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Perhaps the highest cosmic ray dose ever observed in a terrestrial sample

June 17, 2014

Recently at BGC we measured concentrations of cosmic-ray-produced neon-21 in a set of samples collected by John Stone and Perry Spector from nunataks in the Whitmore Mountains, an isolated group of peaks near the center of the West Antarctic Ice Sheet. Two of these samples appear to have the highest cosmogenic Ne-21 concentrations ever measured in terrestrial samples.

I believe the previous record to be held by a sample from the northern Transantarctic Mountains analysed by van de Wateren and others (1999, Global and Planetary Change), that has 2.27 x 10^9 (2.2 billion) atoms/g cosmogenic Ne-21.

Two of the Whitmore Mountains samples have 2.30 x 10^9  and 2.41 x 10^9 atoms/g cosmogenic Ne-21. The following image shows Ne isotope ratios in three heating steps for two aliquots of each of the samples; 400° C in red; 850°C in blue; total extraction at 1200° C in green. The ellipses are 68% confidence regions. One of the samples has very low concentrations of trapped Ne, so the low-temperature steps are extremely highly enriched in cosmogenic Ne. The black lines are proposed atmospheric-cosmogenic mixing lines by Samuel Niedermann (lower slope) and Joerg Schaefer (higher slope). These estimates agree within their respective uncertainties (which are not shown), but these data show better agreement with the Schaefer value.



These do not actually represent the oldest exposure ages ever measured — that record appears to belong to Tibor Dunai and others (Geology, 2005), who f0und surface clasts in the Atacama Desert with apparent Ne-21 exposure ages up to 37 Ma. However, the Atacama samples were collected at a much lower elevation and therefore have much lower Ne-21 concentrations than the Whitmore Mountains samples.

The Whitmore Mountains samples do have apparent exposure ages of 11.3 and 14.1 Ma. The 14 Ma age may be the oldest exposure age observed in Antarctica. It is interesting to think about the fact that this rock — which would have had an excellent unobstructed view of the open-marine seaway that perhaps was West Antarctica at that time — watched the extinction of the tundra ecosystem and establishment of present Antarctic climate at 13.8 Ma and all subsequent events. Want to know if there was a Pliocene collapse of the West Antarctic Ice Sheet? If there was, both of these samples watched it happen.




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