1. 210Pb and Mass Flux Imbalance Between the Settling Particulates and
Sediments at a Sediment Trap Deployment Site in the Northern South
China Sea
C.C. Wu, Y. Chung and G.W. Hung
Institute of Marine Geology and Chemistry
National Sun Yat-sen UniversityKaoshiung, Taiwan
OS11A-06
Introduction
In order to evaluate the mass balance problems between the settling particulates and the underlying sediments in the northern South China Sea, sediment traps were deployed at M1 site (21o32’N, 119o28’E; 2948m) where a box core was taken. Another box core was taken at a nearby station (F, 20o15’N, 118o35’E; 2735m). Their locations are also shown in Fig. 1 together with bathymetry. The sediment trap results were previously published (Chung et al., 2004: Continental Shelf Research). We report here the 210Pb measurements made on these box cores by determining its daughter, 210Po, with alpha spectrometry, assuming the paired nuclides are in radioactive equilibrium. The M1 box core (33 cm long, taken in June, 2004) and F box core (47 cm long, taken in June, 2004) were analyzed for their water contents, loss on ignition (LOI), and 210Pb.
China
Taiwan
South China Sea
Sampling and analysis
The two box cores taken at M1 and F were sampled at 1 cm interval in the upper 10 cm or so and then at 2 cm interval or more below for analyses. The water content of each sample was determined as the weight loss after drying. After drying, the sample was ashed at 550℃ for 6 hours, and the weight loss was designated as the loss on ignition (LOI), which roughly reflected the amount of the combustible organic matter or total organic matter (TOM).
210Po measurements were made on the core samples by conventional alpha counting. As Po isotopes may be subject to evaporation loss at ashing (550℃) temperature, part of the homogenized core sample was digested rather than ashed after a known 209Po spike had been added. The digested sample was conditioned for autoplating of the Po isotopes which were counted with a silicon-surface barrier detector coupled with an alpha spectrometer. The 210Po activity was determined from the known activity of the 209Po spike and their count ratio. With a half-life of days for 210Po, it is assumed that 210Po has been in radioactive equilibrium with 210Pb in these cores. Thus the 210Po activity measured from a sample is equivalent to its 210Pb activity. The verification of this assumption is being conducted with actual 210Pb measurement.
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Figure 1. Map showing locations of the two stations (M1 and F) where box cores were taken.
Results and discussion
The water content in these cores varies with depth, especially at St. F (Fig. 2). At St. M1, it decreases from about 60% to 55% except for the top 2 samples that are over 90%. The water content at St. F is much lower than at St. M1 with a mean of about 40%. After a minimum of about 30% at 25cm, it increases rapidly to about 50% within 2 cm interval before gradual decreasing. The LOI as a measure of the total organic matter (TOM) is fairly constant in both cores at about 12% and 6%, respectively, at St. M1 and St. F. Higher LOIs are associated with higher water content at M1.
The two 210Pb profiles as determined by 210Po measurements show a general exponential decrease with depth within the upper 15cm of the cores. Below 15 cm depth, both cores display a fairly constant 210Pb activity around 10 dpm/g. Thus one can assume that the excess 210Pb has penetrated only to 15 cm depth. Since it takes about 100 years for the excess to decay, one immediately obtains a sedimentation rate of about 0.15 cm/y. The large fluctuation of 210Pb in the upper 10 cm of St. F core may be related to volcanic ash layer(s).
The sedimentation rate at M1, the trap site, translates to a mass flux of about 0.2 g/cm2/y or about 5.5 g/m2/d with a dry bulk density of 1.2g/cm3. Based on the excess 210Pb inventory integrated over the core length, the 210Pb flux is estimated to be about 10 dpm/cm2/y or 270 dpm/m2/d assuming at steady state. The 210Pb and mass fluxes obtained from the deepest trap at M1 were only 129dpm/m2/d and 0.55g/cm2/d, respectively. The large “excess” of the 210Pb and mass fluxes in the sediments over those measured from the sediment trap (2 times in 210Pb flux and 10 times in mass flux) suggests that large additional particulates with less 210Pb content, have been transported laterally below the deepest trap from elsewhere.
Figure 2. Profiles of the water content for the two box cores taken at Station M1 and F.
Figure 3. Profiles of the LOI for the two box cores taken at Station M1 and F.
Figure 4. Profiles of the total 210Pb activity as determined by 210Po activity from the box core
at Station M1 and F.