DETERMINING THE ORIGIN OF SUBSURFACE AMMONIUM SUPPORTING A BIOSPHERE AT HENDERSON MINE, CO

While all life on Earth requires nitrogen as an essential element, deep subsurface environment lie outside the traditionally accepted biological nitrogen cycle. This prompts the question, what are the potential sources of nitrogen for life in the deep subsurface? At Henderson Mine in Colorado, a deep biosphere thrives in fluids circulating among fractures and pore spaces of the granite that hosts the Mo-bearing stocks (Sahl et al., 2008). These fluids are interpreted to be mixtures of meteoric and older, brinier water based on d¹⁸O and dD measurements and mixing trends of rock-derived SO₄, Mn and F. However, brinier fluids also contain correspondingly higher amounts of nitrogen as primarily NH₄⁺ with less NO₃^- and NO₂^-. We propose that NH₄⁺, like SO₄, Mn and F, is released from micas or other K-bearing minerals through water-rock interactions at depth. A possible source of NH₄⁺ to the minerals originally may have been from hydrothermal convection of NH₄⁺-bearing surface fluids. In fact, other studies have found NH₄⁺ in granitic minerals that have interacted with NH₄⁺-bearing hydrothermal fluids (Hall et al., 1991). To test whether the source of NH₄⁺ in the fluids is rock-derived, we will screen micas from a range of unaltered to altered rocks for structural NH₄⁺ using FTIR microscopy. Growth enrichments of microbes from the subsurface waters will be evaluated for their potential to fix N₂ as another possible source of subsurface NH₄⁺. Furthermore, the microbial community’s dependence on NH₄⁺ for processes such as nitrification will also be evaluated, potentially expanding the role of nitrogen from nutrient to energy source. Determining the source of subsurface ammonium will help us to expand our understanding of the biological and rock nitrogen cycles to the subsurface.