Bacterially Induced Spherulites: Examples from Hot Spring, Cold Spring, and Pedogenic Environments

Carbonate spherulites are composed of radiating arrays of crystals and occur in hot spring, cold spring, lacustrine, spelean, and pedogenic environments. Examples from each of these settings display clumps of eubacterial body fossils at their nuclei.

Modern spherulites from hot spring deposits in Yellowstone National Park are composed of well-developed aragonite crystals commonly 5 to 10 microns wide and 500 microns in length. SEM analyses clearly show that coccoid eubacterial bodies occur as clumps, 10 - 20 microns in diameter, in the centers of the spherulites. Radial arrays of aragonite crystals extend outward from these clumps of eubacteria. The aragonite crystals sometimes encase small clumps of eubacterial bodies immediately adjacent to the nuclei. However, there is no evidence of bacterial bodies within the aragonite crystals farther outward from the core region. Thus, eubacterial colonies, either actively or passively, helped to overcome the inhibition for the initial aragonite crystals to nucleate. Once the initial crystals had formed, the aragonite was readily able to continue to grow abiotically.

Spherulites in caliche deposits in Texas are commonly spherical to oval bodies 20 to 50 microns across. They generally have a central cavity and their cortices are composed of radiating bulbous, rod-shaped calcite crystals. The cores are made-up of 0.2 to 0.3 micron calcite crystals that encase eubacteria. In contrast with the hot spring spherulites, the rod-shaped crystals making up the radiating cortices of the pedogenic allochems have encased short linear arrays of coccoid eubacteria. Spherulites from a cold spring setting (Crystal Geyser, Utah) are similar to the pedogenic forms, i.e., the eubacterial bodies are abundant within the radiating calcite crystals.

Differences between these two types of spherulites can readily be due to the vastly different environmental conditions under which they formed. Hot spring precipitation occurred in highly supersaturated water yielding predominantly abiotic aragonitic spherulites whereas growth within cold spring and soil environments produced spherulites predominantly made-up of biotically induced calcitic precipitates. Eubacteria are believed to have been instrumental in the initiation of the carbonate precipitation.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California