Header

Evolution of the Precambrian atmosphere

Past work carried out at CRPG suggests that certain sedimentary rocks (barite and fluid inclusions in hydrothermal quartz) have preserved vestiges from the Archean atmosphere. These studies have determined the isotopic compositions of the Ar and Xe of the air 3.5 Gy ago. The variation in the 40Ar/36Ar ratio of the atmosphere over time is a potentially powerful tracer of the dynamics of the mantle and crustal growth. The stable isotopic composition of atmospheric xenon also appears to record the interactions between UV rays and the atmosphere. We will develop, in the next quadrennial, the analysis of sedimentary rock series that may have retained the characteristics of the ancient atmosphere in their elemental and isotopic noble gas compositions. These samples will be (i) barites from different periods (3.5 to 1.0 Gy), which have already been sampled in South Africa and Australia, and (ii) evaporites, also covering different periods of the Archean and Proterozoic. We hope that these analyses will advance the study of the evolution of the atmosphere during the first billion years of Earth’s history. This work corresponds to a portion of the ERC NOGAT Project.

The evolution of the stable isotope composition of atmospheric xenon (expressed in parts per thousand per unit mass) over time, deduced from the analysis of Archean barite and quartz. These tests suggest that this composition was intermediary between that of chondritic xenon and modern atmospheric xenon in the Archean period. These variations may illustrate certain interactions of the young Sun, which produced more UV rays, with the Archean atmosphere (Pujol et al. EPSL, 2011)

Parallel to the study of atmospheric noble gases, we will continue our research on the mass-independent fractionation (MIF) of isotopes of sulphur. These very specific signatures are produced by the interaction of UV radiation with gaseous sulphur-containing molecules and can only be produced in an atmosphere that is transparent to UV radiation, i.e., devoid of an ozone layer. However, beyond the mere presence or absence of oxygen, the interpretation of the MIFs has the potential to provide essential information on the composition of the primitive atmosphere. Indeed, their production by photolysis and their transfer into the sedimentary column is related to the presence of other molecular species in the atmosphere (for example, pCO2, pO2, pCH4, and pSO2). Variations in the composition of the atmospheric chemistry control the co-variations ∆33S-∆36S-δ34S and, although the qualitative effect of each molecular species is still not known, an accurate and simultaneous measurement of four sulphur isotopes may help clarify the composition of the atmosphere. This method has already been applied to sulphide inclusions in diamonds from the Slave Craton, to sulphides extracted from eclogites, and to a series of metasediments from South Africa. It will be used on the series of Eoarchean metasediments sampled from Nunavik (Canada).