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EXPERIMENTAL PYRITE FORMATION ASSOCIATED WITH DECAY OF PLANT MATERIAL

¹ Research Laboratory for Archaeology and the History of Art, University of Oxford, Dyson Perrins Building, South Parks Road, Oxford, OX1 3QY, UK;
² School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK;
³ Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109, USA derek.briggs{at}yale.edu

Laboratory experiments on microbial decay were used to investigate the conditions required for pyritization of decaying twigs, as it provides an important source of data on the anatomy of fossil plants. Plane (Platanus acerifolia) was chosen as the experimental taxon, because this genus is preserved in pyrite in the Eocene London Clay. Experiments were designed to develop sulfate reduction under marine conditions, and each contained estuarine sediment with added iron oxide (1%) with a layer of pH-buffered artificial seawater medium above, which had a labile organic-matter source (yeast extract) and an inoculum of anaerobic, sulfate-reducing bacteria. Twigs (5) were pressed into the sediment and the systems incubated with a loose lid, in air at 15°C for up to 12 weeks. These conditions were varied to reflect those thought to promote pyrite formation in the natural environment (high concentrations of reactive iron and bioavailable organic matter, local concentration of decaying material, concurrent high concentrations of sulfide and iron, and oxidation of iron sulfides), plus variations in incubation time, anoxia, pH, and sulfate supply. Changes in the chemistry of the decay systems were monitored with oxygen and pH microelectrodes, and concentrations of sulfate, sulfide, ferrous iron, and sedimentary solid-phase sulfide pools were analyzed at the end of each experiment. All systems rapidly developed bacterial sulfate reduction, dissolved iron, and iron sulfides. In only 2 out of 18 reference systems were areas of some twigs pyritized, however, although this did occur rapidly (5.4 weeks). No twigs in the modified systems were pyritized despite up to a 240% increase in solid-phase iron sulfides, the presence of diffusion gradients of ferrous iron and sulfide, the focus of sulfate reduction on the twigs, and pyrite formation in the sediment. Neither slightly oxidizing nor completely anoxic conditions enhanced pyritization. These results suggest that conditions that promote formation of sedimentary pyrite differ considerably from those that facilitate pyritization of twigs. Pyritization can occur rapidly in conditions common in marine sediments with intense microbial activity, but the process is rather random and may be controlled by the nucleation of pyrite on decaying tissue rather than factors controlling pyrite formation.

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