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 reducing lithospheric conditions on the early Earth have been presumed to favour the generation of an ammonia-rich atmosphere. but this hypothesis has not been studied experimentally. Here we demonstrate mineral-catalysed reduction of N-2, NO2- and NO3- to ammonia at temperatures between 300 and 800 degrees C and pressures of 0.1-0.4 GPa-conditions typical of crustal and oceanic hydrothermal systems. We also show that only N-2 is stable above 800 degrees C, thus precluding significant atmospheric ammonia formation during hot accretion, We conclude that mineral-catalysed N-2 reduction might have provided a significant source of ammonia to the Hadean ocean. These results also suggest that, whereas nitrogen in the Earth's early atmosphere was present predominantly as N-2, exchange with oceanic, hydrothermally derived ammonia could have provided a significant amount of the atmospheric ammonia necessary to resolve the early-faint-Sun paradox(3).