ABIOTIC NITROGEN REDUCTION ON THE EARLY EARTH

The production of organic precursors to life depends critically on the form of the reactants, In particular, an environment dominated by N-2 is far less efficient in synthesizing nitrogen-bearing organics than a reducing environment rich in ammonia (refs 1, 2). Relatively reducin g lithospheric conditions on the early Earth have been presumed to fav our the generation of an ammonia-rich atmosphere. but this hypothesis has not been studied experimentally. Here we demonstrate mineral-catal ysed reduction of N-2, NO2- and NO3- to ammonia at temperatures betwee n 300 and 800 degrees C and pressures of 0.1-0.4 GPa-conditions typica l of crustal and oceanic hydrothermal systems. We also show that only N-2 is stable above 800 degrees C, thus precluding significant atmosph eric ammonia formation during hot accretion, We conclude that mineral- catalysed N-2 reduction might have provided a significant source of am monia to the Hadean ocean. These results also suggest that, whereas ni trogen in the Earth's early atmosphere was present predominantly as N- 2, exchange with oceanic, hydrothermally derived ammonia could have pr ovided a significant amount of the atmospheric ammonia necessary to re solve the early-faint-Sun paradox(3).