Geomagnetic reversals, polar ice and cosmic spherules: some recent measurements with a small dedicated accelerator mass-spectrometry facility

• Published in: Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1987
• Subjects: Greenland; Antarc*; Antarctica; ice core
• Online Access: http://dx.doi.org/10.1098/rsta.1987.0075; https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1987.0075

We have developed techniques for measuring the cosmogenic isotopes 10Be (half-life 1.5 M a) and 26A1 (716 ka) by using a small (ca. 2.2 M V) dedicated accelerator mass spectrometer facility. Three recent applications of such measurements are as follows. 1. 10Be has been measured in marine-sediment cores at levels corresponding to three recent geomagnetic reversals. We observe an increase in 10Be production at each of these times. The results give information on the form and length of the geomagnetic intensity changes during a reversal, and the level at which magnetic remanence is acquired in the sediments. 2. 10Be has been measured over a 2083 m ice core, corresponding to the last climatic cycle, recovered from Vostok, Antarctica. The results suggest that the precipitation rate during the last Ice Age was about half of its present rate. There are also some indications of fairly rapid 10Be production changes. 3. 10Be and 26A1 have been measured in ‘cosmic spherules’ (small round objects, ca. 500 pm diameter) recovered in deep-sea sediments and in melt lakes on Greenland ice. The results confirm an extraterrestrial origin for such objects, as well as indicating that the parent bodies of most of them were irradiated in space as small (less than 1 cm) objects. These spherules thus very probably represent cometary debris.