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4.3. Estuarine surface sediment and its sources
Comparison of δ13C and C/N from a wide range of potential sources of sediment to the Pearl River estuary with measurements of bulk sediment samples from the estuary makes it possible to identify the dominant sources of organic sediment to the estuary floor. Organic matter within the surface sediment is mainly composed of the POC, especially from the marine surface sediment (Liu et al., 2007), while the organic matter of the surface sediments from freshwater environments is a combination of freshwater in situ plankton productivity and the TOM. Spatially, δ13C increases from –25.0‰ to –21.0‰ from river tributaries to marine areas with C/N decreasing from 15.2 to 6.8. This trend indicates a decreasing TOM contribution into the surface sediment organic matter, with increasing POC contribution seawards. Inner estuarine areas near the river mouth predominantly receive TOM delivered by the freshwater runoff. The proportion of TOM in the surface sedimentary organic matter decreases seawards as the influence of the freshwater runoff becomes weaker. In the marine area, surface sedimentary organic matter is dominated by the POC as TOM flux is weakened by the tide influence.
Applicability of δ13C and C/N of the surface sediment for indicating sources of the sediment has been explored in previous studies in the estuarine and continental areas based on the sedimentation rates (Hedges and Parker, 1976; Goñi et al.,. 1997; Hu et al., 2006; Ramaswamy et al., 2008). Hu et al. (2006) examined organic carbon, total nitrogen, δ13C and δ15N of surface sediments from the Pearl River estuary and adjacent shelf, suggesting proportionally higher terrestrial-derived organic carbon at the river mouth and the western coast compared to marine-origin organic carbon. Using a mixing model (Schultz and Calder, 1976), Hu et al. (2006) further suggest the algal-derived organic carbon content is estimated to be low (≤0.06%) at the river mouth and higher (up to 0.57%) on the adjacent inner shelf. A decreasing contribution of TOM in the surface sediment organic matter is also suggested by Zong et al. (2006) by examining the bulk organic δ13C and C/N of the surface sediment, as well as diatom records.
Detailed bulk organic δ13C and C/N of the organic matter from potential source areas of the surface sediment in the Pearl River estuary investigated in this study support the spatial distribution of the TOM and POC in the estuarine area. Different dominant organic matter sources between the freshwater and marine environments generate the lower δ13C but higher C/N in the freshwater sediment compared to the marine sediment (Fig. 8b). Surface sediments from the freshwater area of this study have low δ13C (e.g. –23.9‰ for surface sediment and –26.3‰ for the POC on average in G2). Surface sediments from the brackish/marine environment, with little freshwater influence, have high δ13C (e.g. −21.0‰ in G9, Fig. 8b) due in part to higher δ13C of the marine POC (–24.0‰ on average in G5 and –22.4‰ for G6, Fig. 6). C/N is useful for separating algae and TOM, with algae having C/N between 5 and 8, while TOM has C/N typically higher than 15 (Meyers, 1997). Low C/N of the surface sediments at the marine end (e.g. 6.9 for G9, Fig. 8b) suggests higher organic matter contribution from marine algae (Hu et al., 2006; Wu et al., 2007).
Factors that influence the organic carbon isotopic signature of POC, therefore, also become significant for the signature of the surface sediment, such as strength of the freshwater flux and hydrological conditions. The contribution of marine algae to the sedimentary organic carbon is limited by the turbidity and nutrient content of the water column (Yin et al., 2004; Huang et al., 2003; 2004). Turbidity is higher at the river mouth (nearer the source of sediment flux), the contribution of in situ phytoplankton productivity to the sediment signal is less here than further down the estuary towards the inner-shelf area. A similar phenomenon has been reported from other estuaries, e.g. the Gironde estuary, western Europe (Fontugne and Jouanneau, 1987) and the Yangtze River estuary, China (Wu et al., 2007).
4.4. Influences on the bulk organic δ13C and C/N
4.4.1 Anthropogenic influences
It has been suggested that anthropogenic influence from the Pearl River delta, such as the agricultural products, human wastes and industrial inputs, has become an important factor that changes the organic carbon isotopic signature of the surface sediments in the estuary (Jia and Peng, 2003; Owen and Lee, 2004). Our results show that agricultural C3 plants have marginally higher δ13C (–28.2±1.4‰) than general C3 plants (–29.9±1.3‰), as well as the agricultural C4 plants and general C4 grass (Fig. 3). Correspondingly, δ13C decreases from agricultural soil samples to riverbank and mangrove soil, and the forest soil has the lowest δ13C of all (Fig. 4). C/N increases through these soil types, with the forest soil having the highest C/N. This increase in δ13C and decrease in C/N found in agricultural soil might be due to the fertilization process (O’Leary, 1985). Sediments deposited during recent decades tend to have elevated δ13C, which appear to coincide with rapid urbanisation, industrialisation and reclamation in this area (Owen and Lee, 2004; Hu et al., 2008). For example, well-nourished plants show higher δ13C than plants deficient in nitrogen and/or potassium (less fertilized) (O’Leary, 1985).
As a relatively densely populated area, sewage could be a potentially important contributor to the sediment organic matter in the Pearl River estuary. Sediments from Victoria Bay, Hong Kong, were significantly contaminated by sewage (Hsieh, 2006). The δ13C of these sediments varies from −28.59 to −22.60‰ and C/N from 10.71 to 14.73 (Hsieh, 2006), which must include the influence of sewage. Here we have undertaken biomarker analysis of the Pearl River surface samples (David Strong, pers. comm.) and show the lack of coprostanol and other 5-beta stanols indicating that sewage is an insignificant contributor to the organic carbon in these sediments. The main organic pollutants found in the estuarine area are the polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyles (PCBs) and organochlorinate pesticides (Kang et al., 2000). Concentrations of the total PAHs, PCBs and organochlorinate pesticides in surface sediments from the river to the estuarine area are 1 167 – 21 329 µg g-1, 10.2-485.5 µg g-1 and 5.8-1 658 µg g-1 respectively (Kang et al., 2000). When compared to the TOC values from surface samples analysed here of 0.5-2% TOC (i.e. 5 000-20 000 mg kg-1) the concentration of these pollutants is actually very low. Sediments with the highest concentration of these organic pollutants are from areas near Guangzhou and/or Maucau according to Kang et al. (2000). For example, concentration of organochlorinate pesticides in sediments from areas investigated by this study is usually between 5 and 70 µg g-1 (Kang et al., 2000). Overall we consider that the contribution of organic carbon from these pollutants is insignificant.
A possibly more important contributor that might influence δ13C value of the sediment is crude oil. The light oil from the Pearl River estuary is characterized by low density (less than 0.83 g cm-3), high contents of saturated hydrocarbons (71%-93%) and no asphaltenes (Guo and He, 2006). δ13C of the crude oil from Pearl River estuary is between −27.09‰ and –26.48‰ (Guo and He, 2006), slight heavier than that of C3 plants. However, according to biomarker results (David Strong, pers. comm.), the total amount of lipids from crude oil in these samples ranges from 100 to 500 µg/g, which compared to the average TOC values of 0.5-2% (i.e. 5-20 mg/g, Table 5), indicates a maximum concentration of 10% (0.5% - 10%). Furthermore, biomarker results also suggested that the apolar freactions found in the crude oil would most likely already include the PCBs.
These data indicate that sedimentary organic carbon contributors other than plants, planktons and algae, such as human waste, organic pollutants and crude oil, are less important. The maximum concentration of organic carbon from these anthropogenic sources is less than 10%. Thus, in spite of the influences of organic carbon from other sources, bulk organic δ13C and C/N is still a good indicator for the source of estuarine sediment at the broadest scale from terrestrial, lowest deltaic or marine areas.
Change in bulk sediment organic δ13C and C/N over time due to decomposition is an important consideration in coastal palaeoenvironmental research (e.g. Wilson et al., 2005; Lamb et al., 2006). For example, in the coastal wetlands of Louisiana, USA, sediment δ13C shifts between −0.5‰ and −3.3‰ from the surface vegetation (Chmura et al., 1987). Changes in C/N can also occur during decomposition, particularly in the early stages (Valiela et al., 1985). Lamb et al. (2006) reviewed different studies and concluded that in inter-tidal and supra-tidal sediments, this may result in much lower surface sediment C/N compared with the overlying vegetation, whereas in sub-tidal sediments, C/N is commonly slightly higher than in the suspended sediment phase.
The influence of degradation varies across the Pearl River delta area and estuary. In the Pearl River delta, results show that soil organic matter generally has higher average δ13C but lower C/N (Table 2) than that of overlying vegetation (Table 1). For example, average δ13C of mangrove soil is 3.1‰ higher than that of overlying mangrove plants, and average C/N of mangrove soil is 12.4 compared to 31.0 for that of mangrove plants. Degradation effect might be an important reason for differences in δ13C and C/N between SOM and overlying plants. However, SOM receives organic carbon from all overlying vegetation which is a mixture of both C3 and C4 types. Thus differences of δ13C between the mixed type of organic matter (SOM) and individual plants should also be significant.
In the Pearl River estuary, sedimentary organic matter is mainly derived from riverine and marine areas (Zong et al., 2006). Much of the terrigenous component of riverine organic matter may have already been extensively degraded on land or further upstream and should be relatively resistant to further degradation (Hedge and Keil, 1995). Degradation of in situ POC might become significant for surface sediment δ13C. Results show that POC from both seasons exhibit much lower C/N ratios than surface sediments (Table 3, 5). The contrast is particularly obvious in fluvial areas (e.g. G2, G4), and becomes more similar in marine area (e.g. G6). Similar phenomenon in C/N ratio has been reported by Cifuentes (1991) from the Delaware Estuary, southeast USA. Cifuentes (1991) has also reported differences in δ13C values between the two phases. In the upper estuary, δ13C of POC (−28.9‰) was lower than that of surface sediment (−26.3‰), whilst in the lower estuary, suspended sediment δ13C (−20.1‰) was higher than surface sediment δ13C (−23.5‰) (Cifuentes, 1991). However, changes in δ13C between POC and surface sediment in the Pearl River estuary are not so significant when POC from both summer and winter are taken into consideration. This suggests that differential decomposition of organic detritus may partially explain these observations especially in C/N, seasonality may be more important in the Pearl River estuary, and possibly in the Delaware Estuary, USA (Cifuentes, 1991).
Degradation can have a potentially significant influence on both sediment δ13C and C/N. Early loss of labile material in vascular vegetation can lead to significant shifts in soil δ13C and C/N but is insufficient to prevent the distinction between sediments from C3 and C4 vegetated soil. The degradation of POC in estuarine areas suggests using either proxy in isolation might produce misleading results. It is important to combine δ13C and C/N when indicating sediment sources and environmental changes from coastal areas (Wilson et al., 2005; Zong et al., 2006).
This research is part of the PhD project sponsored by NERC/EPSRC (UK) through the Dorothy Hodgkin Postgraduate Award (to FY). This research is also supported by the University of Durham through a special research grant (to YZ), the NERC (UK) Radiocarbon Laboratory Steering Committee (1150.1005) (to YZ) and the NERC Isotope Geosciences Facilities Steering Committee through the organic isotope analyses awarded (IP/883/1105) (to YZ). We also acknowledge support from the QRA (Quaternary Research Association), the DGGA (Durham Geography Graduates Association), University College of Durham University and the BSRG (British Sediment Research Group) for grants awarded to FY to complete fieldwork and all laboratory visits. This research was also supported by the Research Grants Council of the Hong Kong SAR through research grants HKU7058/06P and HKU7052/08P (to W.W.-S. Yim). The authors also thank the director of the Environmental Protection Department, Hong Kong SRA for the collection of surface sediment samples and water salinity from the Hong Kong area.
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