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Apr.  2023
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QIU RuoYuan, FANG LinHao, LU YuanZheng, DENG ShengHui, ZHANG XinZhi, LÜ PeiZong, REN JiaHao, HUANG RuTing, FANG YaNan, ZHANG XiaoYu, LI HongJia, XIAN BenZhong, SHI ShengBao. Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 425-434. doi: 10.14027/j.issn.1000-0550.2021.095
Citation: QIU RuoYuan, FANG LinHao, LU YuanZheng, DENG ShengHui, ZHANG XinZhi, LÜ PeiZong, REN JiaHao, HUANG RuTing, FANG YaNan, ZHANG XiaoYu, LI HongJia, XIAN BenZhong, SHI ShengBao. Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 425-434. doi: 10.14027/j.issn.1000-0550.2021.095

Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin

doi: 10.14027/j.issn.1000-0550.2021.095
Funds:

National Natural Science Fundation of China 41502024

China University of Petroleum (Beijing) Youth Top Talent Project 2462014YJRC027

  • Received Date: 2021-04-01
  • Accepted Date: 2021-08-18
  • Rev Recd Date: 2021-06-19
  • Available Online: 2021-08-18
  • Publish Date: 2023-04-10
  • The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) is believed to be closely related to the Karoo-Ferrar igneous explosion and the global warming caused by the rapid release of greenhouse gases related to methane gas hydrates. This event has been studied in detail in the Tethys Ocean region, but the environmental changes and sedimentary responses in terrestrial ecosystems have received less attention. Here, the organic carbon isotopic stratigraphic data for the Kuqa River area at the northern margin of the Tarim Basin are reported. The overall positive excursion trend of δ13C curves in the East and West sections are interrupted by multi-stage rapid negative excursions, which indicates that the surface carbon cycle has been greatly disturbed. The rapid fluctuation of the δ13C value in the event layer suggests that the carbon source with light isotopic characteristics was injected into the atmospheric carbon pool over a short period of time, and was characterized by unstable and periodic injection. It is speculated that this may be related to the positive feedback caused by warming. The abrupt change in sedimentary facies and the decrease in sporopollenin species indicate that the sedimentary environment has changed greatly. The presence of clastic dolomite and purplish mudstone, the attenuation of wet palynology and the occurrence of mesopalynology all indicate that the climate at the northern margin of Tarim Basin became dry and hot during T-OAE. This study reports the first T-OAE record from the margin of a continental sedimentary basin, and is crucial to understanding the evolution of terrestrial systems in the context of the significant increase of carbon dioxide in the present period of Earth history.
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  • Received:  2021-04-01
  • Revised:  2021-06-19
  • Accepted:  2021-08-18
  • Published:  2023-04-10

Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin

doi: 10.14027/j.issn.1000-0550.2021.095
Funds:

National Natural Science Fundation of China 41502024

China University of Petroleum (Beijing) Youth Top Talent Project 2462014YJRC027

Abstract: The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) is believed to be closely related to the Karoo-Ferrar igneous explosion and the global warming caused by the rapid release of greenhouse gases related to methane gas hydrates. This event has been studied in detail in the Tethys Ocean region, but the environmental changes and sedimentary responses in terrestrial ecosystems have received less attention. Here, the organic carbon isotopic stratigraphic data for the Kuqa River area at the northern margin of the Tarim Basin are reported. The overall positive excursion trend of δ13C curves in the East and West sections are interrupted by multi-stage rapid negative excursions, which indicates that the surface carbon cycle has been greatly disturbed. The rapid fluctuation of the δ13C value in the event layer suggests that the carbon source with light isotopic characteristics was injected into the atmospheric carbon pool over a short period of time, and was characterized by unstable and periodic injection. It is speculated that this may be related to the positive feedback caused by warming. The abrupt change in sedimentary facies and the decrease in sporopollenin species indicate that the sedimentary environment has changed greatly. The presence of clastic dolomite and purplish mudstone, the attenuation of wet palynology and the occurrence of mesopalynology all indicate that the climate at the northern margin of Tarim Basin became dry and hot during T-OAE. This study reports the first T-OAE record from the margin of a continental sedimentary basin, and is crucial to understanding the evolution of terrestrial systems in the context of the significant increase of carbon dioxide in the present period of Earth history.

QIU RuoYuan, FANG LinHao, LU YuanZheng, DENG ShengHui, ZHANG XinZhi, LÜ PeiZong, REN JiaHao, HUANG RuTing, FANG YaNan, ZHANG XiaoYu, LI HongJia, XIAN BenZhong, SHI ShengBao. Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 425-434. doi: 10.14027/j.issn.1000-0550.2021.095
Citation: QIU RuoYuan, FANG LinHao, LU YuanZheng, DENG ShengHui, ZHANG XinZhi, LÜ PeiZong, REN JiaHao, HUANG RuTing, FANG YaNan, ZHANG XiaoYu, LI HongJia, XIAN BenZhong, SHI ShengBao. Responses to the Early Jurassic Oceanic Anoxic Events in the Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 425-434. doi: 10.14027/j.issn.1000-0550.2021.095
  • 近一百年以来,由于人类持续地开发开采和使用化石燃料,导致大气中CO2的浓度不断地增加,造成了越来越严重的温室效应,引起国际社会的广泛关注。以在地质历史上曾经发生过的增温事件为研究对象,对预测由变暖效应带来的一系列影响有重要借鉴意义;地质深时时期的记录为全尺度洞察在温室气候状态下地球系统如何运行提供了一个天然的实验室[1]。早侏罗世Toarcian期大洋缺氧事件(Toarcian Oceanic Anoxic Event,T-OAE)是中生代的一次重要环境演变事件,是显生宙以来最大的两次全球极热事件之一(另一次是古新世和始新世之交的极热事件),其以黑色页岩中大幅度的碳同位素负向偏移(Negative Carbon Isotope Excursion,NCIE)为最显著的特征。Karoo-Ferrar大火成岩省被认为是主要的诱发因素,随后在甲烷水合物释放、岩浆侵入富有机质煤层、冰冻圈融化、湿地野火等其他因素共同作用下进一步引起了多幕次的碳同位素负偏[2]。T-OAE历时不到一百万年,但对气候、环境和生态系统造成了巨大的影响,如全球气温迅速上升[3]、海洋酸化[4]、海洋生物集群灭绝[57]、陆地气候带变化[89]以及海水缺氧与表层富营养化[1012]等。

    目前,关于T-OAE仍有很多关键问题需要解答,例如,大量温室气体的来源和释放的机制是什么?大量的碳源进入大气—海洋系统如何被沉降固定下来?陆地的气候干热还是湿热变化?陆地生态系统在迅速升温的背景下如何响应等问题。因此,开展陆相T-OAE研究,对于解决这些关键科学问题具有重要意义。迄今为止,T-OAE的研究主要针对海相地层,目前报道的陆相盆地记录有三个,分别是中国四川盆地[13]、中国鄂尔多斯盆地[14]、中国东北的金羊盆地[15]。而露头剖面仅有鄂尔多斯盆地,四川盆地和东北的金羊盆地都是岩心记录。在侏罗纪中国主要板块与西伯利亚板块共同构成欧亚大陆的主体(图1a),陆相沉积广泛发育[8],对于开展陆相T-OAE研究具有得天独厚的条件。本研究选取塔里木盆地北缘的库车河剖面为对象,综合前人在该区生物地层工作成果,通过系统地测试分析样品的有机碳同位素,厘定T-OAE在塔里木盆地中准确层位,报道其在陆相盆地中的响应特征,并进一步尝试探索T-OAE成因。

    Figure 1.  Tarim Basin: (a) paleogeography[16]; (b) section location; and (c) stratigraphic outcrops

  • 塔里木盆地位于新疆维吾尔自治区南部,是我国最大的内陆含油气盆地,面积达56万平方千米。盆地北至天山南麓,南至西昆仑山和阿尔金山北麓,总体呈东西向展布的菱形,海拔高度为800~1 300 m,地势西高东低。塔里木盆地内沉积岩厚度超过16 000 m,显示为两套沉积环境,震旦系—下二叠统的沉积环境为海相—海陆过渡相,上二叠统—第四系的沉积环境为陆相。塔里木盆地共发育五个坳陷构造单元[17]。本文主要研究的库车河剖面属于盆地北部的库车坳陷构造单元,下侏罗统阳霞组顶部至中侏罗统克孜勒努尔组中部的地层连续出露(图1b)。

    库车前陆盆地位于塔里木盆地北缘,是一个中、新生代的盆地。库车前陆盆地在侏罗纪处于盆地的坳陷阶段,沉积连续,自下而上依次为阿合组、阳霞组、克孜勒努尔组、恰克马克组、齐古组、喀拉扎组。阿合组为一套灰白色砂岩、砾岩,阳霞组岩性总体以灰绿色砂岩、灰黑色粉砂质泥岩为主,顶部为一套约50 m厚的灰黑色碳质泥岩,克孜勒努尔组为砂岩、粉砂岩、泥页岩夹煤层。本次研究选取库车河地区东边和西边两个剖面(图1c),分别命名为“东剖面”和“西剖面”。“东剖面”位于库车河东侧约10 km,是一个近年来停止开采的煤场,剖面新鲜,但地层出露不够连续,需在煤场内沿地层走向约300 m范围内根据标志层拼接。“西剖面”位于库车河西侧,岩层呈单斜状连续产出,无明显沉积间断,两剖面相距约15 km,剖面位置以及库车河地区地层出露如图1c所示。

    前人曾在阿合组—克孜勒努尔组下部发现了疑源类化石库车孢型体Kuqaia,确定其时代为早侏罗世—中侏罗世[18]。库车河剖面包含两个植物组合,下部阳霞组发育Cyathidites⁃Cibotiumspora⁃Disacciatrileti组合,克孜勒努尔组发育Cyathidites⁃Neoraistrickia⁃Disacciatrileti组合,前者大部分为早侏罗世常见分子,而后者中CyathiditesNeoraistrickia的高含量是中侏罗世孢粉组合的主要特征,这显示克孜勒努尔组地层时代已跨入中侏罗世[19]。前人研究发现[1720],孢粉组合在阳霞组和克孜勒努尔组之间变化明显,阳霞组顶部耐干喜热孢粉组合Classopollis数量增多[21],而且克孜勒努尔组下段153层中出现了碎屑白云岩和紫红色泥岩组合,而在克孜勒努尔组153层的碎屑白云岩之上,孢粉组合同样也发生了明显变化,热带—亚热带湿生孢粉种类减少,而温带湿生—中生孢粉OsmundaciditesPseudoreticulatus出现,均暗示在阳霞组顶部—克孜勒努尔组对应沉积时期可能存在较大的气候转变事件。

    在岩性地层方面,阳霞组岩性总体以灰绿色砂岩、灰黑色粉砂质泥岩为主,顶部为一套灰黑色碳质泥岩;克孜勒努尔组下段146~152层为砂岩、粉砂岩和泥页岩夹煤层,153层出现碎屑白云岩和紫红色泥岩,154~157层出现由现代煤层自燃而烘烤形成的桔红色细砂岩。

    因此,综合生物地层和岩性地层资料,阳霞组顶部—克孜勒努尔组下部是可能对应于早侏罗世晚期—中侏罗世早期,因此将研究层位锚定在阳霞组顶部以上克孜勒努尔组145~157层之间(层号据王招明等[17])。

  • 研究样品取自库车河地区,以阳霞组顶为起点,采样间隔根据岩性确定,其中泥岩的采样间隔为20 cm、砂岩为30~40 cm、砾岩为40~60 cm、煤线层位10 cm,“西剖面”采样38块,“东剖面”采样35块,共采样73块。实验室前处理中,将所有样品用去离子水浸泡冲洗,并反复用超声波清洗除去表面杂质,烘干后磨碎至200目。然后加入3 mol/L的盐酸在60 ℃水浴中加热脱硫并除去可能含有碳酸根的无机物;最后用去离子水漂洗至中性,在低温烘箱中烘干48~72 h,待测。

    碳同位素测试仪器为Thermo Scientific(热电)FLASH HT EA⁃MAT 253 IRMS,碳同位素分析为连续流检测,是将Flash HT元素分析仪与MAT 253同位素质谱仪结合起来分析样品。FLASH HT EA中氧化管填料为线状还原铜、氧化铬、含银氧化钴;反应温度为980 ℃;载气为He(99.999%),载气流量为100 mL/min,辅助气为250 mL/min;助燃为纯氧(99.995%),流量为250 mL/min;色谱柱炉温为恒温50 ℃。MAT 253质谱采用EI源,电离电压为70 eV。以SY/T 5238—2019《有机物和碳酸盐岩碳、氧同位素分析方法》为执行标准,所用标准物质为IAEA-600 Caffeine,USGS24 Graphite。测试中每10个样品中加入一个平行样从而排除仪器造成的误差,最终得到的有机碳同位素(δ13C)的分析结果采用PDB标准,平行样偏差在0.5%以内。总有机碳(Total Organic Carbon,TOC)采用LECO CS230碳硫分析仪进行分析,将处理好的样品称取0.1 g,放入坩埚中,同时加入钨、铁助燃剂,上机测试。所有样品前处理与测试工作均在中国石油大学(北京)油气资源与探测国家重点实验室完成。

  • 从整体来看,“西剖面”的TOC值介于0~4.5%,平均值0.47%,表现为剖面下段高、上段低的特点。TOC含量出现了5个较为显著的峰值,达背景值的10倍以上,其中4个峰值出现在剖面下段的144~152层中,另一个峰值位于153层中,其特点是均表现为脉冲状,仅由一个窄峰组成,且这4次峰值均出现在0~50 m的地层中。TOC第一个峰值出现在145层的黑色泥页岩中,第二个峰值对应着δ13C值在146~152层(约10 m处)中的最负值,其余3个峰值均出现在δ13C值正偏阶段。在153层以上的地层中,TOC迅速降低,尤其在剖面顶部被区域上的煤层自燃烘烤的、呈桔红色的砂岩中,TOC呈平缓低值,在0.3%附近波动,且与δ13C值无明显对应关系。整个剖面TOC仅在144~152层中出现了短暂的高峰值,除此之外的其他层位TOC值也基本位于较低水平。

    在“西剖面”146~157层中,δ13C值表现为一次小幅度的缓慢负偏和三次脉冲状的显著负偏,分别位于146~152层(约10 m处)、153层中部(约60 m处)、153层顶部(约75 m处)以及154~157层(约90 m处)中。146~152层中,δ13C值在-22‰附近波动,且在剖面下部约6 m处的泥岩中出现了一次小幅度的缓慢负偏移,δ13C逐渐负偏至-24‰,随后缓慢正偏,在约22 m处达到-21‰。在153层中,δ13C波动较明显,但整体上呈逐渐正偏的趋势,但在约60 m处的泥岩中,δ13C值出现了极其短暂的负偏峰值,记录了第一次脉冲状的负偏移(幅度达约3‰),随后继续正偏移至约65 m处结束。在153层顶部,紫红色泥岩和碎屑白云岩出现,同时δ13C值由-21.7‰迅速负偏至-26.7‰(幅度达约5‰),第二次脉冲状负偏在泥岩中出现。进入到154~157层中,δ13C值迅速恢复至整个剖面的最正值-20.9‰,又再次迅速负偏至-28.4‰,第三次脉冲状负偏基于剖面δ13C基值(-21‰)负偏了-7.4‰,而且此次负偏出现在154~157层约10 m厚的桔红色细砂岩中,此后δ13C值缓慢正偏,在约100 m处恢复到-22‰。整体来看,碳同位素曲线经历了负偏、正偏、脉冲状负偏三个阶段,而且δ13C值在剖面下端的小幅度负偏与最大幅度负偏均出现在岩性正旋回之后。此外,从144层到153层,湿生孢粉种类不断减少,到153层顶部,碎屑白云岩和紫红色泥岩、温带中生孢粉以及δ13C值大幅度负偏近同步出现。

  • 整体来看,位于煤场内的“东剖面”TOC比“西剖面”含量高呈现出多个峰值,含量约为背景值的5倍,均呈现出脉冲状的特征,且仅由一个窄峰组成,整体TOC介于0.1%~17.1%,高值均出现在碳质泥岩中,其平均值为3.38%。在146~152层中,TOC先增加再减少,与δ13C值先负偏再正偏趋势相对应,同时TOC在15.4 m处出现了一次峰值,约为9.8%,也对应了δ13C值153层中的负偏极值。在153层中,TOC值出现了3个高而窄的峰值,整个剖面最高的TOC值出现在此处,为17.1%,对应于δ13C值正偏阶段。在154~157层中,TOC分别在约70 m和约74 m处出现了2个峰值,分别对应了碳同位素的两个较负值。整个剖面TOC呈现出多个峰值的特点,TOC含量相比于“西剖面”较高。

    在“东剖面”整体约94 m的地层中,δ13C值和“西剖面”表现出非常相似的特征:为一次小幅度负偏移和三次脉冲状负偏移,分别位于146~153层中部(约15 m处)、154~157层(约67 m处、约70 m处、约74 m处)中。在146~152层中,δ13C值有一次小幅度负偏,在约15 m处的碳质泥岩中达到了-24.4‰,基于剖面δ13C基值(-22‰)负偏了约2.4‰,随后,在约30 m处达到较正值-21‰。在153层中,δ13C值整体表现为正偏移,从较负值-23.5‰缓慢增至-22‰,在约60 m处结束正偏移。在154~157层中,δ13C值大幅波动,曲线表现为三次脉冲状的负偏移。在砂岩段中,δ13C值先从-21.9‰降低至-22.8‰,随后迅速恢复至整个剖面正偏极值为-21‰;之后在泥岩段中负偏至-24.4‰,再次迅速恢复至-21.7‰,最终在约74 m处的碳质泥岩段中达到最负值-26.5‰。基于剖面δ13C基值(-22‰)负偏了约4.5‰,与紫红色约20 cm厚的铁质层同时出现。随后,δ13C值逐渐增加,在约93 m处恢复到-22.1‰。从整体来看,“东剖面”碳同位素曲线也经历了小幅度负偏、正偏、脉冲状负偏三个阶段,且小幅度负偏与最大幅度负偏均出现在岩性正旋回之后,与“西剖面”的特征相似,而且在NCIE事件层中碳同位素负值和TOC高峰值有较好的对应关系。

  • 根据前人在库车河地区的研究资料[17],统计了阳霞组顶部—克孜勒努尔组下部的孢粉种类变化趋势,其结果显示孢粉种类从145层顶到157层顶呈衰减的趋势。在145层和146层界限处,热带—亚热带湿生孢粉减少了4种(Comcentrispotires findlaterensis,C.mesozoicus等),温带湿生孢粉减少了3种(Piceaepollenites complanatiformis,P.exloodes等),此后至157层再无出现,同时Peripollenites spp.,Podocarpidites multicinus等热带—亚热带湿生孢粉首次出现,在此层位δ13C值表现出小幅度负偏趋势(图2a,“西剖面”约6 m处)。在152~153层界限处,热带—亚热带湿生孢粉减少了6种(Peripollenites spp.,Podocarpidites multicinus等)热带—亚热带湿生孢粉消失,此后再无出现,而温带湿生孢粉Pseudopinus sp.首次出现,对应地层中δ13C值正偏阶段(图2a,“西剖面”约35 m处)。在153~154层界限处,热带—亚热带湿生孢粉减少了3种(Deltoidospora spp.等)以及温带湿生孢粉Pseudopinus sp.和Deltoidospora spp.等热带—亚热带湿生孢粉消失,同时温带中生孢粉Pseudoreticulatus sp.Osmundacidites wellmanii首次出现,恰好与δ13C值波动强烈层位,脉冲状负偏移同期发生(图2a,“西剖面”约70 m处)。总体上,孢粉种类表现出明显的衰减趋势,其中湿生孢粉种类衰减更加明显,由145~152层的16种衰减到154层的3种,且孢粉种类减少的趋势在NCIE事件层之前就已开始。

    Figure 2.  Comprehensive chart of biostratigraphy, carbon isotope stratigraphy and TOC curves for West and East sections in Kuqa River area

  • 早侏罗世晚期Toarcian期大洋缺氧事件中,碳同位素偏移幅度大、范围广,其δ13C值负偏移超过6‰的被认为是诊断性特征[2]。碳同位素数据显示,塔里木盆地库车河地区克孜勒努尔组146~157层中记录了一次显著的NCIE事件,“东剖面”记录了约4.5‰幅度的NCIE,而“西剖面”中记录了约7‰幅度的NCIE。“东剖面”的NCIE幅度小于“西剖面”的记录,可能是因为位于煤场内的、通过标志层拼接起来的“东剖面”连续性不够,地层记录中缺失了更大的碳同位素负向偏移极值。总体上,“东剖面”和“西剖面”都一致地记录了显著的NCIE,结合生物地层和岩性地层的约束,推测此层位中记录的NCIE事件对应于早Toarcian期大洋缺氧事件。

    将库车河地区“东剖面”和“西剖面”与四川盆地[13]、鄂尔多斯盆地[14]、以及英国Mochras钻孔数据[22]5个T-OAE剖面的记录对比(图3)。5个记录之间也存在明显的差异:δ13C值负偏幅度差异、TOC含量的差异以及δ13C值和TOC值对应模式的差异。塔里木盆地“西剖面”记录了约7‰的NCIE,东剖面记录了约4.5‰的NCIE,四川盆地是最早开始陆相T-OAE研究的盆地,记录了约4.2‰幅度的NCIE负偏极值。鄂尔多斯盆地是目前除塔里木盆地外另一个露头剖面的T-OAE记录,其记录了约12.5‰的NCIE,远大于目前已知的所有T-OAE记录的NCIE。造成碳同位素偏移幅度差异的原因尚不完全清楚,可能是由于温度、湿度引起的碳同位素分馏差异[14]或者藻类、微生物来源的有机碳的混源所导致的[23]

    Figure 3.  Correlation of T⁃OAE records in Kuqa River area with typical records worldwide

    在Toarcian期的地层中,黑色页岩层均以高TOC值为标志, Mochras钻孔的TOC平均值在2%以上,四川盆地和鄂尔多斯盆地的T-OAE/NCIE事件层位TOC平均值约为1.5%和2%。在库车河“西剖面”中,NCIE事件层TOC含量极低,平均值小于0.5%,而“东剖面”的记录显示,其局部TOC可高达16%。此外,另一个显著的特征是,T-OAE剖面记录中TOC高值与NCIE事件层碳同位素有很好的对应,且呈现出“镜像关系”。鄂尔多斯盆地安崖(Anya)剖面NCIE事件层的黑色页岩层段较高的TOC值与碳同位素相对应,但高值大多出现在主NCIE之前,四川盆地以及英国的Mochras钻孔在NCIE事件层中的TOC和碳同位素则显示出很好的镜像对应关系(图3)。在库车河“东剖面”此镜像关系的特征十分明显,但“西剖面”无此特征,推测是因为“西剖面”煤层自燃烘烤的结果,导致在记录NCIE的桔红色细砂岩层中TOC值由于地层被烘烤显著降低(图2)。

    在T-OAE时期加速的水循环和陆地风化,为陆相湖盆提供了养分[1314],鄂尔多斯盆地和四川盆地深湖相沉积中记录的NCIE事件层,对应的高TOC的黑色页岩指示湖相初级生产力的显著提高。库车河地区和鄂尔多斯盆地报道的碳同位素数据中,剖面δ13C值均记录了两个明显的NCIE,其小幅度的NCIE位于剖面下段对应于Pliensbachian-Toarcian界线(图3),这也在英国的Mochras钻孔数据中得到了验证[22,24]。此外,库车河“东剖面”中NCIE事件层碳同位素与TOC高值的对应模式,这种模式与国际典型T-OAE剖面十分吻合,综上所述,基于生物地层资料约束,我们认为本研究报道的库车河地区幅度高达7‰的NCIE事件层位可推定为早侏罗世Toarcian期大洋缺氧事件发生的等时层位。

  • 孢粉种类的第一个主要转变位于剖面下部碳同位素小幅度负偏出现的位置,即Pliensbachian-Toarcian界线处,适应广泛生存环境的裸子植物和适应湿热环境的蕨类植物逐渐减少;第二个转变位于NCIE事件层,孢子花粉的丰度和多样性显著下降,这意味着陆地植被大量损失和植物群落衰退。植被的衰减与气候的变化有密切关系,值得注意的是,孢粉种类的衰减在NCIE事件层之前就已开始,到NCIE事件层中湿热孢粉种类所剩无几,这暗示在NCIE事件层出现之前塔里木盆地气候就已经开始发生变化,推测在Pliensbachian-Toarcian界线处的变化可能与Karoo-Ferrar大火成岩省的第一次火山活动有关[22,25]。在NCIE事件层的变化可能与大量轻碳气体的释放有关,在迅速增温的背景下,植被种群因不适应气候的快速变化而表现出衰减的特征。大火成岩省活动引起了一系列的环境连锁反应[2627],直接或者间接的改变了植物的生存环境,从而引起了植物种类的转变。由于海洋的热缓冲能力高于大气,陆地生态系统对大气变化的反应比海洋生态系统可能更为迅速,因此由火山活动引起的全球变暖对陆地生态系统的影响也更为显著,表明在全球变暖的背景下,陆地生态系统很可能受到较海洋系统更强烈的扰动。

    根据极热事件下海洋—大气模式[28]和大陆性天气的证据[22,26,2930]表明,在T-OAE期间,由于温室效应,全球每年的降水量将增加9 cm[28],而且大陆风化速率是T-OAE之前的3倍[30],这均表明T-OAE期间地球变得更加温暖湿润[31]。由升温导致的海平面上涨,可能使四川盆地暂时与南面的特提斯洋相连[13],虽然鄂尔多斯盆地远离海洋,其盆地的迅速扩张意味着大面积降雨的增加,表明受到了水汽的影响。在全球不同古地理和古纬度的剖面中广泛报道了T-OAE期间水循环加剧的现象——NCIE事件层与较粗颗粒沉积物同时出现,全球粗颗粒沉积物明显同步供应表明其可能受全球性的气候控制而不是局部构造控制[22]。塔里木盆地库车河剖面的记录支持气候主控观点,但表现出了相反的特征。碎屑白云岩和紫红色泥岩的出现指示了地表径流减少,推测为湖盆因气候炎热而收缩的沉积环境,而非水文循环加剧导致湖泊扩张。此外,孢粉资料显示,热带—亚热带湿生孢粉种类的减少,温带中生孢粉的新增,表明塔里木盆地的气候类型发生了显著变化。邓胜徽等[8]在我国Toarcian事件沉积的陆相地层中,发现最明显沉积特征就是聚煤作用的减弱或者中断,这也表明气候类型发生了转变。综上所述,在Toarcian期的塔里木盆地北缘记录了干热的气候特征——这与其他盆地显著不同。笔者推测由于塔里木盆位于欧亚板块内陆,在全球显著增温的背景下水汽无法到达该盆地,因此产生干热的气候。

  • 孢粉种类的演替与碳循环扰动的同期,发生了环境和生态系统变化,碳源可能来自海底甲烷气水合物的释放[2,32]、Karoo-Ferrar大火成岩省的喷发[3334]、岩浆侵位或喷发造成的热成因甲烷[35]等。记录显示,“西剖面”在6 m的地层内记录了约7‰的NCIE,“东剖面”在3 m的地层内完成记录了约4.5‰的NCIE,表明发生过程十分迅速。在如此短的时间内造成该幅度的碳同位素值扰动,目前较合理的推测认为轻同位素特征的碳源(δ13C=-60‰)来自海底甲烷气水合物失稳释放[2]。其迅速释放的触发机质,推测有两种可能:一是变暖驱动的,随着全球变暖,热量传输到洋底,改变温压条件达临界值使甲烷气水合物失稳而分解释放[36];另一种是由某一些特定地质事件导致的,如海底地震、岩浆热液活动触发[37]。从目前的数据推断,尚且无法确定是哪种机制。同时,此过程是否也叠合了天文周期旋回对气候的影响因素,需要未来进一步工作验证[24]

    值得注意的是,“西剖面”NCIE事件层中,δ13C值在脉冲状负偏之后,迅速恢复至整个剖面的正偏极值(图2a,“西剖面”约84 m处;图2b,“东剖面”约70 m处),随后再次迅速负偏出现δ13C负偏极值,“东剖面”也出现了相似的特征(图2)。陆地有机质的碳同位素记录是大气中二氧化碳的碳同位素的信息载体[38],NCIE事件层中δ13C值的迅速波动表明了大气碳库在短时间内的扰动,迅速负偏暗示了轻同位素特征的碳源注入大气;反之,表明重同位素特征的碳源注入或者生产力短时期内快速地增加。δ13C值迅速正偏的重碳气体可能来源于Karoo-Ferrar大火成岩省活动释放的幔源CO2。此外,如果假设轻同位素碳源是甲烷气水化合物,因为甲烷的温室效应是二氧化碳的24.5倍,那么将伴随一个快速地增温正反馈过程。综上,Karoo-Ferrar大火成岩省的喷发可能是T-OAE的导火索,其造成了Pliensbachian-Toarcian界线处出现小幅度的碳扰动,短暂负偏之后又恢复正偏,而大火成岩省的喷发使地表圈层进入相对变热和内陆变干的时期[39],塔里木盆地的干热气候导致研究区孢粉种类出现衰减,盆地的湖平面下降,盆地边缘的沉积相发生改变。全球升温使甲烷气水化合物失稳而分解释放,进一步加剧变暖,在正反馈机制作用下,更多浅海大陆架的甲烷气水化合物幕次地分解释放,同时伴随着岩浆侵入有机质煤层,冰冻圈融化,植被野火等碳源的释放,从而引起脉冲式的碳同位素负偏移。T-OAE期间气候变化的触发模式与如今的变暖有相似性,因此,在应对未来地球气候变化挑战中,由增温引发的一系列过程值得予以关注。

  • (1) 在新疆塔里木盆地库车河地区“东剖面”和“西剖面”同时发现了显著的NCIE事件,推测本研究中库车河地区记录的NCIE事件层与早侏罗世Toarcian期大洋缺氧事件层等时对应,位于克孜勒努尔组154~157层中。

    (2) 陆相有机碳δ13C值记录T-OAE时期大气碳库在短时间内受到扰动,轻同位素特征的碳源以幕次特征的方式注入大气,使得碳同位素地层曲线呈现脉冲状的特点,其注入机制可能受到增温的正反馈作用影响。

    (3) 研究区独特的古地理位置提供了独一无二的T-OAE期间陆相湖盆边缘对气候变化的响应记录。热带—亚热带湿生孢粉的衰减和碎屑白云岩及紫红色泥岩的出现均指示塔里木盆地在T-OAE期间由湿热气候转变为干热气候。推测Karoo-Ferrar大火成岩省活动可能为T-OAE事件的导火索,是早侏罗世环境变化的重要触发因素。

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