BIOGENICITY OF MIXED SILICA-CARBONATE ACCRETIONARY STRUCTURES FROM A HOT SPRING (LITTLE HOT CREEK, CA)

Interesting cm-scale mixed-mineralic accretionary structures were studied in the Little Hot Creek Area, California, to investigate their mode of formation and biogenicity. Little Hot Creek is fed by a series of hot springs located on the eastern edge of the resurgent dome of the Long Valley Caldera. Spring LHC-1 emerged at ~80°C, was circumneutral in pH and had a salinity near 0 ppt. The spring feeds a meter-wide channel that joins other springs that enter Little Hot Creek. This spring was supersaturated with respect to calcite, and contained ~1.0 mM silica (undersaturated for silica at that temperature).

The spring channel was lined with cm-scale accretionary structures that have precipitated from the edge of the stream towards the center. The structures were typically 15 cm wide and partially submerged. The submerged portion was composed of bladed calcite, and revealed progradational growth into the stream, with well-defined mm-scale foresets. Several generations of additional carbonate growth have occurred along the base of the prograded platform. In contrast, the emergent portion of the structure was composed of amorphous silica with subsidiary carbonate. Here, the silica formed mm-scale laminated columnar structures, resembling stromatolites. The structure was relatively porous and carbonate occludes some of the pore space, with filamentous microfossils occuring within the silica-rich region.

Molecular analyses of the structures indicated the microbial community was composed of members of the phyla Cyanobacteria, Armatimonadetes, Deinococcus-Thermus, and Chloroflexi. Despite the presence of these microorganisms, unraveling the biogenicity of the structures was not straightforward. The bladed calcite that composes the bulk of the carbonate portion revealed no microstructural indication of microbial involvement. The silica portion however, resembled lithified microbial mats in some areas and contained filamentous microfossils, but also contained crystalline structures that would typically be interpreted as abiogenic. Thus, it is likely that both abiogenic and biogenic processes operated in tandem during the precipitation of the unique structures.

This research was conducted in association with the 2014 International Geobiology Course.