BACTERIAL NITROGEN AND CARBON DIOXIDE GAS PRODUCTION IN LIMESTONE CAVES: A CASE STUDY IN THE BAHAMAS

The current explanation for the formation of caves in the Bahamas is that mixing of CaCO₃-supersaturated seawater with CaCO₃-saturated groundwater creates under-saturated mixed water capable of dissolving the limestone to produce large voids. Although microorgansism exist within both of these chemically different bodies of water, little is known about what role their metabolic activity might facilitate in the formation of caves.

To better understand microbiological processes that may influence the formation of caves, 23 samples of limestone wall rock were collected from Altar Cave in San Salvador Bahamas. Samples ranging from 0.7 to 5.7 g were placed in 20 ml and 5 ml sterile serum vials that were flushed with helium and incubated at 32ºC from 26 to 45 days. The gas phase was analyzed for the bulk concentration and stable isotope composition of CO₂, N₂ and N₂O using a Gasbench II device attached to a Delta^plusXP isotope ratio mass spectrometer.

The dominant gases produced were N₂ (8.2 x 10^-7 to 2.1 x 10^-4 mM per g), followed by CO₂ (6.3 x 10^-8 to 4.6 x 10^-6 mM per g) and more rarely N₂O (4.7 x 10^-8 to 2.1 x 10^-6 mM per g). The nitrogen isotope composition of N₂ ranged from -2 to 3‰., while the carbon isotope composition of CO₂ ranged from -29 to -6‰. N₂O concentration was insufficient to measure isotopically although it was identified unambiguously by GC retention time. The concentration and isotope composition of all gases were outside the range of values observed for cave atmosphere and inorganic control sample, suggesting that microbial nitrogen and carbon cycles were active in the serum vials under anaerobic conditions during the period of incubation. The source of N and C are presently unknown but it is thought to be organic matter; acid extraction and leaching experiments are presently underway to document their origin. The relative concentrations of N₂ and N₂O suggest that microbial nitrogen dynamics were variable among the serum vials and the relatively low C/N ratios (3.03 x 10^-3 to 7.6 x 10^-1) suggest that a significant carbon sink exists in the vials. The fact that N- and C- bearing gases were produced suggests that an active microbial community exists within the cave wall rock which can significantly influence carbonate dissolution and precipitation processes in caves.