Seawater osmium sizes up meteorites

Osmium isotopes in seawater sediment can be used to reveal ancient meteorite strikes, US-based scientists say. The sensitive technique could provide more details on the size of extraterrestrial rocks which have plunged into the sea and vapourised, leaving no crater-shaped calling card behind.  

Geochemists routinely measure the sediment concentrations of elements such as iridium, in order to infer the presence of meteorites. High levels in ocean sediment indicate that a rock with unnatural concentrations of the element must have dissolved in the sea millions of years ago. But iridium concentrations in seawater vary naturally around the globe, so many monitoring sites are needed.

Fran?ois Paquay, of the University of Hawaii, US, and colleagues say osmium isotope levels are a more sensitive tracer. Natural concentrations of osmium in ocean sediment are low and fairly constant, and the ratio of ¹⁸⁷Os/¹⁸⁸Os is high. A vapourised meteorite adds a spike of osmium to the sea; significantly, with a much lower ¹⁸⁷Os/¹⁸⁸Os ratio. It takes thirty thousand years for ¹⁸⁷Os/¹⁸⁸Os ratios to revert to normal, ample time for the meteorite signal to work its way into ocean sediment around the globe, says Paquay. 

Assuming the osmium-carrying object was a chondrite - the most common type of meteorite, with known average density and osmium concentration - Paquay’s team calculate it is possible to work out the mass and size of an ancient meteorite from its effect on the ocean’s isotopic composition of osmium. 

’We think a lot of deep water impacts will be recognised in the future with this technique,’ Paquay told Chemistry World.

The team have only tested their idea on known impacts so far. Studies on an impact crater in Chixculub, Mexico, suggest a 15 to 19 km diameter meteorite hit Earth 65.5 million years ago, thought to have been one of the impacts that killed off the dinosaurs at the end of the Cretaceous period. That event correlates with a spike in osmium and iridium in ocean sediment, from which the team calculated a much smaller diameter object of 4 to 6 km. Other test cases the team tried out also underestimated meteorite size compared to crater studies - perhaps because not all a meteorite’s osmium ends up deposited in ocean sediment.

Robert Clayton, a geochemist at the University of Chicago, says the work is clever and appears to be sound - at least when applied to geologically important large impacts. But he cautions that if the technique is to prove useful in working out how often unaccounted-for meteorites hit the Earth, it will have to detect much smaller objects. Paquay’s team reckon meteorites as small as 2 km in diameter could leave a noticeable signature in the ocean’s osmium isotope record.

Richard Van Noorden

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