Element Cycling in Bottom Sediments

Significant cycling of trace elements can occur during early diagenesis of oceanic sediments. For some elements this leads to a benthic flux at the water-sediment interface which influences their distributions in the water column. In other cases, redistribution occurs within the sediment column, leading to metal-enriched horizons within sediments. Element cycling in bottom sediments is an important factor in the flow of elements within the oceans. A useful theoretical framework for understanding metal diagenesis is to consider that the early diagenetic reactions involving organic-matter oxidation occur in the sequence of decreasing free-energy yield thus; aerobic oxidation > denitrification >= iodate reduction > manganese oxide reduction > ferric oxide reduction > sulphate reduction > methanogenesis: matrixformatl\ \ CH₂O+O₂-> CO₂+H₂O 5CH₂O+4NO₃^--> 2N₂+4HCO₃^- +CO₂+3H₂O 3CH₂O+2IO₃^--> 2I^-+3CO₂+3H₂O CH₂O+3CO₂+H₂O+2MnO₂-> 2Mn²⁺+4HCO₃^- CH₂O+7CO₂+4Fe(OH)₃-> 4Fe²⁺+8HCO₃^-+3H₂O 2CH₂O+SO₄{}^2--> H₂S+2HCO₃^- 2CH₂O-> CH₄+CO₂. endmatrix This sequence is frequently inferred to have occurred via measurement of pore-water solutes, although historical changes in sedimentation can seriously affect element transport described by steady-state models that are based on this framework. Three factors are considered to be of particular importance in controlling diagenetic reactions in the deep sea: the rate of supply of organic matter, the rate of sediment mixing, and diffusion. Thus, in pelagic sediments at sites where the organic carbon flux to the sediments are similar, contrasting pore-water profiles can occur because enhanced mixing rates can lead to less pre-burial aerobic oxidation and consequent manganese mobilization within the sediment column. Nevertheless, the organic carbon flux to deep-sea sediments (for example, ca. 12-13 μ mol C cm^-2 a^-1 is oxidized dominantly by oxidation by O₂ (for example, ca. 12 μ mol C cm^-2 a^-1 compared with denitrification (for example, ca. 0.1 μ mol C cm^-2 a^-1 and MnO₂ reduction (for example, ca. 0.05-0.1 μ mol cm^-2 a^-1) with a burial flux of organic carbon (for example, ca. 0.1-0.4 μ mol cm^-2 a^-1) of the order of 1% of the flux to the sediment interface. Such factors influence the cycling of certain metals in addition to Mn in bottom sediments because they are coupled with Mn. Thus, Co and Ni show enrichments in pore waters at redox boundaries because they are released with the dissolution of Mn oxides. Other elements (such as Cu and I) show strong interfacial enrichments in pore waters that appear to reflect the breakdown of labile organic material at the water-sediment interface followed by scavenging of some of the released elements. Therefore, the pore-water gradients are supported by active diagenetic recycling at the interface, which sets limits on release of the elements to the oceanic water column.