History of the earth's obliquity

Abstract

The evolution of the obliquity of the ecliptic (e), the Earth's axial tilt of 23.5°, may have greatly influenced the Earth's dynamical, climatic and biotic development. For e > 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 impliese¯≈ 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 e > 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 e > 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 e depends on the relative magnitudes of the rate of obliquity-increasee⋅t caused by tidal friction, and the rate of decrease e⋅p due to dissipative core-mantle torques during precession (e < 90° is required for precessional torques to move e toward 0°). Proterozoic palaeotidal data indicate e⋅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 e⋅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 e⋅p approximate, or exceed by several orders of magnitude, the indicated value of e⋅t. If e⋅p did indeed exceede⋅t in the past, then the obliquity would have decreased during Earth history. It is postulated here that the primordial Earth acquired an obliquity of ∼ 70° (54° < e < 90°) from the Moon-producing single giant impact at ∼ 4500Ma (approach velocity ≈ 5–20km/s, impactor/Earth mass-ratio ≈ 0.08−0.14). Secular decrease in e¯ subsequently occurred under the dominant influence of dissipative core-mantle torques. From 4500-650 Ma, e¯ slowly decreased to ≈ 60° (〈e⋅〉=−0.0009/cy), e¯ then decreased relatively rapidly from ∼ 60° to ∼ 26° between 650 and 430 Ma ((〈e⋅〉=−0.0556/cy)); climatic zonation changed from reverse to normal when e¯ ∼ 610 Ma, and 〈e˙〉 and the rate of amelioration of global seasonality were maxima for e¯= 45°at∼ 550Ma (the precessional rate Ω is maximum when e= 45°, and e⋅p varies as Ω2). Since 430 Ma, 〈e˙〉 has been ≲ −0.0025″/cy and e¯ has remained near its Quaternary range. The postulated relatively rapid decrease in e¯ between 650 and 430 Ma may partly reflect special conditions at the CMB which caused significant increase in dissipative core-mantle torques at that time. This inflection in the curve of e¯ versus time centred at = e¯ 45°also may be partly explained by the function e⋅p ∞ (Ω2/ω)(sin2e), where ω is the Earth's rate of rotation, and other dynamical effects on e⋅p. The Proterozoic-Phanerozoic transition may record profound change in global state caused by reduction in e¯ through the critical values of 54° and 45°. The postulated flip-over of climatic zonation at ∼ 610 Ma (e¯= 54°) coincides with the widespread appearance of the Ediacaran metazoans at ∼ 620−590Ma, and the interval of most rapid reduction of obliquity and seasonality at ∼ 550Ma (e¯= 45°) with the “Cambrian explosion” of biota at 550 ± 20Ma. These two most spectacular radiations in the history of life thus may mark the passage from an inhospitable global state of reverse climatic zonation and extreme seasonality (the Earth's Precambrian “Uranian” obliquity state) to a relatively benign state of normal climatic zonation and moderate seasonality. Further geological, palaeomagnetic and geochronological studies of Precambrian glaciogenic and aeolian deposits can test the predictions of a large obliquity (e > 54°) and reverse climatic zonation and zonal surface winds during the pre-Ediacaran Precambrian.