N-135. Microbially Mediated Cave Formation: The Potential for Alkali-Speleogenesis in Roraima Sur Cave, Venezuela

M. Broering1, E. Banks1, J. Giarrizzo2, P. Suarez2, K. Venkateswaran3, H. A. Barton1;
1Northern Kentucky Univ., Highland Heights, KY, 2Univ. Simon Bolivar, Caracas, VENEZUELA, 3Jet Propulsion Lab., Pasadena, CA.

It has been known for almost two decades that microbial species can contribute to the formation of caves, although such activities generally occur through the production of acids within carbonate rock. Other processes of speleogenesis are less easy to explain, such of the formation of Roraima Sur Cave, Roraima Tepui, Venezuela. This 10.4 km long cave, formed within insoluble silica (alpha-quartz) rock, contains a noticeable microbial population and unusual opal formations. While silica is generally insoluble in water, it can be solubilized if the pH drops below 2.0 or rises above 8.5. While no current conditions within the cave suggest extreme acidity, we believe that microbial nitrogen-fixing activity is leading to an increase pH and generating sufficiently alkaline conditions for silica dissolution. On a recent expedition, we collected both geologic and biological samples from within and around Roraima Sur Cave. Direct measurements indicate that ammonia is accumulating within the silica rock at sites of primary dissolution and increasing the pH of the system. These sites also correlate with high levels of microbial activity within the cave. Numerous bacterial species were isolated from these locations on nitrogen-free media and demonstrated nitrogen-fixation through the accumulation of ammonia within the media. This dissolution of the host rock is producing a high level of silica within the atmosphere of the cave, which we believe is responsible for the formation of unusual opal formations. Together these data suggest a microbially-driven alkali-speleogenesis in the formation of Roraima Sur Cave that has not previously been described.