Using plug-flow column reactor data to constrain calcic mineral weathering rates from watershed mass-balance methods: Lithogenic apatite dissolution and phosphorus fluxes into the Loch Vale Watershed ecosystem, Colorado, USA

Sources of phosphorus (P5+) to ecosystems can be very difficult to identify and quantify due to P5+ being monoisotopic. Previous field-based, watershed mass-balance calculations of the lithogenic P5+ flux in the alpine/subalpine Loch Vale watershed (LVW) of Colorado, USA suggested that weathering was controlled by one of three acid-weathering scenarios: (1) All minerals experience only carbonic acid weathering; (2) all minerals experience both carbonic acid and sulfuric acid weathering; and (3) only silicate minerals experience sulfuric acid weathering. We used a plug-flow column reactor to provide additional constraints that strongly indicate scenario (1) (carbonic acid-only weathering) explains the stream chemistry at the LVW outlet (1984-2008 period of sampling). Therefore, sulfuric acid generated by pyrite oxidation in the LVW is not significantly involved in chemical weathering reactions. In the column reactor, calcite dissolution by carbonic acid seemed to suppress apatite dissolution with the likelihood of this suppression confirmed by thermodynamic modeling and comparison to kinetic data. The comparability between column reactor and watershed-scale data likely reflects the LVW landscape being dominated by relatively high rates of mechanical weathering. The lithogenic P5+ flux from the LVW is solely attributable to apatite (Ca-5(PO4)(3) (OH,F,Cl)) dissolution and is similar to 47 mol ha(-1), yr(-1), similar to 18 times greater than the modeled global average. The relatively high solubility of apatite in the Loch Vale watershed likely results from substantial quantities of radionuclides hosted as substituted elements in the crystal lattice, which may cause loss of crystallinity due to self-irradiation. Aeolian inputs to landscapes may supplement any P5+ from primary phosphate mineral dissolution. These natural sources of P5+ are likely contributing to the increases in algal biomass observed today.