Distribution of microplastics in bathyal- to hadal-depth sediments and transport process along the deep-sea canyon and the Kuroshio Extension in the Northwest Pacific.

Understanding microplastic (MP) behavior in oceans is crucial for reducing marine plastic pollution. However, the complex process underlying MP transportation to the deep seafloor remains unknown despite the deep sea being considered its major sink. We focused on MP distribution in Sagami Bay (adjacent to highly populated areas of Japan), the plate triple junction connected through the Sagami Trough, and the abyssal plain immediately below the Kuroshio Extension. We observed the highest number of MPs in the abyssal stations, more than previously reported. The polymer types and aspect ratio of MPs in the abyssal stations significantly differed from those in the bathyal/hadal stations. The study suggests that MPs accumulated in the open ocean surface layer sink to the abyssal plains immediately below it, while MPs from land sources accumulate in the bathyal depth and are transported to the hadal depth near the coast through turbidity currents along the submarine canyon.

Ultra high-resolution seawater density sensor based on a refractive index measurement using the spectroscopic interference method.

The interference method is one of the most sensitive methods for measuring the refractive index of seawater. We developed a state-of-the-art density sensor for seawater measurements based on measuring the refractive index by the interference method. The resolution of the density sensor is 0.00006 kg/m3 for changing temperature at constant salinity and pressure, 0.00012 kg/m3 for changing salinity at constant temperature and pressure, and 0.00010 kg/m3 for changing pressure at constant temperature and salinity. These resolution values are the best in the history of seawater density measurements. The ultra high-resolution density sensor will contribute notably to climate research at full ocean depth and measurement of seawater sampled from the deep ocean, to research on metrology to establish the traceability of salinity measurements, and to submarine resource exploration to detect spatial changes in the absolute salinity anomaly by combining with conventional conductance-based salinity measurements.

A significant net sink for CO2 in Tokyo Bay.

Most estuaries and inland waters are significant source for atmospheric CO2 because of input of terrestrial inorganic carbon and mineralization of terrestrially supplied organic carbon. In contrast to most coastal waters, some estuaries with small freshwater discharge are weak source or sometimes sink for CO2. Extensive surveys of pCO2 in Tokyo Bay showed that the overall bay acts as a strong net sink for atmospheric CO2. Although small area was a consistent source for CO2, active photosynthesis driven by nutrient loading from the land overwhelmed the CO2 budget in the bay. Here we show a comprehensive scheme with a border where air-sea CO2 flux was ±0 between nearshore waters emitting CO2 and offshore waters absorbing CO2. The border in Tokyo Bay was extremely shifted toward the land-side. The shift is characteristic of highly urbanized coastal waters with an extensive sewage treatment system in the catchment area. Because highly urbanized coastal areas worldwide are expected to quadruple by 2050, coastal waters such as Tokyo Bay are expected to increase as well. Through extrapolation of Tokyo Bay data, CO2 emission from global estuaries would be expected to decrease roughly from the current 0.074 PgC year-1 to 0.014 PgC year-1 in 2050.