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

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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 18O/ 16O 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 Contacts Jacques Laskar (Observatoire de Paris, IMCCE)

Dernière modification le 4 mars 2013