Distribution and transport processes of lithogenic material from the Amur River revealed by the Sr and Nd isotope ratios of sediments from the Sea of Okhotsk

Abstract

Primary productivity is very high in the Sea of Okhotsk, most likely because of terrigenous input from the Amur River that includes dissolved matter and suspended matter. To reveal the transport and deposition processes of the fluvial materials in the northwestern continental shelf of the Sea of Okhotsk, the Sr and Nd isotopes of the detritus in the surface sediments and suspended matter were investigated. The regional variations of the radiogenic isotopes indicate that the detritus in the continental shelf has three main sources: Amur River detritus, with a high 87Sr/86Sr ratio (0.711–0.715) and relatively low εNd value (−7.6 to −7.3); volcanic detritus derived from the Okhotsk-Chukotka volcanic belt to the north of the Okhotsk Sea, with a low 87Sr/86Sr ratio (0.703) and high εNd value (+7 to +8); and detritus from the sedimentary rocks north of Sakhalin Island, with a high 87Sr/86Sr ratio (0.709–0.710) and low εNd value (−10.2 to −9.2). The results indicate that the Amur River detritus is dispersed across the northwestern continental shelf and further transported southward along the east coast of Sakhalin while flowing out of the shelf. Around the northeast coast of Sakhalin Island, the coarse-grained detritus from the north region of Sakhalin Island is distributed only in the narrow region of the shallow depth area and is not transported to the shelf slope. The Amur River detritus flowing out of the shelf is mixed with the Okhotsk-Chukotoka volcanic material transported from the northern area of the Okhotsk Sea by the Okhotsk Sea intermediate water (OSIW). The transport processes of the Amur River detritus in this area are supported by previous studies of turbidity that indicated that the dense shelf water (DSW) entrains resuspended sedimentary particles on the northwestern continental shelf and transports them to the OSIW.