Assessment of Helium Isotopes near the Japan Trench 5 Years after the 2011 Tohoku-Oki Earthquake

The helium isotope ratio is an important tracer of mantle-derived fluids. Different reservoirs in nature have distinct helium isotope signatures that make it possible to identify various sources and their mixing. At 5 years after the 2011 Tohoku-Oki earthquake, we continue to investigate possible fluid venting near the epicenter by measuring helium isotopes. We collected seawater and sediment samples in 2016 at the landward slope of the Japan Trench onboard R/V Shinsei Maru. Sediments were collected using a multiple corer. Seawater samples were obtained from different depths using a CTD rosette system. Pore water was extracted from sediments via centrifugation. Gas from each sample was then exsolved from solution and introduced into the in-house purification system equipped with charcoal traps and Ti getter. Helium isotopes were measured using a noble gas mass spectrometer. Our data show less excess ³He in bottom seawater samples than indicated by helium isotope ratios measured after the earthquake, which suggests the absence of high-pressure fluids that contributed to excess ³He observed previously. Furthermore, ³He/⁴He and ²⁰Ne/⁴He in the pore waters show possible mixing between two sources: Pacific deep seawater and deep fluids. The pore water helium isotope profile follows a non-steady-state diffusion model. One possible reason for this observation is remobilization of the upper ca. 30 cm of sediments during the earthquake, which might have altered the helium isotope profile in the shallow sediment pore waters.