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

本文首先采用基于双声波的海底测深方法对东京湾污染泥层厚度进行了实地调查。然而，这种方法也存在一些问题。因此，我们试图在沉积物岩心取样的基础上直接揭示泥质的空间变化。我们开发了一种岩心采样方法，使用经过改造的Ekman驳船取样器来有效地采集泥浆岩心样本。我们在海湾内部收集了50多个岩心样本，并从沉积物表面到深层每隔1cm切割一次。测定了沉积物含水量、沉积物粒度分布、总有机碳和总有机氮以及碳氮稳定同位素比值。结果表明，内湾中部沉积有机质含量较高的泥质沉积，而内湾中部沉积有机质含量较低的沉积物。为了再现和理解考虑泥沙粒度变化的泥沙淤积过程，建立了三维多级泥沙淤积模型。我们在海湾重现了泥质沉积物的堆积过程。在野外调查和数值实验的基础上，结合以往的野外资料，分析了这些沉积过程的形成机制，认为颗粒越细，有机质吸附量越大，沉降和再悬浮过程是主导因素。此外，中部较低的C/N值也是形成沉积物质量空间差异的主要原因之一，因为中部水域比周围水域暴露于缺氧的时间更长，导致矿化速率较低。

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.