BOUNDARIES AND SCALES OF EXTREME KARST FEATURES IN THE CARBONATE CRITICAL ZONE (Invited Presentation)

In comparing carbonate and silicate settings, one of the substantial differences is the depth at which surface-driven influences can be propagated with timescales varying from individual recharge events to timescales associated with climate change and tectonic processes. Layers within the CZ, including the bottom boundary, are often conceptualized as planar. However, karst settings are highly heterogeneous in their permeability and the corresponding influences from Earth’s surface. Extreme cases of karst features in the carbonate critical zone are large sinkholes more than 100 m deep and wide. Loosing streams, spring discharges, and subterranean rivers are normally associated with these sinkholes, which indicates active conduit flow system developed within these features. Tectonic settings and morphological controls on extremely large sinkholes and tiankengs demonstrate that it took more than a million years of erosion and dissolution to form these features. ²⁶Al/¹⁰Be burial ages in cave sediments (quartz), Pollen compositions in cave deposits, paleomagnetism of sedimentary sequence in cave sediments, and speleothem ages using U/Th or U/Pb isotopes, confirmed that the karstification started in the Miocene or Pliocene in southern China and eastern North America. Exposure age dating of tiankeng rocks by ³⁶Cl-AMS indicates that the collapse of tiankengs could occur in the late Pleistocene.