Project IV.1: Astronomical and geophysical constraints from the sedimentary record (Jacques Laskar)

Objective: We propose to use the available stratigraphic data in order to constraint and improve the geophysical parameters of the astronomical solution. We intend also to use these data to discriminate among different possible planetary orbital evolutions by comparison to stratigraphic data. Ultimately we search for the observational evidence of a transition from libration to circulation in the angle related to the secular resonant argument 2(g4-g3)-(s4-s3). This would be the first observational evidence for the chaotic behavior of the orbital motion of the Earth.

Background: Due to tidal dissipation in the Earth-Moon system, the Earth rotation slows down, and the Earth-Moon distance increases by 3.8 cm per year. Unfortunately, we do not have a precise model of the past evolution of this tidal dissipation. As a consequence the evolution of the precession frequency of the Earth is uncertain beyond 20 Ma. Nevertheless, as the eccentricity signal provides an independent chronology, we aim to constraint the phase shift of the precession induced by the variation of the tidal dissipation by tuning the sedimentary record to the eccentricity signal, thus providing a geological constraint on the geophysical model of the Earth (project I.1) that will allow us to improve the model of the evolution of the Earth-Moon system. Over a longer time scale, we will also search for geological constraints on the chaotic behavior of the Earth orbit resulting from the overlap of secular resonances involving the perihelion and nodes of the Earth and Mars (Laskar, 1990). These secular resonances induce very long term modulations of 1.2 and 2.4 Myr in the eccentricity and obliquity that are visible in some sedimentary records. The variations of these modulations will be used as a constraint for the various computed orbital solutions.

Research strategy and methodology: These studies require refined analysis of the stratigraphic records, but it will also be necessary to analyze the different patterns present in the orbital and precessional solutions. We will perform a statistical study on numerous orbital evolutions, compatible with the best estimate of the accuracy of the solutions. This accuracy will be determined by adjusting directly the orbital solution to all available planetary observations. The most adapted sedimentary records for this study are the Paleogene records (II.1, 2, 3).

Feasibility, innovative aspects and relevance: Attempts have already been made for the determination of geophysical parameters from stratigraphic data (Lourens et al., 2001; Pälike et al., 2000) or for the search for evidence of a chaotic transition (Pälike et al., 2004). The gained experience will be crucial for this new project with improved data and methodology. Constraining the astronomical solution of the Earth, or finding some evidence of its chaotic orbital evolution through geological data are the fundamental questions that motivate this project. The resolution of these questions is difficult, but an additional aspect of this project is the construction of the new generation of astronomical solution for which, for the first time, the accuracy will be fully determined by comparison to observations and will be pushed to its limits.

Link with other projects: This project is directly linked to projects I.1 for the determination of geophysical parameters. The second part has a strong connection with projects II.1, 2, 3 as the use of precise stratigraphic sequences will be essential for searching for observational evidences of chaotic transitions beyond 40 Ma.