王宇航
, 朱园园, 黄建东
Wang Yuhang, Zhu Yuanyuan, Huang Jiandong
中图分类号: P736.4
文献标识码: A
文章编号: 1001-8166(2018)09-0922-11
通讯作者:
收稿日期: 2018-04-19
修回日期: 2018-07-28
网络出版日期: 2018-10-20
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Wang Yuhang(1994-), male, Huanggang City,Hubei Province, Master student. Research areas include carbonate geochemistry. E-mail:z1076614465@163.com
作者简介:王宇航(1994-),男,湖北黄冈人,硕士研究生,主要从事碳酸盐微量元素地球化学及其古环境应用研究. E-mail:z1076614465@163.com
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摘要
海相碳酸盐岩稀土元素(REE)配分模式和含量比值在古海洋环境研究中发挥着重要的作用。尽管海相碳酸盐岩REE主要来源于海水,但是也可能受到陆源碎屑输入和后期成岩改造的影响,因此需要对获得的REE可靠性进行全面评估。与此同时,选取合适的分析方法对获得可靠的碳酸盐相REE含量至关重要。对碳酸盐岩REE的地球化学性质、化学分析与数据处理方法、可行性验证及古环境应用方面进行总结与展望。在未来工作中,应着重于建立更加完善的分析方法,结合宏观地质背景、微观岩相学特征、其他地球化学指标以及同期沉积页岩的REE信息,开展海相碳酸盐岩REE在古海洋环境的全球表现形式研究。
关键词：
Abstract
The pattern, contents and ratios of Rare Earth Elements (REE) from marine carbonates play an important role in the paleo-environmental researches. As result of the REE's source being variable, which includes marine carbonates, detrital input and diagenesis, the overall assessment for the reliability of REE's data is necessary. Furthermore, appropriate analytical method is vital for the reliable contents of REE. This paper reviewed the geochemical properties, analytical and data processing methods, feasibility verification and paleo-environmental application of carbonates REE. The patterns of REE, which provide theoretical basis for the provenance and depositional environment of carbonates, are various with different sources. Cerium, as a redox sensitive element, is a key proxy for the reconstruction of paleo-redox conditions. There are two available analytical methods, acid-leaching method and direct LA-ICP-MS analytical method, to extract REE of seawater preserved in marine carbonate rocks. The contamination from detritus and diagenetic alteration can be detected by the correlations of various elements or element ratios. The REE of marine carbonate has been well applied to reconstruct the environment changes during the Precambrian, Permian-Triassic transition and Cenozoic.
Keywords：
沉积岩稀土元素(Rare Earth Element, REE)含量的变化与物源组成及沉积环境密切相联[1,2],其中碳酸盐岩中的自生沉积矿物能保存原始海水REE信息[3]。随着古海洋环境研究的不断发展,很多重建指标逐渐被开发和应用,学者们开始重视海水中REE的地球化学行为及其在古环境中的指示意义[4,5,6,7]。开展海相碳酸盐岩中REE特征的研究工作,可重建古海洋沉积环境,示踪沉积物源,并且对重建古海水氧化还原条件及其他气候条件有重要的意义[8,9]。本文系统总结了REE的地球化学原理、分析测试方法、数据处理方法及评估等相关内容,对当前开展海相碳酸盐岩REE研究中存在的问题及解决方法进行分析,并对REE在古海洋环境变化中的应用前景做了初步展望。
REE包括镧系元素以及与镧系元素密切相关的钪(Sc)等元素。钇(Y)也属于REE,因为它与钬(Ho)具有非常相似的地球化学特性。REE的离子半径和电价相近或相同,因此具有相似的地球化学行为,并且化学性质相对稳定,溶解度低[10]。它们的地球化学行为与沉积环境密切相关,因此可以用来示踪沉积环境[11,12,13,14,15]。
REE对水深、盐度以及氧逸度都非常敏感,不同类型沉积水体的REE特性存在系统偏差,这些差异在其REE配分模式上较易识别[16,17]。因而REE配分模式可以示踪海相碳酸盐岩中REE的不同来源,主要包括海水、河水或风携带的灰尘以及海相热液输入等。几种典型的REE配分模式(澳大利亚后太古宙平均页岩(Post-Archean Australian Shale,PAAS)标准化结果),如图1所示[18],由图1可以看出不同来源的稀土配分模式存在明显差异。
图1 典型自然环境和矿物中REE的PAAS标准化配分模式
Fig.1 PAAS-normalized REE patterns in typical natural environments and minerals
海相自生碳酸盐岩REE的PAAS标准化配分模式与现代海水类似:①富集重稀土元素(Heavy Rare Earth Element, HREE);②镧(La)正异常;③轻微的钆(Gd)正异常;④高Y/Ho比值(44~74)[19,20]。与海水不同,淡水碳酸盐呈现轻微的轻稀土元素(Light Rare Earth Element, LREE)亏损或富集,也可能存在中稀土元素(Middle Rare Earth Element, MREE)富集,无明显的元素异常,接近球粒陨石的Y/Ho比值(25~28)[21,22]。因此,自生碳酸盐REE配分模式特点可以有效判断其是否沉积于开放海水条件以及该碳酸盐稀土组成是否因后期成岩改造影响而发生改变[17,23]。
海水中的REE主要来自河水、风尘和海底热液等[24,25]。河水REE的PAAS标准化配分模式与海水存在明显差异,表现为统一轻微的LREE亏损和无明显元素异常[26,27]。该特点被广泛应用于区分海相碳酸盐岩与湖相碳酸盐岩[17]。但是,当淡水在入海口与海水混合相互作用后,REE则会迅速变为典型的海水REE配分模式[25]。
热液输入的REE相对于海水则表现出类似于次生流体的特征。前人研究发现还原性的酸性热液流体与海水的REE配分模式相差较大[28,29],表现出明显程度不一的铕(Eu)正异常,PAAS标准化后呈现轻微的MREE富集,HREE亏损以及具有更高的REE含量等特点[30]。Eu3+通常在极端还原高温环境下才会被还原成Eu2+,因此,Eu的正异常一般不代表海水的缺氧环境,而被认为是海水与热液流体混合的结果[31,32]。值得注意的是,明显的Eu异常也可能是测试过程中Ba的干扰造成的[18,33]。
海水中溶解态REE主要以三价离子形式存在。从LREE到HREE,其4f电子层填充电子数越来越多,使得它们在碳酸盐络合物中的含量逐渐上升[24,34],因而导致海水富集HREE。由于部分REE具有与其他REE不同的地球化学性质,因此当海水环境发生变化时,特定的REE会体现出一定的异常现象。现代海水中常常会呈现Y, La, Eu, Gd, 镥(Lu)的正异常以及铈 (Ce) 的负异常。
Y异常通常用Y/Ho表示。Y与Ho具有相似的地球化学性质,它们的地球化学行为通常保持一致,但在海水中Y与Ho的表面络合行为不同,由表层海水至深层海水的过程中,Ho的沉降速率是Y的2倍,导致海水中Y/Ho高于淡水,因而Y/Ho可以用来指示不同水体类型[35]。开放大洋会出现较强的Y异常(Y/Ho为40~80),近岸或特定背景下Y异常程度较低(Y/Ho为33~40)[36]。除盐度以外,化学风化、生物改造和氧化还原反应等过程引起的分馏作用也是引起Y/Ho变化的原因[37,38]。La,Gd和Lu的富集可能与它们的4f电子排布有关,电子轨道上空白、半充满或是全充满状态使其在海水中的溶解态相对稳定[23]。自然海水条件下Gd异常程度通常较小,但人为因素可能引起较大程度异常。
氧化还原条件是反映古海洋环境的重要指标之一,利用相关的替代性指标是指示古氧化还原环境的必要手段[39]。Ce是一种氧化还原敏感元素[40,41]。Ce主要以Ce3+和Ce4+存在,在氧化水体中,可溶的Ce3+会被氧化成Ce4+,而Ce4+不可溶且易吸附于细微颗粒物上[42],导致氧化性海水中普遍存在Ce的负异常,并且该信息被保存于海水的自生碳酸盐岩中;相反,吸附Ce4+的铁锰沉积物则显示Ce的正异常现象[43,44]。在缺氧的环境下,富Mn与Fe的沉淀在还原条件下溶解,Ce的负异常较弱[10]。因此,Ce异常是示踪氧化还原环境变化的重要指标[45,46,47]。Ce在海水中的滞留时间仅为几十年,远远短于海水的混合时间,因此海水中Ce含量及Ce异常的波动一般反映局部环境的变化情况[48,49]。
有效提取海相碳酸盐相态中的REE是开展该项研究的关键。由于非碳酸盐矿物在提取过程中会被部分溶解,进而对最终的分析结果造成干扰,因此在样品选取、预处理和化学提取过程中需小心谨慎[50],样品选取和预处理过程中可采取的措施包括:①选择新鲜的碳酸盐岩,避免碎屑岩;②正交偏光镜下观察微晶方解石和白云石无明显的重结晶作用以及变质作用;③小心挑除非碳酸盐矿物。
由于碳酸盐岩中REE含量比碎屑岩含量低很多,小部分碎屑REE成分的混入会极大地影响最终结果。因此,利用碳酸盐岩全岩REE指示海水的组成显然不太合适。为准确获取碳酸盐相态的REE信息,相关学者主要利用不同浓度的酸进行选择性提取[51],即利用不同浓度的硝酸、盐酸或者醋酸对样品进行溶解,然后利用ICP-MS测试其REE含量。Ellingboe等[52]利用不同浓度、不同种类的酸(盐酸、甲酸、乙酸)在不同条件(反应时间、温度)下分别对碳酸盐矿物进行溶解,发现10%的盐酸在室温条件下反应24 h能够有效分离碳酸盐矿物与非碳酸盐矿物。但是后来的研究工作和我们最近的研究表明,该提取方法会造成少量非碳酸盐矿物溶解,可能会影响最终数据结果[53,54]。Chen等[53]采用5%的硝酸对样品溶解2~3 h,然后分别测定其可溶相、残渣以及全岩中的REE含量,发现它们之间REE的PAAS标准化模式各不相同,同样说明非碳酸盐相矿物的混入对数据结果有明显影响。Rongemaille等[54]通过系统研究发现醋酸溶解提取的REE含量明显低于硝酸与盐酸处理的结果,表明硝酸与盐酸处理可能导致非碳酸盐矿物的溶解(图2)。最终结果显示体积百分比为5%的醋酸在室温下溶解24 h效果最好。虽然Rongemaille等[54]采用不同处理方法获取的REE配分模式相似,但是他们的研究对象是纯度较高的冷泉碳酸盐岩,因此相同的配分模式可能不具有代表性,不同种类和浓度酸对REE的PAAS配分模式的影响仍需开展进一步的研究工作。
图2 不同酸溶解碳酸盐岩碳酸盐矿物提取液REE页岩标准化模式(据参考文献[
Fig.2 Shale-normalized REE patterns of leachates from carbonate minerals in carbonate rocks treated by different acids (modified after reference[54])
为此,Zhang等[55]开展了更为系统的研究,他们分别选取5%和10%的醋酸对灰岩、钙质白云岩、硅质白云岩及纯白云岩等不同碳酸盐岩进行逐步溶解和分析,获取其REE含量和PAAS配分模式,结果显示5%醋酸提取效果优于10%醋酸,并发现次生碳酸盐相和吸附物会优先溶解,非碳酸盐矿物则最后溶解,进一步证明溶解过程中段得到的溶液PAAS配分模式更能够代表原始海水的信息,但此方法操作起来非常繁琐。
需特别说明的一点,前处理过程使用的器具本身会对REE数据结果产生影响,但关于此方面的研究较少。Lawrence等[27]处理时分别采用0.22 μm和0.45 μm孔径的滤膜,发现0.22 μm滤膜过滤出来的滤液,相较于0.45 μm滤膜,其REE含量低于5%~15%,并存在轻微的HREE富集现象,预示LREE优先于HREE被胶质物吸附,这与前人结果一致[56]。2种条件下的REE比值模式较为平缓,异常元素(La,Ce,Eu,Gd,Y/Ho,Lu)几乎没有变化,所受影响较小。此外,Lawerence等[27]还对比了溶液放置于离心管中14天的变化,其REE组成并无明显变化。
图3 0.22/0.45 μm过滤河水样品REE模式[
0.22/0.45 μm过滤代表过滤后样品相同元素含量之比
Fig.3 REE patterns for 0.22/0.45 μm filtered river samples[
0.22/0.45 μm filtered represent the ratios of same elements filtered samples
综上所述,在采用酸溶法提取碳酸盐相态REE时,低浓度的醋酸提取可有效避免非碳酸盐相成分的干扰,现有的结果表明5%的醋酸提取效果最佳,其缺点是可能溶解不完全,造成总稀土含量偏低,但是不会对元素比值和PAAS配分模式造成影响。硝酸、盐酸等虽然能够充分溶解样品,但其溶解的非碳酸盐相成分会对最终的结果造成较大的干扰。
LA-ICP-MS(激光剥蚀电感耦合等离子体质谱)法是分析碳酸盐岩样品REE含量的另外一种方法,与化学溶解不同,该方法可以直接对碳酸盐岩样品表面进行剥蚀然后测定元素含量。Chen等[57]比较了2种不同分析方法(LA-ICP-MS法及酸溶法)获取的碳酸盐矿物中REE信息,发现LA-ICP-MS测试的REE含量略高于酸溶法,但这2种方法得到REE的PAAS标准化配分模式几乎完全相同(图4),据此推断LA-ICP-MS分析碳酸盐矿物REE含量的方法可靠有效。该方法与酸溶法相比,省去了较为繁复的前处理过程,但是无法消除非碳酸盐矿物的影响。
图4 酸溶法与LA-ICP-MS法测定碳酸盐矿物REE页岩标准化模式(据参考文献[
Fig.4 Shale-normalized REE patterns of carbonate minerals analyzed by acid-leaching method and LA-ICP-MS method (modified after reference[57])
总而言之,如果研究对象(灰岩、白云岩等)是单一的纯海相自生碳酸盐矿物,我们可以利用稀硝酸、盐酸或醋酸对样品进行溶解并利用ICP-MS分析测试,或者也可以采用LA-ICP-MS法进行分析测试。然而自然界中纯碳酸盐岩是不存在的,因此作者认为,Zhang等[55]利用5%醋酸提取的分析方法在实际研究工作中最为适用,既能够准确获取高纯度碳酸盐岩中的REE信息,又适用于低纯度碳酸盐岩(泥灰岩、硅质白云岩等)中REE的提取。
沉积岩的REE配分模式一般通过PAAS进行标准化,以消除元素奇偶效应引起组成模式图的锯齿状变化,同时也可以展示其相对于标准物质的分异程度。采用其他的标准化方式,如北美页岩(North American Shale Composites, NASC),一般也不会改变整体的趋势以及关键的元素异常[42]。但是,当海水以NASC为标准时会显示Ho的正异常,这与其他的标准参考值不同。通常采用(La/Sm)N,(La/Yb)N和(Sm/Yb)N分别表示轻中稀土之比(LREE/MREE)、轻重稀土之比(LREE/HREE)、中重稀土之比(MREE/HREE),其中N代表PAAS标准化后结果。也有学者采用Er/Nd代表海水的LREE/HREE[58],正常海水中的Er/Nd约为0.27[59]。
海水中存在部分REE的异常,这些元素的异常值一般通过其相邻元素进行计算。由于La的相邻元素仅有Ce,因此计算La异常时一般选取Pr和Nd这2种元素,具体公式为La/La*=LaN/(3PrN-2NdN)。传统的Ce异常(Ce/Ce*)计算方法包括3CeN/(2LaN+NdN)[59,60]和2CeN/(LaN+PrN)[61]等。但是海水中存在La异常,采用La计算时可能会对Ce异常结果产生影响。因此Bau等[61]认为只有在Pr/Pr*>1.0的情况下Ce的负异常才被认为是可靠的,其中Pr/Pr*=2PrN/(CeN+NdN)(图5)。
图5 Ce/Ce*与Pr/Pr*相互关系图[
阴影部分代表现代海水范围
Fig.5 Corrections of Ce/Ce* and Pr/Pr* for marine carbonates[
The shaded area represents the range of modern seawater
某些异常元素的相邻元素在地质过程中可能出现富集或亏损现象,为避免该情况对元素异常值计算造成的影响,部分学者通过非异常相邻元素的几何平均算法计算REE的异常值,例如,Ce异常的计算公式Ce*=PrN×(PrN/NdN)[27]。目前使用普遍的计算方法如表1总结,但总体来说,这2种计算方法获得的结果差异小于5%[62]。
表1 REE常见参数计算公式
Table 1 Common methods of calculation for REE parameters
| 参数 | 传统计算方法 | 几何平均算法 |
|---|---|---|
| LREE/MREE | (La/Sm)N | |
| LREE/HREE | (La/Yb)N; (Nd/Yb | |
| MREE/HREE | (Sm/Yb)N | |
| La/La* | LaN/(3PrN-2NdN)[16] | LaN/[PrN×(PrN/NdN)2][27] |
| Ce/Ce* | 2CeN/(LaN+PrN) [61]; CeN/(2PrN-NdN)[6]; 3CeN/(2LaN+NdN) [45] | CeN/(P |
| Pr/Pr* | 2PrN/(CeN+NdN) [31] | |
| Eu/Eu* | 2EuN/(SmN+GdN)[27]; EuN/(0.67SmN+0.33TbN)[6]; EuN/(1.5SmN-0.5NdN)[27] | EuN/(S EuN/[SmN×(SmN/NdN)1/2][27] |
| Gd/Gd* | GdN/(2TbN-DyN)[6] GdN/(0.67TbN+0.33SmN) [3] | GdN/(T GdN/[TbN×(TbN/DyN)][27] |
| Lu/Lu* | LuN/(2YbN-TmN) [27] | LuN/(Y |
后期成岩作用是造成海相自生沉积碳酸盐的REE发生变化的原因之一[63,64],另外,分析测试过程中非碳酸盐相态成分的混入也会造成最终的分析结果发生变化[65]。因此,在利用REE组成和Ce异常开展古海洋环境研究工作时,必须充分探讨分析结果是否能够代表原始海水的信息。
REE在碳酸盐晶格中以类质同象的方式替代Ca2+[34],这种赋存状态使得它们在成岩过程相对稳定,并且成岩流体中∑REE非常低(1×10-12~100×10-12)[66]。Webb等[67]研究成岩蚀变对碳酸盐岩REE组成的影响时发现,即使成岩蚀变程度相对较强,其对碳酸盐岩中的REE组成和配分模式的影响较小。由此可以初步推断,后期成岩改造对碳酸盐相态REE组成影响很小[68,69,70]。
虽然在成岩过程中,碳酸盐矿物中REE并不会轻易随流体迁移[71],但是对于自生碳酸盐矿物而言,其中包含的原始海水REE信息仍然可能会发生改变。Shields等[63]发现沉积作用之后的成岩交换会慢慢使得自生碳酸盐或磷酸盐的REE组成配分模式发生改变,具体表现为Ce逐渐富集,Eu逐渐亏损和DyN/SmN相应减小。所以,当地质样品中Ce/Ce*与Eu/Eu*及DyN/SmN表现出负相关时,可能反映其REE模式已经受到成岩作用的影响。Bolhar等[17]认为如果碳酸盐中的REE信号未受到明显的成岩改造影响,页岩标准化后的Eu/Eu*与(Pr/Nd)N之间会呈现正相关性。此外,成岩流体改造可能会造成碳酸盐相态的∑REE上升[72]。部分学者认为成岩流体作用的影响不可忽视,而这种影响可以通过REE配分模式与主微量元素的对比等对之加以评估和限制[73]。例如,成岩流体作用通常导致Sr的亏损,Mn的富集和δ18O的降低等[74,75,76],所以Mn/Sr比和Rb/Sr比常常被用来判断成岩作用的强弱。因此,我们可以利用REE与反映成岩作用强弱的指标之间的相互关系进一步验证其是否受到后期成岩改造作用的影响。
总之,现有的研究表明成岩流体对海相碳酸盐相态REE含量及配分模式影响不显著,但仍不能排除。我们在开展碳酸盐相态REE研究工作时,仍需要讨论成岩流体对REE的影响。
非碳酸盐成分的混入是改变最终分析结果的主要因素,该部分物质可以分为3类:①陆源碎屑颗粒,其主要特征为∑REE明显高于碳酸盐相,并且其REE配分模式较为平缓,与碳酸盐相区别明显[25];②Fe,Mn氧化物[19],也具有较高∑REE,同时可能出现Ce的正异常以及Y的负异常;③磷酸盐矿物[77],同样具有高∑REE,通常P浓度与∑REE呈正相关性。这些干扰物质都表现出∑REE明显高于碳酸盐的特征,但是∑REE高并不一定代表非碳酸盐相物质的干扰。除此之外,相比碳酸盐岩,微生物岩的∑REE更高,并且不同类型的微生物岩的REE分配系数也可能不同[3]。
一般情况下,改变自生碳酸盐REE分布特征的主要影响因素是来自陆壳碎屑岩的干扰,特别是黏土矿物。因为陆源碎屑岩的REE含量明显高于碳酸盐矿物,并且REE配分模式与海相碳酸盐岩及海水差别显著。Nothdurft等[3]研究显示少量的碎屑岩混入就可以改变海相碳酸盐岩的整个REE配分模式。碎屑岩在沉积过程中会释放出REE,并迁入到次生碳酸盐矿物相中,在化学分析过程中随次生碳酸盐矿物溶解进入提取液中,此外其本身也可能存在部分溶解,从而对最终的分析结果产生影响。研究人员通常根据碳酸盐全岩中Al含量、溶解碳酸盐组分中Th和Sc含量,或这些元素含量与ΣREE之间的相关性来评估陆源碎屑岩的影响[9,67]。
除此之外,Zr,Fe和Th等元素在碎屑岩中的丰度也相对较高。例如,PAAS中的Zr元素含量(210×10-6)明显高于碳酸盐岩沉积岩,因此可以作为评估页岩干扰的一个指标。当样品数据受到页岩干扰时,它的Y/Ho与Zr含量呈现出负相关性;与此同时,Zr含量越高,REE总量也将越高,已有学者通过部分相关性指标,如(La/La*) vs. (Y/Ho)PAAS,(La/La*) vs. Zr和Nd/Yb vs. Zr等,指示碳酸盐相态REE是否受到页岩污染[62]。硫化物与氧化物中的REE信号与原始海水信号也存在显著差异,这些因素对碳酸盐样品的污染可以通过组分中相关元素与Y/Ho之间的相关性排除[17],例如,富集于氧化物中的Ni和Cu、富集于硫化物中的Pb和Sc。假若样品的REE信号受到这2个因素的污染,Y/Ho会与Pb以及Cu之间存在明显的相关性。
大量的研究表明海相碳酸盐相态REE组成及配分模式可以保留原始海水的信息,因此在古氧化还原环境重建中发挥着重要的作用[78,79]。目前,该指标已经被应用在前寒武纪[80,81,82],二叠纪—三叠纪之交[83,84,85]和新生代[67]的古海洋环境变化研究中。但是这些研究中所采用的提取方法各不相同,可能引起最终分析结果和结论的差异。并且,一些研究直接利用全岩的REE组成来代表原始海水的信息,这样得到的古环境信息显然存在问题,需进一步研究来校验。
因此,我们认为该项研究工作仍需进一步开展的研究包括以下几个方面:
第一,系统开展碳酸盐相态REE分析方法研究工作,建立可靠的分析方法,保证准确提取能够反映原始海水的信息。
第二,利用可靠的分析测试技术对关键地质时期的碳酸盐岩样品进行研究,重新审视古海洋环境的具体变化情况。
第三,由于某些REE异常(如Ce异常)指示的是区域性环境变化,因此有必要对全球不同地区同时期的碳酸盐岩开展对比研究,这样更能够揭示海洋环境的全球表现形式。
第四,在利用海相碳酸盐相态REE重建古海洋环境时,一方面需要紧密结合研究区域的地质背景(如沉积背景,构造背景等),获取宏观尺度上的环境变化信息。此外还需要综合参考其他地球化学指标,如风化指标和氧化还原指标等,这样可以获得更加全面和可靠的古环境信息。
第五,由于海相碳酸盐岩的REE特征代表古海水的信息,而同时期沉积页岩中REE的变化情况则反映海水中REE向沉积物中迁移的情况,因此研究同时期沉积页岩中的REE组成及其变化可以进一步佐证海相碳酸盐岩REE变化的可靠性。
The authors have declared that no competing interests exist.
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REE characteristics and its geological significance of the Permo-Carboniferous in Bohaiwan Basin [J].渤海湾盆地石炭二叠系稀土元素特征及其地质意义 [J].
采集了渤海湾盆地周缘露头剖面和盆内钻井岩心的泥岩及少量灰岩样品,采用电感耦合等离子质谱技术对样品进行了系统的稀土元素测试分析。测试样品稀土总量平均值为290.39μg/g,明显大于北美页岩组合样。各样品δEu为0.42～0.83,平均0.67,中度Eu负异常;大多样品Ce含量正常,δCe为0.51～1.22,平均0.97。稀土元素配分模式总体为轻稀土富集、重稀土亏损型,轻稀土段曲线明显"右倾",重稀土段则较为"平坦"。灰岩及部分泥岩中Ce的负异常指示了晚石炭世本区陆表海的沉积环境特点,古海水为氧化环境。根据稀土元素的Eu异常及总的元素组合特点,判断渤海湾盆地石炭—二叠系沉积物主要来自盆地北缘的阴山古陆,母岩类型为花岗岩。
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Distribution characteristics of rare earth elements in surface sediment and their provenance discrimination in the Eastern Bohai and Northern Yellow Seas [J].渤海东部和黄海北部表层沉积物稀土元素的分布特征与物源判别 [J].
为了探讨渤海东部和黄海北部稀土元素分布特征及其影响因素,对渤海东部和黄海北部138个表层沉积物样品进行电感耦合等离子体质谱法分析.结果表明:渤海东部和黄海北部沉积物的配分模式与中国黄土的接近,与周边的鸭绿江、黄河等河流输入物质也有相似性,表明研究区沉积物主要来自于周边大陆.沉积物物源判别结果显示:山东半岛近岸及其西南部沉积物主要来源于黄河,该区域沉积物分布受山东半岛沿岸流及近岸潮流影响;研究区东部沉积物稀土元素组成特征与鸭绿江沉积物组成相近,表明研究区东部沉积物可能由鸭绿江供应;研究区西北部沉积物主要来源于黄河及辽宁沿岸物质输入,局部有滦河、六股河物质存在;研究区东南部物质主要来源于黄河.对沉积物稀土元素百分含量及参数特征值进行了Q型聚类分析,将研究海域沉积物划分为4个不同的区域,分别为北黄海东部区域(Ⅰ)、山东半岛近岸周边区域(Ⅱ)、渤海东部中部区域(Ⅲ)及研究区的西北部与东南部分布区(Ⅳ).
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Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: Confirmation of a seawater REE proxy in ancient limestones [J]. |
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Characteristics of rare earth elements in the sediments from the middle Okinawa Trough and its application in the paleoenvironment evolution [J].冲绳海槽中部沉积物稀土元素地球化学特征及其在古环境变化研究的应用 [J].
冲绳海槽巨厚的沉积物蕴藏着丰富的、长时间尺度的区域古海洋和古环境演化地质信息。对采自海槽中部的A7孔柱状沉积物进行了高分辨率的稀土元素、碳酸盐、常量元素组成的测试与分析,以深入探讨沉积物稀土元素组成对古环境变化的响应问题。结果表明,A7孔沉积物在球粒陨石标准化的稀土元素配分形式上与典型上陆壳物质非常相似,而与富火山灰层和小型浊流沉积存在明显差异。通过分析,认为冲绳海槽沉积物REEs组成主要是受陆源碎屑组分所控制的,与黏土组分含量相关性较大,但与沉积物的Fe-Mn组分含量关系较弱,同时受到生源组分的"稀释作用",其REEs组成可以用于反映陆源物质来源及其输入过程的研究。排除火山灰和浊流沉积物质的影响,A7孔沉积物的REEs含量(∑REE,LREE,HREE),LREE/HREE,(La/Yb)n,Eu/Eu~*等参数的垂向变化曲线在175 cm层位表现出明显的差异。这一层位的沉积年龄为11.2 cal.ka.BP,正好是冰后期和末次冰消期的转换期(LGIT),表明在LGIT期间海槽内的沉积环境和陆源物质的来源发生了较大改变,而造成这种变化的最可能直接因素是东海海平面的快速上升(WMP-1B事件)造成的沉积格局改变,但与黑潮"摆动"和H1,YD事件等古气候与古海水因素的关系并不显著。研究结果证实,沉积物稀土元素组成及其特征参数的变化可作为研究长时间尺度(冰期—冰间期)冲绳海槽古环境变化的有效指标。
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Cerium negative anomaly just before the Permian and Triassic boundary event—The upward expansion of anoxia in the water column [J]. |
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Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator [J].
A first study of REE + Y distribution in a variety of Neoproterozoic (Cryogenian and Ediacaran) carbonates from different settings in the Saldania, Gariep, Damara and West Congo Belts in southwestern and central Africa revealed systematic differences that can be explained by varying palaeoenvironmental factors. The majority of samples display relatively unfractionated, flat shale-normalised REE + Y patterns that cannot be ascribed solely to shale contamination but are interpreted as resulting from the incorporation of near-shore colloids, possibly related to Fe-oxihydroxide scavenging. Only few carbonate units yielded trace element distributions that conform to a typical seawater composition. Those carbonates that were affected by stratiform, syn-sedimentary hydrothermal mineralisation are distinguished by Eu anomalies. Considering the similarity in residence time between REE and carbon, the strong influence of river-born particles on the REE + Y distribution in the analysed carbonates casts considerable doubt over the usefulness of these carbonates for stratigraphic correlation of Neoproterozoic sediment successions based on carbon isotopes.
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Oceanic environmental changes on a shallow carbonate platform (Yangou, Jiangxi Province, South China) during the Permian-Triassic transition: Evidence from rare earth elements in conodont bioapatite [J]. |
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REE geochemical characteristics of carbonate rocks in Southern Qinling, Shaanxi Province and their Paleo-Oceanologic significance [J].陕西南岭碳酸盐岩的稀土元素特征及其古海洋学意义 [J]. |
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Cerium anomaly variations in Ediacaran-earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater [J]. |
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Geochemical characteristics of REE in limestone of Jiudingshan Formation,Northern Anhui Province and their constraint on the seawater [J].皖北九顶山组灰岩稀土元素地球化学特征及对古海水的制约 [J].
采集皖北新元古代九顶山组灰岩进行了系统的岩石学和稀土地球化学分析,研究了灰岩中稀土元素 的来源,并对海水的稀土配分模式进行反演,结果表明：灰岩中稀土元素主要受控于古海洋,基本不受陆源物质影响;灰岩样品中稀土含量很低,平均为 8.54×10-6,轻重稀土分异明显,为正常浅海沉积;海水稀土配分模式与灰岩相似,为重稀土亏损,灰岩和海水的Y/Ho、Y/Dy值呈正相关;海水具 有较高的Y/Ho比、Ce负异常,且海水的δEuNASC值小于1,表明了新元古代海水与现代海水的稀土元素特征基本相似。
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| [11] |
REE geochemistry of the Upper Jurassic-Lower Cretaceous carbonate rocks and palaeoclimates in the Nadigangri region,Qiangtang Basin [J].羌塘盆地那底岗日地区上侏罗统—下白垩统碳酸盐岩稀土元素地球化学与古气候 [J]. |
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Rare earth element complexation by carbonate and oxalate ions [J].
Rare earth carbonate and oxalate complexation constants have been determined through ex-amination of distribution equilibria between tributyl phosphate and an aqueous perchlorate phase. Carbonate complexation constants appropriate to the REE in seawater (25°C, 35%., 1 atm) can be described in terms of atomic number, Z. nlog sw β 1 = 4.853 + 0.1135( Z 61 57) 61 0.003643( Z 61 57) 2log sw β 2 = 80.197 + 0.1730( Z 61 57) 61 0.002714( Z 6157) 2 where swβ 1 = [MCO + 3] [M 3+][CO 261 3] T, swβ 2 = [M(CO 3) 61 3] [M 3+][CO 261 3] 2' T [ M 3+] is an uncomplexed rare earth concentration in seawater, [ MCO + 3] and [ M( CO 61 3) 2] are carbonate complex concentrations, and [CO 261 3] T is the total (free plus ion paired) carbonate ion concentration in seawater (molal scale). Our analyses indicate that in seawater with a total carbonate ion concentration of 1.39 × 10 614 moles/Kg H 2O, carbonate complexes for the lightest rare earth, La, constitute 86% of the total metal, 7% is free La 3+ and the remaining 7% exists as hydroxide, sulfate, chloride and fluoride complexes. For Lu, the heaviest rare earth, carbonate complexes are 98% of the total metal, 0.3% is uncomplexed and 1.5% is complexed with hydroxide, sulfate, chloride and fluoride. Oxalate and carbonate constants are linearly correlated. This correlation appears to be quite useful for estimating trivalent metal-arbonate stability constants from their respective oxalate stability constants.
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| [13] |
Complexation of trivalent rare earth elements (Ce, Eu, Gd, Tb, Yb) by carbonate ions [J].
Carbonate stability constants for five rare earth elements (Ce , Eu , Gd , Tb , and Yb ) have been determined at t = 25° C and 0.70 ± 0.02 M ionic strength through solvent exchange techniques. Estimated stability constants for Ce, Eu, and Yb are in close agreement with previous work. Analyses using Gd and Tb provide the first carbonate stability constants for these elements based on direct measurements. Our measured stability constants were used to estimate carbonate stability constants for the entire suite of REEs. Our Eu, Gd, and Tb carbonate stability constants demonstrate the existence of a "Gd-break": Carbonate stability constants for Gd are smaller than those for Eu and Tb. In analogy to Gd concentration anomalies reported in field observations, Gd stability constant anomalies have been defined in terms of the difference log β (Gd) - log { ( β (Eu) + β (Tb)) /2} , where β( M) = [ ML][ M] [ L] . Examinations of REE-organic stability constants demonstrate that 106 out of 125 organic ligands have negative Gd anomalies in their first stability constants. The magnitudes of negative Gd anomalies generally become greater with increasing magnitude in Gd-ligand stability constants. Field observations of positive anomalies in shale-normalized Gd concentrations can be explained in terms of REE scavenging by organic surface ligands, such as polyaminocarboxylic acids, which possess a more negative Gd anomaly than carbonate ligands. Our modeling efforts indicate that a mixture of strongly complexing organics such as polyaminocarboxylic acids, and weakly complexing organics such as mono- and dicarboxylic acids is consistent with the pattern of REE scavenging by marine particulate matter.
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| [14] |
Competition between humic acid and carbonates for rare earth elements complexation [J].
Competition between humic acid (HA) and carbonates (Carb) for rare earth elements (REE) complexation.
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| [15] |
Chemical environment of cold seep carbonate formation on the northern continental slope of South China Sea: Evidence from trace and rare earth element geochemistry [J]. |
| [16] |
Characterisation of early Archaean chemical sediments by trace element signatures [J].
Rare earth element (REE) plus yttrium (Y) patterns of modern seawater have characteristic features that can be used as chemical fingerprints. Reliable proxies for marine REE+Y chemistry have been demonstrated from a large geological time span, including Archaean banded iron formation (BIF), stromatolitic limestone, Phanerozoic reef carbonate and Holocene microbialite. Here we present new REE+Y data for two distinct suites of early Archaean (ca. 3.7鈥3.8 Ga) metamorphosed rocks from southern West Greenland, whose interrelationships, if any, have been much debated in recent literature. The first suite comprises magnetite-quartz BIF, magnetite-carbonate BIF and banded magnetite-rich quartz rock, mostly from the Isua Greenstone Belt (IGB). The REE+Y patterns, particularly diagnostic anomalies (Ce/Ce*, Pr/Pr*), are closely related to those of published seawater proxies. The second suite includes banded quartz-pyroxene-amphibole卤garnet rocks with minor magnetite from the so-called Akilia Association enclaves (in early Archaean granitoid gneisses) of the coastal region, some 150 km southwest of the IGB. Rocks of this type from one much publicised and highly debated locality (the island of Akilia) have been identified by some workers [Nature 384 (1996) 55; Geochim. Cosmochim. Acta 61 (1997) 2475] as BIF-facies, and their 13C-depleted signature in trace graphite interpreted as a proxy for earliest life on Earth. However, REE+Y patterns of the Akilia Association suite (except for one probably genuine magnetite-rich BIF from Ugpik) are inconsistent with a seawater origin. We agree with published geological and geochemical (including REE) work [Science 296 (2002) 1448] that most of the analysed Akilia rocks are not chemical sediments, and that C-isotopes in such rocks therefore cannot be used as biological proxies. Application of the REE+Y discriminant for the above two rock suites has been facilitated in this study by the use of MC-ICP technique which yields a more complete and precise REE+Y spectrum than was available in many previous studies.
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| [17] |
Van Kranendonk M J. A non-marine depositional setting for the northern Fortescue Group, Pilbara Craton, inferred from trace element geochemistry of stromatolitic carbonates [J]. |
| [18] |
Effective use of cerium anomalies as a redox proxy in carbonate-dominated marine settings [J].
Rare earth elements and yttrium (REY) have a distinct distribution pattern in seawater, and this pattern may be faithfully preserved in carbonate sediments and rocks. Anomalous concentrations of redox-sensitive cerium (Ce) compared with neighbouring REY originate in oxic water column conditions, and as such, Ce anomalies can provide a potentially useful redox proxy in carbonate-dominated marine settings. The methods used to extract REY from carbonates vary widely, and may suffer from widespread leaching of REY from accessory non-carbonate minerals and organic matter, limiting the application of Ce anomalies for palaeo-redox reconstruction. We have systematically compared different methods on 195 carbonate samples with varying purity (% carbonate) from both modern and ancient environments. We used sequential leaching experiments in nitric acid to identify the most eawater-like portion of the carbonate sample where contributions from non-carbonate minerals and organic matter are minimised. We also compared the results of sample dissolution in different types and strengths of acid. Our results confirm that REY concentrations can be inadvertently contaminated by partial leaching of clays and Fe (oxyhydr)oxides during a single-step digestion, and we suggest a pre-leach of 20% of the sample, followed by a partial leach of 40% of the sample to selectively dissolve carbonate. We suggest that REY studies are optimised in carbonates with >85% CaCO 3 , and show that dolomites behave differently during the leaching process and must be treated separately. We present REY patterns for modern carbonate-rich sediments from a range of environments, and show that seawater REY are faithfully preserved in some non-skeletal carbonate, but modified leaching procedures are necessary for impure, unlithified or organic rich carbonate sediments. We combine REY with Fe-speciation data to identify how Fe oxides and clays contribute to the REY signal and explore how the two proxies can be used together to provide a complex and high-resolution redox reconstruction in carbonate-dominated marine environments.
|
| [19] |
Comparison of the partitioning behaviours of yttrium, rare earth elements, and titanium between hydrogenetic marine ferromanganese crusts and seawater [J]. |
| [20] |
Has the REE composition of seawater changed over geological time? [J].
Rare earth element (REE) contents of marine biogenic apatites have been shown to record seawater compositions. A data base of available and newly acquired rare earth element (REE) contents of marine biogenic apatites has been created to assess the past seawater REE compositions. To ensure that this data base contains only the pristine REE signals, altered samples, characterized by very low (La/Sm)(N) ratios (where N stands for REE ratios normalized to the NASC composition; Gromet et al., 1984, Geochim. Cosmochim. Acta 48 (1984) 2469) acquired during apatite recrystallization, are not included in the database. These data reveal a change in the Tethyan seawater composition between 110 and 80 Ma. After the end of the Cretaceous, the REE chemistry of seawater remains constant until present-day. This seawater composition change is likely due to concurrent changes in REE scavenging processes. A strong correlation between decreasing (Sm/Yb)(N), ratios and increasing Cc anomalies for samples deposited during the 80-110 Ma period is observed. As Cc anomalies are attributed to ocean water redox conditions, changes in REE scavenging could reflect an evolution from stratified and poorly oxygenated waters towards well-mixed and oxygenated waters. This could have resulted from changing current patterns stemming from the contemporaneous opening of the Atlantic Ocean. A major change in Middle and Late Cretaceous oceanic circulation linked to plate tectonics would have favored the colonization of pelagic environments. (C) 2003 Elsevier B.V. All rights reserved.
|
| [21] |
Rare earth elements and yttrium in seawater: ICP-MS determinations in the East Caroline, Coral Sea, and South Fiji basins of the western South Pacific Ocean [J]. |
| [22] |
Sources of dissolved REE in mountainous streams draining granitic rocks, Sierras Pampeanas (Córdoba, Argentina) [J]. |
| [23] |
On the reliability of stable carbon isotopes for Neoproterozoic chemostratigraphic correlation [J].
The reliability of 13 C trends in Neoproterozoic carbonate-dominated successions for regional and global chemostratigraphic correlation is discussed. In the light of recent findings of a predominantly non-marine rare earth element and yttrium signature in most Neoproterozoic carbonates and a comparatively short oceanic residence time of carbon, trends towards enrichment in 13 C seen in many of these carbonates are considered to reflect facies variations rather than temporal signals of ocean chemistry. Positive 13 C Carb excursions are explained by elevated bioproductivity and/or increased evaporation in shallow marine, near-coastal, temporarily restricted depositional environments. Examples are provided that illustrate that C isotope trends can be highly ambiguous temporal markers and are in the absence of other chemostratigraphic data, such as Sr isotope ratios, and radiometric age control of only limited use for stratigraphic correlation. The overall enrichment in 13 C recorded by most Neoproterozoic carbonates, except for those in close stratigraphic proximity to glacial deposits, is suggested to reflect a dominance of microbially mediated carbonate formation in the Neoproterozoic. This might explain why C isotope chemostratigraphy in Neoproterozoic successions is less reliable than in Phanerozoic successions in which carbonates are, with only few exceptions, biogenic products of shelly fossils.
|
| [24] |
Rare earth element scavenging in seawater [J].
Examinations of rare earth element (REE) adsorption in seawater, using a variety of surface-types, indicated that, for most surfaces, light rare earth elements (LREEs) are preferentially adsorbed compared to the heavy rare earths (HREEs). Exceptions to this behavior were observed only for silica phases (glass surfaces, acid-cleaned diatomaceous earth, and synthetic SiO 2 ). The affinity of the rare earths for surfaces can be strongly affected by thin organic coatings. Glass surfaces which acquired an organic coating through immersion in Tampa Bay exhibited adsorptive behavior typical of organic-rich, rather than glass, surfaces. Models of rare earth distributions between seawater and carboxylate-rich surfaces indicate that scavenging processes which involve such surfaces should exhibit a strong dependence on pH and carbonate complexation. Scavenging models involving carboxylate surfaces produce relative REE abundance patterns in good general agreement with observed shale-normalized REE abundances in seawater. Scavenging by carboxylate-rich surfaces should produce HREE enrichments in seawater relative to the LREEs and may produce enrichments of lanthanum relative to its immediate trivalent neighbors. Due to the origin of distribution coefficients as a difference between REE solution complexation (which increases strongly with atomic number) and surface complexation (which apparently also increases with atomic number) the relative solution abundance patterns of the REEs produced by scavenging reactions can be quite complex.
|
| [25] |
The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters [J]. |
| [26] |
Rare earth elements in river waters [J].
We measured rare earth element (REE) concentrations in river waters to characterize the suspended and dissolved river flux of the REE to the oceans. The REE pattern of river suspended materials is sensitive to drainage basin geology. A positive correlation is observed between La/Yb ratios and Nd model ages for the rivers studied. Major rivers have light REE enriched patterns relative to the North American Shale Composite (NASC), with (La/Yb) N = 1.6612.7. River water dissolved material ( < 0.2 μm) is heavy REE enriched relative to suspended material, and the most pronounced negative Ce anomalies occur in rivers of high pH. Light REE concentrations vary by approximately 3 orders of magnitude and are inversely related to pH and major cation concentrations. From these data, we estimate that typical major river runoff has heavy REE depleted suspended material with (La/Yb) N ≈ 1.9. We conclude that the terrigenous input to the oceans from major rivers is heavy REE depleted relative to shales. From the available data, average river water dissolved material appears to be heavy REE enriched with (La/Yb) N ≈ 0.4. Estuarine removal processes lower the dissolved REE river flux by approximately 60% and result in a flux that is more heavy REE enriched with (La/Yb) N ≈ 0.2. Calculated oceanic residence times with respect to river input range from 2300 to 21,000 years, are shortest for Ce, and greatest for the heavy REE and La. Such long residence times may suggest the presence of additional sources of REE in seawater.
|
| [27] |
Rare earth element and yttrium variability in South East Queensland waterways [J].DOI URL Magsci [本文引用: 14] 摘要
We present data for the rare earth elements and yttrium (REY) in the National Research Council of Canada natural river water reference material SLRS-4 and 19 natural river waters from small catchments in South-East Queensland, Australia, by a direct ICP-MS method. The 0.2202μm filtered river water samples show a large degree of variability in both the REY concentration, e.g., La varies from 13 to 115702ppt, and shape of the alluvial-sediment-normalised REY patterns with different samples displaying light, middle or heavy rare earth enrichment. In addition, a spatial study was undertaken along the freshwater section of Beerburrum Creek, which demonstrates that ~75% of the total REYs in this waterway are removed prior to estuarine mixing without evidence of fractionation.
|
| [28] |
Comparing yttrium and rare earths in hydrothermal fluids from the Mid-Atlantic Ridge: Implications for Y and REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater [J]. |
| [29] |
Trace element and REE composition of a low-temperature ridge-flank hydrothermal spring [J].
Warm (25°C) hydrothermal springs have been sampled on Baby Bare, a basaltic outcrop on 3.5-Ma-old crust 65100-km east of the Endeavor Segment of the Juan de Fuca Ridge. The source for these springs is a 62 to 64°C formation water that has cooled conductively as it ascends to feed the springs. This water originated as bottom seawater that probably descended into basement 6552 km to the southwest at another, much larger outcrop called Grizzly Bare. As this seawater flows towards Baby Bare, it is heated and altered by reactions within basaltic basement and by diffusive fluxes to and from the overlying sediment. Concentrations of Mn, Co, Ni, Zn, Cd, and Mo in the spring waters are greater than in bottom seawater, indicating that the oceanic crust is a source for these elements to the oceans. At least a portion of this increase probably results from the redox cycling of Mn in sedimentary sources near the basement interface that produces a diffusive flux to basement formation waters. Additional removal of Mo and inputs of the other five elements to two of the three springs are observed locally near sites of venting, where density gradients can form shallow circulation cells within the sediment and diffusive exchange occurs. Concentrations of Cu, U, V, Y, and the rare earth elements (REEs, excluding Ce) in these samples are less than in bottom seawater, indicating that the oceanic crust is a net sink for these elements in this environment. Copper is probably removed into newly formed carbonate and/or sulfide phases. Removal of the oxyanions U and V is consistent with a net removal of phosphate demonstrated previously for ridge-flank hydrothermal systems. Similarly, removal of Y and the REEs is associated with carbonate, phosphate-rich, and oxide phases. Calculated maximum global chemical fluxes from “warm” ridge-flank hydrothermal systems such as Baby Bare are insignificant relative to riverine fluxes for these elements, except possibly for Mn and Mo. The impact on global geochemical budgets for these elements from lower temperature (<25°C) alteration on ridge flanks is still unknown, but it may well be larger than for warm ridge flanks.
|
| [30] |
Yttrium and rare earth elements in fluids from various deep-sea hydrothermal systems [J].
Rare earth element (REE) and yttrium (Y) concentrations were measured in fluids collected from deep-sea hydrothermal systems including the Mid-Atlantic Ridge (MAR), i.e., Menez Gwen, Lucky Strike, TAG, and Snakepit; the East Pacific Rise (EPR), i.e., 13°N and 17–19°S; and the Lau (Vai Lili) and Manus (Vienna Woods, PacManus, Desmos) Back-Arc Basins (BAB) in the South-West Pacific. In most fluids, Y is trivalent and behaves like Ho. Chondrite normalized Y-REE (Y-REE N ) concentrations of fluids from MAR, EPR, and two BAB sites, i.e., Vai Lili and Vienna Woods, showed common patterns with LREE enrichment and positive Eu anomalies. REE analysis of plagioclase collected at Lucky Strike strengthens the idea that fluid REE contents, are controlled by plagioclase phenocrysts. Other processes, however, such as REE complexation by ligands (Cl 61 , F 61 SO 4 261 ), secondary phase precipitation, and phase separation modify REE distributions in deep-sea hydrothermal fluids. REE speciation calculations suggest that aqueous REE are mainly complexed by Cl 61 ions in hot acidic fluids from deep-sea hydrothermal systems. REE concentrations in the fluid phases are, therefore, influenced by temperature, pH, and duration of rock-fluid interaction. Unusual Y-REE N patterns found in the PacManus fluids are characterized by depleted LREE and a positive Eu anomaly. The Demos fluid sample shows a flat Y-REE N pattern, which increases regularly from LREE to HREE with no Eu anomaly. These Manus Basin fluids also have an unusual major element chemistry with relatively high Mg, SO 4 , H 2 S, and F contents, which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation. REE distribution in PacManus fluids may stem from a subseafloor barite precipitation and the REE in Demos fluids are likely influenced by the presence of sulfate ions.
|
| [31] |
Trace and rare earth elemental investigation of a Sturtian cap carbonate, Pocatello, Idaho: Evidence for ocean redox conditions before and during carbonate deposition[J]. [J]. |
| [32] |
Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China [J]. |
| [33] |
Avoidance of spectral interference on europium in inductively coupled plasma mass spectrometry by sensitive measurement of the doubly charged ion [J]. |
| [34] |
Partitioning of Rare Earth Elements (REEs) between calcite and seawater solutions at 25 ℃ and 1 atm, and high dissolved REE concentrations [J]. |
| [35] |
The fractionation between Y and Ho in the marine environment [J].
New measurements of Y and Ho in seawater, rivers and rain are presented. Based on the data and a two-box model calculation, we suggest that fractionation between Y and Ho takes place during their removal by particulate matter from the surface ocean to the deep sea. The fractionation factor, K is calculated to be 0.50, implying that Ho is scavenged two times faster than Y. This presumably occurs due to differences between Y and Ho complexation behavior with respect to seawater inorganic ligands (mainly carbonate ions) and soft organic ligands (though unspecified) of the surface of particulate matter. Fractionation of Y and Ho during weathering and fluvial transport to the ocean appears to have minor influence on the relative abundance of Y and Ho in seawater. We also estimated the mean oceanic residence time to be 5100 years for Y and 2700 years for Ho. Y is less effectively scavenged from seawater than any of the trivalent rare earth elements and theY/Ho ratio in seawater is higher than those in rain, rivers and estuarine waters.
|
| [36] |
Rare earth elements in the Pacific and Atlantic Oceans [J].
The first profiles of Pr, Tb, Ho, Tm and Lu in the Pacific Ocean, as well as profiles of La, Ce, Nd, Sm, Eu, Gd and Yb, are reported. Concentrations of REE (except Ce) in the deep water are two to three times higher than those observed in the deep Atlantic Ocean. Surface water concentrations are typically lower than in the Atlantic Ocean, especially for the heavier elements Ho. Tm, Yb and Lu. Cerium is strongly depleted in the Pacific water column, but less so in the oxygen minimum zone. The distribution of the REE group is consistent with two simultaneous processes: 1. (1) cycling similar to that of opal and calcium carbonate 2. (2) adsorptive scavenging by settling particles and possibly by uptake at ocean boundaries. However, the first process can probably not be sustained by the low REE contents of shells, unless additional adsorption on surfaces is invoked. The second process, adsorptive scavenging, largely controls the oceanic distribution and typical seawater pattern of the rare earths.
|
| [37] |
Rare earth and yttrium phosphate solubilities in aqueous solution [J].
Rare earth and yttrium phosphate solubility products range over more than 1 order of magnitude. Minimum solubilities are observed for light rare earths between Ce and Sm. For the elements Ce, Pr, Nd, and Sm solubility products (log K( M) = log ([ M] [PO ])) at zero ionic strength and 25°C can be approximated as log K,( M) = -26.3 ± 0.2. Rare earth phosphate solubility products for well-aged, coarse precipitates increase substantially between Sm and Lu, with log K(Lu) estimated as -24.7. The solubility product of Y is similar to that of Ho (log K(Y) = -25.0) and is much higher than those of all light rare earths. The solubility product of La is substantially larger than that of Cc (log K(La) - log K(Ce) ≈ 0.5). Solubility products are strongly dependent on the conditions of solid phase formation. Fresh precipitates are much more soluble than slowly formed, well-aged, coarse precipitates. The pattern of rare earth and yttrium phosphate solubility products is generally similar to the fractionation patterns which are developed during phosphate coprecipitation.
|
| [38] |
Yttrium: The immobility-mobility transition during basaltic weathering [J].
Examines the yttrium immobility-mobility transition during basaltic weathering. Use of yttrium as tectonic and geochemical discrimination diagrams for igneous rocks; Details of the assemblage evolution during lateritization; Comparison between major and individual trace element variation during lateritization.
|
| [39] |
Research advances in sedimentary records of coastal bottom-water Hypoxia [J].近岸底层水体低氧沉积记录研究进展 [J].
长时间尺度的沉积记录可以提升对近岸低氧形成机制的理解,从而为预报、预测和发展减缓低氧措施等提供必要的依据。综述了近岸低氧发育与演化历史的主要研究进展,尤其侧重于沉积记录的研究。首先按水体受限程度将近岸低氧发育环境划分为半封闭型海盆/海湾和开阔陆架海2类,分别探讨了二者低氧发育的主要特征。其次分析总结了低氧沉积记录的替代性指标,包括对水体氧化还原环境具有较好指示意义的生物学、矿物学和地球化学指标,着重分析了各类替代性指标的适用性。最后对长江口外海域底层水体低氧发生机制与沉积记录的研究现状作了回顾与分析。目前长时间尺度的低氧沉积记录研究仍然较少,鉴于长江口外海域低氧的动态发育特征,提出了多钻孔、多参数结合的研究方案。
|
| [40] |
Solution chemistry of the rare earth elements in seawater [J]. |
| [41] |
Secular change in northwestern Tethyan water-mass oxygenation during the late Hauterivian-early Aptian [J]. |
| [42] |
Rare earth elements in seawater: Particle association, shale-normalization, and Ce oxidation [J].
Dissolved (<0.04 渭m, not <0.4 m) and total acid-soluble concentrations of rare earth elements (REEs) and yttrium were measured by using ICP mass spectrometry in the seawaters obtained from various depths in the western North Pacific near Japan. The difference, i.e., acid-soluble particulate fraction, was found to be small, 2-5% for all tri-valent light and middle REEs and less than 1% for heavy REEs and yttrium. The high particulate fraction of 31% for Ce is consistent with its predicted oxidation state of tetra-valence. Elevated particulate fraction of all REEs was found within 80 m above the bottom due to contribution of flocculated resuspended particles. The vertical profiles of REE(III)s show smoothly increasing convex curves with depth similar to those reported previously. Dissolved Ce concentration decreases from 6 pmol/kg near the surface to a minimum at 2.5 pmol/kg around 400 m where the North Pacific Intermediate Water penetrates, and then approaches to nearly constant value of 4 pmol/kg below 800 m. Particulate Ce concentration significantly increases from the surface to 200 m depth suggesting oxidation of Ce(III) to Ce(IV) and subsequent scavenging in the upper water column. However, there is no evidence in our data showing that Ce oxidation is continuously taking place even in the deep sea. Shale-normalized patterns of dissolved REEs were examined in detail, based on three datasets of composite shales available in the literature. Distinctively positive La and only slightly positive Gd anomalies were identified together with well-documented negative Ce-anomaly as common features of seawater. These anomalies systematically change with depth. Rapid changes occur in the upper several hundred meters suggesting that their distributions are largely governed by ocean circulation and biogeochemical cycling.
|
| [43] |
Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments [J]. |
| [44] |
Changing redox conditions at cold seeps as revealed by authigenic carbonates from Alaminos Canyon, northern Gulf of Mexico [J]. |
| [45] |
Geochemical indices for redox conditions of marine sediments [J].海相沉积氧化还原环境的地球化学识别指标 [J].
全球海洋在10～5．4亿年间演变成氧化环境，此后历经多次全球性的缺氧事件后演变到现在的氧化环境。海水和沉积物中多种元素的循环、分异和富集明显受氧化还原条件的影响。Mn、Mo、Cr、V和u等变价元素的溶解度随氧化还原条件的改变产生极大变化，导致沉积物中的元素含量分异；Ni、Co、Cu和Zn等在还原条件下形成硫化物沉淀，导致沉积物中对应元素的富集。这些元素的地球化学行为是古海洋氧化还原条件变化的灵敏指示剂，可以作为恢复古海洋氧化还原环境变化的地球化学指标。黄铁矿化程度（DOP）、生物标志化合物和Ce异常等也是沉积环境氧化还原条件的常用判别指标。泥岩研究中通常采用DOP、U/Th、自生U、V/Cr、Ni/Co和生物标志化合物等指标，碳酸盐岩则主要采用Ce异常指标。当前各种指标的定性分析都取得比较一致的结果，但是用一种或几种定量的地球化学指标来恢复整个古海洋的氧化还原环境目前还有很大的问题。
|
| [46] |
Trace element compositions in conodont phosphates responses to biotic extinction event:A case study for main act of global boundary Stratotype Section and point of the Permian-Triassic [J].牙形石微量元素对生物绝灭事件的响应:以二叠—三叠系全球层型剖面第一幕绝灭事件为例 [J]. |
| [47] |
Differences between the Upper and Lower Kellwasser events and productivity variations during the Late Devonian Frasnian-Famennian transition in Yangdi of Guilin,South China [J].广西桂林杨堤晚泥盆世弗拉期—法门期之交上、下Kellwasser事件的差异与海洋生产力变化 [J]. |
| [48] |
Rare earth elements and their isotopes in the ocean [J].
The rare earth elements comprise fifteen lanthanide elements (atomic number, Z= 57–71) as well as yttrium (Z= 39) and scandium (Z= 21), although promethium (Z= 61) does not appear in nature due to its radioactive instability (Figure 1). They are an extremely coherent group
|
| [49] |
Geochemical evidence from bio-apatite for multiple oceanic anoxic events during Permian-Triassic transition. [J]. |
| [50] |
Trace element and strontium isotope constraints on sedimentary environment of Ediacaran carbonates in southern Anhui, South China [J]. |
| [51] |
Progress of analytical methods of rare earth elements of carbonate minerals in carbonate rocks [J].碳酸盐岩中碳酸盐矿物稀土元素分析方法进展 [J].
Solution-ICP-MS and LA-ICP-MS are the most common analytical methods for measuring rare earth elements in carbonate minerals of carbonate rocks.HCl and HNO not only can completely dissolve carbonate minerals in carbonate rocks but also dissolve some other minerals,and such will disturb the analytical results.CHCOOH, on the other hand,can avoid interferences from non-carbonate minerals,but probably cannot completely dissolve carbonate minerals in carbonate rocks,and such will also cause bias from the true result.LA-ICP-MS,which uses laser beam to ablate carbonate minerals and directly analyze rare earth elements contents in the evaporation,is an effective method for analyzing carbonate mineral crystals in carbonate rocks.But this method may also be questionable since the diameter of the laser beam may be too large to avoid tiny clay minerals in carbonate minerals.In summary,the results obtained by solution-ICP-MS and LA-ICP-MS on the shale-normalized REE patterns and their main parameters of carbonate minerals in carbonate rocks are comparable,and both methods can be used to trace the sedimentary environment.
|
| [52] |
A quantitative separation of non-carbonate minerals from carbonate minerals [J]. |
| [53] |
Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the Northeastern South China Sea [J]. |
| [54] |
Rare earth elements in cold seep carbonates from the Niger delta [J].
78 We developed a procedure for leaching authigenic carbonates and avoid contaminations. 78 REE are used as tracers of fluid sources. 78 REE composition in authigenic carbonates is controlled by fluid and alkalinity.
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| [55] |
A refined dissolution method for rare earth element studies of bulk carbonate rocks [J].
61Extraction of primary REE signals from bulk carbonate rocks is a key task in palaeoenvironmental studies.61A series of experiments have been performed to investigate the REE behaviours during dissolution of carbonate rocks.61Contaminative REE are mainly released at the anterior and posterior dissolution stages.61A refined dissolution method for REE studies of bulk carbonate rocks has been proposed.
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| [56] |
The aquatic chemistry of rare earth elements in rivers and estuaries [J].
Laboratory experiments were carried out to determine how pH, colloids and salinity control the fractionation of rare earth elements (REEs) in river and estuarine waters. By using natural waters as the reaction media (river water from the Connecticut, Hudson and Mississippi Rivers) geochemical reactions can be studied in isolation from the large temporal and spatial variability inherent in river and estuarine chemistry. Experiments, field studies and chemical models form a consistent picture whereby REE fractionation is controlled by surface/solution reactions. The concentration and fractionation of REEs dissolved in river waters are highly pH dependent. Higher pH results in lower concentrations and more fractionated composition relative to the crustal abundance. With increasing pH the order of REE adsorption onto river particle surfaces is LREEs > MREEs > HREEs. With decreasing pH, REEs are released from surfaces in the same order. Within the dissolved ( MREEs > LREEs, is most pronounced in the solution pool, defined here as MREEs > HREEs. While the large scale removal of dissolved river REEs in estuaries is well established, the release of dissolved REEs off river particles is a less studied process. Laboratory experiments show that there is both release and fractionation of REEs when river particles are leached with seawater. The order of sea water-induced release of dissolved REE(III) (LREEs > MREEs > HREEs) from Connecticut River particles is the same as that associated with lowering the pH and the same as that associated with colloidal particles. River waters, stripped of their colloidal particles by coagulation in estuaries, have highly evolved REE composition. That is, the solution pool of REEs in river waters are strongly HREE-enriched and are fractionated to the same extent as that of Atlantic surface seawater. This strengthens the conclusions of previous studies that the evolved REE composition of sea water is coupled to chemical weathering on the continents and reactions in estuaries. Moreover, the release of dissolved Nd from river particles to sea water may help to reconcile the incompatibility between the long oceanic residence times of Nd (7100 yr) and the inter-ocean variations of the Nd isotopic composition of sea water. Using new data on dissolved and particle phases of the Amazon and Mississippi Rivers, a comparison of field and laboratory experiments highlights key features of REE fractionation in major river systems. The dissolved pool of both rivers is highly fractionated (HREE enriched) with respect to the REE composition of their suspended particles. In addition, the dissolved pool of the Mississippi River has a large negative Ce-anomaly suggesting in-situ oxidation of Ce(III). One intriguing feature is the well developed maximum in the middle REE sector of the shale normalized patterns for the dissolved pool of Amazon River water. This feature might reflect competition between surface adsorption and solution complexation with carbonate and phosphate anions.
|
| [57] |
Accurate determinations of fifty-four major and trace elements in carbonate by LA-ICP-MS using normalization strategy of bulk components as 100% [J]. |
| [58] |
Geochemistry of Neoproterozoic limestones of the Shahabad Formation, Bhima Basin, Karnataka, southern India [J].
Major, trace and rare earth element (REE) geochemistry of carbonate rocks of the Neoproterozoic Shahabad Formation, southern India were studied in order to investigate the depositional environment and source for the REEs. The PAAS (Post Archaean Australian Shale) normalized REE + Y pattern of Shahabad limestones have consistent seawater-like pattern i.e., i) LREE depletion (average (Nd/Yb)(SN) = 0.64 +/- 0.08), ii) negative Ce anomaly, iii) positive Gd anomaly (average Gd-SN/Gd* = 1.05 +/- 0.16), iv) superchondritic Y/Ho ratio (average Y/Ho = 38.13 +/- 21.35). The depletion of LREE and enrichment of HREE are clearly indicated by the (La/Yb)(SN), (Dy/Yb)(SN) and (Nd/Yb)(SN) ratios, which suggest the retention of seawater characteristics in these limestones. The negative Ce anomaly reflects the incorporation of REE directly from seawater or from the pore water under oxic condition, and also reveals the mixing of two-component systems with terrigenous clay (detrital) in the marine sediments. The terrigenous input in these limestones is confirmed by positive correlation of I REE pound with Al2O3, negative correlation of I REE pound with CaO and differences in Y/Ho ratios. V, Cr, and Sc, are positively correlated with Ti, and strong positive correlation of I REE pound with Fe2O3, Ni, Cr, Sc, and Y also indicate the presence of terrigenous materials in the Shahabad limestones.
|
| [59] |
Rare earth element distributions in anoxic waters of the Cariaco Trench? [J].
The concentrations of dissolved and suspended particulate rare earth elements in the Cariaco Trench are reported. In solution all REE, notably Ce, show a sharp increase just at or below the oxic anoxic interface at 300 meters depth. Particulate concentrations show a complementary decrease at the same depth. The overlying oxic waters exhibit a negative Ce anomaly; the anoxic waters carry a slightly positive Ce anomaly. The particulate Ce anomaly reaches a maximum just above the interface, coinciding with maxima for particulate Mn and Fe and minima for the dissolved Ce anomaly and dissolved Ce, Mn and Fe. Thermodynamic calculations predict that the solubility of Ce(IV)O 2 is greatly enhanced by the steep p系-drop at the O 2 H 2S interface whereas oxygenated seawater is grossly oversaturated with dissolved Ce(III). However, the dramatic shifts in pe and absolute total Ce concentration do not affect the relative speciation of Ce in solution, which varies only with pH and not with p系 because dissolved Ce(IV)-species are negligible. The cycling of Ce across the oxic anoxic boundary is driven largely by its own redox chemistry as with Fe and Mn. The remaining REE, being strictly trivalent, are recycled in association with the dissolution-precipitation of (ferro)manganese oxyhydroxides. The relative turnover rates at the interface are ranked as Mn > Ce = Ce-anomaly > Nd, Sm, Eu, La, Dy, Er. The observed absence of Eu anomalies would render in situ reduction of Eu(III) to Eu(II) unlikely; thermodynamic considerations also rule out the existence of Eu(II) species in low temperature reducing environments.
|
| [60] |
Rare earth elements in deep-water articulated brachiopods: An evaluation of seawater mass [J].
61REE distribution coefficients of modern Rhynchonellids and Terebratulids shells61Open water brachiopods display vertical Ce/Ce* profiles similar to that of seawater.61HREESNvalues define a seawater–depth relationship trend.61REEs in deep-water brachiopods are a robust paleoceanographic proxy.
|
| [61] |
Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa [J]. |
| [62] |
Paleoenvironments in Meso-Neoproterozoic carbonates of the Mbuji-Mayi Supergroup (Democratic Republic of Congo)-Microfacies analysis combined with C-O-Sr isotopes, major-trace elements and REE+Y distributions [J]. |
| [63] |
Diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies: An isotopic and REE study of Cambrian phosphorites [J]. |
| [64] |
Deciphering the diagenetic alteration degree in thrombolites across the Permian-Triassic boundary and the evaluation of REY as a proxy of palaeoseawater [J]. |
| [65] |
Rare earth element geochemistry of shallow carbonate outcropping strata in Saudi Arabia: Application for depositional environments prediction [J]. |
| [66] |
Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis [J]. |
| [67] |
Rare earth elements in Holocene reefal microbialites: A new shallow seawater proxy [J].
The concentration of rare earth elements and yttrium (REE + Y) was determined in Holocene Mg-calcite microbialites from shallow reef framework cavities at Heron Reef, Great Barrier Reef. Shale-normalized REE + Y patterns of 52 microbialite samples show: (1) uniform heavy REE enrichment (Nd SN/Yb SN = 0.236, SD = 0.026); (2) consistent negative Ce and positive La anomalies; (3) marine Y/Ho ratios (56.17, SD = 2.66); and (4) slightly positive Gd anomalies. All of these features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater. REE partition coefficients were calculated relative to shallow Coral Sea seawater. They are uniform (relative SD = 10.2%) across the entire mass range and almost two orders of magnitude higher than those between coral and seawater. Hence, terrigenous detritus-free, modern microbialites are a more reliable proxy for seawater REE chemistry than are skeletal carbonates. Ancient limestones have been considered largely problematic as sources for REE proxies owing to perceived problems with diagenesis, partly on the basis of relatively high REE concentrations in some limestones compared to modern skeletal carbonates. However, high REE concentrations in modern microbialites suggest that ancient limestones with relatively high REE concentrations, if not contaminated by terrigenous detritus, may reflect original seawater chemistry. Terrigenous contamination, if present, is readily detectable on the basis of co-occurring trace element concentrations (Sc, Hf, Th) and Y/Ho ratio. Hence, ancient, particularly reefal, limestones may provide reliable seawater REE proxies. The occurrence of microbialites in clean limestones as old as 3.5 Ga suggests the possibility of reconstructing shallow marine REE chemistry over most of Earth history with important implications for paleogeography and paleoredox studies.
|
| [68] |
Preservation of primary REE patterns without Ce anomaly during dolomitization of Mid-Paleoproterozoic limestone and the potential re-establishment of marine anoxia immediately after the "Great Oxidation Event" [J]. |
| [69] |
Post-depositional origin of highly 13C-depleted carbonate in the Doushantuo cap dolostone in South China: Insights from petrography and stable carbon isotopes [J]. |
| [70] |
The diagenesis of coral Reefs [J].珊瑚礁的成岩作用 [J]. |
| [71] |
Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: A sequence through neomorphism of aragonite to calcite [J]. |
| [72] |
Geochemistry of carbonate beds in the Neoproterozoic Rosh Pinah Formation, Namibia: Implications on depositional setting and hydrothermal ore formation [J]. |
| [73] |
Diagenetic uptake of rare earth elements by bioapatite, with an example from Lower Triassic conodonts of South China [J]. |
| [74] |
Chemical diagenesis of a multicomponent carbonate system-1. Trace elements [J]. |
| [75] |
Global events across the Mesoproterozoic-Neoproterozoic boundary: C and Sr isotopic evidence from Siberia [J]. |
| [76] |
Diagenetic alteration affecting δ18O, δ13C and 87Sr/86Sr signatures of carbonates: A case study on Cretaceous seep deposits from Yarlung-Zangbo Suture Zone, Tibet, China [J]. |
| [77] |
Application of the Ce anomaly as a paleoredox indicator: The ground rules [J].
Much attention has been paid, in recent years, to the potential application of the Ce anomaly, measured in various marine phases, as a paleoceanographic indicator of widespread marine anoxia. In this paper we present and discuss results from recent studies of present-day rare earth element (REE) distributions (and hence Ce anomaly distributions) in the marine environment which are particularly pertinent to paleoceanography. Subsequently, we review and discuss the validity of the recent literature in which Ce anomalies have been employed as paleoredox indicators.
|
| [78] |
Geochemistry of rare earth elements in carbonate rocks of the Mirga Mir Formation (Lower Triassic), Kurdistan Region, Iraq [J]. |
| [79] |
New recognition of carbonate nodules genesis in Sinian Doushantuo Formation in Zigui Area and its geological implication [J].秭归地区震旦系陡山沱组碳酸盐岩结核成因新认识及其地质意义 [J].
扬子克拉通秭归地区震旦系陡山沱组第四段黑色泥页岩中广泛发育具明显δ-（13） C负异常的碳酸盐岩结核,其是否与古甲烷天然气水合物渗漏有关值得深入研究.对该碳酸盐岩结核开展的沉积结构构造、岩相学和地球化学研究表明,碳酸盐岩结核具典型韵律环带结构,普遍发育有亮晶球体结构、草莓状黄铁矿,以及与渗漏系统有关的凝块组构,δ-（13） C具明显负异常（-5.65‰--6.76‰）,U、Mo元素强烈富集（U（EF）=8-26,Mo（EF）=99-320）,Y/Ho比值为31.05-37.31,稀土配分型式为平缓左倾,主微量元素K、Sc、V、Cr、Co、Ni、Rb、Sr、Ba、Th、U和Mo等总体显示为缺氧-硫化环境,与冷泉碳酸盐岩的形成环境和特征一致.碳酸盐岩结核环带SiO2、MgO、CaO、CO2等地球化学元素含量呈阶段性连续增减变化,显示碳酸盐岩结核形成经历了初始形成、成岩-交代、成岩后改造3个连续演化阶段.据此,提出碳酸盐岩结核是新元古代末噶斯奇厄斯冰期（582-551Ma）结束温度回暖,黑色泥页岩中低温封存固态天然气水合物发生分解释放和成岩-交代形成的冷泉碳酸盐岩结核,也是古天然气水合物存在的重要地质记录和标志,这一新认识为华南扬子克拉通在震旦系和下古生界沉积盖层中寻找页岩气（甲烷天然气）储集层位提供了重要地质依据.
|
| [80] |
Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China [J]. |
| [81] |
Chromium isotope, REE and redox-sensitive trace element chemostratigraphy across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia [J]. |
| [82] |
Extremely low oxygen concentration in mid-Proterozoic shallow seawaters [J].
The mid-Proterozoic (1.8610.8 Ga) witnessed the first appearance but unusually low diversification of eukaryotes. The stagnant biotic evolution during this billion-year-long period (commonly referred to as the “Boring Billion”) was arguably ascribed to low oxygen levels in atmosphere and ocean. However, evidence supporting low oxygen in shallow-marine environments where early eukaryotes first evolved is generally lacking or insufficient. Here we report REE+Y (Rare Earth Element and yttrium) data, particularly cerium (Ce) anomalies, from a suite of mid-Proterozoic sedimentary rocks of the North China platform. The new data from North China, in combination with available Ce anomaly data from other Proterozoic successions, demonstrate that during mid-Proterozoic, negative Ce anomalies did not occur until 651.53 Ga and after 651.5 Ga, only episodic negative Ce anomalies were present in shallow-water carbonates. Trace element enrichments (UEF, VEF, and MoEF) remained at the average continental crust level before 651.53 Ga but showed a sudden increase at 651.53 Ga. The data suggest that oxygen concentration in shallow-marine environments of the mid-Proterozoic ocean was extremely low, probably <0.2μM prior to 651.53 Ga (based on minimal oxygen concentration requirement for Ce (III) oxidation) and fluctuating around 0.2μM afterwards. The low oxygen concentration (650.2μM) in shallow waters of the mid-Proterozoic ocean accounts for only 650.1% of the modern surface ocean oxygen level (65280μM) and may help explain the evolutionary stasis of eukaryotes during the mid-Proterozoic.
|
| [83] |
The Permian-Triassic boundary in Western Slovenia (Idrijca Valley section): Magnetostratigraphy, stable isotopes, and elemental variations [J]. |
| [84] |
Stable isotope and trace element stratigraphy across the Permian-Triassic transition: A redefinition of the boundary in the Velebit Mountain, Croatia [J]. |
| [85] |
Shallow water redox conditions from the Permian-Triassic boundary microbialite: The rare earth element and iodine geochemistry of carbonates from Turkey and South China [J]. |
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