Abstract
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).
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