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A Different Angle on Earth’s Climate History

Credit: pongpongching/Adobe

Credit: pongpongching/Adobe

By George Williams, Phillip Schmidt & Grant Young

Earth’s axial tilt affects our environment in many ways, but a much greater tilt in the remote geological past may have strongly influenced the planet’s climate history and the evolution of life.

Do you know what controls:

  • the seasons, with warm summers and cool winters in middle latitudes, large seasonal temperature changes in high latitudes, and a consistent equatorial climate?
  • the meridional temperature gradient, which runs north–south and explains why Darwin is warmer than Hobart, and why high latitudes have ice sheets while the equator is hot?
  • the general direction of global oceanic circulation?

These important features of the environment are controlled by the tilt of Earth’s spin axis relative to the perpendicular to the plane of Earth’s orbit around the Sun. This tilt is termed the obliquity of the ecliptic, and it varies between 22° and 24° over a cycle of 41,000 years (ka).

For the present-day obliquity of 23.4°, global seasonal temperature variation is mostly moderate, with the poles cold and the equator hot. The meridional temperature gradient controls the general direction of oceanic circulation, with cold, dense water sinking at the poles and upwelling at lower latitudes where it is driven by surface winds. This process transports heat, fresh water and carbon around the globe.

The obliquity cycle of 41 ka, together with other orbital cycles affecting Earth and its movement around the Sun, has influenced the planet’s climate cycles, as shown by the advance and retreat of glaciers during the past few million years. But has the obliquity remained near 23° throughout Earth’s 4.54 billion-year history? After all, the outer planet Uranus has an obliquity of 97.9°, so it seems to be lying on its side. Likewise Mars, which lacks the stabilising gravitational influence of a large moon, reached an obliquity greater than 70° since 3000 million years ago (Ma), with a mean of 41.8° compared with the current obliquity of 25.2°. Intriguingly, geological and geophysical observations imply that Earth’s obliquity in the remote past was also much greater than the current value.

During the past 40 years we have studied glacial deposits of Neoproterozoic age (720–635 Ma) in South Australia, Canada and Alaska, and of Palaeoproterozoic age in Western Australia (1800 Ma) and Canada (2300 Ma). Our work using palaeomagnetism – the study of ancient magnetism of rocks – showed that, curiously, marine glacial deposits older than 635 Ma in Australia and North America formed at palaeolatitudes lower than 20°.

Palaeolatitudes are revealed by determining the direction of magnetism locked in iron-bearing minerals within rocks at or soon after the time the rocks were formed. Numerous independent palaeomagnetic studies on other continents support our findings. Importantly, no glaciation has been identified in palaeolatitudes higher than 40° prior to 635 Ma, when only primitive life forms like bacteria and algae prevailed on Earth.

A frigid, strongly seasonal late Neoproterozoic (645–635 Ma) glacial climate is indicated by sand wedges several metres deep in South Australia, north-west Africa, Europe and Greenland. The sand wedges in South Australia are identical in dimensions and structure to sand wedges now forming in polar permafrost regions, such as Antarctica’s Dry Valleys, under extreme seasonal temperature changes (winter–summer mean monthly air temperature difference of about 40°C).

Such wedges form from vertical thermal contraction cracks several metres deep in the upper permafrost during severe winters. The cracks fill with windblown sand, and repeated cracking and filling over many years produces sand wedges that push aside and upturn adjacent material as they grow. A strongly seasonal climate is also implied by the existence of ancient laminated marine or glacial-lake deposits that are comparable to modern seasonally laminated deposits found in polar regions.

The seemingly contradictory findings of low-latitude marine glaciation with strongly seasonal climates may be explained if Earth’s obliquity was greater than 54°, with computer models finding that:

  • the equator would be cooler than the poles, on average, and low latitudes would be glaciated preferentially;
  • high latitudes around times of summer solstices would be too hot for the accumulation of snow and the development of high-latitude ice sheets;
  • very large seasonal air temperature ranges (mean monthly difference at least 40°C) would affect all continents, including those at low latitudes;
  • the meridional temperature gradient and the general direction of global oceanic circulation would be reversed relative to the present day; and
  • only primitive life forms could exist because of the extreme seasonal stresses.

Significantly, glacier landforms in low latitudes on Mars are interpreted by planetary scientists as evidence of glacial activity at times of high Martian obliquity. These findings support the concept of low-latitude glaciation at times of high planetary obliquity.

So how might Earth have acquired a high obliquity in the remote past? Computer simulations have found that the impact of a Mars-sized body with Earth 4.51 billion years ago, which many astronomers think resulted in the Moon’s formation, would likely have imparted a high obliquity on the early Earth. This would explain why all glacial deposits older than 635 Ma that have yielded high-quality palaeomagnetic data indicate that glaciation occurred at low palaeolatitudes.

The high-obliquity hypothesis for low-latitude glaciation prior to the Ediacaran Period (635–541 Ma) requires Earth’s obliquity to have decreased to a value near that of today between the end of Neoproterozoic low-palaeolatitude glaciation after 635 Ma and the advent of high-palaeolatitude glaciation in the early Cambrian at 535 Ma. This obliquity decrease would have greatly increased Earth’s habitability by reducing seasonal stresses.

Several mechanisms for such a decrease have been suggested, but none is established. However, many decades passed before a mechanism for the lateral movement of continents – slow convection of Earth’s mantle that is driven by internal heat – was identified and accepted.

The Ediacaran Period marks the transition from an alien world with primitive life forms and marine glaciations in low latitudes under strongly seasonal climates, to the familiar world of complex life, high-latitude glaciations and mostly temperate climate. The available palaeomagnetic data suggest that marine glaciation and ice rafting of stones during the Ediacaran Period occurred over a range of latitudes, which is consistent with a transition occurring between those contrasting worlds.

Global oceanic circulation may have undergone a revolution during the Ediacaran as the meridional temperature gradient reversed. Such a dramatic change in oceanic circulation would have brought oxygenated surface waters to the previously oxygen-depleted deep ocean and been a catalyst for the evolution of complex marine life at that time, including the world famous Ediacara biota in South Australia.

The high-obliquity hypothesis offers explanations for many puzzling features of Earth’s climatic and biological records. It provides an alternative to the “snowball Earth” hypothesis, which proposes that Earth was at times frozen-over from pole to pole prior to the Ediacaran, with sea-ice 1 km thick on average, ice sheets more than 2.5 km thick covering all continents, and the hydrological cycle shut down for many millions of years.

However, geological observations indicate that the Neoproterozoic glacial environment included ice-free continental regions with widespread windblown dune fields, extensive and long-lived open seas that permitted wave-generated structures to form in marine deposits, an active hydrological cycle, and repeated glacial advances and retreats. Nor do palaeontological data permit a snowball Earth because life forms require open oceanic refugia for their long-term survival.

Moreover, the thick ice sheets covering all continents in a Snowball Earth scenario would cause extreme sea level falls of up to 1 km. However, recent studies of late Neoproterozoic glacial sequences in southern Africa and South Australia found no evidence of extreme sea-level variation. These observations together militate against the snowball Earth scenario.

Indeed, most glacial sedimentary geologists oppose the snowball Earth hypothesis and favour an Earth with some open seas, terming it a “waterbelt Earth”’. However, neither a snowball Earth nor a waterbelt Earth can explain the extreme seasonal mean monthly air-temperature difference of 40°C in low palaeo­latitudes.

The high obliquity and snowball Earth hypotheses each face challenges, and continuing investigation of ancient glaciations promises to further illuminate Earth’s climate history.

George Williams is Visiting Research Fellow in Earth Sciences at the University of Adelaide and Fellow of the Australian Academy of Science. Phillip Schmidt is Adjunct Professor in Earth and Planetary Sciences at Macquarie University, and Grant Young is Emeritus Professor in Earth Sciences at the University of Western Ontario.