Estimates of surface peat accumulation rates in wetlands on plateaus without direct sediment supply from rivers have been scarcely reported. However, assessing the potential of peatlands as global carbon sinks is essential to understand the global carbon cycle. Here, we employed multiple isotopes to investigate surface peat accumulation on a plateau in eastern Hokkaido, Japan. Our results suggest that the constant rate of the supply model, corrected for the ¹⁴C bomb peak, can provide accurate accumulation rate estimates. The top 13 cm of the peat layer formed over the last 70 years, with mean accumulation rates of 8.1 ± 3.2 and 1.3 ± 0.5 mm yr^–1 in the acrotelm and catotelm, respectively. Comparisons with a previously published study suggest that peat accumulation rates were constant within the evaluated study area, when external factors were excluded. Additionally, our estimated carbon accumulation rates in the acrotelm and catotelm were 110 ± 37 and 33 ± 9 gC m^–2 yr^–1, respectively, providing a valuable example of short-term carbon accumulation rate estimations in the East Asian monsoon region.

A geochemical mapping study was conducted to investigate the spatial distribution of ⁸⁷Sr/⁸⁶Sr ratios in Kyushu. The study aimed to understand the factors controlling the geochemical distribution of Sr isotopes in stream sediments more completely. Using a thermal ionization mass spectrometer, we analyzed the ⁸⁷Sr/⁸⁶Sr of 307 stream sediments collected for geochemical mapping in Japan. The results showed that systematic variations in ⁸⁷Sr/⁸⁶Sr ratios correlate with surface geology. Stream sediments derived from mafic volcanic and pyroclastic rocks displayed low ⁸⁷Sr/⁸⁶Sr ratios (0.704–0.708), whereas those derived from felsic volcanic, granitic, and metamorphic rocks showed intermediate Sr isotope ratios (0.705–0.711). In the northwestern and southeastern areas underlain with accretionary complex and non-accretionary sedimentary rock, high ⁸⁷Sr/⁸⁶Sr ratios (0.705–0.721) were observed in the stream sediments. Based on the ⁸⁷Sr/⁸⁶Sr data of stream sediments derived from these rock types, we found a predominant supply of clastic materials originating from the Asian continent to the Paleogene back-arc basin in northwest Kyushu and the Paleogene subduction zone in southwest Kyushu. This suggests that the Sr isotope map is useful for the provenance analysis of clastic particles in sedimentary rocks. Furthermore, we present a map of the ⁸⁷Sr/⁸⁶Sr ratio in the exchangeable fraction of stream sediments. This fraction, which dissolves easily in water and is accessible for plants, is important for provenance studies of agricultural products. The ⁸⁷Sr/⁸⁶Sr ratio in the exchangeable fraction was predominantly affected by the presence of plagioclase in stream sediments and exhibits a positive correlation with that in the bulk fraction.

The oxygen isotope composition of diatom frustules is a potential paleoclimate archive; however, the temperature-dependent gradient of the oxygen isotopic records of diatoms is not supported by abiotic experiments. We synthesized amorphous silica under controlled water temperatures (10–40°C) using the traditional sol-gel method to understand the temperature-dependent oxygen isotopic fractionation. Spherical silica particles were precipitated in the solution containing a relatively low proportion of water, whereas flake-like fragments were precipitated at a high proportion of water (> 60 vol. %). The oxygen isotopic values of the flake-shaped silica demonstrated a clear temperature dependence (R² = 0.87), except for one sample that showed an extremely high value. The observed temperature gradient (–0.092‰/°C) was consistent with the kinetic isotopic fractionation model between orthosilicic acid and water, suggesting two possibilities: 1) spontaneous hydrolysis of tetraethyl orthosilicate to form orthosilicic acid and 2) less temperature-dependent isotopic fractionation between orthosilicic acid and amorphous silica. This is the first study to directly evaluate the oxygen isotopic fractionation between synthesized amorphous silica and water, supporting the idea that the temperature-dependent fractionation of oxygen isotopes in freshwater diatoms is based on abiotic processes.