Submarine sediments in Ariake Bay, Kyushu Japan, were examined to assess geoenvironmental changes in the area. 6 core samples from representative stations (Stas. 1-6) were analyzed for 16 minor elements (S, Cl, Ba, Rb, Th, Nb, Zr, Y, Ga, V, Zn, Cu, Ni, Co, Cr, and Pb) using XRF. Based on measurements of 210
Pb radioactivity, sedimentation rates at 2 stations, one off the Chikugo River (Sta. 2) and the other at the mouth of Isahaya Bay (Sta. 4), were calculated, respectively, at 0.24 cm/year and a maximum of 0.15 cm/year.
The concentrations of heavy metals, such as Cr, Ni, Zn, and Pb, in the marine sediments are about one-third of those of the highly polluted marine sediments recorded in Tokyo Bay around 1970. The vertical profile of a minor element can be divided into an upper section and a lower section. The upper section indicates various gradual changes, but the lower section is almost constant. The boundaries between the sections roughly correspond to the beginning of the 20th century, based on the sedimentation rate.
Zn and Pb in the submarine sediments can be useful indicators to trace suspended mud derived from the Chikugo River. The complementary relationship of the vertical profiles in the spatial distribution suggests that the continuous migration of the sedimentary province may be governed by the geometrical arrangement of the estuaries of major rivers and by the tidal circulation pattern in Ariake Bay. Probably, large dams built on the major rivers have hampered the sedimentation process dominated by traction flow and have decreased the maximum drainage speed. As a result, such processes have promoted the expansion of the muddy area in Ariake Bay.
Planktonic dinoflagellata (Sta. 2) and diatom (Sta. 4) fossils were also analyzed. Assuming such sedimentation rates at the stations, marine phytoplanktons (Brigantedinium
spp. and Skeletonema
costatum), which indicate eutrophication of the sea water, have increased in relative abundance during the past 10 years.
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