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In search of cosmic footprints using neutrons


Some 12,900 years ago, the earth’s climate cooled significantly within just a decade, which very likely led, among other things, to the extinction of the mammoths. The cause of this is thought to be a powerful meteorite impact, which covered Europe and America with a thin layer of rock. Neutron research has now thrown up clues about the origin of the cosmic chunks: the composition of the rock stratum shows an amazing similarity to moon rock.

PGAA/NAA-experiments PGAA/NAA-experiments

Fig. 1: PGAA/NAA analysis of the YD layer at many sites shows: (a) numerous chemical similarities and (b) negative Eu anomalies observed only in lunar KREEP and in SAU 169, a meteor from Oman, believed to have originated from the Procellarum Lunar KREEP Terrane region of the moon. | © Richard B. Firestone, Lawrence Berkeley National Laboratory

Some 12,900 years ago, mammoths and other large animals became extinct and the prehistoric Paleo-Indian Clovis culture disappeared from the Americas. At the same time, the Laurentide Ice Sheet in Canada suddenly failed and affected the North Atlantic currents, leading to a 1300 year return of ice age conditions known as the Younger Dryas (YD), named after a flower which was common to the icy tundra climate. Simultaneously, a thin sediment layer was deposited across the Americas and Europe containing magnetic grains, microspherules enriched in iridium, carbon spherules, nanodiamonds, and a host of other markers consistent with a massive extraterrestrial impact. According to a new hypothesis, this impact was directly responsible for extinctions and changes in climate that occurred at that time.

A neutron technique called Prompt Gamma Activation Analysis (PGAA) proved to be an important tool for determining the meteorite’s composition and samples from where it impacted Earth. PGAA utilizes the absorption of neutrons by atomic nuclei followed by the emission of characteristic gamma radiation. PGAA is non-destructive and provides a unique opportunity for the analysis of light elements, especially hydrogen and boron, which is difficult to analyse using routine instrumental analytical techniques. The method is also very sensitive for rare-earth elements. A set of archaeological samples, soils and meteoritic particles, originating from different excavation sites in the USA, have been analysed using the PGAA instrument at the Heinz Maier-Leibnitz Zentrum.

Fig. 2: Searches of fossilized mammoth tusks and bison skulls as evidence of damage from the YD impact led to the discovery of fossils with high velocity impact pits only on one side of the object. PGAA analysis of particles embedded in these fossils indicated that they were mainly Fe/Ni enriched with the nickel content as high as 20%, consistent with normal iron meteorites. This is very different from the Ti-rich, Ni-poor composition of the YD magnetic grains. Subsequent radiocarbon dating indicated that these fossils were about 37,000 years old and originated from a different impact, possibly associated with the Sithylemenkat Crater in Alaska. | © Richard B. Firestone, Lawrence Berkeley National Laboratory

Analysis of the YD impact layer (Fig. 1) shows remarkable similarity to the lunar terrain visited by the Apollo astronauts. It is characterized by the presence of a magmatic rock called KREEP, which contains Potassium (K), Rare Earth Elements, and Phosphorus. High water content in the magnetic grains determined by PGAA suggests they were formed in wet conditions consistent with a scenario involving forcible contact with the Laurentide Ice Sheet. The distribution of the impact material and the orientation of the Carolina Bays, suspected impact structures on the East Coast of the US, are also consistent with an impact near the Great Lakes. Searches for more YD impact evidence have led to the discovery of other impact events (Fig. 2).

Original publication:
Ted E. Bunch et al.: Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago, Proc. Nat. Acad. Sci. 109 (2012) E1903, DOI: doi: 10.1073/pnas.1204453109

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