275-9 Bedrock Fluorapatite Dissolution Rates and Phosphorus Fluxes from the Granitic Periglacial Headwater Loch Vale Watershed, Rocky Mountain National Park, Colorado, USA in a Warming Climate

Abstract:

The global phosphorus and carbon cycles are intimately linked through marine photosynthetic productivity with bedrock chemical weathering being the sole source of phosphorus to natural waters and ecosystems.  Solute-based watershed mass-balance modeling is one of the most accurate means for quantifying mineral weathering rates and elemental transfers, including phosphorus, at the Earth’s surface.  Such methods have been utilized for the 1984-2008 measurement period to calculate the fluorapatite dissolution rate, and associated phosphorus flux, for the 680 ± 2 hectare granitic headwater periglacial experimental Loch Vale watershed (LVW) in Colorado, USA.  Because phosphorus primarily leaves the LVW adsorbed to sediment, the fluorapatite dissolution rate was determined from the calcium, bicarbonate, and fluoride stream fluxes.  Two standard deviation uncertainty on all hydrogeochemical kinetic calculations were determined using bootstrapping statistical methods.  It was impossible to achieve a watershed mass-balance after 2008, likely a result of escalating loss of the permanent cryosphere disrupting the watershed-scale geochemical weathering stoichiometry.  Despite the LVW bulk bedrock containing only ~0.07 weight % P₂O₅ and ~0.1 % modal primary fluorapatite, the 1984-2008 fluorapatite dissolution rate for the LVW was 16 ± 1 mol ha^-1 yr^-1 with an associated phosphorus flux of 48 ± 2 mol ha^-1 yr^-1.  This area-normalized watershed-scale phosphorus flux for the LVW is almost double that of any other location reported in the literature to date.  However, because fluorapatite dissolution in the LVW is controlled by winter frost-cracking, climatic warming will likely reduce future phosphorus fluxes which may adversely impact downstream natural terrestrial and aquatic ecosystems, agriculture, and aquaculture.  

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