The State of the Earth's Atmosphere 3.5 Ga Ago

The atmosphere evolved through time upon escape to space and exchange of volatile elements with the Earth's solid reservoirs (mantle, crust). We are currently investigating the composition of the ancient atmosphere by analyzing gases trapped in Archean chemical sedimentary rocks, including fluid inclusions in Archean hydrothermal quartz. The rationale of the study is that trapped noble gases, as well as nitrogen for some of the samples, represent ancient atmospheric gases that were dissolved in surface fluids (seawater, freshwater), so that their isotope compositions and elemental ratios reflect those of the Archean atmosphere at the time of trapping. Particular care was given to select samples for which fluids did not exchange significantly since the epoch of trapping. Data show that: (i) the felsic continental crust grew up mostly during the time interval 3.5-2.7 Ga ago [1]; (ii) the atmospheric nitrogen isotope composition and density were comparable to the modern one, implying the existence of a significant terrestrial magnetic field at that time [2]; (iii) Archean neon and krypton isotopic ratios were similar to their modern ones; but (iv) the xenon isotopic composition was isotopically fractionated and intermediate between those of potential cosmochemical (chondritic, solar) end-members and of the present-day atmosphere[3,4]. These differences are attributed to different ionization efficiencies for atmospheric species, and subsequent escape to space for Xe [3-5]. Together with the constant Ne, Kr and N isotope compositions, they shed strong constraints on the nature of the solar UV flux 1Ga after Earth's formation. [1] Pujol. M. et al. (2013), Nature 498, 87-90. [2] Marty B. et al. (2013), Science 342, 101-104. [3] Pujol M. et al. (2011), Earth Planet. Sci. Lett., 308, 298-306. [4] Avice G. & Marty B. Phil. Trans. R. Soc. A, In press. [5] Hébrard E. & Marty B. (2014), Earth Planet. Sci. Lett., 385, 40-48.