History of the earth's obliquity

The evolution of the obliquity of the ecliptic (ε), the Earth's axial tilt of 23.5°, may have greatly influenced the Earth's dynamical, climatic and biotic development. For ε > 54°, climatic zonation and zonal surface winds would be reversed, low to equatorial latitudes would be glaciated in preference to high latitudes, and the global seasonal cycle would be greatly amplified. Phanerozoic palaeoclimates were essentially uniformitarian in regard to obliquity, with normal climatic zonation and zonal surface winds, circum-polar glaciation and little seasonal change in low latitudes. Milankovitch-band periodicity in early Palaeozoic evaporites implies ε ¯ ≈ 26.4 ± 2.1°at ∼ 430 Ma, suggesting that the obliquity during most of Phanerozoic time was comparable to the present value. By contrast, the paradoxical Late Proterozoic (∼ 800−600Ma) glacial environment— frigid, strongly seasonal climates, with permafrost and grounded ice-sheets near sea level preferentially in low to equatorial palaeolatitudes—implies glaciation with ε > 54° (assuming a geocentric axial dipolar magnetic field). Palaeotidal data accord with a large obliquity in Late Proterozoic time. Indeed, Proterozoic palaeoclimates in general appear non-uniformitarian with respect to climatic zonation, consistent with ε > 54°. The primordial Earth's obliquity is unconstrained by the widely-accepted single-giant-impact hypothesis for the origin of the Moon; an impact-induced obliquity ≳ 70° is possible, depending on the impact parameters. Subsequent evolution of ε depends on the relative magnitudes of the rate of obliquity-increase ε ⋅ t caused by tidal friction, and the rate of decrease ε ⋅ p due to dissipative core-mantle torques during precession (ε < 90° is required for precessional torques to move ε toward 0°). Proterozoic palaeotidal data indicate ε ⋅ t ≈ 0.0003−0.0006″/cy (seconds of arc per century) during most of Earth history, only half the rate estimated using the modern, large value for tidal dissipation. The value of ε ⋅ p resulting from the combined effects of viscous, electromagnetic and topographic core-mantle torques cannot be accurately determined because of uncertainties in estimating, at present and for the geological past, the effective viscosity of the outer core, the nature of magnetic fields at the core-mantle boundary (CMB) and within the lower mantle, and the topography of the CMB. However, several estimates of ε ⋅ p approximate, or exceed by several orders of magnitude, the ind