biostudies-literatureObase TMEXT Integrated Program for Next Generation Volcano Research and Human Resource Development17967https://www.ebi.ac.uk/biostudies/studies/S-EPMC9678900biostudies-literatureUnknown12(1)We repeatedly measured isotopic compositions of noble gases and CO₂ in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in ³He/⁴He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in ³He/CO₂ ratios and carbon isotopic ratios of CO₂. The ³He/⁴⁰Ar* ratios (⁴⁰Ar*: magmatic ⁴⁰Ar) show significant increases in the period of high ³He/⁴He ratios. The temporal variation in ³He/⁴⁰Ar* ratios may reflect chanScientific reportsMonitoring of magmatic-hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases.PMC9678900JPJ005391Sumino HKawana KYamane KYaguchi MObase TToyama KTerada AOhba TfalseMonitoring of magmatic-hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases.We repeatedly measured isotopic compositions of noble gases and CO₂ in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in ³He/⁴He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in ³He/CO₂ ratios and carbon isotopic ratios of CO₂. The ³He/⁴⁰Ar* ratios (⁴⁰Ar*: magmatic ⁴⁰Ar) show significant increases in the period of high ³He/⁴He ratios. The temporal variation in ³He/⁴⁰Ar* ratios may reflect chan2022-01-01T00:00:00Z2022 Nov2025-04-19T00:36:10.376Z2025-04-07T11:43:09.748ZS-EPMC96789003641129410.1038/s41598-022-22280-3