Vertical Flux of Microplastics in the Deep Subtropical Pacific Ocean: Moored Sediment-Trap Observations within the Kuroshio Extension Recirculation Gyre.

The Kuroshio Extension recirculation gyre in the western North Pacific is an accumulation site of plastic litter transported by the Kuroshio Current. A sediment trap was moored at a depth of 4900 m at Station KEO within the Kuroshio Extension recirculation gyre, and the vertical flux of microplastics in sinking particles of size <1 mm was observed. Forty-one sediment-trap samples collected from July 1, 2014, to October 2, 2016, were analyzed with a micro-Fourier transform infrared spectrometer and microplastics were detected in all samples. Seventeen polymer types were identified, and 90% of the microplastics were less than 100 µm in size. Microplastic sinking was driven by the action of the biological pump, which was in turn driven by seasonal variations in solar radiation and increased surface primary production typical of the spring season. Microplastic mass flux varied from 4.5 × 10-3 to 0.38 mg m-2 day-1 during the sampling period, with a mean and standard deviation of 0.054 ± 0.075 mg m-2 day-1. Extrapolating the annual microplastic mass flux at Station KEO to the entire Kuroshio Extension recirculation gyre, it is estimated that 0.028 million metric tons of microplastics are transported annually to 4900 m depth in this area.

Migration processes of radioactive cesium in the Fukushima nearshore area: Impacts of riverine input and resuspension.

It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (137Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the 137Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%-83% of secondary 137Cs migration in nearshore areas, whereas the influence of riverine 137Cs input on the sediment was only 7% per year.

Radiocesium in Japan Sea associated with sinking particles from Fukushima Dai-ichi Nuclear Power Plant accident.

This study examined the temporal variations in radiocesium concentration associated with sinking particles in the northeastern Japan Sea between September 2010 and July 2012. We analyzed sediment trap samples from this period after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011. Cesium-134 was detected in samples collected between May and July 2011 at a depth of 1100 m (4.2-11 mBq g-dry-1) but not in other periods at 1100 m or deeper (3100 and 3500 m). These results confirmed the deposition of FDNPP-derived radiocesium on the surface water in the late April 2011, which rapidly sank with sinking particles to a depth of at least 1100 m, in the northeastern Japan Sea, about 40 days after the deposition in the North Pacific. If FDNPP-derived 137Cs was excluded, no seasonal changes were detected in the 137Cs activity concentration of the sinking particles, and the 137Cs activity concentration of the particles increased with increasing depth. Judging from the concentration of 137Cs of sinking particle and seasonal variation of total mass flux and organic matter content, the lithogenic particle seems to be important for radiocesium associated with sinking particles. These data also strongly suggest a difference in sinking features of particles between 2010-2011 and 2011-2012 deployments. Due to the existence of benthic front, shallow water (1100 m) and deep water (3500 m) are separated during 2010-2011 deployment, but in the winter of 2011-2012, this front disappeared and the particles in surface water seem to have sunk to the depth of 3100 m. The sinking velocity of the particles at 1100 m was estimated to be 33-62 m day-1, with a mean sinking velocity of 43 m day-1. These values were comparable to those estimated at depths shallower than 1000 m in the North Pacific after the FDNPP accident, or in the Mediterranean, North, and Black Seas after the Chernobyl accident.

On the role of 210Bi in the apparent disequilibrium of 210Pb-210Po at sea.

The disequilibrium of the grandparent-daughter pair 210Pb (t1/2=22.3 years)-210Po (t1/2=138 days) has been used to estimate the export fluxes of particulate organic carbon in the ocean using particulate-matter-associated 210Po. 210Po is produced from 210Bi, not from 210Pb. The half-life of 210Bi (t1/2=5.01 days) is sufficiently long compared to the rates of biological particle formation and decomposition or dissolution occurring at sea. The role of 210Bi has not yet been assessed quantitatively in the apparent disequilibrium between 210Pb and 210Po, partly due to the non-existence of 210Bi depth profile measurements at sea up to now. However, greater affinity of 210Bi over 210Po and 210Pb was found recently in coastal waters and phytoplankton 207Bi uptake experiments. Build upon these findings, we developed a primitive and simple analytical approach to elucidate the role of 210Bi in the 210Po-210Pb pair in the ocean using a simplified two-box irreversible steady-state ocean model. We assumed that the activity concentrations in the dissolved and particulate phases of 210Pb, 210Bi, and 210Po in a given water column are solely determined by the concentration of the particles, their input and output, the distribution coefficients between dissolved and particulate phases, and decay constants of these radionuclides in the steady-state ocean. The 210Bi contribution to the 210Pb-210Po activity difference in seawater is found to be significant, therefore, it needs to be considered in estimating particle fluxes using 210Pb-210Po secular equilibrium at sea.