Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Abstract

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.