Variabilities of particulate flux and ~(210)Pb in the southern East China Sea and western South Okinawa Trough

摘要

Variabilities of the particulate mass flux and associated ~(210)Pb activity were examined on samples collected from time-series sediment traps deployed in the continental slope area of the East China Sea off northeast Taiwan and in the western South Okinawa Trough (SOT) as a part of the KEEP program. The particulate flux decreases laterally from the continental slope toward the trough, increases vertically toward the bottom at most of the mooring sites, and shows fairly synchronous temporal variations among the traps in each mooring for most of the deployments. The temporal variation of ~(210)Pb is generally smaller than that of the particulate flux in terms of relative change as expressed in standard deviation of the mean of each trap. In the lower slope area, the mean particulate flux varies between 9.5 and 79 g/m~2/d, and the mean ~(210)Pb varies between about 44 and 81 dpm/g. In the trough, the mean particulate flux has the smallest values, from 0.06 to 5.7 g/m~2/d, while the mean ~(210)Pb has the highest, from 82 to 192 dpm/g. The particulate flux decreases while the ~(210)Pb activity increases from the slope toward the trough. In the trough, ~(210)Pb increases with depth while the particulate flux may show a mid-depth maximum of a nepheloid layer. Although the ~(210)Pb activity increases from the slope toward the trough the ~(210)Pb flux decreases toward the trough because of much greater particulate flux in the slope area. Within the trough the ~(210)Pb flux as determined from the traps is comparable to that determined from the underlying sediments, but on the lower slope, the ~(210)Pb flux determined from the traps is much higher than that obtained from the underlying sediments. Thus a large portion of the particulates and the associated ~(210)Pb may have been transported laterally from elsewhere to the slope area in transit without depositing onto the underlying sediments. Plots of the mean ~(210)Pb flux versus the mean mass flux for each of the traps show positive linear correlation with four distinctive slopes, each representing a mean ~(210)Pb activity of a particular area or a group of traps. In the Mien-Hua canyon the mean activity is about 40 dpm/g; in the SOT the mean value is about 130 dpm/g; on the lower slope the mean values fall in-between those of the canyon and the trough. In the middle traps where the mass flux is higher in a nepheloid layer the mean activity is about 47 dpm/g, while the remaining traps yield a mean of about 65 dpm/g. Taking the mean activity of the canyon and that of the trough as the two end-member values, the slope with a nepheloid layer is a mixture of about 90% canyon and 10% trough materials, indicating the canyon as the major particulate provider for the nepheloid layer. The slope without a nepheloid layer is a mixture of about 70% canyon and 30% trough materials.