Nitrogen Partial Pressure in the Archean Atmosphere From Analysis of Hydrothermal Quartz

Atmospheric nitrogen constitutes the main pool of one of the major biogenic elements, it may provide a record of the terrestrial magnetic field, and it might have been instrumental to maintain clement temperatures despite a lower energy provided by the ancient Sun. We have investigated the partial pressure of di-nitrogen in the Archean atmosphere by analyzing the ratio between N2 and argon-36 (a primordial, chemically inert noble gas for which there is no reason to suspect abundance variation in the atmosphere through time) in fluid inclusions (FI) trapped in two hydrothermal quartz from the 3.5 Ga-old Dresser formation, North Pole, Pilbara (NE Australia). These samples have different histories of fluid trapping and deposition, and their ages are constrained within 3.0-3.5 Ga from U-Xe, and Ar-Ar dating. FI nitrogen is a mixture between hydrothermal and air-saturated water (ASW) end-members, that can be identified through geochemical correlations between Cl/36Ar, 40Ar/36Ar and N2/36Ar ratios. The ASW component has a N2/36Ar ratio within 30 % of the modern value, implying a N2 partial pressure during the Archean was within 0.6-1 bar. The nitrogen isotopic composition of the ASW end-member is also found similar to the present-day one within 3 permil. Combined with the recent proposal from ancient raindrop imprints that the total atmospheric pressure was 0.5-1.14 bar, this leaves less than 0.7 bar for the pressure of other atmospheric gases including CO2. Thus nitrogen did not play a significant role in the thermal budget of the ancient Earth, and the terrestrial magnetic field was already strong enough at that time to shield the upper atmosphere from interaction with the solar wind, and therefore to prevent atmospheric escape that would have been recorded otherwise in the nitrogen composition. These results also imply that exchanges of nitrogen between the Earth's mantle and the surface were limited, or proceeded at similar rates in both ways from the Archean to Present.