Full-depth profiles of hydrographic and geochemical properties at the Izu-Ogasawara Trench were observed for the first time using a CTD-CMS (Conductivity Temperature Depth profiler with Carousel Multiple Sampling) system, supplemented by some comparative samplings at the northern Mariana Trench using the same methods. The CTD sensor, calibrated from measurements of seawater samples, demonstrated a well-mixed hydrographic structure below 7000 m within the Izu-Ogasawara Trench.

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... samples collected from this well-mixed hadal layer exhibited constant concentrations of nitrate, phosphate, silicate, and nitrous oxide as well as constant nitrogen and oxygen isotopic compositions of nitrate and nitrous oxide. These results agree well with previous observations of the Izu-Ogasawara hadal waters and deep-sea water surrounding the Izu-Ogasawara Trench. In turn, methane concentrations and isotopic compositions indicated spatial heterogeneity within the well-mixed hadal water mass, strongly suggesting a local methane source within the trench, in addition to the background methane originating from the general deep-sea bottom water. Sedimentary compound releases, associated with sediment re-suspensions, are considered to be the most likely mechanism for generating this significant CH₄ anomaly.