PRECISE DETERMINATION OF VARIATIONS IN THE 138CE142CE RATIOS OF MARINE FERROMANGANESE NODULES

Abstract

New and precise data on the 138Ce142Ce 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 ϵCe (one tenth of per mil difference from the standard). The nodule Ce isotopic data are used as a surrogate of surrounding seawater, and the ϵCe, 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 ϵCe with ϵNd for 143Nd/144Nd, 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 138Ce142Ce and 143Nd144Nd ratios are determined by mixing of two end-members, i.e. the continental crust and MORB (mid-ocean ridge basalt), the oceanic variation of ϵCe is greater than that of ϵNd 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 138Ce142Ce and 143Nd144Nd ratios modified by scavenging and oceanic mixing are recorded in the manganese nodules.