New method evaluates past meteorite strikes from changes
in sea chemistry

Ancient meteorite impacts can be detected from minute changes in the composition of sea water, according to American researchers.

By analysing changes in the ratio of osmium isotopes you can work out the timings of past impacts of stony meteorites called chondrites. What's more, you can also tell how large these impacts were, claims François Paquay of the University of Hawaii, Honolulu.

Paquay and his colleagues used the method to study known impacts, including the blast thought to have wiped out the dinosaurs 65 million years ago, and another one thought to have hit Earth some 35 million years ago. As the team reports in Science ¹, the chemicals of seabed sediments can give an indication of the size of what hit us.

Indirect estimates

The new method produces estimates that are substantially different from those based on crater sizes, Paquay’s team admits. But they argue that the new method has substantial advantages over previous techniques. If the new method is correct, it could mean revising the estimated sizes of several past blasts.

“We do not need a preserved impact crater,” explains Paquay. “We just need a site or two with well-preserved and well-dated deep marine sediments, as the osmium isotopes are well mixed in the global ocean.”

Because the new method does not study impact craters directly, it could identify impacts that have so far gone undetected. “We do hope to find unrecognized impact events, especially deep-ocean impact events, as 70% of Earth’s surface is made of water,” says Paquay.

Normally, sea water has a high ratio of osmium-187 to osmium-188. Chondrite meteorites, which represent roughly 80% of known impacts, according to Paquay, generally have a lower ratio. When a large meteorite hits and vaporizes and its osmium ends up in the sea, this will alter the ratio of isotopes left behind in the sea.

How far back it will be possible to go is up for debate.

“It’s an interesting and new method to arrive at such numbers,” says Christian Köberl, head of the Department of Lithospheric Studies at the University of Vienna, Austria. “I am a little more sceptical, though, that too many impacts will be studied that way, because the osmium isotopic signal in the oceans only goes back maybe 100 million years, and there are not too many large impacts during that time period besides the two already studied by Paquay et al .”

Does size matter?

Previously, the diameter of impacting meteorites has been estimated in two ways. The first involves looking at iridium levels around the world. Iridium is relatively scarce naturally, so relatively large amounts of it in a soil layer can indicate a big rock smashing into Earth.

The second method involves modelling the impact, mainly on the basis of the size of the crater.

“Each of those methods has its problems,” says Köberl.

With the iridium method you have to assume the element is distributed homogeneously around the world, he explains, and you have to know what type of meteorite it was. You also need to assume no iridium has been removed since it was deposited.

For impact simulations, he says, the variety of velocities and angles involved means that estimated diameters can sometimes be wrong by a factor of three.

One problem in evaluating the new method is that, as estimated in the cases investigated in the new paper, the different methods don’t all give the same answer.

While Paquay’s osmium estimates are similar to those arrived at from iridium data, they differ substantially from those generated using impact simulations.

For example, for the Late Eocene ‘Popigai’ impact, his 3-kilometre diameter for the meteorite is roughly similar to the 4-kilometre estimate from the iridium method, but it is some way off the 8 kilometres given by impact simulations.

If the impact simulations are correct, it means that more than 90% of the osmium and iridium carried by these projectiles remains hidden somewhere in the Earth system,” Paquay says. “We think that this is unlikely, but we can’t rule this possibility out without additional work.”

A major problem with the osmium method is the number of uncertainties involved, says Graham Pearson, an expert in isotope geochemistry at Durham University, UK. These include not only exactly what the ratio of isotopes in any given meteorite actually is, but also how much osmium actually ends up in the sea.

“It [the paper] is a nice illustration of the potential of finding even very small impacts in the stratigraphic record. I’m kind of worried their estimates differ so much from the ballistic estimates,” says Pearson.

From Nature News