Abstract
The distribution of δ13C and δ15N in sedimentary organics from the Tama River, Sumida River and Arakawa River estuaries and Tokyo Bay was determined to establish an isotope biogeochemical structure to assess the fate of organic matter in the estuary-bay system. Variation in isotope ratios in intertidal river sediment was primary governed by the mixing of terrestrial and riverine organic matter. Fluctuation in isotopic composition in the intertidal zone may possibly result from different flushing times at each site.
Variation in δ13C and δ15N was remarkable in the vicinity of the river mouth and inner bay. Enhanced δ15N at the river mouth of the Tama River may possibly have been due to oxidation reduction-reactions through which 15N depleted N2O and N2 were emitted from the aquatic system. The very low values for δ15N and high δ13C in the inner bay may reflect isotope fractionation during assimilation of inorganic nitrogenous compounds with eutrophication and rapid growth of marine phytoplankton, respectively. The variation in isotopic composition was schematically illustrated based on available data.
