Ice an Asteroid Impact and the Rise of Complex life
By Victor Gostin, David McKirdy and George Williams
An asteroid impact in southern Australia is redefining the conditions that preceded the explosion of multicellular life more than 500 million years ago. Some of the major events during the Earth’s history have occurred simultaneously or in close succession. The consequences of these events have led scientists to conclude that the synchronicity of these global disasters has been a powerful driver of ecological and evolutionary change.
For example, the impact of a huge asteroid measuring ~12 km in diameter at the end of the Cretaceous Period 65 million years ago was the death-knell for the dinosaurs. The impact on Mexico’s Yucatan Peninsula coincided with extremely large and extensive basaltic eruptions in India known as the Deccan Traps. The combined effect of these two events on the Earth’s atmosphere and oceans evidently spelt the end not only of the dinosaurs but also of the marine ammonites and most other creatures in the upper few metres of all oceans and inland waters. This set the scene for the subsequent explosion of mammalian evolution.
Several geological eras earlier, before the dawn of multicellular animal life, a smaller but significant asteroid measuring ~4.7 km in diameter hit South Australia around 580 million years ago at Lake Acraman in the Gawler Ranges. The impact site in ancient volcanic rocks now bears the deeply eroded scar of the initial transient cavity, measuring ~40 km in diameter, that led to the formation of a larger collapsed crater ~90 km across. Because of its equatorial position at that time – a product of the drift of continents through plate tectonics – the devastating effects of the impact probably spread globally.
The turbulent mix of fragmented and melted volcanic bedrock and asteroid were scattered widely and fell into bordering shallow seas, and are now preserved in thick shale sequences 238– 385 km away in the Flinders Ranges to the east and over 540 km away in the Officer Basin to the north-west of the impact site. An intense earthquake, much stronger than even the strongest internally generated earthquake, shook the whole planet and left its mark in distinct wave generated tsunami deposits. Atmospheric effects involved significant injections of stratospheric water vapour, widespread fires and the destruction of the ozone shield. The global dispersal of a dust cloud would have lowered light levels below the threshold required for photosynthesis.
In the Flinders Ranges, thick deposits of marine mud and silt (now shale) created the Bunyeroo Formation, where an extensive layer up to 20 cm thick of red volcanic fragments match their Gawler Ranges source rocks. Visually, the shale overlying the impact layer is no different from what was deposited before the violent event, indicating that no obvious change in climate or sea level took place. However, the story recorded by the little-altered Dey Dey Mudstone of the same age in the Officer Basin is enlightening.
Using the impact ejecta horizon as a reference point in cores recovered from drill holes in the Officer Basin, palaeontologist Dr Kath Grey at Macquarie University noticed a remarkable change in the planktonic unicellular microfossils, from simple smooth-walled leiospheres below the ejecta horizon to an abundance and wide variety of acanthomorphs (spiny acritarchs) above it. Geochemical analyses of the same drill cores revealed a parallel change in molecular fossils. The distribution of steranes was at first typical of normal marine phytoplankton, but this changed to a distribution largely derived from green algae.
High-resolution carbon isotope studies of the organic material and coexisting marine carbonates show that major fluctuations followed the impact. On this basis geochemists concluded that there are two biomarker anomalies linked to the Acraman impact event. The first biomarker revealed the initial collapse of primary production in the ocean. The second biomarker reveals the appearance and radiation of algae that had an encystment stage in their life cycle, and hence were able to survive the environmental shock.
An alternative explanation for these observations is that the massive impact induced combustion of the contemporary nearshore and onshore biomass (comprising microbial mats and perhaps fungi), thus perturbing the global carbon cycle. Evolution of the diverse microflora was soon followed by the first appearance of large, multicellular, soft-bodied animals known as the Ediacaran biota. The International Union of Geological Sciences has recently approved a new subdivision of geological time spanning the interval from about 635 million years ago to the start of the Cambrian period 542 million ago years ago. The new subdivision is named the Ediacaran Period after a sequence of sedimentary strata in the Flinders Ranges. The Bunyeroo Formation, with its Acraman impact ejecta horizon, and the coeval Dey Dey Mudstone in the Officer Basin are of mid-Ediacaran age.
The exact timing of the Acraman impact has been a frustrating unknown. This is because the least altered minerals in its ejecta – zircons – provide the age of the original target volcanic rocks in the Gawler Ranges. Nevertheless, recent discoveries have opened the door to new interpretations. Although rare isolated pebbles were observed some years in sequences hosting the Acraman ejecta, last year we described ice-rafting of debris in the Australian Journal of Earth Sciences. Our evidence suggests that debris from the melting of seasonal and glacial ice was deposited before, during and after the Acraman impact. But was this freezing a worldwide phenomenon?
Elsewhere in Australia, potential equivalent deposits may be seen in the northwest of Tasmania as well as in the Kimberley region of Western Australia. Further a field, several countries including Canada, China, Norway, Scotland and Brazil also have Ediacaran glacial deposits that are all commonly assigned to the same event – the Gaskiers glaciation in Newfoundland, Canada. However, until we can successfully date the Australian Ediacaran glacial deposits, it remains possible that these deposits were not contemporaneous but occurred as multiple glacial advances and retreats in different locations at different times. Two major glacial events that preceded the Ediacaran period, the Sturtian and Elatina glaciations about 700 and 630 million years ago, respectively, had no discernable effect on the contemporary biosphere. However, we think that the eventual easing of environmental stresses brought about by the less intense Ediacaran glaciation and Acraman impact triggered a major evolutionary advance.
Since 1986, Dr Phil Schmidt from the CSIRO has conducted detailed studies of the directions of the Earth’s magnetic field in these ancient glacial strata. To the surprise of many, it turns out that the ejecta were all deposited in low latitudes. The Sturtian, Elatina and mid Ediacaran glaciations all point to glacial conditions at sea level near the Equator. Such a situation is easily explained by assuming that the whole planet was frozen over as described in the “Snowball Earth” hypothesis. However, the evidence now points to glacial ice entering open ocean waters. The observation that the Bunyeroo Formation exhibits tsunami-like reworking of the Acraman ejecta layer indicates that, despite the very cold conditions, the mid-Ediacaran ocean was not totally frozen over as required in a snowball Earth.
Geoscientists will continue to probe the ancient rock record to clarify and confirm essential observations regarding the timing of glaciations, large asteroid impacts and other atmospheric and environmental crises that eventually led to the amazing explosion of multicellular life during the Ediacaran Period, now firmly defined in Australia’s backyard.
Victor Gostin, David McKirdy and George Williams are with the School of earth & environmental Sciences at the University of Adelaide.
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Updated 22nd May 2012