Hydrological processes in microbial preservation

摘要

Extreme environments, such as aqueous, high temperature, mineralizing systems (thermal springs) are the focus of the search for evidence of life on early Earth or on Mars. Mineral deposition from saturated waters potentially entombs these organisms complicating hydrological control of the fossilization process. Near-surface and subsurface hydrology of these systems is governed by the porosity and continuity of pore spaces in microbial mats and associated sinter deposits. Herein we examine the evolution of pore space in microbial mats with emphasis on the relationship between pore size and geometry, and silica deposition. Microbial mats living in the outflow channels of silica-rich thermal springs in Yellowstone National Park, WY, and Steamboat Springs, NV are best preserved under conditions of intermittent inundation and drying and/or cooling. This leads to periodic deposition of silica initially as a coating on the cells and eventually as an infilling in the cells. As a consequence, pore spaces between microbial filaments retain characteristic configurations and are filled with silica crystals of different size and morphology than that of the coatings or fillings. The nature of the pore-filling silica is controlled by the temperature and chemistry of the water flowing through the sinter mound and is indicative of the environment of preservations.