Particulate flux and 210Pb determined on the sediment trap and core samples from the northern South China Sea

Abstract

A string of four time-series sediment traps was deployed at each of the two mooring sites located in the deep basin of the northern South China Sea for 6 months. Each of these traps was equipped with 12 sampling cups and each cup collected the settling particulates for 15 days. Large temporal and spatial variations in particulate mass flux were observed and the temporal variations were generally not synchronous at each site. The time-series particulate flux varies between about 50 and 2000 mg/m 2/d for all the traps and generally increases with depth. The mean particulate flux of each trap varies from about 200 to 550 mg/m 2/d. The total organic matter in the trapped particulates as indicated by the loss on ignition decreases from 25% to 35% at the shallow traps to about 7–12% at the deep traps. These values are much higher than those observed in the southern East China Sea. 210Pb activity in the trapped particulates varies from about 40 to 370 dpm/g, and the mean activity increases with depth from about 70 to 300 dpm/g. Although 210Pb activity at each depth varies inversely with the particulate flux, the 210Pb flux correlates with the particulate flux positively and the slope increases with depth. The mean fluxes of particulates and 210Pb obtained from the traps are compared with the fluxes determined from the box cores taken in the same deep basin environment. The deepest particulate flux measured at 3240 m depth averages about 260 mg/m 2/d while the mass accumulation rate (MAR) estimated from the box cores is at least over 3 g/m 2/d, an order of magnitude higher. The high MAR derives from the very high sedimentation rate which is based on the excess 210Pb distribution assuming no mixing effect on the sediments. The sedimentation rate can be reduced by an order of magnitude if a reasonable mixing rate in the sediments is adopted. Thus, the MAR of the sediments can be quite variable and made comparable to the mean mass flux as observed from the deep traps. However, the 210Pb inventory and so the 210Pb flux at steady state can be obtained by integrating the excess 210Pb over the core depth regardless of the mixing effect. The 210Pb flux determined this way varies between 230 and 310 dpm/m 2/d although the cored sites are far from each other. The 210Pb flux measured from the deepest sediment trap at M1 and M2 sites (over 300 km apart) averaging over 6 months is about 130 and 80 dpm/m 2/d, respectively. Thus, the 210Pb flux determined on the cored sediments is significantly higher than that measured from the trapped particulates, suggesting that a large 210Pb input into the sediments is required. This additional input may be derived from lateral transport, probably via benthic nepheloid layers which may exist below deepest traps.