K. Nishiizumi, R.C. Finkel, J. Klein, C.P. Kohl

Journal of Geophysical Research 101 (1996) 22,225-22,232

We have measured ¹⁰Be(t_1/2 = 1.5 x 10⁶ years) and ⁷Be (t_1/2 = 53.28 days) concentrations in water targets exposed for 1 to 2 years at Echo Lake, Colorado (elevation = 3246 m) and at La Jolla, California (140 m). Neutron monitor data were used to normalize the measured concentrations in order to calculate production rates equivalent to the cosmic ray flux averaged over four solar cycles (43 years). The ⁷Be production rates thus obtained correspond to 6.03 +- 0.07 x 10^-6 atom g^-1.O s^-1 at La Jolla. When compared with ¹⁰Be production rates determined in ¹⁰Be-saturated rocks from the Antarctic and with theoretical calculations based on meteorite and lunar sample data, we find that the million-year average production rate is about 14-17% greater than the present production rate averaged over the last four solar cycles. Comparison with production rates determined by measuring glacially polished rocks from the Sierra Nevada in California indicates that average production (based on a revised 13,000-year deglaciation age and a geographic latitude correction) is about 11% greater than the average over the last four solar cycles. The measured ¹⁰Be/⁷Be production ratio in oxygen is 0.52 +- 0.03 at Echo Lake and 0.55 +- 0.07 at La Jolla.