Particulate inorganic carbon flux and sediment transport dynamics in karst: Significance to landscape evolution and the carbon cycle.

Abstract

Research focused on three areas of karst hydrogeology and sediment transport that have been poorly studied in the past: the role of particulate inorganic carbon transport in calculating carbon sink rates in karst; rapid changes in surface vs. subsurface sediment mixing in karst conduits; and comparison of landscape denudation calculations using dissolved carbonate load vs. total dissolved/sediment load. Carbonate bedrock weathering is a significant component of the atmospheric carbon sink. Particulate inorganic carbon (PIC) in bed and suspended sediment load of karst waters is frequently dismissed as insignificant for calculating denudation and carbon transport/sink rates, but PIC flux has not been adequately studied. PIC fluxes were quantified in fluviokarst settings using RFID-tagged cobbles and gravel to track subsurface bed load flux, along with remote loggers and lab analysis of sediment and water chemistry for suspended and dissolved loads. PIC contributed about 10.3 percent annually to total inorganic carbon removal and additional carbon sequestration at Blowing Cave basin. Cosmogenic and fallout radionuclides 7Be and 137Cs were also utilized to develop a better understanding of rapid, short-term changes in sediment flux and transport through karst aquifers. The ratio of surface-derived sediment to stored and remobilized subsurface sediment in cave streams was estimated by tracking isotopic changes across storm runoff events. During a large storm event, approximately 34% of the total sediment flux during the 4-day runoff event originated from surface erosion. Landscape denudation rates in karst settings have been calculated using a number of different methods. The most widely applied basin-wide method is measurement of dissolved carbonate load at base-level springs, calculating denudation using the area of exposed carbonate outcrop, and assuming other outcrop types are in equilibrium. In mixed carbonate/noncarbonate lithologies (which includes most fluviokarst settings), much of the landscape is also eroded by transport of sediments through conduits. The basin average rate of landscape denudation (aggregating both carbonate and non-carbonate areas) was 48.5 mm/ka, but denudation rates for carbonate and non-carbonate outcrop areas were not in equilibrium.