Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Abstract

Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2-5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe,\0.15 lm) mainly ([85%) consisted of Fe(II) and gradually increased from 0.4 to 15 lM at the sediment surface to *100-170 lM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) ([85% sFe, \0.02 lm). Sub-surface dFe(II) maxima were enriched in light isotopes (d 56 Fe -2.0 to -1.5%), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in light isotopes (down to -3.0%). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water Responsible Editor: Martin Solan.Electronic supplementary material The online version of this article (doi:10.1007/s10533-017-0309-x) contains supplementary material, which is available to authorized users. 49-67 DOI 10.1007/s10533-017-0309-x oxygen in late spring can lead to a substantial release of Fe(II). Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after [7 h. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23-31 lmol m -2 day -1 ) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusion-reaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.

Seasonal iron depletion in temperate shelf seas

“…Site A (51°12′N, 6°8′W), ~100 m depth, was the primary sampling site of the study conducted by Klar et al . [].…”

Oxygen dynamics in shelf seas sediments incorporating seasonal variability

“…The ability to sequester organic carbon is influenced by the presence and activity of macrofauna and meiofauna (Woulds et al 2007), and the type of sediment (cohesive vs permeable (Fuchsman et al 2015;Glud 2008)). The efficiency of organic carbon burial has been directly linked to the pore water oxygen concentration and exposure time (Hartnett et al 1998), and the availability of oxygen regulates different benthic biogeochemical processes (Glud 2008;Klar et al 2017;Scholz et al 2014;Serpetti et al 2016). Oxygen distribution in sediments influences denitrification, trace metal speciation and release of iron in pore waters (Kitidis et al 2017;Klar et al 2017;Rabouille et al 2003; Thompson et al submitted, this issue).…”

“…The efficiency of organic carbon burial has been directly linked to the pore water oxygen concentration and exposure time (Hartnett et al 1998), and the availability of oxygen regulates different benthic biogeochemical processes (Glud 2008;Klar et al 2017;Scholz et al 2014;Serpetti et al 2016). Oxygen distribution in sediments influences denitrification, trace metal speciation and release of iron in pore waters (Kitidis et al 2017;Klar et al 2017;Rabouille et al 2003; Thompson et al submitted, this issue). In coastal sediments and shallow waters, oxygen production through photosynthesis may exceed consumption.…”

Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability

“…If the sediments are too well flushed, do not contain enough organic matter, or are too frequently disturbed, the clear zone of iron or magnesium reduction will be pushed deeper into the sediment. This is also evident from the profiles in Klar et al (2017) discussed above, as they were able to profile to 12 cm and show the Fe reduction peak did not occur until *6 cm. As additional data becomes available, the interpretation of the lack of variability of pH at depth will be improved.…”

“…This interpretation links well with Fe and Mn pore-water profiles taken at Site A, which are presented in Figs. 3 and 4 in Klar et al (2017). The profiles show dissolved Fe release into pore waters at depth (4-6 cm) which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter.…”