Les iguanodons de Bernissart: leur évolution diagénétique et les processus de dégradation

About thirty complete articulated skeletons of iguanodons have been excavated from Bernissart. This unique collection is conserved at the RBINS. The bones excavated were the object of diverse physical and chemical treatments to protect them from their degradation. These bones were not the object of detailed mineralogical analyses allowing to understand either diagenetic mechanisms involved or to understand the specific mechanisms of their change. This work thus aims at filling this lack. It integrates petrographic (POM, SEM), mineralogical (DRX, Raman Spectroscopy) and chemical (EDS, X-ray fluorescence, microprobe) techniques. By these methods, 30 different minerals were identified.
The diagenesis of Iguanodon bones is the result of cumulative physical, chemical and biological processes that will modify the original chemical and/or structural properties of this organic object. Bone diagenesis is a complex process that includes the degradation of organic matter, the dissolution and recrystallization of bone apatite, the enrichment in trace elements, the precipitation of new minerals in the bone cavities (permineralisation), the fracturation process. A plan is proposed positioning in a relative timescale minerals formed during 3 diagenetic stages and the degradation processes engaged following the bones exhumation.
During burial variable recrystallisation took place: the slightly crystallized carbonated hydroxylapatite (the mineral phase of fresh bone tissue) was replaced by well crystallized carbonated fluorapatite currently present in the Iguanodon bones. Whereas some minerals infiltrated in the bone during cavity filling by the sediment (“mainly” quartz, “argillaceous” phyllosilicates), others are authigenic and precipitated in the cavities during burial (pyrite, barite, sphalerite, celestine, iron oxi-hydroxides, authigenic apatite, calcium sulfate and to a lesser extent vivianite). Pyrite is the most abundant authigenic mineral in the Iguanodon fossil bones. It occurs in different morphologies (crystals, framboids, sticks, fibroradial structure, thin coating…). Four kind of pyrite were determined according to the diagenesis phase during which they were formed and the source of iron and sulfur. Besides pyrite barite is the second most abundant phase. When both minerals are present pyrite has been formed first. The distribution of these minerals within bones is more controlled by micro-environments than by particular structure of the bone.
Since their discovery the Iguanodon fossil bones were treated for conservation in several ways. This however did not prevent the alteration of pyrite into an assemblage of 16 different secondary minerals. The ferrous sulfates (szomolnokite and rozenite) are the most abundant of these minerals and can be found in nearly all samples sulfatized. Other sulfates were formed as the result of alteration of the wealdian shale by acid produced by pyrite oxidation: e.g. tschermigite, natrojarosite and halotrichite. Gypsum, anhydrite, iron phosphate and diadochite are the result of dissolution of bone apatite.
The variations of relative humidity in collections and in showrooms are responsible for the pursuit of the bones degradation (sulfatation of the residual sulfides ; fracturing by dilation / contraction). The relative humidity, conjugated to the temperature variations are the parameters to be controlled for a better preservation of these invaluable fossils. Techniques implementing some chemicals products proposed in the literature are applicable only to certain objects of limited size.