T143 Geochemistry of Karst Waters: A Window on Hydrogeology and Biota

Wednesday, 7 November 2001
1:30 PM - 5:30 PM

CHEMICAL WEATHERING OF CARBONATE PHASES AT KONZA PRAIRIE LTER SITE—DEPENDENCE ON IMPORTED MATERIAL

Paper number: T143-12
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Chemical weathering rates (calculated using stream chemistry, precipitation chemistry, stream discharge rates, and wet precipitation rates) at the Konza Prairie LTER Site generally fall within the range of the few rates calculated in similar ways for carbonate-based river basins. Konza is underlain by alternating limestones and shales of Permian age. Shales are twice as thick as limestones, but limestones host the aquifers and thus both ground- and surface-water chemistry is dominated by calcium and alkalinity. Konza weathering rates are strongly dependent upon precipitation amount (and, thus, stream discharge). Chemical weathering rates for years of near-average precipitation are similar to other carbonate-basin rates, but wet precipitation in any single year at Konza is typically well above or well below average, rather than average. As a result, the range of annual weathering rates calculated (about 300 – 2300 kg/hc/yr) for a five-year period is greater than reported rates for carbonate-based river basins.

Recent work on Sr isotope ratios in Konza ground water, surface water, bedrock, and soils has shown that about 40% to 90% of the solutes in the stream water and ground water result from dissolution of a soil carbonate phase that is not derived from local bedrock. The soil carbonate, presumably originating from atmospheric dust, thus has a strong effect on calculations of landscape denudation rates. We present weathering rates for limestone bedrock at Konza and rates for dissolution of imported dust that show the strong dependence of landscape dissolution on rates of dry deposition and wet precipitation. Discharge-weighted rates show that about 65% to 80% of the dissolved mass exiting one watershed at Konza is not derived from local bedrock. The high percentage of dissolved mass that is not derived from bedrock accounts for the thin soils and persistence of landforms at Konza and, more generally, east central Kansas.