Iron inputs from sediments to the oceans

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Homoky, William Bela