HYDROCHEMICAL CHARACTERISTICS OF A UNIQUE SULFUR-RICH KARST SYSTEM IN SOUTHERN MEXICO

Over 25 sulfur-rich, subterranean “springs” emerge within the 2 km long cave, Cueva de Villa Luz (a.k.a. Cueva de la Sardina) in Tabasco, Mexico. The sulfur-rich waters provide energy for a unique and robust, chemoautotrophic ecosystem. The stream typically flows from the cave resurgence at ~290 L/sec regardless of the season, with minor fluctuations during intense, local storms. Extended monitoring of selected inlets within the cave (n=10938) demonstrated that the water temperature ranged from 26.76-28.28oC (mean=28.20 oC), total dissolved solids from 2.02-2.91 g/L (mean=2.81 g/L), salinity from 1.61-2.37 ppt (mean=2.28 ppt), dissolved oxygen from 0.80-89.2 mg/L (mean=4.02 mg/L), and pH from 6.63-7.56 (mean=6.75). Field titrations showed a range of dissolved carbon CO2 in 43 measurements was 33-330 mg/L (mean=155 mg/L) and sulfide ranged from 0-28 mg/L (mean=4 mg/L). Concentration of sulfate determined by a spectrophotometer extended from 1000-6400 mg/L (mean=2200). Nitrate and nitrite were present in all samples. The spectrophotometer also reported formaldehyde in remarkably high concentrations varying from 0.11-33700 mg/L (mean=2300 mg/L).

Logging O2, pH, temperature, TDS, and salinity every minute over several days during two dry seasons has shown that these parameters frequently change, with dissolved O2 concentrations shifting as much as 3-fold within 5 minutes. Air monitors occasionally reveal >100x increases in the release of CO2 and >10x increases in atmospheric H2S within a few minutes. Atmospheric oxygen levels have twice been observed to plummet to below 10%. These fluctuations have no apparent association with rainfall, diurnal, or barometric changes. Variations in water chemistry from visit-to-visit and minute-to-minute demonstrate the importance of long-term monitoring to understand this novel, chemoautotrophic environment.