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晚古生代大冰期是陆地生态系统繁盛以来最显著的冰川事件,持续时间近亿年[1⁃3](360~260 Ma)。研究表明晚古生代大冰期并不是冈瓦纳大陆上长期稳定存在的持续性冰川作用,而是由多个离散的冰期和间冰期组成的动态变化过程[4⁃6]。一般认为冈瓦纳大陆冰川从密西西比亚纪晚期开始启动,在二叠纪早期达到了最高峰,并在早二叠世的中晚期开始逐渐消退[2,7⁃9]。晚古生代大冰期也发生多次全球变暖事件,这些变暖事件及其触发机制和对陆地—海洋系统造成的影响等方面,近年来引起学界的广泛关注[10⁃15]。
其中,在晚宾夕法尼亚世卡西莫夫期和格舍尔期界线附近(Kasimovian-Gzhelian Boundary,KGB),全球多个陆块记录了一次短暂而显著的碳同位素负漂事件[16]。同时期,大气二氧化碳浓度(pCO2)由~350 ppmv快速上升到~700 ppmv[17⁃18],腕足壳氧同位素恢复的表层海水温度(SSTs)由~25 ℃上升到~29 ℃[19],从宾夕法尼亚世中期持续到晚期的海侵达到了最大[20],古热带植物群发生了向北半球中纬度地区的扩散[21],最早出现于古热带地区的䗴类Triticites也向北美地区进行了迁移[22],这说明该碳同位素负漂对应着显著的全球变暖,称之为KGB变暖事件[11]。
冰室气候间冰期背景下的KGB变暖事件与地球现在所处的气候状态十分类似,且具有相近的大气pCO2[18],对KGB变暖事件的研究不仅能够帮助探索晚古生代大冰期的演化机制,也能为理解当今全球变暖提供新思路。然而,KGB变暖事件研究程度还较低,仅Chen et al.[11]通过地球系统模拟的方法对碳同位素负漂的碳源和海洋缺氧的程度开展了定量计算并指出,这次碳同位素负漂事件是由约9万亿吨有机质来源的轻碳注入大气—海洋系统引发的,伴随着近20%的海底缺氧面积。但目前在KGB变暖的启动机制、识别标准以及全球不同地区对该事件的响应等方面还不明确。因此,还需要对该变暖事件开展进一步的研究,以揭示其形成机制和对表层地球系统的影响。
本文通过对相关研究剖面开展的详细的沉积学和碳同位素地层学工作,结合前人开展的生物地层学和旋回地层学研究,分析KGB变暖事件的沉积过程,并与全球不同地区的沉积盆地进行对比,为后续进一步开展相关研究打下基础。
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华南板块由扬子板块和华夏板块于新元古代早期沿江南造山带拼合而成[23⁃24](~830 Ma),宾夕法尼亚亚纪晚期,华南板块孤立地漂泊在赤道附近的低纬度地区(图1a),位于古特提斯洋东北部,与泛大洋连通,广泛分布的巨厚碳酸盐岩沉积以及同期岩浆、构造和变质作用记录的缺失说明华南板块在此时期属稳定的被动大陆边缘背景[23,26⁃27]。沉积环境方面,华南板块被广阔的陆表海所覆盖,以碳酸盐岩开阔台地沉积为主,台地中有部分滩相沉积和盆地相沉积[28⁃29](图1b)。
晚古生代大冰期高纬度地区的冰川进退会引起低纬度地区旋回性的海平面变化,使得低纬度地区出现明显的沉积间断和频繁暴露改造[30⁃31],华南板块的众多台地相剖面几乎都存在明显的沉积间断或暴露改造,造成了地球化学信号的偏移以及地层对比的困难[32⁃36]。与之相比,位于开阔台地内部的裂陷盆地发育近乎连续的斜坡—盆地相沉积,能够完整地记录整个时期的沉积环境变化和化学信号。因此,研究选取华南板块西南部罗甸盆地内的纳庆(25°14′55″ N,106°29′35″ E)、上隆(25°21′3″ N,106°30′4″ E)、纳绕(25°24′48″ N,106°36′13″ E)三个不同深度的斜坡相剖面为研究对象,三个剖面均位于贵州省黔南布依族苗族自治州罗甸县域内,沿路边出露(图2),上隆剖面分别位于纳庆剖面北方和纳绕剖面西南方约11 km和12.5 km处,距东侧的罗甸县城约27 km。本文在精细的牙形刺生物地层、碳同位素化学地层等框架下,对研究层段进行了详细的沉积学工作及全球对比研究。
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沉积学方面,对纳庆、上隆、纳绕三个剖面的上宾夕法尼亚统卡西莫夫阶和格舍尔阶界线附近约20 m的地层进行了逐层野外观察和描述,并绘制了详细的沉积柱状图(图3)。同时采集了大量的手标本,在室内进行切割、抛光后制作岩石光面用以扫描观察;在手标本切面上选择区域制作岩石薄片,使用奥林巴斯(Olympus)BX53光学显微镜进行观察。
图 3 纳庆、上隆和纳绕剖面牙形刺生物地层、沉积柱状图和碳同位素(生物地层数据引自文献[37⁃38];纳庆和纳绕碳同位素数据引自文献[37])
Figure 3. Conodont biostratigraphy, sedimentological column, and carbon isotopes of the Naqing, Shanglong, and Narao sections (biostratigraphy data from reference [37⁃38]; carbon isotope data of the Naqing and Narao sections from reference [37])
沉积地球化学方面,在上隆剖面系统采集了90个灰岩样品,切开新鲜面后钻取粉末用于无机碳和氧同位素分析,取样时避开了方解石脉和硅化部分。无机碳、氧同位素在中国科学院南京地质古生物研究所实验技术中心稳定同位素比质谱仪实验室内测试,使样品粉末在Kiel IV Carbonate Device装置中与磷酸反应,生成的CO2通过Thermo Fisher Scientific MAT-253质谱仪测定样品的δ13Ccarb和δ18Ocarb值,测试结果标准参照GBW-04405,测定值偏差分别优于0.02‰和0.05‰。
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三条研究剖面的KGB层位均开展过牙形刺生物地层工作[37⁃38,41],包括卡西莫夫阶的Idiognathodus guizhouensis带、Heckelina eudoraensis带和I.naraoensis带,以及格舍尔阶的H.simulator带,I.nashuiensis带和Streptognathodus vitali带(图3)。其中,纳庆剖面作为格舍尔阶全球界线层型剖面与点位的候选之一,研究程度高;而上隆和纳绕剖面的格舍尔阶目前仅识别出了H.simulator带,之上地层的生物地层工作尚未开展。纳庆和纳绕剖面的KGB层位曾开展过高密度、连续采样的生物地层研究,H.simulator的首现层位分别为220.45 m和229.61 m[37,41]。上隆剖面采样密度较低,目前仅在186.40 m发现了典型的格舍尔阶分子,包括两枚H.simulator的标本[38]。之下至182.00 m之间,仅在184.90 m的样品中获得了极少量亚成年体和幼年体标本,无法明确指示是卡西莫夫阶或格舍尔阶,因此上隆剖面H.simulator的首现层位可能低于186.40 m,需要逐层采集较大重量的样品进行分析方可确定。
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碳酸盐岩碳同位素的变化,不仅受全球碳循环影响,也与区域性的水循环及成岩改造密切相关[33,36,42⁃43]。因此,要保证碳同位素信息的可靠性,将其应用到地层对比之前,需要对其数值进行评估。一方面,经过野外和室内的详细观察,研究层段未发现明显的暴露构造(如古喀斯特、古土壤等)和成岩作用改造(如后期重结晶化、白云岩化等),因此认为其地球化学信号能够反映海水的原始信息。另一方面,与大气淡水相关的成岩作用,会使得碳同位素值和氧同位素值降低,根据水岩比率、大气淡水淋滤作用及陆地有机碳氧化程度等的不同,无机碳、氧同位素之间的相关关系也呈现出不同的特征,受成岩改造强烈的样品往往会在碳氧同位素值中呈现正相关关系[43]。通过建立碳氧同位素交会图(图4)可以发现,三个剖面碳氧同位素值之间均无明显的相关关系,并与北美中大陆盆地经过成岩筛选的腕足壳碳同位素值[19]具有相似的特征,说明其受成岩作用的影响较小,可以用于全球对比。
图 4 (a)纳庆、(b)上隆、(c)纳绕剖面和北美中大陆盆地腕足壳碳氧交会图(蓝色部分为纳庆和纳绕剖面碳、氧同位素数据引自文献[37];灰色部分为北美中大陆盆地腕足壳碳氧同位素数据,引自文献[19])
Figure 4. Cross⁃plot of carbonate δ13C and δ18O values of the (a) Naqing, (b) Shanglong, and (c) Narao sections[37] (blue) and comparison with those from brachiopods of the North American Midcontinent Basin[19] (gray)
上隆剖面碳同位素分析结果显示,碳同位素范围为-0.2‰~5.0‰,其平均值为4.0‰。氧同位素范围为-9.2‰~-1.7‰,其平均值为-3.5‰。碳同位素记录在剖面182.75~185.00 m牙形刺化石带Idiognathodus naraoensis内识别出了一次显著的负漂事件(图3),负漂幅度可达4.3‰。碳同位素值稳定在4.6‰~2.2‰之间,平均值约为4.0‰。碳同位素值在182.75 m从4.1‰快速下降到-0.2‰,之后碳同位素值逐渐恢复背景值,在185.00 m处达到4.4‰,整个负漂事件碳同位素平均值为3.0‰。185.00~196.00 m,碳同位素整体介于5.0‰~2.0‰,平均值约为4.3‰。整体来说,上隆剖面171.00~195.00 m的碳同位素记录与同沉积盆地纳庆、纳绕剖面的碳同位素记录[37]可以进行良好的对比,彼此之间变化趋势一致。
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在野外详细观察和描述的基础上,室内对样品进行切割、抛光、制作薄片和详细的镜下观察分析,根据经典的邓哈姆碳酸盐岩岩相分类方案[44],依据岩石的宏观展布、颗粒类型及微相特征等,在三个研究剖面共识别出4种岩相,分别为泥状灰岩相(LM)、生物碎屑粒泥灰岩—生物碎屑泥粒灰岩相(W-Pb)、正粒序生物碎屑泥粒灰岩相(Png)和黑色钙质泥岩相(CMd)。
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泥状灰岩主要呈薄层状发育(图5a),部分为中层,极少见厚层,层与层之间常夹有毫米级的泥页岩层。颜色多为浅灰色,成分均一,主要为泥晶或粉屑(图5b),极少含生物碎屑等颗粒,部分受到扰动(图5c),或可见轻微的水平层理(图5d),整体波浪或流水构造以及生物扰动发育较少。
生物碎屑较少、未发育大量的波浪流水构造和生物扰动说明泥状灰岩相主要沉积在低水动力的静水环境,鉴于整体的环境背景,最有可能位于风暴浪基面以下,夹层的泥页岩可能是风或河流带来的陆源碎屑物质沉积而成[33,35]。
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该岩相主要呈中厚层状发育(图5e),由以生物碎屑为主的颗粒和杂基组成,颗粒分选磨圆中等。其中,生物碎屑粒泥灰岩呈杂基支撑,颗粒含量较少,颗粒间彼此不相接触,颗粒类型多为海百合茎和藻类碎屑,粒径约0.2 mm(图5f),小部分可达0.5~1.0 mm(图5g)。生物碎屑泥粒灰岩呈颗粒支撑,颗粒间互相接触。不同层位颗粒粒径与类型略有差异,可分为两类:一类颗粒粒径一般不超过0.5 mm,颗粒成分多为海百合茎碎片、藻类碎片、泥球和小型的䗴等(图5h);另一类颗粒粒径较大,多在1.0 mm以上,也可达2.0 mm,较大粒径的颗粒主要是大䗴、海百合茎碎片和苔藓虫等生物碎片(图5i)。
较为破碎、杂乱的生物碎屑可能来源于水动力较强的相对浅水的环境,而中等的分选磨圆以及未明显发育的波浪流水构造说明整体沉积环境水动力中等,可能是重力流沉积物在斜坡远端沉积而成[45⁃46]。
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正粒序生物碎屑泥粒灰岩从薄层至厚层均有发育(图6a),一般从底部到顶部呈一个期次的旋回,底部为颗粒支撑的生物碎屑泥粒灰岩,其特征与W-Pb中的生物碎屑泥粒灰岩基本相同,有时还含内碎屑(图6b),向上颗粒粒径逐渐减小,灰泥基质含量逐渐增多,过渡到粒度更细的生物碎屑泥粒灰岩、生物碎屑粒泥灰岩或泥状灰岩(图6c,d),为典型的正粒序构造,部分层位与下伏岩层有截然的接触面(图6c)。
截然的接触面说明上覆沉积物沉积时对下伏沉积物有强烈的侵蚀作用,结合正粒序构造,指示该岩相为浊流沉积的产物,浅水区域未固结的沉积物受到风暴、地震或海平面变化等扰动后,向下滑动形成沉积物重力流,在斜坡上堆积[33,35,47⁃48]。
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黑色钙质泥岩主要呈薄层状发育,有机质含量高,新鲜面呈暗黑色(图6e),最典型的黑色钙质泥岩集中发育在三个剖面的KGB界线之下。黑色钙质泥岩由陆源碎屑和泥质成分组成,粒径细小,陆源碎屑粒径普遍在0.1 mm以下(图6f)。部分层位含特殊的蚯蚓状构造,形状各异,但基本为扁平的长条形,成分以泥质为主,具有一定的定向性(图6e,g)。
高有机质含量以及底栖生物的缺少指示了较为还原的沉积环境,细粒陆源碎屑和泥质的成分指示黑色钙质泥岩是悬浮沉降缓慢堆积而成,且对应着碳酸盐的生产力下降[49],可能代表了研究剖面为较深水的沉积[20]。蚯蚓状的构造可能是遗迹化石Nereites isp.,该遗迹化石往往出现在深海、半深海的沉积环境中[50⁃51]。
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高精度的生物地层对比是不同地区相互对比的重要前提,也是建立石炭纪年代地层框架的基础。在石炭系海相地层中,牙形刺是最重要的标准化石之一,石炭系的大多数全球年代地层单位界线层型剖面和点位(GSSP,俗称“金钉子”)都已经用牙形刺的序列来确定。然而,石炭纪的牙形刺生物地层工作还存在不足,特别是宾夕法尼亚亚纪牙形刺的研究相较于密西西比亚纪非常滞后,晚宾夕法尼亚世更是如此。造成这种现象的其中一个重要原因是,在早期石炭系的生物地层研究中,宾夕法尼亚亚系已经建立了完备的䗴和菊石的生物带[52],导致牙形刺的研究程度较低。目前,全球上宾夕法尼亚统建立了较为精确的牙形刺化石带的主要地区为北美中大陆盆地(Midcontinent Basin)、莫斯科盆地(Moscow Basin)、顿涅茨盆地(Donets Basin)和我国华南地区[37,53⁃55](图1a),除了这四个主要的区域,其他地区上宾夕法尼亚统一般都缺乏牙形刺化石带研究。
北美中大陆盆地发育有大规模的旋回地层,被认为是冈瓦纳冰川进退引起的全球海平面变化导致的,并与天文轨道参数相关联,前人已经对这些旋回进行了详细的划分和命名[31,40]。在这些旋回中发育有大量深灰色到灰色的深水页岩[56](core shales),其中含大量牙形刺化石,对牙形刺化石分带厘定具有重要作用,也是整个北美地区进行地层对比的重要参考[52]。中大陆盆地的地方性年代地层单位中与卡西莫夫阶和格舍尔阶大概相对应的分别是Missourian和Virgilian这两个阶,但其延限有所区别[52]。中大陆盆地Missourian阶最顶部的带是Heckelina eduoraensis带,紧接着为Streptognathodus zethus带,其首现定义了Missourian和Virgilian的界线。Heckelina simulator带位于S.zethus带之上,首次出现在Oread旋回的Heebner黑色页岩中[54,57]。
莫斯科盆地上宾夕法尼亚统现有的标准牙形刺化石带主要来源于南乌尔地区的Usolka剖面和Nikolsky剖面,该地区具有更加深水的环境且包含了更加丰富的牙形刺类群,能够进行区域间的对比,此外Usolka剖面还有精确的放射性同位素年龄[58]。莫斯科盆地的地方性年代地层单位划分较细,卡西莫夫阶和格舍尔阶分别被分为3个和4个亚阶,其中与本文研究层段相对应的是卡西莫夫阶顶部的Dorogomilovian亚阶和格舍尔阶底部的Dobryatinian亚阶,二者界线对应Heckelina simulator带底界。但在H.simulator带之下并未发现华南和中大陆盆地都有的H.eduoraensis带,而是被Streptognathodus firmus带所替代,在该带上部也含有大量的S.zethus分子,因此可与中大陆盆地的S.zethus带和华南地区的I.naraoensis带进行对比。
顿涅茨盆地位于东欧台地的最南缘,出露有近乎完整的石炭系,并且已经开展过详细的层序地层学的研究[20],顿涅茨盆地内与本文研究层段相对应的地方性年代地层单位为卡西莫夫阶顶部的Toretzian亚阶和格舍尔阶底部的Kalinovskian亚阶。Toretzian亚阶最顶部为Streptognathodus firmus⁃Idiognathodus kalitvensis带,Kalinovskian亚阶底部为Heckelina luganca带,虽然并没有建立H.simulator带,但这一层位出现的H.simulator类群也指示了格舍尔阶的底界[59]。Heckelina luganca带一直持续到H.simulator类群的消失,之上的牙形刺化石带还未建立[52]。
目前对Heckelina simulator以及其演化谱系的研究取得了较大的进展[37,41],而且其作为一个全球广泛分布的物种在各个地区都能进行识别,能够为KGB的洲际地层对比提供重要参考(图7)。除了牙形刺,有孔虫也是石炭纪一种重要的生物化石,尤其是到了宾夕法尼亚亚纪,以䗴类为主的有孔虫快速兴起并繁盛,主要分布在浅水区域,在一些既含有牙形刺又含䗴的剖面中,往往作为浅水相地层和深水相地层对比的“桥梁”[60]。由于全球构造运动的影响,北方劳亚大陆和南方冈瓦纳大陆之间连通泛大洋和古特提斯洋的瑞克洋在石炭纪中期完全关闭[61],改变了密西西比亚纪全球海洋充分贯通的状态,使得底栖有孔虫产生了明显的生物地理分区[60],再加上海平面频繁变化导致的浅水相剖面的暴露改造或沉积间断,致使䗴生物带自身的对比以及䗴与牙形刺间的对比存在一定问题。多位学者指出,在不同沉积盆地间使用䗴生物带进行对比可能会存在5~10 Ma的误差[62⁃63],因此本文进行对比讨论的年代地层学框架还是以牙形刺化石带为准。
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从整体岩相来看,研究层段以泥状灰岩相为主,水动力较低,同时发育有多套重力流沉积,形成典型的正粒序构造并带来浅水区域的生物碎屑,生物碎屑类型包括䗴、有孔虫、苔藓虫和海百合茎等,大多较破碎,这一系列特征表明研究层段主要为水体较深的斜坡相沉积环境。黑色钙质泥岩相的出现及其特征则代表台间盆地相的沉积环境,体现了水体的加深[64]。
研究层段4种岩相的交替说明该时期海平面的频繁波动:当相对海平面较低时,碳酸盐岩台地进积,位于斜坡上部的沉积物更容易受到扰动,供给的碳酸盐颗粒增多,同时未固结的沉积物沿斜坡向下滑动,形成大量的重力流沉积,甚至滑塌沉积[65],但由于研究层位该时期水深总体较深,并未出现像同剖面其他层位的大型滑塌变形[33,66⁃67],原先台地的区域则可能受到暴露改造(图8a);相对海平面较高时,重力流沉积显著减少,供给的碳酸盐颗粒也减少,主要发育悬浮沉积形成的均质薄层泥状灰岩(图8b)。当海平面升高到一定程度时,研究剖面古水深可能位于氧化还原界面以下,同时浅水的碳酸盐台地被淹没,供给的碳酸盐颗粒进一步减少,悬浮的黏土和泥质沉积物以及未被氧化的有机质就会沉积为黑色钙质泥岩(图8c)。
由于上隆剖面和纳绕剖面格舍尔阶的生物地层研究程度不足,Heckelina simulator带的具体延限还无法确定,卡西莫夫阶和格舍尔阶界线之上三个剖面的生物地层并不能进行精确的对比,因此本文对研究剖面古水深变化的分析对象主要是Idiognathodus naraoensis带和Heckelina eudoraensis带内的地层。通过对牙形刺生物地层、碳同位素地层和详细的沉积岩相分析对比可以发现,虽然H.simulator带的延限无法进行精确的对比,但其底部均发育相对浅水的沉积。而在I.naraoensis带内,三个剖面在卡西莫夫阶和格舍尔阶界线之下均发育了一套黑色钙质泥岩和薄层泥状灰岩的岩相组合,说明在这一层位的古水深较深,并伴随着碳同位素的显著负漂。其下则为相对浅水的沉积,普遍发育正粒序生物碎屑泥粒灰岩相。纳庆剖面的215.30 m、上隆剖面的179.20 m和纳绕剖面的223.10 m也存在与界线下黑色钙质泥岩相类似的对应关系,指示古水深的加深,而且在上隆和纳绕剖面出现碳同位素的微小负漂,纳庆剖面碳同位素未见明显变化。在H.eudoraensis带内,三个剖面均存在一套正粒序生物碎屑泥粒灰岩的沉积,表明这一时期的古水深较浅,尤其是纳绕剖面厚度可达一米,这可能是因为纳绕剖面相对上隆剖面和纳庆剖面水深更浅导致的。H.eudoraensis带下部为薄层的泥状灰岩,说明此时古水深相对较深。
综上所述,基于纳庆、上隆和纳绕三个剖面的沉积分析,认为在本文研究层段H.eudoraensis带到H.simulator带底部经历了3次古水深的旋回性变化,其中在卡西莫夫阶和格舍尔阶界线处的古水深最大,沉积了一套黑色钙质泥岩,这3次古水深的旋回性变化与Wu et al.[39]建立的405 ka长偏心率周期天文旋回E70、E71和E72具有良好的对应关系(图3)。
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宾夕法尼亚亚纪全球海平面变化主要是根据北美中大陆盆地和顿涅茨盆地的旋回地层重建的[40,68⁃70]。前文提到,通过对纳庆、上隆和纳绕剖面进行详细的岩相和沉积过程分析,认为在研究层段Heckelina eudoraensis带到H.simulator带底部经历了3次古水深旋回性变化。其中,卡西莫夫阶和格舍尔阶界线处的古水深最大,沉积一套黑色钙质泥岩。结合牙形刺化石带,该黑色钙质泥岩可与北美中大陆盆地Oread旋回中的Heebner页岩相对应,均沉积于海侵—最大海泛期,整个界线处的水深变化则可以与Oread旋回相对应[40,52,57]。从H.eduoraensis带到H.simulator带在北美中大陆盆地由下而上可对应Stanton、Cass和Oread旋回[31,40,71],Schimtz et al.[58]认为这三个旋回代表了两个405千年的长偏心率周期,Oread旋回在一个周期内,与顿涅茨盆地的SG2对应,Santon和Cass旋回在一个周期内,与SG1对应[72⁃73](图9)。然而,该划分方案与Wu et al.[39]在纳庆剖面通过天文旋回划分的从H.eduoraensis带到H.simulator带为E70到E72三个长偏心率周期是相矛盾的。考虑到中大陆盆地更为成熟的旋回地层学和生物地层学工作,笔者认为Santon和Cass旋回各自代表一个长偏心率周期更为合理,即与Wu et al.[39]的划分方案相吻合(图9a,b),而顿涅茨盆地旋回地层与中大陆盆地旋回地层的对应关系[20,58]可能需要根据新的牙形刺生物带划分标准[52]重新厘定。因此,研究层段H.eduoraensis带的浅水沉积可能对应Stanton旋回的海退阶段,Idiognathodus naraoensis带的中下部对应Cass旋回,I.naraoensis带的中上部则对应Oread 旋回并一直持续到H.simulator带的顶部(图3)。Eros et al.[20]在当时的年代地层学框架下认为顿涅茨盆地的卡西莫夫阶和格舍尔阶之交是短暂的低水位沉积,随后格舍尔阶下部出现高水位沉积,而此时的卡西莫夫阶和格舍尔阶界线位于O6灰岩之下。牙形刺生物地层的进一步研究和修订后,目前顿涅茨盆地代表格舍尔阶底界的Heckelina luganca带与O7灰岩相对应,因此研究层段的黑色钙质泥岩可以与O7灰岩对应的海侵序列中的最大海侵相对应[20,52,76](图9a)。综上所述,研究剖面古水深的波动代表了此时全球海平面的波动变化,尤其是卡西莫夫阶和格舍尔阶界线处黑色钙质泥岩,与北美中大陆盆地和顿涅茨盆地的旋回地层具有很好的对应关系,同时期的华北板块也表现为较高海平面的沉积[77⁃78],说明海侵在此时达到了最大。
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无机碳同位素也是一种重要的对比工具,在石炭系地层对比中得到了广泛的应用[33,55,74,79],同时碳同位素负漂也是KGB变暖事件的重要识别标准。Grossman et al.[19]对石炭纪到中二叠世的碳同位素记录进行了总结和评估,通过与全球典型沉积盆地的碳同位素数据对比发现,虽然不同区域碳同位素的绝对值和变化幅度存在差异,但基本都在卡西莫夫阶和格舍尔阶界线处(303.7 Ma)存在显著的负漂,大气二氧化碳浓度和氧同位素也出现了协同变化,指示KGB变暖事件的普遍存在(图9c,d,e)。Chen et al.[11]研究认为该次负漂的形成需要有大量的轻碳输入海洋,轻碳的来源可能是北欧以Skagerrak地区为中心的大火成岩省(Skagerrak-centered LIPs)侵入富含有机质的沉积物释放的热成因甲烷或由变暖导致的冻土消融释放的有机碳。
总体来说,KGB变暖事件是一次全球性的变暖事件,由全球范围内的碳扰动驱动,伴随着碳同位素的负漂移,气温的快速升高可能使得冈瓦纳大陆的冰川消退,造成了全球海平面的明显上升,在多个沉积盆地内形成了一套深水沉积。KGB变暖事件相关层位的生物地层学和旋回地层学等的全球对比研究还存在一定问题,需要开展更深入的工作,为KGB变暖事件的深入研究打好基础。
华南罗甸盆地纳庆、上隆和纳绕剖面发育有连续的牙形刺化石带,提供了全球对比的基础。出露岩层受成岩改造较少,记录的地球化学信号更可能反映原始的海水信息。海平面变化主要受控于冰川进退引起的全球海平面升降。因此,在华南罗甸盆地开展相关研究可作为KGB变暖事件研究和全球对比的重要参考。
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(1) 华南板块罗甸盆地的纳庆、上隆和纳绕剖面上宾夕法尼亚统共发育4种沉积岩相,分别是泥状灰岩、生物碎屑粒泥灰岩—生物碎屑泥粒灰岩、正粒序生物碎屑泥粒灰岩和黑色钙质泥岩,揭示了研究层段主要为海平面频繁波动的深水斜坡沉积环境,从H.eudoraensis带到H.simulator带底部记录了3次古水深的旋回性变化。
(2) KGB变暖事件是一次全球性的气候事件,出现了明显的变暖效应,可能导致了冈瓦纳大陆冰川消退,使得全球海平面明显上升,致使卡西莫夫阶和格舍尔阶界线处黑色钙质泥岩的沉积,并伴随着显著的碳同位素负漂移,这在全球典型的沉积盆地均有记录。
(3) KGB变暖事件相关层位的研究目前还存在诸多问题,如生物带和旋回地层的划分与对比等方面,进一步的研究工作仍需开展。华南罗甸盆地发育有连续的牙形刺化石带,出露岩层受成岩改造较少,海平面变化主要受控于冰川进退引起的全球海平面升降。在华南罗甸盆地开展相关研究可作为KGB变暖事件研究和全球对比的重要参考。
Sedimentary Response and Global Correlation of the Late Pennsylvanian Warming Event
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摘要: 目的 晚古生代大冰期是陆地生态系统繁盛以来最显著的冰川事件,是由多个离散的冰期和间冰期交替组成的动态变化过程。这一时期也发生了多次全球变暖事件,近年来引起学界的广泛关注。其中,在晚宾夕法尼亚世卡西莫夫期和格舍尔期界线(Kasimovian-Gzhelian Boundary,KGB)附近记录了一次短暂而显著的碳同位素负漂事件,并伴随显著的全球变暖,称之为KGB变暖事件。这一冰室气候背景下的变暖事件具有重要的研究意义,然而对该变暖事件的研究仍处于起步阶段。 方法 选取华南板块罗甸盆地纳庆、上隆和纳绕剖面,对各自上宾夕法尼亚亚统卡西莫夫阶和格舍尔阶界线附近约20 m厚的地层开展详细的沉积学和碳同位素地层学工作,分析KGB变暖事件的沉积响应,并与全球不同地区的进行对比,为进一步开展相关研究打下基础。 结果 共识别出泥状灰岩、生物碎屑粒泥灰岩—生物碎屑泥粒灰岩、正粒序生物碎屑泥粒灰岩和黑色钙质泥岩4种沉积岩相,指示了海平面频繁波动的深水斜坡沉积环境。上隆剖面新报道的碳同位素记录与纳庆和纳绕剖面已发表的碳同位素记录显示出一致的变化趋势,KGB附近的碳同位素负漂移在全球多个不同剖面均有记录。研究层段牙形刺Heckelina eudoraensis带、Idiognathodus naraoensis带和H. simulator带底部记录的3次古水深的周期性变化与前人建立的天文旋回和全球典型沉积盆地的沉积旋回具有良好的对应关系。 结论 宾夕法尼亚亚纪晚期变暖事件(即KGB变暖事件)是一次全球性的气候事件,华南罗甸盆地相关研究可作为该事件研究和全球对比的重要参考。Abstract: Objective The Late Paleozoic Ice Age is the most remarkable icehouse period since the flourish of the terrestrial ecosystem and is characterized by multiple discrete glacial and interglacial periods. Several global warming events occurred during the Late Paleozoic Ice Age, and these events have received extensive attention in recent years. An abrupt negative excursion in carbon isotopes (δ13C) was recorded near the Late Pennsylvanian Kasimovian-Gzhelian Boundary (KGB), accompanied by significant global warming. The KGB warming event against the background of the interglacial period of the icehouse climate is of great interest, but the study of this warming event is still in its initial stages. Methods Here, detailed sedimentological and carbon isotope stratigraphic studies were carried out on the ~20 m-thick strata across the KGB in the Naqing, Shanglong, and Narao sections of the Luodian Basin in South China. Results Four sedimentary lithofacies were identified-lime mudstone facies, bioclastic wacke to packstone facies, normal-graded packstone facies, and dark calcareous mudstone facies-indicating a deep-water slope environment with frequent sea-level fluctuations. The newly obtained carbonate δ13C record from the Shanglong section can be compared with the previously published records from the Naqing and Narao sections, and the negative excursion in δ13C across the KGB is recorded around the world. Three cycles of paleo-water-depth variation at the
conodont Heckelina eudoraensis zone, Idiognathodus naraoensis zone, and the bottom of the H. simulator zone in the study interval showed similar pace with astronomical cycles and can be correlated to those of the North American Midcontinent. Conclusions The studied successions of the Luodian Basin provide an important reference for the study of the KGB warming event. -
Key words:
- Late Paleozoic Ice Age /
- warming event /
- South China Block /
- sedimentary response /
- carbon isotope
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图 3 纳庆、上隆和纳绕剖面牙形刺生物地层、沉积柱状图和碳同位素(生物地层数据引自文献[37⁃38];纳庆和纳绕碳同位素数据引自文献[37])
Figure 3. Conodont biostratigraphy, sedimentological column, and carbon isotopes of the Naqing, Shanglong, and Narao sections (biostratigraphy data from reference [37⁃38]; carbon isotope data of the Naqing and Narao sections from reference [37])
图 4 (a)纳庆、(b)上隆、(c)纳绕剖面和北美中大陆盆地腕足壳碳氧交会图(蓝色部分为纳庆和纳绕剖面碳、氧同位素数据引自文献[37];灰色部分为北美中大陆盆地腕足壳碳氧同位素数据,引自文献[19])
Figure 4. Cross⁃plot of carbonate δ13C and δ18O values of the (a) Naqing, (b) Shanglong, and (c) Narao sections[37] (blue) and comparison with those from brachiopods of the North American Midcontinent Basin[19] (gray)
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