The discovery by Farquhar et al. (1) of mass-independent isotope fractionation of S and S in rocks formed more than ;2 billion years ago (Ga), but not in younger rocks, has boosted the theory postulating a dramatic change from an anoxic to an oxygen-rich atmosphere about 2 Ga. That is because the only known natural process that may cause mass-independent fractionation of both S and S involves a...

The foundations of stable isotope geochemistry were laid in 1947 by Urey’s classic paper on the thermodynamic properties of isotopic substances and by Nier’s development of the ratio mass spectrometer. Before discussing details of the naturally occurring variations in stable isotope ratios, it is useful to describe some generalities that are pertinent to the field of non-radiogenic isotope geoc...

The ratio of l*C : 13C was measured for Emiliania huxleyi cells grown under a range of irradiances in batch culture. Based on the assumption that HCO, was the carbon source, the maximum discrimination against 13C in the decalcified E. huxleyi cells was 24.6?& at 4.8 mol photons m-* d-l and the minimum discrimination was 17.9?& at 0.5 mol photons m-* d-l. In calcified cells, the range of 13C iso...

Boron isotope fractionation during brucite deposition from artificial seawater J. Xiao, Y. K. Xiao, C. Q. Liu, and Z. D. Jin State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, 710075, China Qinghai Institute of Salk Lakes, Chinese Academy of Sciences, Xining, Qinghai, 810008, China State Key Laboratory of Environmental Geoch...

We have detected circumstellar absorption lines of the CN and CN Violet and Red System in the spectrum of the post-AGB star HD 56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 ± 0.04 km s, CN/CN= 38±2, and a lower limit of 2000 on CN/CN and CN/CN. A simple chemical model has been computed of the circumstellar shell surrounding HD 56126 that takes i...

The hydrogen isotope ratio (D/H) is commonly used to reconstruct the chemical processes at the origin of water and organic compounds in the early solar system. On the one hand, the large enrichments in deuterium of the insoluble organic matter (IOM) isolated from the carbonaceous meteorites are interpreted as a heritage of the interstellar medium or resulting from ion-molecule reactions taking ...

We investigate the gas-phase and grain-surface chemistry in the inner 30AU of a typical protoplanetary disk using a new model which calculates the gas temperature by solving the gas heating and cooling balance and which has an improved treatment of the UV radiation field. We discuss inner-disk chemistry in general, obtaining excellent agreement with recent observations which have probed the mat...

High precision oxygen three-isotope ratios were measured for four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in equilibrated ordinary chondrites (EOC) using a secondary ion mass spectrometer. Eleven EOCs were studied that cover all groups (H, L, LL) and petrologic types (4, 5, 6), including S1-S4 shock stages, as well as unbrecciated and brecciated meteorites. SIMS a...

Introduction: Methanogenic archaea, which consume CO2 and H2, have been considered models for possible life forms on Mars for a long time, even before the discovery of methane in the Martian atmosphere (1-8). Several intensive experimental studies have been conducted to investigate survivability of methanogens under conditions that approach those found on Mars (2, 3, 9). Various potential sourc...

The stable carbon isotope (13)C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against (13)CO(2) so that source sugars (photosynthates) are on average (13)C depleted by 20‰ compared with atmospheric CO(2). The carbon isotope distribution within sugars has been shown to be heterogeneous, with rel...