Distribution of mercury in sediments from Kagoshima Bay, Japan, and its relationship with physical and chemical factors

Kagoshima Bay is located on the south coast of Kyushu Island, Japan. Submarine fumaroles present in the northern area have been identified as a natural source of mercury in the bay. The aim of the present work is to study the spatial distribution of mercury in the bottom sediments of the bay and its relationship with major physicochemical properties that might affect the transport of mercury in the ecosystem surrounding this area. Sediment core samples were collected in the north and central areas in surveys performed during 2011 and 2013. The individual core samples were sliced and processed for the analysis of total mercury (T-Hg), chemical composition, total organic carbon (TOC), particle size and magnetic susceptibility. The horizontal distribution of T-Hg indicates that fumaroles are the primary source of Hg in the area; the results indicate a key role of organic matter binding to Hg. In the area to the west of the fumaroles, the relationship between Hg and TOC is characterized by two linear trends that correspond to the upper and lower sections of the cores. From the physicochemical properties, it was suggested that the two distinct linear relations might be caused by a change of the volcanic activity at the Aira Caldera, which is located at the northern part of the bay. The extent of the bay impacted by the Hg generated from the hydrothermal vents has decreased in recent years and is limited to the vicinity of the emission point.