CONTROLS ON MICROBIAL BIOSIGNATURE PRESERVATION IN SUBAERIAL HYDROTHERMAL SPRINGS, YELLOWSTONE NATIONAL PARK

Previous paleontological studies of cyanobacterial mat systems in thermal spring environments have revealed the dominant fossilization process to be pervasive encrustation (templating) of organic surfaces, followed by rapid organic degradation to produce external molds. Organically preserved microfossils are uncommon, being largely restricted to lower temperature microfacies where permineralization processes are more common.

Microelectrode measurements of rapidly mineralizing hot spring mats in Yellowstone have revealed that mat/sinter frameworks retain anomalously high photosynthetic oxygen concentrations (between 3-7 times atmospheric) down to a depth of several centimeters. In these systems, the presence of a slime (exopolymer) matrix appears to play a key role in the retention of photosynthetic oxygen by reducing mat permeability.

Thermal spring mats show clear taphonomic contrasts with well-studied laminated mat systems of coastal marine salars in Central Baja. High sulfide concentrations in these rapidly accreting systems is associated with slower rates of organic matter degradation. Early diagenetic mineralization of analogous laminated mat systems in late Proterozoic peritidal environments produced many well documented examples of excellent cellular-level preservation.

The high oxygen concentrations observed in rapidly-mineralizing thermal spring mat systems appear to account for the rapid degradation of organic matter that occurs during early diagenesis and is consistent with observations of ancient thermal spring sinter analogs. However, a deeper understanding the taphonomic processes of thermal spring environments will require clarification of the balance between carbon burial and oxidation and a knowledge of the actual processes of organic matter degradation. In theory, high oxygen concentrations should restrict sulfate reduction, while sustaining high rates of aerobic heterotrophy and inorganic oxidation. Future experiments will seek to define the relative contributions of these processes.