INSIGHTS INTO INACTIVE SPRING CARBONATES BASED ON THE CHARACTERISTIC MICROSTRUCTURE OF THE DESMID OOCARDIUM STRATUM

Currently inactive carbonate spring deposits have been found north of Santa Barbara, California, but the nature of their deposition (cold or hot spring) is unknown. The spring deposits formed upon a steep slope composed of the Monterey Formation, with a vertical drop of at least 10m. The thickness of the spring deposit is not yet known, but is likely on the order of 1-2m thick. SEM and light microscopic analyses of the carbonate microstructure reveal a distinct crystal morphology strikingly similar to structures formed by the desmid Oocardium stratum—a unicellular, round, micro-alga known for its ability to secrete mucilage tubes that become encrusted in the growth of metabolically-induced calcite. Living cells of Oocardium stratum are known exclusively from active, carbonate-depositing, cold water springs. Due to its restricted habitat, the detection of the Oocardium fabric at this site limits the depositional environment of these carbonates to cold, calcium-rich, flowing water—illustrating an example of using microbe-mineral interactions as a tool to reconstruct ancient carbonate environments.