A sediment trap experiment in Funka Bay, Japan: “upward flux” of particulate matter in seawater

Sediment trap experiments were carried out ten times in one year (1977) at three depths in Funka Bay. The material obtained in the traps was analyzed for metals, organic elements and radionuclides, together with the suspended matter in the overlying water column. Two groups with extremely different downward fluxes were found, a group with a small flux increasing with depth, and another with a large flux that is rather constant with depth and is observed only in winter. The flux in winter, and sometimes in the bottom layer below the summer thermocline was larger than the net sedimentation rate for total dry matter or for each chemical constituent. The flux was also larger than the net removal flux for ²³⁴Th. A most striking fact is that the specific activity of short-lived ²³⁴Th did not decrease in winter, indicating that the large flux in winter was not caused by the re-suspension of old bottom sediments. The concentration of suspended matter in winter was not much greater than that in other seasons. These results suggest that the downward flux observed in sediment trap experiments is not a net removal rate and that there must be an upward particulate flux in the bay.