Early diagenetic reactions mentioned above are not limited to the surface sediment; they occur already while the particles are suspended in the bottom water. Nearly everywhere in the ocean we observe a bottom nepheloid layer, a layer of one to several hundred meters thickness with higher load of suspended particles than in the clear water above. This layer is maintained by bottom friction and is clearly better developed in areas with strong bottom currents, as observed in the FRAM Strait. However, it is unclear over what distances particles can be transported in this layer and how much they are affected by diagenetic reactions during their residence in suspension.

Again we use the natural tracer ²³⁴Th to quantify the exchange rate of particles between sediment and suspension in the loop of settling and resuspension. This yields the residence time of particles in suspension. Additionally, we measure depth gradients in particle composition and then estimate the reaction rates of particulate substances, like the decomposition of organic matter, opal dissolution or the exchange of trace metals between particles and solution.

The transport of trace metals to the sediments is strongly controlled by the abundance, production, sinking and decomposition of particulate matter. Besides terrigenous material originating from river and atmospheric input, particles are produced primarily by phytoplankton in the surface water of the oceans. Together with the nutrients, metals are taken up actively or passively by these primary producers and are incorporated into their cells. Upon sinking through the water column, metals can be released from particles through degradation of organic matter or dissolution of inorganic carrier phases (metal oxides, opal, calcite). The vertical and horizontal distributions of dissolved metals in sea-water are a reflection of the whole range of chemical, oceanographic and sedimentary controls on their supply to, distribution in and their removal from the ocean.

Once resuspension and bioturbation rates are known from the radionuclide measurements (see natural radionuclides, ²³⁴Th), the turnover rates of other trace elements can be calculated if their concentration gradients in the bottom water are known. Our main interest is the turnover of substances that are released in relation to the decay of organic matter in the surface sediment or in the bottom water. This includes elements that were component of or adsorbed to the degraded organic material (e.g. Cu, Cd, REE), as wellas secondary reaction products of diagenetic reactions in the sediment (Mn, CH₄).

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Bottom water sampler
Methods for pore water analyses compiled by Martin Kölling (University of Bremen)