Nitrogen Isotopic Composition and Density of the Archean Atmosphere
1CRPG-CNRS-Université de Loraine, 2School of Earth, Atmospheric and Environmental Sciences, University of Manchester UK. 3Institut de Physique du Globe Paris.
Article at a glance
The first life on Earth blossomed up during the Archean, between 3.8 and 2.4 billion years ago, at a time when climatic conditions were made favourable by the absence of any major glaciations. Researchers at the CRPG-CNRS University of Lorraine (France), in collaboration with the University of Manchester (UK) and the Institut de Physique du Globe of Paris, have measured the pressure of nitrogen (N2, which forms 78 % of the present-day atmosphere) and the isotopic composition (15N/14N ) of this element in the Archean atmosphere by analyzing water bubbles trapped in rocks formed near the surface of the Earth 3.5 billion years. These measurements provide important clues to reconstruct ancient climatic conditions, including the level of ancient atmospheric carbon dioxide (CO2), a major greenhouse gas in the atmosphere, as well as the strength of the Earth’s magnetic field at that time.
Stratigraphic section of the 3.5 Gigayears old Dresser Formation through which the PDP2 scientific drilling was carried out. On the forefront, basalts hosting fluid inclusions analyzed in this study are crosscut by chert and barite dikes on which lie the sediments (photo: P. Philippot).
Quartz-carbonate aggregate filling up vacuoles in 3.5 Ga-old basalts on the top of the Dresser Formation. Quartz contains microbubbles of ancient water that were analyzed in this study (photo B. Marty).
The faint young Sun paradox
During the Archean the young Sun was much dimmer than it is today and the solar energy received at the surface of the Earth was about 20-25% lower than at present. If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated. However, there is no geological evidence for any global glaciations before the end of the Archean, in fact the all the signs are that liquid water was widespread. This climate puzzle is known as the «faint young Sun paradox». There have been many suggestions to solve this conundrum. Perhaps there were higher concentrations of greenhouse gases such as CO2, which is one of the main regulators of the current climate, a concentration about 1000 times higher today would be needed. The best indicators of ancient CO2 levels are ancient «fossil» soils that depend on the partial pressure of atmospheric CO2 at the time, and these soils suggest only modest CO2 levels in the atmosphere during the Archean. Maybe other atmospheric greenhouse gases were also present, in particular ammonia (NH3) and methane (CH4), but these gases are fragile and easily destroyed by ultraviolet solar radiation. Intriguingly, methane is a chemical «fingerprint» of microbial activity (methanogens), so if there was a continuous supply of this gas in the atmosphere during the Archean, it would be a strong signal of early life on Earth. Another climate-warming suggestion is that the pressure of nitrogen was three times higher than present, as this much nitrogen would amplify the greenhouse effect of atmospheric CO2 and allow the Earth to remain ice-free.
The experimental approach
In the study published in the journal Science, the team analyzed tiny samples of water, containing trapped air, measuring the amount and isotopic abundances of nitrogen and argon, the latter a noble gas which, being a chemically inert gas, is an ideal element to monitor atmospheric change. The samples are from a region of northern Australia that is extremely old and contains exceptionally well-preserved rocks. The ancient air is trapped in the form of bubbles in quartz (a mineral form of silica) that precipitated during circulation of heated waters in the crust. Using the nitrogen and argon measurements the team were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and from that, the atmosphere that was once in equilibrium with the water.
Fluid inclusions (5-20 micrometers in size) in quartz filling vaccuoles (here, a few mm-sized) in basalts analyzed in this study. Fluids trapped in these inclusions contain a mixture of hydrothermal fluids and Archean surface water in the age range 3.0-3.5 Ga. Atmospheric gases from the ancient atmosphere were dissolved in the latter and analyzed in this study (Photo P. Philippot).
The results and their implications
The researchers found that the partial pressure of N2 in the Archean atmosphere was similar, possibly slightly lower, than it is at the present day. This rules out nitrogen as one of the main contenders for solving the early climate puzzle, the amount of this gas in the atmosphere was too low to enhance the greenhouse effect of CO2 sufficiently to warm the planet. The results also give an upper limit of 0.7 bar for the partial pressure of CO2 in the Archean atmosphere, enough to counteract the effects of the faint young Sun, but currently at odds with CO2 estimates based on fossil soils. Another important finding of the study is that the nitrogen isotope ratio (14N and 15N) of the Archean atmosphere was very similar to that of modern-day nitrogen. The 14N and 15N ratio is a sensitive probe of atmospheric loss from the upper atmosphere into space. This loss can occur by interaction of the atmosphere with the solar wind (the flux of material released from the Sun’s atmosphere) and the Earth’s magnetic field effectively shields the Earth to prevent this happening. If the early Earth did not have a strong magnetic field then the interaction would have been much more intense and during the loss of atmosphere to space leading to a significant change in the isotopic composition of N, because these two isotopes have different rates of loss, this is exactly what is believed to have happened to the atmosphere of Mars, a planet with a weak magnetic field. The constancy of the isotopic ratio of nitrogen from the Archean to the present-day attests to the presence of a significant geomagnetic field since that time, requiring an intensityof at least 50% of the current magnetic field.
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