Rock magnetic signatures reveal the dissociated gas distribution in the gas hydrate reservoir off SW Taiwan

The first shallow hydrate sample off southwest Taiwan has been collected at core site MD18-3542 in cruise MD214. Attractive features are present at ~5.5 m core depth where an unconformity lies, and sediment properties are distinctively different above and below the layer. In the work, we perform rock magnetic analyses to study the core magnetic behavior, including magnetic susceptibility (MS), hysteresis parameters, and X-ray diffraction spectra. Combining the core features and rock magnetic results with the shallow hydrate formation off southwest Taiwan, we consider that variation of the core magnetic property could relate to the pyritization of magnetic minerals caused by the dissociated gas, and summarize the gas distribution at the shallow strata: Abundant methane with hydrogen sulfide trapped below the unconformity could form an anoxic setting and activate the pyritization. Dissociated gas may have also leaked to its above layer, featuring the MS anomaly at ~3.5 m core depth. Magnetite appears in addition to iron sulfides at the depth between ~6.5 and ~10.0 m, indicating a transition layer and implying the insufficiency of the dissociated gas to complete the pyritization. Iron sulfides become dominant again at the core bottom, revealing that the dissociated gas mainly migrates from deep strata.