  Recent work indicates the presence of substantial geologic nitrogen
reservoirs in the mantle and continental crust. Importantly, this geologic
nitrogen has exchanged between the atmosphere and the solid Earth over time.
Changes in atmospheric nitrogen (i.e. atmospheric mass) have direct effects on
climate and biological productivity. It is difficult to constrain, however, the
evolution of the major nitrogen reservoirs through time. Here we show a secular
increase in continental crust nitrogen through Earth history recorded in
glacial tills (2.9 Ga to modern), which act as a proxy for average upper
continental crust composition. Archean and earliest Palaeoproterozoic tills
contain 66 $\pm$ 100 ppm nitrogen, whereas Neoproterozoic and Phanerozoic tills
contain 290 $\pm$ 165 ppm nitrogen, whilst the isotopic composition has
remained constant at ~4$\permil$. Nitrogen has accumulated in the continental
crust through time, likely sequestered from the atmosphere via biological
fixation. Our findings support dynamic, non-steady state behaviour of nitrogen
through time, and are consistent with net transfer of atmospheric N to geologic
reservoirs over time.
