1 The Australian National University, ACT, Australia.

2 School for Earth and Ocean Sciences. University of Victoria, BC, Canada. E–mail: crbarnes@uvic.ca

Key words: Conodonts. Geochemistry. Palaeoenvironments. Ordovician. Silurian.

A resurgence in studies using conodont apatite as a proxy for seawater composition has prompted a re–assessment of their suitability as geochemical tracers. This has not been adequately addressed in the past and requires clarification; an effective sampling protocol is also needed to ensure data integrity. Sample integrity is critical for determining primary geochemical signatures to evaluate environmental conditions and processes in Earth history. Trace element and isotopic heterogeneity of conodont tissues are described and interpreted in the context of palaeoenvironmental analysis.

Previous conodont geochemical studies have utilized conventional geochemical approaches (e.g., REEs, Sr and O isotope analyses), with isotopic work usually based on bulk samples (>1 to 10’s or 100’s of conodont elements) of mixed and undisclosed taxa. Recent technical advances afford high–resolution, in–situ, micro–analysis (e.g., laser ablation ICPMS), and has been used by the present study to determine trace element and isotopic compositional profiles of single conodont elements. Compositional relationships to ultrastructure, taxonomy, and histology (albid, hyaline crown tissues) have been recognized and their implications for geochemical studies are discussed. Various trace element and isotopic systems in conodont apatite are assessed then applied to continuous stratigraphic sections from low latitude, Ordovician and Early Silurian sequences. These are described in the context of palaeocean reconstruction and environmental processes that were likely operating at key times within this interval.