CLUES TO THE ORIGIN OF SUBAQUEOUS CAVE MICROBIALITES: THE LIVING POOL FINGERS OF THE HERBSTLABYRINTH/ADVENTHÖHLE CAVE SYSTEM, HESSE, GERMANY

Pool fingers are subaqueous cave microbialites that hang from shelfstone and other overhangs in cave pools. Based on morphology, microbial fabrics, and presence of fossil microbes, pool fingers are interpreted as mineralization around hanging filaments and/or biofilm. However, previous studies are all based on fossil examples. Living, active pool fingers are needed to test this model. In 2010, German cavers identified the first known active pool fingers in the Herbstlabyrinth/Adventhöhle (HA) Cave System in Hesse, Germany.

In the Rätselhalle chamber, seven pools are present; one contains clay and pool fingers of smooth biofilm, the rest lack either clay or obvious biofilm. In Adventhöhle, three pools have been described to date, all with clay. One pool is dry, has no overhang and has no pool fingers. One is full of water and contains pool fingers with lumpy biofilm; a second was dry in July 2011, but was full of water in Nov 2010. In this pool, completely mineralized pool fingers were found, each with a soft clay core.

SEM examination of the smooth pool fingers found two morphologies of reticulated microbial filaments in EPS. The lumpy pool fingers were similar but also contained euhedral crystals of CaCO₃which, based on the crystal morphology, were determined to be calcite. The clay core of the completely mineralized pool fingers also contained reticulated filaments. Etching the calcite portion revealed entombed filaments and EPS, similar to those found in previous studies of fossil pool fingers.

All three examples of living pool fingers in the HA Cave System are associated with clay-rich biofilms: pools without clay lack pool fingers and are coated in pool spar. Mineralization ranges from none (smooth biofilm), to just starting (lumpy biofilm), to fully formed pool fingers, albeit with soft clay cores. Mineralization appears to be enhanced by seasonal drying of one pool but clearly starts within living biofilm, supporting a key role for the microbial community.