In order to understand the source, transport pathway and sink of settling particulates in the sea off northeast Taiwan, three moorings with six sediment traps (about 100 m and 300 m above bottom) were deployed on the northern slope of the Ilan ridge (T17S and T18S) and in the South Okinawa trough (T19S). The mooring deployed at T17S was lost due to a connecting ring which was rusted and broken. The duration of the mooring was 6 months (late November, 1999 to early May, 2000) with sampling intervals at 15 days each. When the moorings were recovered, seawater samples were collected for filtration near each station and a box core at T19S was also taken so that the settling particulates, suspended particulates and bottom sediments can be compared for their characteristics. The results show that higher apparent mass fluxes were observed in January, February and early April, 2000 at T18S. The apparent mass fluxes vary between 11 and 91 g/m2/d at T18S, and between 6 and 22 g/m2/d at T19S. The time-series variations of the Pb-210 activity are between 46 and 76 dpm/g at T18S, and between 79 and 122 dpm/g at T19S, the values of which are much higher than those of T18S. The temporal Po-210 variations range between 18 and 90 dpm/g at T18S, and from 65 to 193 dpm/g at T19S. The Po-210 activity and its variation are greater at T19S than at T18S. The 210Po/210Pb activity ratios are less than unity at about 0.8 at T18S, and greater than unity at about 1.3 at T19S. The total Po-210 activity in all water columns varies between 0.2 and 16 dpm/100kg while the total Pb-210 activity varies between 28 and 43 dpm/100kg. The mean total 210Po/210Pb activity ratio is about 0.3, indicating that Po-210 was easily scavenged and removed from water columns by particulate matter. The box core taken at T19S has Po-210 activity ranging from 54 to 121 dpm/g and Pb-210 activity from 51 to 90 dpm/g. The 210Po/210Pb activity ratio is greater than unity in the upper 6 cm, indicating an excess of Po-210 activity. Below 6 cm Po-210 and Pb-210 are nearly at equilibrium. The 210Po/210Pb activity ratios in the water columns observed after the first deployments were higher than those observed after the second deployments. However, the 210Po/210Pb activity ratios of the settling particulates collected from the first trap deployments were lower than those from the second trap deployments. These changes might be attributed to earthquakes which occurred mainly before the second mooring period. These earthquakes could have caused a large quantity of terrigenous particulates with higher 210Po/210Pb activity ratio to be transported out to the study area in addition to resuspended shelf and slope sediments. These settling particulates, being enriched in organic matter with higher activity ratio, could preferentially scavenge Po-210 from the water column, resulting in much higher 210Po/210Pb activity ratios.

Abstract In order to understand the source, transport pathway and sink of settling particulates off northeast Taiwan, time-series sediment traps (PPS-3/3) were deployed on the north slope of Ilan ridge (T18) and in the Okinawa trough (T15 and T16) to collect settling particulates. The trapped particulate samples were determined for apparent mass flux (T18), particle size distribution, and contents of lithogenic portion, metals (Al, Mg, Ca, Fe, Mn, Cu), opal, OC (organic carbon) and IC (inorganic carbon). The results show that mass flux in winter is higher than in summer except for typhoon or rainstorm in summer which may cause high particulate flux. At mooring T11 and T17, only 2 cups had collected particulates but with rapid decrease, and the remaining cups were empty. It is not clear whether Kuroshio plays a role and further investigation is needed. The trapped particulates were mainly clay and silt in the north slope of Ilan ridge and south Okinawa trough, and were sand and silt in the Mien-Hwa canyon. The portion of coarse silt and larger particles (>31 mm) decreases with an increase of distance from the land (from T11 to T18). The grain size distribution of particulate at T18 is similar to that at T15 and T16, but the mass flux in the north slope of Ilan ridge is an order of magnitude higher than in the south Okinawa trough. The size distribution pattern below 600m water depth is very similar at both T15 and T16, but the coarse fraction (> 31 mm) is more at the upper than lower traps, presumably due to lateral transport. The Mn content of the trapped particulates in the south Okinawa trough is twice as hign as that in the north slope of Ilan ridge. High Mn/Al ratio in the trough suggests that Mn is supplied by hydrothermal activities. Lithogenic portions occupy about 84~88.5% at T18, T16 and T15. Relative to other marginal seas biogenic contributions are clearly lower.

Abstract The research was conducted as a part of the phase III of the KEEP(Kuroshio Edge Exchange Processes)program. Two strings of sediment traps were deployed simultaneously: one near the outlet of North Mien-Hua Canyon (T15), and the other at the south rim of the South Okinawa Trough (T16). The purposes are to collect settling particulates at various depths for the studies of temporal and spatial variations of the particulate flux, 210Pb activity, 210Pb flux and size distribution. The particulate fluxes showed synchronous variations among the traps deployed at different depths of the same site. In general, the particulate flux in the marginal sea areas increases with depth but the maximum values may be at the mid-depth rather than at the deepest trap and remain so for the entire mooring period as observed at T15 and T16. Unusually high particulate fluxes could occur within the same period, probably reflecting an episodic event, such as typhoon or rain storm. 210Pb activity measured from different depths at T15 and T16 increases generally with depth along with the increase of the particulate flux toward the bottom. The temporal variation of 210Pb activity is generally smaller than that of the particulate flux in terms of relative amplitude. At the outlet of the canyon, T15, the temporal mean particulate flux of each trap ranged from 0.16 to 3.3 g/m2/d(a factor of ~ 20), while the mean 210Pb activity of each trap varied from about 98 to 168 dpm/g. At T16 located at the south rim of the South Okinawa Trough, the temporal mean particulate flux ranged from 0.06 to 5.7 g/m2/d(a factor of ~ 100), while the mean 210Pb activity varied from 82 to 192 dpm/g. The particulate 210Pb flux was smaller at T15 than at T16 because T15 has smaller particulate flux. The trapped particulates in the canyon(T1-T3) are mostly silt and sand, while the particulates collected from the Okinawa Trough(T15, T16) are mainly silt and clay. The 210Pb activity of the former is much lower than that of the latter, indicating the enrichment of 210Pb on the fine particulates. As the fine-grained particulates away from landmass have longer residence time in the ambient water, they can scavenge 210Pb more effectively.