扬子地区震旦纪—寒武纪转折期大陆风化研究进展与展望

付勇，夏鹏，龙珍，张恭境，谯文浪，郭川，杨镇

1) 贵州大学资源与环境工程学院，贵阳，550025； 2) 喀斯特地质资源与环境教育部重点实验室，贵阳，550025； 3) 中国科学院地质与地球物理研究所新生代地质与环境院重点实验室，北京，100029； 4) 贵州省地矿局102地质大队，遵义，563003； 5) 贵州民族大学生态环境工程学院，贵阳，550025

内容提要: 通过调研扬子地区震旦纪(埃迪卡拉纪)—寒武纪(E—C)转折期大陆风化、海洋环境和有机质富集特征研究。结果显示，扬子地区E—C转折期遭受了较强烈的化学风化作用，且在这一时期区内兼具浅水碳酸盐台地和深水盆地环境。在台地相区，下寒武统牛蹄塘组黑色岩系不整合接触于震旦系灯影组白云岩之上，该不整合与E—C转折期全球“大不整合”具有紧密的关系，盆地区发育的完整的沉积地层记录了台地区被风化地层的痕迹。现有大陆风化的地球化学证据集中于n(87Sr)/ n(86Sr)、δ13C、CIA和εNd(t)等，同时这些地球化学数据也局限在少数的剖面和层段，因此迫切需要更多的地球化学参数来反映该风化作用的影响范围和演化特征。准确认识扬子地区E—C转折期大陆风化作用与海洋环境演变间的耦合关系，是揭示古生物演化、有机质富集机制的重要环节。

震旦纪(埃迪卡拉纪)—寒武纪(E—C)转折期同时发生了超大陆的裂解(Rodinia)与聚合(Gondwana)(Li Da et al., 2013; Yao Weihua et al., 2014)、海洋化学与生物化学的动荡变化(Hoffman et al., 1998; Fike et al., 2006; McFadden et al., 2008; Och et al., 2013; Sahoo et al., 2012; Li Da et al., 2013)、生命的幕式更替(张文堂，1997; Zhang Xingliang et al., 2014; 朱茂炎等，2019)等重大地质事件。其中，最引人注目的是新元古代末期埃迪卡拉纪动物群的消失，以及随后从早寒武纪开始的骨骼化动物爆发辐射(即“寒武纪生命大爆发”)。普遍认为，E—C转折期大气氧浓度的持续增加(图1)以及海水氧化还原条件的动态变化在早期生命演化过程中起着显著的作用(Li Chao et al., 2010; Och et al., 2013; Wen Hanjie et al., 2015; Li Weiping et al., 2017; 朱茂炎等，2010，2019)。然而，目前对这一时期大气氧浓度和海洋氧化还原条件变化的驱动力仍然没有定论。

图1 新元古代以来构造、地球化学特征及大气氧气浓度(大气氧含量参考Kump et al., 2007; Lyons et al., 2014；海水环境参考Canfield et al., 2008；全球n(87Sr)/n(86Sr)、εNd数据及构造活动等参考Hoffman and Li, 2009; Peter and Gaines, 2012；华南E—C转折期n(87Sr)/n(86Sr)、εNd参考Wei Guangyi et sl., 2019; Li Meng et al., 2020)Fig. 1 Characteristics of tectonic movements, geochemistry, and atmospheric oxygen concentration (oxygen concentration is from Kump et al., 2007, Lyons et al., 2014; Marine environment is from Camfield et al., 2008; Globaln(87Sr)/n(86Sr), εNd, and tectonic movements are from Hoffman and Li, 2009, Peter and Gaines, 2012; n(87Sr)/n(86Sr) and εNd in South China are Wei Guangyi et al., 2019, Li Meng et al., 2020)

前寒武纪末期，地球气候曾发生剧烈的波动，并出现四次影响深远的冰期气候(图1)(Hoffman and Li, 2009; Zhou Chuanming et al., 2019; Li Minglong et al., 2019；李明龙等，2021)。在冰期气候之后，冰川的迅速消融使地球进入温室气候，导致前寒武纪末期化学风化强度迅速增强(Hoffman et al., 1998; Peter and Gaines, 2012)。随后在早寒武世期间广泛的海侵和基底改造，使寒武系不整合沉积于前寒武纪风化地层之上，导致E—C转折期“大不整合(Great unconformity)”的形成(Peter and Gaines, 2012; Shahkarami et al., 2020)。“大不整合”具有全球规模，在E—C转折期各主要大陆均有分布(图2)，并且该不整合面具有穿时特征。例如，在西伯利亚、外蒙古、挪威等地发育在埃迪卡拉系上统内部，以碎屑角砾岩层不整合覆盖于碳酸盐岩层之上为特征(Nielsen and Schovsbo, 2011; Smith et al., 2015; Markov et al., 2019)；在加拿大麦肯锡、美国加利福尼亚等地区表现为黑色细粒沉积岩系与下伏碳酸盐岩或角砾岩之间的不整合接触(Peter and Gaines, 2012; Smith et al., 2016)；在澳大利亚弗林德斯，该不整合面为埃迪卡拉系与寒武系的界线，寒武系含砾砂岩不整合覆盖于埃迪卡拉系碳酸盐岩之上(Mapstone and Mcllry, 2006)；我国扬子地区，该不整合面同样是埃迪卡拉系与寒武系的界线，但不整合面之上为下寒武统黑色细粒岩系，有机质丰富(Zhu Maoyan et al., 2010, 2019; Li Chao et al., 2020)；在纳米比亚，该不整合面位于埃迪卡拉系与寒武系的界线以上，为寒武系底砾岩与下伏埃迪卡拉系碳酸盐岩的不整合接触(Linnemann et al., 2019)。E—C转折期“大不整合”的出现与这一时期海水环境和大气氧浓度变化存在较好的对应关系(图1)，暗示“大不整合”形成过程中产生的大量风化产物输入海洋，驱动早寒武世海洋环境变化，触发“寒武纪生命大爆发”(Peter and Gaines, 2012)。然而，“大不整合”的分布特征(图2)反映E—C转折期不同地区大陆风化特征存在明显差异。

本文以扬子地区E—C转折期为例，学习和总结了关于这一时期古大陆风化、古海洋环境演化以及有机质富集特征方面的研究进展，讨论了大陆风化与古海洋环境的协同演化关系及相关问题。

2 大陆风化作用强度研究方法

2.1 化学蚀变指数

上地壳遭受化学风化的过程中，K+、Na+、Ca2+等离子活性强，容易随地表流体大量流失，然而，Al3+、Ti4+等离子较稳定，容易保存在风化残留物中，导致风化残留物中主成分Al2O3所占的比重随风化作用强度而不断变化。据此，Nesbitt和Young(1982，1989)将“化学蚀变指数”(chemical index of alteration,CIA)作为评价物源区风化强度和气候条件的指标，即：

其中n(CaO*)为硅酸盐矿物中的CaO。高的CIA值反映较强烈的化学风化作用，对应温暖潮湿的气候条件，K+、Na+、Ca2+等易迁移阳离子大量去除，Al3+、Ti4+等难迁移阳离子逐渐在风化残余物中富集。低CIA值反映化学风化作用较弱甚至不存在，对应寒冷干燥的气候条件。未经风化的沉积岩CIA标准值为48(Rudnick and Gao Shan, 2014)，遭受风化后的沉积物CIA值高于该标准值，并且随风化强烈程度增加而不断升高(Nesbitt and Young, 1982, 1989)。

基于上述理论基础，CIA在化学风化强度研究方面得到了广泛应用，例如，我国南方新元古代古城冰期和南沱冰期细碎屑岩CIA值较低，分布在56.5～64.6，均值约59.8，反映物源区经历了轻微的化学风化作用(冯连君等，2003；熊晨，2019；李明龙等，2021)，与冰期寒冷干燥的气候相对应；晚奥陶世五峰组—早志留世龙马溪组碎屑岩CIA值为75～90，反映物源区经历了较强烈的化学风化(Yan Detian et al., 2010)。然而，CIA是基于化学组成相对均一的岩石建立的(Ohta et al., 2007)，受原岩成分影响大，钾交代作用、变质作用、古老沉积岩的再循环沉积等会改变原岩成分，进而制约CIA判断风化强度的准确性。因此在应用CIA指标时不仅对样品的选取有严格的要求，还需结合校正公式去除钾交代的影响(Panahi et al., 2000; Zhai Lina et al., 2018)，以及结合ICV(成分变异指数，index of compositional variability)和Th/Sc—Zr/Sc、A—CN—K等图解评价沉积分选及再循环作用(冯连君等，2003；Yan Detian et al., 2010；吴蓓娟等，2016)。结合这些参数能够在一定程度上强化CIA判断化学风化的准确性，但在评价化学组成极不均一的黑色页岩时仍然存在较大分歧，以我国南方主要页岩气储层—龙马溪组黑色页岩—为例，经过钾交代校正后CIA值可达90，反映极强烈的化学风化作用(Yan Detian et al., 2010)，但近期的研究显示该黑色页岩受成岩及交代作用影响较小，CIA平均值为66，风化作用强度较小(张茜等，2020)，这可能是受样品较高的碳酸盐矿物含量的影响，同时黑色页岩的强非均质性也是可能的影响因素。针对湘中下寒武统黑色页岩，吴蓓娟等(2016)成功构建了WB指数评价其风化程度，但该指数是基于现今黑色页岩风化特征的评价，对其评价地质历史中黑色页岩风化强度的适用性还需进一步证实。可见，采用CIA单一指标评价风化强度很难得到可靠的结果，通常将CIA与同位素地球化学指标相结合判断大陆风化作用。

2.2 同位素地球化学指标2.2.1 Sr同位素

海水中的Sr主要有大陆风化输入和海底热液两种来源，其中，大陆风化作用提供的Sr相对富87Sr [具放射性，n(87Sr)/n(86Sr)=0.7119] (Palmer and Edmond, 1989)，海底热液活动提供的Sr相对富86Sr(n(87Sr)/n(86Sr)=0.703)(Holland, 1984)。溶解Sr在海水中停留时间(2～3 Ma)远长于海水混合时间(1～1.5 ka)，所以海水的Sr同位素比值n(87Sr)/n(86Sr)在任何时间均被认为是均一的(McArthur et al., 2012)，那么原生海底沉积物和自生矿物中Sr同位素比值，反映的是大陆风化作用来源Sr和热液活动来源Sr的动态平衡。因此，海水n(87Sr)/n(86Sr)值能够反映风化作用强度(或造山作用)与洋壳增生速度(或火山—热液活动强度)，并对构造演化和气候变化(或P(CO2))进行约束(Jenkyns et al., 2002; Wang Wei et al., 2007；王文倩等，2014)。

如图1，新元古代至寒武纪，古海洋n(87Sr)/n(86Sr)持续升高，这一时期正好对应Rodinia大陆的裂解、剥蚀，形成“大不整合”(Peters and Gaines, 2012)，显示了Sr同位素变化对大陆风化作用的良好响应。在华南、北美、蒙古、西伯利亚、阿拉伯半岛和南非等地均存在E—C转折期高n(87Sr)/n(86Sr)值(0.707～0.711)(Sawaki et al., 2008; Li Da et al., 2013; Wei Guangyi et al., 2019)，表明在E—C转折期，强烈的化学风化作用遍及全球大部分地区，形成的“大不整合”具有全球规模(Shahkarami et al., 2020)(图2)。尽管海水Sr同位素组成在示踪大陆风化方面取得了较多成功的运用，但Sr同位素易受原岩(主要是碳酸盐岩)性质的影响，因此通过海洋沉积的Sr同位素示踪大陆风化过程存在多解性。

2.2.2 Os同位素

海水中Os元素主要有三种来源：① 大陆地壳中Os经河流带入；② 洋中脊热液蚀变来源；③ 宇宙尘埃来源。大陆地壳Os同位素富集放射性187Os，n(187Os)/n(188Os)值较高，现今陆源输入n(187Os)/n(188Os)平均值为1.54(Levasseur et al., 1999; Cohen et al., 2004)(图3)。洋中脊热液蚀变来源和宇宙尘埃来源的Os为非放射性成因，并且两种来源Os具有相近的n(187Os)/n(188Os)值，约为0.126，远低于大陆地壳风化来源Os的n(187Os)/n(188Os)值。海水中Os元素的组成特征主要是这三种来源的综合结果，其中约80%来自陆源输入，仅20%来源于海底热液蚀变和宇宙尘埃(Sharma and Wasserburg, 1997)，因此可以通过海水Os同位素的变化来约束大陆风化强度和海底热液喷发等过程(Cohen, 2004; Zhu Bi et al., 2013; Tripathy et al., 2018)。

图3 前寒武纪海水初始n(187Os)/n(188Os)值(数据引自Kendall et al., 2009; Rooney et al., 2010, 2011; Zhu Bi et al., 2013)Fig. 3 Initialn(187Os)/n(188Os) of marine water of Precambrian (data are from Kendall et al., 2009; Rooney et al., 2010, 2011; Zhu Bi et al., 2013)

强烈的化学风化作用对应高的n(187Os)/n(188Os)值，例如，近50 Ma以来，海水n(187Os)/n(188Os)值逐渐升高，与海水的Sr同位素组成升高的趋势一致，反映这一时期喜马拉雅抬升运动造成的大陆风化强度逐渐增强(Pegram et al., 1992)。英格兰Yorkshire早侏罗世经历了温暖湿润的古气候和强烈的化学风化，对应的含砾石黑色页岩段n(187Os)/n(188Os)值为0.8～1.0，n(87Sr)/n(86Sr)值由0.70706迅速升至0.70720(Cohen et al., 2004)。新元古代晚期，苏格兰、爱尔兰、毛里塔尼亚以及中国等均显示海水初始n(187Os)/n(188Os)值迅速升高(图3)，反映这一时期存在强烈的化学风化作用(Zhu Bi et al., 2013)，与“大不整合”的时间相当(Peters and Gaines, et al., 2012; Li Meng et al., 2020; Shahkarami et al., 2020)，说明海洋n(187Os)/n(188Os)值的变化对“大不整合”有较好的响应，能够反映地质时期的化学风化强度。然而，黑色岩系(主要是黑色页岩)富集有机质和硫化物会大量吸附海水中Os，导致黑色岩系中Os异常富集，因此Os同位素示踪黑色岩系的大陆风化过程存在多解性。

2.2.3 Li同位素

Li同位素在示踪大陆硅酸岩风化方面具有以下优势：① 化合价单一，不受氧化还原状态影响；② 大陆硅酸岩地壳具有相对较高的Li含量(李东永等，2019)，且在风化过程中可以产生极大分馏(Rudnick et al., 2004; Tomascak., 2004)；③ 不受生物过程影响(Rudnick et al., 2004; Penniston-Dorland et al., 2017)。因此，所有地质过程中Li同位素的分馏发生在大陆风化作用过程中(Rudnick et al., 2004; Henchiri et al., 2014)。目前已经基本获得自然储库中Li的丰度和δ7Li值(图4)，为Li同位素示踪大陆风化研究奠定了基础。由于Li是水溶性元素，受淋溶作用易迁移至溶液中，经搬运注入海洋。Li的同位素分馏主要发生在搬运过程中，在淋溶过程中仅仅发生微弱的同位素分馏(Wimpenny et al., 2010a, 2010b; Verney-Carron et al., 2011)。搬运过程中，6Li优先在次生黏土矿物中富集，造成黏土矿物中Li含量高(平均约为80 μg/g)，δ7Li值相对较低(1.6‰～5.0‰)；大部分7Li跟随流体注入海洋，并在俯冲过程中被带到下地壳或者地幔，导致上地壳富集6Li(Marschall et al., 2007; Steinhoefel et al., 2021)。

图4 自然储库中Li同位素(苟龙飞等，2017)和Mg 同位素(Huang Jinxiang et al., 2016)的分布Fig. 4 Lithium(Gou Longfei et al., 2017&) and magnesium (Huang Jinxiang et al., 2016) isotopic composition in natural reservoir

Li同位素已被成功运用到示踪大陆风化过程的多项研究中：太古宙3.0～2.9 Ga期间，快速的大陆风化作用导致海水δ7Li值降低，远低于现代海水(付露露等，2020)；在445 Ma的Hirnantian冰期，大陆风化强度降低，海水δ7Li值明显升高(von Strandmann et al., 2017)；晚白垩世生物大灭绝及大洋缺氧事件(OAE2)前夕，大陆风化强度增加，海水δ7Li值显著降低(von Strandmann et al., 2013; Sun He et al., 2018)。然而，也有研究表明细粒级(<63 μm)沉积物中δ7Li对气候变化不敏感，因此δ7Li示踪大陆风化的可靠性还有待进一步证实。

2.2.4 Mg同位素

Mg有三个稳定同位素，即24Mg(78.99%)、25Mg(10.00%)和26Mg(11.01%)，它们之间存在较大的相对质量差，如24Mg与26Mg之间相对质量差约达8%，在地质作用过程中可以发生显著的Mg同位素质量分馏(Catanzaro et al., 1966；朱祥坤等，2013)。同时，Mg不仅是地壳和地幔中的主量元素，也是主要的流体活动性元素，这决定了它在化学风化过程中会伴随明显的同位素分馏。已有研究表明，Mg同位素在化学风化过程中会产生高达2‰的分馏，其中轻同位素24Mg、25Mg更易随流体迁移，而重同位素26Mg不易被溶解迁移，保留在风化残余物中，导致风化残余物中具有高δ26Mg值(图4)，搬运介质中具有低δ26Mg值(Wimpenny et al., 2010a; Teng Fangzhen et al., 2010; von Strandmann et al., 2012; Liu Xiaoming et al., 2014)，因此风化残余物中更高的δ26Mg值指示了温暖湿润的气候条件，更低的δ26Mg值则指示了寒冷干燥的气候条件(Huang Jinxiang et al., 2016)。

黏土矿物对不同同位素吸附能力差异可能也是导致风化过程中Mg同位素质量分馏的原因。Huang Kangjun 等 (2012)、Liu Xiaoming 等 (2014)对玄武岩风化剖面的研究显示高岭石、三水铝石优先吸附26Mg，造成26Mg在风化残余物中的富集。Wimpenny 等 (2014)对黏土矿物中不同赋存形态Mg同位素的测量结果表明，黏土矿物晶体结构中Mg同位素组成较重，黏土矿物表面和层间的Mg同位素组成较轻，黏土矿物吸附过程并不产生同位素分馏。可见，目前对风化作用过程中Mg同位素的分馏机制还不完全清楚，同时现有Mg同位素示踪大陆风化的研究集中于玄武岩、安山岩、花岗岩、碳酸盐岩等岩石风化剖面，还有待更深入和更广泛的研究。

2.2.5 K同位素

K是地表河流和地壳中的主量元素，约90%的河流溶解K来自于硅酸盐的风化(Meybeck, 1987; Berner and Berner, 2012)，因此K稳定同位素(39K和41K)可以示踪大陆硅酸盐风化。在硅酸盐风化过程中，轻K同位素优先迁移到水溶液中，河流溶解负荷δ41K值降低，风化残余物具有较高的δ41K值，河流溶解K同位素与风化强度负相关(图5)(Hu Yan et al., 2020)。重K同位素随河流输入海洋造成海水δ41K值的变化，因此，利用古海水δ41K记录可以从地球历史的角度推断大陆风化强度(Hille et al., 2019; Teng Fangzhen et al., 2020)。

图5 自然储库δ41K(引自Hu Yan et al., 2020; Teng Fangzhen et al., 2020)以及河流沉积物中δ41K 与CIA关系(引自Hu Yan et al., 2020)Fig. 5 Potassium isotopic composition in natural reservoir (Hu Yan et al., 2020; Teng Fangzhen et al., 2020), and the plot of δ41K vs. CIA (from Hu Yan et al., 2020)

2.2.6 Cu同位素和Zn同位素

Cu和Zn均属于过渡金属元素。大陆风化作用是海洋Cu和Zn地球化学循环主要的“源”，风化过程中Cu和Zn分馏的可能原因包括: ① 主要造岩矿物的溶解过程发生同位素分馏； ② 溶解态以及次生矿物吸附态的同位素分馏；③ 大气浮沉的输入；④ 植物的吸收作用(Moynier et al., 2017)。

岩石氧化淋滤过程中，重Cu同位素被优先释放进入河流体系，进而注入海洋，造成河水和海水中相对较重的Cu同位素组成(图6)。氧化淋滤过程对Zn同位素的分馏程度较低，水体中Zn同位素组成和原岩几乎一致(图6)，不会超过0.1‰～0.3‰(Weiss et al., 2014)。Cu、Zn同位素在示踪大陆风化强度方面的应用还较匮乏，关于大陆风化对Cu和Zn同位素分馏影响机制还不清楚。但是已有的研究(吕逸文，2018)证实在黑色页岩和碳酸盐岩的风化过程中，存在Cu和Zn同位素分馏现象，值得再做更加深入和广泛的研究。

图6 表生环境自然样品Cu、Zn同位素组成(引自Moynier et al., 2017；吕逸文，2018)Fig. 6 Copper and zinc isotopic composition in natural reservoir under supergene environment (from Moynier et al., 2017; Yiwen et al., 2018&)

上述研究表明，目前没有任何地球化学参数能够完全准确地示踪大陆风化强度，需要多参数结合，互相验证，才能保证解释结果的准确性。有研究显示Tethyan南部黏土矿物的分布与气候分带之间相关性非常明显，揭示黏土矿物的分布能够反映古气候和大陆风化特征(Chenot et al., 2018)。因此，矿物学与地球化学参数的有效结合，有望为大陆风化强度研究提供更可靠的手段。

3 扬子地区E—C转折期大陆 风化作用

“雪球地球”理论认为在新元古代成冰纪结束后，距今652.5 Ma左右(邓俊等，2020，及该文中相关引用文献)，地球迅速转入温室气候(Hoffman et al., 1998)，E—C转折期全球化学风化强烈(Shield, 2005)。然而有研究发现在埃迪卡拉纪晚期亦有冰期沉积物的存在，如我国华北的正目观组、罗圈组和凤台组(Le Heron et al., 2018；岳亮等，2020)，以及西北的汉格尔乔克组和红铁沟组(Xiao Shuhai et al., 2004; Shen Bing et al., 2010)。在高纬度地区的爱沙尼亚(位于波罗的大陆，60°S)，有研究发现其寒武纪早期黑色页岩样品的CIA值低至59，指示寒冷干燥气候下较弱的化学风化作用(Tosca et al., 2010)。因此埃迪卡拉纪晚期—寒武纪早期可能并非长期处于稳定的超级温室气候环境中(Shen Bing et al., 2010；岳亮等，2020)。

对于我国华南扬子地区而言，其在埃迪卡拉纪晚期发育台内凹陷和大规模开放盆地，台地区发育灯影组白云岩，盆地区发育老堡组硅质岩(图7a)；早寒武世早期，全球海平面上升，碳酸盐台地遭到广泛淹没，到牛蹄塘沉积期形成以黑色细粒沉积为主的陆架环境(戴传固等，2013；Yeasmin et al., 2017)。快速海侵后保存了之前的地形地貌，黑色岩系在台地区直接不整合沉积于白云岩之上(图7b)，如上扬子台地区域灯影组顶部广泛发育有不整合面或岩溶面(朱东亚等，2014；杨雨等，2014；刘宏等，2015；丁一，2018)，在盆地区则与下伏硅质岩呈整合接触(图7c)。可见，扬子地区在E—C转折期经历了风化作用和快速海侵。近年来，我国扬子地区大陆风化强度研究已积累了一定的基础。前人通过对三峡地区与云南东部E—C地层序列的研究，发现在埃迪卡拉纪末期n(87Sr)/n(86Sr)显著增高并于E—C转折期附近达到最大值，而在进入寒武纪之后，n(87Sr)/n(86Sr)又逐渐减小(Sawaki et al., 2008, 2014; Li Da et al., 2013)。最近，Stammeier et al.(2019)统计了全球各地E—C时期的n(87Sr)/n(86Sr)数据，同样发现了在E—C转折期n(87Sr)/n(86Sr)迅速增高，进入寒武纪又明显降低的变化规律。说明在E—C转折期大陆风化作用较为强烈，但可能存在明显的波动。Chen Can 等(2020)通过对三峡地区多个剖面的陡山沱组地层开展研究，系统地重建了埃迪卡拉纪末期CIA变化曲线，并识别出了三次CIA降低阶段，结合岩石矿物学、地球化学(n(87Sr)/n(86Sr)，δ18O)指标指出这三次CIA的降低对应三次气候变冷事件。贵州铜仁道坨剖面埃迪卡拉系陡山沱组CIA值总体较高，位于70～85之间(图8)，指示其源区气候温暖湿润，化学风化程度较强，寒武系九门冲组下部(黑色页岩段)CIA值降至55～70，反映源区气候转为寒冷干燥，风化作用以物理风化为主，而九门冲组上部CIA值再次升高，表明E—C转折期风化作用经历了强—弱—强的波动(Zhai Lina et al., 2018)。另外有研究发现在道坨和坝黄剖面的牛蹄塘组底部，Ti/Al值增加且高于平均值，表明此时风尘输入增强，气候变得相对干燥(Yeasmin et al., 2017; Zhai Lina et al., 2018)。广西省三江县石门剖面和泗里口剖面清溪组页岩CIA值显示稳定的高值(76～84)，指示寒武纪早期其源区中等至较强程度的化学风化作用，源区古气候条件以温暖湿润为主，与扬子陆块中部上斜坡(贵州东北部)源区存在显著差异，与同时期位于赤道附近的阿曼地区化学风化程度和古气候一致(张子虎，2018；熊晨，2019)。E—C转折期碳酸盐岩Nd元素丰度和εNd(t)值均显著降低，表明陆相风化物质向陆架海水的输出逐渐加强(Wei Guangyi et al., 2019)。考虑到不同剖面距离扬子或华夏板块的位置，Li Chao 等(2020)推测石门剖面和泗里口剖面主要记录了风化作用强烈的华夏板块的源岩信息，而靠近扬子一侧的道坨、硝滩、龙鼻嘴等剖面则反映了在E—C时期扬子区域的化学风化作用相对较弱，但具体造成化学风化强度不同的原因还不明晰，有待进一步探究。

图7 (a)扬子地区E—C转折期地层划分对比(改自陈建书等，2020；年龄数据来自朱日祥等，2009； Xu Lingang et al., 2011; Wang Xinqiang et al.,2012; Zhu Bi et al., 2013; Chen Daizhao et al., 2016; Fu Yong et al., 2016; Zhou Chuanming et al., 2020)；(b)灯影组与牛蹄塘组不整合接触，遵义松林；(c)老堡组与牛蹄塘组整合接触，铜仁Fig. 7 (a) Stratigraphical division of E—C strata in Yangtze area (revised from Chen Jianshu et al., 2020&, and age data are from Zhu Rixiang et al., 2009&; Xu Lingang et al., 2011; Wang Xinqiang et al.,2012; Zhu Bi et al., 2013; Chen Daizhao et al., 2016; Fu Yong et al., 2016; Zhou Chuanming et al., 2020)

现有示踪扬子地区E—C转折期大陆风化的地球化学证据不足，集中在n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等，需要更多的地球化学参数来反映该风化作用的影响范围和演化特征，同时n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等数据也局限在少数剖面和层段，如εNd(t)仅涉及晚埃迪卡拉纪碳酸盐岩，缺少对早寒武纪黑色页岩的分析。风化作用与扬子地区古海洋环境、古生物演化和有机质富集等有什么影响？

图9 风化作用对海洋环境及有机质富集影响模式示意图(注：OM为有机质；DIC为溶解无机碳)Fig. 9 Affecting pattern of continental weathering onmarine environment and organic matter enrichment (note: OM is organic matter; DIC is dissolved inorganic carbon)

扬子地区E—C转折期大陆风化对古海洋环境、有机质富集有何影响？准确认识该问题有助于揭示古海洋环境演变和古生物演化规律，能为区内油气资源的勘探开发提供新的思路。

4 结论与展望

笔者等学习和总结了前人对扬子地区震旦纪(埃迪卡拉纪)—寒武纪(E—C)转折期大陆风化作用的研究成果，取得以下主要认识：

(1)扬子地区震旦纪(埃迪卡拉纪)—寒武纪(E—C)转折期遭受了较强烈的化学风化作用，在台地相区，牛蹄塘组黑色岩系不整合接触于灯影组白云岩之上，斜坡—盆地相区，该黑色岩系与下伏老堡组硅质岩呈整合接触。台地区不整合与E—C转折期全球“大不整合”具有紧密的关系，而盆地区发育的完整的沉积地层则记录了台地区被风化地层的痕迹。扬子地区E—C转折期兼具浅水台地和深水盆地环境，是研究这一时期全球大陆风化作用的重要窗口。

(2)现有扬子地区E—C转折期大陆风化的地球化学证据集中于n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等，同时n(87Sr)/n(86Sr)、δ13C、CIA和εNd(t)等数据也局限在少数剖面和层段。因此迫切需要更多的地球化学参数来反映该风化作用的影响范围和演化特征。

(3)扬子地区E—C转折期大陆风化作用与海洋环境演变存在耦合关系，准确认识这种关系有助于揭示古生物演化、有机质富集的机制，能够为扬子地区油气资源勘探开发提供新思路。

致谢：感谢评审专家和责任编辑对本文修改提出的宝贵建议。

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