Sedimentation rates on the continental slope off eastern Taiwan

Abstract

230Th and 210Pb were measured to determine sedimentation rate and mass flux for several cores collected from the continental slope off eastern Taiwan. In the slope northeast of Taiwan at 490 m depth, the sedimentation rate, as determined from a box core by the excess 210Pb method, was 0.28 cm/yr for the past 60 years. In the deep slope off eastern Taiwan, the rates as determined from two piston cores by the excess 230Th method were similar at about 2–3 cm/kyr. In gravity core located near Lutao farther south, the sedimentation rate dropped to 0.6 cm/kyr. These rates suggest that the river input and runoff from the east coast of Taiwan contribute a significant mass of sediments to the slope off the east coast. The sedimentation rate obtained on the northern continental slope is about 100 times greater than that on the slope off the east coast, suggesting that the major sediment source for the former site is probably the East China Sea shelf. There are indications that the sediments from both the Yangtze and Yellow rivers are transported southward by the coastal current and then deflected eastward just north of Taiwan when the coastal current encounters the north-flowing Taiwan Warm Current. Sediment traps deployed near the coring site on the northern slope yield an apparent particulate flux that varies periodically with time. The flux ranges between 2.2 and 2.8 g/m 2/day. The high variability of trapped particulate flux was found to be closely correlated with the tidal current. The mass flux estimated from the sedimentation rate (0.28 cm/yr) and dry bulk density is about 11 g/m 2/day, about four to five times higher than the trapped particulate flux. This discrepancy may be mainly due to horizontal transport near bottom below the trap deployment depth. An alternative interpretation for the high mass flux on the northeastern slope is that the sediments have been transported from north to south along the shelf break by the counter current just immediately to the west of the Kuroshio.