44. Helium Isotope Ratios of Pore Gases in Deep-sea Sediments, Leg 128

We have measured the He/He and He/Ne ratios and chemical compositions of gases exsolved from deep-sea sediments at two sites (798 and 799) in the Japan Sea. The He/He and He/Ne ratios vary from 0.642 Ratm (where Ratm is the atmospheric He/He ratio of 1.393 × IQr) to 0.840 Ratm, and from 0.41 to 4.5, respectively. Helium in the samples can be explained by the mixing between atmospheric helium dissolved in bottom water of the Japan Sea and crustal helium in the sediment. The sedimentary helium is enriched in mantle-derived He compared with those from the Japan Trench and the Nankai Trough. This suggests that the basement of the Japan Sea has relatively large remnants of mantle-derived helium compared with that of the Pacific. Major chemical compositions of the samples are methane and nitrogen. There is a positive correlation between methane content and helium content corrected for air component. Based on the ^erHe-ZC^He diagram, the major part of methane can be attributed to crustal and/or organic origin.