Precise determination of variations in the Ce-138/Ce-142 ratios of marine ferromanganese nodules

New and precise data on the Ce-138/Ce-142 ratio in marine manganese nodules obtained from the Atlantic and Pacific Oceans fall within a narrow range of -0.6 to +1.1 in terms of epsilon(Ce) (one tenth of per mil difference from the standard). The nodule Ce isotopic data are used as a surrogate of surrounding seawater, and the epsilon(Ce) values ranging from -0.6 to -0.2 for the central Pacific are clearly more negative than those (+0.2 to +1.1) for the Atlantic. By coupling the epsilon(Ce) with epsilon(Nd) for Nd-143/Nd-144, it is inferred that the relative contribution of the continental-derived to mantle-derived sources is smaller in the Pacific than in the Atlantic. If we assume that the oceanic Ce-138/Ce-142 and Nd-143/Nd-144 ratios are determined by mixing of two end-members, i.e. the continental crust and MORB (mid-ocean ridge basalt), the oceanic variation of epsilon(Ce), is greater than that of epsilon(Nd) relative to their difference in the two end-members. This strongly suggests that Ce and Nd are fractionated within the ocean because of the difference in their mean residence times (90-165 yr for Ce and 1000-1500 yr for Nd) and the Ce-138/Ce-142 and Nd-143/Nd-144 ratios modified by scavenging and oceanic mixing are recorded in the manganese nodules.