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Isotopic Equilibration (isotopic + equilibration)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralization

GEOFLUIDS (ELECTRONIC), Issue 2 2001
S. H. Bottrell
Abstract Mineral deposits in the Cupp-Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre-existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp-Coutunn. The predominance of sulphate mineralization in Cupp-Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the ,34S of sulphate minerals and a corresponding ,34S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in 18O (,18OSMOW , + 10,) relative to meteoric groundwater and seawater. Estimated values for ,13CDIC (,13CPDB , , 13,) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for ,13CDIC and ,18Owater indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water,rock interaction. Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher ,34S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate-rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in 13C compared to most cave deposits, but can be explained by normal speleothem-forming processes under thin, arid-zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO2 derived by reaction of limestone with sulphuric acid (,condensation corrosion') contributed to the formation of 13C-enriched speleothem deposits. [source]

Non-steady state effects in diurnal 18O discrimination by Picea sitchensis branches in the field

PLANT CELL & ENVIRONMENT, Issue 5 2006
U. SEIBT
ABSTRACT We report diurnal variations in 18O discrimination (18,) during photosynthesis (18,A) and respiration (18,R) of Picea sitchensis branches measured in branch chambers in the field. These observations were compared with predicted 18, (18,pred) based on concurrent measurements of branch gas exchange to evaluate steady state and non-steady state (NSS) models of foliage water 18O enrichment for predicting the impact of this ecosystem on the ,18O of atmospheric CO2. The non-steady state approach substantially improved the agreement between 18,pred and observed 18, (18,obs) compared with the assumption of isotopic steady state (ISS) for the ,18O signature of foliage water. In addition, we found direct observational evidence for NSS effects: extremely high apparent 18, values at dusk, dawn and during nocturnal respiration. Our experiments also show the importance of bidirectional foliage gas exchange at night (isotopic equilibration in addition to the net flux). Taken together, neglecting these effects leads to an underestimation of daily net canopy isofluxes from this forest by up to 30%. We expect NSS effects to be most pronounced in species with high specific leaf water content such as conifers and when stomata are open at night or when there is high relative humidity, and we suggest modifications to ecosystem and global models of ,18O of CO2. [source]

A simple method for oxygen-18 determination of milligram quantities of water using NaHCO3 reagent

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 13 2003
Akira Ijiri
This paper presents a modified H2OCO2 equilibration method for stable oxygen isotopic composition (,18O) analysis of water. This method enables rapid and simple ,18O analysis of milligram quantities of water, by employing solid reagent NaHCO3 as the CO2 source, a small (0.6,mL) glass vial for the equilibration chamber, and an isotope-monitoring gas chromatography/mass spectrometry (irm-GC/MS) system for analysis. This method has several advantages, including simple handling for the H2OCO2 equilibration (without purging and/or evacuation treatments), rapid and easy ,18O analysis of equilibrated CO2, and highly sensitive and highly precise ,18O analysis of H2O, using samples as small as 10,mg and with a precision of less than ±0.12,. The time needed to attain oxygen isotopic equilibration between CO2 and water is also comparable (17,h for 10,mg H2O and 10,h for 100,mg H2O) to other previous methods using CO2 gas for the CO2 source. The extent of ,18O variation of sample water from its initial ,18O value due to isotope exchange with added NaHCO3 is also discussed. It is concluded that the correction needed is negligible (less than 0.1,) as long as the oxygen atom ratio () is less than 3.3,±,10,3 and provided the determination is made by comparing ,18O of CO2 equilibrated with sample water and that equilibrated with standard water of a moderately close ,18O value, less than 30, difference. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Evolution of the stable water isotopic composition of the rain sampled along Sahelian squall lines,

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010
Camille Risi
Abstract In the Tropics, the stable isotopic composition (HDO, HO) of precipitation is strongly modulated by convective activity. To better understand how convective processes impact the precipitation isotopic composition, we analyze the isotopic composition of rain collected during the passage of four squall lines over the Sahel (Niamey, Niger) in August 2006 during the African Monsoon Multidisciplinary Analysis (AMMA) campaign. The high-frequency sampling (5,10 min) of the precipitation allows us to investigate the evolution of the precipitation isotopic composition in different phases of the squall lines. Despite a large variability among the different squall lines, some robust isotopic features appear: the W shape of the ,18O evolution and the deuterium excess decrease in the first part of the stratiform zone. To understand more quantitatively how convective processes impact the precipitation isotopic composition, a simple stationary two-dimensional transport model including a representation of cloud microphysics and isotopic fractionation is developed and forced by three-dimensional winds retrieved from the Massachusetts Institute of Technology (MIT) radar on 11 August 2006. The model reproduces the robust observed features and a large sensitivity to the squall-line dynamics. This model suggests that the main controlling factors of the isotopic evolution are (1) squall-line dynamics, especially the downward advection of air at the rear of the squall lines, affecting the vapour composition and, by isotopic equilibration, the subsequent precipitation composition and (2) rain re-evaporation. This suggests that water isotopes have the potential to better constrain squall-line dynamics and rain re-evaporation, and to evaluate the representation of convective processes in numerical models. Copyright © 2009 Royal Meteorological Society [source]