尚墨翰, 汤冬杰, 史晓颖, 魏昊明, 刘安琪. (2018)I/(Ca+Mg)作为指示碳酸盐沉积氧化还原条件的重要指标: 研究进展与问题评述* [J]. 古地理学报, 20(4): 651-664
Shang Mo-Han, Tang Dong-Jie, Shi Xiao-Ying, Wei Hao-Ming, Liu An-Qi. (2018)I/(Ca+Mg)as an important redox proxy for carbonate sedimentary environments: Progress and problems[J]. Journal Of Palaeogeography, 20(4): 651-664.   
Doi: 10.7605/gdlxb.2018.04.047
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I/(Ca+Mg)作为指示碳酸盐沉积氧化还原条件的重要指标: 研究进展与问题评述*
尚墨翰1,2, 汤冬杰1,3, 史晓颖1,2, 魏昊明2, 刘安琪3
1 生物地质与环境地质国家重点实验室,中国地质大学(北京),北京 100083
2 中国地质大学(北京)地球科学与资源学院,北京 100083
3 中国地质大学(北京)科学研究院,北京 100083
通讯作者简介： 汤冬杰,男,1985年生,中国地质大学(北京)副教授,主要从事地球生物学和前寒武纪地质研究工作。E-mail: dongjtang@126.com。

第一作者简介： 尚墨翰,男,1993年生,古生物学与地层学博士研究生。E-mail: shangmohan126@126.com。

收稿日期:2018-03-06
基金资助:*国家自然科学基金项目(编号:41672336,41402024),中央高校基本科研基金(编号:2652018005,2652017050,2652017256)共同资助;
摘要

海水氧化还原条件显著影响真核生物的起源与早期演化,但以往有关早期海水氧化还原条件研究的对象,主要依赖相对深水的细粒碎屑岩沉积(如黑色页岩),而对真核生物集中分布的浅水环境中的碳酸盐岩关注不够且手段缺乏。这显著制约了对真核生物起源与早期演化机理的认识。近年来,有学者提出碳酸盐岩的 I/( Ca+ Mg)值可作为反映海洋氧化还原条件的重要指标,并将其广泛应用于海相碳酸盐岩的古氧相研究中。该指标的提出主要基于对现代海洋碘组分的观测以及室内方解石合成实验结果: 观测结果表明,海水中的碘主要以氧化态( IO3-)和还原态( I-) 2种形式存在,随着氧含量的下降(如在氧极小带),氧化态的碘被逐步转换为还原态的碘,且海水中的 IO3-浓度与海水氧含量大体呈正相关。实验研究证明, IO3-可按一定的分配系数进入到碳酸盐矿物晶格中,但 I-则不能。由于 IO3- /I-的还原势能与 O2/H2O的还原势能接近,因此 I/( Ca+ Mg)值是最早响应海洋氧含量下降的指标之一,可用于表征深时(如前寒武纪)次氧化环境中表层海水的氧含量波动。此外,学者们也尝试建立 I/( Ca+ Mg)值与氧含量之间的半定量关系,如 I/( Ca+ Mg)值大于 0和 2.5μmol/mol这两个临界值所对应的海水氧含量。结合大量现代缺氧水体和氧极小带中碘组分与溶解氧浓度相关关系的研究,作者提出 I/( Ca+ Mg)= 1.5μmol/mol为重要的临界值之一,可用于限定初级生产力在表层海水中所能产生的最大氧浓度值( ~10 μM),并能进一步区分海水和大气的氧化。此外,对 I/( Ca+ Mg)值的应用进展及潜在问题进行评述,并对可能的发展方向进行展望。

关键词: 浅海碳酸盐岩; 氧化还原状态; I/( Ca+ Mg)值; 初级生产力
中图分类号:P534.41 文献标志码:A 文章编号:1671-1505(2018)04-0651-14
I/(Ca+Mg)as an important redox proxy for carbonate sedimentary environments: Progress and problems
Shang Mo-Han1,2, Tang Dong-Jie1,3, Shi Xiao-Ying1,2, Wei Hao-Ming2, Liu An-Qi3
1 State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences(Beijing),Beijing 100083
2 School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083
3 Institute of Earth Sciences,China University of Geosciences(Beijing),Beijing 100083
About the corresponding author： Tang Dong-Jie,born in 1985,is an associated professor of China University of Geosciences(Beijing). He is engaged in geobiology and Precambrian geology. E-mail: dongjtang@126.com.

About the first author： Shang Mo-Han,born in 1993,is a Ph.D. candidate of paleontology and stratigraphy. E-mail: shangmohan126@126.com.

Fund:Co-funded by the National Natural Science Foundation of China(Nos. 41672336, 41402024), and the Fundamental Research Funds for the Central Universities(Nos. 2652018005, 2652017050, 2652017256)
Abstract

The redox conditions of seawater play a pivotal role in influencing the origin and early evolution of eukaryotes. However,previous studies regarding ocean redox conditions mainly focus on fine-grained siliciclastic rocks(e.g.,black shale)deposited in relatively deep seawater,rather than carbonates formed in eukaryote-concentrated shallow seawater,due largely to a lack of valid method,significantly limiting our understanding of the mechanisms concerning the origin and early evolution of eukaryotes. In recent years,I/(Ca+Mg)was proposed as a proxy for redox conditions of seawater,and has been widely employed in carbonates to analysis seawater redox conditions. The proposal of this proxy is mainly based on measurements of iodine speciation in modern oceans and experiments of calcite synthesis in laboratory. The measurements demonstrate that marine iodine composition mainly occur in two states, namely, Oxidized-state iodate(IO3-)and reduced-state iodide(I-). With the decrease of oxygen concentration(such as in an oxygen minimum zone,OMZ),the oxidized-state iodate,which is proportional to the oxygen concentration,would be gradually reduced into reduced-state iodide. The experiments confirm that only IO3- could be incorporated into the lattices of carbonate minerals with a fixed distribution coefficient,but I-would be excluded. Because of the high redox potential of IO3-/I-,which is close to that of O2/H2O,I/(Ca+Mg) is one of the proxies earliest responding to the decrease of ocean oxygen concentration. I/(Ca+Mg) is therefore sensitive to the variation of oxygen concentrations in weakly oxidized surface seawaters in deep time(e.g.,Precambrian). Furthermore,some scholars attempted to establish semiquantitative relationships of I/(Ca+Mg)values to oxygen concentrations,and two threshold values of I/(Ca+Mg)>0 and 2.5μmol/mol have been proposed as the semiquantitative constraints for the oxygen concentrations in ancient ocean waters. In addition,in the light of the study of iodine speciation and dissolved oxygen concentrations in modern anoxic basins and water columns within OMZs,our results suggest that I/(Ca+Mg)=1.5μmol/mol could be used as the threshold between atmosphere and surface seawater. This threshold value may be used to reflect that the oxygen concentration of surface ocean is up to 10 μM,which is the maximum oxygen concentration increased by the primary productivity,and therefore to distinguish the potential variations of oxygen concentration between atmosphere and surface seawaters. In this paper,some of the recent progress and potential problems in redox analysis using I/(Ca+Mg)in ancient carbonates were briefly reviewed,and some tentative suggestions for future study were also put forward.

Key words: marine shallow-water carbonates; redox conditions; I/(Ca+Mg)proxy; primary production

海水的化学条件对真核生物的起源和演化具有重要影响(例如, Knoll and Carroll, 1999; Anbar and Knoll, 2002; Glass et al., 2009; Guan et al., 2014), 尤其是海水的氧化还原条件, 直接控制了新元古代晚期后生动物的兴起与生态分布(例如, Knoll and Carroll, 1999; Canfield et al., 2007; Sahoo et al., 2012, 2016)。因此, 针对元古代海水氧含量进行研究, 将对认识真核生物和后生动物的起源和早期演化具有重要的科学意义。尽管近年来对早期大气(Partin et al., 2013; Planavsky et al., 2014)和海洋(Johnston et al., 2010; Li et al. 2010; Poulton et al., 2010; Planavsky et al., 2011; Poulton and Canfield, 2011; Canfield et al., 2013; Partin et al., 2013; Reinhard et al., 2013)氧含量的研究取得了重大进展和新认识, 但对许多时段的认识目前仍限于理论推测(Planavsky et al., 2011; Reinhard et al., 2013)且争议很大(Planavsky et al., 2014; Cole et al., 2016; Gilleaudeau et al., 2016; Tang et al., 2016; Zhang et al., 2016, 2017; Hardisty et al., 2017)。而在分析方法上, 对海水氧化还原的研究主要运用的是铁组分(例如, Poulton et al., 2004, 2010; Canfield et al., 2007, 2008; Li et al., 2010; Planavsky et al., 2011)、氧化还原敏感元素丰度(例如, Scott et al., 2008; Scott and Lyons, 2012; Reinhard et al., 2013; Wang et al., 2015b)、N同位素(例如, Stü eken, 2013, 2016; Ader et al., 2014; Wang et al., 2015a; Koehler et al., 2017)、Cr同位素(例如, Frei et al., 2009; Planavsky et al., 2014)和Mo同位素(例如, Arnold et al., 2004; Kendall et al., 2015; Kurzweil et al., 2015)等。这些方法主要适用于相对深水的细粒碎屑岩样品(如页岩), 而对于广泛发育的浅水碳酸盐岩, 仍缺乏有效的反映周围海水氧化还原状态的地球化学指标。事实上, 早期真核生物的生态空间可能被局限在氧含量相对较高的浅水区域(Adam, 2014)。因此, 针对浅水碳酸盐岩所记录的水体氧化还原状态的研究, 将为探讨海水氧化还原状态以及真核生物与后生动物的演化提供重要、直观的信息。

早期针对浅水碳酸盐岩的氧化还原研究, 采用的指标主要为稀土元素Ce异常, 但是该指标极易受到陆源碎屑尤其是黏土矿物的影响。已有的一些研究结果表明, 那些陆源碎屑含量较高的碳酸盐岩样品, 主要记录的是碎屑的稀土组成, 而非海水的氧化还原信号(例如, Planavsky et al., 2010; Ling et al., 2013; Tang et al., 2016)。对现代海洋中氧极小带和缺氧盆地碘(I)组分的早期观测表明, 随着O2的耗尽, 碘酸根离子(I O3-)几乎会被完全还原为碘离子(I-)(Wong and Brewer, 1977; Emerson et al., 1979; Wong et al., 1985; Luther Ⅲ and Campbell, 1991)。近年来的方解石沉淀观测和模拟实验发现, 方解石晶体内的碘含量随介质中I O3-浓度的升高而呈线性增加, 但I-却被完全排除在外(Lu et al., 2010)。碘在现代海洋中的浓度为~0.45 μ M, 在海水中的驻留时间约为300 kyr(Broecker and Peng, 1982)。根据碘的这些化学特性, 有研究提出, 碳酸盐岩中的I/(Ca+Mg)值, 可反映沉积环境水体中是否存在I O3-及其大致浓度, 并可指示水柱中游离氧存在与否及其含量(Lu et al., 2010)。进一步的研究证明, 成岩作用只可能降低岩石中原始的I/(Ca+Mg)值(Loope et al., 2013; Hardisty et al., 2017)。因此, 近年来I/(Ca+Mg)值被应用于表征古代碳酸盐岩沉积环境的氧化还原情况(例如, Lu et al., 2010, 2016; Hardisty et al., 2014, 2017; Zhou et al., 2014, 2015, 2017; Edwards et al., 2018), 它的提出为地质时期氧化还原条件的研究提供了一种新手段。

为更好地应用这一指标, 文中总结了I/(Ca+Mg)值作为氧化还原指标的基本原理、碳酸盐岩中碘含量的测试方法、I/(Ca+Mg)值中若干重要临界值的古环境氧含量指示意义、前寒武纪海洋中I/(Ca+Mg)值的长期演变特征以及将这一指标作为古代碳酸盐岩氧化还原指标可能存在的问题, 并对下一步的研究方向予以展望。

1 基本原理

现代海洋的观测表明, 虽然碘元素能以多种价态存在, 但具有热稳定性的溶解无机碘仅有氧化态的碘酸根离子(I O3-)和还原态的碘离子(I-)2种(Wong and Brewer, 1977; Emerson et al., 1979; Rue et al., 1997; Farrenkopf and Luther Ⅲ , 2002; Chance et al., 2014)。现代海洋中的溶解碘浓度在全球海洋中相对均一, 这可能是由于海洋中碘的滞留时间(~300 kyr)远大于海水混合时间所致(Broecker and Peng, 1982; Moran et al., 2002; Kü pper et al., 2011)。方解石合成实验表明, 在方解石沉淀过程中, 进入到碳酸盐矿物内的氧化态碘酸根离子含量与溶液中的碘酸根离子浓度呈正相关, 但还原态的碘离子则会被排除在碳酸盐矿物之外(Lu et al., 2010)。目前对碘酸根离子进入碳酸盐矿物的机理尚不明确, 推测可能是通过取代碳酸根离子和/或占据晶格缺陷而进入到碳酸盐矿物中, 与硫酸根离子进入碳酸盐矿物的机理类似(Staudt and Schoonen, 1995)。碘离子被排除在外, 可能是因为碘离子中的碘相对于碘酸根离子中的碘具有更大的原子半径(Lu et al., 2010)。

碘酸盐是一种海洋生物的微营养物质(Kü pper et al., 2010), 初级生产力和有机质降解可分别导致表层海水中少量至~50%碘酸盐的损失以及深层水柱中碘化物的析出。这种情况在现代海洋的许多区域均可观察到, 包括夏威夷和百慕大群岛(Campos et al., 1996)、威德尔海(Campos et al., 1999; Bluhm et al., 2011)、地中海(Tian et al., 1996)、阿拉伯海(Farrenkopf and Luther Ⅲ , 2002)及南极洲沿岸水域(Chance et al., 2010)等。另一方面, 深部氧化水体的上涌可补偿表层海水中碘酸盐的减少(Truesdale and Bailey, 2002)。因此, 尽管受生物摄取/释放的影响, 但仍普遍认为海水中碘组分的主要控制因素为局部海水的氧化还原状态。目前已知I O3-仅存在于氧化水体中, 且在弱氧化水体中先于铁和硫酸盐被还原(图 1; Wong and Brewer, 1977; Emerson et al., 1979; Kennedy and Elderfield, 1987a, 1987b; Rue et al., 1997; Farrenkopf and Luther Ⅲ , 2002)。例如, 在低氧(hypoxic)条件下, 超过75%的碘酸盐会转化为碘化物(Truesdale and Bailey, 2000); 在缺氧(anoxic)条件下, 如缺氧盆地(Wong and Brewer, 1977; Emerson et al., 1979)和还原性孔隙水中(Kennedy and Elderfield, 1987a, 1987b), I O3-会被完全还原。由于在碘组分中I O3-是唯一能与碳酸盐矿物同沉淀的组分, 因此浅海碳酸盐岩中的I/(Ca+Mg)值可指示浅海中是否存在游离氧(Hardisty et al., 2014)以及水柱中氧化还原界面的相对位置(Lu et al., 2016)。这一认识与以往报道的大氧化事件(Great Oxidation Event, GOE, ca. 2.4Ga)之前碳酸盐岩中缺乏碘、显生宙碳酸盐岩中普遍具有较高I/(Ca+Mg)值的结果一致(Hardisty et al., 2014, 2017)。

图 1 各种元素的还原电位和氧化还原对(据Lu et al., 2010, 有修改)Fig.1 Reduction potentials and redox couples of various elements(modified from Lu et al., 2010)

每一种氧化还原指标, 均在不同的电位范围内响应环境的变化。那些具有低还原电位的指标(如, Mo O42-/MoO2), 要比具有高还原电位的指标(如, Ce4+/Ce3+)更早地响应环境的氧化, 从而在进一步的氧化过程中始终保持高价态(图 1)。理论上, I O3-/I-的标准还原电位高于N O3-/N2(图 1), 因此随着海水中氧含量逐渐升高, I-向I O3-的氧化晚于N2向N O3-的氧化。这可以用来解释为何地质记录中有氧氮循环可见于太古宙氧浓度小于微摩尔浓度级别的表层海水中, 但同时却不存在I O3-或氧气的广泛积累(Thamdrup et al., 2012; Hardisty et al., 2014)。再例如, 由于 Ce4+/Ce3+的标准还原电位较Fe3+/Fe2+高(图 1), 因此在海水含氧量逐渐上升的过程中, Fe2+向Fe3+的氧化将早于Ce3+向Ce4+的氧化。这可以用来解释为何铁组分数据能够记录伊迪卡拉纪中期(580-560ma)的深海氧化事件(Canfield et al., 2008), 但在同期的浅水样品中则表现不明显, 而只有具有更高还原电位的指标(Ce4+/Ce3+)才可较灵敏地示踪该时期浅海和大气的增氧过程(Ling et al., 2013)。由于I O3-/I-的标准还原电位与O2/H2O接近, 且能够有效反映氧化还原阶段介于Ce4+/Ce3+和N O3-/N2之间的元古宙海水氧化情况, 故可以用来指示元古宙大气和浅海氧气含量的明显增加(Hardisty et al., 2014, 2017)。由于当前针对浅水碳酸盐岩沉积环境氧化还原状态的研究手段较少, 因此I/(Ca+Mg)值是用来表征元古宙台地(浅海)碳酸盐岩沉积环境氧化还原状态较为理想的指标。

2 测试方法

碳酸盐岩的碘含量测试分析, 在国内开展的相对较少, 但在国际上已有较为成熟的方法。现有的标准方法中, I/(Ca+Mg)值可使用四级杆电感耦合等离子体质谱仪测量(ICP-MS, 如Bruker M90)(Lu et al., 2010, 2016; Hardisty et al., 2014, 2017; Zhou et al., 2014, 2015):首先, 称3~5mg样品粉末, 用1mL去离子水漂洗4次, 并离心倾析上层清液, 以去除样品表面可能附着的可溶性碘盐。然后, 配加3%硝酸并置于超声水浴中~10min, 以完全溶解碳酸盐; 残留的非碳酸盐杂质通过离心去除, 保留上层清液。最后, 溶液被稀释至含有~50 μ g/g的Ca、~0.5%的三甲胺(用于稳定碘酸盐)(Schnetger and Muramatsu, 1996)以及内标(如In)。在ICP-MS测试过程中, 对于每个批次的测试, 均采用不同浓度的分析纯碘酸钾建立标准曲线。

当使用Bruker M90型ICP-MS上机测试时, 应首先将设备的灵敏度调至1 ng/g浓度的127I所对应的设备计数率为~80-100 kcps。测试结果表明, 127I的精确度普遍优于1%, 长期准确性可通过参考物质监控, 如Jcp-1(Lu et al., 2010)。该标样国内较难获得, 可使用GSR-12(GBW07114)替代。I/(Ca+Mg)值的检测限通常低于0.1μ mol/mol。对于灵敏度较低的ICP-MS, 可以提高样品质量至40mg, 减少溶液稀释倍率至含有~400 μ g/g的Ca, 灵敏度调至1 ng/g浓度的127I所对应的设备计数率为~2-3 kcps, 此时测试的127I精确度可普遍优于6%。需要注意的是, 本处理方法仅限于受黏土、硅酸盐及有机质影响较小的碳酸盐岩, 因为在化学分析时, 附着于非碳酸盐岩的碘有可能影响所测试的I/(Ca+Mg)值。若难以筛选出符合条件的样品, 需要评估非碳酸盐岩对I/(Ca+Mg)值的影响。

3 碳酸盐岩中I/(Ca+Mg)指标若干临界值的古氧相指示意义

研究表明, 未遭受显著成岩作用改造的碳酸盐岩, 若其具有高的I/(Ca+Mg)值, 便可明确指示沉积水体具有高氧气含量(例如, Lu et al., 2016)。尽管全球标定结果表明I/(Ca+Mg)值并不能完全线性、定量地反映水体溶解氧浓度, 但基于现代海洋碘组分的研究成果, 笔者依然能够确定一组I/(Ca+Mg)的临界值用于半定量地限定沉积水体的溶氧量(例如, Hardisty et al., 2014, 2017; Lu et al., 2016)。

3.1 I/(Ca+Mg)> 0μ mol/mol

前人指出, 海洋中I O3-的积累与碳酸盐中碘的检出, 至少需要1~3μ M的氧浓度条件(Hardisty et al., 2014, 2017)。这一认识得到了北太平洋东岸氧极小带(oxygen-minimum zone, OMZ)观察的证实。在这一水域, 海洋水体中的氧气浓度从表层的~225 μ M降至OMZ中心的1~3 μ M, 与此相对应的I O3-浓度降至~0.01 μ M, 但仍大于0 μ M(图 2; Rue et al., 1997)。值得指出的是, 这个氧浓度值并不代表能够形成I O3-的最小值, 而是使海水中I O3-能够积累的最小值。类似的情况也广泛地发现于氧浓度低于3 μ M的现代缺氧盆地和OMZ(Wong and Brewer, 1977; Wong et al., 1985; Luther Ⅲ and Campbell, 1991; Farrenkopf et al., 1997; Rue et al., 1997; Farrenkopf and Luther Ⅲ , 2002)。因此, I/(Ca+Mg)> 0μ mol/mol, 可用来指示海水中溶解氧浓度大于 1~3μ M。

图 2 秘鲁远海OMZ中溶解的[I O3-]与[O2]纵向变化关系(据Rue et al., 1997)Fig.2 Vertical variation of dissolved[I O3-] and [O2] in water column of the Peruvian OMZ (after Rue et al., 1997)

依据这一指标, Hardisty等(2014)提出, GOE期间海水表面氧气浓度高于1 μ M。另外, Hardisty 等(2017)指出, 大部分元古宙碳酸盐岩单元的I/(Ca+Mg)> 0μ mol/mol, 是表层海水[O2]> 1~3μ M的有力证据。这一认识得到了中元古界发现真核生物微体化石的支持(Knoll, 2014), 也得到了模拟结果的支持, 即在完全缺氧的大气环境下, 表层水体内的光合作用也可导致氧气浓度升高至数微摩尔浓度(Reinhard et al., 2016)。

3.2 I/(Ca+Mg)> 1.5μ mol/mol

区分海水氧化与大气氧化十分必要, 因为二者的氧化一般并不完全同步。研究表明, 当大气氧含量小于 2.5% PAL时, 表层海水的氧含量主要受控于初级生产力, 从而很难与大气达到平衡(Reinhard et al., 2016)。换言之, 当大气氧含量小于2.5% PAL时, 表层海水的氧化还原状态并不能直接指示大气氧的含量。但是, 模型研究表明, 初级生产力所导致的局部氧气浓度升高仅限于1~10 μ M之间, 不可能超过10 μ M(Kasting, 1991; Olson et al., 2013; Reinhard et al., 2013)。因此, 建立表层海水氧浓度为10μ M时对应的海水I O3-浓度以及碳酸盐岩I/(Ca+Mg)临界值, 对于区分表层海水与大气中的氧含量变化具有重要意义。

由于受生物吸附作用及其之下水体是否存在氧极小带等因素的影响, 表层海水的I O3-浓度与氧气浓度虽然大体上呈正相关, 但并不完全线性相关(例如, Zhou et al., 2014, 2015; Lu et al., 2016)。这个结论, 为笔者建立表层海水氧气浓度为10μ M时海水中的I O3-临界浓度提供了理论基础。结合目前所获得的多个现代缺氧盆地和氧极小带区域海水I O3-和氧气浓度剖面数据, 对比发现, 当[I O3-]> 0.15μ M时, I/(Ca+Mg)=~1.5μ mol/mol(Lu et al., 2010), 对应的水体[O2]> 10μ M(Wong and Brewer, 1977; Emerson et al., 1979; Smith et al., 1990; Luther Ⅲ and Campbell, 1991; Rue et al., 1997; Lu et al., 2010, 2016; Glock et al., 2014)。由于表层海水中的生物吸附或下覆OMZ带内I O3-的还原作用, 只能降低表层海水的I O3-浓度, 因此, [I O3-]=0.15μ M(即, I/(Ca+Mg)=~1.5μ mol/mol), 是确保表层海水中[O2]> 10μ M的最保守估计值。

3.3 I/(Ca+Mg)> 2.5μ mol/mol

I O3-在缺氧水体中被完全还原仅需数小时(Farrenkopf et al., 1997), 而I-在氧化水体中被完全氧化是一个相对缓慢的过程, 估计需要数周至数年时间(Luther Ⅲ et al., 1995; Chance et al., 2014)。由于碘组分氧化与还原速率的不对称性, 导致了海水中I O3-浓度在OMZ或缺氧盆地的氧跃层中具有最大梯度(图 2; Rue et al., 1997; Farrenkopf and Luther Ⅲ , 2002)。因此, 碘酸盐浓度在垂向剖面上除受生产力影响外, 还受是否存在OMZ的影响。如果不发育OMZ, 碘酸盐浓度将从初级生产力活跃的混合层向下逐渐升高(图 4-A, Jickells et al., 1988); 由于呼吸作用的存在, 海水中的氧浓度一般随深度增加而减少, 但碘酸盐在氧气完全被消耗前并不作为氧化剂, 因此其浓度会大体增加至海水的平均水平(~0.45 μ M)。反之, 当水体中存在相对较浅的OMZ时, 碘酸盐浓度从混合层向OMZ上部快速下降(图 4-B, Smith et al., 1990); 当OMZ较深时(图 4-C), 碘酸盐浓度从混合层向下先增加, 随后在OMZ降至0(Rue et al., 1997)。

图 3 低氧水体中[I O3-]与[O2]浓度相关性及底栖有孔虫钙质壳中的I/Ca值与[O2]协变图
A— 黑海[I O3-]与[O2]协变图(Luther Ⅲ and Campbell, 1991, 及其引用的文献); B— 北太平洋[I O3-]与[O2]协变图(Rue et al., 1997); C— 阿拉伯海[I O3-]与[O2]协变图(Farrenkopf and Luther Ⅲ , 2002, 及其引用的文献); D— 缺氧水体和大西洋表层水体中[I O3-]与[O2]的比较(据Lu et al., 2010修改), 黑色圆圈代表不同水深的水样结果; E-H— 底栖有孔虫壳体I/Ca与[O2]协变图 (据Glock et al., 2014, 修改), 不同形状标记代表来自不同样品集群的结果; 示氧气浓度10 μ M的最高临界[I O3-]为0.15 μ MFig.3 Correlation of[O2] with [I O3-] for the hypoxic waters and [O2] with I/Ca for the analyzed benthic foraminiferal species

图 4 海水碘酸盐和氧浓度变化示意图(据Zhou et al., 2014修改)Fig.4 Schematic seawater iodate and oxygen concentration profiles(modified from Zhou et al., 2014)

就存在OMZ条件下的I O3-深度剖面而言(图 4-B, 4-C), 下层水体中可能存在临界氧浓度, 导致I O3-被完全还原。相应地, 表层水体可能同时存在另一个临界值, 指示当表层水体中I O3-浓度低于该临界值时, 下层水体中I O3-被完全还原。据现代海洋I O3-和O2数据的对比关系分析, Lu等(2016)提出, 表层水体[I O3-]< ~0.25μ mol/L(对应碳酸盐岩I/(Ca+Mg)< 2.5μ mol/mol), 可作为指示下层水体[O2]< 20~70μ M的临界值(图 5)。与现代海洋中的OMZ和缺氧盆地的氧跃层类似, 古代碳酸盐岩中I/(Ca+Mg)值非0且低于~2.5μ mol/mol时, 可指示水体中氧气水平超过了维持I O3-积累的浓度(即> 1~3 μ M), 但原位或邻近可交换水体中的I O3-还原十分活跃(这种水体条件对应于[O2]< 20~70 μ M; Lu et al., 2016; Hardisty et al., 2017)。相反, I/(Ca+Mg)> ~2.5 μ mol/mol, 则可反映邻近区域不含低氧水体, 即[O2]> 20~70μ M。

图 5 现代开放海洋和具OMZ海洋的表层水体I O3-浓度(据Lu et al., 2016, 有修改)
A— 现代海洋表层海水中I O3-浓度与最小O2浓度比较(据 Emerson et al., 1979; Wong, 1985; Nakayama et al., 1989; Smith et al., 1990; Campos et al., 1996; Rue et al., 1997; Truesdale and Bailey, 2000; Truesdale et al., 2000; Farrenkopf and Luther Ⅲ , 2002; Waite et al., 2006; 资料汇编)。棕色虚线表示表层水体中区分OMZ型和开放海洋型I O3-剖面的[I O3-]临界值(约为0.25 μ M); B— 海洋中I O3-浓度随深度变化曲线, 分别来自赤道东太平洋(Rue et al., 1997)、阿拉伯海N8站点OMZ带(Farrenkopf and Luther Ⅲ , 2002)和威德尔海PS71/179-1站点 (Bluhm et al., 2011)附近的高纬度富氧地点Fig.5 Concentration of I O3- in surface seawaters of modern open ocean and OMZs(modified from Lu et al., 2016)

4 前寒武纪海水中I/(Ca+Mg)指标的长期演化

碳酸盐岩中I/(Ca+Mg)值的长期演化序列表明, 元古宙碳酸盐岩中I/(Ca+Mg)平均值较显生宙偏低(图 6)。Hardisty等(2017)认为这一特征指示, 在元古宙大部分时期表层海水仅微弱氧化, 且在邻近区域存在缺氧水体。这可能说明, 元古宙海洋表层水体的氧化还原状态, 类似于现代海洋的缺氧盆地和OMZ附近的氧跃层(图 2)。在这些环境中游离氧可局部存在, 但同时在该部位或相邻地区的可交换水体中, I O3-的还原反应十分活跃。这种环境以I O3-可持续积累但浓度很低为特征, 使得对应的I/(Ca+Mg)值在元古宙始终保持在较低范围内(图 6, 黄色正方形)。如图 6所示, 虽然元古宙表层海水以低的I/(Ca+Mg)值为主要特征, 但也存在间歇性的I/(Ca+Mg)高值, 与过去认为的氧化事件相对应。这些事件, 包括约在2.4Ga发生的GOE(Lyons et al., 2014)、Lomagundi碳同位素漂移(图 6; Bekker and Holland, 2012)以及发生在约1.4Ga(Mukherjee and Large, 2016)、新元古代(Planavsky et al., 2014)和伊迪卡拉纪中期(Shuram)的同位素异常事件(Fike et al., 2006)。综合来看, 元古宙表层海水I/(Ca+Mg)值的长期演化, 表明这一时期的海洋以缺氧为主, 伴生有动态的脉冲式氧化事件, 因此增氧过程可能并不是单向的持续状态(Reinhard et al., 2013; Sahoo et al., 2016)。

图 6 海洋I/(Ca+Mg)值的长期演变趋势
A— 大气氧含量长期演化趋势(Lyons et al., 2014; Planavsky et al., 2014), 虚线表示缺乏定量约束的不确定性; B— δ13Ccarb和I/(Ca+Mg)长期演变趋势(Lu et al., 2010, 2016; Loope et al., 2013; Glock et al., 2014; Hardisty et al., 2014; Zhou et al., 2014, 2015)与已知的重要事件的时间关系(Knoll, 2014)。蓝色圆圈代表现代氧化环境且未受成岩改造的样品值, 黄色方块代表钙质有孔虫样品值; 灰色圆圈代表碳酸盐岩样品的值; C— 深海主体的氧化还原特征, 据铁组分数据而定(Sperling et al., 2015)。红色和紫色方条分别代表铁化和硫化条件, 蓝色方条代表氧化条件Fig.6 Secular trends of I/(Ca+Mg)ratios in ocean seawater

5 存在的问题
5.1 海洋碘库的稳定性

虽然现代海洋碘库的大小尚未被精确限定(约为0.45 μ M), 但据估算, 进出海水的碘通量(如河流及地壳的输入、有机质埋藏的输出), 可能比生物生产力相关的碘通量低1至2个数量级(表 1; Lu et al., 2010及其引用的文献)。高生产力区域混合层水体内的生物泵作用, 会造成水体中碘酸盐浓度降低, 但大部分被有机质吸收的碘, 都会随着有机质在氧化水柱中的分解而被重新释放到海洋(Lu et al., 2010, 及其引用的文献)。因此, 一般认为海洋的碘库, 在显生宙期间大体稳定。

表 1 据海水输入/输出量估算的碘通量与水柱中有机质循环、火山排放碘通量的对比(据Lu et al., 2010, 有修改) Table 1 Comparison of iodine fluxes estimated from in/out of seawater with those recycled by organic matter in water column and those released by volcanic emission(modified from Lu et al., 2010)

然而, 前寒武纪海洋的碘库, 很可能由于缺乏或仅含有有限的富碘藻类, 而比现代或显生宙的碘库整体偏大(Hardisty et al., 2017)。此外, 显生宙全球性的氧化还原转化和有机碳埋藏事件, 也很可能导致碘库的变化。例如, 初级生产力的上升, 将导致从混合层输出的有机碘增加, 从而降低总无机碘的浓度, 进而降低碳酸盐岩的I/(Ca+Mg)值(Zhou et al., 2015)。

如果在事件期, 碳酸盐岩的I/(Ca+Mg)值主要受控于碘库的变化(有机碳埋藏和大陆风化变化)而非氧化还原状态的变化, 那么, 不同剖面中的I/(Ca+Mg)值应该具有相似的变化趋势。但实际情况并非如此。如在白垩纪森诺曼阶— 土仑阶(Cenomanian-Turonian)的OAE2事件期, I/(Ca+Mg)值在不同剖面上有显著差异, 而且I/(Ca+Mg)值与风化指标钙、锂及碳同位素变化趋势并不一致。因此, 碳酸盐岩的I/(Ca+Mg)值, 主要受水体的氧化还原状态控制, 而与有机碳埋藏或风化相关的全球海洋碘库波动的影响, 均会被局部或区域性水体的氧化还原信号所覆盖(Zhou et al., 2015)。

5.2 成岩作用的影响

由于碘离子是缺氧水体中碘的唯一存在形式(Wong and Brewer, 1977; Wong et al., 1985; Kennedy and Elderfield, 1987a, 1987b; Luther Ⅲ and Campbell, 1991; Farrenkopf et al., 1997; Rue et al., 1997; Farrenkopf and Luther Ⅲ , 2002), 但它不能进入碳酸盐矿物内(Lu et al., 2010), 因此, 碳酸盐岩成岩作用或原生碳酸盐矿物在缺氧孔隙水中重结晶, 都会导致全岩的I/(Ca+Mg)值下降(Hardisty et al., 2017)。古代碳酸盐岩样品中碘的存在, 可作为其沉积水体氧化条件的有力证据。

对新近纪样品的研究表明, 白云岩化作用会显著降低样品的I/(Ca+Mg)值(Hardisty et al., 2017)。但由于白云岩相较于其他类型碳酸盐岩具有更长期的稳定性, 因此早期成岩阶段形成的白云岩, 被认为是比灰岩更可靠、甚至首选的古海洋化学条件记录载体。事实上, 元古宙碳酸盐岩中高的I/(Ca+Mg)值, 经常发现于白云石中, 而不是方解石中。由于晚期成岩改造流体(如缺氧卤水, Derry, 2010)不可能含有高的I O3-浓度, 因此部分元古宙白云岩的高I/(Ca+Mg)值, 不可能由这些过程所导致(Hardisty et al., 2017)。考虑到I O3-的还原在水体氧含量降低初期就会发生, 以及具有高I/(Ca+Mg)值的元古宙白云石沉淀可能发生在沉积— 水界面附近, 因此I/(Ca+Mg)值至少可以部分地记录海水信号(Tucker, 1982; Fairchild et al., 1991; Kah, 2000; Hood and Wallale, 2011; Hardisty et al., 2017)。

因此, 成岩作用不可能导致碳酸盐岩中I/(Ca+Mg)值的增加, 同一地层剖面中同一层位样品的I/(Ca+Mg)最大值, 反映的是该层位局部海水的实际I O3-浓度以及[O2]的下限。

6 结语与展望

碳酸盐岩的I/(Ca+Mg)值, 是指示其沉积期海水氧化还原条件的重要指标。这个指标的开发和利用, 弥补了长期以来碳酸盐岩中指示氧化还原条件指标的不足, 显著拓展了古海洋氧化还原条件研究的方法。由于I O3-/I-具有类似于O2/H2O的高还原势能, 导致I/(Ca+Mg)指标对弱氧化条件下的氧含量变化十分敏感, 从而可广泛应用于浅海环境海水氧化还原条件的限定。目前, 针对I/(Ca+Mg)指标已初步建立了3个具有重要意义的临界值, 分别为0、1.5和2.5μ mol/mol, 它们的意义在于:

当I/(Ca+Mg)=0μ mol/mol时, 表明I O3-在海水中不能积累, 海水中的[O2]< 1~3 μ M。

当0< I/(Ca+Mg)< 1.5μ mol/mol时, 表明I O3-在海水中可以积累, 虽然[O2]> 1~3 μ M, 但很可能小于10μ M, 低于表层海水中初级生产力所能产生的最大氧浓度值。此时大气氧含量可能还比较低, 与海水交换尚未达到平衡。

当1.5< I/(Ca+Mg)< 2.5μ mol/mol时, 表明海水中[O2]> 10μ M, 超过了初级生产力所能产生的表层海水氧浓度最大值, 但同时也表明下伏或相邻水体中存在OMZ([O2]< 20~70μ M)。

当I/(Ca+Mg)> 2.5μ mol/mol时, 表明海水的氧化较充分, 下伏或相邻水体中不存在OMZ([O2]> 20~70μ M)。

应该说明, 这些临界值, 是依据现代海洋中缺氧盆地或OMZ内碘组分与氧含量的相关性研究以及方解石合成实验结果分析而提出的。由于地质历史时期海水物理化学条件可能与现代海洋存在显著差异(如碘库规模、海水氧化还原分层), 就有可能导致地质历史时期的这些临界值与现代海洋的状况存在一些偏差。因此, 将这些临界值应用于古代海水氧化还原条件的定量分析, 仍处于尝试阶段。对现代海洋碘氧化还原过程以及古代海洋碘组分的进一步深入研究, 将有可能使这些指标的应用更加可信。

作者声明没有竞争性利益冲突.

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