With the onset of plate tectonics, surface elements began cycling into deep Earth via subduction and out again at volcanoes, gradually changing the atmosphere and the planet’s interior.
Gaseous nitrogen constitutes 78% of today’s atmosphere, but that was not always the case. The nitrogen concentration in Earth’s atmosphere has gradually declined, and a new reference model from DCO collaborators Peter Barry (University of Oxford, UK) and David Hilton (Scripps Institution of Oceanography, USA) explains how .
Subduction takes place at convergent margins, as tectonic plates collide. Where they meet, crust material from the more dense plate is pushed beneath the opposing plate and into Earth’s mantle. In this new research study, Barry and Hilton show that nitrogen has gradually accumulated in the mantle, thus depleting supplies at the surface over geologic time.
The paper, published this month in the journal Geochemical Perspectives Letters, uses samples collected at the Central Indian Ridge, its off-axis ridges and at Réunion Island. Barry and Hilton report new nitrogen and noble gas data from basalts and phenocrysts, showing strong evidence for both plume-derived and recycled volatiles in these samples. Their reference model reveals how N and potentially other volatile elements (e.g., C, O) behave during subduction, suggesting they are preferentially recycled into the deep plume-source mantle.
“Our data suggest there is an isotopically-high recycled nitrogen component present in these plume-influenced basalts,” said Barry. “This means that the deep Earth nitrogen cycle favors regassing of Earth’s lower mantle, which in turn has slowly changed atmospheric concentrations as well.”
This has far-reaching consequences for our understanding of deep Earth volatile cycles. The new reference model is consistent with the higher nitrogen content in Earth's early atmosphere. Such elevated nitrogen concentrations potentially instigated nitrogen-enhanced greenhouse warming, thus explaining the lack of global glaciations on early Earth as a consequence of a faint young Sun.
Image: Peter Barry and colleagues collect samples from the Central Indian Ridge on board the R/V Roger Revelle in November 2007. Credit: KNOX11RR Shipboard Scientific Party