Geoscientist Online

Lena Z Evins* investigates a Large Igneous Province that might have been responsible for the first Phanerozoic mass extinction.

Geoscientist 19.10 October 2009

In a rarely visited corner of the Purnululu National Park, Western Australia, well preserved remnants of huge volcanic eruptions form an undulating terrain, and tell a story of ancient calamity. The eruptions occurred as animal life was just starting out. This was the time of the early Cambrian fauna, when trilobites were the latest thing. Reefs were not built of coral as they are today, but sponge-like creatures called Archaeocyatha. All was well until, around 510 million years ago, disaster struck. Massive lava flows buried the land, sulphurous gases poisoned the atmosphere and enormous amounts of carbon dioxide caused a greenhouse effect, heating up the oceans to levels intolerable for reefs and their inhabitants.

Well, that’s the hypothesis. There are still many uncertainties, but we know that at the time of the eruptions many of the early animal species expired. This one affected creaters a lot less spectacular than the dinosaurs, but it profoundly affected the evolutionary course of early animals. The reef-building Archeocyatha, for example, vanished for ever.

Extinctions and volcanoes

There have been several mass extinctions during the course of evolution. The most famous (though not the biggest) of these, was the K-T extinction, which has since 1987 become associated with a giant meteorite that collided with the Earth at about the same time as the dinosaurs finally became extinct. That event has also been linked to the Chicxulub crater, offshore the Yucatan Peninsula, in the Caribbean.

Doubts now surround the correspondence of the crater with the end-Cretaceous impact. According to Gerta Keller (Princeton University) and co-workers, the Chicxulub meteorite impact occurred 300 000 years before the dinosaurs expired. But whatever the truth of that particular controversy, it remains the case that the K-T extinction is one of many mass extinctions that are demonstrably contemporaneous with major volcanic events.

These huge events, unparalleled in human experience, formed vast landscapes of mainly basaltic lava, called Large Igneous Provinces (LIPs). One example of a LIP is the Deccan plateau, which covers almost a quarter of India. The Deccan plateau is, in fact, the LIP that is considered by some to have been the main cause of the K-T extinctions.

Australia’s LIP

The LIP that formed in Australia during the early Cambrian stretches from East Kimberley in Western Australia (WA) to westernmost Queensland. Recently it was shown that rocks in southern WA and South Australia (SA) also belong to this province, which means that it originally covered an area of ca. 2 million km² (about ten times the area of the UK). Australian researchers Linda Glass and David Phillips recently named the province the “Kalkarindji Continental Flood Basalt Province” after an aboriginal community in the area. The province is now much eroded and buried; but the major part of outcrop is found in the East Kimberley, WA, and western Northern Territory (NT). Basalts locally can reach a thickness of c. 1km.

So – did the Kalkarindji LIP do to the Archaeocyatha like the Deccan Traps probably did for the dinosaurs? One thing that could help us link the LIP to the early Cambrian mass extinction is the mix of gases that it erupted into the atmosphere. Before eruption, magma contains a certain amount of volatiles, such as water, sulphur dioxide, carbon dioxide, and hydrochloric acid. As magma moves upwards and cools, these volatiles tend to separate into fluid phases, and are released as gases from the erupting magma like bubbles from a champagne bottle. However when the magma reaches shallower depths, it starts to crystallise and some remnants of these volatiles may be trapped. We can study these in the laboratory and determine what gases and fluids were present during magma evolution. Glassy inclusions can even tell us about the pre-eruptive composition and volatile content of the original magma.

Erupted gases influence Earth’s climate and environment. Sulphur, for example, rapidly forms sulphate and sulphuric acid, which form aerosols that reflect the Sun’s rays back into space, cooling the Earth. Luckily for us, this effect tends to be short-lived, and may not cause long-term global cooling. On the other hand, carbon dioxide stays in the atmosphere for a long time, and is of course a greenhouse gas. If a vast amount of carbon dioxide is released in a short time, the result may well be a shift towards warmer global climate. The faster the eruption, the higher the potential for climate change. To find out if this happened 510 million years ago, we need to know three things: the amount of dissolved gas, the amount of magma erupted and the rate of eruption.

Size matters, but not exclusively

This is why, in many LIP studies, one of the first questions raised are: how big is this province? The size of the province indicates the amount of magma erupted. By looking at the bubbles of fluids and glass in the volcanic rocks, we will know what gases were released, as well as get an estimate of volatile concentration in the magma. To establish the rate of eruption is quite tricky; it requires high resolution age dating of volcanic rocks from the oldest and the youngest lava flows of the province. However, during the building of a LIP there are many individual eruptions and eruptive events.

It has been shown that although the most magma is erupted in a period of less than 1 million years, the beginning and ending stages may be more prolonged. Even during the main eruptive phase, individual eruptive events are the most significant for determining gas flux. This is especially true since individual eruptions may involve lavas of somewhat different composition, and therefore, different volatile content. If individual flows or flow fields (large areas of lava, from the same magmatic system, erupted within a short period) can be identified and their volume estimated, then one has a way of calculating the probable gas flux for that eruptive event. With this in mind, it is clear that knowing not only the size, but the internal architecture of the province is an essential part of any study aiming to evaluate the environmental effect of a LIP.

In the Kalkarindji LIP one lava unit stands out as an individual eruptive event. The Bingy Bingy member is a porphyry, a fine grained volcanic rock dotted with larger crystals and is thus quite easily distinguished. It crops out over an area of at least 10,000 km² (about half the size of Wales) and, with an average thickness of c.40m, corresponds to about 400km³ of erupted lava. Using a model developed by the Icelandic researcher Thorardsson and colleagues in 2003, we can estimate that the Bingy Bingy eruption released ca. 2600 megatons of SO₂ to the atmosphere. The modern atmosphere contains about three megatonnes SO₂ at any one time. No such quick-and-easy model has been developed for CO₂, so the CO₂ emissions of most LIPs are still much more uncertain.

Type of eruption is also an important factor when it comes to climatic influence. Highly explosive eruptions can inject gas, ash and aerosols into the stratosphere (the atmospheric layer 15 -50 km up, above the troposphere), which maximises their climatic influence. Previously, it had been thought that most, if not all, LIP eruptions were non-explosive. However, recent studies and reviews suggest that explosive eruptions may have played a significant role in some LIP eruptions. What about 510 million years ago?

Explosive magmatism leaves its mark - as brecciated rock, glass shards and sometimes rounded particles called accretionary lapilli. I believe that explosive volcanism may have played an important role in the late stages of the Kalkarindji event. In the East Kimberley and western NT, an extensive volcanic breccia up to 70 m thick could have formed from explosive eruptions. This has previously been interpreted as forming on the top and front of a lava flow as it cooled, solidified, and was fragmented by the flow’s continued movement; but recent work has cast doubt on this hypothesis.

Genetic investigation of a fragmentation process usually involves some assessment of the shapes of the fragments. Quenching, (ie rapid contraction of the hot magma due to contact with surface waters) produces shapes different from those created by explosive gas release and expansion. When particles in the Kalkarindji breccia (the Blackfella Rockhole member) were analysed in this way, researchers concluded that the fragments had probably been formed mainly by magmatic blasts and surges, and by phreatomagmatic processes, (explosions that occur when hot magma interact with water). Only a minority had been formed non-explosively by quenching. This indicates that the Kalkarindji breccia was indeed formed by explosive activity. If this is true, stratospheric disturbance and the related environmental effects would have been prolonged.

Some environmental effects from the Kalkarindji LIP were recorded in the Georgina basin, a sedimentary basin in northern Australia. Researcher Michelle Hough and her colleagues at James Cook University (Townsville, Queensland) studied phosphor-rich sedimentary rocks from the Georgina basin, were deposited in the aftermath of the Kalkarindji eruptions, 510-505 million years ago. They found sulphur with an exceptional isotopic composition - a composition that could only have been due to a global change in ocean chemistry. The studies show that at the time immediately after the Kalkarindji eruptions, the oceans did not contain much oxygen. The easiest explanation for this would be higher temperature: warm waters can hold less oxygen than cold water. A similar shift in the sulphur isotope composition has been noted for other episodes of environmental crisis, for example the end Permian, the most severe of the “big five” mass extinctions. The eruption of the Siberian traps (a truly huge LIP) coincided with this mass extinction.

If the oceanic anoxia noted during these times were due to higher temperatures, then it is tempting to point an accusing finger towards carbon dioxide, also emitted in vast quantities from the LIP eruptions. This, however, would be somewhat premature. More research is needed before we can fully evaluate the different effects that the gases and particles emitted by LIP eruptions had on the global environment.

Humans have never witnessed eruptions like those that formed the Large Igneous Provinces. For this, we ought to be grateful. We may, however, be in the middle of an environmental crisis comparable to those in the geological past. Our modern lifestyle may well be the cause of elevated ocean temperatures and ongoing mass extinction. About this, we ought to feel alarmed - and be ready to change if we are to avoid the fate of the humble archaeocyathid.

I thank the people who help me during my field trip to the east Kimberleys in 2005: my husband Paul for driving and taking care of our baby and the camp, Kimberley Specialists for help and sponsorship, Triple J tours for the extra equipment, Lindsay the ranger in Purnululu National Park, and Bill and Bev, volunteers at the Kurrajong campsite. Great thanks also go to Jane and Mike Shaw at Spring Creek station for their hospitality and friendliness, and to the Westerways at Inverway station. This research is supported by a Marie Curie International Fellowship within the 6th European Community Framework Programme.

• Courtillot, V, 1999. Evolutionary catastrophes: The science of mass extinction. Cambridge University Press, 173 p.
• Shields, G and Evins, L 2004. Mass extinctions caused by Australian volcanism. Australasian Science 25 (4), 14-18
• Glass, L M , Phillips, D , 2006. The Kalkarindji Continental Flood Basalt Province: A new Large Igneous Province in Australia with possible links to end-Early Cambrian faunal extinctions. Geology 34, 461–464
• Hough, M L , Shields, G A , Evins, L Z, Strauss, H , Henderson, R A and Mackenzie, S , 2006. A major sulphur isotope event at c 510 Ma: a possible anoxia-extinction-volcanism connection during the Early-Middle Cambrian transition? Terra Nova 18, 257-263
• Evins, L Z , Jourdan, F , Phillips, D (2009). The Cambrian Kalkarindji Large Igneous Province: extent and characteristics based on new 40Ar/39Ar and geochemical data. Lithos 110, 294-304

Author affiliation

* Dr. Lena Zetterström Evins, Department of Mineralogy Swedish Museum of Natural History Box 50007 SE-104 05 Stockholm SWEDEN tel +46-8-51954041