Stable isotope systematics of two cenotes from the northern Yucatan Peninsula, Mexico

Abstract

Deep water-filled sinkholes, cenotes, are common in the northern Yucatan Peninsula. At least five of these cenotes are deep enough to extend through a freshwater lens of meteoric origin in which ␦ 18 O and ␦D follow the trend ␦D ϭ 8.11 ϫ ␦ 18 O ϩ 10.4. Below this freshwater lies saline water that originated as seawater and has retained its seawater isotopic identity. Deep cenotes, characterized by input of variable amounts of organic debris from tropical vegetation and by poor circulation below the fresh-/saltwater interface, provide excellent water columns in which to study sulfur redox phenomena. Measurements include O, H, and S isotope composition, conductivity, sulfur speciation, and pH from two cenotes (Ucil, 98 m deep, and Xcolac, 125 m deep). Strong 34 S enrichment of sulfate and 34 S depletion of sulfide indicate anaerobic bacterial reduction of sulfate. A shift in the isotopic composition of sulfur in Xcolac from a seawater value of ϩ21.0‰ (CDT) to ϩ41.8‰ indicates conversion of sulfate to isotopically light sulfide. Mass balance calculations indicate that escape of isotopically light sulfur from the system is a slow process. At 80 m in Xcolac, a difference in sulfur isotope composition between sulfate and sulfide (⌬ 34 S) of 63.2‰ is observed and could be the result of multiple sulfate reduction reactions. Higher in the water column, sulfide oxidation occurs, probably the result of bacterially mediated sulfide oxidation processes. A deep observation well (lacking organic matter input) shows only a slight deviation in sulfur isotope composition of sulfate from seawater values.We report here the relations between depth and the parameters O, H, and S isotopic composition; conductivity; sulfur speciation; and pH of two deep, water-filled karst sinkholes (known locally as cenotes from the indigenous Maya word tzonot ϭ lake) in the north-central Yucatan of Mexico. These cenotes, Xcolac and Ucil, both extend through the regional freshwater lens into an extensive saltwater intrusion containing water isotopically indistinguishable (H, O, and S) from local seawater. Although they are not the deepest cenotes in northern Yucatan (Andreas W. Matthes describes a 168-mdeep cenote, Sabak-Ha, unpubl.), they are the most accessible deep cenotes that can be sampled conveniently from the surface.Ucil and Xcolac Cenotes offer an unusual, perhaps unique, opportunity to study bacterially mediated redox reactions between sulfur-and carbon-bearing components in natural lowtemperature aqueous systems. Whereas elsewhere such redox reactions commonly take place within a sediment column over a depth range of a few centimeters or in water columns that experience strong seasonal changes, in the Yucatan, cenote redox processes are spread through meters of 1 Corresponding author (rsocki@ems.jsc.nasa.gov). AcknowledgmentsWe acknowledge Jaime Durazo, Miguel Villasuso, and Salvador Gaona for help with field work associated with this study. A previous version of this manuscript was reviewed by Hazel Barton and Pat Shanks and two a...

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

“…Stem water 18 O values were signiWcantly diVerent from groundwater for all the tree species investigated (P < 0.001; Mann-Whitney U-test). Groundwater 18 O and D values plotted on the meteoric water line (MWL; Socki et al 2002), as shown in Fig. 4.…”

“…While most native woody species in northern Yucatan are drought deciduous, many of them Xush new leaves during the dry season, which also hints at root access to moist subsoil layers or groundwater. Northern Yucatan is underlain by a permanent groundwater table within 5-20 m of the land surface (Socki et al 2002). There are numerous examples in the literature of woody plants sending roots to water tables at similar or greater depths (Lewis and Burghy 1964;Canadell et al 1996 and references therein;Zencich et al 2002;Gries et al 2003;Ludwig et al 2003).…”

“…During the dry season, the upper soil layers become enriched in the heavier isotopes (D and 18 O) due to evaporative fractionation, resulting in steep gradients in oxygen and hydrogen isotopic signatures with depth (Barnes and Turner 1998). In the Yucatan the isotopic composition of soil water is expected to be most distinct from that of groundwater at the peak of the dry season, as the aquifer is not subject to evaporative enrichment (Socki et al 2002). Phreatophytic plants may use soil water during wet periods, but they generally switch to groundwater sources as the soil dries out (e.g., Zencich et al 2002).…”

Biología y calidad del agua del acuífero norte de Quintana Roo

“…Los estudios realizados en sistemas acuáticos cársticos de la Península de Yucatán se han enfocado particularmente en dos aspectos: 1) geohidrológicos, geoquímicos y de contaminación (Marín y Perry, 1994: 623;Steinich et al, 1996a: 159;Alcocer et al, 1998: 301;Herrera y Comín, 2000: 227;Socki et al, 2002Socki et al, : 1818; y 2) taxonómicos (Suárez et al, 1996: 296;Schmitter, 1998: 239;Suárez y Reid, 1998: 265;Alcocer et al, 1999: 25;Sarma y Elías, 1999: 196;Elías y Suárez, 2000:14).…”

El balance hídrico en cuerpos de agua cársticos de la Península de Yucatán

“…Los eT pueden agruparse en tres categorías: 1) aquellos que tienen alta variabilidad espacial y temporal y pueden cambiar significativamente entre un sistema y otro; por ejemplo NO 3 , Na-, K+, Mg+, Cl-y Sulfatos (reactivos); 2) aquellos con poca variación y una distribución homogénea espacial y temporalmente (no reactivos) por ejemplo el pH, Ca++, HCO 3 ; y 3) los conservativos como el oxígeno isotópico (Socki et al, 2002(Socki et al, : 1812Perrin et al, 2003: 677). Los eT han sido una herramienta importante para evaluar el flujo subterráneo, evapotranspiración y escorrentía en otras regiones cársticas (Perrin et al, op.…”