A secular increase in continental crust nitrogen during the Precambrian

TL;DR

This study reveals a long-term increase in nitrogen content in the Earth's continental crust from the Archean to the present, indicating a net transfer of atmospheric nitrogen to geological reservoirs over Earth's history.

Contribution

It provides the first evidence of a secular increase in continental crust nitrogen through Earth history using glacial tills as proxies.

Findings

Nitrogen content in tills increased from 66 ppm in the Archean to 290 ppm in the Neoproterozoic and Phanerozoic.

Isotopic composition of nitrogen remained constant at ~4‰ over time.

Results suggest a net transfer of atmospheric nitrogen to the crust over geological time.

Abstract

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.