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Title: Iron inputs from sediments to the oceans
Author: Homoky, William Bela
ISNI:       0000 0004 2696 8263
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2009
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This thesis explores the nature and ubiquity of iron (Fe) inputs from sediments to the oceans. In the last 10 years continental shelf sediments have become widely recognised as important vectors for dissolved Fe inputs to the oceans, where bacterial dissimilatory Fe-reduction (DIR) promotes the flux of Fe to the water column during the oxidation of sedimentary organic matter. Deep-sea and volcanogenic sediments however, are important reservoirs of Fe, which have not yet been investigated as sources of Fe to seawater. Furthermore knowledge of the nature of Fe phases involved in sediment, porefluid and 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 near the active volcanic island of Montserrat, Caribbean Sea, and mixed biosiliceous sites around the dormant Crozet Island archipelago, Southern Ocean. Analyses reveal both regions maintain high pore-fluid Fe concentrations close to the sediment surface (up to 20 μM 0-5 cmbsf), despite relatively low organic carbon supply and contrasting oxygen utilization pathways. The oxidation of young tephra is thought to maintain the steep oxygen gradient measured in Montserrat sediments, and is considered to be an important component 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 colloidal phases (0.02-0.2 μm), and in both oxic and sub-oxic sediment layers. Thus mixed biosiliceous-volcaniclastic sediments are shown to host important colloidal-Fe generating reactions, which it is argued, promote the exchange of Fe with the overlying bottom waters. Re-cycling processes close to the seafloor are likely to determine the impact of this flux on seawater Fe budgets. Low-cost ex-situ incubation experiments were used to measure a benthic Fe flux on sediments from the river-dominated Californian margin (6.3 ± 5.9 μmol Fe m-2 yr-1) consistent with previous studies. Fe and Mn fluxes from Montserrat tephra deposits were also assessed; Differences in oxidation kinetics are shown to prevent the accumulation of Fe, yet permit the accumulation of Mn (~27 μmol m-2 yr-1) in Montserrat bottom waters. Studies indicate temporospatial variations to bioirrigation and sediment re-suspension are important aspects of sedimentary Fe inputs that are poorly represented by conventional sampling 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 their kind in deep-sea pore-fluids. Unique relative to previous studies of pore-fluid Fe isotopes, the near-crustal δ56Fe compositions, demonstrate that DIR does not impart the same light Fe-isotopic signature that characterises previous sub-oxic pore-fluids. Comparison of reactive Fe contents between Crozet and pacific margin sediments indicates pore-fluid Fe isotopes reflect the extent to which Fe is recycled by redox processes. This discovery brings 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 (δ56Fe) is predicted to be -0.1 to -3.2‰ depending on the balance of uncertainty in input terms. 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 inventory already constrained for this region.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; GC Oceanography