Rapid alteration of cortical bone in fresh- and seawater solutions visualized and quantified from the millimeter down to the atomic scale

Abstract

It is widely known that post mortem diagenetic alteration processes cause modifications and overprinting of the chemical and isotopic proxies incorporated in vivo in bone apatite and collagen. Understanding the processes occurring during the interaction between fluids and bones in an early diagenetic setting is fundamental to determining the extent to which the commonly-used geochemical proxies in bone get modified during fossilization. The present study experimentally investigates the structural and chemical changes in bone induced by controlled in vitro aqueous alteration experiments under simulated early diagenetic conditions. It is intended to derive a deeper phenomenological and quantitative understanding of the transport and reaction processes that occur in the early stage of fossilization. For this purpose, 3.5 mm-sized cylinders were drilled from modern ostrich cortical bone and immersed in different experimental solutions enriched with tracers such as Zn, Sr, rare earth elements, and U. The experiments ran for several hours to weeks at 30, 60, and 90 °C - the latter two temperatures were chosen to accelerate anticipated early diagenetic modifications of the bone samples. Both the bone samples and the experimental solutions were analyzed using micro-analytical techniques such as Raman spectroscopy, electron microprobe, high-resolution inductively coupled plasma-mass spectrometry, nanoscale ion microprobe, and atom probe tomography to assess mineralogical, chemical, and structural changes from the millimeter to the atomic scale. The results show that element uptake into the bone samples occurs within hours after they have been exposed to an aqueous solution instead of years, as previously assumed. Additionally, distinct modifications of the organic phase were observed, accompanied by the growth of new apatite phases by dissolution-reprecipitation and recrystallization processes. Carbonate-poor or -free hydroxylapatite formed in the sample center and more stable carbonated fluorapatite in the sample rim. From these data, a phenomenological model is derived that explains the interaction between bone and aqueous solutions during the earliest stages of fossilization. This study also demonstrates the importance of using a comprehensive methodological approach when investigating alteration processes whose effects range from the millimeter down to the atomic scale.