Nitrogen Isotope

(Submitted Abstract to the 2003 Geological Society of America Meeting, Seattle, Washington)

Nitrogen isotope evidence of ammonia vapor assimilation by cave wall microbial biofilms in a sulfidic cave, a novel mechanism of nutrient acquisition

* Stern, L A, Engel, A S, Bennett, P C
University of Texas, Department of Geological Sciences, Austin, TX 78712-0254 United States

Ammonia volatilization provides a source of fixed nitrogen to the microorganisms living at low pH on the walls of Lower Kane Cave. Sulfuric acid speleogenesis is actively enlarging this cave; hydrogen sulfide dissolved in ground water is oxidized to sulfuric acid both in the cave streams and after volatilization to the cave walls. The result of this process is replacement the host limestone with a rind of gypsum on the cave walls upon which droplets with pH values of 1-2 sulfuric acid accumulate. The radically different pH values of the cave stream and cave wall habitats results in an unusual mechanism of fixed nitrogen acquisition by the cave wall microbial community, accumulation of volatilized ammonia. The spring water entering the cave has 25 to 40 mM NH4+ and circum-neutral pH allowing a small but significant amount this ammonium to volatilize as NH3. This gaseous ammonia may then partition into the acidic cave wall droplets accumulating to concentrations of up to 800mM where it serves as a nitrogen source to the cave wall microorganisms. The effects of this ammonia volatilization may be seen in the extremely low d15N value of the cave wall biofilms, as low as -16 permi$. These d15N values are among the lowest observed in organic materials. Therefore nitrogen isotope ratios represent a marker of ammonia volatilization in subaerial sulfuric acid speleogenesis that may be applied to cave systems that were once sulfidic to delineate regions of subaerial versus subaqueous cave formation.