In-situ determination of dissolved iron production in recent marine sediments

Fe-DGT (Diffusive Gradient in Thin-films), as a measure of in-situ net dissolved Fe production, together with conventional pore water ions, were determined in the upper 40 cm of organic rich sediments from Loch Duich, Scotland. There was an increase in net Fe production towards the base of the apparent Fe-R zone, which was interpreted as either being due to rapidly decreasing dissolved Fe removal and/or to an increase in sulphate reducing microniches towards the base of the Fe-R zone. A sharp Fe-DGT minimum, due to non-steady state Fe removal processes, was found at the boundary between the Fe-R and S-R zones. The Fe-DGT profile showed clear evidence of decreasing dissolved Fe production in the upper layers of the S-R zone. A comparison of the calculated net rate of iron pyritisation (0.23 moles Fe m^-2 yr^-1) with the net production rate (2.0 moles Fe m^-2 yr^-1), as measured by Fe-DGT, showed that there was significant recycling of iron before final burial in the sediment as pyrite. The Fe-DGT profiles provide in-situ data which is consistent with previous interpretations of iron production and recycling in marine pore waters obtained by less direct means.