地群岛与巴塔哥尼亚新生代关键构造演化

南美最南端(53°S以南)作为德雷克海峡北边界,构造演化与德雷克海峡打开息息相关,其从大地构造上可分为5部分,从西到东分别为:(1)古太平洋板块俯冲形成的晚侏罗世—新近纪岩浆弧(又称巴塔哥尼亚岩基)^[67];(2)由残余洋壳和上覆沉积物组成的晚侏罗世—早白垩世洛卡斯维迪斯盆地^[68];(3)古生代变质基底(达尔文山脉)^[69];(4)晚白垩世—中新世海相沉积褶皱与逆冲断层带(麦哲伦褶皱和逆冲断层带)^[70];(5)未发生强烈变形的弧前盆地(麦哲伦—奥斯特拉尔盆地)(图4^[24,71])。侏罗纪与冈瓦纳大陆裂解有关的岩浆作用导致南美东部岩石圈发生伸展作用,控制了麦哲伦—奥斯特拉尔前陆盆地早期形成^[72-73]。洛卡斯维迪斯盆地形成于晚侏罗世伸展事件峰期^[74-75],盆地沉积延续到早白垩世^[76-77]。南大西洋在约100 Ma开始打开之后,其挤压应力传递到洛卡斯维迪斯盆地,产生一系列逆冲断层^[78-79]。这一阶段变形使得洛卡斯维迪斯盆地逐渐关闭,产生区域变形与变质改造^[80],并使达尔文山脉高角闪岩相变质基底在逆冲过程中被抬升至地表^[77,81]。

南美板块主体呈现南北走向,但到了南美南端变为近东西走向(图4c),古大陆重建显示南美板块在约50 Ma从南向转变为西北西向运动,导致德雷克海峡逐步打开^[82]。南美南端洛卡斯维迪斯盆地以西的巴塔哥尼亚地区(麦哲伦地块)相对于南美北部的逆时针旋转主要发生在100~90 Ma,这导致洛卡斯维迪斯盆地关闭,新生代以后只经历了小规模逆时针旋转^[83],而洛卡斯维迪斯盆地以东的麦哲伦—奥斯特拉尔盆地弯曲形态可能为冈瓦纳大陆裂解期间形成的原始形态(图4a,b)。地质与地球物理证据显示在南美南端火地群岛东南部存在一系列新生代伸展盆地,伸展作用发生在晚古新世—早始新世(56~49 Ma),可能与德雷克海峡早期打开有关^[84](图5a^{[55,84⇓⇓⇓⇓⇓-90]})。区内深海至浅海沉积地层广泛发育同沉积构造、褶皱和叠覆构造,并且晚白垩世—古近纪海相地层向内陆逐渐变厚,代表逐渐海侵^[91]。南美南部新生代期间分别经历了凤凰板块、法拉隆(纳兹卡)板块和南极板块俯冲改造。其中法拉隆—凤凰板块洋中脊与南美南部在中始新世碰撞之后继续俯冲形成板片窗,导致岩浆作用在中始新世—晚渐新世期间自北西向南东迁移^[67,92-93](图5b^[93]),在约22 Ma时板片窗到达南美板块最南端,而凤凰板块、法拉板块和南极板块3联点在18~17 Ma俯冲至南美南端之下,导致俯冲板片大规模断离,板片窗快速扩大和软流圈物质上涌,地表岩浆作用范围扩大并向北东方向扩展^[93](图5b)。南美南部在40~25 Ma期间为岩浆削弱期^[92],对应凤凰板块—南美板块汇聚速率较低阶段^[55],而约23和29 Ma峰期岩浆作用^[93]对应凤凰板块—南美板块汇聚速率即将发生变化的时期^[55],这与南极半岛北部—南设得兰群岛峰期岩浆作用发生时间相近(图3b,c),说明南美板块内岩浆作用同时受到洋中脊俯冲和板块汇聚速率影响。凤凰板块俯冲导致的构造变形自西向东逐渐传递至南美大陆内部,对应里奥图尔维奥角度不整合与生长断层^[94]、梅塞塔斯始新世玄武岩高原^[95]。Ghiglione和Ramos^[86]提出新生代南美南部的造山带经历了三阶段主要变形,早期的两阶段变形发生在61~55(圣文森特逆冲事件)和49~34 Ma(里奥布埃诺逆冲事件),导致基底变质岩快速抬升。

其中第二期快速抬升也被磷灰石和锆石热年代学数据记录了下来^[85,87](图5a),而第一期变形事件对应南极半岛北部—南设得兰群岛在62~55 Ma期间的快速顺时针旋转(图3a),说明这是南美南部—南极半岛北部重要的一期构造事件。麦哲伦—奥斯特拉尔盆地新生代沉积地层物源分析显示在约39 Ma发生了一次快速物源变化,这可能与构造作用导致地壳快速抬升、剥蚀有关^[60]。29 Ma至今发生第三期构造事件,峰期事件是发生在24~16 Ma的走滑挤压事件(蓬塔格鲁撒走滑事件)^[86],主要与麦哲伦—法尼亚诺左旋走滑断裂运动有关^[96](图1b,4c)。沉积相分析显示麦哲伦—奥斯特拉尔盆地在37 Ma至29~24 Ma期间为深海沉积,之后地层由于构造事件影响逐渐抬升至海表,对应火地群岛海道逐渐关闭,约11 Ma之后,由于构造沉降或者海平面变化,又恢复为浅海—河口环境^[86](图5a)。在火地群岛以西的巴塔哥尼亚,沉积记录同样显示在晚始新世—早渐新世地壳快速抬升,这与纳兹卡板块加速向南美板块俯冲时间相对应^[55,97],代表南美南部在这段时期经历了广泛构造挤压,导致巴塔哥尼亚的地层在23~20 Ma期间从海相转变为河流相沉积^[88⇓-90]。最终,早中新世之上覆盖的近水平地层代表麦哲伦—奥斯特拉尔盆地内挤压变形阶段的结束^[98]。这些峰期构造事件对应法拉隆—凤凰板块与南美板块之间汇聚速度高值期^[55](图5a),证明南美板块内部构造事件由洋壳俯冲所控制。南美南端在渐新世—早中新世的强烈构造抬升导致巴塔哥尼亚与火地群岛相关区域发生海退,减少了德雷克海峡净流量,削弱了南极绕极流,被用于解释晚渐新世时全球突然增温^[91]。

3 德雷克海峡打开过程的古构造重建

德雷克海峡由斯科舍板块和南桑威奇板块组成(图1b)。在斯科舍海西部,沙克尔顿断裂将其与凤凰板块分开,以南通过南斯科舍洋脊中左旋走滑断层将其与南极半岛分隔开,以东通过东斯科舍洋脊将其与南桑威奇板块分隔,以北通过麦哲伦—法尼亚诺和北斯科舍洋脊断裂系统将其与南美板块分隔开^[82,99]。斯科舍海由西、中、东斯科舍海盆组成,它们形成于斯科舍板块伸展时期,通过海底磁异常条带等资料限制的3个海盆的打开时间分别是26.5至6.6~5.9、21~7和15~0 Ma^{[100⇓⇓⇓⇓-105]}。另有研究对西斯科舍海中相对较老的磁异常条带分析得出其形成于30.5~28 Ma^[106-107],而中斯科舍海还可能残存有白垩纪洋壳^[108]。南斯科舍洋脊由弧后残余陆块和洋盆组成,自西向东包括泰罗隆起、普罗泰科特盆地、皮里海底高地、多芬盆地、布鲁斯海底高地、斯甘盆地和迪士卡沃瑞海底高地^[12,99]。南斯科舍洋脊以南至南极半岛之间的区域自西向东分布有鲍威尔盆地、南奥克尼微陆块、简盆地和简海底高地。板块重建、地震、岩石学和物源对比证据显示泰罗隆起、皮里海底高地、布鲁斯海底高地、迪士卡沃瑞海底高地和简海底高地拥有陆壳基底,在新生代岩石圈伸展时期分裂自南极半岛北部地区^[109⇓-111]。南奥克尼微陆块拥有古生代基底,形成于古太平洋板块俯冲增生过程中^[112],在新生代从南极半岛北部分离出去^[111]。斯科舍板块北边界为北斯科舍洋脊,包括伯德伍德海底高地、戴维斯海底高地、巴克海底高地、沙格洛克海底高地和南乔治亚微陆块^[99]。岩石地球化学、地层对比、热年代学和古大陆重建等证据显示南美板块俯冲到斯科舍海导致的弧后伸展使得北斯科舍洋脊附近陆块和南乔治亚微陆块从南美南部分离出去,并逐渐运动到当前位置^[12,113-114]。ACC穿过斯科舍海时,其自北向南分布的极锋(Polar Front,PF)、亚南极锋(Sub-Antarctic Front,SAF)和南极绕极流南锋(South Antarctic Circumpolar Current Front,SACCF)携带南太平洋海水流经斯科舍海后再穿过北斯科舍洋脊的一系列狭窄海道进入南大西洋(图1b)。而威德尔海深层水(Weddell Sea Deep Water,WSDW)和威德尔海底层水(Weddell Sea Bottom Water,WSBW)则沿逆时针方向从威德尔海流经南斯科舍洋脊不同海道后进入斯科舍海(图1b)。因此,北斯科舍洋脊、南斯科舍脊相关盆地和斯科舍海打开时间是重建ACC演化的关键。尽管前人对这些盆地开展了大量研究,但由于缺少钻探样品验证,目前对于北斯科舍洋脊和南斯科舍脊相关盆地打开时间仍存在较大争议,其中北斯科舍洋脊打开与西斯科舍海扩张有关,可能开始于约26 Ma^[103,113],关于南斯科舍脊相关盆地打开时间总结如下(图6d^{[101,103,115⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-127]})。

普罗泰科特盆地:34~30(磁异常条带)^[115]、28~25(热流数据)^[116]、21~17(异常条带)^[103]、17.533~13.608(磁异常条带)^[117]、16~13.1(磁异常条带)^[101]和17.4~13.8 Ma(多波束测深法、多道地震反射、重力异常和磁异常条带)^[118];多芬盆地:43~42(热流数据)^[116]、41~34.7(磁异常条带重建)^[115]、24.5~21.7(底拖岩石、富钴铁锰结壳和磁异常条带)^[119]和23.03~20.709 Ma(拖网岩石样品年代校正磁异常条带)^[117];斯甘盆地:35.7~29.5(磁异常条带和多道地震反射)^[120]、30.2(热流数据)^[116]和26 Ma—中新世(多道地震反射与重力)^[121];简盆地:32~25(热流数据)^[122]和17.6~14.4 Ma(磁异常条带)^[123];鲍威尔盆地:38~32(热流数据)^[124]、29.7~21.8(磁异常条带)^[125]、29~23(根据经过沉积校正的洋壳深度推断)^[126]和27~18 Ma(磁异常条带、重力测量和地震反射剖面)^[127]。

尽管不同研究的结论不同,但越来越多的证据支持30~20 Ma是南斯科舍脊相关洋盆打开关键时间,对应凤凰板块—南极半岛之间汇聚速率和角度的增加(图3b),这会导致南极半岛北部—南设得兰群岛内产生垂直于俯冲带方向的岩石圈伸展,形成与俯冲方向近垂直的岩脉,如利文斯顿岛赫德半岛和巴纳德角31~29 Ma东南东和南东走向的岩脉^[53]。

除了通过海底磁异常条带重建德雷克海峡打开过程外,对南极半岛和南美南部开展陆地构造古地磁研究能重建海峡宽度变化,进而限制海峡流量,但由于恶劣的野外工作环境与有限的后勤保障,目前南极半岛和南设得兰群岛新生代古地磁数据主要集中在65~46 Ma^{[30,34-35,71,128⇓-130]},南美南部古地磁数据集中在55~40 Ma^[83,131],这些数据可以重建海峡打开前两地区古地理位置,但尚不足以控制海峡打开过程,这导致目前南极半岛和南美南部古大陆位置重建多依靠东南极与南美板块视极移曲线^[61],因此古地磁数据缺失阻碍了对德雷克海峡打开过程的认识。

4 古海洋学制约南极绕极流与大西洋经向翻转流形成

除了通过对洋盆重建和陆地古地磁研究来限制德雷克海峡打开外,前人还根据古海洋学和古生物学等方法来限制海峡演化(图6d)。智利南部巴塔哥尼亚晚白垩世—始新世浅海沉积中甲藻孢囊分析^[132]和南极半岛与南美南部麦哲伦—奥斯特拉尔盆地(图4)中始新世甲藻孢囊分析^[133]支持古新世—早始新世时德雷克海峡已经有浅水贯通的观点。Reguero等^[134]通过陆地动物群分析提出南美南部和南极半岛北部分离始于57~56 Ma。这些海峡早期打开时间对应南极半岛北部—南设得兰群岛62~55 Ma期间快速顺时针旋转(图3a),因此这两者之间或许存在一定联系。前人根据海底磁异常条带和卫星重力异常^[106,115]等数据提出南美板块和南极半岛之间的岩石圈伸展可追溯至约50 Ma。van de Lagemaat等^[82]通过磁异常条带和古地磁开展古大陆重建推断约50 Ma伸展作用是由南美板块从南向运动转变为西北西方向运动所致。Mao和Mohr^[135]对布鲁斯滩岩心中孢粉与微体古生物的研究显示其水深在46.3~44.5 Ma期间已经达到800~2 000 m。Brown等^[136]提出岩石圈伸展导致德雷克海峡深度在43~32 Ma已经超过2 000 m。Lawver和Gahagan^[10]综合古海洋和古大陆重建数据提出德雷克海峡的深部洋流在约32 Ma已经形成,之后,Lawver等^[137]根据古大陆重建和德雷克海峡内不同洋盆打开时间又将深部洋流的形成时间限制在约28 Ma。Pfuhl和McCave^[138]将189航次1170站位在约23.9 Ma发生的沉积物粒度变粗归因于德雷克海峡深部洋流增加。Roberts等^[139]以及Florindo和Roberts^[140]根据凯尔盖朗深海高原(120航次;744和748站位)和毛德海隆(113航次;690站位)沉积间断时间,提出ACC始于31~30 Ma。Wei和Wise^[141]通过对凯尔盖朗深海高原(120航次;744和748站位)超微化石研究,提出南极极锋形成于晚中新世—早上新世。Persico和Villa^[142]通过对毛德海隆和凯尔盖朗深海高原(689,690,744和748站位)超微化石研究,提出德雷克海峡浅部洋流形成于约33.5 Ma。Diester-Haass和Zahn^[143-144]通过对毛德海隆763,592(113航次;689站位)古生产力变化研究提出原始ACC形成于约37 Ma。Latimer和Filippelli^[145]通过对厄加勒斯海岭(Agulhas Ridge)(177航次;1090站位)沉积物物源变化和古生产力变化研究,提出ACC深部洋流形成于32.8 Ma。Diekmann等^[146]通过对Agulhas Ridge(177航次;1090站位)古生产力变化研究,提出ACC形成于33~30 Ma。Scher和Martin^[147-148]通过毛德海隆(689站位)和厄加勒斯海岭(177航次;1090站位)ε_Nd变化,提出太平洋海水在约41 Ma通过德雷克海峡进入大西洋。Gamboa等^[149]提出南巴西盆地(515站位)渐新世/中新世沉积增强是ACC导致南极底层水(Antarctic Bottom Water,AABW)加强导致。Beu等^[150]提出太平洋和大西洋在晚渐新世水体贯通可以解释新西兰与南美之间的软体动物扩散。Kennett和Barker^[151]提出毛德海隆(113航次;689和690站位)和福克兰深海高原(36和71航次;511和512站位)在渐新世拥有类似古地理记录,并根据渐新世—中新世沉积间断限制ACC形成时间为晚渐新世之后(图6d)。

除了通过古大陆重建和古海洋学等指标限制ACC形成外,对与ACC有密切关系的大西洋经向翻转流(Atlantic Meridional Overturning Circulation,AMOC)形成时间进行制约也可以在一定程度上理解ACC的形成。前人曾提出3个主要的AMOC形成模式:一是ACC形成导致大洋深层水上涌到南大洋,这种深水补偿机制迫使AMOC形成^[152-153];二是格陵兰—苏格兰海脊(Greenland-Scotland Ridge,GSR)下沉导致北大西洋与北欧海之间双向环流形成,促使北大西洋深层水(North Atlantic deep water,NADW)形成^[154];三是北极—大西洋海峡关闭,导致盐度较低的北极海水无法进入大西洋,所以大西洋海水盐度增加,导致NADW形成^[155]。Cramer等^[156]与Abelson和Erez^[154]总结全球大洋底栖有孔虫δ¹⁸O发现南大洋高纬度底栖有孔虫δ¹⁸O在约40 Ma时相对于其他大洋开始增加(图6f),代表南大洋高纬度地区相对于其他地区迅速变冷,支持德雷克海峡在约40 Ma打开,这与通过ε_Nd获得的德雷克海峡打开时间基本一致^[147-148](图4g)。但是,北大西洋与南大西洋底栖有孔虫δ¹⁸O没有明显差别,数值模拟显示AMOC形成会导致南北半球大气与海洋温度不均一,其中北半球温度高于南半球^{[152,156⇓⇓-159]},所以南、北半球一致的底栖有孔虫δ¹⁸O表明,AMOC在约40 Ma时并没有形成。另外,西南太平洋地震剖面、北大西洋ASP-5钻孔和1053站位底栖有孔虫δ¹³C变化支持AMOC形成于约35.5 Ma^{[147,154,160-161]}。因此,目前大洋ε_Nd、底栖有孔虫δ¹⁸O和δ¹³C支持德雷克海峡浅部洋流形成于约40 Ma,但地质证据^[154,161]和数值模拟^{[153,155,162⇓-164]}显示只有ACC时无法形成AMOC。除了ACC,还需要有其他因素导致AMOC形成。AMOC形成有赖于NADW形成。NADW形成需要GSR下沉到一定深度,使得北欧海海水通过GSR进入大西洋^[154]。大洋钻探获得的证据,包括硅藻变化^[165-166](DSDP 338站位,ODP 151航次913和908站位)、有孔虫^[167-168](162航次;ODP 985站位)、底层水变化^[165,168](ODP 151航次)、沉积相^[168-169]和海底侵蚀面^[170-171]都支持GSR在始新世—渐新世时发生明显沉降,使北大西洋高盐度、温暖浅层水通过GSR后进入北欧海,冷却并下沉后从GSR流出,形成NADW。基于此,Abelson和Erez^[154]提出始新世—渐新世期间GSR沉降以及北大西洋和北欧海之间环流形成导致AMOC形成,而塔斯马尼亚海峡在35~32 Ma期间深部洋流形成^[10],进一步加强了ACC与AMOC。但Hutchinson等^[155]通过数值模拟发现GSR下沉无法形成NADW,只有北极—大西洋海峡关闭,导致盐度低的北极海水无法进入大西洋,使北大西洋海水盐度增加,才能使北大西洋海水下沉,最终形成NADW^[155]。尽管关于NADW形成机制还存在争议,但越来越多的地质^[154]和数值模拟^[155]结果支持AMOC在约34 Ma形成,并且ACC不能单独导致AMOC形成。AMOC形成与ACC加强进一步减少了中纬度向南极输送热量,增强了南大洋深层水上涌,为南极带来丰富水汽,这可能促进南极大范围冰盖在约34 Ma形成。

AMOC形成使更多低ε_Nd的海水通过绕极深层水(Circumpolar Deep Water,CDW)上涌进入南大西洋,导致南大洋生产力提高,并导致南大西洋海水ε_Nd(ODP 689站位,ODP 1090站位,沃尔维斯Walvis站位)持续降低^[154]。由于太平洋海水ε_Nd值比南大西洋高,因此,当太平洋海水通过德雷克海峡进入南大西洋时,南大西洋的海水ε_Nd会增加。所以,南大西洋比较明显的3次ε_Nd升高^[147-148](40~35、30~25和20~18 Ma;图6g^[147-148])可能是德雷克海峡变宽或变深,使更多南太平洋海水进入南大西洋所致。这与南极半岛北部—南设得兰群岛、南斯科舍脊和其附近区域发生强烈伸展时期(31~24 Ma),以及Hodel等^[172]和Barker^[103]的研究结果基本一致。Hodel等^[172]通过对IODP 320航次(1334C钻孔)和ODP 113航次(689B钻孔)底栖与浮游有孔虫Mg/Ca、Ce/Ce^*和⁸⁷Sr/⁸⁶Sr值提出德雷克海峡在31~26 Ma打开并逐渐加深;Barker^[103]根据北斯科舍脊戴维斯滩、奥若拉滩构造和古深度变化推断ACC深部洋流的形成时间为22~17 Ma。而40~35 Ma也是凤凰板块向南极半岛和南美南部俯冲速度快速降低的阶段(图3b),凤凰板块俯冲速度降低导致其回卷并发生软流圈物质上涌,导致德雷克海峡地区强烈构造伸展和快速打开。德雷克海峡没有明显变宽或变深的时间段(35~30和25~20 Ma),ε_Nd持续降低可能是在通过德雷克海峡的太平洋的海水(高ε_Nd)没有明显增加的情况下,NADW(较低的ε_Nd)还在持续上涌所致。通过与深海底栖有孔虫δ¹⁸O^[173]对比发现,在ACC加强阶段,全球温度并没有明显变化(图6b^[173]),但与δ¹³C对比后发现,其中30~25和20~18 Ma两阶段明显对应δ¹³C低值期^[173](图4c)。深海底栖有孔虫δ¹³C的大小取决于有孔虫造壳时海水溶解无机碳(Dissolved Inorganic Carbon,DIC)δ¹³C(¹³C/¹²C)值,增强的ACC尽管能通过加强CDW进而增加南大洋生产力(图7a^{[143,146,174]},b^[175]),但会限制中纬度高盐水向南进入南大洋,因此,南大洋海水盐度将降低,这会减弱AABW,进而减弱AABW在赤道地区上涌,这解释了这些时期赤道地区降低的生产力(图7c^[176],d^[177]),也使得大洋底部与浅部海水间循环减弱。海表植物在光合作用过程中优先从大气CO₂中吸收¹²C,产生动力学分馏。当这些吸收了¹²C的植物死亡后沉到海底,在底层水循环不畅情况下,它们会在深海中积累得越来越多,在分解过程中消耗氧气(图6e^[178]),释放出大量的¹²C,导致深海δ¹³C值减小(图6c^[173])。因此,德雷克海峡打开后不仅改变了大洋环流模式,还可能进一步主导大洋碳循环演化和全球大洋生产力变化。大洋生产力变化影响全球海洋生物分布和演化,因此在研究新生代长时间尺度全球海洋生物时需要考虑德雷克海峡演化导致的影响。

尽管全球大气CO₂重建^[6]显示出与深海底栖有孔虫δ¹⁸O较为一致的变化趋势(图6a^[6],b),似乎支持CO₂变化导致全球温度和冰量变化,但并不能排除德雷克海峡变化导致CO₂变化,尤其是在ACC减弱的两个阶段(25~20和18~14 Ma)对应CO₂高值期与底栖有孔虫δ¹³C低值期,但是ACC加强阶段对应CO₂低值期和底栖有孔虫δ¹³C高值期(图6a)。德雷克海峡打开导致南大洋生产力增加,而AABW能有效且快速地将更多有机碳输送至深海,此时较弱的表层和底层海水交换能将这些有机碳长期封存在深海,使大气中CO₂减少(图6a)。这一过程持续到ACC停止增强阶段,AABW与赤道上涌水增强,使封存在深海中的碳释放到大气中,导致大气CO₂浓度增加,这一过程可以解释大气CO₂与底栖有孔虫δ¹³C同步变化(图6a,c)。另一方面,德雷克海峡加深增强了AMOC,提高了北半球温度和降雨量,增强风化剥蚀量,也会消耗大气CO₂,促进大规模南极冰盖在始新世—渐新世形成^[179],相同地质过程可能发生在40~35、30~25、和20~18 Ma ACC加强阶段。

中新世约14 Ma之后,底栖有孔虫δ¹³C持续降低(图6c),其变化模式相对于始新世—早中新世期间发生了变化,对应大气CO₂快速减少,NADW增强^[180],南大洋生产力快速增加^[181],南极永久冰盖形成^[1](图6)和相对稳定大洋底层氧含量的形成^[178](图6e),尽管斯科舍弧在约15 Ma之后东移^[103]、东斯科舍海自约15 Ma之后打开^[182]和中斯科舍海在约11.6 Ma时去除火山弧对绕极洋流的阻挡^[183]都会增加通过德雷克海峡净流量,但与古大洋演化有关的指标演化模式的改变说明除了德雷克海峡还有其他海峡发生了关键变化。这对应特提斯海道在约15 Ma之后永久关闭^[184],印尼海道在约14 Ma之后开始逐步关闭^[185],以及11.5~2.9 Ma期间中美海道逐渐变浅至关闭^[186],数值模拟显示这些海峡逐渐关闭会改变全球大洋环流模式,增强AMOC和ACC^{[163,187-188]},进一步阻碍了中低纬度高温高盐海水进入南大洋,加速南极变冷,促进南极永久冰盖在约14 Ma时形成。中美海道和印尼海道在上新世最终关闭也被认为与北极冰盖形成有关^[189],因此低纬度海道的两次关键关闭时间分别对应南极和北极永久冰盖形成。低纬度海峡关闭改造大洋环流模式也被用来解释石炭—二叠纪、晚奥陶世和埃迪卡拉纪冰期事件^[190],尽管两者在时间上有一定对应关系,但仍需要更多的地质证据和数值模拟来回答两者之间真正的关系。因此,在解释中新世之后全球变化时需要综合评估德雷克海峡、中美海道、印尼海道和特提斯海道等变化对全球大洋环流和南北极冰盖形成等的影响。

5 德雷克海峡影响下的构造—古海洋—古气候演变:问题与展望

德雷克海峡打开是新生代大洋和气候演化的重要一环,通过重建德雷克海峡不同洋盆打开时间来重建德雷克海峡打开过程仍存在很大争议,主要原因之一在于缺乏洋盆中原位钻探得到的基岩样品来验证海底磁异常条带年龄。南极半岛在62~55 Ma期间的快速顺时针旋转对应南极半岛和南美南部之间的分离,可能导致德雷克海峡早期打开。海峡持续打开与南极半岛北部—南美南部构造演化息息相关,尤其是俯冲大洋岩石圈减速会导致俯冲板片回卷、软流圈物质上涌和岩石圈伸展,其中40~35 Ma期间凤凰板块俯冲速度降低导致岩石圈伸展可能加速了南极半岛和南美板块分离以及海峡打开。30~25 Ma期间凤凰板块—南极半岛之间汇聚速率和角度增加导致南极半岛北部发生垂直于俯冲带方向伸展作用,可能促进了南斯科舍脊相关洋盆打开。凤凰板块—南美板块之间汇聚速度增加解释了麦哲伦—法尼亚诺走滑断裂在24~16 Ma期间的运动,进一步导致西斯科舍海和北斯科舍脊一系列盆地打开,使ACC深部洋流贯穿德雷克海峡。而约50 Ma以后南美板块从南向运动转变为西北西方向运动使其更快与南极半岛发生分离,最终形成目前的德雷克海峡。

南极半岛和南美南部陆上古地磁研究可制约海峡宽度变化,但目前新生代古地磁研究多集中在古新世—早始新世,仍难以准确重建海峡打开过程。从大洋沉积物中获得的数据存在多解性,并且南大洋深海钻探工作相对于其他大洋较少,因此未来需要在南大洋开展更多钻探工作,尤其需要在德雷克海峡内部和两侧布置更多钻探站位,对获得的数据进行综合解译,识别出代表ACC演化的关键指标。总之,尽管前人对德雷克海峡演化开展了大量研究,但同时也带来了更多争议,考虑到德雷克海峡在新生代全球变化中的重要地位,未来有必要加强德雷克海峡演化研究。

6 结语

本文总结了南极半岛北部和南美南部新生代构造演化以及与德雷克海峡打开过程相关的研究,提出40~35、30~25和20~18 Ma这3期与海峡打开和南极绕极流加强有关的阶段,对应南极半岛北部和南美南部重要构造事件。其中后两期ACC增强事件发生在AMOC形成之后,对应大气CO₂低浓度,大洋底栖有孔虫低δ¹³C、深海溶解氧含量低值和南大洋高生产力阶段。ACC增强导致进入南大洋的中纬度高盐海水减少,降低了南大洋海水盐度,减弱了南极底层水强度,降低了赤道生产力,但促进了绕极深层水上涌,增加了南大洋生产力。减弱的南极底层水削弱了大洋底层水与表层水之间的交换,使深海积累并分解了更多富¹²C的有机质,降低了深海δ¹³C和溶解氧含量。更多有机碳埋藏在深海可能导致大气中CO₂减少和全球温度降低。因此,板块构造控制下的德雷克海峡逐步打开在新生代古海洋和古气候变化中占有重要地位。

感谢审稿人和编辑在审稿过程中提供的宝贵意见。

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基金资助

国家自然科学基金项目(41930218)

国家自然科学基金项目(42322607)

国家自然科学基金项目(42076223)

中央高校基本科研业务费专项(265QZ2021013)