Microstratigraphic evidence of oceanographic and tectonic controls on hydrogenetic ferromanganese crusts in the NW Pacific seamounts

Selected hydrogenetic ferromanganese crusts were characterized at a microstratigraphic scale across various NW Pacific seamounts. The crusts encompassed over 100 sites, and were mostly found on old rock outcrops. The crusts also exhibit spatial compositional variation and are largely enriched in Co and Ni. However, the parameters controlling this compositional variation remain unclear. To address this issue, we characterized the microstructure, mineralogical and chemical compositions of seven carefully selected samples from topographic highs in the mid-latitudes on the Philippine Sea and Pacific plates and revealed the following trends: (1) the lower phosphatized layer formed in the middle Miocene or earlier and only on the Pacific Plate, (2) an increasing Co/Mn ratio with younger age from the substrate to the surface, and generally higher in the Pacific Plate than the Philippine Sea Plate, and (3) an increase of detrital quartz and plagioclase since approximately 5 Ma, with high Al/Mn ratios near the continental margin and to a lesser extent in pelagic areas. These results are partly congruent with those of other sediment cores from the other NW Pacific basins. Furthermore, the secular increase of Co and detrital minerals in the crust samples suggests a strong influence of oceanographic and tectonic conditions, especially the expansion of the oxygen minimum zone (OMZ) for the former and a significant input of detrital materials from the continents for the latter. •Secular changes in chemical and mineralogical compositions are well correlated among the hydrogenetic ferromanganese crusts in the NW Pacific.•The trends of the changes are characterized by phosphatization, expansion of the oxygen minimum zone, and detrital materials input.•The changes are ideal markers of long-term oceanographic and tectonic conditions during the ultra-slow and uninterrupted growth of the crusts.