长江口缺氧区沉积物-水界面Fe的迁移

Changes in iron (Fe) supply from continental margin sediments are suggested to affect primary production and carbon export over large areas of the ocean. However, accurate quantification of benthic fluxes of Fe from coastal sediments is a major challenge in marine chemistry, and the mechanisms that control Fe transfer across the sediment-water interface remain poorly understood. In this project, I propose to utilize a newly developed approach – the 224Ra/228Th disequilibrium approach to evaluate the transfer efficacy of Fe across the sediment-water interface in a hypoxia area off the Changjiang River Estuary. Sediment columns as well as overlying water samples will be collected along a dissolved oxygen (DO) gradient in two transects across the hypoxia area. I plan to carry out two sampling campaigns: one in summer when hypoxia prevails in the study region, the other in late summer when bottom waters are saturated with oxygen. Dissolved Fe, oxygen, nutrients, and H2S will be measured. With the 224Ra/228Th disequilibrium approach, I will quantify benthic fluxes of Fe from the seafloor and examine the key processes that regulate the transfer efficacy of Fe across the sediment-water interface. In particular, I will assess how the redox potential affects the sediment-water transfer of Fe in a coastal system, like the hypoxia area off the Changjiang River Estuary.

沉积物-水界面Fe的迁移及其调控机制的研究，对于理解地质历史时期以及未来年代里，近岸沉积物作为大洋水体Fe源之量级的大小和变动，有着极重要的科学意义。本项研究拟选择长江口缺氧区为研究对象，在这一区域里设定1“十”字断面，分别在底部水体处于充氧和低氧-轻度缺氧状态的季节，借助申请人所开拓的天然放射性同位素新体系—224Ra/228Th不平衡法，确定底部沉积物Fe的释放通量，进而研究缺氧区内外沉积物-水界面Fe迁移效率的空间变化格局；结合其他天然放射性同位素手段（如7Be、234Th），探讨界面动力学过程对Fe迁移的影响；最后通过不同季节的对比研究，揭示界面氧化还原状态对Fe迁移的作用机制。

铁（Fe）是海洋初级生产力的限制因子之一，大洋Fe 输入量的波动，控制着海洋初级生产力及颗粒有机碳输出通量的变动，最终导致了冰期-间冰期大气CO2浓度的周期性振荡。近年来国际GEOTRACES计划的推进，发现近岸底部沉积物溶解Fe向水体的迁移是维系上层海水高含量Fe的重要原因。因此，定量研究沉积物–水界面Fe 的迁移及其控制机制，对于理解地质历史时期以及未来年代里，近岸沉积物作为大洋水体Fe 源之量级的大小和变动，均有重要的科学意义。.本项目以长江口缺氧区为重点研究对象，应用项目负责人开拓的天然放射性同位素体系——224Ra/228Th不平衡法，定量研究了长江口底部沉积物Fe的释放通量和迁移效率，发现长江口的底部Fe通量与其他浅海陆架释放通量相当，但较河口通量低近一个量级。进一步汇总了本项目课题组近10年在中国近海积累的相关数据，揭示了沉积物–水界面Fe 的迁移受氧化还原条件和底部边界层的动力条件所共同控制，二者的竞争消长决定了沉积物–水界面Fe的迁移效率，为定量研究全球尺度下陆架沉积物对海洋Fe的补充机制提供了关键理论依据。本项目发现传统的沉积物培养方法可能低估了沉积物–水界面间Fe的交换通量，近海沉积物是海洋中的Fe不可忽视的来源，其在全球海洋铁循环中的作用需要重新评估。