Figure 11. Methane ¹³C values of gas samples from experiments on degassing of CH₄-saturated water. Data are in Table 3. "Standard" refers to a sample of gas that was isolated at pressure in the T above the inlet port to the pressure vessel while the vessel was equilibrating (see Fig. 9). This gas was released into a syringe before the samples of headspace gas and CH₄-saturated water were taken. Similarly, "gas out top" refers to high-pressure methane that was extracted from the pressure vessel through the vertical portion of the inlet tube after the "standard" sample was taken. "Water + gas" refers to CH₄-saturated water extracted out of the top of the inlet port. This water effervesced vigorously as it entered the syringes, and all gas released from solution was trapped in the syringe. "Headspace gas" is a sample of gas that was isolated in the sampling tube (see Fig. 9) before release of CH₄-saturated water. This gas was released into the bubbling chamber where it could be sampled using syringes. Release of CH₄-saturated water caused a large pressure drop inside the pressure vessel. Several minutes after this pressure drop occurred, a new sample of headspace gas (which undoubtedly contained some CH₄ just released from solution) was isolated in the sampling tube and then released to the bubbling chamber. In the second and third experimental runs (B and C), an additional sample of CH₄-saturated water at lower pressure was released out the top of the inlet port. The average (±2s uncertainty) CH₄ ¹³C of methane used in the experiments, based on data from the pressurized methane release experiments (Table 2; Fig. 10), is shown by the solid and dashed lines.