Conodont Geochemistry - proxies for understanding palaeoenvironments, bio-events and geo-events of the Palaeozoic

Conodont Geochemistry - proxies for understanding palaeoenvironments, bio-events and
geo-events of the Palaeozoic

J A Trotter

The ubiquity and biostratigraphic significance of conodonts underscore their potential importance for geochemical studies, especially as potential recorders of past oceanic composition, with the aim to better reconstruct and understand palaeoenvironmental change during the Palaeozoic and early Mesozoic. They have great potential to help define the processes that were operating in palaeoceans, both regionally and globally, particularly in the context of bio- and geo-events, and thus the driving forces that determine the evolution of life. Understanding extinction events and the processes controlling life during Earth history has immediate implications for present and future life on Earth.

Despite the increasing focus on conodont apatite for such studies, their reliability as proxies of ambient seawater chemistry has been questioned by the geochemistry and palaeontology communities due to issues of data reproducibility and internal consistency, and is therefore yet to be effectively demonstrated. Chemical systematics of marine biogenic apatite in general is not well understood due to the lack of rigorous and systematic investigations to address potential diagenetic effects. Studies to date incorporate many assumptions on the integrity of conodont tissues and are based on

bulk specimens of which details regarding taxonomy and morphology are generally lacking, yet are likely to have significant implications for the integrity of data generated. Detailed and high resolution geochemical studies are therefore required to determine the significance of such variables (including histology) that have been thusfar overlooked, to address these critical issues of sample and data integrity.

This study aims to determine the suitability of conodont apatite as a recorder of ambient seawater chemistry and the criteria to discriminate primary geochemical signatures from secondary effects and background noise. A multi-proxy approach of trace element and isotope geochemisty using an array of high-resolution instrumentation are being applied to key intervals through the Ordovician and Early Silurian to identify potential relationships between climate cycles, tectonics, fluxes in ambient seawater chemistry and the biosphere. Initial work has produced continuous, high-resolution chemical profiles of different components within single conodont elements using in-situ laser ablation ICPMS. Systematic variations in the chemistry of the component tissues have been recognised and related to conodont ultrastructure (SEM), and have significant implications for the integrity of conodont tissues and sampling strategies for further conodont geochemical studies. Current work is also utilising Transmission Electron Microscopy, Laser Raman Spectroscopy, Electron and Ion Micropobes and MC-ICPMS, targeting specific tissues to assess their chemical and isotopic heterogeneity, preserved textures, and relative susceptibility to diagenesis.