东赤道太平洋高生产力和热液...境铁组分特征及其古海洋意义_郭柏.pdf

第 52 卷 第 1 期 Vol.52,No.1,120134 2023 年 1 月 GEOCHIMICA Jan.,2023 收稿日期:2021-03-30;改回日期:2021-05-14 项目资助:国家自然科学基金项目(41807435)资助。第一作者简介:郭柏(1996),男,硕士研究生,地球化学专业。E-mail: 通信作者:陈天宇(1986),男,教授,主要从事深海地球化学和古海洋研究。E-mail: Geochimica Vol.52 No.1 pp.120134 Jan.,2023 东赤道太平洋高生产力和热液活动叠加沉积环境 铁组分特征及其古海洋意义 郭 柏1,胡 镕2,魏广祎1,曲 扬1,李 达3,殷一盛1,凌洪飞1,陈天宇1*(1.南京大学 地球科学与工程学院,内生金属矿床成因机制国家重点实验室,江苏 南京 210023;2.南京大学 地理与海洋科学学院,江苏 南京 210023;3.南京师范大学 海洋科学与工程学院,江苏 南京 210023)摘 要:沉积物 Fe 组分是研究不同地质历史时期古海洋氧化还原环境的重要指标,而现代海洋不同沉积环境下的 Fe 组分变化特征则是限定这一指标适用范围的基础。东赤道太平洋巴拿马海盆作为高生产力和热液活动的叠加作用沉积区,代表了地质历史上大陆边缘海的一类典型沉积环境,但前人对这类沉积环境的Fe组分研究较为缺乏。本次研究分析了巴拿马海盆氧化底水下 ODP 677 钻孔(112.138N,8344.220W,3461 m)约300 m 长的岩心柱有机 C、Fe 组分及氧化还原敏感元素(U、Mo)含量的变化。结果表明,ODP 677 钻孔 Fe/Al值大约为 0.780.14(n=35,),明显高于上部陆壳的平均值 0.44,而 Al/Ti 值与上部陆壳接近且保持稳定在22.41.3(n=35,),可以推测,相当一部分 Fe 应来源于活跃热液活动和高生产力背景下有机碳颗粒对水柱中Fe 氢氧化物微粒的吸附或 Fe 的自聚集/沉淀。高活性 Fe 与总 Fe 的比值(FeHR/FeT)大约为 0.300.10(n=35,),远低于活性 Fe 加入通量的预期,表明一部分活性 Fe 可能在高生产力背景下转化为活性较差的组分。同时,部分活性 Fe 发生了黄铁矿化,黄铁矿 Fe 与高活性 Fe 比值(Fepy/FeHR)大约为 0.390.14(n=35,),最高可达 0.7左右。ODP 677 钻孔在 1.52.2 Ma(钻孔深度 90 m 处)记录到强烈的有机碳通量增高现象,有机 C 含量可达4.61%,其高 Mo和 U通量指示了该时期早期成岩中孔隙水可能处于高硫化环境,但钻孔中黄铁矿埋藏通量或Fepy/FeHR的高值则出现在钻孔更深的位置,表明孔隙水中部分还原态硫和铁可能发生迁移并在更深处沉淀。与目前已发表的氧化性底水环境下沉积物的 Fe 组分相比,ODP 677 钻孔部分样品具有较高程度的Fepy/FeHR和 FeHR/FeT值同时升高的现象,与典型的“铁化海洋”沉积特征吻合。本次研究进一步证实在利用 Fe组分解释古深水的氧化还原环境时,需可靠分析沉积环境并排除高生产力和热液活动叠加作用的潜在影响。关键词:Fe 组分;孔隙水硫化;热液活动;高生产力;东赤道太平洋 中图分类号:P736 文献标志码:A 文章编号:0379-1726(2023)01-0120-15 DOI:10.19700/J.0379-1726.2021.06.011 Characteristics of sedimentary Fe speciation under superimposed high productivity and hydrothermal activity in the eastern equatorial pacific and their paleoceanographic implications GUO Bai1,HU Rong2,WEI Guangyi1,QU Yang1,LI Da3,YIN Yisheng1,LING Hongfei1,CHEN Tianyu1*(1.State Key Laboratory for Mineral Deposit Research,School of Earth Science and Engineering,Nanjing University,Nanjing 210023,Jiangsu,China;2.School of Geography and Ocean Science,Nanjing University,Nanjing 210023,Jiangsu,China;3.College of Marine Science and Engineering,Nanjing Normal University,Nanjing 210023,Jiangsu,China)Abstract:Fe speciation of marine sediments is an important proxy for reconstructing redox states of the oceans in 第 1 期 郭 柏等:东赤道太平洋高生产力和热液活动叠加沉积环境铁组分特征及其古海洋意义 121 Geochimica Vol.52 No.1 pp.120134 Jan.,2023 geological history.Thus,the characteristics of Fe speciation in sediments of present-day oceans in different sedimentary environments provide fundamental constraints for such application.The Panama basin in the eastern equatorial Pacific,being characterized by high productivity and hydrothermal activity,represents a typical sedimentary environment of the continental marginal seas throughout geological history.However,studies on Fe speciation of the sediments under such conditions are still rare.Here,we discuss data on the organic carbon content,Fe speciation,and contents of redox-sensitive elements(U,Mo)in a 300-m-long core of ODP 677 (112.138N,8344.220W,water depth 3461 m)recovered from the Panama Basin.Our results show that the Fe/Al ratio of ODP 677 is about 0.780.14(n=35,),which is significantly higher than the average ratio of 0.44 in the upper continental crust,the Al/Ti ratio is close to that of the upper continental crust and remains stable at 22.41.3(n=35,).It is inferred that a considerable amount of Fe is derived from adsorption onto organic particles or from the self-aggregation/precipitation of dissolved Fe in the deep-water column against a background of high hydrothermal activity and productivity.The ratio of reactive Fe to total Fe(FeHR/FeT)is about 0.300.10(n=35,),which is considerably lower than the expected input flux of reactive Fe,indicating that some reactive Fe may be converted to less reactive components under high productivity.Furthermore,a proportion of the reactive Fe is pyritized,and the ratio of pyrite Fe to total reactive Fe(Fepy/FeHR)is approximately 0.390.14(n=35,),with the highest ratio of about 0.7.A substantial increase in biological productivity is recorded in ODP 677 during the period 1.5 to 2.2 Ma,with the organic carbon content reaching up to 4.61%.The high Mo and U fluxes indicate that pore water in the early diagenesis during this period may be in a highly sulfidic environment;by contrast,the high burial flux of pyrite or high degree of pyritization may have occurred at the deeper depths of the core,indicating that some of the reduced S and Fe in pore water may have migrated and precipitated at deeper depths.Compared with the reported Fe speciation of global marine sediments in the oxic bottom water environment,some of our samples in ODP 677 exhibit a high degree of a concurrent increase in Fepy/FeHR and FeHR/FeT,which is consistent with the typical“ferruginous ocean”depositional characteristics.We thus emphasize the importance of understanding the depositional environment and distinguishing the potential superimposed effects of high productivity and hydrothermal activities when applying Fe speciation,in order to reliably interpret the redox s