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Geochemical zoning in a partially fossilized Pleistocene artiodactyl metapodial was contrasted with a fully fossilized Cretaceous ceratopsian femur as distinct stages along the fossilization process to investigate the physicochemical effects of fossilization and fossilization mechanisms. Bone fragments were analyzed via laser ablation ICP-MS, FT-IR, and stable isotope microanalysis to determine zoning patterns in trace elements, relative carbonate content and collagen contents, and stable isotopes of both the CO3 and PO4 components (δ13CCO3, δ18OCO3, and δ18OPO4). The Pleistocene sample shows a pronounced step in U concentration from high values (up to 90 ppm) within 1 mm of the outer bone surface and inner medullary cavity to ppb-level concentrations typical of in-vivo bone in the interior (inner ∼6 mm). High U broadly coincides with reduced collagen, higher δ13CCO3, and lower δ18OCO3. Rare earth elements (REE), however, show typical exponential decreases from bone edges inward, and δ18OPO4 shows no clear trend. The Cretaceous bone shows high REE and U contents throughout, a complete absence of collagen, and relatively uniform isotope compositions. The Pleistocene data, especially U zoning, implicate a diffusion-reaction (DR) fossilization mechanism, in which collagen degradation facilitates a recrystallization front that propagates as a front towards the bone interior, reducing OH-site CO3 content, and shifting δ13CCO3, δ18OCO3, and δ18OPO4. Disequilibrium O-isotope partitioning between the CO3 and PO4 components in the interior of the Pleistocene bone suggests that both the CO3 and PO4 components have exchanged with diagenetic waters, likely through abiotic and microbially-induced exchange, respectively. Strong partitioning between apatite and soil water, coupled with uptake of REE in apatite from surrounding sediment matrix during fossilization, leads to exponential decreases in concentrations, which are passively recorded in apatite in the bone interior. The Cretaceous data do not implicate a unique fossilization mechanism but instead reflect the consequences of protracted fossilization and the final state that the Pleistocene bone might eventually have reached had the bone not been excavated. That is, the Cretaceous bone fossilization history resulted in no collagen, reduced or no OH-site CO3, and more uniform isotopic compositions. The multi-analytical methods employed in this study may benefit studies of diagenetic alteration of other apatitic tissues, e.g., tooth enamel.

A bone fragment (CGDQ-3) of Falcarius utahensis , a therizinosaur from the Early Cretaceous Cedar Mountain Formation, Utah, contained within a carbonate nodule, was analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in order to investigate REE variability within a thin-walled phalanx. Previous studies have found depth-related REE pattern variations; however, in CGDQ-3 variation occurs along the circumference of the bone. NASC-normalized REE patterns and concentrations vary between two apparent end members. A light-REE enriched (LREE) pattern, similar to solution ICP-MS analysis of this bone, characterizes approximately two-thirds of the bone fragment. Total REE concentrations are high and do not vary significantly from the periosteal surface to the medullar surface. Conversely, the remaining one-third of the bone has REE patterns that are MREE-depleted and low in total REE concentrations. REE concentrations in this part of the bone do not vary significantly from the periosteal to the medullar surface. A positive Ce anomaly is found throughout the entire bone, and is greatest within the LREE-enriched portion of the bone. This, in combination with the LREE-enrichment, suggests that the bone fossilized under reducing conditions. The distance between the LREE-enriched and MREE-depleted regions is less than 1 mm. Isotopic and petrographic analyses of the bone and surrounding carbonate matrix suggest the REE patterns in the bone were the result of partial fossilization/incomplete filling of micro-pore spaces around bone crystallites in an environment with changing redox conditions. The lower, MREE-depleted part of the bone fossilized contemporaneously with a pendant cement that formed on the underside of the bone in the vadose zone. Formation of the pendant cement restricted water flow through the bone, isolating the lower portion, which incorporated a MREE-depleted pattern. The upper part of the bone (LREE-enriched side) fossilized under more reducing conditions than the lower part. This resulted in reductive dissolution of minerals (such as hydrous ferric oxides and manganese-oxides) which were LREE-enriched with positive Ce anomalies. These REE were then incorporated into the upper part of the bone. This likely occurred after the bone entered the saturated zone (below the water table). The thinness of the bone and presence of a pendant cement helped facilitate partial fossilization of the bone, preserving the REE signature of an earlier fluid. As demonstrated by this case study, differences in REE patterns within a fossil may record changes in geochemical conditions during fossilization, particularly, when bones are encased in a material that reduces fluid permeability, such as micritic vadose calcite. Analysis of such bones may offer the opportunity to decipher complex fluid histories that occur during fossilization. LA-ICP-MS should be utilized to confirm solution ICP-MS analysis if it yields high REE variability, prior to an interpretation of reworking or time-averaging.