Abstract
The stable isotope ratios of biogenic carbon and nitrogen were studied along the Otsuchi watershed with emphasis on the area drained by the Unosumai River. Plant remains collected from the mountain soil exhibited the lowest values (δ13C=-27.5±1.0‰, δ15N=-1.8±0.8‰) in the watershed. Both isotope ratios increase along the watershed from the upper reaches to the Otsuchi Bay but a dramatic change for the river and inner bay sediments near the river mouth. A biogeochemical framework for assessing the variation of the both isotope ratios is presented. The relationship between the size of particles in the sediments and isotope ratios was studied in detail. The plant remains and organo-silty-clay minerals were emphasized to be responsible for the land-derived refractory organic matters in the bay sediments. Carbon and nitrogen isotope ratios were shown to be useful to trace the behavior of organic matter along the watershed.
