Ammonium in thermal waters of Yellowstone National Park: Processes affecting speciation and isotope fractionation

Abstract

Dissolved inorganic nitrogen, largely in reduced form ( NH 4 ( T ) ≈ NH 4 ( aq ) + + NH 3 ( aq ) o ), has been documented in thermal waters throughout Yellowstone National Park, with concentrations ranging from a few micromolar along the Firehole River to millimolar concentrations at Washburn Hot Springs. Indirect evidence from rock nitrogen analyses and previous work on organic compounds associated with Washburn Hot Springs and the Mirror Plateau indicate multiple sources for thermal water NH 4(T), including Mesozoic marine sedimentary rocks, Eocene lacustrine deposits, and glacial deposits. A positive correlation between NH 4(T) concentration and δ 18O of thermal water indicates that boiling is an important mechanism for increasing concentrations of NH 4(T) and other solutes in some areas. The isotopic composition of dissolved NH 4(T) is highly variable (δ 15N = −6‰ to +30‰) and is positively correlated with pH values. In comparison to likely δ 15N values of nitrogen source materials (+1‰ to +7‰), high δ 15N values in hot springs with pH >5 are attributed to isotope fractionation associated with NH 3 ( aq ) o loss by volatilization. NH 4(T) in springs with low pH typically is relatively unfractionated, except for some acid springs with negative δ 15N values that are attributed to NH 3 ( g ) o condensation. NH 4(T) concentration and isotopic variations were evident spatially (between springs) and temporally (in individual springs). These variations are likely to be reflected in biomass and sediments associated with the hot springs and outflows. Elevated NH 4(T) concentrations can persist for 10s to 1000s of meters in surface waters draining hot spring areas before being completely assimilated or oxidized.