Osmium geochemistry of modern estuarine sediments from the Tama and Yasaka rivers in Japan

•We measured Os contents and 187Os/188Os ratios on two estuarine sediment profiles.•Os in Tama River sediments is mainly originated from human activity.•Redox change induced Os enrichment at depth 25cm in Tama sediments.•Os in upper and lower intervals suggested constant burial Os fluxes in Tama sediment.•Os in Yasaka River sediments may be mainly from natural sources. Concentrations of Re and Os and 187Os/188Os ratios were obtained on the modern sediment samples from two estuarine areas: the Tama River in Tokyo and the Yasaka River in Oita, Japan. The shallow (depth 25cm) intervals are 99.2–105.5 ppt Os that are more than 3 times higher than UCC. However, all samples have 187Os/188Os ratios within a relatively narrow range of 0.3029–0.4035, much lower than that of UCC (1.0–1.3), indicating a similar source of Os in both parts. The lower 187Os/188Os ratios and high Os concentrations of the sediments in the Tama River suggest the impact from human activities. The sudden enrichment of Os at the deep interval may be related to the redox condition change. In addition, the Os concentration is vertically constant in either the upper and lower intervals of the sediment profile, suggesting constant burial fluxes of Os in the sediment. The Os in the Yasaka River estuary sediments (9.22–29.4ppt) are generally lower than that of the Tama River, either similar or much lower (∼1/3) than the average UCC Os concentrations. The samples with high Os concentrations have lower 187Os/188Os ratios whereas the samples with lower Os concentrations have relatively higher 187Os/188Os ratios, probably indicating different Os sources. The relatively higher 187Os/188Os ratios and lower Os concentrations in some Yasaka River estuarine sediments than the Tama River may also suggest less pollution from human activity to the Yasaka River compared to the Tama River. There is a large decrease of Re concentrations with depth in the Yasaka River estuarine sediment whereas less variations of Re in the Tama River estuarine sediments. The ratio of Re/Os is also sharply different between the two estuarine sediments, also indicating different sedimentary environments. The Tama River sediments may receive much more anthropogenic influence while the Yasaka River sediments get strong impact from modern volcanic activity due to their