Sedimentary environments based on textures of surface sediments and sedimentation rates in the South Yatsushiro Kai (Sea), southwest Kyushu, Japan

Abstract

The main purpose of this study is to clarify the relation between characteristics of sediments and environmental conditions, based on the analysis of core samples from the South Yatsushiro Kai (Sea) which is located off the west coast of central Kyushu Island, Japan.Bottom sediment samples were collected at 74 stations in the southern part of the Yatsushiro Kai using gravity corers in March 1996. 62 core samples were used for the mechanical analysis by the settling-tube method, and Q-mode cluster analysis was carried out based on the proportion of grain size distribution (Wentworth size classes) . 61 core samples were available to estimate the sedimentation rates (during c. 50 years) which were recognized from the first appearance of high mercury contents of cores. On the other hand, the topmost one centimeter of 73 core samples, which is approximately 10 cc of wet sediments, was utilized for foraminifera) study.The general trend of the bottom current system in the area was discussed, based on the mechanical analysis data of bottom sediments, the distribution of sedimentation rates and the distributions of the number of individuals of planktic and living benthic foraminifera. Based on the results mentioned above, the South Yatsushiro Kai is subdivided into the following five areas:1) the area near the straits characterized by coarse sediments under the influence of strong tidal and bottom currents; 2) the southern part and the area off the mouth of the Minamata River, both of which are characterized by relatively coarse sediments under the influence of longshore currents and poorly sorted sediments supplied by the Komenotsu and Minamata Rivers; 3) the northwestern and eastern parts characterized by fine-grained sediments accumulated under conditions of rather stagnant water masses; 4) the central area, characterized by relatively fine sediments and a large number of planktic empty tests deposited on the sea bottom under the condition of rather weak tidal current systems; 5) the northern part characterized by gravels or gravelly sands which were derived from shallow coastal areas.Judging from the distribution pattern of the maximum mercury content at some horizons of each core, the fine-grained sediments polluted by mercury were transported northeastward and southward by weak longshore currents and spread toward north and west across the northern and southern parts of the South Yatsushiro Kai.

Sandbar Formation in the Mesjid River Estuary, Rupat Strait, Riau Province, Indonesia

“…Other areas occupied by finer grained-sediments which are influenced by weak tidal currents less than 0.3 m/s (Figures 5 and 6). Strong relationship between the distribution of median diameter of bottom surface sediments and currents system was also found in the Dumai Coastal Waters of Rupat Strait which located close to the Mesjid River Estuary and in the strait which connected to the estuary [Rifardi and Syahrul, 2013], and this relationship is also recognized in the South Yatsushiro Sea, southwest Kyushu Japan [Rifardi et al, 1998]. The relationship between particle size and water velocity was also explained by Koiter et al [2013].…”

“…The samples were used for analysis of mechanical grain size. The gravel and sand proportions were determined by sieving and were weighed oven dried (Rifardi et al,1998). The settling tube method was utilized to determine the mud proportion.…”

Impact of Reclamation Project on Mercury-Contaminated Sediment Transport from Minamata Bay into the Yatsushiro Sea

“…Based on Rajar et al (2004), contaminated area has reached the Yatsushiro Sea, mostly before, or during the dredging process 7) . Rifardi et al, (1998) also reported based on their field survey result, there were southern and northern sediment transports that contained mercury in Yatsushiro Sea 8) . Recently, Snyder (2012) 9) mentioned that the Asahi Shinbun reported that between January 2005 and June 2010 around 568 people who live in the designated area for relief measures in Amakusa region, the western part of Yatsushiro Sea, plus 160 people who live outside of the designated area have shown similar symptoms with Minamata Disease patients 9) .…”

“…(1998) 8) . But here, regardless the mercury concentration, we only consider to the similarity of the pattern and direction of the sediment transport around Minamata Bay and in the Yatsushiro Sea.…”

“…They conducted sediment core sampling in the Yatsushiro Sea, resulting horizontal distribution of maximum total Hg concentration in bottom sediment. Based on their research, the contaminated sediment was transported both northeastward and southward by longshore current and spread north and west across the sea in the northern and southern parts of Yatsushiro Sea 8) . The results from our simulation both for before and after reclamations in 30 days show the sediment transport move almost to all over the Yatsushiro Sea.…”