Chemical composition of ferromanganese crusts and manganese nodules from the Indian and Southern Ocean

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.

过剩钍-230（Excess 230Th）主导了锰结核与深海沉积物表层区域的α活度（alpha activity）。若维持铀系平衡（uranium series equilibrium），则总α活度深度剖面在沉积速率测定中，其效用与钍-230测量方法相当。
本研究采用紧贴锰结核切片的硝酸纤维素α径迹记录仪（cellulose nitrate alpha track recorders），对大量锰结核获取了高分辨率的α活度“图谱”，同时还测定了多根沉积物岩芯的总α活度深度剖面。
截至目前，本研究已测量的约90条锰结核剖面中，73条呈现出简单的近似指数型深度依赖性；其中约2/3的剖面推断沉积速率为4~10 mm/百万年。另有10条剖面存在统计学意义显著的斜率突变，表明其生长速率发生了变化。在单次拖网采获的样品中，锰结核生长速率的差异极小，但同一锰结核的不同区域有时会表现出不同的生长速率。除本研究测量的少量大西洋锰结核普遍呈现出略低于太平洋、印度洋样品的生长速率这一趋势外，测得的生长速率与埋藏深度之间不存在相关性。
本研究将多个锰结核与沉积物的α径迹测量结果，与其他研究者基于钍-230（230Th）得到的沉积速率测定结果进行了对比。有2个锰结核的钍-230沉积速率（引自W. S. Broecker等人的私人通信），其误差范围内与α径迹测定结果完全一致。另有1个锰结核（引自S. Krishnaswami的私人通信）以及此前由Goldberg与Koide在《地球科学与陨石学（E. Sci. and Meteoritics）》1963年第90页中测量的2根沉积物岩芯，其基于钍-230测量得到的沉积速率均远低于α径迹结果。