The Sistema Zacatón karst area in northeastern Mexico (Tamaulipas state) is limited to a relatively focused area (20 km2) in a carbonate setting not prone to extensive karstification. The unique features found here are characteristic of hydrothermal karstification processes, represent some of the largest phreatic voids in the world, and are hypothesized to have formed from interaction of a local Pleistocene magmatic event with the regional groundwater system. Aqueous geochemical data collected from five cenotes of Sistema Zacatón between 2000 and 2009 include temperature (spatial, temporal, and depth profiles), geochemical depth profiles, major and trace ion geochemistry, stable and radiogenic isotopes, and dissolved gases. Interpretation of these data indicates four major discoveries: 1) rock-water interaction occurs between groundwater, the limestone matrix, and local volcanic rocks; 2) varying degrees of hydrogeological connection exist among cenotes in the system as observed from geochemical signatures; 3) microbially-mediated geochemical reactions control sulfur and carbon cycling and influence redox geochemistry; and 4) dissolved gases are indicative of a deep volcanic source. Dissolved 87Sr/86Sr isotope ratios (mean 0.70719) are lower than those of the surrounding Cretaceous limestone (0.70730-0.70745), providing evidence of groundwater interaction with volcanic rock, which has a 87Sr/86Sr isotope ratio of 0.7050. Discrete hydraulic barriers between cenotes formed in response to sinkhole formation, hydrothermal travertine precipitation, and shifts in the local water table, creating relatively isolated water bodies. The isolation of the cenotes is reflected in distinct water chemistries among them. This is observed most clearly in the cenote Verde where a water level 4-5 meters lower than the adjacent cenotes is maintained, seasonal water temperature variations occur, thermoclines and chemoclines exist, and the water is oxic at all depths. The surrounding cenotes of El Zacatón, Caracol, and La Pilita show constant water temperatures both in depth profile and in time, have similar water levels, and are almost entirely anoxic. A sulfur (H2S) isotope value of δ34S = -1.8 ‰ (CDT) in deep water of cenote Caracol, contrasted with two lower sulfur isotopic values of sulfide in the water near the surface of the cenote (δ34S = -7 ‰ and -8 ‰ CDT). These δ34S values are characteristic of complex biological sulfur cycling where sulfur oxidation in the photic zone results in oxidation of H2S to colloidal sulfur near the surface in diurnal cycles. This is hypothesized to result from changes in microbial community structure with depth as phototropic, sulfur-oxidizing bacteria become less abundant below 20 m. Unique microbial communities exist in the anoxic, hydrothermal cenotes that strongly mediate sulfur cycling and likely influence mineralization along the walls of these cenotes. Dissolved CO2 gas concentrations ranged from 61-173 mg/L and total dissolved inorganic carbon (DIC) δ13C values measured at cenote surfaces ranged from -10.9 ‰ to -11.8 ‰ (PDB), reflecting mixed sources of carbon from carbonate rock dissolution, biogenic CO2 and possibly dissolved CO2 from volcanic sources. Surface measurements of dissolved helium gas concentrations range from 50 nmol/kg to 213 nmol/kg. These elevated helium concentrations likely indicate existence of a subsurface volcanic source; however, helium isotope data are needed to test this hypothesis. The results of these data reflect a speleogenetic history that is inherently linked to volcanic activity, and support the hypothesis that the extreme karst development of Sistema Zacatón would likely not have progressed without groundwater interaction with the local igneous rocks 

The Yucatán Peninsula is one of the largest limestone platforms in the world. The Mexico isthmus region of the peninsula is a low-relief pitted karst plain containing few surface drainage systems or lakes. Karst windows punctuate the scrubby terrain, exposing a shallow aquifer that engulfs an ancient dry cave environment. These openings, called cenotes, allow modern explorers to document a growing assemblage of deep underwater sinks, and exceptionally long and complex underwater cave systems. Deep classic sinks are common to the interior of the isthmus. Long horizontal caves remain a coastal phenomenon. Their complexity is derived from irregular sea-level fluctuations produced during Pleistocene glaciations. Fractures within the parent strata and tidal fluctuations of the halocline are crucial elements in sustaining extant speleogenesis.

Assessing water quality in aquifers has become increasingly important as water demand and pollution concerns rise. In the Yucatan Peninsula, sinkholes, locally known as cenotes, are karst formations that intercept the water table. Cenotes are distributed across the peninsula, but are particularly dense and aligned along a semicircular formation called the Ring of Cenotes. This area exhibits particular hydrogeological properties because it concentrates, channels, and discharges fresh water toward the coasts. In this study, we identify spatial and temporal variations in chemical and physical variables at twenty-two cenotes to identify groups that share similar characteristics. Water samples from each cenotes were taken at three depths (0.5, 5.5, and 10.5 m) and during three seasons (dry, rainy, and cold-fronts season). Field measurements of pH, temperature, electrical conductivity, and dissolved oxygen were taken, and the concentrations of major ions (K+, Na+, Mg2+, Ca2+, HCO{ 3 , SO2{ 4 , Cl2 and NO{ 3 ) were quantified. Identifying regions of the cenotes were done by applying multivariate statistical techniques (PCA, PERMANOVA, CAP). The chemical variables revealed spatial trends among the cenotes. We identified three main regions. Region 1 is associated with sea-water encroachment and high levels of sulfate that travel through preferential groundwater flowpaths from evaporites in the southern Yucatan Peninsula; Region 2 is a recharge zone, and Region 3 is characterized by sea water encroachment and by the high chemical and physical variability associated with groundwater flow from the east.