In the Pacific,ore grade manganese nodules enriched in Mn,Ni,and Cu,occur at the margins of the equatorial high productivity zone on sea floor near the CCD.This distribution is thought to result from labile organic matter being supplied by the elevated biological productivity in the overlying waters and being concentrated in the sediments near the CCD as a result of calcium carbonate dissolution.The decay of this material promotes diagenetic Mn,Ni and Cu uptake in the nodules.

A paradox in relating nodule compositional variability to biological productivity is that bulk nodule compositions appear to reflect modern productivity conditions and yet the nodules have been forming in some cases since the Miocene.During these long periods,environmental conditions will have varied and would have been different to those prevailing now.A possible explanation is that post-depositional changes in nodule(and crust if buried)composition may occur over time,resulting in them equilibrating with current high productivity related depositional conditions.The open porous nature of the nodule structure would facilitate exchange reactions between dissolved elements in the sediment interstitial waters and the nodule solid phases, transforming the latter to ore grade.