ENVIRONMENTAL SCANNING ELECTRON MICROSCOPY AS A TOOL FOR ASSESSING TAPHONOMIC BIAS IN PROTEROZOIC ACRITARCHS

The presence of eukaryotic biomarkers in Archean sedimentary rocks (Brocks et al., 1999) has given new life to the question of whether eukaryotic fossils can be reliably identified in the geologic record. TEM and SEM analyses of acritarchs in Mesoproterozoic shales show promise for elucidating eukaryotic cell wall ultrastructure in ancient samples (Javaux et al., 2003). At present, however, modern-fossil comparisons are difficult to make because of the technical challenges of imaging modern microbes by conventional electron microscopy techniques. The Environmental Scanning Electron Microscope (ESEM) allows high-resolution imaging of uncoated, delicate, and hydrated samples. Using this tool, fresh microbial cultures can be imaged directly and their surface morphology compared to fossil microbes in chert or shale. Using this technique, taphonomic alterations to microbial ultrastructure can be assessed in situ and compared to observed fossil ultrastructure.

We conducted a series of ESEM analyses of modern and fossil microbes, with the aim of identifying common taphonomic and taxonomic features. Live cultures of the green alga Eremosphaera were assessed using the ESEM in environmental mode (hydrated). Uncollapsed cells exhibited the typical smooth-walled structure that characterizes this alga. Fresh (undecomposed) cells collapsed by changing chamber pressure during imaging displayed a fine-scale reticulate surface pattern, and decomposing cells display such a pattern to varying degrees. Some acritarchs possess a similar reticulate texture, which has been described as a taxonomic feature. Our results suggest that this surface pattern could be created by diagenetic processes such as compaction or desiccation. We are currently investigating other algal taxa to determine whether the observed reticulate pattern forms commonly during collapse, or whether its expression is limited to certain algal groups. These preliminary results suggest that ESEM studies are useful in understanding taphonomic pathways in modern microbial eukaryotic organisms and might provide insight toward potential taphonomic bias when examining the Proterozoic acritarch record.

Javaux, EJ et al., 2003, GSA Abst. Prog. 35(6):456. Brocks, JJ et al., 1999, Science 285:1033.