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Variability of bulk organic δ13C and C/N in the Pearl River delta and estuary, southern China and its indication for sources of the estuarine sediment
Fengling Yua*, Yongqiang Zongb, Jeremy M. Lloyda, Guangqing Huangc, Melanie J. Lengd,e, Christopher Kendrickd, Angela L. Lambd, Wyss W.-S Yimb
a Department of Geography, University of Durham, South Road, Durham DH1 3LE, UK
b Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
cGuangzhou Institute of Geography, 100 Xian Lie Road, Guangzhou 510070, China
dNERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
e School of Geography, University of Nottingham, Nottingham NG7 2RD, UK
*Corresponding author. Tel: +44 (0)191 3341817; Fax: +44 (0)191 3341801; Email address: firstname.lastname@example.org
This paper presents carbon isotope composition (δ13C) and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of δ13C and C/N as indicators for sources of estuarine sediment. In addition to the 91 estuarine surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment as well as mangrove leaves and sediment from the salt marshes; estuarine and marine particulate organic carbon (POC) from both summer and winter. Results show that average δ13C of estuarine surface sediment increases from –25.0 ±1.3‰ from the freshwater environment to –21.0 ±0.2‰ from the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ±0.2. In the source areas, C3 plants have lower δ13C than C4 plants of –29.0 ±1.8‰ and –13.1 ±0.5‰ respectively. δ13C increases from –28.3 ±0.8‰ in the forest soil to around –24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C4 grasses. The δ13CPOC increases from –27.6 ±0.8‰ in the freshwater areas to –22.4 ±0.5‰ in the marine-brackish-water areas in winter, and ranges between –24.0‰ and –25.4‰ from freshwater areas to brackish-water areas in summer. Comparison of the δ13C and C/N between sink and source indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that the bulk organic δ13C and C/N can be used as indicators for sources of the sedimentary organic matter. Organic carbon in surface sediments derived from anthropogenic sources such as human waste, organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated δ13C of sedimentary organic matter due to some agricultural products such as sugarcane, C3 plants are still the dominant vegetation type in this area, and the bulk organic δ13C and C/N is still an effective indicator for sources of estuarine sediments.
Keywords: δ13C, C/N, sediment source, Pearl River, estuary, southern China
Preservation of organic matter in estuarine and coastal sediments is an important process in the global carbon cycle as more than 90% of the carbon buried in the oceans occurs in continental margin sediments (Emerson and Hedges, 1988; de Haas et al., 2002). In estuarine areas, organic matter can be supplied both from autochthonous sources (such as plants growing on the sediment surface) and allochthonous sources (organic material transported predominantly by the tide or a river). Better constraints on the sources of organic matter in marine sediments are needed to understand the processes responsible for its preservation (Mayer, 1994). Such understanding will help estimate the possible contribution of different sources of organic matter to marine organic matter, relating to the global cycle of carbon (Schlunz et al., 1999; Gaye et al., 2007). Bulk organic δ13C and C/N have been widely used to elucidate the source and fate of organic matter in the terrestrial, estuarine and coastal regions (e.g. Hedges and Parker, 1976; Fontugne and Jouanneau, 1987; Goñi et al., 1997; Bianchi et al., 2007; Middelburg and Herman, 2007; Harmelin-Vivien M. et al., 2008; Ramaswamy et al., 2008) and also as proxies in palaeoclimatic studies (e.g. Chivas et al., 2001; Lamb et al., 2006; Zong et al., 2006; Wilson et al., 2005; Mackie et al., 2007).
The Southeast Asian area is one of the most densely-populated areas in the world, as well as one of the most industrialized regions. The contribution of organic carbon from land to the ocean via major fluvial systems in this area has not been well estimated. Some studies have been carried out to assess the contribution of terrigenous organic matter to the South China Sea (e.g. Hu et al., 2006), as well as a number of studies investigating the modern-day organic carbon isotopic signature from the Pearl River delta and estuary (Dai et al., 2000; Chen et al., 2003, 2004; Jia and Peng, 2003; Callahan et al., 2004; Zong et al., 2006). However, a detailed understanding of the range of sources of organic matter and their relative contribution to the bulk sediment organic carbon within an estuary is still rather poorly constrained. Detailed investigation of δ13C and C/N of the organic matter through the full estuarine complex (from freshwater to marine environments) is one way to address this problem. Here we aim to use this technique to establish the full range of δ13C and C/N of the organic matter in the Pearl River estuary from source to sink. We will then assess the suitability of this proxy as an indicator for dominant vegetation type, sediment source and environmental conditions as well as the role of anthropogenic influence on the δ13C and C/N in the Pearl River estuary.
2.1. The Pearl River delta and estuary
The Pearl River delta is located at 21°20´-23°30´N and 112°40´-114°50´E (Fig. 1a), and formed during the last 9000 years (Zong et al., 2009). The Pearl River (Zhujiang) is the general name for the three rivers (the East, the North and the West, Fig. 1a) that flow into the Pearl River delta-estuary before entering the South China Sea. It is the second largest river in China in terms of water discharge (about 330 ×109 m3/year; Hu et al., 2006). The River is 2214 km in length (the West River), and it drains an area of 425,700 km2 (Li et al., 1990). The suspended sediment concentration in the Pearl River is relatively low compared with other major Asian rivers (Zong et al., 2009), with a mean concentration of about 0.172 kg m-3 and an annual flux of about 30.64 106 t (Wai et al., 2004). About 92-96% of the suspended sediment is discharged during the wet season (from April to September, monsoon summer), with maximum river discharge occurring in July. The warm/hot wet season is followed by a cool/cold dry season (monsoon winter from October to March). Approximately 80% of sediment influx to the estuary is deposited within the Pearl River estuary, the remaining being transported to the South China Sea (Xu et al., 1985).
The coastal waters in the vicinity of the Pearl River estuary are influenced by three water regimes: the Pearl River discharge, oceanic waters from the South China Sea and coastal waters from the South China Coastal Current (Morton and Wu, 1975; Yin et al., 2004). These water regimes are subjected to two seasonal monsoons. In winter, the northeast monsoon prevails, the China Coastal Current dominates the coastal waters of Hong Kong, and freshwater discharge is at its lowest. In summer when the monsoon blows from the southwest direction and the Pearl River discharge reaches the maximum, the interaction of the estuarine plume and oceanic waters from the South China Sea dominate the coastal water regime (Yin et al., 2004).
The Pearl River delta is within the tropical climate zone, with mean annual temperature ranging from 14 to 22°C across the basin and precipitation ranging from 1200 to 2200 mm year-1 (Zhang et al., 2008). Subtropical and tropical forests are the dominant vegetation type in the Pearl River catchment (Winkler and Wang et al, 1993). The natural plants in this area are dominantly C3 plants, mixed with some C4 grasses, while sugarcane (a C4 plant) is one of the major agricultural products in the lower deltaic area today.
2.2. Filed data collection
2.2.1. Terrestrial organic matter
Terrestrial organic matter (TOM) includes land plants and soil organic matter (Fig. 1b; Table 1, 2). Plant samples collected include: 1) C4 plants, including general C4 grasses, such as Panicum maximum Jacq. and sugarcane (Saccharum officinarum). As this study also examines δ13C and C/N of agricultural plants, it is necessary to separate sugarcane from other non-agricultural grasses. The ‘general C4 grasses’ are the non-agricultural, naturally-grown C4 grasses in this area; and 2) C3 plants, including general C3 plants (e.g. Pteris semipinnata, Pinus sp.), mangrove (Avicennia, Kandelia obovata, Bruguiera gymhorrhiza, Acanthus ilicifolius, Aegiceras corniculatum, Avicennia marina, Ficus microcarpa) and agricultural plants e.g. banana (Musa acuminata), lotus (Nelumbo nucifera), reed (Phragmites australis) and rice (Oryza sativa). Here, the ‘general C3 plants’ are the non-agricultural, naturally-grown C3 plants in this area. Representative plant organs were collected and initially stored in paper bags until dried at 50ºC over night in an oven. Soil samples were collected from c. 3 cm depth from under the surface (to avoid fresh plant roots and disturbed soil, Liu et al., 2003), and stored in polyethylene tubes in a fridge before sample preparation and analysis. As both C3 and C4 plants grow in the terrestrial area, organic matter in terrestrial soil samples is a mixture of both kinds of plants. Soil samples are clustered into the following groups: forest soil, mangrove soil, riverbank soil and agricultural soil, depending on sample location as well as the dominant vegetation type. Plants and soil samples were collected in June 2006, except for agricultural plants (banana, lotus, reed, rice and sugarcane) and agricultural soil samples which were collected in August 2007.
2.2.2. Estuarine organic material
Estuarine organic material includes seasonal particulate organic carbon (POC; mainly derived from freshwater or marine phytoplankton and terrigenous organic matter) and estuarine surface sediments (Fig. 1b; Table 3, 4; Table 5). A total of 49 POC samples (PE41-92, without PE76-77) were collected in winter (December 2006), and 45 POC samples (PE41-75) in summer (June 2006) including 35 samples on filter paper and 10 samples using a 70 µm net. POC samples were collected by filtering water samples using the fibreglass filter paper (Fisher Brand MF200) in the laboratory. POC samples were also collected using a 70 µm net from some sites in June 2006 (no POC on net was collected during winter due to small size of the boat). Samples were then washed from the net into a sampling tube, and refrigerated prior to analysis. A total of 91 surface sediment samples (PE1-PE92, without PE43; Fig. 1b) were obtained using a grab sampler from a boat. The top 10 cm sediment was collected, which potentially represent sediment deposited during the past 6-10 years, according to the sedimentation rate of around 0.8 cm/year on shoals within the estuary (Li et al., 1990). Samples were sealed in polyethylene tubes and stored in fridge at 2-3ºC before analysis.
A total of three field campaigns were carried out to collect these surface sediment and POC samples. Surface sediment samples PE01-40 were collected by Zong et al. (2006) in June 2005. Surface sediment sample PE41-77 were collected by this study in June 2006, and PE78-92 were collected in December 2006. Summer POC samples PE41-75 were collected in June 2006 and winter POC samples PE78-92 were collected in December 2006.
2.2.3. Estuarine environmental variables
Environmental variables including water salinity (Sal), water depth (WD), pH, total dissolved solid (TDS) and water temperature (Temp), were measured from the same sampling sites as the POC samples (Fig. 1b; Table 3). Summer water salinity and mean water depth from PE78-PE92 were interpolated based on a bathymetry map of the area. Mean water salinity for sites of PE41-PE92 is the mean value of seasonal values measured. Water salinity was measured using the Practical Salinity Scale. Mean water salinity and mean water depth from PE1-PE40, PE76 and PE77 were interpolated based on bathymetry maps of this area. These variables were measured at 50 cm below water surface using a YSI meter.
2.3. Laboratory methods
2.3.1. Sample preparation
Soil and sediment samples were prepared using 100 ml 5% HCl to remove carbonates. They were then washed 3 times with deionised water using the fibreglass filter paper, before being dried down at 50ºC overnight, homogenised in a pestle and mortar, and weighed (25-50 mg) for δ13C and C/N analysis.
Plant samples are placed in a –80ºC freezer overnight followed by freeze drying for 24 hours. These samples were then crushed and homogenised in a ball mill at a speed of 500 rpm for 5-15 minutes (depending on type of plants) and then weighed (1-2 mg) for δ13C and C/N.
POC samples on the filter paper were dried in the field, and were difficult to remove from filter papers. Some were successfully removed by gently scrapping off the filters while others that could not be removed had to be analysed while attached to the filters. The POC on the filters could only be measured for C/N ratios (rather than TOC and TN) as absolute weights could not be measured (Table 3).
2.3.2. Sample analysis
Carbon isotopes and C/N analyses were performed by combustion in a Carlo Erba NA1500 (Series 1) on-line to a VG Triple Trap and Optima dual-inlet mass spectrometer, with δ13C calculated to the VPDB scale using a within-run laboratory standards (BROC) calibrated against NBS-19 and NBS-22. Replicate analysis of well-mixed samples indicated a precision of ±<0.1‰ (1SD). C/N was determined by reference to an Acetanilide standard. Replicate analysis of well-mixed samples indicated a precision of ±<0.1%. The isotopic ratios are expressed as δ13C, in units of per mille (‰).
Organic matter is depleted in 13C relative to V-PDB, so δ13C of organic material is negative. The weight ratio of total organic carbon (%TOC) to total nitrogen (%TN), giving C/N is usually measured along side δ13C and help to distinguish carbon sources.
Particle size analysis was carried out using a laser granular meter (Coulter LS 13200). Values of the δ13C, C/N, particle size and environmental parameters in this paper are presented in the format of ‘average value ±standard deviation (SD)’. Values of δ13C or C/N of some samples are not available due to low content of organic matter in those samples.
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