Quantification of Dynamic Water-Rock-Microbe Interactions in a Travertine-Depositing Hot Spring, Mammoth Hot Springs, Yellowstone National Park, USA

Filamentous microbial mats in a travertine-depositing hot spring at Mammoth Hot Springs in Yellowstone National Park exert primary controls on the growth rate, mineralogy, and crystal fabric of calcium carbonate minerals (travertine) that precipitate in the spring. Filaments directly affect porosity and permeability of travertine by providing a structural framework consisting of "ropes" of microbial cells around which carbonate minerals precipitate, creating a uniquely biogenetic mineral fabric characterized by horizontal layers of large tubular pores. Nanometer scale microscopy reveals that these mineral fabrics may be directly tied to microbial activities, as aragonite crystals precipitating directly on filaments are smaller and more densely packed than crystals precipitating on extra-polymeric substances (EPS) between filaments. In order to more closely examine the processes which control calcium carbonate crystallization dynamics in this system, a high-resolution transect of water and travertine was sampled for geochemistry, microscopy, and microbial biomass along the primary flow path from upstream to downstream of Narrow Gauge spring at Mammoth Hot Springs. Travertine samples were analyzed for petrography using transmitted light, cathodoluminescence, and laser confocal microscopy to examine crystal morphology and associations with microbial filaments and provide insight on pore network distributions. Additionally, travertine and spring water geochemistry was also analyzed for major and trace ions, δ34S, δ13C, and δ18O, to identify any trends that may relate to crystallization rates, microbial biomass, or crystal habit. Total biomass was determined using dried weight. Water-rock-microbe interactions result in upstream-to-downstream variations in travertine crystal morphology and water chemistry that are directly related to systematic changes in microbial biomass and community respiration. Geochemical modeling lends insight into the biogeochemical reactions that affect travertine crystallization in this dynamic spring system. Quantification of these processes allows development of a dynamic process-based biomarker that can be used to more accurately identify and interpret ancient microbial ecology and evolution on Earth or potentially other planets.