ALBERT

Description: Excess 230Th dominates the alpha activity of the surface regions of manganese nodules and deep sea sediments. If uranium series eguilibrium is maintained, total alpha activity depth profiles should be as useful in determining sedimentation rates as are 230Th measurements. We have used cellulose nitrate alpha track recorders pressed against manganese nodule slabs to record high resolution alpha activity "maps' of a large number of nodules. Total alpha activity profiles have also been obtained for several sediment cores. From the approximately 90 nodule profiles measured to date, 73 show simple, approximately exponential depth dependence; about 2/3 of these have inferred deposition rates of 4-10 mm/10power6 yr. Ten profiles show statistically significant breaks in slope, suggesting growth rate changes. Within single dredge hauls there is little variation in growth rates, although different portions of the same nodule sometimes exhibit different growth rates. There is no correlation between depth and measured growth rate, except for a general tendency for the few Atlantic Ocean nodules we have measured to have slightly lower rates than Pacific or Indian Ocean samples. For a number of nodules and sediments the alpha track results have been compared with 230Th-based deposition rates measured by other workers. Two nodules have 230Th rates (W. S. Broecker et al., pers. comm.) identical within errors to the alpha track determinations. A third nodule (S. Krishnaswami, pers. comm. ) and two sediment cores previously measured by Goldberg and Koide (E. Sci. and Meteoritics, 1963, p. 90} all have much slower rates based on 230Th measurements compared with the alpha track results.