Iron inputs from sediments to the oceans

This thesis explores the nature and ubiquity of iron (Fe) inputs from sediments to theoceans. In the last 10 years continental shelf sediments have become widely recognisedas important vectors for dissolved Fe inputs to the oceans, where bacterial dissimilatoryFe-reduction (DIR) promotes the flux of Fe to the water column during the oxidation ofsedimentary organic matter. Deep-sea and volcanogenic sediments however, areimportant reservoirs of Fe, which have not yet been investigated as sources of Fe toseawater. Furthermore knowledge of the nature of Fe phases involved in sediment, porefluidand seawater cycling is limited.The nature of Fe cycling was investigated in deep-sea volcaniclastic surface-sediments(0-20 cmbsf). Pore-fluid and sediment samples were collected from tephra-rich sites nearthe active volcanic island of Montserrat, Caribbean Sea, and mixed biosiliceous sitesaround the dormant Crozet Island archipelago, Southern Ocean. Analyses reveal bothregions maintain high pore-fluid Fe concentrations close to the sediment surface (up to20 ?M 0-5 cmbsf), despite relatively low organic carbon supply and contrasting oxygenutilization pathways. The oxidation of young tephra is thought to maintain the steepoxygen gradient measured in Montserrat sediments, and is considered to be an importantcomponent of Fe, and in particular manganese (Mn), cycling with local bottom water.Unlike Montserrat dissolved Fe and Mn in Crozet pore-fluids are dominated by colloidalphases (0.02-0.2 ?m), and in both oxic and sub-oxic sediment layers. Thus mixedbiosiliceous-volcaniclastic sediments are shown to host important colloidal-Fe generatingreactions, which it is argued, promote the exchange of Fe with the overlying bottomwaters. Re-cycling processes close to the seafloor are likely to determine the impact ofthis flux on seawater Fe budgets.Low-cost ex-situ incubation experiments were used to measure a benthic Fe flux onsediments from the river-dominated Californian margin (6.3 ± 5.9 ?mol Fe m-2 yr-1)consistent with previous studies. Fe and Mn fluxes from Montserrat tephra deposits werealso assessed; Differences in oxidation kinetics are shown to prevent the accumulation ofFe, yet permit the accumulation of Mn (~27 ?mol m-2 yr-1) in Montserrat bottom waters.Studies indicate temporospatial variations to bioirrigation and sediment re-suspension areimportant aspects of sedimentary Fe inputs that are poorly represented by conventionalsampling methods.In an effort to trace the biogeochemical processing of pore-fluid Fe in Crozet sediments,its isotopic composition was determined, representing the first measurements of theirkind in deep-sea pore-fluids. Unique relative to previous studies of pore-fluid Fe isotopes,the near-crustal ?56Fe compositions, demonstrate that DIR does not impart the same lightFe-isotopic signature that characterises previous sub-oxic pore-fluids. Comparison ofreactive Fe contents between Crozet and pacific margin sediments indicates pore-fluid Feisotopes reflect the extent to which Fe is recycled by redox processes. This discoverybrings to light the potential for Fe isotopes to trace the input of Fe from shelf sediments,where redox re-cycling of Fe is extensive. The mean oceanic Fe isotope composition(?56Fe) is predicted to be -0.1 to -3.2‰ depending on the balance of uncertainty in inputterms. The predicted surface water Fe isotope composition in the Crozet region (-2.0 to -2.2‰) is shown to reflect the light composition of shelf-derived Fe for a Fe inventoryalready constrained for this region