THIN LIMESTONES, A MISSING KARST END MEMBER IN UNDERSTANDING CRITICAL ZONE DYNAMICS (Invited Presentation)

Thin limestones interbedded with shales are common, yet their hydrogeologic characteristics are poorly known. Although thin, these limestones develop karst features and groundwater interacts with both limestone and shale, thus representing end members of karst and shale terrains. We will summarize the hydrogeology and hydrochemistry of thin (~1-2 m), Permian-aged limestones that crop out and dominate the subsurface in northeastern Kansas, at the 35 km² Konza Prairie Biological Station and Long-Term Ecological Research Site. The stratigraphy is nearly flat-lying although the field site is characterized by deeply dissected hills (60 m of relief, slope gradients up to 25%) with bench (limestones) and slope (shales) topography. We present data and models to demonstrate that thin limestones should be classified as karst with complex hydrology and rapid weathering, both of which are responsive to short time-scale climate changes. Specifically, streams and water-table elevation respond quickly to precipitation, producing flashy hydrographs similar to those found in thicker, karstified limestones. Further, fluorescent dye tracing indicates: 1) diametrically opposed, horizontal flow directions in two limestones separated by a shale; and 2) vertical flow through a shale effecting subsurface bypass of the flume recording surface discharge. Thus, as in traditional karst regions, groundwater flow can be complicated. The water chemistry from Konza indicates three important end members, limestone water, soil water, and an unknown third member, possibly shale water; and various lines of evidence including ⁸⁷Sr/⁸⁶Sr suggest more than half of the supposed limestone contribution is not from the limestone, but potentially from dust. Finally, groundwater temperature is linked to timing of recharge, where cool-season recharge can lower groundwater temperature due to dominance of advective transport. Therefore, in areas where cool-season precipitation is expected to increase with climate change, groundwater warming may be less than might be predicted by warming air temperature alone. Together these results highlight the usefulness of thin limestone terrains as an end member in elucidating the sensitivity of critical zone dynamics to hydrologic perturbations, be it climate or human induced.