Stable isotope ratios of nitrogen gas in ice core air samples represent a valuable tool for studying the past climate change. In this study, we investigated the stable isotope fractionation of nitrogen between the gas and hydrate phases during nitrogen hydrate formation. Nitrogen hydrate samples were formed in the temperature range of 225–271 K and pressure range of 13–19 MPa. The isotopic compositions of the original, residual (unreacted), and hydrate-bound gases were measured. The nitrogen isotope ratio of the hydrate-bound gas was greater than that of the residual gas by 0.2‰, indicating that the nitrogen hydrate was slightly more susceptible to 14N15N inclusion. This suggests that the equilibrium pressure of the 14N15N hydrate is slightly lower than that of the 14N2 hydrate.

Isotopic fractionation of methane between gas and solid hydrate phases provides data regarding hydrate-forming environments, but the effect of pressure on isotopic fractionation is not well understood. In this study, methane hydrates were synthesized in a pressure cell, and the hydrogen isotope compositions of the residual and hydrate-bound gases were determined. The δ2H of hydrate-bound methane formed below the freezing point of water was 5.7–10.3‰ lower than that of residual methane, indicating that methane hydrate generally encapsulates lighter molecules (CH4) instead of CH32H. The fractionation factors αH-V of the gas and hydrate phases were in the range 0.9881–0.9932 at a temperature and pressure of 223.3–268.2 K and 1.7–19.5 MPa, respectively. Furthermore, αH-V increased with increasing formation pressure, suggesting that the difference in the hydrogen isotopes of the hydrate-bound methane and surrounding methane yields data regarding the formation pressure. Although the differences in the hydrogen isotopes observed in this study are insignificant, precise analyses of the isotopes of natural hydrates in the same area enable the determination of the pressure during hydrate formation.