Abstract: The vertical profiles of 230Thexcess and 230Thexcess/232Th in a sediment core have been used to determine the sedimentation rates of Terengganu River estuary, Malaysia. Applying the 230Thexcess and 230Thexcess/232Th method, U and Th isotopic involved complete dissolution of the samples, followed by separation on anion exchange, average sedimentation rates of 1.02 and 1.01 cm year-1 were obtained, respectively. The results of the accretion rate obtained from the both methods are consistent with average sedimentation rates of 1.01 cm year-1. Assuming that the accretion rate values are accurate, this may imply that the sediments at the deepest core at 14 cm were deposited during the last 14.2 years ago.
INTRODUCTION
Recently, much less effort has been focused on estuaries, embayment and coastal waters. It is known that many of these estuaries are heavily or moderately eutrophied (Weckstrom, 2006) but due to the lack of sufficient monitoring data there is no evidence whether the small and shallow estuaries are naturally eutrophic or to the degree they are affected by cultural eutrophication (Vaalgamaa and Conley, 2008). Coastal sediments are important sinks for a wide spectrum of contaminants (Grant and Middleton, 1993; Lee and Cundy, 2001). Human influence has been successfully traced using sediment geochemistry (Cundy et al., 1997; Chague-Goff et al., 2000).
Estuarine areas where freshwater encounters seawater are characterized by a lateral variation in salinity and can represent as a transfer box for the sediments between land and the open ocean (Forstner et al., 1990; Carvalho, 1995). Estuarine sediments also act as a temporary store of inorganic and organic materials. In high productive system, the decomposition of organic matter consumes oxygen and the sediment becomes anoxic. The reduced conditions cause chemical transformations of metals and important anions in the sediment, as phosphate, ammonium, iron, manganese and bicarbonate could be released to the water and production increases in an amplified positive feedback (Hinga, 1990; Rubio et al., 2003; Chau and Jiang, 2004).
Despite the acceptance that the estuary is an important sink for sediments, few studies have addressed sediment accretion using the 210Pb, 137C, 230Th and 7Be (DeMaster, 1981; Andersson et al., 1995; Peter et al., 2000). One possible way to date sediments is with the 230Thexcess (non-supported 230Th in sediments) method which can be used to date sediments up to 300 000 years old. The 230Thexcess method relies on a constant production rate of 230Th from the radioactive decay of dissolved 234U in the water column (Walter et al., 2000; Cheng et al., 2000). The measurement of 230Th concentrations in sediments provides one method of developing accretion histories. 230Th is valuable tracer of the processes whereby reactive elements are scavenged from seawater (Clulow et al., 1998). These isotopes are produced in seawater by the radioactive decay of dissolved uranium, which has a long residence time of about 4x105 years and uniform distribution in the oceans (Ku and Broecker, 1965). Thus, 230Th is produced at constant rates throughout the oceans. Following their production in seawater, 230Th is rapidly hydrolyzed and subsequently removed to sediments on a time scale of a few decades in the deep ocean and weeks to months in surface water, transported to some depositional sink. The aim of this study was to determine the accretion rate of sediment which can give detailed estimated information on both the age of the sediments and the paleoceanographic conditions in the study area.
MATERIALS AND METHODS
Description of the study area: The study site area is near to the Paka town, the southern city of Terengganu, Malaysia (Fig. 1). A sediment core sample was collected in the estuary using a piston corer in August 2004. In recent years, the study area especially for the first km along the river has been heavily impacted by discharges from municipal and industrial outflows. This was due to the rapid development of the area via expansion of the industrialization area as well as the increase in population. Steel and petro-chemicals are the main industry in the area and is the catalyst for other supportive industries to develop around the same area. The outfall of Paka River to estuary was usually influenced by the monsoon seasons which prevailed from October to March.
| Fig. 1: | The location of core (•) in the Terengganu Estuary, Malaysia |
Being, a river with smaller discharge rate, the Paka river morphology and hydrological conditions are much influenced by the seawater, even though the maximum tidal range of the area almost exceeds 2 m, thus making the estuary as a microtidal estuary. In this study a 50 cm sediment core was collected with a core sampler in the Paka estuary (Fig. 1). The core was cut into segments of approximately 5 cm interval, labeled and stored until analysis in the laboratory.
Analytical method for 232Th and 230Th.: The analytical method of 230Th and a total Th (232Th) in the sample was carried out according to the published method (Tsunogai et al., 1980; Taguchi et al., 1989; Veeh et al., 2000) with some modifications. An inductively coupled plasma mass spectrometer (ICP-MS) was used, for the quick and precise determination of Th in the digested sediment. The method involved heating 1-2 g of dried sediment and digesting it with a mixture of solution of concentrated HF, HNO3 and HCl. The solution containing Th was heated to make the solution clear before being treated with anion and cation exchange resins for the separation and purification. The effluent containing Th was then heated to dryness and finally dissolved in 5% HNO3. The concentration of 230Th was then measured with a fast and sensitive ICP-MS. The precision assessed by the replicate analyses was less than 3%. The accuracy was also examined by analyzing duplicately a Canadian Certified Reference Materials Project standard (DL-1a) and the results coincided with the certified values within a difference of ±3%.
RESULTS
230Thexcess was used to determine the accretion rates of the study areas (Suman and Bacon, 1989; Scholten et al., 1994; Thomson et al., 1999). The amounts of 230Thexcess are calculated using the following equation:
|
| (1) |
where, 230Thtotal and 232Thtotal are the measured concentrations of 230Th and 232Th, respectively and 234U and λ230 are the concentration of 234U (of which radioactivity is assumed to be 1.1 times the 238U concentration) and the decay constant of 230Th (9.24x10-6 year), respectively. The second term on the right hand side of the equation (232Thtotal) is necessary in order to subtract the lithogenic fraction and the assumed coefficient, 0.8, is a mean activity ratio of 230Th/232Th for the lithogenic fraction as reported by Anderson (1982). The third term (234U (1 - exp {- λ230t}) is for the correction of 230Th produced from 234U in the sediments, which is necessary because 230Th is produced from authigenic U contained in the sediment.
The determination of average sedimentation rate is based on the assumption that the 230Thexcess is incorporated into the sediments with a constant rate (Osmond, 1979; Ku et al., 1968). For the estimation of the sedimentation rate, both concentrations of 230Thexcess and the 230Thexcess /232Th ratio were used. The later method was used in order to minimize the limitation associated with the 230Thexcess that were produced by the decay of the uranium. Assuming, if the value of 230Thexcess derived from Eq. 1 is correct, the radioactivity of 230Thexcess in sediment core which decrease exponentially with depth and the sedimentation rates can be calculated from the following equation:
| Fig. 2: | (a) The concentration of 230Thexcess and (b) ratio 230Thexcess/232Th versus depth |
| (2) |
where, b is a gradient of the 'best-fit' curve in the plot of log concentrations of 230Thexcess against depth (cm). In this study the concentrations of 230Thexcess and the ratio value were plotted linearly with some points displayed anonymous values. As shown in Fig. 2a and b, the calculated sedimentation rates for 230Thexcess and 230Thexcess/232Th were 1.01 and 1.02 cm year-1, respectively.
DISCUSSION
By the estuary definition, estuaries are influenced by the marine and freshwater systems. Within any one estuary, the importance of marine versus freshwater influence varies both spatially and temporally. In Paka River, natural variations in freshwater flow play a significant role in influencing suspended sediment levels, siltation rates, bed composition and the position of estuary channel. However, marine influences are also important in supplying sediment to the estuary and suspending sediments through tidally generated currents and wave disturbance.
The results of the study give us a better understanding of the controls on deltaic sedimentation in the study area, which will be useful for evaluating recent deltaic morphological changes, accumulation and erosion budgets in relation to environmental modification at human and catchment dimension (Goodbred and Kuehl, 1998; Syvitski et al., 2005). The sedimentation rate recorded in this study was generally comparable to the values reported by other scientist at the Huelva Estuary, Spain (San Miguel et al., 2004), Yangtze Estuary (Chen et al., 2004) and in Palmones River (Rubio et al., 2003). However, the sedimentation rate obtained was relatively much higher when compared with other estuaries such as Rother River and Culm River (Zwolinski, 1992). Our higher value can be explained by the geographical position of our study area, which is located close to the mouth of the estuary where the 2 main rivers meet, allowing more fine sediment to be deposited. The tides also play a significant role in transporting sediments offshore into the estuary, thus the offshore materials consisting of mostly fine sediments would also be transported into the estuary but only little would reach further upstream due to the opposing river currents. Assuming the sedimentation rate values for the both methods were accurate, the age of sediment at the deepest core at 14 cm were estimated to be 101 and 102 years, respectively.
CONCLUSION
The application of 230Thexcess and 230Thexcess / 232Th in river shows the similar trend of radionuclide decay behavior although there was some anonymous existence in the graph. This anonymous somehow assisting in determining the mixing layer of the core sample. The variations in 230Thexcess activities could be caused by bioturbation in the core as well the sedimentation rates. In this future study, the application geochronology method of 210Pb could be reference in establishing the method of 230Thexcess and 230Thexcess/232Th to estimate the sedimentation rates in the river and estuary.