COSMOGENIC BE-10 AND HE-3 ACCUMULATION IN PLEISTOCENE BEACH TERRACES IN DEATH-VALLEY, CALIFORNIA, USA - IMPLICATIONS FOR COSMIC-RAY EXPOSURE DATING OF YOUNG SURFACES IN HOT CLIMATES

To investigate the history of Pleistocene lake levels in the southwestern U.S.A., and to improve our understanding of the systematics of in situ produced cosmogenic nuclide accumulation in hot climates and young rocks, we examined the He-3, Be-10 and Al-26 contents of quartz separates from metamorphic quartzites and quartz-rich felsic rocks incorporated in a series of stranded conglomerate beach ridges ranging from 3 to 160 m above sea-level in Death Valley, California. Calculated Be-10 ages range from 17 to 135 kyr, with most ages between 20 and 60 kyr. In contrast, cosmogenic 3He contents indicate exposure ages of <1 kyr, except for one sample at 18 +/- 4 kyr for which the Be-10 age is still significantly greater (44+/-11 kyr). Al-26/Be-10 ratios close to the cosmogenic production ratio rule out the presence of meteoric Be-10 and point to He-3 loss as the cause of the age discrepancies. Comparison to laboratory diffusive loss measurements using a diurnal direct solar heating model implies the He-3 loss occurs because the effective quartz grain sizes are <100 mu m - far smaller than the 1-3-mm fracture spacings estimated from thin sections. Because direct insolation enhances He-3 loss, sampling from shallow depths (similar to 20-40 cm) may be useful in some cases. He-4 and Ne-20 measurements demonstrate that small amounts of radiogenic and atmospheric He-3 are also present at levels equivalent to a few kiloyears sea-level exposure, which should be taken into account when dating surfaces of similar age. The lack of any dear progression with terrace altitude and the occurrence of two-fold variations in apparent exposure ages within individual terraces suggest that the Be-10 contents do not solely represent accumulation in the present sample locations. Instead, accumulation probably also occurred during sample exposure in the higher-altitude beach rock source regions and during fluvial transport to the ancient lake shore. Thus the beach terrace exposure ages of similar to 20-60 kyr are probably maximum ages for the times of these high lake level stands in Death Valley.