摘要
埃迪卡拉纪记录了地球历史上最显著的无机碳同位素负异常,其变化幅度从≥+5‰降到≤-12‰。这种巨大的碳同位素波动很显然代表了异常的埃迪卡拉纪海洋地球化学环境。传统上认为碳同位素负异常可以作为埃迪卡拉纪地层对比的有利工具,尤其是对于这些缺乏生物地层控制和精确同位素定年的地区。但是全球埃迪卡拉纪地层已有的同位素数据资料表明:碳同位素异常不仅在数量上而且在变化幅度上都存在着显著的变化(Jiang et al., 2007, 2008)。例如,在华南和印度北部地区Marinoan冰期盖帽碳酸岩之上至少发现了两个δ~(13)C_(carb)负异常(Jiang et al., 2002; Kaufman et al., 2006; Zhu et al., 2007; Jiang et al., 2007);在美国西部加利福尼亚地区Johnnie组和Stirling石英岩中也曾报道过两个δ~(13)C_(carb)同位素负异常(Corsetti and Kaufman, 2003; Kaufman et al., 2007),但它们的时限与华南及印度有明显不同;纳米比亚北部和南部的埃迪卡拉纪地层分别记录了一个变化幅度较小的δ~(13)C_(carb)异常,但两者之间的对比关系以及不整合面所造成的地层缺失仍待厘定(Saylor et al., 1998; Halverson et al., 2005);与上述地层不同的是,在阿曼的埃迪卡拉系中仅发现了一个变化幅度达17‰的δ~(13)C_(carb)负异常,且这个负异常所代表的时间可能长达12~50 Ma(Fike et al., 2006; Le Guerroue et al. 2006; Bowring et al., 2007);在澳大利亚南部也发现了一个与阿曼变化幅度相似的δ~(13)C_(carb)异常,但其持续时间还不能确定(Calver, 2000)。造成这种差异的很大一部分原因可能是目前对于碳同位素异常的原因目前还存在着很大的争论。现有的几种模型如:“雪球地球”假说(Hoffman et al., 1998; Hoffman and Schrag, 2002),甲烷渗漏(Kennedy et al., 2001; Jiang et al., 2003a),海水翻转(Kaufman et al., 1991; Grotzinger and Knoll, 1995; Knoll et al., 1996)及“plumeworld”(Shields, 2005)都是针对盖帽碳酸盐中的碳同位素异常提出来的,它们对于解释盖帽碳酸盐之上发现的碳同位素异常在理论上均存在着困难。最近有人提出在埃迪卡拉纪深海中存在一个巨大的有机碳溶解库(DOC),这个有机碳溶解库(DOC)可能相当于现代海洋的10-100倍(Rothman et al., 2003; Fike et al., 2006)。当时的海洋处于分层状态,上部是有氧环境,而下部为缺氧环境,中间被一个氧化界面所分开,并且认为埃迪卡拉纪大多数无机碳同位素异常均与这个有机碳溶解库(DOC)的氧化有关。尽管其中详细的氧化过程还不是很清楚,但是这个模型却得到两方面证据的支持:1)在阿曼和华南地区的埃迪卡拉纪地层中先后发现了有机碳和无机碳同位素的不协调变化(Fike et al., 2006; Mcfadden et al. 2008);2)华南埃迪卡拉纪陡山沱盆地不同沉积环境中存在着巨大的无机碳同位素梯度(Jiang et al., 2007;Wang and Shi, 2009)。如果这个推测巨大的有机碳溶解库(DOC)确实存在的话,那么一个很关键的问题是,什么时候它才最终消失的?一种观点认为其结束的时间大概在551 Ma左右,这种解释的主要依据是有机碳和无机碳同位素在陡山沱顶部和Shuram末期逐渐趋于一致变化的趋势(Fike et al., 2006; Mcfadden et al., 2008),然而其最大的局限在于同位素数据的不完整性。最近的研究发现,在华南地区的早寒武世的某些剖面,有机碳和无机碳同位素依然呈不协调的变化关系(Guo et al., 2005),其它的一些地球化学数据如Fe,Mo和S同位素的研究也表明早寒武世的海洋也处于强烈的分层状态(Goldberg et al., 2005, 2007; Lehmann et al., 2007; Canfield et al., 2007; Wille et al., 2008)。究竟这种特点是继承于早埃迪卡拉纪还是寒武纪重新形成的?要回答这一问题就需要对晚埃迪卡拉纪(551 Ma之后)的碳同位素做详细的研究,而这一块正是目前研究所比较缺乏的。
华南地区是世界上埃迪卡拉纪地层剖面保存最好的地区之一,它保存了台地相至盆地相区一系列连续的剖面,是解决埃迪卡拉纪许多重大科学问题的理想地区之一。过去华南地区埃迪卡拉纪碳同位素化学地层曾做过大量的工作,但是这些研究存在着以下三个方面的问题:1)过去的研究多强调碳同位素时间上的变化,而对空间上的变化研究相对较少;2)研究的时限多集中在陡山沱时期,灯影期研究很少,而这也是认为DOC是在551 Ma消失的重要原因之一;3)有机碳同位素的研究相对比较缺乏。最近的研究恰恰表明有机碳和无机碳同位素的变化关系可能是解决碳同位素异常及海洋环境变化的关键(Fike et al., 2006; Mcfadden et al., 2008; Ader et al., 2009)。为了验证DOC模型的合理性及海洋分层的持续时间,本文选择了华南地区两条保存于不同沉积环境的晚埃迪卡拉-早寒武剖面进行了高分辨率的有机碳及无机碳同位素分析。一条是九龙湾-计家坡剖面,位于湖北省西部扬子三峡地区。剖面的主体部分为灯影组,厚度约340 m,总体上为一套厚层的碳酸盐岩沉积,代表了明显的浅水沉积环境。从下向上依次可以分为蛤蟆井段,石板滩段和白马沱段。其中蛤蟆井段和白马沱段以厚层状、浅灰色白云岩、粉砂质白云岩为主,沉积水深相对较浅;而石板滩段为深灰色、纹层状泥晶灰岩为主,富含沥青质,沉积环境相对较深。灯影组下伏地层为陡山沱组四段富含有机质的黑色泥岩沉积,其上被早寒武世岩家河组硅质白云岩,泥质白云岩和泥灰岩所覆盖。另一条是龙鼻嘴剖面,位于湖南省西部吉首地区。剖面总厚度从陡山沱顶部到早寒武世牛蹄塘组下部约50 m,主要沉积是一套黑色硅质岩、黑色泥岩和硅质泥岩的沉积组合,见大量的黄铁矿,为典型的深水缺氧环境沉积。
同位素分析结果显示,在浅水台地相剖面(图1),δ~(13)C_(carb)在整个灯影期均为正值,其波动范围在0‰和+6‰之间。可以识别出三个明显的变化阶段,在下部的蛤蟆井段,δ~(13)C_(carb)数值相对比较分散,但均落在0‰和+3‰之间,很少超过+4‰;而在中部的石板滩段,δ~(13)C_(carb)值总体上有个明显上升,多数数值在+4‰和+5‰之间变化;到了白马沱段,δ~(13)C_(carb)又小幅回落到+2‰左右并且向上保持相对比较稳定。δ~(13)C_(org)的变化趋势正好与δ~(13)C_(carb)相反,在下部的蛤蟆井段及上部的白马沱段,δ~(13)C_(org)值相对较大,其变化范围在-24‰和-27‰之间,平均值约-25‰;而在中部的石板滩段,其值相对较小,平均值在-28‰左右。总体上δ~(13)C_(org)与δ~(13)C_(carb)呈不协调的变化。值得注意的是在台地相剖面上同时发现了两次重大的有机碳和无机碳同位素负异常,变化幅度均超过了8‰,其分别位于陡山沱组与灯影组界线之交以及前寒武纪/寒武纪界线附近,其中陡山沱灯影之家的两次同位素异常和前寒武寒武界线附近的无机碳同位素负异常在以前都有广泛的报道,而前寒武/寒武界线附近的有机碳异常是首次发现。从变化特点来看,无机碳同位素的异常变化相对比较缓和,往往持续了一段相对较长的时间,而有机碳同位素的变化比较突然,似乎代表了瞬时事件。在盆地相剖面(图2),由于其沉积组合以黑色硅质岩和黑色泥岩为主,很难用来做无机碳同位素的分析,因此,我们只作了有机碳同位素的测试。在整个剖面上,δ~(13)C_(org)都相对比较稳定,大部分数据均落在-32‰至-35‰这个区间,与台地相比较一个很重要的特点是并没有发现明显的δ~(13)C_(org)负异常。在与灯影组相当的留茶坡组,δ~(13)C_(org)的平均值在-34‰左右,这与以前相邻地区研究的结果十分相似,而在台地相区,同时期的δ~(13)C_(org)平均值约为-27‰,二者存在着高达7‰的δ~(13)C_(org)差异(图3)。
以上几个显著的特点在以前的研究中很少报道。如果有机碳和无机碳的不协调变化确实反映了海洋中巨大有机碳溶解库(DOC)的氧化,那么这个推测的巨大有机碳溶解库(DOC)并没有如以前报道的在551 Ma左右被完全消耗掉,它可能持续了整个埃迪卡拉纪,区域上甚至持续了更长的时间,早寒武世的海洋分层很可能是继承了埃迪卡拉纪的特点。对于前寒武纪/寒武纪附近的有机碳和无机碳的变化,我们给出了如下解释(图4):(A)在埃迪卡拉纪末期,由于大量的有机质埋藏和高的生物生产力导致了重的δ~(13)C值。海洋环境继承了早期埃迪卡拉纪海水分层的特点;(B)海平面上升把大量的贫~(13)C的海水带到相对浅水地区,导致δ~(13)C_(carb)开始向负向变化以及不稳定的δ~(13)C_(org)值(CN1a);(C)全球变暖及高生物生产力造成了大量的氧气消耗和持续的缺氧状态,有机质通过硫酸盐还原(SO_2~(4-) +2CH_2O→2HCO_3- +H_2S)导致δ~(13)C_(carb)进一步向负向漂移(CN1b),而光和作用对重碳酸盐的吸收(2HCO-3 +Ca2+→CaCO3+CO2+H2O)将必然降低有机碳和无机碳同位素的分异以及相对稳定的δ~(13)C_(org)值;(D)持续的缺氧环境导致磷的释放和营养物质堆积,进一步促进生物生产力和对氧气的需要,硫酸盐还原造成硫化物的大量堆积(Kump et al., 2005; Meyer and Kump, 2008,使得透光带也处于缺氧状态,从而导致非常负的δ~(13)C_(org)(CN1c)。灯影期巨大的δ~(13)C_(org)梯度很可能反应了在氧化还原界面之下缺氧环境中有强烈的化能自养生物的活动,因为化能自养生物过程产生的有机质往往会比初始生产者的光和作用产生的有机质贫~(13)C,最高可达15‰。
本文在前人对华南埃迪卡拉纪碳同位素研究的基础上,结合自己的研究成果,系统总结了碳同位素(包括有机碳和无机碳)在埃迪卡拉纪的变化,在浅水台地相区,目前华南共发现了四次δ~(13)C_(carb)负异常和2次δ~(13)C_(org)负异常,这些异常(盖帽除外)均与DOC库有很大的关系;而在盆地相区,研究的分辨率很低,现有的结果发现无机碳同位素在陡山沱组中部有一个很大的跳跃,有机碳整体上比较稳定。另外,华南埃迪卡拉纪保存了该时期最丰富的化石,大部分化石都是来自浅水台地剖面,本文对台地相无机碳同位素的变化与生物的演化关系作了详细的探讨,进一步发现该时期重要生物类型与生态变化的出现几乎总是在碳同位素负异常之后(图5),从而提出他们可能代表了两种截然不同的环境条件:前者代表有氧环境而后者代表缺氧环境。同时,重要的生物类型出现之前往往伴随有大量黄铁矿的析出,首次提出硫化环境的消除可能是驱动生物演化的一个重要环境因素。
The Ediacaran period is characterized by the extremely negative carbon isotope anomalies with magnitude from≥+5‰down to≤-12‰which have been documented almost all over the world. It is traditionally argued that the carbon isotope records can be used as a useful tool for global correlation of Ediacaran strata, particularly for those sections lacking age-diagnostic biostratigraphic markers and/or precise radiometric age determinations. However, published Ediacaranδ~(13)C_(carb) curves from different continents show major inconsistencies not only in age but also in number and magnitude, this is partially because the origin of these anomalies remains largely controversial. South China is one of the most well-preserved location for the Ediacaran succession, it contains relatively conformable carbonate/shale sections along a platform-to-basin transect and can sever as an ideal place to solve a series key questions in this period. During the past two decades, numerous studies have been done on the Ediacaran carbon isotope chemostratigraphy in South China, however, most of the previous researches only emphasize the carbon isotope temporal variation but ignored the spatial variation, and put much energy on the Doushantuo period but overlook the late Ediacaran. Moreover, there is a generally lack of paired carbonate and organic carbon isotope research in South China. Here we present high-resolution carbon isotope records from the late Ediacaran to the early Cambrian in two different sedimentary environments. Our results show that: 1) theδ~(13)C_(org) profile from the platform section overall displays an obvious decoupling relationship with that ofδ~(13)C_(carb); 2) two negativeδ~(13)C_(org) excursions associated with two negativeδ~(13)C_(carb) anomalies are recognized at the uppermost Doushantuo Formation and Precambrian/Cambrian(P(?)/C) boundary respectively in the platform section, during which theδ~(13)C_(org) excursion at P(?)/C boundary was rarely reported; and 3) an up to 7‰δ~(13)C_(org) gradient exists between shallow water platform and deep water basin during the Dengying period. These three characters were barely reported in the previous studies. If the decoupling ofδ~(13)C_(carb) andδ~(13)C_(org) did result from the remineralization of a large DOC reservoir just as suggested, then the DOC reservoir may not have been completely consumed at 551 Ma, it may last to at least early Cambrian, or even longer. We argue that the carbon isotope excursions of both carbonate and organic carbon at P/C boundary were caused by the sustained anoxic deep ocean formed by the ocean stratification. Theδ~(13)C_(org) gradient in the Dengying period indicate perhaps strong chemoautotrophic recycling of organic carbon in anoxic/euxinic conditions below the chemocline because chemoautotrophic organisms including sulfur oxidizing bacteria use recycled carbon during carbon fixation and their biomass is up to 15‰depleted in ~(13)C relative to primary photosynthate.
引文
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