congresos y reuniones científicas
ineralogical analysis in bones of Cretaceous reptiles: geochemical attack during fossil diagenesis or depositational nucleus of Fe and Mn oxides-hydroxides?
Congreso; 18th International Sedimentological Congress - Mendoza, Argentina, 2010; 2010
Institución organizadora:
Asociación Argentina de Sedimentología
Presence of Fe and Mn oxide-hydroxides on bones of fossil vertebrates is frequent in sedimentary depositssuch as concretions lying in mottled beds and directly associated to the fossil-diagenetic process of the remains.These concretions are a consequence of precipitation of minerals present in gels formed within aqueous solutions.Bone pieces may be unaltered or modified – at least superficially – and are usually found within the samebed. Such a taphonomic attribute elicits questions on the driving process that concentrates these minerals on thebone nuclei. To understand this pattern, the chromatic variation of the bones of Kaikaifilusaurus avelasi(Apesteguía and Novas, 2003) [MPCA-365] was analyzed. The specimens were collected from the CandelerosFormation (Cenomanian–Turonian) exposed at “Huene” quarry (S39º36.831´/W68º40.254´), La Buitrera locality,Rio Negro Province. Analyses included X-ray diffraction [XRD], infra-red spectroscopy [IR-FT] and scanningelectron microscope [SEM]. Following the Cailleux soil color chart, they were classified into 5 categoriesfrom A to E: very pale chestnut-brown (10 YR 8/3); weak red (10 R 5/3); reddish chestnut-brown (5 YR 4/3);dark reddish chestnut-brown (5 YR 3/2) and very dark gray (2.5 Y 3/0). Finally, following the “Bone Weather -ing Stages” proposed by Behrensmeyer (1978), they were grouped into stages 1 (A-B) and 2 (C-D-E). From theXRD analysis it was determined that all the pieces contained varieties of Apatite [Francolite, Carbonatehydroxyapatiteand Carbonatefluoroapatite] in different proportions. In addition, A presents traces of Analcime; Ccontains a higher Hematite concentration than B; D is richer in this oxide than in phosphate, with subordinatePyrolusite. Finally, E has subordinate Hematite to Pyrolusite and Manganite, with scarce Apatite. The IR spectrumindicates that A is composed of 65% Apatite and 35% Carbonatehydroxyapatite; and that in D and E thereis a relative decrease of the phosphate bands. Macroscopically, A and B show fractures parallel to the bone fibresand SEM of A shows unaltered periosteal bone. On the other hand, damaged and mosaic-patterned cracking andflaking bone surfaces are clear in C, D and E. This is more clearly noticeable in C because of the thinner coating,while in D and E it is obscured by the almost complete absence of bone material. This study suggests a differen -tial preservation of fossil remains from the same strata. Such a difference appears early in the biostratinomic processwith the weathering of the carcass altering the surfaces of the bones. The extended exposure of the remainswould cause the pre-burial damage observed in the periosteal bone. This renders the formation of oxide and hydroxidecoatings over the material easier. During fossil-diagenesis the bones act as crystallization nuclei, and theformation of concretions over them is enhanced by circulation of fluids loaded with oxides and hydroxides withinthe fossil-bearing layer. This differential precipitation produces anomalous concentrations (mottles) of Hematite,Pyrolusite and Manganite over the most heavily weathered bones. A remarkable direct correlation existsbetween the weathering stage and the increasing (from C to E) clouding of boundaries between bone and oxide-hydroxides resulting from phosphate/Fe - Mn reactions (Staley et al., 1992; Cornell & Schwertmann, 2003).