Current World Environment

Introduction

Continental margins from a link between the ocean and land is an important areas for understanding the biogeochemical cycles of heavy metals, carbon and other materials (Anderson et al., 1994; Yamada and Aono, 2003; Masuzawa et al., 2003). In addition to land-derived materials, a large quantity of particles are produced in situ by high primary production and transported away from the land to the sea through river and estuary (Kato et al., 2003). Mechanisms that promote sedimentation lead to the sequestration of large amounts of organic matter as well as mad-made heavy metals and other contaminants.

Sedimentation rate and mass accumulation rate are useful to understand sediment-transport processes and the flux of themajor constituents of the sediments, as lithogenic and organic components, both important for geochemical studies (Alperin et al., 2002; Borole, 2002; Gorgas and Wilkens, 2002). Aelian transport of lithogenic particles could constitute an important input to the deep sea sediments. Size, shape and composition of settling particles are important factors that determine transport and preservation in the sediments. Moreover, morphologic characteristic of the seafloor, such as the presence of canyons, slope terraces and govern sediment accumulation patterns (Masqué et al., 2002; Muñoz et al., 2004).

Naturally occurring radioisotope ²¹⁰Pb has been widely utilized to determine sedimentation rates in coastal continental margin sediments and despite some limitations of the methodology, it has demonstrated to be a useful tool to estimate recent sedimentation rates (White et al., 2002; Moon et al., 2003; Shotyk et al., 2005; Crusius et al., 2004; Huh and Chen, 1999; Oguri et al., 2003; Masqueet al., 2003). ²¹⁰ Pb is a natural product of the ²³⁸U decay series with a half-life of 22.3 years. Basically, ²¹⁰Pb content in seawater is supplied from the insitu decay of ²²⁶Ra in seawater and ²²² Rn in the atmospheric and subsequent rainout to the ocean surface. ²¹⁰Pb is efficiently scavenged by suspended fine particles and thus actively removed from seawater on timescales comparable to its half-life (Henderson and Maier-Reimer, 2002); it is deposited on the seafloor, producing an excess with respect to ²²⁶ Ra decay within the sediments. This allows the estimation of sedimentation rate where productivity and adjective processes are considered to be important aspects in the distribution and removal of ²¹⁰ Pb in the water column (Nozaki et al., 1997; Thomson and Turekian, 1976).

South China Sea Terengganu waters are located in eastern peninsular Malaysia. The surrounding community of Terengganu waters is mainly depends on the sea as the source of income particularly the fisherman. In recently year, the coastal area of Terengganu was rapidly developed in order to maximize the employment of natural resources in the marine environment. Importance of this coastal area as a fishing area yet is exposed to large different of sediment transportation pattern as the great development is carrying out along the Terengganu river, Marang river, Merchang river and Paka river.

Material and Method

Collection and Sampling of Sediments

Core sample was collected in Terengganu waters in August 2004 using gravity corer (Fig. 1). Samples was then stored in the PVC pipe and keep in freezer in order to minimize contamination. In the laboratory, sediment cores were sliced into 2cm intervals and were dried at 60^oC to a constant weight and homogenized to analyses the ²¹⁰Pb activities, determined mainly by alpha spectrometry of its daughter, ²¹⁰Po.
 

figure 1: Map showing sampling location T1 and T2 off the Terengganu waters click here to view figure

Chemical Analysis

For the chemical analysis, 2g of dried sample was placed in a beaker and digested with HNO₃ with a spike of 0.275ml of ²⁰⁹Po. The solution was heat on the hot plate in the temperature of less that 60°C (²⁰⁹Po will be lost in temperature above 60°C) for 24 hours. Time and date are necessarily to be recorded. Each additive of acid required 24 hours for digesting such as HClO₄, HCl, and 6M of HCl. After that, sample solution was centrifuged and plated onto the silver foil with the presence of ascorbic acid for 24 hours.

After the deposition process, deposited silver plate will be dried in room temperature before detect by Alpha Spectrometry, Canberra model. The activities ²⁰⁹Po and ²¹⁰Po will be appeared in a peak form which will be assisting in the calculation of ²¹⁰Pb (dpm/g). The duration of counting will takes for at least 3 days

Data Analysis

The activity of ²¹⁰Pb is obtained by the formula below:



Neither estimation considers the mixing effect. Since other radionuclides are not available for this study, we worked with the ²¹⁰ Pb activities below the mixing layer (Lewis et al., 2002; Oguri et al., 2003). Thus our estimates on sedimentation rate assume that mixing is negligible below the mixed layer, where sedimentation is the dominant process. On the other hand, the use of a constant value to estimate the unsupported ²¹⁰ Pb (in excess) where no ²²⁶Ra data are available does not greatly affect sedimentation rate estimations; ²²⁶ Ra is usually constant with depth and ²²⁶Ra value agree well with ²¹⁰ Pb values at the base of the sediment cores (Radakovitch and Heussner, 1999; Oguri et al., 2003).

Results and Discussion

Figure 2 shows the activities of ²¹⁰Pb_xs was decreasing exponentially with depth from 0 – 8cm depth. The activities were later increased from 8cm onwards to 18cm and finally reaching constant values at ~18cm to 26cm. A higher value of ²¹⁰ Pb activity on the surface might be caused by the fresh deposition particle which basically derived from the atmosphere and land. Besides that, this might be also resulting from ²¹⁰ Pb initial activity of settling particles and additional scavenging in the water column, or from older sediment issued from resuspension (Miralles et al. 2005). It is clear that ²¹⁰ Pb increases monotonically with depth, suggesting of the particulate scavenging (Chung et al. 2004). Thus, the scavenging of ²¹⁰ Pb from the sediment and water column might be a reason to the increases of ²¹⁰ Pb value from 8 cm to 18 cm depth.

Figure 2: 210Pbxs distribution with depth in Terengganu South China Sea click here to view figure

Conclusion

²¹⁰Pb established method deployed in the Terengganu South China Sea water indicates the distribution of ²¹⁰ Pb activities was varies according to depth. Core sample did not indicate a significant of mixing layer as well as bioturbation activities. The sedimentation rate in Terengganu South China Sea was mainly influenced by the ambient forces such as current motion (lateral transport) and river input especially during the northeast monsoon season. Top core layer experiencing the freshly deposition of ²¹⁰ Pb sources from terrestrial and atmosphere. Meanwhile the increases of ²¹⁰ Pb activity in the deep core were suggested due to the particulate scavenging from the water column and the deeper sediment pore.