Search for supernova-produced 60Fe in Earth’s microfossil record


Search for supernova-produced 60Fe in Earth’s microfossil record

Ludwig, P.; Bishop, S.; Egli, R.; Chernenko, V.; Faestermann, T.; Famulok, N.; Fimiani, L.; Gomez-Guzman, J. M.; Hain, K.; Korschinek, G.; Hanzlik, M.; Merchel, S.; Rugel, G.; Frederichs, T.

INTRODUCTION
The radioisotope 60Fe (T1/2 = (2.62+0.04) Ma [1]) can be produced in copious amounts during different phases of evolution of massive stars. It is possible that 60Fe-rich supernova debris is deposited into solar system reservoirs [2]. Samples from Pacific Ocean sediment were chosen as sample material for this work. Considering an enrichment of the ocean water with 60Fe after deposition of SN material on Earth, all minerals being formed during that time in the sediment will incorporate 60Fe and preserve the original concentration of 60Fe/Fe, except for natural radioactive decay. One particularly interesting reservoir of in situ formed iron-bearing minerals are magnetosomes. Chains of these magnetite crystals are built up by magnetotactic bacteria, who use it similar to a compass needle for magnetotaxis. After cell death, the magnetosome chains can be preserved over geologically significant timescales and represent a very interesting sample material to look for a 60Fe isotope anomaly.
MATERIALS AND METHODS
As sample material for this project, two sediment cores from the Eastern Equatorial Pacific (ODP Leg 138, Sites 848 and 851) were chosen. In order to extract iron from 60Fe rich minerals, the chemical CBD extraction procedure was employed. It was carefully calibrated to only dissolve particles of 200 nm diameter. In this way, dilution of 60Fe from iron-bearing grains from other sources (wind, water), which are typically larger, is reduced. A thorough study of the magnetic properties of the samples was also performed [3]. The concentration of 60Fe/Fe was then measured using accelerator mass spectrometry (AMS) at the GAMS setup at the MLL. Its unique ability to separate isobaric background in a gas-filled magnet allows for sensitivities reaching down to 60Fe/Fe ~10-16 and even lower.
RESULTS AND OUTLOOK
At this point, AMS measurements on the smaller one of the two cores have been completed. Measurements of the larger core are underway and are expected to be completed in mid 2014. The 60Fe/Fe concentration determined in core 848 (smaller core) can be seen in Fig. 2. A total of 7 counts of 60Fe have been observed in a depth range corresponding to an age of 2.0 to 2.6 Ma. The signal is above the expected background. Both, this age and the observed average concentration in this range (60Fe/Fe ~1x10-15) agree well with earlier results from a ferromanganese crust [2]. In order to improve statistics and time resolution, the larger core has to be examined as well. In this larger core 851, a total of 12 counts of 60Fe have been detected so far, but measurements have not been completed yet. In addition to measurements of 60Fe, 10Be and 26Al are also currently being measured in the smaller core, to obtain an independent dating, at the DREAMS facility in Dresden.
REFERENCES
[1] G. Rugel et al., Phys. Rev. Lett., 103(2009) 072502
[2] K. Knie et al., Phys. Rev. Lett., 93(2004) 171103
[3] P. Ludwig et al., Global Planet. Change 110(2013) 321-339

Keywords: accelerator mass spectrometry; supernova; cosmogenic radionuclide

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Publ.-Id: 20151