Iron inputs from sediments to the oceans

• Main Author: Homoky, William Bela
• Format: Thesis
• Language: English
• Published: 2009
• Subjects: Southern Ocean
• Online Access: https://eprints.soton.ac.uk/169045/; https://eprints.soton.ac.uk/169045/1/Homoky_PhD_2010.pdf

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 ?M 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 ?m), 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 ?mol Fe m-2 yr-1) ...