Micro-heterogeneity of carbon mineralization and metal mobilisation in marine sediments

海洋是二氧化碳的主要汇。浮游动物以微小的浮游植物为食，通过光合作用将其从大气中移除。当它们死亡时，一部分浮游生物下沉到沉积物中，在那里它们遇到大量的微生物，这些微生物在一个被称为矿化的过程中积极分解有机物质。在这个过程中，氧气被消耗掉了，所以它在沉积物中的浓度几乎为零。然后使用其他氧化剂，包括铁和锰的氧化物。铁和锰被转化为可溶的、还原的形式，同时与氧化物结合的微量金属被释放出来。传统上，我们通过分析一系列垂直层中的沉积物，并考虑各种成分的垂直输送和反应来研究这些过程。最近出现了可以提供氧气、二氧化碳和pH值的二维图像的新技术。他们使用数码相机记录被测量组件触发的荧光信号，被称为平面光电器件。它们被用来证明矿化在沉积层中不是均匀分布的。相反，它最有效地发生在离散的，毫米或亚毫米大小的位置，称为微孔。他们还表明，沉积物中的生物，如特殊的蠕虫，引入了局部结构。另一项新技术是薄膜中的扩散梯度（DGT），它通过在金属穿过一层水凝胶（如隐形眼镜）后将金属积聚在薄结合层中来测量金属的动员。使用DGT获得的二维移动金属图像表明，金属在微微米处被移动。这一新的证据表明，碳循环的这一重要部分并不像以前认为的那么简单。需要进行系统和仔细的研究，以找出在三维空间中考虑这种较小规模的过程是否会影响我们对过程速率的理解以及我们建模和预测矿化和金属动员的能力。要做到这一点，我们需要能够研究单个微孔，这意味着我们必须能够在完全相同的位置进行所有测量。我们已经开展了DGT与平面光电器件相结合的前期工作。我们希望使用这种组合的二维探针来研究矿化和金属动员之间的关系。一系列系统的测量将在实验系统中进行，以复制微生境和蠕虫的洞穴。研究结果将用一个新建立的三维输运和反应模型来解释。然后，我们将使用光电器件和DGT直接在现场进行测量，使用从实验室研究中获得的新理解来解释研究结果。这项工作将促进对矿化和金属动员以及两者之间联系的理解，包括在三维框架中发生的速率的量化。它将为全球碳循环和微量金属动员的研究提供信息。后者涉及金属与上覆水的交换以及微孔对矿物形成过程的影响。

The oceans are major sinks of carbon dioxide. It is removed from the atmosphere during photosynthesis by microscopic phytoplankton, which are grazed by zooplankton. As they die, a proportion of the plankton sink to the sediment where they encounter large microbial populations that actively decompose the organic material in a process known as mineralisation. Oxygen is consumed in this process so that its concentration within the sediment is virtually zero. Other oxidants, including iron and manganese oxides are then used. Iron and manganese are transformed into their soluble, reduced forms and simultaneously trace metals that were bound to the oxides are released. We have traditionally studied these processes by analysing the sediments in a series of vertical layers and considering the vertical transport and reactions of the various components. New techniques that can provide two dimensional (2-D) images of oxygen, carbon dioxide and pH have recently become available. They use a digital camera to record a fluorescent signal triggered by the measured component and are known as planar optodes. They have been used to show that mineralization is not uniformly distributed in a layer of sediment. Rather it occurs most efficiently at discrete, mm or sub-mm sized sites, known as microniches. They have also shown that the organisms within the sediment, such as specialised worms, introduce localised structure. Another new technique is diffusive gradients in thin-films (DGT), which measures metal mobilisation by accumulating the metal in a thin binding layer after it has passed through a layer of hydrogel (like a contact lens). Images of mobilised metals in 2-D obtained using DGT show that metals are mobilised at microniches. This new evidence indicates that this important part of the carbon cycle is not as simple as previously thought. Systematic and careful studies need to be undertaken to find out whether consideration of the processes on this smaller scale in three dimensions will affect our understanding of process rates and our ability to model and predict mineralisation and metal mobilisation. To do that we need to be able to study individual microniches, which means that we must be able to make all measurements at exactly the same location. We have undertaken preliminary work which has combined DGT and planar optodes. We wish to use this combined 2-D probe to investigate the relationship between mineralization and metal mobilisation. A systematic series of measurements will be made in experimental systems that replicate microniches and worm burrows. The results will be interpreted using a newly constructed model of transport and reaction occurring in three dimensions. We will then perform measurements using optodes and DGT directly in the field, using the new understanding gained from the laboratory studies to interpret the findings. The work will advance understanding of mineralisation and metal mobilisation and the linkages between the two, including quantification of rates occurring in a 3-D framework. It will inform studies of the global carbon cycle and of trace metal mobilisation. The latter relates to exchange of metals with the overlying water and the effect of microniches on mineral formation processes.