PLEISTOCENE GLACIATION AND THE EARTHS PRECESSION CONSTANT

We predict time-dependent variations in the Earth's precession constan t arising from the ice and ocean mass fluctuations associated with the Late Pleistocene glacial cycles. Our predictions incorporate contribu tions from both the surface mass load redistribution and the adjustmen t of the solid earth. In this regard, we adopt spherically symmetrical , self-gravitating, Maxwell viscoelastic earth models and obtain resul ts for a large suite of radial viscosity profiles. These profiles incl ude a set obtained from published inferences based on post-glacial rel ative sea-level and uplift histories, as well as a set intended to sam ple the sensitivity of the results to variations in the viscosity with in a number of major subregions of the mantle (e.g. the transition zon e, the upper mantle, and the lower mantle). A more detailed measure of this sensitivity is also obtained by computing Frechet kernels for th e predictions. We construct an ice model which incorporates the ICE-3G model for the final deglaciation event and which is constrained to yi eld a eustatic sea-level variation which matches observed fluctuations in oxygen isotope records over the last 800 kyr. In all cases, the oc ean mass redistribution is constrained to be gravitationally self-cons istent, The computed Frechet kernels indicate that the predictions are most sensitive to variations in viscosity in the deepest regions of t he mantle; indeed, in some cases the sensitivity peaks at the core-man tle boundary. Both positive and negative perturbations to the precessi on constant are predicted, with the maximum peak-to-peak (relative) va riation being similar to 0.20 per cent for the published viscosity mod els and similar to 0.32 per cent for all other models. Furthermore, th e mean relative perturbation in the precession constant, with respect to the present-day value, is found to reach similar to -0.08 per cent for the published viscosity models, and similar to -0.20 per cent for other models. Recent solutions for the Earth's precession, obliquity a nd insolation parameters (Laskar, Joutel and Boudin 1993), indicate a passage through resonance, associated with a perturbation of Jupiter a nd Saturn, in the case when the mean relative perturbation in the prec ession constant is less than or similar to -0.15 per cent. We find tha t this threshold is not achieved for any of the published viscosity mo dels; however, it is reached, for example, for earth models with lower mantle viscosities which exceed 30-50 x 10(21) Pa s, or models charac terized by large (similar to two orders of magnitude) jumps in viscosi ty at mid-lower mantle depths.