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

我们首先利用基于两个超声波的海底测深方法对东京湾的污染泥层进行了现场调查，揭示了污染泥层的厚度。然而，这种方法存在一些问题。因此，我们试图在沉积物岩心取样的基础上，直接揭示泥浆质量的空间变化。我们开发了一种岩心采样方法，通过对Ekman驳船采样器进行改造，可以有效地采集泥岩岩心样品。我们在海湾内部采集了50多个岩心样本，从沉积物表面到深层每隔1厘米进行一次切片。然后测量了沉积物的含水率、沉积物粒度分布、总有机碳和总有机氮、碳氮稳定同位素比值。阐明了内湾中部为高有机质淤泥堆积，低有机质沉积为周边堆积。为了再现和理解泥沙淤积过程，建立了考虑泥沙粒度变化的三维多级泥沙淤积模型。我们重现了海湾泥沙堆积的过程。在野外调查和数值实验的基础上，结合前人的野外资料，分析了这些沉积过程的形成机制，认为细颗粒吸附较大量的有机质，沉降和再悬浮过程是主导因素。此外，观测到的中部较低的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在较长的低氧环境中，这使得矿化率较低。

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.