Compared to its nearest planetary neighbors, Venus and Mars, Earth’s atmosphere is unusually enriched in nitrogen relative to primordial noble gases. While many unique facets of Earth have been considered as responsible for this distinctive atmosphere, including the planet’s size and its distance from the Sun, none have satisfactorily answered this conundrum. In a new paper in Nature Geoscience, DCO’s Sami Mikhail (University of Bristol, UK) and Dimitri Sverjensky (Johns Hopkins University, USA) outline a compelling model for nitrogen accumulation in Earth’s atmosphere, suggesting subduction, and subsequent degassing at arc volcanoes, is key .
Nitrogen is stored deep in Earth, particularly in the mantle, and cycles in and out of the planet’s interior much like other volatile elements (such as hydrogen, carbon, and oxygen). However, understanding how nitrogen is stored and cycled in the mantle has been challenging. In an earlier publication, Sverjensky and colleagues described a Deep Earth Water model allowing them to predict the behavior of water and its reactions with rocks at mantle-like temperatures and pressures . This breakthrough permitted Sverjensky and Mikhail to constrain nitrogen speciation in Earth’s mantle with confidence.
They found that nitrogen stored in the supercritical, aqueous fluids of Earth’s mantle is a mixture of N2 and NH4+. In more reducing conditions, the mixture comprises almost entirely NH4+, which tends to stay in minerals, unlike the noble gases which can escape into the atmosphere. But more N2 occurs in the relatively oxidizing conditions of mantle wedges beneath arc systems. From this environment both N2 and noble gases are easily degassed into the atmosphere through volcanism, and therefore over time the ratio of N2 to noble gases became enriched in Earth’s atmosphere. Further calculations confirmed that such degassing from arc volcanoes could result in enrichment of atmospheric nitrogen over a realistic timeframe (on the order of two billion years).
“Subduction zones ‘mix’ the solid Earth, and here we were able to show how they have altered the chemistry of the atmosphere,” said Mikhail. “Therefore, without subduction the chemistry of Earth’s atmosphere would be different, which raises the question: what would life be like without it?”
Oxidizing mantle conditions are found only at areas of active subduction. This important geologic process lies at the heart of plate tectonics, and importantly does not take place on Mars or Venus.
These data also provide tantalizing clues as to when plate tectonics initiated on Earth. Current estimates are vague, and range from 2 to 4.4 billion years ago. A single dataset suggests that Earth’s atmosphere, at least in terms of its nitrogen:argon ratio, resembled the present day between 3 and 3.5 billion years ago. If correct, this chronology considerably refines our understanding of the history of plate tectonics on Earth.
“Our results also suggest that the ratio of N2 to noble gases in a planetary atmosphere may be a marker of plate tectonic activity on that planet,” added Sverjensky.
Image: Schematic representation of the types of nitrogen molecules that are stabilized within the interiors of Earth, Mars, and Venus, where all three planets have similar nitrogen in their interiors with the sole exception being induced by of subduction zones on Earth. This causes the relative amount of nitrogen emitted by volcanoes to differ, thus the atmospheric composition of the three planets diverged once plate tectonics got going on Earth. In contrast, the atmospheres, and potentially the habitabilities, of Venus and Mars have evolved along different paths through geologic time because they lack plate tectonics.