MICROBIAL INFLUENCE OF AQUEOUS GEOCHEMICAL ENVIRONMENTS: SISTEMA ZACATON, MEXICO

Sistema Zacatón is an isolated hydrothermal karst area in Tamaulipas Mexico containing a series of water-filled sinkholes, or cenotes. Water chemistry in these cenotes is spatially variable, due in part to the geomorphic evolution of the karst and additionally by microbial processes taking place both in the water column and along the walls and floors of the cenotes. The deepest cenotes, including El Zacatón (319 m deep), have highly homogeneous water chemistry vertically in the water column, resulting from a steady flux of convective heat sourced from recently active local volcanism. El Zacatón and the adjacent cenote Caracol share similar geochemical environments that are dominated by microbial cycling of sulfur and carbon, which occur in strong reducing, anoxic conditions throughout the water column. Microbial sulfide oxidation in these two cenotes creates clouds of colloidal sulfur that develop on regular diurnal cycles, but only in shallow, photic zones, indicating activity from photosynthetic sulfur oxidizing bacteria. Adjacent to Caracol, the cenote Verde is the only body of water to have discernable thermoclines and chemoclines, and seasonal variability indicating isolation from the deeper hydrothermal water feeding Zacatón and Caracol. The water is oxidizing, with dissolved oxygen values from 7 mg/L at the surface to 2 mg/l 45 m deep on the bottom. Verde does contain a hydrothermal spring on the northwest wall that feeds water similar to that in the other cenotes, but at rates lower than can overwhelm the physical isolation of this cenote. Just east of Verde, the cenote La Pilita is more similar to Zacatón and Caracol in that it is convectively mixed and has reduced, anoxic water. The four deepest cenotes of El Zacatón, Caracol, Verde, and La Pilita have similar ion geochemistry of a typical calcium-bicarbonate karst aquifer, however all contain abnormally high levels of dissolved carbon dioxide and methane, likely a result of microbial activity. Carbon-13 isotopes from dissolved inorganic carbon at each of the four cenotes present values from -10 ⁰/₀₀ to -12 ⁰/₀₀ PDB. These values are likely reflective of mixing between two sources of inorganic carbon: one from dissolution of source rocks at depth and the other from microbial cycling of carbon.