OXIDATION-REDUCTION COUPLES IN THE FORMATION OF STROMATOLITES: AN EXAMPLE OF IRON CYCLING IN OBSIDIAN POOL PRIME HOT SPRING, YELLOWSTONE NATIONAL PARK, WYOMING (Invited Presentation)

Siliceous stromatolites form around the edge and just ‘off shore’ in Obsidian Pool Prime Hot Spring, Yellowstone National Park, Wyoming. Laminations within the stromatolite are composed of silicified cyanobacterial sheaths arranged in light-dark couplets (a very porous light colored set oriented primarily in the growth direction, and a much thinner, less porous, dark set oriented perpendicular to growth). Large (100-200 um), round cavities are abundant in certain light layers, which we interpret as molds of bubbles formed via oxygenic photosynthesis. Iron oxides are preferentially concentrated in the bubble molds vs. elsewhere in the stromatolite. Providing platforms for stromatolite growth in the spring reveals that growth is not continuous nor always prevalent, but when growth does occur, the overall vertical growth velocity is cm’s/year. We’ve argued that temperature (T) plays a large role in driving dissolved Si precipitation, as we know the solubility of Si is a strong function of T. Cooling by a few degrees T can drive supersaturation increase by 10x. Yet we are also exploring the role of cyanobacterial photosynthesizers lowering pCO₂, elevating pH and driving the precipitation of Fe(III) from dissolved Fe(II). The oxidation rate of Fe(III) is dependent on both pH and oxygen concentration, hence these environments are strongly favorable for this reaction. Under high dissolved Si conditions, Fe(III)-oxide formation can scavenge SiO₂, aiding the precipitation process. All of the above occurs during daytime. In the dark, Fe(III)-oxide reduction may occur via C_org respiratory heterotrophy. It is not lost on us that these coupled reactions, also likely to occur in carbonate systems, will promote calcium carbonate mineral precipitation by driving up saturation state. A test of the Fe-hypothesis is underway. Preliminary results are encouraging: [Fe] in spring water is ~18 uM. It has a δ⁵⁶Fe isotopic value of -0.55±0.06 ‰. Interstitial stromatolite water is slightly lighter, -0.69±0.10, acid leachable solid Fe from the stromatolite is considerably heavier 0.71. This isotope pattern is consistent with the redox cycle we predicted, and overall, the process provides a new way to think about stromatolite formation through time.