Are Sulfur Bacteria Associated With Ancient Phosphogenesis? An Example From the Phosphatic Shale Member, Monterey Formation

Frank A. Corsetti, William Berelson, Jake Bailey, Sarah Greene, and Victoria Petryshyn
Dept. Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740

We have examined a section of the Phosphatic Shale Member at Shell Beach from a geobiological perspective, in order to investigate the potential origins of the phosphate-rich facies. Studies of recent phosphogenesis indicate the presence of Thiomargarita, a giant chemoautotrophic sulfur-oxidizing bacterium correlates with increased pore water and mineral phosphorous. Other sulfide oxidizers (Thioploca, Beggiatoa) are commonly associated with similar environmental conditions, although their potential involvement in phosphogenesis is still under investigation. All of these taxa live at the interface between oxidizing and sulfidic conditions, contain large vacuoles used to store nitrate, and include globules of sulfur and/or polyphosphate within their cytoplasm. Thioploca and Beggiatoa are also known to form extensive microbial mats and appear septate in longitudinal section. Where well preserved, samples from the Phosphatic Shale Member reveal a tangled, mat-like texture of organic filaments on the order of 3 to 10 microns in diameter. In addition, we have discovered larger phosphatized, septate structures that resemble bundles of filaments studded with small opaque spheres. When qualitatively examined with EDS, the opaque spheres are significantly enriched in sulfur. The hemipelagic depths represented by the Phosphatic Shale member make it unlikely that such filaments represent cyanobacteria or other phototrophic organisms, but rather some chemolithotrophic microbe. The size, morphology, and association with sulfur rich globules is consistent with the sulfur oxidizing bacteria. The organic rich setting of the Monterey Formation finds analogue with the loci of modern phosphogenesis in upwelling zones. Therefore, we suggest that the phosphatic rich facies in the Monterey Formation may have been in some measure microbially mediated. We have also identified similar structures from Cretaceous and Neoproterozoic phosphorites, suggesting the possibility of 600 million years of bacterially mediated phosphogenesis.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California