PARTICULATE INORGANIC CARBON FLUX IN KARST AND ITS SIGNIFICANCE TO KARST DEVELOPMENT AND THE CARBON CYCLE

Carbonate bedrock weathering has been recognized as a significant component of the global atmospheric carbon sink. Studies of atmospheric carbon sink processes and landscape evolution in carbonate bedrock terrains have focused primarily on dissolved inorganic carbon flux. Particulate inorganic carbon (PIC) in the sediment load of karst waters is frequently dismissed as insignificant for calculating denudation and carbon transport/sink rates, because chemical processes are assumed to greatly dominate. PIC flux from carbonate terrains may be an important missing term in carbon cycle calculations because carbonate sediment continues to dissolve in undersaturated water downstream of sampling points.

PIC loads are being quantified by measuring sediment load entrainment and transport in fluviokarst settings in Kentucky and Missouri. Bed load transport in underground streams and surface overflow channels is tracked with RFID tagged cobbles and gravel. Automatic samplers and data loggers are used for suspended load and water chemistry. The mass of PIC within suspended sediments is quantified by titration and cation/anion analysis of dual filtered/unfiltered samples. The mass of PIC in the bed load is quantified by separation and measurement of carbonate material from representative samples. Total PIC flux is then compared to dissolved inorganic carbon flux to determine the significance of each component of carbon removal.

Initial analysis of storm events at the Eastern Kentucky site indicates that PIC flux can be significant during peak storm discharges. For example, a total PIC flux of 64 g/s was measured during a moderate storm with a 0.95-month return period and a peak discharge of 1756 l/s, with 127 g/s peak dissolved carbonate flux. The largest storm measured to date had a return period of 1.38 months, a peak discharge of 3880 l/s, and peak PIC flux of 361 g/s. Peak PIC flux for this storm exceeded the peak dissolved inorganic carbon flux of 224 g/s. Bed load movement is the most significant factor contributing to PIC flux at the Eastern Kentucky site during moderate storms, exceeding the suspended load flux by an order of magnitude. When calculated on an annual basis, current data show the additional inorganic carbon load contributes 8.8 percent to total inorganic carbon removal at the Eastern Kentucky site.