Biogeochemical characteristics of the Fujimae tidal flat sediments indicated by vertical distribution of elements, stable isotope ratios of carbon and nitrogen, and five types of phosphorus fractions

Abstract

In order to understand the biogeochemical characteristics of urban tidal flat sediments, distributions of organic and inorganic elements, carbon and nitrogen isotopic compositions, and their relationships with the five different fractions of phosphorous were analyzed for five cores of 15 cm long collected at the Fujimae Tidal Flat in the Port of ( n = 50 ) The Fujimae tidal flat sediments are characterized by the enrichment in Pb and Zn, with factors of 1.72 and 1.66 relative to the upper continental crustal values respectively, implying anthropologic contamination. The sediments have a C:N mole ratio of 15.1 and a 13 C value of -25.81 ‰ on average, which are almost the same as the riverine suspended particulate matter ( SPM ) . This suggests that organic matter in the sediments is largely of terrestrial origin. The results from the fraction analyses of phosphorus show that the exchangeable or loosely adsorbed P and Fe-bound P account for about 70 % of total P at the sediment surface ( 0 - 1.5 cm ) . Such available phosphorous fractions decrease by depth and remain constant at around 38 % below 6 cm from the surface. The upper layer of the sediment is likely oxidized and it can be assumed that there is a Fe-redox boundary around 6 cm depth in the sediment. In addition to exposure to the air during the ebb, the expected predominance of terrestrial organic matter with low decomposability, which would slowly consume dissolved oxygen in bottom waters and sediment interstitial waters, could play an important role for the oxidizing condition of the upper sediment.