Iron inputs from sediments to the oceans

Abstract

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)
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 ?mol 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 ?56Fe 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
(?56Fe) 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.

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