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

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.