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113 11 Diagenesis of the Fossil Bones of Iguanodon bernissartensis from the Iguanodon Sinkhole Thierry Leduc We investigate the bone diagenesis of the Iguanodon skeletons discovered in the Iguanodon Sinkhole in 1878–1881. By means of x-ray diffraction and energy-dispersive spectrometry analysis, about 30 mineral phases were identified in the fossil bones of the Bernissart iguanodons. During burial, recrystallization took place: the slightly crystallized carbonated hydroxylapatite (the mineral phase of fresh bone tissue) was replaced by wellcrystallized carbonated fluorapatite, currently present in the Iguanodon bones. Whereas some minerals infiltrated the bone during cavity filling by the sediment (detrital quartz, “argillaceous” phyllosilicates, anatase, and rutile), others are authigenic and precipitated in the cavities during burial (pyrite, barite, sphalerite, celestine, and to a lesser extent quartz) or after the exhumation (other sulfates, oxides, and/or hydroxides). Pyrite is the most abundant authigenic mineral in the Iguanodon fossil bones, making them brittle. Different pyrite morphologies can be observed, such as crystals, framboids, thin coatings, and fibroradial structures. Barite is the second most abundant phase. When both minerals are present, pyrite was formed first. Their distribution throughout the bones seems to be random. Since their discovery, the fossil bones have been treated for conservation in several ways. However, this treatment did not prevent the alteration of pyrite into an assemblage of 13 different secondary sulfate minerals, more particularly szomolnokite (FeSO4·H2 O) and rozenite (FeSO4·4 H2 O) and in a less way roemerite (Fe2+ Fe3+ 2 SO4·14 H2 O), jarosite [KFe3 (SO4 )2 (OH)6 ], natrojarosite [NaFe3 (SO4 )2 (OH)6 ], halotrichite [Fe2+ Al2 (SO4 )4·22 H2 O], tschermigite [(NH4 )Al(SO4 )2·12 H2 O)], melanterite (FeSO4·7 H2 O), coquimbite [Fe3+ 2 (SO4 )3·9 H2 O], metavoltine [(K,Na)8 Fe2+ Fe3+ 6 (SO4 )12 O2·18 H2 O], gypsum (CaSO4·H2 O), anhydrite (CaSO4 ), and pure sulfur (S). Szomolnokite and rozenite are the most abundant of these minerals and can be found in nearly all samples. These two minerals differ only by their degree of hydration and can easily transform into each other. This mechanism depends on the degree of humidity in the environment. Barite and anhydrite are the result of the neutralization of the acid by phosphate or carbonate. Taphonomy studies the passage of organisms from the biosphere to the lithosphere. These processes include diagenesis. The diagenesis of animal or vegetal remains reflects the postmortem modification of their chemical composition after their burial (Sandford, 1992). Specifically, bone diagenesis is the result of cumulative physical, chemical, and biological processes that modify the original chemical and/or structural properties of an organic Introduction Leduc 114 object and that determine its ultimate fate in terms of preservation or destruction . Existing studies mainly report discrete aspects of bone diagenesis, including the permineralization (Briggs et al., 1996; Pfretzschner, 2000, 2001; Parker and Toots, 1970), the recrystallization of bone apatite (Bonar et al., 1983; Chipera and Bish, 1991; Gillette, 1994; Jackson et al., 1978; Kolodny et al., 1995; Lees and Prostak, 1988), and the enrichment of the bone in trace elements (Parker and Toots, 1972; Sillen and Sealy, 1995; Tuross et al., 1989; Williams and Marlow, 1987). Most researches have been conducted so far on archaeological bone material or on a limited quantity of samples. From this perspective, the complete skeletons of Bernissart iguanodons constitute a rich source of information. Although the presence of pyrite and the influence of its oxidation on the conservation of the bones are well known, no detailed mineralogical study of this material has been undertaken so far. The present study was thus undertaken to obtain a better knowledge of the mechanisms of the diagenetic processes so that better conservation techniques could be developed for fossilized bones. Bone diagenesis is a complex process that includes the degradation of organic matter, the recrystallization of bone apatite, the enrichment in trace elements, the precipitation of new minerals in the bone cavities (permineralization ), and the compaction process—and on top of that, all the mechanisms that took place more recently during the storage in natural history museums. Only some of these processes will be discussed in this article. Institutional abbreviation. RBINS, Royal Belgian Institute of Natural Sciences, Brussels, Belgium. Material When the iguanodons were discovered in the Iguanodon Sinkhole, pyrite was abundantly present in the fossil bones. This made them brittle. The clay blocks that contained the fossils remains were covered with plaster to preserve the cohesion of the bones. Afterward, in the museum, all the visible pyrite was...

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