Field investigation and numerical simulation of long-term sedimentary processes in a bay

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We first conducted a field survey to reveal the thickness of polluted mud layer in Tokyo Bay using a method of bottom sounding based on two supersonic waves. There were, however, some problems for this method. We thus tried to reveal spatial variation in mud quality directly on the basis of sediment core sampling. We developed a core sampling method using an Ekman barge sampler altered to take mud core samples efficiently. We collected more than 50 core samples in the inner part of the bay and sliced them every about 1cm from the surface of the sediment to the deep layer. Then we measured water content, sediment grain size distribution, total organic carbon and nitrogen, and carbon and nitrogen stable isotope ratio. We elucidated higher organic content mud is accumulated around the central part of the inner bay while lower organic content sediment is accumulated around it. A three-dimensional and multi-class sediment accumulation model was developed to reproduce and understand the sediment accumulation processes considering the variation in sediment grain size. We reproduced muddy sediment accumulation processes in the bay. On the basis of field investigation and numerical experiments as well as former field data, we analyzed the mechanism of the formation of these sedimentary processes such that finer particles adsorb larger amount of organic matter and settling and resuspension processes are dominant factors. Moreover, observed lower C/N values at the central part are also one of the major causes of the formation of spatial variation in sediment quality because the waters in the central part are exposed to hypoxia for longer time than those of their surrounding waters, which makes the rate of mineralization lower.