The classical scheme of biogeochemical zones (BGZ) is known to be an oversimplification\ud of the microbial processes that occur in organic-rich marine sediments. Results from a coupled\ud deployment of pore-water gel probes in Loch Duich, Scotland, provide direct evidence for rapid\ud recycling within the iron reduction (FeR) and sulphate reduction (SR) zones. High resolution porewater\ud profiles obtained using diffusive equilibrium in thin films (DET) gel probes found a nitrate peak\ud at the boundary between the FeR and SR zones. This non-steady state feature is consistent with recycling\ud of reduced N occurring throughout the FeR zone. Both conventional pore-water iron profiles\ud and results from diffusive gradient in thin films (DGT) probes indicate that iron is solubilised and precipitated\ud in rapid Fe/S recycling reactions throughout the SR zone. The presence of such complex\ud recycling reactions confirms the oversimplification of the classical BGZ scheme
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