Geoscientist Online

A five-million year-long, mid-Ordovician meteorite shower has now been positively linked to the creation of the Gefion Family of asteroids – created in the Solar System’s biggest bang for a billion years. Ted Nield writes from the 72nd Annual Meteoritical Society meeting in Nancy.

Geoscientist Online Special 17 July 2009

In the early eighties, research student Birger Schmitz (picture) decided to take the K-T boundary impact idea (then newly published by Luis and Walter Alvarez, Helen Michel and Frank Asaro) seriously. So seriously, in fact, that he fought against opposition from his peers at Lund University, Sweden, and changed his topic. “They said it was all “geofantasy””, he told Geoscientist Online. Now, Schmitz is professor of Lithosphere and Paleobiosphere Sciences at Lund University’s Department of Geology, and is leading one of the biggest scientific stories of the decade. And it all stems from that one courageous decision.


While still writing his thesis, Schmitz published two sole-author papers, one in the prestigious journal Geochimica et Cosmochimica Acta, another in Geology, no mean feat in itself. He also senior-authored another paper in Cosmochimica - and all were critical of the impact hypothesis. About the same time, Luis Alvarez, the Nobel prize-winning physicist who, with his geologist son Walter, was the driving force behind the K-T impact theory, came to Stockholm to be entertained at dinner by the Swedish Academy. Luis surprised the Academy by asking if this young Schmitz character could be invited along.

“They put me really far away, at the end of the table” says Schmitz. “Luis was surrounded by all the big professors the Academy had invited, but really the only person he wanted to speak to was me!” Eventually, at coffee afterwards they managed to talk. “His kids just loved it, you know? Here was this guy, not much older than them, taking on the great Nobel prizewinner!” he recalls with a smile. The encounter won him Alvarez’s respect, with the result that he was invited to do his post-doc at Berkeley on the K-T boundary.

Shortly after his move to the US, evidence of shocked quartz grains began to turn up. Schmitz realised that although he may have been right in geochemical detail, he had been wrong about the big picture. However, when he returned to Sweden, he was “mentally prepared” for what was about to come at him, from much older rocks of his native country.

Back in 1952, Per Thorslund, Professor of Geology at Uppsala, had been sent a mysterious slab from a dimension stone quarry in central Sweden. The Ordovician limestone contained an anomalous black clast, almost 10cm across. This was, in fact, the first known example of a fossil meteorite from rocks of any age; but at first it was mis-identified as a "strongly metamorphosed ultramafic rock". Thorslund thought that maybe it had been rafted into the gentle, sediment-starved Ordovician sea from the eastern shore of Iapetus (then about 14km distant) on a mat of seaweed. The explanation was not very satisfying, particularly because no rocks of that type were known to have cropped out on that shore. The slab was put away – but not quite forgotten – for 25 years.

By some kind of coincidence, Thorslund had been the first to describe the impact breccia of the Lockne crater (though without realising its impact origin). As ideas about terrestrial impacts became more widespread during the 1970s, and people began to look at the Siljan Ring in this context, Thorslund was struck by the notion that perhaps that mysterious clast was in fact a fossil meteorite. The slab was dusted off and re-examined – and although its mineralogy was entirely pseudomorphed by common minerals like calcite and barite, clearly chondritic structures were identified. Thorslund had found the first ever confirmed fossil meteorite – now known as Brunflo, from the nearest town to the Rödbrottet Quarry, Gärde, where it had been unearthed all those years before. Thorslund’s paper appeared in Nature, eleven months before his death, in 1979. “He did so much to build the foundations” says Schmitz. “If only he had lived a little longer, to see where it would all lead!”

Soon after his return to Sweden from the US, Schmitz got to hear of the discovery of another Ordovician fossil meteorite, from slightly older rocks, several hundred kilometres to the south of Brunflo. Once again, the host rock was a limestone, a condensed deposit laid down at the rate of only two millimetres every thousand years. The pinkish Orthoceratite Limestone, quarried for building and paving since the 12th Century, came from Thorsberg Quarry. Local geologist Mario Tassinari came to hear about it from his local newspaper in Linköping, and was quickly on the scene asking the quarrymen if they knew of any more. It wasn’t long before more appeared.

Before long, Schmitz had begun a systematic sampling project to find more meteorites. Since this project started, in 1993, 90 individual meteorites have been recovered. “It is remarkably predictable” Schmitz told the Meteoritical Society yesterday. “Each year in Thorsberg Quarry, another 750 square metres of Ordovician sea floor is quarried, and each year it produces six, plus or minus two meteorites. Up till now, 12,000 square metres of seafloor have been excavated. Of the 90 meteorites found, 65 have been analysed and all have been diagnosed as L chondrites.”

Schmitz admits this side of the work is becoming a little boring. “I think there is a very high probability that they are all L chondrites!” he says.

L chondrite meteorites are among the most common meteorites among present-day falls. They are a undifferentiated meteorites with a distinctive low-iron chemistry, whose chemical, isotopic and textural signatures make it a certainty that they all derive from the same parent body. The Brunflo meteorite, was originally designated by Thorslund as an H-chondrite (another common family) but it too has now proved to be an L chondrite¹. This remaining anomaly was removed last year.

Schmitz says that for him, the scientific turning point that turned his investigation from a geological curiosity into a major interdisciplinary project with far-reaching implications for solar system history, meteoritics, astronomy and even evolutionary biology, came after they had found about nine meteorites at Thorsberg.

“I thought suddenly – hell, isn’t this a lot of meteorites?” At the time Dr Phil Bland, now with the Open University and then of the Natural History Museum in London, had been publishing results from his work, counting the flux of incoming meteorites over the Nullarbor Plain, southern Australia. “I did some quick calculations comparing areas, sedimentation rates, numbers of meteorite finds and so on, and quickly decided that the flux must have been amazingly high at this period in the mid Ordovician” Schmitz says.

Subsequent work has confirmed that in the three million years after 468 (plus or minus 1.6Myr, approximately equivalent to the bases of the Eoplacognathus variabilis conodont and Expansograptus hirundo graptolite zones of the Darriwillian), meteorite flux to Earth rose by at least two orders of magnitude, and remained one order of magnitude greater than it is today for a further million years after that.

Schmitz and co-workers have also discovered that the cosmic ray exposure (CRE) ages of the fossil meteorites increases up the section. So, the younger their “terrestrial” age (ie, the later they fell), the longer time they spent in space. This has been possible to determine because, although nearly all the original meteorite material has been replaced, the most resistant mineral, chromite, has survived – enabling the appropriate isotopic studies to be performed.

Catastrophe

All this evidence pointed to a big cosmic event that had resulted in the Earth's being bombarded for several millions of years by meteorites ranging in size from the very minute to (perhaps) the enormous. One of the most useful facts uncovered by the investigation has been the finding that chromite grains were also arriving in increased numbers – at a rate 100 times the background from rocks pre-dating E. hirundo zone, and sometimes much more. Studies of solar-wind-implanted neon isotopes (which only affect materials close to surfaces exposed in space) show that these grains did indeed arrive as micrometeorites. But – to go from the minute to the enormous - the stratigraphic range covered by the raised meteorite flux also encompasses the estimated ages of no less than four major impact structures in Baltoscandia alone – namely Lockne, Kärdla, Tvären and Granby.

Schmitz and his co-workers have now found that the meteorite flux increase can be traced all over the globe – first to Killeröd Quarry in southern Sweden, and later to the Puxi River section in similar-age rocks of central China (which was not, though, quite so far away from Baltoscandia in the Mid-Ordovician, both being located in the southern hemisphere along with the other break-up products of the supercontinent Rodinia).

Schmitz’s work has led to the drawing together of many multidisciplinary strands. It had been noted,as long ago as 1964 by the great meteorite scientist Edward Anders, that the shock ages of L chondrites clustered together between four and five hundred million years ago, suggesting some massive catastrophe had occurred. Indeed, the effects of the break-up of the L chondrite parent body are still being felt today, since 20% of all meteorite falls (40% of all chondrite falls) consist of L chondritic material. One such fall occurred in on August 14, 1992,  – when a fragment from the exploded bolide, measuring about one centimetre across, landed on the head of a small boy playing football in Mbale, Uganda (fortunately, after first bouncing off the leaves of a tree - picture).

Estimates from the meteoritic material found on Earth so far indicates that the enough material from L chondrite parent reached our planet to account for a body at least 100-150km in diameter. It is reasonable to suppose that such a catastrophe would also have resulted in the creation of a large asteroid “family”, still orbiting together somewhere in the asteroid belt. But which family?

Originally, interest centred on the well-known Flora Family, situated in the inner main belt of the Asteroid Belt. Meteorites that enter Earth-crossing orbits reach our planet when they become expelled from the Belt by straying into one of a number of clear zones that divide the Belt into distinct stripes, rather resembling the rings of Saturn. These clear zones, called Kirkwood Gaps, are swept clear of debris because the orbits of any body within them experience orbital resonance with the nearby giant planet, Jupiter. Since orbits in these lanes cause asteroids within them to line up more frequently with Jupiter, they tend to get pulled out. However - they may instead find themselves being expelled from the Belt altogether.

The Flora Family of asteroids lies close to one of these gaps; but the required ejection speeds and the composition of the asteroids (as determined by reflectance spectra) turned out to be wrong for L6-Chondrites. As has been revealed this year, however, the Gefion Family (labelled Ceres in the diagram above, which was the family's old name) provides a much better fit. The family’s age is estimated at 485Ma (to a tolerance of plus 40 or minus 10 Myr), which is in the right ball-park. Their mineral composition is also consistent with L chondrites, and the independently-determined minimum size of their parent (estimated from putting back together all the asteroids now comprising the family) comes in at 100-150km diameter - in other words, about the same mass as ended up on Earth during the Mid Ordovician immediately followng the catastrophe.

The short Cosmic Ray Exposure ages observed in the fossil meteorites (from 0.05 to 1.5Myr) mean that Gefion fragments would have to be able to evolve quickly into Earth-crossing orbits once expelled into their nearby Kirkwood Gap. Simulations have now shown that escape velocities of only 50m/s are required to reach resonant orbits from the core of the Family, that most Gefion fragments would reach Earth a mere 50kyr after expulsion, and that their flux would peak at 1-2Myr. These findings fit the observed CRE ages of the fossil meteorites, and the stratigraphic range observed². As Schmitz put it to the MetSoc delegates in Nancy: “This was the largest documented destructive event to occur in the Solar System during the last billion years.”

Future

Were there any wider implications of this catastrophic break-up? Many think the fossil meteorites of the mid Ordovician may have much more to tell us. Schmitz and others have already hypothesized that a rain of meteorites large and small over a period of five million years may have had a profound effect on life on Earth³. It so happens that this prolonged bombardment coincides with one of the great unexplained biodiversity increases in the history of life – the Great Ordovician Biodiversity Event.

Hitherto, no convincing reason for the sudden spike in biodiversity at species and genus level has been forthcoming, even though the original discovery (through the taxonomic database work carried out by the late Jack Sepkoski) has been amply confirmed by later studies. Ecological disturbance is a well-known stimulus to biodiversity, creating new habitats for colonisation and allowing opportunistic species to move into areas that were previously filled. Sub-lethal bombardment may have had such an effect over a long period – and might in turn have resulted in greater evolutionary diversification.

Impacts were perhaps not the only trigger. Recent literature surveys by John Parnell (University of Aberdeen) have turned up a coincidental worldwide occurrence of megabreccias in the period 470 through 460Ma³. The stupendous downslope movements of sediment and rock which these represent may have been triggered by seismic activity as a result of frequent impacts, Parnell suggests.

More acid needed

Many problems remain. For instance, rather obviously it takes two objects to make a collision. If one was the L6 parent, what was the other one, and where is it now? Schmitz thinks this problem might be resolved by looking more closely at clast inclusions in the L chondrites.

What most excites him now, though, is the prospect of dissolving even more of the stratigraphic column in acid.

“We now have a tool”, he says, referring to extraterrestrial chromite grains, “that can enable us to work out precise meteorite flux rates for the whole geologic column. This is amazing!”

Schmitz has plans to get going on this project soon. “We’re going to need a lot more acid” he says.

* Ted Nield is currently writing a book about meteorites, and the influence they have had on Earth (and human) history. Provisionally entitled Incoming – or why we should stop worrying and learn to love the meteorite it will be published by Granta in 2010.

Selected references

1. Alwmark, C and Schmitz, B 2008 The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden). Meteoritics & Planetary Science 44, 1 pp95-106.
2. Bottke W. F., Nesvorn´y, D. Vokrouhlick´y D., Morbidelli A. 2009 The Gefion Family as the Probable Source of the L Chondrite Meteorites. Proceedings of the 40th Lunar and Planetary Science Conference.
3. Parnell, J, 2008 Global Mass wasting at continental margins during Ordovician high meteorite flux. Nature Geoscience Online 14 December.