A new astronomical solution for the calibration of a geological timescale (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Jacques Laskar, Observatoire de Paris, IMCCE
Tél: 33 1 40 51 21 14
Fax: 33 1 40 51 20 55
E-mail: Jacques.LASKAR @ obspm.fr

25 October 2004

A new astronomical solution for the calibration of a geological time scale

A team led by Jacques Laskar, from IMCCE/CNRS and Paris Observatory has released
a new solution for the long term evolution of the orbital and rotational motion
of the Earth. Using Milankovitch paleoclimate cycles, this solution has already
been used as a reference time scale for the calibration of the sedimentary
records over the Neogene period (0-23.03 Myr) in the new geological time scale
(GTS2004) adopted by the International Comission of Stratigraphy (ICS) and the
International Union of Geological Sciences (IUGS). It is the first time that an
astronomical solution has been used to establish the ICS geological chronology
over a full geological period.

Due to gravitational planetary perturbations, the orbit of the Earth is slowly
changing in time, so as the orientation of the planet's spin axis. These changes
induce variations of the insolation received on the Earth's surface that are,
according to Milankovitch theory of paleoclimate (1941), responsible for some of
the large climate changes in the past. Since the landmark work of Hays, Imbrie,
and Shackleton, (1976), which established a clear correlation over 500 kyr
between astronomical forcing and the ratio 18-O/16-O [Oxygen-18 / Oxygen-16] in
marine sediments, the Milankovitch theory has been confirmed overall (see Imbrie
and Imbrie, 1979, for historical details). Geologists are now currently using
the computed evolution of the Earth orbit and rotational parameters for the
calibration of sedimentary cores over several millions of years.

Paris Observatory has been involved for a long time in the computation of
planetary orbits variations over an extended time span. Indeed, Le Verrier,
famed for the discovery of Neptune in 1846, and the former director of Paris
Observatory computed in 1856 an analytical solution for the long term evolution
of the Earth orbit. This solution was used by Milankovitch to establish his
theory of paleoclimates. More recently, the paleoclimate community has used for
their calibration of sedimentary cores the orbital solution from Paris
Observatory derived by Bretagnon (1974) and Laskar et al. (1993). This latest
solution was estimated to have a length of validity of about 10 Myrs, but the
improvement in collecting of the geological data was urging for a new solution.
As a result of the chaotic behaviour of the planetary orbits (Laskar, 1989), the
uncertainty in these computations is multiplied by 10 every 10 Myr. It is thus
hopeless to search for a precise solution of the Earth past evolution beyond 100
Myr, but it is possible to obtain a precise solution over a few tens of millions
of years. The new solution published in this issue of Astonomy and Astrophysics
can be used for the calibration of paleoclimate data for the last 40 - 50 Myr.

This solution has indeed already been used for the establishement of the new
Geological Time Scale GTS2004 for the Neogene period (0-23.03 Myr) (Lourens et
al, 2004). This new time scale, adopted by the International Union of Geological
Sciences (IUGS) results from a cooperative effort among sedimentologists around
the world in view to obtain an overall view of the Earth' history from now to
about 3.8 Gyr. The adoption of the astronomical solution for the calibration of
the Neogene period allows to obtain a precision of about 40 kyr (one obliquity
cycle) over the full range of the Neogene. This adoption resulted in a change of
about 0.8 Myr of the Neogene/Paleogene limit with respect to the previous
determination, obtained through radiogenic data.

The authors of the present paper also show that if we do not search for a
complete solution of the Earth orbit, but just for the main variation of its
orbit eccentricity, a relatively stable modulation of 405 kyr, resulting from
the perturbations of Jupiter and Saturn (that are more stable than the inner
planets) can be used over the full Mesozoic era (up to about 250 Myr) for the
astronomical calibration of sediments with an uncertainty of about 0.5 Myr after
250 Myr. This term is actually related to a geological cycle that is present in
some jurassic and triassic sediments.

Due to the tidal dissipation in the Earth-Moon system, the Earth's rotation
slows down, and the Moon is receding away at about 3.82 cm/yr. This induces a
slow increase in the obliquity (angle between the Earth's equator and the orbit)
of about 2 degrees per Gyr. However, J. Laskar and his colleagues show that in
the near future, a resonance with a small gravitational perturbing effect of
Jupiter and Saturn, will make the obliquity decrease of about 0.4 degrees within
a few millions of years, with some possible impact on the climate. When looking
for the evolution of the obliquity of the Earth, it is surprising to see (Fig.
2) that this crossing of resonance is the only noticeable singularity from -250
Myr to + 250 Myr. Nevertheless, as this change occurs in the future, the authors
assume that unless some new results of the past evolution of the dynamical shape
of the Earth show that the crossing of this singularity could have also existed
in the past, one should consider that the proximity of this resonance is pure
chance.

The solution and associated files are freely available on the website,
http://www.imcce.fr/Equipes/ASD/insola/earth/earth.html

Press release of Astronomy & Astrophysics,

http://www.edpsciences.org/journal/i...iv3=PRaa200410


References

A long term numerical solution for the insolation quantities of the Earth,
Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A., Levrard, B.:
2004 (A&A in press)

Orbital, precessional, and insolation quantities for the Earth from -20 Myr to
+10 Myr,
Laskar, J., Joutel, F., Boudin, F.: 1993, A & A, 270, 522-533

www.stratigraphy.org

Time Lords, Whitfield, J. : 2004, Nature, 429, 124-125

The Neogene Period,
Lourens, L.J., Hilgen, F.J., Shackleton, N.J., Laskar, J., and Wilson, D.: 2004,
in F. Gradstein, J. Ogg et al. - A Geologic Time Scale 2004.Cambridge University
Press, UK (in press)

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/oct04/geo-f1.jpg (41KB)]
Evolution of the obliquity of the Earth in degrees, from -1 to +1 Myr (bottom),
and of the insolation in summer at the Earth's surface at a latitude of 65
degree North (top). These variations result from the gravitational perturbations
on the orbital motion of the Earth from all the planets of the Solar System.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/oct04/geo-f2.jpg (92KB)]
Evolution of the obliquity of the Earth in degrees, from -250 to +250 Myr. The
grey zone is the actual obliquity, while the black curve is the averaged value
of the obliquity over 0.5 Myr time intervals.