Paper No. 10
Presentation Time: 3:45 PM


SPEAR, John R.1, CORSETTI, Frank A.2 and BERELSON, William M.2, (1)Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887, (2)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089,

Siliceous stromatolites were collected from a hot spring (75°C) in Yellowstone National Park in order to investigate stromatolite morphogenesis and growth rate. The majority of the examined stromatolites are composed of relatively porous light and dark layers of silica-coated filaments. The light layers (~150 um thick) predominantly consist of surface normal filaments and the dark layers (~50 um thick) are composed of reclining filaments. The main body lamination is interrupted by another style that drapes the entire structure, contains coccoidal as well as filamentous microbial forms, is well-cemented with silica, and includes a significant population of pennate diatoms. Over the course of stromatolite growth, the main body style and the drape style lamination appear to alternate, but the majority of the growth is composed of the light/dark couplets. Radiometric dating (228Th/228Ra, 228Ra/226Ra, and 137Cs) indicate that the growth of a 5 cm stromatolite occurred on the order of years, and in situ growth experiments reveal that the light/dark laminae couples do not represent daily, weekly or seasonal cycles.

14C analysis reveals that organic matter from the main body consistently contains lower D14C versus the drape fabric. As CO2 from the hydrothermal vent waters is radio-carbon dead (which would result in lower D14C values), we interpret the main body to have formed when spring level was deeper/hotter, resulting in a greater influence from spring derived CO2 during photosynthetic CO2 uptake, and the drape to have formed when spring level was shallower/cooler, resulting in a greater incorporation of atmospheric CO2.

Combining the radiometric age dating, in situ growth experiments, and D14C analysis suggests that growth of the light/dark couplets is sporadic and linked to rising water levels, whereas formation of the drape style lamination represents falling water levels and possible emergence. It is possible that the changing water levels, and thus stromatolite growth rate and morphology, record the larger scale geologic processes in Yellowstone National Park.