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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...

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Abstract-Samples from a suite of Shergotty-Nakhla-Chassigny (SNC) meteorites wzre analyzed for their 0 isotopic ratios by a modified version of the laser fluorination technique. Measured isotopic ratios (I7O/l6O and 180/*60) from bulk samples of the Shergottites, EETA79001, Shergotty and Zagami; the Nakhlite Lafayette; and Chassigny are similar to those reported in the literature, as are those from olivine and pyroxene mineral separates from Lafayette.Iddingsite, a preterrestrial alteration product of Lafayette, was measured for the first time as a separate phase. Oxygen isotopic ratios increase with the percentage of iddingsite in a sample to a maximum 6180 of 14.4%0 for a -90% separate. Based on these measurements, end-member iddingsite has a 6180 of 15.6%0, which places it among other 180-enriched secondary phases (carbonate and silica) observed in SNC meteorites. The relatively large difference in d18O between iddingsite and the olivine and pyroxene it replaces (-1 I%o) is typical of low-temperature alteration products.A range of crustal fluid B18O values can be interpreted from the B18O for end-member iddingsite, assuming isotopic equilibrium was achieved during low-temperature hydrous alteration ( 4 0 0 OC; Treiman et al., 1993). The calculated range of values, -1 5 to 5%0, depends on many factors including: (1) the modal mineralogy of iddingsite, (2) potential isotopic exchange among other 0-bearing phases such as host silicate and carbonate, and (3) exchange with evolved or exotic 0 reservoirs on Mars. Despite the lack of constraints, the calculated range is consistent with isotopic exchange, and possibly equilibria, among components of the C02-carbonate-iddingsite-H20 system at low temperature.The SNC meteorite samples in this study have A170 values that are indistinguishable from bulk Mars (0.30%0), except for a single, small sample of iddingsite that has an anomalous A170 of -1.4%0. While analytical difficulties make isotopic measurements for this sample problematic, the A170 is similar in direction to AI7O reported for waters extracted from bulk samples of Lafayette (Karlsson er af., 1992). If the A170 for iddingsite is confirmed, it can be concluded that evolved or exotic fluids on Mars have contributed volatiles to the 0 reservoir from which iddingsite formed 130 to 700 Ma ago.