Systematic correlation of the Ce anomaly with the Co/(Ni+Cu) ratio and Y fractionation from Ho in distinct types of Pacific deep-sea nodules

Chemical characteristics of "diagenetic" and "hydrogenous" type deep-sea no dules from the central and northeastern Pacific were studied regarding thei r Ce anomalies and Y fractionation from heavy REE. Pacific nodules of both types show low Y/Ho ratios less than average shales or chondrites, contrast ing to the other marine samples (seawaters, limestones, and phosphorites) h aving higher Y/Ho ratios than average shales or chondrites. The "hydrogenou s" type nodules show large positive Ce anomalies up to log(Ce/Ce*) = +0.6, whereas the "diagenetic" type nodules display only small positive Ce anomal ies or even small negative ones. Two nodule subsamples, which are character ized by 10 Angstrom manganate but are chemically the intermediate between t he two types, show Ce anomalies of log(Ce/Ce*) = +0.3 in the middle of the two end members. Interestingly, the Ce anomalies of the nodules vary cohere ntly with their logarithmic Co/(Ni+Cu) ratios. This positive correlation is valid even after combing many literature data of Pacific nodules with our data. In the plot of Ce anomaly vs, log[Co/(Ni+Cu)], the three distinct typ es of Pacific nodules are systematically distinguished: "suboxic-diagenesis " less than or equal to "diagenetic" less than or equal to "hydrogenous". T he systematics strongly suggest that: (i) the Ce anomaly and log[Co/(Ni+Cu) ] are similar geochemical indexes showing how effectively oxidative uptake of Ce and Co occurred in each nodule relative to nonoxidative uptake of nut rient-type metals in the respective metal groups, and (ii) there exists an initial source supplying metals common to all the types of Pacific nodules. We inferred from various reasons that the common initial source is biogeni c particulates delivered from overlying surface water. Oxidative uptake of Ce and Co by fast sinking large biogenic particulates is less effective, bu t such particulates can more effectively convey nutrient-type metals involv ed with them to the sea floor because of their shorter residence time in ox ic water. However, the relationship between metal transports of scavenged- and nutrient-type elements is reversed in the case of slowly sinking biogen ic particulates. High surface productivity inevitably provides high flux of fast sinking large organic particulates, whereas low productivity gives ri se to organic particulate flux dominated by slowly sinking small ones. Thes e mechanisms explain the observed systematics of Ce anomaly vs. log[Co/(NiCu)] plots for Pacific nodules.