¹⁰Be (t_1/2=1.5x10⁶ yr) and ³⁶Cl (3.0x10⁵ yr) show concentration peaks at ~40 kyr in ice cores from both the Antarctic and Arctic.  These peaks are of uncertain origin.  They may have been caused by a supernova event, a geomagnetic excursion, or through some other process.  We have been measuring continuous records of ¹⁰Be and ³⁶Cl in the GISP2 ice core.  The 41 kyr peak is clearly seen in the ¹⁰Be and ³⁶Cl data from this core.  In order to investigate the cause of this event we have extended our measurements to include ²⁶Al (t_1/2=7.05x10⁵ yr) in this same core.  To enable the ²⁶Al measurements to be made, we have combined the aluminum fraction from between 6 and 15 ¹⁰Be and ³⁶Cl ice samples.  For the Holocene period these combined samples averaged 27 kg of ice; for the glacial period, when snow accumulation rates were lower, the combined samples averaged 11 kg of ice.

The ²⁶Al concentrations at the 41 kyr peak are a factor of ~3 times (as ¹⁰Be and ³⁶Cl) higher than before and after the peak but the ²⁶Al/¹⁰Be ratio remain constant.  The mean ²⁶Al/¹⁰Be atom ratio in 5 samples of Holocene ice (4,991-7,635 yr BP) is 1.4±0.3x10^-3 and in 15 samples of glacial ice (34,472-52,074 yr BP) is 1.3±0.1x10^-3.  The ²⁶Al/¹⁰Be ratio of the GISP2 ice core is slightly lower than that of Antarctic ice, 2.1±0.2x10^-3 (average of 3 samples) and similar to rain water collected at La Jolla (1986-1988), 1.5±0.3x10^-3 (average of 8 samples) but near half of the ratio of stratospheric air filtered sample (17.1 km, 71-75ūN), 2.9±0.2x10^-3.  All those results are unpublished by Nishiizumi et al.

Constant ratios of ²⁶Al/¹⁰Be and ³⁶Cl/¹⁰Be in and outside the 41 kyr peak in GISP2 ice core clearly indicate that the enhancement of cosmogenic nuclide concentrations at the peak was not caused by increasing influx of extraterrestrial materials such as interplanetary/interstellar particles, or nearby supernova ejecta because those materials may contain ²⁶Al/¹⁰Be ratio >1.  The enhancement was caused by increasing production rates in atmosphere due to higher cosmic ray intensity at the peak period.  Furthermore, near the constant low ²⁶Al/¹⁰Be ratio in GISP2 ice core indicates that extraterrestrial materials in the ice core are negligible and relatively constant with observed time period.

This work was supported by NSF and NASA.