Wakamiko Caldera is an isolated basin located in the northern part of Kagoshima Bay. The underwater caldera is known for the active fumaroles that continuously discharge volcanic substances into the overlying water. Due to the limited exchange of waters in the caldera with the surrounding area, the water column gets stagnant and stratified from spring to autumn. Previous studies have shown that overturning occurs in the winter. Based on new data obtained by field observation in Wakamiko Caldera, we show that during winter, vertical mixing “resets” the helium isotope profile. After winter, as the water column regains its stratification, fluids with mantle helium slowly build up near the seafloor, resulting in the high ³He/⁴He ratio previously observed. We now use the temporal variation of ³He/⁴He ratio in different seasons to calculate the flux of ³He. Using this method, we estimate the ³He flux at approximately 0.013 mol/year.

Variations in Mo isotopic ratios (δ^98/95Mo values) in volcanic arc lavas reflect various processes occurring beneath island arcs, including slab dehydration, mantle melting, and crystal fractionation, making this tracer important in recent geochemistry. Previous studies of Mo isotopes have focused primarily on samples from the volcanic front (VF) of oceanic island arcs, and only a limited number of studies have been conducted on volcanoes in the rear arc (RA) regions. To better understand the behavior of Mo isotopes in subduction zones, we focused on the Northern Izu (N-Izu) arc and determined major and trace element abundances and δ^98/95Mo values in basaltic samples from one VF (Izu-Oshima) and two RA (Niijima and Kozushima) islands. The N-Izu basalts are characterized by relatively uniform Zr/Hf ratios with systematic decrease in Ba/Th and increase in Ce/Mo ratios with increasing depth of the Wadati-Benioff zone, indicating a gradual decrease in the contribution of aqueous fluids from the subducting slab to a chemically homogeneous mantle wedge. Similarly, the δ^98/95Mo values in the N-Izu basalts decrease with increasing slab depth from Izu Oshima (+0.11 ± 0.09‰) to Niijima (–0.14 ± 0.05‰) and Kozushima (–0.20 ± 0.21‰). The most straightforward interpretation for the elevated δ^98/95Mo values in the N-Izu VF basalts is the contribution of slab-derived aqueous fluid enriched in heavy Mo isotopes to the source mantle of the VF volcanoes, due to the presence of residual minerals in the slab in which light Mo isotopes are preferentially distributed. In contrast, the low δ^98/95Mo values in the RA basalts, which are as low as the mean MORB value (–0.18‰), suggest that the slab-derived fluid in the RA region is depleted in heavy Mo isotopes as a result of the continuous loss of heavy Mo isotopes via slab dehydration in the VF region, while other possibilities, including the incorporation of slab melts containing sedimentary components, cannot be ruled out.

Iron (Fe) in seawater is essential for marine phytoplankton. The bioavailability of Fe to phytoplankton largely depends on its chemical form in seawater. Fe(II) is an important component of surface water because photochemical reduction processes are related to the Fe acquisition mechanism of phytoplankton. However, the marine biogeochemical processes of Fe(II) have not been thoroughly investigated. This study applied the luminol chemiluminescence method to directly determine Fe(II) in acidified seawater in the Kuroshio area, subarctic Pacific, and the Bering Sea. We successfully obtained the vertical profile of Fe(II) in the Kuroshio region, where Fe oxidation has been studied; oxidation rates were consistent with previous studies. The prolonged half-life of Fe(II) in the Bering Sea suggests the possible influence of dissolved organic matter from the marginal sea on Fe(II) oxidation. The long half-life of Fe(II) in the high nutrient, low chlorophyll (HNLC) water may be crucial for supplying Fe to phytoplankton.

Lead-zinc tailings represent an important source of Cd and other heavy metals in the environment. The active geochemical process in the tailings plays a critical role in these heavy metals’ fate and ecological risk. However, a substantial study gap occurs in carbonate-rich Pb-Zn tailings compared to carbonate-poor ones. In this contribution, we conducted XRD mineralogy, particle distribution, ICP-MS and ICP-AES bulk chemical analyses, and BCR geochemical fractions for Niujiaotang Cd-rich Pb-Zn tailings to decipher the geochemical process operating in the tailings. The result demonstrates that dolomite and pyrite dominated the mineralogy of tailings with an average pH of 6.36. The average Cd, Zn, and Pb in 17 contents tailings samples are 141, 8118, and 340 mg/kg, respectively. The significant correlation between Cd vs. Zn and Zn vs. Pb and varied Zn/Cd and Zn/Pb ratios in the four BCR fractions from the tailings samples were revealed. Pyrite weathering and associated dissolution and oxidation of sphalerite are responsible for the release of Cd and Zn from the tailings. Dolomite dissolution and associated acid neutralization caused Cd and Zn attenuation by the formation of carbonate minerals. The differences in weathering and oxidation rates of sulfide minerals and the pH-dependent geochemical behavior of secondary minerals lead to varied mobility among Pb, Zn, and Cd. We proposed that these geochemical processes operating in the tailings significantly lowered the ecological risk of heavy metals. This contribution sheds new insight into heavy metal pollution control for Pb-Zn tailings from carbonate-rich deposits.