The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions refl ect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m 2 area in Harrison's Cave, Barbados, West Indies. Only some of the plate δ 13 C values and none of the plate δ 18 O values correspond to equilibrium values, indicating signifi cant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects.On each plate, speleothem calcite was deposited with distinct δ 18 O and δ 13 C compositions that increase progressively away from the growth axis, with up to 6.6‰ 13 C and 1.7‰ 18 O enrichments. The positive δ 13 C versus δ 18 O trends are likely a result of 18 O and 13 C Rayleigh-distillation enrichment in the HCO 3 reservoir owing to progressive CO 2 degassing and CaCO 3 precipitation. The magnitude of the δ 13 C versus δ 18 O slope is likely controlled by the extent to which CO 2 hydration-hydroxylation reactions buffer the oxygen isotope composition of the HCO 3 reservoir during calcite precipitation. Complete oxygen isotopic buffering of the HCO 3 reservoir by CO 2 hydration-hydroxylation reactions will produce a vertical δ 13 C versus δ 18 O slope in calcite sampled along a growth layer. As oxygen isotope buffering of the HCO 3 reservoir decreases to no buffering, the δ 13 C versus δ 18 O slope in calcite sampled along a growth layer will decrease from vertical to 0.52 at the cave temperature. In this study, modern speleothem calcite sampled along the growth layer produced a δ 13 C versus δ 18 O slope of 3.9, indicating incomplete oxygen isotope buffering of the HCO 3 reservoir during calcite precipitation.Both modern and Holocene speleothem calcite from Barbados, sampled temporally along the growth axis, shows similar positive δ 13 C versus δ 18 O slopes. These results, along with the spatial variations in glass plate calcite carbon and oxygen isotope compositions, suggest that the isotopic composition of the Holocene speleothems is in part controlled by non-equilibrium isotope effects. In addition, there is a correlation between stalactite length and oxygen and carbon isotope ratios of calcite precipitated on the corresponding stalagmite and glass plate, which may be due to 13 C and 18 O enrichment of the HCO 3 reservoir during CO 2 degassing-calcite precipitation along the overhanging stalactite.We compiled 165 published speleothem stable isotope records with a global distribution and found that most of these records show a positive covariation between δ 13 C and δ 18 O values. Speleothem stable isotope records may be infl uenced by kinetic isotope effects such...

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This study places constraints on the source and transport mechanisms of methane found in groundwater within the Barnett Shale footprint in Texas using dissolved noble gases, with particular emphasis on Kr andXe. Dissolved methane concentrations are positively correlated with crustal He,Ne, and Ar and suggest that noble gases and methane originate from common sedimentary strata, likely the Strawn Group. In contrast to most samples, four water wells with the highest dissolved methane concentrations unequivocally show strong depletion of all atmospheric noble gases (Ne, Ar,Kr, Xe) with respect to air-saturated water (ASW). This is consistent with predicted noble gas concentrations in a water phase in contact with a gas phase with initial ASW composition at 18 °C-25 °C and it suggests an in situ, highly localized gas source. All of these four water wells tap into the Strawn Group and it is likely that small gas accumulations known to be present in the shallow subsurface were reached. Additionally, lack of correlation ofKr/Ar and Xe/Ar fractionation levels along with He/Ne with distance to the nearest gas production wells does not support the notion that methane present in these groundwaters migrated from nearby production wells either conventional or using hydraulic fracturing techniques.