Recycling mercury at a convergent margin from Nankai Trough to southwest Japan

In the southwest Japan, located at the eastern edge of Eurasian plate, dehydrated water from subducting Philippine Sea Plate from the Nankai Trough issues without magmatic activity as thermal brines. Those brines contain high amounts of mantle derived components, and mercury may be one of those components. Mercury contamination is found in shallow groundwaters (>0.1 µg/kg) and soils (>200 µg/kg) along peripheral active faults of Osaka Basin, where subducting slab from the Nankai Trough appears deeper than the surroundings. Occurrence of the mercury contaminated groundwater also corresponds to the areas of deep low frequency tremor, which is known as a phenomenon occurring in relation to dehydration from subducting slab.

In order to specify the source(s) of mercury above described, mercury concentration and stable isotopes were analyzed for the sediments down to 2200 mbsf (meters below seafloor) taken from drilled cores at Kumano-nada basin IODP Site 0002, where accreted oceanic and forearc sediments are deposited. The mercury concentration, ranging 30-240 µg/kg, except three of 330-820 µg/kg of samples taken from ≥2000 mbsf. The range of 30-240 µg/kg is the same as those of surface sediments of ocean bottom of the study area. The high concentrations seemed due to high contribution of volcanogenic materials. The mercury stable isotopes values, –0.26 to –0.83 ‰ for δ²⁰²Hg and no shift of Δ¹⁹⁹Hg and Δ²⁰¹Hg, indicating geogenic origin without biogenic and/or photochemical alteration. The isotope values are in the similar range of those in the shallow groundwater in the Osaka Basin. The isotope values of δ²⁰²Hg are slightly smaller than that of mantle (0 ‰). These observations would be direct evidence that the mercury deposited in the subducting slab comes up with upwelling fluids.

Ocean scientific drillings have dissolved material cycles and the associating geological phenomenon, especially related to volcanic and seismic activities, at convergent margins related to subduction factory. From the point of views of concentration of useful elements, subduction factory is an important role for formation of ore deposits mainly via magmatic and the associating hydrothermal activities, which also cause the contamination of toxic elements. However, this study gives possible contamination of hydrosphere and pedosphere with toxic elements via non-volcanic tectonic activity.