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.

Abstract

Chemical characteristics of “diagenetic” and “hydrogenous” type deep-sea nodules from the central and northeastern Pacific were studied regarding their Ce anomalies and Y fractionation from heavy REE. Pacific nodules of both types show low Y/Ho ratios less than average shales or chondrites, contrasting to the other marine samples (seawaters, limestones, and phosphorites) having higher Y/Ho ratios than average shales or chondrites. The “hydrogenous” type nodules show large positive Ce anomalies up to log(Ce/Ce∗) = +0.6, whereas the “diagenetic” type nodules display only small positive Ce anomalies or even small negative ones. Two nodule subsamples, which are characterized by 10 Å manganate but are chemically the intermediate between the 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 coherently 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 types of Pacific nodules are systematically distinguished: “suboxic-diagenetis” ≤ “diagenetic” ≤ “hydrogenous”. The 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 non-oxidative uptake of nutrient-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 biogenic particulates delivered from overlying surface water. Oxidative uptake of Ce and Co by fast sinking large biogenic particulates is less effective, but such particulates can more effectively convey nutrient-type metals involved with them to the sea floor because of their shorter residence time inoxic water. However, the relationship between metal transports of scavenged- and nutrient-type elements is reversed in the case of slowly sinking biogenic particulates. High surface productivity inevitably provides high flux of fast sinking large organic particulates, whereas low productivity gives rise to organic particulate flux dominated by slowly sinking small ones. These mechanisms explain the observed systematics of Ce anomaly vs. log[Co/(Ni+Cu)] plots for Pacific nodules.