Complex factors contribute to the chemical composition of clastic sediments traveling from rivers to the ocean along island arcs: A case study of the Otsuchi area, Northeast Japan

The chemical composition of coastal sediments and river sediments is influenced by the geological constitution of provenance and fractionation of clastic particles during sedimentary processes. The intricate nature of the geology in active subduction zones has complicated the comprehension of detrital sediments. More geochemical case studies in such areas are necessary to enhance the understanding of the coastal detrital sediments. This study presents the results of a geochemical and particle size analysis of sediment samples from the Sanriku coastal area of Northeast Japan, including inside and outside of Otsuchi Bay and the three rivers that flow into the bay. Otsuchi Bay area is known to have been severely affected by the 2011 off the Pacific coast of the Tohoku Earthquake and tsunami, and tsunami deposits on its seabed have been reported. The present geochemical analysis indicates that the sediments along those three rivers are well explained by the mixing of rocks occurring in their respective provenance. The chemical composition and particle size distribution of the sediments within Otsuchi Bay suggested the removal of coarse-grained quartz. They increased the abundance of fine-grained mafic minerals from Unosumai River having the largest catchment area among the three rivers. During shell debris in the seabed sediment outside of the bay made interpretation of the results was difficult; the sediments collected closer to the bay mouth contained more coarse-grained material with a composition closer to granitoid and sandstone in the provenance. In contrast, offshore sediments consisted of finer grains with a composition closer to mudstone. Notably, the sediments within Otsuchi Bay did not exhibit the characteristic compositional fractionation or particle size distribution associated with tsunami deposits. These findings underscore the importance of considering the geological diversity of provenances and the particle size distribution of minerals in understanding coastal sediments in subduction zones, including tsunami deposits.