USING PLUG-FLOW COLUMN REACTOR DATA TO CONSTRAIN CALCIC MINERAL WEATHERING RATES FROM WATERSHED MASS-BALANCE METHODS: LITHOGENIC PHOSPHORUS FLUXES FROM APATITE DISSOLUTION INTO LACUSTRINE ECOSYSTEMS OF THE LOCH VALE WATERSHED, COLORADO, USA

Cirque lakes within the Loch Vale watershed of Colorado, USA are experiencing increases in algal biomass despite being located in high-elevation remote wilderness. Hence, quantifying the lithogenic phosphorus (P⁵⁺) flux to ground and surface waters attributable to apatite (Ca₅(PO₄)₃(F,Cl,OH)) dissolution is critical. Determination of the apatite weathering rate will be governed, at least in part, by the acids involved in chemical weathering. Previous watershed mass-balance calculations of the apatite weathering rate, and consequently the P⁵⁺ flux from apatite dissolution, in the Loch Vale watershed identified three equally plausible acid-weathering scenarios. These scenarios are: (1) All minerals experience only carbonic acid (H₂CO₃) weathering; (2) all minerals experience both H₂CO₃ and sulfuric acid (H₂SO₄) weathering; and (3) only silicate minerals experience H₂SO₄ weathering. Since bicarbonate (HCO₃^-) fluxes in the watershed could result from carbonate mineral dissolution and/or the H₂CO₃-promoted dissolution of other minerals, watershed HCO₃^- flux data are insufficient to provide evidence for which of the three acid-weathering scenarios is most probable.

Results from a laboratory plug-flow column reactor strongly indicate that scenario (1) (H₂CO₃ weathering) explains the stream water chemistry at the Loch Vale outlet for the 1984 – 2008 period of sampling. Therefore, H₂SO₄ generated by pyrite (FeS₂) oxidation in the Loch Vale watershed is not significantly involved in chemical weathering reactions. Such comparability between column reactor and watershed-scale data likely reflects the Loch Vale watershed landscape being dominated by tundra, bare rock, and relatively high rates of mechanical weathering.

The flux of phosphate (PO₄^3-) out of the Loch Vale bedrock is ~47 mol ha^-1 yr^-1, ~18 times greater than the modelled global average. Apatite hosts significant quantities of radionuclides and its relatively high solubility in the Loch Vale watershed may reflect that it has experienced loss of crystallinity from self-irradiation. These findings demonstrate the importance of mineralogic characterization in studies of P⁵⁺-release from bedrock during chemical weathering.