EVALUATING SIGNIFICANCE OF PARTICULATE CARBONATE TRANSPORT TO THE KARST CARBON SINK AND COMPARISON OF KARST LANDSCAPE DENUDATION RATES USING DISSOLVED LOAD VS. TOTAL LOAD CALCULATIONS

Two areas of karst hydrogeology and sediment transport have been little studied in the past. This research measured the role of particulate inorganic carbon (PIC) transport in carbon removal/carbon sink rates in karst, and compared mixed-bedrock landscape denudation rates from dissolved carbonate load (DIC) to rates calculated with total dissolved/sediment loads.

PIC fluxes were quantified in fluviokarst basins by analysis of sediment entrainment and transport using multiple methods including bed load RFID tagging, data logging, and cation/anion analysis of suspended load. Measurements indicated that PIC flux can be significant during peak storm flow. At Tumbling Creek karst basin in Missouri, PIC flux is dominated by surface overflow transport. At Blowing Cave (BC) karst basin in Kentucky, a storm with a 0.87-month return period produced a peak PIC flux of 13.9 g/s and a peak DIC flux of 147 g/s at 1.44 m³/s flow. The largest storm at BC had a return period of 7.7 months, a peak discharge of 4.57 m³/s, and peak PIC flux of 621 g/s compared to a peak DIC flux of 353 g/s. Bed load transport was the most significant PIC component at the BC site. Annually, PIC contributes about 10.3% to total inorganic carbon removal and additional carbon sequestration at BC.

Landscape denudation in karst is commonly determined by measurement of DIC load at base-level springs and calculation based on exposed carbonate outcrop area. This assumes that non-carbonate bedrock is in erosional equilibrium with carbonate bedrock. In mixed carbonate/non-carbonate lithologies, the landscape is also eroded and transported in sediment. By measuring all dissolved, suspended and bed load components at BC, a rate of 60.3 mm/ka was calculated for the 2.31 km² carbonate outcrop surface, and 41.7 mm/ka for the 3.95 km² non-carbonate surface. The basin average denudation rate was 48.5 mm/ka. The rate differences are reflected in the landscape, which exhibits steep-walled escarpment retreat and piracy of allogenic runoff into well-integrated karst conduits. Landscape evolution in fluviokarst settings is more complex than can be measured by solute load analysis alone, and complete mass flux measurement of all erosion components will help improve future studies.