EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 35, 1095–1102 (2010) Copyright © 2010 John Wiley & Sons, Ltd. Published online 2 June 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/esp.1970
Sedimentary evidence of Late Holocene human activity in the Pearl River delta, China Y. Zong,1* F. Yu,2 G. Huang,3 J. M. Lloyd2 and W. W.-S. Yim1 Department of Earth Sciences, The University of Hong Kong, Hong Kong, SAR China 2 Department of Geography, University of Durham, Durham, UK 3 Guangzhou Institute of Geography, Guangzhou, China
1
Received 1 April 2009; Revised 15 September 2009; Accepted 4 November 2009 *Correspondence to: Yongqiang Zong, Department of Earth Sciences, The University of Hong Kong, Hong Kong, SAR China. E-mail:
[email protected]
ABSTRACT: This study examines the sedimentary evidence of human activities during the last 4000 years in the Pearl River deltaic area. The analyses are focused on indentifying agricultural signatures present in the sedimentary record and establishing the timing of a change from a simple, rice-based agriculture to a more advanced, diverse agriculture. The examination is based on modern sediment and plant samples and a sediment core collected from the deltaic area. The analyses include particle size and diatom analysis to determine the environmental conditions that were associated with the period of human activities. Organic carbon isotope ratios and major metal elements reveal an expansion in commercial crop production and metal smelting in the Pearl River delta area about 2000 years ago. The input of organic matter from introduced sugarcane, a C4 plant, elevates the bulk organic carbon isotope values in the estuarine sediments above that represented by other common agricultural crops in the study area, including rice, banana and lotus, which are all C3 plants. The increase in bulk organic isotopic value coincides with the rise in the concentration of copper, iron and lead in the sedimentary sequence, suggesting a wider use of metal tools. These results indicate that advanced agriculture started about 2000 years ago as an expansion in human population took place in the area. This record also provides sedimentary evidence that help ascertain the timing and type of human activities that are linked to subsequent land reclamation on the deltaic plain, resulting in rapid shoreline advancement in the last 2000 years. Copyright © 2010 John Wiley & Sons, Ltd. KEYWORDS: sedimentary record; human activity; Late Holocene; deltaic landform; Pearl River delta; organic carbon isotopes
Introduction Deltas are fast evolving landforms. Many modern deltas have formed at river mouths that received large quantities of sediment during the Holocene (Woodroffe et al., 2006). The formation and evolutionary history of modern deltas are influenced by a number of driving mechanisms including sealevel change (Stanley and Warne, 1994; Hori and Saito, 2007), water/sediment discharge (Goodbred and Kuehl, 2000; Woodroffe, 2000; Saito et al., 2001; Ta et al., 2002; Tanabe et al., 2006) and human activities (Zong et al., 2009), the latter having transformed the landscape characteristics of many deltaic plains. Human activities in deltas have been intensive because of the highly productive nature of the deltaic wetlands. At present, many deltas are densely populated and form important economic centers for many countries. The Pearl River delta is an example that demonstrates the important influence of human activities in shaping the history of the delta’s landform evolution. In this case, archaeological and historical records suggest an acceleration in shoreline
advance since agriculture in the deltaic area was expanded as a result of an increase in population (Zong et al., 2009). Despite evidence of possible signs of human activity identified in the top meter of core V37 from the mouth area of the estuary (Zong et al., 2006), the sedimentary evidence for the change from a primitive form of agriculture in the late Neolithic and Bronze Age to an advanced form of agriculture during the historical time is generally lacking. In this study we aim to identify sedimentary signatures of various agricultural crops common in the Pearl River deltaic area and, based on sedimentary records, establish the timing for the major expansion of a more advanced, diverse agriculture, and wider use of metal tools in the study area. In this paper, we present results from a sediment core obtained from the Pearl River estuary to show the environmental conditions of the estuarine area during the Late Holocene and evidence of a major expansion in the production of commercial crops, such as sugarcane and metal smelting. The interpretation of core data is supported by analyses of modern sediment and plant samples collected from the deltaic area. This work is part
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Plants & soil samples Core UV1 Surface sediment deltaic distributaries shallow subtidal deep subtidal shallow marine Delta plain Solid outcrop
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Figure 1. (A) The map indicates the location of the Pearl River delta. (B) The map shows locations of modern surface sediment and soil samples, plant samples and the sediment core. The modern surface sediment samples are divided into four groups, the deltaic distributaries (delta plain), shallow subtidal (delta front), deep subtidal (pro-delta) and shallow marine environments.
of a research project that aims to further distinguish the influence of human impacts from the effects of natural processes in deltaic landform changes in the recent past.
The Study Area The modern Pearl River delta comprises the East River deltaic plain and the North and West Rivers deltaic plain (Figure 1A), both having formed during the past 6800 years since postglacial sea level rise stabilized (Zong, 2004; Zong et al., 2009). Between 6800 and 2000 calendar year (cal. yr) BP the delta plains developed in the up-river areas of the receiving basin, but the progradation rate gradually reduced as the summer monsoon started to weaken and freshwater/sediment discharge declined. During the last 2000 years, shoreline advancement accelerated despite continued weakening of the summer monsoon. Such acceleration in shoreline progradation is attributed to human activities (Zong et al., 2009). Throughout the last 2000 years, local people employed a series of techniques to reclaim newly emerging parts of deltaic wetlands for agriculture. One such method involved emplaceCopyright © 2010 John Wiley & Sons, Ltd.
ment of lines of gravel and stone along the low tide mark on a tidal flat, with their height being raised each year. As a result, more and more sediment was trapped behind the ridge of stones, and the ground altitude of the tidal flat rose. Finally, as the land surface of the tidal flat rose to the height above mean tide level, people completed reclaiming the tidal flat by constructing an earth bank (sea wall) on the stone ridge. These active land reclamation activities have resulted in accelerating shoreline advancement (Zong et al., 2009). At present, these two delta plains cover about 5650 km2, separated by the estuary which occupies about 1740 km2 (Figure 1B).
Methods In order to obtain the sediment sequences that may capture signatures of agricultural change taking place on the deltaic plain we collected a drill core (UV1) on the subtidal flat at the mouth area of the estuary (Figure 1B). This pro-delta area saw steady vertical sedimentation during the Late Holocene (Zong et al., 2009), and is away from the two main tidal channels. The delta plain itself was considered unsuitable for this study Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
SEDIMENTARY EVIDENCE OF LATE HOLOCENE HUMAN ACTIVITY IN THE PEARL RIVER DELTA, CHINA
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Table I. The radiocarbon dates for the sediment core
Laboratory code
Depth (m)
Sample type
GZ2211 GZ2212 SUERC-9602 GZ2213 SUERC-9605
0·51–0·52 1·31–1·33 1·91–1·93 2·61–2·63 4·51–4·53
Foraminifera Foraminifera Foraminifera Foraminifera Foraminifera
δ13CVPDB‰ ± 0·1
Carbon content (percentage by weight)
−3·1
9·8
−4·0
8·8
Radiocarbon age (yr BP) 108 2254 ± 3019 ± 2974 ± 3963 ±
30 35 33 35
Central calibrated age (cal. yr BP)a Modern 2010 2930 2870 4150
a
Reported to nearest decade. Note: GZ, Geochemistry Institute of Guangzhou, China; SUERC, NERC (UK) radiocarbon dating laboratory.
due to problems of disturbance of sediments by human activities and reworking by deltaic processes. The core site lies behind the Lantau Island (N22°17′10″, E113°51′49″) and is protected from typhoon storms. Water depth at the coring site is 9 m. The drill core was obtained using a vibracorer with a plastic tube inside the corer to capture and protect the sediment samples from disturbance. The top 0·35 m of sediment was not recovered due to high water content. Between 0·35 m and 5·0 m, the sediment consists of undisturbed very soft to soft, dark greenish grey, silt and clay. The core was sealed at the field site to maintain sediment moisture content. Once the tubes were opened in the laboratory, the sediment cores were scanned using a multi-sensor core scanner for magnetic susceptibility, followed by sampling at 2 cm intervals. The subsamples were analyzed for particle size, microfossil diatoms, major metal concentrations and organic carbon isotopes. To establishing a chronology for the sediment sequence, foraminifera specimens from five depths were taken for radiocarbon dating using the accelerator mass spectrometry (AMS) method. The dates were converted to calendar years using the CALIB5.10 marine04 program (Stuiver et al., 1998) with a correction factor, ΔR − 128 ± 40 years according to Southon et al. (2002). To help determine the environmental conditions and interpretation of the core data, surface sediment samples across the estuary were collected using a grab sampler (Figure 1B). Also collected were soil samples from forest and riverbank sites landward from the delta plain, as well as soil samples from agricultural fields on the delta plain. Plant samples collected for the analysis are common C3 and C4 plants, mangroves, and common agricultural plants from the delta plain, including rice, sugarcane, banana and lotus. These samples were all analyzed for organic carbon isotopes, and a detailed analysis is presented in Yu et al. (submitted for publication). Particle size distribution for the sediment was measured using a laser particle size analyzer, and classified into clay, silt and sand fractions. The technical procedure for diatom sample preparation followed those described by Palmer and Abbott (1986). A minimum count of 300 diatom valves was reached for all samples. Diatoms were identified to species level (e.g. van der Werff and Huls, 1958–1966; Jin et al., 1982) and grouped into three categories, marine water, brackish water and freshwater according to their salinity preferences (e.g. Denys, 1991–1992). Concentration of major metals, including copper, iron and lead, was measured for all samples from the sediment core using an inductively coupled plasma (ICP) mass spectrometer. Total organic carbon and nitrogen from the core samples, surface sediments, soil samples, and plant samples were analyzed using a Carlo Erba elemental analyzer, calibrated through an internal standard. Bulk organic carbon isotope measurements were carried out using a Carlo Erba 1500 online to a VG Triple Trap and Optima dual-inlet mass specCopyright © 2010 John Wiley & Sons, Ltd.
trometer. The δ13C values were calculated to the ViennaPeeDee Belemnite Standard (VPDB) scale using a within-run laboratory standard calibrated against NBS-19 and NBS-22. Replicate analyses of samples gave precision of ± 0·1‰.
Results Based on the five radiocarbon dates (Table I), two chronological models for the sediment core are established, and both suggest a gradual reduction in sedimentation rate towards present (Figure 2). The youngest date at 0·52 m shows a modern age, indicating recent disturbance (possibly trawling) at the top of the core. Such reduction in sedimentation rate is likely to be the result of a large amount of fluvial sediment being trapped in wide expanses of tidal flats under the process of reclamation (Zong et al., 2009). Diatom results show generally little change in water salinity in the past 4000 years (Figure 3). In detail, the data show there is almost no change in water salinity between 5·0 m and 1·5 m of the core, i.e. between 4000 and 2000 years BP. Over the last 2000 years, there was an initial reduction in water salinity, suggested by the lower number of marine diatoms. But salinity has increased since, despite a small increase in freshwater taxa in the top meter of the core. Similar to the diatom results, the particle size results show little change in the upper part of the core (Figure 3). The sediment is composed of mainly silt (>50%) and clay (20–30%), with a varying amount of sand (10–20%). At about 1 m, sand content increases, suggesting a short period of stronger current activity, possibly a storm. As a whole, the sedimentary environment in the coring site has not changed significantly during the last 4000 years. The concentration of a number of metals, including vanadium (V), chromium (Cr), nickel (Ni), zinc (Zn), barium (Ba), magnesium (Mg) and titanium (Ti), changes little throughout the core. According to Woods (2009) in a study from three short cores across the estuary some of these metals, such as Cr, Ni and Zn, have shown significant increases in the estuary since the late 1980s, reflecting the development of light industry in the delta area over the last three decades. Other metals, such as copper (Cu), iron (Fe) and lead (Pb), which are related to more traditional metal usage, have shown increases in the top 1·3 m of the core (Figure 4). The increase of these metals is also supported by the increase in magnetic susceptibility, which shows a significant up-core increase from c. 1·3 m depth (Figure 4). The organic carbon isotope values of the core samples show a steady trend between 5 m and 1·5 m depth (Figure 5), with an average value of −25‰. From c. 1·3 m to the top of the core, a significant change is recorded, with the organic carbon isotope values becoming heavier increasing to an average of −23·3‰. It is also noted that the variability in the δ13C values Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
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Age (Cal.yr BP) 2000 3000
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R2 = 0.9764
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20
40
60
80 100
50 100
50 100
50 100
4.5 5 Age (Cal.yr BP) 0
1000
2000
3000
4000
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Figure 3. The diatom results are presented as percentages of total diatoms in three categories: marine, brackish and freshwater. Results of particle size analysis are expressed as percentages of sand, silt and clay.
0 B 0.5 1 1.5 Depth (m)
2.0 2.5 3 3.5 4 4.5
y = 2E-07x2 + 2E-05x2 + 0.4932 R2 = 0.9951
5 Figure 2. The graph shows the two chronological models for the sediment core. (A) This graph is based on all five dates and fitted with an exponential trend line. (B) This graph is based on four dates and fitted with a polynomial trend line. Both models suggest a gradual decline in sedimentation rate.
is much greater in the top section of the core than in the lower part of the core. Specifically, two samples between 1·00 m and 1·02 m show significantly heavier values of −17‰. The carbon/nitrogen (C/N) ratios do not change much throughout the core, but show a slight increase from c. 1·0 m to the top of the core. Similar to the δ13C values, the two samples between 1·00 m and 1·02 m show abnormal values. These two samples also have an abnormally high percentage of sand (Figure 3). Bulk organic carbon isotope results of the modern sediment, soil and plant samples are summarized in Table II and preCopyright © 2010 John Wiley & Sons, Ltd.
sented in Figure 6. Samples of C4 grasses and sugarcane show very similar δ13C values around −13‰. Samples of C3 grasses and mangroves also show similar δ13C values around −28‰. Both sets of samples have C/N ratios ranging between 10 and 40. The δ13C values of the common agricultural crops, except sugar cane, vary between −26‰ and −29‰, with lower C/N ratios commonly between 10 and 20. Thus, there are two distinctive groups of plant samples according to their δ13C values and C/N ratio (Figure 6A). The soil samples from forested sites show values of δ13C and C/N ratio similar to those of the C3 plant group, including C3 grasses, mangroves and the common agricultural plants. The soil samples from riverbank and delta plain, however, show δ13C values that are more positive than the C3 plant group and more negative than the C4 group (Figure 6A). The δ13C values of the modern deltaic distributaries and the estuary sediment samples fall between the C3 and C4 plant groups (Figure 6B), ranging from −24‰ to −21‰. The C/N ratios of these samples are generally below 15 (Table II).
Discussion The sedimentary environment Recent research indicated that the water salinity in the Pearl River estuary between c. 6800 and 4000 years BP was lower due to the stronger monsoon precipitation and the higher freshwater discharge (Zong et al., 2009). During the last 4000 years monsoon precipitation declined and freshwater discharge reduced. However, the deltaic shoreline has advanced continuously over the past 4000 years (Li et al., 1990). Due to the specific methods of land reclamation, the shoreline advance in the Pearl River delta has accelerated during the last 2000 years (Zong et al., 2009). The coring site has thus become c. 550 km closer to the outlets of the deltaic distributaries. Yet the water salinity has changed little in the mouth area of the estuary over the last 4000 years (Figure 3), Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
SEDIMENTARY EVIDENCE OF LATE HOLOCENE HUMAN ACTIVITY IN THE PEARL RIVER DELTA, CHINA
Depth (m)
20
Cu (mg/kg) 30 40
Fe (mg/kg) 300 400
50
Pb (mg/kg) 40
30
Magnetic Susceptibility (SI) 10 20 30
50
1099
40
0 110
1 2010
2
2870 3
4
4150 5
Figure 4. The concentration of copper (Cu), iron (Fe) and lead (Pb) and the magnetic susceptibility of the core are shown. Samples for copper and iron are taken at 2 cm intervals, while samples for lead are taken at 12 cm intervals. The scanning resolution for magnetic susceptibility is 0·5 cm.
13
Depth (m) -26
-25
-24
-23
C/N -22
-21
-20
0
4
8
12
16
20
0 110 1 2010
2
2870 3
4 4150 5
Figure 5. The organic carbon isotope and C/N ratios for the core are shown at a sampling interval of 2 cm.
reflecting the combined effect of a reduction in freshwater discharge and continuous shoreline advancement (Zong et al., 2009). Both the diatom and particle size data indicate the sedimentary environment at the coring site has been stable and changed little over the past 4000 years. This relatively stable sedimentary environment together with a progressively lowering sedimentation rate (based on the chronology of the Copyright © 2010 John Wiley & Sons, Ltd.
core) suggests that the coring site is suitable for recording the sedimentary signature of human activity taking place in the delta plain. Soils and plants are the main sources of fluvial sediment draining into the estuary. These sediments are mixed with material from a marine source within the estuary before being deposited (Zong et al., 2006). Therefore, recognizing the Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
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Table II. Organic carbon isotopes and C/N ratios of plants and sediment samples from the Pearl River delta region Code A B C D E F G H I J K L M N O P
Sample type
Number of samples
Common C4 grasses (leaves) Sugarcane (leaves) Common C3 grasses (leaves) Rice and reeds (leaves) Banana and lotus (leaves) Mangrove (leaves) Forest soil Riverbank soil Delta-plain agricultural soil Distributaries’ sediment Mangrove flat sediment Shallow subtidal sediment Deep subtidal sediment Marine sediment Upper part of the core (0·3–1·3 m) Lower part of the core (1·3–5·0 m)
6 3 32 5 2 12 3 7 3 26 12 33 24 11 49 186
21·5 30·6 21·6 13·0 15·4 30·5 16·2 12·5 8·9 12·7 11·5 11·6 10·1 7·5 11·3 10·4
-20.0
A
-10.0
δ13C
C/N ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
8·9 8·4 10·8 2·1 4·0 12·1 3·9 2·5 1·1 2·3 5·1 2·5 2·2 1·1 1·7 1·2
−13·2 −12·7 −29·8 −28·9 −26·5 −27·5 −27·4 −23·9 −21·7 −23·8 −24·1 −23·4 −23·1 −21·4 −23·3 −25·0
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0·5 0·2 1·3 0·8 1·1 2·1 1·6 0·8 0·7 1·5 1·9 0·8 0·6 0·5 0·7 0·3
B
B -21.0
A
-15.0
-22.0
C4 plant group I
13
13
-20.0
N
-23.0
M O L
H -24.0
-25.0
E D
-30.0
G
F -25.0
C
J
K P
-26.0 C3 plant group -35.0 0.0
10.0
20.0
30.0 C/N
40.0
50.0
-27.0 5.0
10.0
15.0
20.0
C/N
Figure 6. (A) This graph shows the organic carbon isotope and C/N ratios, with standard deviations, of the modern soil and plant samples (A – common C4 grasses, B – sugarcane, C – common C3 grasses, D – rice and reeds, E – banana and lotus, F – mangrove, G – forest soil, H – riverbank soil, and I – delta-plain agricultural soil). (B) This graph presents the organic carbon isotope and C/N ratios, with standard deviations, of modern surface and core sediment samples (J – deltaic distributaries’ sediment, K – mangrove flat sediment, L – shallow subtidal sediment, M – deep subtidal sediment, N – shallow marine sediment, O – sediment samples from 0·3 to 1·3 m of the core, and sediment samples from 1·3 to 5·0 m of the core).
characteristics of the material from various sources is important in an attempt to interpret palaeo-sedimentary data. As the modern plant data show in Figure 6(A), the dominant organic sources for modern estuarine sediments are from the C3 group of plants, including mangroves, tropical-subtropical trees and common agricultural crops. This group of plants has a common signature of δ13C, which is around −26‰ to −28‰ (Table II). However, another group of plants, the C4 grasses and sugarcane, are also present in the area. C4 grasses are less common in the catchment area of the Pearl River and its delta plain which lie in the tropical and subtropical zone under humid monsoon climate. Sugarcane is native to the region and one of the main crops in the delta plain. This group of plants has a very different δ13C signature, centering on −13‰ (Table II), significantly heavier than the first group. Modern surface sediments from the deltaic distributaries and the estuary have an organic carbon isotopic signature ranging from −21‰ at the marine end to −24‰ in the distributaries. The sediment samples from the marine area (Figure 1B) are unaffected by the terrestrial organic carbon input from the Pearl River (Zong et al., 2006). This group of sediment samples Copyright © 2010 John Wiley & Sons, Ltd.
has a δ13C signature of −21·4‰ (Table II) and a C/N ratio of 7·5, confirming the organic matter in these samples is mainly from marine algae (Fontugne and Jouanneau, 1987; Meyers, 1994). The sediment samples from the deep and shallow subtidal sites as well as the mangrove flats have δ13C values around −23‰ to −24‰ and C/N ratios between 10 and 12. These values suggest a mixture of sources of organic matter, including terrestrial, marine and in situ productivity (Middleburg and Nieuwenhuize, 1998; Lamb et al., 2006; Zong et al., 2006). However, the sediment samples from the head of the estuary and the distributaries have a δ13C value around −23·8‰ and a relatively high C/N ratio, 12·7 (Table II). The C/N ratios from these samples suggest a greater amount of terrestrial input, but the δ13C values are 2–4‰ more positive than the common terrestrial organic matter which has an organic carbon isotope value ranging between −26‰ and −28‰ (C3 group of plants, Figure 5A). Such departure from the norm of the δ13C values for common terrestrial organic matter is possibly a result of the input of organic matter from sugarcane which has a significantly heavier δ13C value averaging −12·7‰ (Table II). Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
SEDIMENTARY EVIDENCE OF LATE HOLOCENE HUMAN ACTIVITY IN THE PEARL RIVER DELTA, CHINA
Evidence of human activity Most common agricultural plants are C3 vegetation and have a δ13C value between −26‰ and −28‰, similar to many other C3 plants, including mangroves (Table II). Thus, it is difficult to distinguish them from the general C3 vegetation based on their organic carbon isotope values alone. C/N ratios are not so useful either because the C/N ratios of these plants are all similar. However, among the common agricultural crops grown in the Pearl River area, sugarcane is the only C4 plant and has a δ13C value around −13‰, significantly heavier than other common agricultural plants. The input of organic matter to the estuary from sugarcane would, therefore, have an effect of increasing the organic carbon isotope values of bulk organic sediment samples. As presented in Figure 6(A) and Table II, the agricultural soil samples have values between those found from sugarcane and other agricultural plants of around −21·7‰. This is because the soil is a mixture of organic matter from both C3 (rice, banana and lotus) and C4 (sugarcane) agricultural plants. Furthermore, because of the input of the C4 (sugarcane) organic matter from the delta plain, the bulk organic samples from the deltaic distributaries, a freshwater environment, have a δ13C value of −23·8‰, much heavier than expected (e.g. −26‰ to −28‰: Thornton and McManus, 1994; Wilson et al., 2005). Therefore, in an area where sugarcane is a common agricultural crop, agricultural activity can be detected from the sediment sequences based on organic carbon isotopes. The organic carbon isotope results from the core show a significant increase in the δ13C values from c. 1·3 m upwards (Figure 5), suggesting an increase in the proportion of sugarcane grown indicating human activity which involved a diverse form of agriculture in the Pearl River delta plain. The average value of δ13C in the upper part of the core is 1·7‰ higher than that of the lower part of the core (Figure 6B and Table II). Such an elevation in δ13C values took place during a period when environmental conditions have changed little (Figure 3). The organic carbon isotope data from the sediment core, therefore, provides clear evidence for a major expansion of commercial crop production, including sugarcane, in the Pearl River deltaic area, i.e. an advanced form of agriculture that started about 2000 years ago. Furthermore, the concentration of copper (Cu), iron (Fe) and lead (Pb) in the sediment core was relatively constant prior to 2000 years ago at a background level of 20 mg/kg, 33,000 mg/kg and 30 mg/kg respectively. These concentrations all increased from about 2000 years ago towards present to levels of about 40 mg/kg, 40 000 mg/kg and 50 mg/kg respectively (Figure 4). These results suggest mining of metal ores in the catchment area and metal tool manufacture and use to aid cultivation in the Pearl River delta area started from about 2000 years ago. This strongly suggests the expansion of the advanced agriculture in the Pearl River delta from 2000 years ago was supported by a metal industry. This result is supported by the pollen data collected from sediment cores in the Pearl River delta (Zheng, 2009, personal communication) and the Song Hong (Red River) delta (Li et al., 2006), where pollen results suggest a significant expansion in rice cultivation soon after 2000 years ago.
Archaeological and historical records The sedimentary evidence presented here identifying developments in agricultural and metal working technologies in the Pearl River delta during the past 2000 years is supported by archaeological and historical evidence. Before c. 2000 years ago, the deltaic region of the Pearl River was occupied by a Copyright © 2010 John Wiley & Sons, Ltd.
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sparse tribal society, loosely organized, without a dominating ruler (Anon, 1999). This society started rice cultivation in the northern fringe of the delta plains around 4000 to 3000 years ago (Zheng et al., 2003), but fishing and gathering were still the dominant activities. According to historical records, infrequent cultural exchanges with the Bronze-age communities in the middle Yangtze already existed in this period (Anon, 1999). By the Qin Dynasty (221–207 BC), a regional authority, called Nanhai (meaning river networks and extensive waters) was established by the central government. The capital of this region was set up at the northern edge of the deltaic lowlands, called Panyu (meaning a village surrounded by saline waters). This ancient capital has evolved throughout the last 2000 years and become the modern city of Guangzhou. Because of wars in north and central China, there was a continuous influx of migrants into the Pearl River deltaic area. The first population expansion took place during the Han Dynasty (206 BC–AD 220; Anon, 1999). This was followed by a major expansion in agriculture on the emerged deltaic plains (Li et al., 1990). The timing of this expansion agrees well with the suggested agricultural and metal working developments identified from the sedimentary record presented here. Subsequently, the population increased dramatically through influx of migrants during the Tang (AD 618–907), Southern Song (AD 1127–1279) and Qing Dynasties (1644–1911; Anon, 1999). Such increase in population resulted in intensifying agricultural activities in the deltaic area, including active land reclamation on tidal flats (Zong et al., 2009). Once an area of the tidal flat was reclaimed, local farmers would plant lotus for a few years, harvesting the lotus seeds, leaves and roots, at the same time as the soils were being desalinized. This was followed by planting sugarcane for a further few years, before the soils were ready for rice cultivation and other commercial cropping, aquaculture and planting mulberry for silk making. The migrants from the north and central China brought with them these cultivation techniques. The sedimentary record from core UV1 presented here shows that an expansion of sugarcane production along with other common agricultural crops took place at the time when human population of the region increased significantly during the Han Dynasty, during which, human activity was concentrated in the deltaic area of the Pearl River basin (Li et al., 1990). Since this time, the structure of the agriculture, with rice as the main crop and lotus, banana and sugarcane as supplementary crops, has not changed significantly (Anon, 1999), despite the continuous expansion in population, land reclamation, agricultural production and the silk industry. In terms of the metal industry, the sedimentary record from the core suggests its development in the Pearl River area is over 1000 years later than that in north and central China. It is likely that the southward migrating people brought the techniques to the Pearl River area and developed an industry to support the expansion of agriculture and land reclamation in the deltaic region.
Conclusions The sedimentary evidence of human activities in the Pearl River delta area has been investigated through examination of surface and core sediment, and plant samples for the organic isotope signatures. Also investigated are the metal concentration and the magnetic susceptibility of the sediment core. The results show a clear, significant change in agricultural activity in the area about 2000 years ago. The sedimentary evidence suggests that the increase in population in the region has not only resulted in intensifying human activity, but also Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
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diversifying the agricultural production, with the addition of sugarcane as a major crop. It also shows the development and increase in metal working technology in the area that may have helped cultivation and land reclamation efforts. This is the first attempt to use a multi-proxy approach to identify the signature of human activities in the Pearl River delta area. Further research is needed to quantify the impacts of human activities in shaping the deltaic landform and changing the deltaic landscape. Acknowledgements—This research is supported by the University of Durham through a special research grant to Zong, a research grant from the National Science Foundation of China (No. 40771218) to Huang and Zong, two research grants from the Research Grants Council of the Hong Kong SAR, China (No. HKU7058/06P and HKU7052/08P) to Yim and a NERC/EPSRC 05-08 (UK) PhD studentship from the Dorothy Hodgkin Postgraduate Award to Yu. This research is also partly supported by radiocarbon dates and organic isotope analyses awarded to Zong by the Natural Environment Research Council (UK) Radiocarbon Laboratory Steering Committee (No. 1150.1005) and the Natural Environment Research Council (UK) Isotope Geosciences Facilities Steering Committee (IP/883/1105). The authors thank the director of the Environmental Protection Department, Hong Kong SAR for the collection of surface sediment samples and water salinity in the Hong Kong area. This manuscript is improved by the detailed, constructive comments from the reviewers.
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Earth Surf. Process. Landforms, Vol. 35, 1095–1102 (2010)
