C R P G
Centre de Recherches Pétrographiques et Géochimiques
www.crpg.cnrs-nancy.fr UMR 5873
«Science» september 20
th
2013
To appear in the journal Science:
Nitrogen Isotopic Composition and Density of the Archean
Atmosphere
Marty1 B., Zimmermann1 L., Pujol1 M., Burgess2 R.,Philippot3 P.
press release
1 : CRPG-CNRS-Université de Loraine, 2 : School of Earth, Atmospheric and Environmental
Sciences, University of Manchester UK. 3 : Institut de Physique du Globe Paris.
Contact:
Bernard Marty 03 83 59 42 22, 06 78 03 28 99
[email protected]
a professor at the Institut Universitaire de France and the Centre de
Recherches Pétrographiques et Géochimiques (CRPG-CNRS),
University of Lorraine
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 CRPGCNRS 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.
Michel Champenois : Chargé de communication CRPG/CNRS/Univ. Lorraine
tél : 03 83 59 42 36/06 - 06 84 64 39 14
[email protected]
C R P G
Centre de Recherches Pétrographiques et Géochimiques
www.crpg.cnrs-nancy.fr UMR 5873
«Science» september 20
th
2013
press release
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.
Michel Champenois : Chargé de communication CRPG/CNRS/Univ. Lorraine
tél : 03 83 59 42 36/06 - 06 84 64 39 14
[email protected]
C R P G
Centre de Recherches Pétrographiques et Géochimiques
www.crpg.cnrs-nancy.fr UMR 5873
«Science» september 20
th
2013
The results and their implications
press release
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/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 intensity
of at least 50 % of the current magnetic field.
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).
Michel Champenois : Chargé de communication CRPG/CNRS/Univ. Lorraine
tél : 03 83 59 42 36/06 - 06 84 64 39 14
[email protected]
C R P G
Centre de Recherches Pétrographiques et Géochimiques
www.crpg.cnrs-nancy.fr UMR 5873
«Science» september 20
th
2013
press release
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).
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).
Michel Champenois : Chargé de communication CRPG/CNRS/Univ. Lorraine
tél : 03 83 59 42 36/06 - 06 84 64 39 14
[email protected]