Manganese (Mn) is a crucial element in global metallic cycling owing to its redox-sensitive dynamics; however, determining the redox state of abundant Mn-microparticles in deep-sea oxic sediments is challenging because of high-energy beam irradiation sample processing, such as focused-ion-beam and/or electron beam processing. Our study demonstrates an improved sample preparation method for accurately determining the redox state of Mn-microparticles in deep-sea oxic sediments. We could enrich Mn-microparticles without altering their redox state by employing a novel beam-free approach, as confirmed by scanning transmission X-ray microscopy (STXM) analyses. Our methodological advancements revealed that the Mn-microparticles predominantly existed in the Mn⁴⁺ oxidation state. The new technique, which allows us to accurately determine the redox state of Mn in these particles, provides critical insights into the geochemical processes of Mn oxide precipitation and the environmental conditions that favor its formation and preservation, thereby highlighting the potential of these particles as indicators of oxic conditions in geological environments.

We measured Raman spectra of calcite at various exposure times using a spectrometer with two gratings (1800 and 1200 grooves/mm) to evaluate the precision of the relative ratio of Raman peaks (¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃), which is attributed to ¹⁸O/¹⁶O. Preliminary calculations showed that the uncertainty in the Raman peak ratio is a function of the Raman peak intensity and the pixel resolution. The relation between the Raman peak ratio and the exposure time obtained with two gratings showed that the precisions of both the intensity and area ratio of the Raman peaks improved with increasing exposure time for both gratings, which is consistent with the calculations. However, for exposure time greater than approximately 90 s, the rate of improvement in precision with increasing exposure time decreased for both gratings. Continuous monitoring of the Raman peak ratio for more than 60 hr revealed that the decrease in precision improvement can be attributed to external factors such as room temperature fluctuation. Nevertheless, we obtained the best precision values of 2.69 ± 0.84‰ and 3.26 ± 0.74‰, respectively, for the ¹²C¹⁶O¹⁶O¹⁸O/¹²C¹⁶O₃ intensity and area ratio when measuring using a 1200 grooves/mm grating and 900 s exposure time. Those findings are 70 times better than the best precision achieved to date. They might be applicable to studies using natural calcites with large oxygen isotopic variation.

Minerals formed in high-temperature environments are distributed along faults on Yonaguni Island. Volcanic activity is thought to be a possible heat source. In this study, samples from the carbonate veins distributed around the basement rocks and faults of Yonaguni Island were melted in vacuum, and the concentrations and isotope ratios of the noble gases in the furnace were investigated. The observed isotope ratios suggest that the noble gases were dominated by crustal helium, with little influence from mantle helium. These results suggest that there was some kind of non-volcanic heat supply at Yonaguni Island (e.g., eruption of non-volcanic hot springs).

Groundwater and river water are essential in geochemical transportation from land to sea. Freshwater discharge to the coastal sea as groundwater sometimes has large geochemical fluxes comparable to river water; however, it is more challenging to monitor groundwater than river water. In this study, we assessed the carbon cycle including an underground system in a highly porous coastal area along Mt. Chokai, northern Japan, where abundant submarine spring water emerges. Groundwater and river water chemistries are generally characterized by silicate weathering, reflecting the andesitic lava that occupies the basin. Groundwater discharge was determined using mass balance calculations, including precipitation, river water discharge, and evapotranspiration. Considering the underground dissolved bicarbonate flux from land to ocean, the CO₂ consumption by silicate weathering in the western foothills of Mt. Chokai is calculated to be 8.8±0.15 t-C/km²/yr, which is significantly larger than the case where only river water flux was evaluated (4.7±0.25 t-C/km²/yr). Therefore, considering underground flux when estimating the amount of chemical weathering in coastal watersheds is crucial.