2017 Volume 68 Issue 3 Pages 87-110
The authors present comprehensive terrestrial and marine geochemical maps of the Tohoku region and examine how marine sedimentary environments affect the spatial distribution of elemental concentrations. Marine sediments from the Sea of Japan are relatively enriched in elements abundant in felsic igneous rocks such as K2O, Ba, and REEs. These elements are highly enriched in sediments around submarine topographic highs, and are products of the denudation of Neogene sedimentary rocks associated with dacitic tuff and barite nodules. The silt and clay in deep basins are abundant in elements such as MnO, Cd, and Pb because of early diagenetic processes. Silty sediments on the continental shelf are supplied by large rivers; however, their geochemical features are influenced not by the adjacent terrestrial materials but simply by the effects of grain size. The sandy sediments that extensively cover the seafloor of the Pacific Ocean in the study area were sampled from depths below wave base (100–1,000 m). Their chemical compositions are homogenous and similar to those of stream sediments that originate from mafic volcanic rocks. Therefore, they are inferred to have been deposited as relict sediments under the influence of Quaternary mafic volcanic activity that have been transported from the shelf to the deep sea by gravity currents. Although 81% of the total terrestrial sediment yield is discharged into the Sea of Japan, the spatial distributions of elemental concentrations are not always continuous between land and sea. Similar findings are recognized for the Pacific Ocean side. Notably, coarse sands distributed offshore from Kamaishi are enriched in elements abundant in sedimentary rocks of the accretionary complexes and granitic rocks that crop out in adjacent terrestrial areas. Because the river system in this region is small and has low sediment yields, these coarse sands may have been produced via coastal erosion or denudation of parent rocks during past regression and transgression.