A secular increase in continental crust nitrogen during the Precambrian
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 c...
| Published in: | Geochemical Perspectives Letters |
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| Main Authors: | , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Geochemical Perspectives Letters
2017
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| Subjects: | |
| Online Access: | http://hdl.handle.net/1828/11015 https://doi.org/10.7185/geochemlet.1731 |
| Summary: | 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 ± 100 ppm nitrogen, whereas Neoproterozoic and Phanerozoic tills contain 290 ± 165 ppm nitrogen, whilst the isotopic composition has remained constant at ∼4 ‰. 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. The authors thank Richard Gaschnig and Roberta Rudnick for providing glacial till samples as well as Dante Canil for the initial suggestion to use glacial tills as a crust composition proxy. We also thank Natasha Drage at the University of Victoria for assistance with sample compilation. Andy Schauer at the University of Washington assisted in isotopic analyses. Funding was provided in an NSERC Discovery Grant to CG. We thank Sami Mikhail and an anonymous reviewer for constructive feedback, as well as Helen Williams for editorial support. Faculty Reviewed |
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