U-Pb systematics of fossilized biogenic tissues as a tracer of long-term diagenetic processes

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Fossilisation of mineralised (bone, tooth) and non-mineralised organic tissue (wood) is a complex, post-mortem alteration or diagenetic process that involves a number of parallel or subsequent transport and reaction steps. These processes are related to certain geological processes, such as the mobilisation of a fluid phase and/or temperature changes, may run over short time scales of days to even several millions of years and are not yet fully understood. Although relative permineralisation and replacement sequences can be identified in fossil wood and bones by detailed petrographic and mineral-chemical and -structural investigations, the absolute time scales and durations of such processes are not yet known, particular for pre-Pleistocene fossils. If a biogenic template has incorporated radioactive isotopes of trace elements during early diagenetic processes, relatively rapidly after death, and remained a closed system throughout geologic time, then geochronological methods should return a radiometric age consistent with the near-depositional or stratigraphic age. The available data, however, suggests that fossil tissues do not necessarily record dates that are concordant with the expected stratigraphic age. Instead, they might provide a tool to unravel a sequence of different diagenetic alteration processes, not otherwise identifiable, and eventually consistent with the earliest diagenetic state accessible. The U-Pb system is well suited to study low temperature processes. Modern mass spectrometric analytical techniques such as laser ablation inductively-coupled plasma mass spectrometry, ion microprobe, nano-sized secondary ion mass spectrometry, and atom probe tomography opens up new avenues to investigate the U-Pb isotope system of complexly-structured amorphous or micro- to nano-crystalline phases that are involved in the fossilisation of organic tissue. The prime objective of this project is to unravel the timing of different diagenetic processes such as element transport, mineralisation, or recrystallisation at different length scales in order to increase our understanding of the long-term fossilisation process. In addition, we want to find geochemical, textural, or mineralogical criteria for screening the most promising petrified wood and fossil bone samples or sample domains for "direct" U-Pb dating. Domains in fossilised bone that give near depositional ages are expected to also remain close with respect to stable isotope systems, which is critical for the application of proxies for palaeodietary, palaeoenvironmental, and palaeoecological reconstructions.