Allan Hills (ALH) 84001 is the most recently recognized member of a suite of meteorites--the SNCs--that almost certainly originated on Mars. Several factors distinguish ALH84001 from the other SNC meteorites. Preliminary studies suggest that it may be older than other martian meteorites. Moreover, it contains abundant, zoned domains of calcium-iron-magnesium carbonate that are indigenous to the sample and thus may hold important clues regarding near-surface processes on Mars and the evolution of the martian atmosphere. We report here analyses of the carbon and oxygen stable-isotope compositions of the carbonates that place constraints on their formation conditions. Our results imply the presence of at least two chemically distinct carbonates--one Ca,Fe-rich, the other Mg-rich--that are enriched in 13C relative to terrestrial carbonates (delta 13C approximately +41/1000), consistent with martian atmospheric CO2 as the carbon source. The oxygen isotope compositions of the carbonates indicate that they precipitated from a low-temperature fluid in the martian crust. Combined with textural and bulk geochemical considerations, the isotope data suggest that carbonate deposition took place in an open-system environment in which the ambient temperature fluctuated.

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Organic carbon-rich sediments from the surface of fresh, intermediate, brackish and salt marshes of coastal Louisiana were sampled and analyzed for their C content. The average ∂C from all sites within each wetland type was-27.8‰,-22.1‰,-16.9‰, and-16.2‰, for fresh, intermediate, brackish and salt marshes, respectively. Means from the fresh, intermediate and brackish marshes were significantly different at the 0.01 level. A mixing model using measurements of standing crop and ∂C of plant carbon was applied to estimate the contribution of each species to the sedimentary carbon at four of the marsh sites. Sedimentary ∂C values generally reflected that of the dominant species present at each site. Brackish and salt marsh samples, however, showed a negative shift of ∂C with respect to whole plant carbon. We interpret these depeleted ∂C values to be the result of more extensive organic matter decomposition and selective preservation of C-depleted refractory components in sediments from saline sites. The results of this study suggest that ∂C composition of sedimentary carbon may offer a valuable tool for distinguishing subtle changes in paleohydrology of wetlands resulting from relative sea level changes.

The need for a rapid, inexpensive technique for routine 18O/16O extraction from water has arisen recently through applications in the medical sciences and in hydrology. The traditional experimental technique for determining the oxygen isotope composition of water, the CO2-H2O equilibration method, is tedious, time consuming, and involves the use of custom-made glass apparatus. Furthermore, because of potential memory effects from one sample to the next, the glassware needs to be thoroughly cleaned between runs. A few attempts have been made to improve upon the method. Attempts to analyze water directly in the source of the mass spectrometer produced large memory effects and questionable results. Commercially available apparatus for automated extraction of 18O/16O from water is generally prohibitively expensive and often is designed to interface only with the manufacturer's own mass spectrometer. The method described in this paper utilizes inexpensive, off-the-shelf, preevacuated, glass vials. Preevacuated vials have been used by others for the isotopic analysis of breath CO2 and are well tested. The vials can be purchased in bulk from scientific apparatus suppliers at a relatively low cost. These are coupled with a simplified extraction line consisting of a stainless steel syringe needle and a glass cold trap. Vials are filled with CO2 and H2O and shaken in a constant-temperature water bath for at least 90 min. Since the vials are discarded after use, no cleaning is necessary, essentially eliminating any memory effect. Reproducibility is generally better than +/- 0.05%. The only reagents required are gaseous CO2 for equilibration, a dry ice/alcohol mixture for trapping water, and liquid nitrogen for transferring the CO2.

The use of doubly labeled water (DLW) to measure energy expenditure is subject to error if the background abundance of the oxygen and hydrogen isotope tracers changes during the test period. This study evaluated the accuracy and precision of different methods by which such background isotope changes can be corrected, including a modified method that allows prediction of the baseline that would be achieved if subjects were to consume water from a given source indefinitely. Subjects in this study were eight women (4 test subjects and 4 control subjects) who consumed for 28 days water enriched to resemble drinking water aboard the United States space shuttle. Test subjects and control subjects were given a DLW dose on days 1 and 15, respectively. The change to an enriched water source produced a bias in expenditure calculations that exceeded 2.9 MJ/day (35%), relative to calculations from intake-balance. The proposed correction based on the predicted final abundance of 18O and deuterium after equilibration to the new water source eliminated this bias, as did the traditional use of a control group. This new modified correction method is advantageous under field conditions when subject numbers are limited.

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...

Energy requirements during space flight are poorly defined because they depend on metabolic-balance studies, food disappearance, and dietary records. Water turnover has been estimated by balance methods only. The purpose of this study was to determine energy requirements and water turnover for short-term space flights (8-14 d). Subjects were 13 male astronauts aged 36-51 y with normal body mass indexes (BMIs). Total energy expenditure (TEE) was determined during both a ground-based period and space flight and compared with the World Health Organization (WHO) calculations of energy requirements and dietary intake. TEE was not different for the ground-based and the space-flight periods (12.40 +/- 2.83 and 11.70 +/- 1.89 MJ/d, respectively), and the WHO calculation using the moderate activity correction was a good predictor of TEE during space flight. During the ground-based period, energy intake and TEE did not differ, but during space flight energy intake was significantly lower than TEE; body weight was also less at landing than before flight. Water turnover was lower during space flight than during the ground-based period (2.7 +/- 0.6 compared with 3.8 +/- 0.5 L/d), probably because of lower fluid intakes and perspiration loss during flight. This study confirmed that the WHO calculation can be used for male crew members' energy requirements during short space flights.

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.