中国科学院海洋研究所机构知识库

氧化还原敏感痕量元素（RSEs）从水体到沉积物的迁移与富集受自身地球化学性质和海洋氧化还原条件的控制，因此常被用于指示海洋环境的氧化还原状态，其中常用的元素主要为Mo、U、V。为了重建长江口邻近海域低氧的发展历史，本研究借助氧化还原敏感痕量元素来指示长江口邻近海域低氧的演变。通过系统分析长江口邻近海域低氧频发区表层沉积物及柱状沉积物中氧化还原敏感痕量元素的分布和富集特征，并深入探讨研究区域氧化还原敏感痕量元素自生富集的控制机制，本研究不仅阐明了氧化还原敏感痕量元素自生富集程度与底层水最低溶解氧含量的耦合关系，还进一步揭示了Mo-U共变体系在指示长江口邻近海域氧化还原状态以及低氧区迁移方面的潜在应用价值，并在此基础上重建了近年来长江口邻近海域氧化还原状态的演变。本研究的主要认识和研究结论如下：

（1）长江口邻近海域表层沉积物的RSEs含量呈现“离岸降低”的趋势。其中，V的分布主要受有机质和陆源碎屑输入的强烈控制，U的分布主要受陆源碎屑输入的强烈控制，而Mo的分布受“粒控效应”、有机质、Fe-Mn循环和陆源碎屑输入的影响相对较小。

长江口邻近海域14个站位表层沉积物的平均RSEs/Al值均高于PAAS/Al值，呈现由近岸到远岸降低的趋势，高值区主要集中在浙江沿岸海域。其中，V/Al值与TOC含量（r=0.759，p＜0.01）和Al含量（r=0.628，p<0.01）呈现显著的正相关关系，表明V的分布受有机质和陆源碎屑输入的强烈控制，U的分布同样受陆源碎屑输入（r=0.542，p<0.05）的强烈控制。而Mo的分布受“粒控效应”、有机质、Fe-Mn循环和陆源碎屑输入的影响相对较小（r=-0.119~0.458）。研究区域RSEs的分布并未显示出明显的“粒控效应”，推测为采样前后台风活动的影响所致。

（2）沉积短柱3050-2中V受有机质、Fe氧化还原循环和陆源碎屑输入的强烈影响，而Mo和U的富集则主要受氧化还原条件的控制，可用作研究区域的氧化还原代用指标。沉积短柱3050-2中Mo_EF/U_EF比值的年代变化趋势表明近十五年来研究区域的低氧现象呈现逐渐加剧的趋势，同时Mo-U共变体系与长江口低氧区底层水最低溶解氧浓度具有良好的耦合关系，使得其可用于指示研究区域海洋氧化还原状态的演变过程，同时其在近岸低氧区的迁移和不同区域低氧程度的对比方面也具有潜在的指示意义。

沉积短柱3050-2中Mo、U、V的富集程度普遍较弱，V受有机质、Fe氧化还原循环和陆源碎屑输入的强烈影响（r=0.717~0.811，p＜0.01），不适合作为该区域的氧化还原代用指标。而Mo和U受有机质、Fe-Mn氧化还原循环、陆源碎屑输入和“粒控效应”的影响相对较小（R²=0.001~0.377），其自生富集主要受氧化还原条件的控制，可用作研究区域的氧化还原代用指标。

沉积短柱3050-2的Mo_EF/U_EF比值基本介于0.1×Mo/U_现代海水~0.3×Mo/U_现代海水，指示氧化-次氧化状态，其年代变化趋势表明近十五年来研究区域的低氧现象呈现逐渐加剧的趋势。长江口低氧区的底层水最低溶解氧浓度（DO_min）与沉积短柱3050-2中Mo_EF/U_EF比值的年代变化趋势吻合良好，沉积短柱3050-2的Mo-U共变体系可以记录采样站位邻近海域低氧区的迁移。此外，通过比较沉积短柱3050-2与邻近沉积岩芯E3、E4的平均Mo_EF/U_EF比值，还可以评估研究站位相对于低氧区中心的偏移程度：相对于E4（1.118）、3050-2（1.052）站位，E3站位（1.327）很可能更为靠近历年的低氧区中心。

（3）重力柱3100-1中Mo和V受Fe-Mn氧化还原循环的影响，U和V受陆源碎屑输入的强烈影响，重力柱3100-1中Mo_EF/U_EF比值的年代变化可能是多种因素综合作用的结果，因此Mo-U共变体系不适合作为研究站位的氧化还原代用指标。重力柱3100-1中U_EF值、（Cu+Mo）/Zn比值、V/Cr比值、Ni/Co比值的结果均表明沉积时的氧化环境，其稳定的年代变化趋势表明，近五十年来3100-1站位邻近海域底层水体普遍处于氧化状态。

重力柱3100-1中Mo、U、V的富集程度普遍较弱，V受到Fe-Mn氧化还原循环和陆源碎屑输入的强烈影响（r=0.693~0.923，p＜0.01），不适合作为该区域的氧化还原代用指标。Mo受到Fe-Mn氧化还原循环的强烈影响（r=0.738~0.829，p＜0.01），以及U受到陆源碎屑的强烈影响（r=0.837，p＜0.01），这些因素可能干扰Mo和U的自生富集记录。因此，重力柱3100-1中Mo_EF/U_EF比值的年代变化可能并非氧化还原自生富集的单一结果，而是多种因素共同作用的结果。

重力柱3100-1（U_EF值为0.776~1.127）及其邻近沉积岩芯E2（U_EF值为0.702~1.121）中较弱的U自生富集程度与稳定的U_EF值年代变化趋势表明，近五十年来3100-1站位邻近海域整体低氧程度较弱，普遍为氧化状态。此外，重力柱3100-1中（Cu+Mo）/Zn比值（0.321~0.419）、V/Cr比值（1.256~1.423）和Ni/Co比值（2.479~2.711）均指示沉积时的氧化状态。与U自生富集的指示结果相一致，RSEs比值的稳定年代变化趋势也表明近五十年来3100-1站位邻近海域底层水体普遍处于氧化状态。

The migration and enrichment of redox-sensitive trace elements (RSEs) from seawater to sediments are controlled by their geochemical properties and marine redox conditions. Therefore, RSEs, such as Mo, U, and V, are generally used to indicate redox states of marine environments. To reconstruct the historical redox states in hypoxic zone, RSEs were used to indicate the evolution of hypoxia in the Changjiang River Estuary Adjacent Waters. Through the systematic analyses of surface sediments and sediment cores collected from the recurrently hypoxic region in the Changjiang River Estuary Adjacent Waters, the distribution and enrichment of RSEs have been obtained, and the authigenic enrichment mechanism of RSEs in the study area has been deeply discussed in the study area. This research illustrated the coupling relationship between authigenic enrichment degree of RSEs and the minimum dissolved oxygen concentrations of bottom seawater. Further, the potential application of Mo-U covariation system in indicating redox states and the migration of hypoxic zone in the Changjiang River Estuary Adjacent Waters has been revealed. Based on the above, this research reconstructed the historical redox states in recent years near the Changjiang River Estuary Adjacent Waters. The main results and conclusions obtained from this study are as follows:

(1) The RSEs contents of surface sediments in the Changjiang River Estuary Adjacent Waters show an obvious “offshore decrease” trend. The distribution of V is strongly affected by organic matter and terrigenous detritus input. The distribution of U is strongly affected by terrigenous detritus input, while the distribution of Mo is relatively less affected by clay proportion, organic matter, Fe-Mn redox cycling, and terrigenous detritus input.

The average RSEs/Al values of fourteen surface sediments in the Changjiang River Estuary Adjacent Waters are higher than PAAS/Al values. The RSEs contents of fourteen surface sediments show an obvious “offshore decrease” trend, and the high-value area is mainly located in the coastal waters of Zhejiang Province. The V/Al values have a significant positive correlation with TOC contents (r=0.759, P<0.01) and Al contents (r=0.628, P<0.01), indicating that the distribution of V is strongly affected by organic matter and terrigenous detritus input. The distribution of U is also strongly affected by terrigenous detritus input (r=0.542，p<0.05), while the distribution of Mo is relatively less affected by clay proportion, organic matter, Fe-Mn redox cycling, and terrigenous detritus input (r=-0.119~0.458). It is speculated that due to the disturbances by typhoon activities during sampling, the obvious “grain size effect” of RSEs has not been observed in the study area.

(2) V is strongly affected by organic matter, Fe redox cycling, and terrigenous detritus input in sediment core 3050-2, while the enrichment of Mo and U are mainly controlled by redox conditions. Therefore, Mo and U could be used as redox proxies in the study area. The chronological variation of Mo_EF/U_EF values in sediment core 3050-2 reveals gradually intensified hypoxia in the study area during the last 15 years. The good coupling relationship between the Mo-U covariation system in sediment core 3050-2 and the minimum dissolved oxygen concentrations of bottom seawater in hypoxic zone near the Changjiang River Estuary Adjacent Waters makes it possible to indicate the evolution of marine redox states in the study area. Besides, the Mo-U covariation system has a potential indication in the migration of coastal hypoxic zone and the comparison of hypoxic intensity in different areas.

The enrichment degree of Mo, U, and V in sediment core 3050-2 are generally mild. V is strongly affected by organic matter, Fe redox cycling, and terrigenous detritus input (r =0.717~0.811, p<0.01), so it is not suitable to be used as a redox proxy in the study area. However, Mo and U are relatively less affected by organic matter, Fe-Mn redox cycling, terrigenous detritus input, and “grain size effect” (R²=0.001~0.377). Their authigenic enrichments are mainly controlled by redox conditions and thus could be used as redox proxies in the study area.

The Mo_EF/U_EF values in sediment core 3050-2 are basically between 0.1 and 0.3×Mo/U_{modern seawater} value, indicating an overall progressive transition from oxic to suboxic conditions in this site. And its chronological variation reveals gradually intensified hypoxia in the study area during the last 15 years. There has a good coupling relationship between the chronological variation of Mo_EF/U_EF values in sediment core 3050-2 and the minimum dissolved oxygen concentrations of bottom seawater in hypoxic zone near the Changjiang River Estuary Adjacent Waters. The Mo-U covariation system in sediment core 3050-2 could indicate the migration of hypoxic zone in the Changjiang River Estuary Adjacent Waters. In addition, the average Mo_EF/U_EF values in sediment core 3050-2 and its adjacent cores could be used to evaluate the deviations from the center of hypoxic zone at different stations. Station E3 might be closer to the center of hypoxic zone over the years than stations E4 and 3050-2.

(3) Mo and V are affected by Fe-Mn redox cycling, and U and V are strongly affected by terrigenous detritus input in sediment core 3100-1. Therefore, the chronological variation of Mo_EF/U_EF values in sediment core 3100-1 might not be a result of the concerted action of manifold factors, and the Mo-U covariation system is not suitable for the redox reconstruction of sediment core 3100-1. The results of U_EF value, (Cu+Mo)/Zn ratio, V/Cr ratio, and Ni/Co ratio in sediment 3100-1 indicate an oxic environment during deposition. And their stable chronological variations illustrate that the bottom water of sea area adjacent to station 3100-1 has been generally in an oxic state during the past fifty years.

The enrichment degree of Mo, U, and V in sediment core 3100-1 are generally mild. V is strongly affected by Fe-Mn redox cycling and terrigenous detritus input (r=0.693~0.923, p<0.01), so it is not suitable to be used as a redox proxy in the study area. Mo is strongly affected by Fe-Mn redox cycling (r=0.738~0.829, p<0.01), and U is strongly affected by terrigenous detritus input (r=0.837, p<0.01). These factors could also interfere with the authigenic enrichment records of Mo and U. Therefore, the chronological variation of Mo_EF/U_EF values in sediment core 3100-1 might not be a single result of redox authigenic enrichment, but a result of the concerted action of manifold factors.

The weakly authigenic enrichment of U and the stable chronological variations of U_EF values in sediment core 3100-1 and E2 show that the hypoxia has been generally mild in the study area and the sea area adjacent to station 3100-1 has been generally in an oxic state during the past fifty years. In addition, the results of (Cu+Mo)/Zn ratio (0.321~0.419), V/Cr ratio (1.256~1.423), and Ni/Co ratio (2.479~2.711) in sediment 3100-1 indicate an oxic environment during deposition. Consistent with the indication result of U’s authigenic enrichment, the stable chronological variations of RSEs’ ratios reveal that the bottom water has been generally in an oxic state in adjacent water of station 3100-1 during the past fifty years.

第1章  绪论………………………………………………………………………1

1.1  海洋低氧……………………………………………………………………...1

1.2  长江口低氧区………………………………………………………………...4

1.3  氧化还原敏感痕量元素……………………………………………………...6

1.3.1  海洋环境行为特征………………………………………………………6

1.3.2  环境判别方法……………………………………………………………8

1.4  国内外研究进展……………………………………………………….........10

1.5  研究内容与研究意义………………………………………………….........12

第2章  材料与方法……………………………………………………………15

2.1  样品采集与预处理…………………………………………………….........15

2.2  分析测试方法……………………………………………………………….16

2.2.1  沉积粒度分析…………………………………………………………..17

2.2.2  碳氮含量及同位素分析………………………………………………..17

2.2.3  元素分析………………………………………………………………..17

第3章  表层沉积物氧化还原敏感痕量元素的分布与控制因素…....21

3.1  表层沉积物沉积地球化学特征…………………………………………….21

3.1.1  沉积粒度特征…………………………………………………………..21

3.1.2  碳氮含量及同位素组成………………………………………………..21

3.1.3  氧化还原敏感痕量元素分布特征……………………………………..24

3.2  表层沉积物氧化还原敏感痕量元素分布的控制因素……………….........25

第4章  近十五年来氧化还原敏感痕量元素对低氧的响应………….27

4.1  沉积短柱3050-2的沉积地球化学特征……………………………….......27

4.1.1  沉积粒度特征…………………………………………………….........27

4.1.2  沉积年代学特征………………………………………………….........27

4.1.3  碳氮含量及同位素组成……………………………………………….27

4.1.4  氧化还原敏感痕量分布特征………………………………………….29

4.2  沉积短柱3050-2氧化还原敏感痕量元素富集的控制因素………….......30

4.3  近十五年来氧化还原敏感痕量元素共变体系对低氧的响应…………….32

第5章  近五十年来氧化还原敏感痕量元素对环境条件的重建…....37

5.1  重力柱3100-1的沉积地球化学特征……………………………………....37

5.1.1  沉积粒度特征……………………………………………………..........37

5.1.2  沉积年代学特征…………………………………………………..........38

5.1.3  碳氮含量及同位素组成…………………………………………..........38

5.1.4  氧化还原敏感痕量分布特征……………………………………..........40

5.2  重力柱3100-1氧化还原敏感痕量元素富集的控制因素…………………41

5.3  近五十年来氧化还原敏感痕量元素对氧化还原状态的重建………..…...43

第6章  结论与展望………………………………………………………........47

6.1  主要结论……………………………………………………………….........47

6.2  不足与展望…………………………………………………………….........50

参考文献…………………………………………………………………………...51

致  谢…………………………………………………………………………........57

作者简历及攻读学位期间发表的学术论文与研究成果………………...59