Y. Kato's research while affiliated with Tokai University and other places

Profiles of the radioisotopes 210Pb and 137Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210Pbex (unsupported) in core top sediments increased with water depth from 25 dpm g−1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g−1 at the deep-sea station SB. The high 210Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137Cs fallout.The mean sedimentation rates estimated by the 210Pbex inventory method, rather than using 210Pbex profiles, ranged from 0.06 g cm−2 y−1 to 0.14 g cm−2 y−1. The average value of the rates in SB cores was calculated to be 0.11 g cm−2 y−1, which was similar to that calculated under the assumption that the age of the 137Cs peak corresponds to its maximum fallout year in 1963.Although 137Cs inventories represented one tenth of the anthropogenic fallout of 137Cs until 1997, they correlated with the increase in 210Pbex inventory. This suggests that the scavenging of refractory 137Cs as well as 210Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.

The seasonal carbon cycle was studied in the bathyal environment of Sagami Bay, Japan, to determine whether “benthic–pelagic coupling” takes place in this eutrophic marginal oceanic setting. Both Japanese sea color observation satellite (ADEOS) photography and sediment trap moorings have been used since 1996 for monitoring sea surface primary production. Video records at a real time deep-sea floor observatory off Hatsushima Island in Sagami Bay were also used to monitor the deposition of phytodetritus on the sea floor. At this location, a spring bloom starts in mid-February and ends in mid-April. About 2 weeks after the start of the spring bloom, phytodetrital material is deposited on the sea floor. Video records clearly show that phytodetritus deposition has taken place in the spring of every year since 1994, even though the exact timing is different from year to year. The population size of benthic foraminifera is highly correlated to this phytodetritus deposition. The phytodetritus triggers rapid, opportunistic reproduction of the shallow infaunal taxa, Bolivina pacifica, Stainforthia apertura and Textularia kattegatensis. Shallow infaunal species mainly occur in the phytodetrital layer or just below this layer during the spring. This indicates that such opportunistic species are key indicators of phytodetrital deposition. The deep infaunal taxa Globobulimina affinis and Chilostomella ovoidea show less pronounced seasonal fluctuations in population size, but nevertheless exhibit some response to phytodetrital deposition. Thus the seasonal flux of organic matter is the most important determinant of population size, microhabitats and reproduction of benthic foraminifera in Sagami Bay.