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Iron(III)-Reduction in a Low-Organic-Carbon Brackish–Marine System

¹ University of Manchester, Department of Earth Sciences, Williamson Building, Oxford Road, Manchester, M13 9PL, U.K.; present address: Rice University, 6100 Main Street, Department of Civil and Environmental Engineering, Mechanical Lab, Houston, Texas 77005, U.S.A.; Laura.Adams{at}rice.edu
² University of Manchester, Department of Earth Sciences, Williamson Building, Oxford Road, Manchester, M13 9PL, U.K.
³ Liverpool University, Department of Earth and Oceanographic Sciences, 4 Brownlow Street, Liverpool, L69 3GP, U.K.

Siderite rhizocretions are generally considered to be an indicator of fresh-water conditions. The presence of siderite rhizocretions with a marine ¹⁸O isotope signature in the Rutland Formation, Ketton, U.K. seems to contradict this belief. Commonly, in marine settings pyrite is more prevalent than siderite because of the high concentrations of sulfate in seawater. The Rutland Formation is a fine-grained mixed carbonate-clastic succession with interbedded coals that was deposited in marginal marine conditions. Analysis of siderite revealed that it was chemically zoned, predated pyrite, and has an average ¹⁸O signature of +0.44. This siderite is interpreted as having precipitated during early diagenesis from brackish to marine porewaters containing low concentrations of bioavailable organic matter. Despite the porewaters being dominantly marine, under conditions of restricted organic-matter quality and/or quantity Fe(III)-reducing bacteria can outcompete sulfate-reducing bacteria for the organic substrate, resulting in the precipitation of siderite at the expense of pyrite.