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Diamonds in glaciers

You can find all sorts of things in glaciers if you look hard enough. Among the oddities that come to mind are volcanic sulphur, soot, and bacteria and fungi.

But now Andrei Kurbatov and co-authors, writing in the Journal of Glaciology about fieldwork on the western margin of the Greenland Ice Sheet, have found something really surprising: diamonds. Don’t get too excited. If you look in just the right place you can expect to find trillions of them per litre of melted ice, but these are nanodiamonds, the biggest only a few hundred billionths of a metre across. There is no danger of prices collapsing in the international diamond market.

However there is definitely a likelihood of a diamond rush spearheaded by scientists. The stimulus for this work was the discovery of nanodiamonds in ordinary sediments from several sites across North America. At all of the sites, much of the diamond is actually lonsdaleite, and there are other indications that point to the material being non-terrestrial. Lonsdaleite is elemental carbon that has crystallized in the hexagonal system, so it is a polymorph of the more familiar diamond belonging to the cubic system. Cubic diamond forms at temperatures and pressures appropriate to depths greater than about 150 km beneath the Earth’s surface. To make lonsdaleite it appears that you need much greater temperatures and pressures even than that. At any rate, it is known only from meteorites and impact craters. We conclude that either it arrived with the meteorite or it formed during the impact.

The next exciting thing about these non-terrestrial diamonds is their age. They are found exactly at the base of the Younger Dryas cold snap, dating to about 11,000 BC. You could not ask for a sharper spike in abundance than the one shown in the Kurbatov paper, and it matches the evidence from elsewhere perfectly.

The first synthesis of this evidence showed that there are non-terrestrial “event markers” all across North America at the base of the Younger Dryas. It was a bold, if partly conjectural, synthesis, linking the impact not just to the cold snap but to the extinction of the mammoths, the disappearance of the palaeoamerican Clovis culture and the formation of the Carolina Bays.

The Carolina Bays can be seen in the atlas as the multiple arcs that form the coastline of the two Carolinas, but inland from the coast there are also numerous lakes of elliptical outline. They might be just quirks of Nature, but they would also be consistent with the putative Younger-Dryas impact having been in fact an airburst, followed by the impact of multiple smaller fragments.

We are now unambiguously in the realm of conjecture, but the Carolina Bays have been a geographical puzzle for centuries. Perhaps they are about to turn out to be not just a puzzle, as happened with the jigsaw fit of western Africa and eastern South America. Whatever their status, we can expect an energetic search over the next few years for the locality of the impact or airburst at the base of the Younger Dryas. We can also expect energetic discussions about its efficiency as a trigger for cooling.

The Greenland nanodiamonds are thus a small part of what is beginning to look like a much bigger picture, but they also represent a glaciological tour de force. I said that the Kurbatov spike was found “exactly” at the base of the Younger Dryas. So it was, but not in an ice core, as you might have guessed. The authors went to the ice exposed in the ablation zone about 1 km in from the margin of the ice sheet. All of it must have travelled some hundreds of kilometres from where it fell as snow in the ice-sheet interior. The text is rather coy here: “One of the authors (Jorgen Steffensen) … identified a candidate for the Younger-Dryas-age section based on visual inspection of dust stratigraphy.”

The atmosphere is slightly dustier when it is colder, and the dust makes ice that accumulated during cold episodes greyer. The nanodiamonds were found at the base of a band of greyish ice bounded above and below by whiter ice. There is therefore a sense in which the “exact” location of the base of the Younger Dryas has only been pinpointed circumstantially. But I doubt that there will be much questioning of the identification, and the success of the search is not less astonishing and gratifying for being due to use of the human eyeball as a search tool.

Posted by Graham Cogley on October 18, 2010 3:00 PM | Permalink