Abstract
Four-hundred sixty marine sediment samples were collected in the western Sea of Japan and analyzed for 53 elements for a marine geochemical mapping project associated with a nationwide terrestrial geochemical map. Grain size and chemical compositions of marine sediments vary significantly with location of origin (shelf, marginal terrace, slope, or basin). Sandy sediments distributed on the shelf do not likely reflect the geochemical features of river sediments, which are the dominant source of sands in the shelf. Most of the shelf sediments sampled are composed of relict sediments (little contribution of stream sediments) formed between the regression age and the transgression age because they contain a large amount of quartz coated by iron hydroxide and highly enriched in As. The marginal terrace is covered by modern silty sediments that are selectively deposited at the water mass boundary between Tsushima Current (surface water) and Japan Sea Proper Water (deep water). Silty sediments in the western portion of marginal terrace are highly enriched in Nb, rare earth elements, Ta, and Th, which are supplied from Quaternary alkaline volcanic rocks by a denudation process. They are carried eastward by as much as 200 km by oceanic currents. In contrast, the eastern marginal terrace is covered by silty sediments that are highly abundant in Cu, Zn, and Hg, which is attributed to biogenic remains in sediments (organic complex formation). Clayey sediments are widely distributed in the Oki Trough and basin where a hemipelagic environment and highly oxic conditions are found because of the influence of Japan Sea Proper Water. A thin Mn oxide layer in the uppermost 0–4 cm and extreme enrichment of V, Co, Ni, Mo, Sb, and Pb would be caused by an early diagenetic process. Thus, the spatial distribution patterns of elements in marine sediments in the study area are strongly controlled by their depositional environments.
