The History of Water Salinity in the Pearl River Estuary, China, During the Late Quaternary

EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2010) Copyright © 2010 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/esp.2030

The history of water salinity in the Pearl River estuary, China, during the Late Quaternary

Yongqiang Zong,1* Fengling Yu,2,4 Guangqing Huang,3 Jeremy M. Lloyd2 and Wyss W.-S. Yim1 1 Department of Earth Sciences, University of Hong Kong, Hong Kong, SAR China 2 Department of Geography, University of Durham, Durham, UK 3 Guangzhou Institute of Geography, Guangzhou, P.R. China 4 Earth Observatory of Singapore, Nanyang Technological University, Singapore

Received 25 June 2009; Revised 2 March 2010; Accepted 9 March 2010 *Correspondence to: Y. Zong, Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China. E-mail: [email protected]

ABSTRACT: This research reconstructed the Late Quaternary salinity history of the Pearl River estuary, China, from diatom records of four sedimentary cores. The reconstruction was produced through the application of a diatom–salinity transfer function developed based on 77 modern surface sediment samples collected across the estuary from shallow marine environment to deltaic distributaries. The statistical analysis indicates that the majority of sediment samples from the cores has good modern analogues, thus the reconstructions are reliable. The reconstructed salinity history shows the older estuarine sequence formed during the last interglacial was deposited under similar salinity conditions to the younger estuarine sequence, which was formed during the present interglacial. Further analysis into the younger estuarine sequence reveals the interplays between sea level, monsoon-driven freshwater discharge, and deltaic shoreline movement, key factors that have inﬂ uenced water salinity in the estuary. In particular, a core from the delta plain shows the effects of sea-level change and deltaic progradation, while cores from the mouth region of the estuary reveal changes of monsoon-driven freshwater discharge. This study demonstrates the advantages of quantitative salinity reconstructions to improve the quality of reconstruction and allow direct comparison with other quantitative records and the instrumentally observed values of salinity. Copyright © 2010 John Wiley & Sons, Ltd.

KEYWORDS: salinity; diatoms; deltaic shoreline; sea-level change; freshwater discharge

Introduction qualitative approaches in, for example, the North Sea coast (Vos and de Wolf, 1993; Long et al., 1998; Zong, 1998) and the Salinity variability in a given location within an estuary on cen- Paciﬁ c coast (Shennan et al., 1999; Ta et al., 2001; Zong, 1992). tennial to millennial timescales during the Late Quaternary is a However, the development of quantitative methods such as result of changes in several external forcing mechanisms such as transfer functions has greatly improved reconstructions of pal- sea-level change, freshwater discharge and shoreline migration. aeo-salinity and coastal history. The ﬁ rst such attempt using The variability in salinity reconstructed from estuarine sedimen- diatom-based transfer functions was achieved in the Thames tary sequences can help identify interactions between these estuary, which helped reconstruct the water salinity related to external forcing mechanisms and the evolutionary history of an an archaeological site (Juggins, 1992). Subsequently, such estuary (Woodroffe et al., 2006; Zong et al., 2009a). Thus, recon- methods were applied to sea-level reconstructions in a number struction of palaeo-salinity in a river mouth region is an important of estuarine and coastal environments (Zong and Horton, 1999; step towards understanding the sedimentological and hydrologi- Zong et al., 2003; Hamilton et al., 2005; Horton et al., 2007). cal processes operating within an estuary. Furthermore, estuarine The diatom–salinity relationship in the Pearl River estuary has sediments are good archives for climate variability (Zong et al., been tested statistically, and the quality of a diatom-based salin- 2006) and human activities (Chen et al., 2007; Zong et al., 2010) ity transfer function, examined (Zong et al., in press). This paper within a drainage basin. Long-term change of salinity in an presents an application of the transfer function to reconstruct estuary is, thus, important information for understanding the palaeo-salinity from sedimentary sequences which cover history of water cycles and anthropogenic impacts upon the periods of sea-level change in the last glacial cycle and a period environment. This paper aims, therefore, to reconstruct the Late of stable sea level since the mid Holocene. Quaternary salinity history of the Pearl River estuary and assess the effects of millennial scale sea-level change and freshwater discharge on salinity in the river mouth area, evaluating the The Study Area research methods and paving the way towards reconstruction of longer-term changes in salinity on the continental shelf. The Pearl River estuary, located on the south coast of China, Reconstruction of salinity changes within an estuary using drains into the South China Sea (Figure 1A) from its drainage microfossil diatom proxies have mostly been attempted by basin between 26°N and 22°N, a transitional area between Y.Q. ZONG ET AL.

113º00' E 113º30' E 114º00' E A

North River Guangzhou

East River West River

JT81

Shenzhen 22º30' N 23º00' N

BVC UV1 Hong Kong V37

Macau 0km 30 -10 m South China Sea -5 m -20 m 105º E 110º E 115º E B Modern samples Sediment cores C H I N A 25º N Bedrock

North River East River Water depth West River -5 m -10 m V I E T N A M Pearl River delta and estuary -20 m 0km 200 South China Sea 20º N

Figure 1. (A) The Pearl River estuary and locations of the modern surface sediment samples and cores. (B) The Pearl River drainage basin and the south coast of China.

the tropical and the temperate zones (Figure 1B). Three main Table I. Present-day environmental characteristics of the Pearl River rivers drain into the drowned coastal basin and have created estuary two deltaic complexes that are separated by the estuary. Tidal range within the estuary is low, ranging between 1·3 m at the Summer salinity Winter salinity Water depth mouth and 1·9 m at the head of the estuary. Water depth Environment (‰) (‰) (m) varies from ca 4 m within the deltaic distributaries and ca Distributaries 2·1 ± 2·3 7·5 ± 5·5 3·9 ± 2·3 10 m at the mouth area (Figure 1A). Water salinity at the head Delta front 12·7 ± 4·3 21·2 ± 3·6 7·9 ± 5·0 of the estuary and within the lower reaches of the distributaries Pro-delta 25·0 ± 6·0 30·0 ± 3·7 11·1 ± 6·4 is generally low and variable between seasons (Table I). In the Marine 33·8 ± 0·1 33·1 ± 0·2 27·0 ± 3·2 middle part of the estuary, or the delta front environment, water salinity changes greatly between seasons, while in the mouth area, or the pro-delta environment, water salinity appears high all year round. In general, annual variability of (M1) was deposited during the present interglacial (Zong et water salinity is dependent on the amount of freshwater dis- al., 2009a). These two estuarine sequences are well preserved charged (Zong et al., 2006). across the estuary and separated by the younger terrestrial Two terrestrial and two estuarine sediment sequences, sequence (T1), comprising either ﬂ uvial sand/gravel or weath- dating from the Late Quaternary, were deposited in the Pearl ered clay (Zong et al., 2009b). Within the M1 sequence, Zong River estuary. The older terrestrial sequence (T2) lies on et al. (2009b) identiﬁ ed a sub-unit (M1a) which was depos- bedrock and comprises sands and gravels, overlain by the ited during the period of postglacial sea-level rise, preceding older estuarine sequence (M2) which was formed during the the M1 unit, which formed during the period of stable sea last interglacial (Yim, 1994). The younger estuarine sequence level over the last 8000 years (Yim, 1994). These sequences

Table II. Lithology of the four sediment cores

Core BVC (Alt. −7·6 m, N22°20′09″, E114°01′46″)

Depth (m) Description Unit*

0·0 – 0·9 Disturbed sediments M1 0·9 – 2·4 Soft, light brownish grey, silt and clay 2·4 – 8·1 Soft to ﬁ rm, greenish grey to grey, silt and clay 8·1 – 12·3 Firm, grey to brownish grey, silt and clay with small clumps of weathered clay increasing towards the lower boundary M1a 12·3 – 19·9 Firm, grey to brownish grey, silt and clay with plentiful plant fragments M2 19·9 – 21·7 Firm, brown, organic rich sand with clay T2 21·7 – 22·4 Firm, brown to grey, clayey sand with gravel 22·4 – Residual soil of bedrock (granite)

Core V37 (Alt. −1·5 m, N22°15′02″, E113°51′29″) 0·0 – 2·0 Very soft to soft, dark greenish grey, slightly sandy clayey silt M1 (M1a) 2·0 – 10·1 Soft, dark greenish grey clayey silt 10·1 – 10·6 Firm, dark grey, clayey silt with gravel and light brown clay pockets T1 10·6 – 13·4 Firm, light yellowish brown and spotted red silt and clay 13·4 – 16·0 Firm, light grey, silt, sand and gravel with large plant fragments 16·0 – 18·0 Stiff, yellowish brown, mottled light greenish grey, silty clay 18·0 – End of coring

Core UV1 (Alt. −9·0 m, N22°17′10″, E113°51′49″) 0·0 – 10·2 Soft, dark greenish grey, silt and clay M1 10·2 – 10·6 Firm, bluish grey, silt and clay with small gravel and coarse sand T1 10·6 – 23·2 Soft to ﬁ rm, bluish grey, silty clay with occasional shell fragments M2 23·2 – 24·3 Firm, bluish grey, mottled yellow, silty clay T2(?) 24·3 – 35·3 Soft to ﬁ rm, bluish to greenish grey, silty clay ? 35·3 – 35·5 Soft, greenish grey, sandy, silty clay 35·5 – End of coring

Core JT81 (Alt. 0·5 m, N22°56′29″, E113°29′35″) 0·0 – 1·2 Disturbed sediments due to cultivation 1·2 – 14·1 soft, dark grey, silt and clay, with organic rich bands at various depths M1 14·1 – 15·8 Firm, yellowish grey, ﬁ ne sands with silt and clay M1a 15·8 – 18·0 Soft, grey, silt and clay M2 18·0 – 21·3 Firm, yellowish grey, coarse sand T2 21·3 – Residual soil of bedrock (sandstone)

* Geological units defi ned by Yim (1994). provide a good sedimentary archive for palaeo-salinity recon- The Holocene chronology of each core is established by a struction and examination of the effects of sea-level change. number of radiocarbon dates using either bulk organic (for Secondly, the drainage basin is under the infl uence of humid JT81 only) or a benthic, calcareous foraminifera assemblage summer monsoon from the south and dry winter monsoon (dominantly Ammonia beccaii, Elphidium hispidulum and E. from the north and thus, sensitive to monsoon variability. advenum). The radiocarbon dates (Table III) were calibrated Therefore, salinity changes during a period of stable sea level, according to CALIB5.10 (Stuiver et al., 1998) using the i.e. the last 7000 years (Zong, 2004), can be used to refl ect IntCal04 programme for terrestrial organic matter and the the variations of monsoonal freshwater discharge after the Marine04 program for marine organisms with a correction factor of shoreline advancement is considered (Zong et al., factor, ∆R, −128 ± 40 years based on the average radiocarbon 2009a). age of modern corals from Hong Kong (Southon et al., 2002). The relatively low marine reservoir effect is a result of the constant supply to the study area of easterly surface water Materials and Methods which is driven by winter monsoon and Kuroshio Current and rather well equilibrated with atmospheric 14C. This correction The sediment samples factor is the best estimate to date for the reservoir effects on calcareous materials in the shallow marine environment of the Sediment cores were recovered using a push corer for soft and northern South China Sea, and is believed to have changed fi ne sediments and a rotary corer for stiff and coarse sedi- little during the last 7000 cal. years because water depth and ments. Cores JT81 and BVC were selected for analysis to coastal current formation of the study area have changed little reconstruct water salinity history of the upper and lower estua- in the same period. A central calibrated age is given for each rine areas respectively (Figure 1A). The two cores are from date and reported to the nearest decade. To estimate the age sites where both the M1 and M2 units are preserved (Zong et uncertainty for the sediment samples analysed between dated al., 2009b). Cores UV1 and V37 are both from the pro-delta depths, a method proposed by Heegaard et al. (2005) was area, where continuous sedimentary accretion was recorded adopted. This involves running a free, open-source statistical (Zong et al., 2009a), thus they are suitable for reconstruction package R (Parnell et al., 2008). This method takes account of detailed Holocene salinity history. The lithology of these of the uncertainty in the dated and calibrated ages and, at the cores is described in Table II. same time, provides estimates of the uncertainty on the

Table III. Radiocarbon dates from the four sediment cores

Conventional age Calibrated age Central cal. age Laboratory Core Depth (m) Material dated Method (years BP) (years BP) (1σ) (years BP)1 code

BVC 3·0 Foraminifera AMS 14C 4191 ± 30 4518–4386 4450 GZ2210 BVC 7·5 Foraminifera AMS 14C 6973 ± 34 7633–7531 7580 GZ2209 BVC 8·8 Foraminifera AMS 14C 8071 ± 34 8784–8591 8690 GZ2208 V37 2·0 Foraminifera AMS 14C 3470 ± 40 3564–3420 3490 Beta193746 V37 2·9 Foraminifera AMS 14C 4330 ± 40 4725–4555 4640 Beta193747 V37 7·0 Foraminifera AMS 14C 7020 ± 40 7666–7565 7620 Beta193748 V37 9·7 Foraminifera AMS 14C 7970 ± 40 8631–8474 8550 Beta193749 UV1 0·5 Foraminifera AMS 14C modern GZ2211 UV1 1·3 Foraminifera AMS 14C 2254 ± 30 2082–1943 2010 GZ2212 UV1 1·9 Foraminifera AMS 14C 3019 ± 35 3009–2851 2930 SUERC9602 UV1 2·6 Foraminifera AMS 14C 2974 ± 33 2942–2800 2870 GZ2213 UV1 4·5 Foraminifera AMS 14C 3963 ± 35 4229–4060 4150 SUERC9605 UV1 7·5 Foraminifera AMS 14C 4847 ± 35 5412–5274 5340 SUERC9606 UV1 9·5 Foraminifera AMS 14C 5633 ± 36 6259–6129 6190 SUERC9607 JT81 3·9 Bulk organic Conv. 14C 1310 ± 65 1299–1225 1260 KWG-693 JT81 5·9 Bulk organic Conv. 14C 2430 ± 90 2519–2359 2440 KWG-690 JT81 10·7 Bulk organic Conv. 14C 3840 ± 95 4416–4154 4290 KWG-700 JT81 14·9 Plant fragment Conv. 14C 7390 ± 140 8351–8157 8250 KWG-890

1 Dates are expressed as central calibrated ages to nearest decade.

interpolated age estimates for each sediment sample between aging with partial least squares (WA-PLS) is preferred (ter the dated depths. Thus a complete chronology for the sedi- Braak and Juggins, 1993). Due to the geographical distribution ment cores was generated with Monte Carlo tests and pre- of modern diatom samples, some spatial autocorrelation is sented in the form of 95% highest posterior density range likely present in the dataset. Such autocorrelation may cause (HDR) and a mean (50%) curve (Parnell et al., 2008). unrealistically optimistic estimates of transfer function perfor- In order to develop a diatom-based water salinity transfer mance (Telford and Birks, 2005). We applied the graphical function, 77 modern surface sediment samples were collected method developed by Telford and Birks (2009) to investigate using a grab sampler from the marine environment southeast the infl uence of spatial autocorrelation on the dataset. This of Hong Kong through the estuary to deltaic distributaries procedure compares cross-validated transfer function perfor- (Figure 1A), along with salinity and water depth measured mance when a subset of sites is deleted randomly (the most during sampling (Zong et al., in press). The modern surface common method of evaluation) against performance when samples represent sediments deposited in the past 6–10 years samples are deleted geographically or based upon environ- according to the sedimentation rates of around an average mental similarity. The result of removing samples at 5 km 1·8 mm year−1 on the shoals within the estuary (Li et al., 1991) intervals from 0–50 km (the maximum distance between any and generally below 1·0 mm year−1 from the area of Hong two samples in the study area was approximately 110 km) was Kong (Owen, 2005). investigated. Once the transfer function was developed, we assessed the precision of the reconstruction using the calibration program Laboratory and statistical analyses C2 (Juggins, 2004) with Monte Carlo simulations using leave- one-out cross-validation (Table IV) to estimate realistic sam- Both modern and core sediment samples were prepared for ple-specifi c errors of prediction for all inferred values from diatom analysis using standard methods (Zong et al., 2006). fossil diatom assemblages (Birks, 1995, 1998). To ensure Diatom taxa were identifi ed under a light microscope with accurate reconstruction, the reconstructions were validated by 1000× magnifi cation and minimum 300 valves per sample comparing the fossil diatom data with the modern diatom were counted. The counts were converted into percentages of data, i.e. whether or not there is a good modern analogue for total diatom valves for each assemblage. The diatoms found each fossil sample used for reconstruction, using a method from each sample are likely to be a mixture of locally pro- called ‘goodness-of-fi t’ (Birks, 1998) which applies two simple duced taxa and transported taxa by tidal current (Zong, 1997; but useful measures of reconstruction reliability: the percent- Horton et al., 2007). However, locally produced taxa tend to ages of the total fossil assemblages that consist of taxa that are dominate diatom assemblages found in each sediment sample not represented at all in the calibration dataset (1) and that are (Zong, 1998; Zong et al., 2006). poorly represented (e.g. <5 % occurrences) in the calibration In order to develop a diatom-based transfer function, we dataset (2). fi rst tested the relative strength of relationships between the modern diatoms and the environmental variables (salinity, water depth and sand content) using canonical correspon- Results and Interpretations dence analysis (CCA) based on the CANOCO 4.5.1 program (Ter Braak and Smilauer, 1997–2003). We then applied Fossil diatoms from the sediment cores detrended canonical correspondence analysis (DCCA) to determine whether or not the modern diatom data set was Cores BVC and JT81 reached the bedrock and captured both unimodal. If it is proven to be unimodal, a diatom-based salin- Late Quaternary estuarine sediment sequences, including the ity transfer function using regressions known as weighted aver- sub-unit (M1a) (Table II). However, there is a hiatus above the

Table IV. Apparent and jack-knifed errors of estimation and prediction for the diatom-based water salinity transfer function produced by the calibration programme C2 (Juggins, 2004)

Component RMSE (‰) RMSEP (jack ‰) Apparent (r2) Prediction (jack r2) Jack_Ave_Bias Jack_Max_Bias

1 2·52 2·66 0·946 0·940 −0·046 3·281 2 1·70 1·96 0·975 0·967 0·039 1·841 3 1·43 1·80 0·983 0·973 −0·135 1·600 4 1·29 1·68 0·986 0·976 0·043 1·611 5 1·16 1·63 0·989 0·977 −0·035 1·202

M2 unit in these sediment cores, and in both cases the T1 unit age of marine diatoms increases, characterised by the rise in is missing. Core V37 did not reach the bedrock and only number of Thalassionema nitzschioides (Figure 2), indicating penetrated the younger terrestrial and younger estuarine a more saline environment. sequences. Core UV1 did not reach the bedrock too, but In core UV1, the diatom assemblages are well preserved penetrated into or through the older estuarine sequence (Table and diverse, but only confi ned to the M1 sedimentary unit II). Diatoms are recovered from both estuarine sequences of (Figure 3). The assemblages are dominated by marine and cores BVC and JT81, while in cores V37 and UV1 diatoms are brackish water planktonic diatoms, particularly Cyclotella only found in the younger estuarine sequence. striata, Cyc. stylorum and Paralia sulcata, which are com- In core JT81, the diatom assemblages are dominated by monly found at present in the central and outer parts of the both brackish water and freshwater taxa (Figure 2). Within the estuary (Zong et al., in press). From the base to the top of the M2 section, diatoms are poorly preserved. Only two species section, subtle changes in the assemblages can be identifi ed, (Coscinodiscus divisus and Aulacoseira granulata) appear in i.e. the percentage of both marine and freshwater diatoms high numbers, and the section indicates an estuarine environ- increase slightly, particularly marine species Chaetoceros ment. Diatoms in the M1a section of the core are much better radians and Thalassionema nitzschioides as well as freshwater preserved, and the assemblages are characterised by the domi- species Synedra acus and Syn. ulna. This change suggests that nance of Aulacoseira granulata. This freshwater planktonic marine infl uence on the site became progressively stronger at species appears in high numbers in tidal channel environ- the same time as the site became closer to the sources of ments of the present-day inner estuary (Zong et al., 2006). freshwater. Given the sandy nature of the deposit, the M1a section is Finally, core V37 covers both the M1a and M1 sequences considered to represent a tidal channel environment close to and records a diverse and well preserved diatom assemblage the mouth of deltaic distributaries. The diatom assemblages in (Figure 3). The marine planktonic species (Paralia sulcata and the M1 section comprise diverse, dominantly brackish water Thalassionema nitzschioides) decrease in number upwards in planktonic taxa, including Coscinodiscus divisus, Cos. blandus, the M1a section, giving way to the brackish water planktonic Cyclotella striata and Cyc. stylorum. These four taxa are found taxa (Cyclotella striata, Cyc. stylorum, Coscinodiscus divisus in the central part of the estuary at present (Zong et al., in and Cos. blandus) commonly found in the central part of the press). In this section of the core, freshwater diatoms are all Pearl River estuary (Zong et al., in press), indicating a change in low number. Near the top of this section, however, the from marine to estuarine conditions. These taxa, particularly frequency of freshwater diatoms increases, indicating stronger Cyclotella striata and Cyc. stylorum, continue to dominate the freshwater infl uence from 3 m up core (Figure 2). The diatom M1 section, i.e. an estuarine environment. At the top of the data therefore suggest a brackish water environment that core, the marine species, Thalassionema nitzschioides, existed during the last interglacial (M2). Based on the radio- increases as other taxa decrease or change little, suggesting a carbon dates, the diatom data can be summarised as: the site more saline environment. changed from deltaic distributaries’ channel in the early Holocene to an estuarine environment in the middle to late Holocene, and fi nally a freshwater environment as a result of Validation for quantitative reconstructions deltaic progradation. Diatoms are very well preserved in the sedimentary The diatom-based salinity transfer function was subjected to a sequences of core BVC. At the base of the core, the T2 section series of statistical tests that suggest accurate reconstructions are is characterised by a group of benthic diatoms (Figure 2), with possible (Zong et al., in press). First, the CCA shows that water one brackish water species (Achnanthes brevipes) and three salinity is the most important environmental factor, and the freshwater taxa (Cymbella affi nis, Fragilaria pinnata and associated Monte Carlo permutation tests indicate that water Synedra ulna), suggesting a dominantly freshwater environ- salinity accounts for a signifi cant portion of the total variance ment. In the M2 section, the diatom assemblages are much in the diatom data (P < 0·005 with 199 random permutations), more diverse, and composed of marine, brackish and fresh- while both water depth and sand content gradients are not water taxa. These mixed assemblages are characterised by the signifi cant (P > 0·030 with 199 random permutations). Secondly, brackish water planktonic Coscinodiscus divisus and the fresh- the DCCA result suggests that the primary axis is highly corre- water benthic Synedra ulna, suggesting shallow estuarine lated with water salinity (the weighted correlation, r = 0·97) and environments close to freshwater sources. The M1a section is represents 85·7% of total variance in the diatom–environment dominated by marine planktonic Paralia sulcata, a marine relationship. It also suggests that the diatom distribution along species that can tolerate lowered salinity in an estuarine envi- the salinity gradient is unimodal, and accordingly a WA-PLS ronment (Zong, 1997). Nevertheless, this section seems asso- transfer function model is developed. The transfer function ciated with a strong marine infl uence. The M1 section of the relating diatom assemblages and water salinity has high correla- core is composed of a group of marine and brackish water tion (r2 as 0·94–0·98 for WA-PLS) and relatively low root mean diatoms, with Cyclotella striata, Cyc. stylorum and Paralia squared error for prediction (RMSEP as 2·66–1·63‰ for WA-PLS) sulcata dominating. Towards the top of the core, the percent- (Table IV). When the effect of autocorrelation is considered, r2

s us rant Core JT81 lata m blandu sa mulata loru o alt tole er r sty ira granu radi er s a e lla ell e ia subha Depth (m) ot ac rine watckish water yclotella meneghiniana ycl ra reshwate Coscinodiscus divis CoscinodiscusCyclotella striata Cyclot C Aulacos C Fall Ma B FreshwatF 0 1 2 3 1260 4 5 2440 6 7 8 M1 9 10 4290 11 12 13 14 M1a 8250 15 16 17 18 M2 19 20

20 40 60 80 20 20 40 60 20 40 20 20 40 60 80 20 20 20 40 60 80 100 20 20 40 60 80

Core BVC t ran sus e ipes ol nitzschioides ta t r t iata ta s divi e t a us blandus ffinis a r t sal str scu a ln te a r en onema ulc i pinna u a w ter te er la s a fasciculata thes brevlla ria h a ol tel r n a ra e w is wa w t Depth (m) lo cinod be in k h h n m gil ned r c s lt i aralia os y y a a a ThalassiCyc Cyclotella stylorum P Syned CoscinodiscC Achna C Fra S M Br Fre Fres S 0 1 2 4450 3 4 M1 5 6 7 7580 8 8690 9 10 M1a 11 12 13 14 15 16 M2 17 18 19 20 T2 20 20 20 40 60 80 20 40 60 20 20 20 40 20 20 20 20 20 40 60 20 40 60 80 100 20 20 40 60 80 20

Figure 2. Diatom diagrams showing the major diatom taxa (over 10% abundance) for cores JT81 and BVC. Radiocarbon dates are expressed as central calibrated ages to nearest decade.

decreases to 0·90 and RMSEP increases to 5·41‰. Thus the recent fossil samples. The results show the correlation coef- 2 diatom–salinity relationship remains appropriate for recon- fi cient (r jack, Table IV) is suffi ciently high, and the random structing palaeo-salinity. mean squared-root of errors for prediction (RMSEPjack, Table Before the transfer function is applied to the fossil diatom IV) is reasonably low. The second method is to measure the records for palaeo-salinity reconstructions, it is necessary to reconstruction reliability by calculating the goodness-of-fi t evaluate the suitability of the transfer function for the fossil between the fossil assemblages and the calibration dataset data from the sediment cores. There are a number of means (Birks, 1998). The results of these two calculations for each of numerical evaluation. The fi rst method is to compare the fossil assemblage are presented in Figure 4 for the four sedi- reconstructions against known recorded recent past environment cores analysed. mental records (Birks et al., 1995). This is carried out by In core JT81, the percentages of the total fossil assemblages computing the correlation coeffi cient between the observed that consist of diatom taxa absent from the calibration data set values and the model-predicted values and the potential errors are all very low, i.e. all below 1%. Similarly, less than 10% for prediction with the modern samples being used as the most of the total fossil assemblages consist of taxa that are poorly

Core UV1 m u sp. s a ter r a te la striata a uln Depth (m) r ish water k hwa rine w a Paralia sulcata ChaetocerosThalassionema radiansAnomoeoneisCoscinodiscus nitzschioides Coscinodiscus blandus Cyclotel divisus Cyclotella stylor SynedraSyned acu M Brac Fres 0

1 2010

2 2870 3

4 4150 5 M1

7 5340 8

9 6190 10 T1

20 20 20 20 20 40 20 40 20 40 20 40 60 80 20

ioides h Core V37 us nd m p. bla divisus ta s a i r ter eis cus e s str styloru r onema nitzsc di la la te Depth (m) i l l wat inodiscus te e wa cino ote n h s l clo ckish wa o yc ari a res Paralia sulcata Thalass AnomoeonC Cosc C Cy M Br F 0

3490 2

4640 3 M1

7620 7

8 M1a

9 8550 10 T1 11 20 40 20 20 20 20 40 60 20 40 20 40 60 20 40 60 80 100 10

Figure 3. Diatom diagrams showing the major diatom taxa (over 10% abundance) for cores UV1 and V37. Radiocarbon dates are expressed as central calibrated ages to nearest decade.

represented (e.g. <5% occurrences) in the calibration dataset. reliable reconstruction for these samples is possible. In In other words, the fossil diatom dataset of core JT81 matches summary, the majority of samples from the four sediment closely the modern diatom calibration data set, so the recon- cores have good modern analogues, and thus reliable reconstruction using the calibration data set for core JT81 can be structions are realistically possible. considered reliable. Similar measurements for cores BVC, UV1 and V37 are shown in Figure 4. In these cores, most samples have less than 4% diatoms from taxa absent from the Quantitative reconstruction of calibration dataset, and less than 10% diatoms from taxa palaeo-salinity history poorly represented in the calibration dataset. The lowest four samples from core BVC and the top and bottom three samples The chronology of the Holocene section of these cores is of core V37 are higher than 10%, suggesting a less-than- established by age models based on a number of radiocarbon

Core JT81 Core BVC Depth (m) Depth (m) A B A B 0 0 1 1 2 2 3 4450 3 1260 4 4 M1 5 5 2440 6 6 M1 7 7 7580 8 8 9 8690 9 10 10 M1a 4290 11 11 12 12 13 13 14 M1a 14 8250 15 15 16 16 M2 17 17 18 M2 18 19 19 20 20 T2 210 2468 1020304050 Core UV1 Core V37 Depth (m) Depth (m) A B A B 0 0

1 1 2010 2 3490 2 M1 2870 4640 3 3

4 4 4150 5 5 M1 6 6 7620 7 7 5340 8 8 M1a 9 9 8550 6190 10 10 T1 T1 11 24 10 21020

Figure 4. Graphs showing the reconstruction reliability according to a method called ‘goodness-of-ﬁ t’ (Birks, 1998) between the fossil diatom data from the four sediment cores and the modern calibration diatom data set (for details see Zong et al., in press). Column A: the percentages of total fossil assemblages that consist of taxa not represented at all in the calibration data set. Column B: the percentages of total fossil assemblages that are poorly represented (e.g. <5 % occurrences) in the calibration data set.

dates on each core (Table III). To estimate the age uncertainty 9500 and 10 000 cal. years BP according to the sea-level for each sediment sample analysed between dated depths, the history of the region (Zong, 2004). age model for each core is calibrated according to Parnell et The reconstructed salinity for cores BVC and JT81 is pre- al. (2008) and presented as an age–depth plot (Figure 5). The sented in Figure 6, which illustrates the Late Quaternary results show that the chronology of both cores UV1 and V37 history of water salinity in the Pearl River estuary. The lowest is well constrained, while the lower parts of cores JT81 and four sediment samples of core BVC are from the organic-rich BVC are less well established. In particular, the age uncer- sand and clay (Table II) with diatom assemblages dominated tainty of sediment samples between 8·9 m and 12·5 m of core by freshwater species (Figure 2). Although the salinity recon- BVC is too large, and thus the chronology of this section of struction for these samples is possibly less reliable (Figure 4), the core must be taken with great caution. Given the lowest the reconstructed salinity values of 10‰ or lower (Figure 5) sample of this section was collected at −19·9 m (Table II), the seem reasonable, representing a terrestrial environment close sedimentation of this section is considered to start between to sea level. The reconstructed salinity for the M2 unit of core

Core JT81 Core BVC 0 0 Modal age (n=28) Modal age (n=48) 2 95% HDR 2 95% HDR Unrestricted calibrated dates 4 Unrestricted calibrated dates 4 6 6 8

Depth (m) Depth (m) 8 10 10 12

14 12

16 14 0 2000 4000 6000 8000 10000 0 5000 10000 15000 20000 25000 Age (cal. yr BP) Age (cal. yr BP)

Core UV1 Core V37 0 0

2 2

4 4

6 6 Depth (m) Depth (m) 8 8 Modal age (n=86) Modal age (n=39) 10 95% HDR 10 95% HDR Unrestricted calibrated dates Unrestricted calibrated dates 12 12 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 12000 Age (cal. yr BP) Age (cal. yr BP)

Figure 5. The 95% highest posterior density ranges (HDR) for the Holocene section of the four sediment cores indicate the uncertainty of the ages assigned to each samples between the dated depths, together with the mean modelled age for each sample. In each graph, the number of samples involved in the calibration are shown as ‘n’. The modelled age curves for the cores are established based on the 50% chronology via Bchron (Parnell et al., 2008).

BVC varies between 20‰ and 25‰, comparable with modern salinity was at its lowest at both cores V37 and UV1, both delta-front to pro-delta environments (Table I). were around 22‰ or delta front conditions. Water salinity The M1a unit in core BVC contains high percentages of then rose with fl uctuations gradually towards the present, as marine planktonic Paralia sulcata (Figure 2), and shows salin- indicated specially by the increases of marine planktonic ity around 30‰, which is similar to modern outer estuarine Chaetoceros radians and Thalassioname nitzschioides in both environments. The same unit in core JT81, however, is associ- cores, a change back to pro-delta environments. ated with dominant freshwater planktonic Aulacoseira granulata and a salinity <10‰, suggesting tidal channel conditions between distributaries and delta front environments. In the M1 Discussion unit of core BVC, the reconstructed salinity fl uctuates between 20‰ and 25‰ and increases slightly towards the top of the The Late Quaternary palaeo-salinity history core, while in the same unit in core JT81 the salinity varies between 15‰ and 20‰. In other words, the M1 units in the The above results show that the history of palaeo-salinity in two cores represent a delta front environment (JT81) and a the Pearl River estuary is clearly related to glacial to intergla- pro-delta environment (BVC), respectively. During the past cial changes in sea level. As demonstrated in cores BVC and 2000 cal. years, the salinity at core JT81 decreased signifi - JT81, brackish water conditions were established during both cantly, indicating a change from estuarine to distributaries periods of sea-level highstands (i.e. M2 and M1). In both cores environments. the composition of the diatom assemblages for the M2 unit is For cores V37 and UV1, the reconstructed salinity is shown somewhat different from that of the M1 unit (Figure 2), indicat- in Figure 7. For the fi rst three millennia of the Holocene, the ing different local conditions. For instance, the M2 unit in core reconstructed salinity as shown in core V37 suggests a declin- BVC contains some benthic freshwater diatoms, yet the M1 ing trend from about 30‰ to around 20‰, as refl ected in the unit has few benthic diatoms (Figure 2), i.e. the former belongs reduction in the proportion of marine planktonic Paralia to a shallower environment than the latter. Nevertheless, both sulcata in the M1a unit (Figure 3), i.e. a change from marine units represent estuarine conditions with salinities around to delta front environments. From 7000 to 6000 cal. years BP, 20‰ to 25‰ (Figure 6).

Core BVC Core JT81 Salinity (‰) Salinity (‰) 010203040051015202530 0

2000

4000 M1

6000

M1a Age (cal. yr BP) 8000

M2 17 10000 18 M1a 19 Depth (m)

12000 20

M2 16

18 Depth (m)

20 T2 21

Figure 6. Reconstructed paleo-salinities for cores BVC and JT81 plotted with error ranges with boxes indicating common errors estimated by the WA-PLS model and thin solid bars showing additional errors due to autocorrelation. The age model of the Holocene section of these cores is established using the method suggested by Heegaard et al. (2005) and Parnell et al. (2008). Note the large age uncertainty for the M1a section of core BVC (Figure 5).

The Holocene history revealed from the sediment cores decline as recorded in the M1a unit of cores V37 indicates interplays between several factors. By about and BVC. 8200 cal. years BP, relative sea level rose to ca −12 m (Zong, During the middle Holocene, sea level was stable (Zong, 2004) and caused marine inundation in the area distal to river 2004) and deltaic shoreline advanced (Zong et al., 2009a). In mouths (i.e. cores V37 and BVC) where water salinity reached theory, water salinity should have decreased as core sites were 30‰ (Figures 6 and 7). Around the same time, the sea getting closer to the freshwater sources. However, diatoms advanced up-estuary, and the area proximal to river mouths from all four cores suggest a slight increase in salinity instead (i.e. around core JT81) changed from delta front tidal channel (Figures 6 and 7). In comparison, water salinity at core JT81 to an estuarine environment (Figures 2 and 6). As sea level was about 5‰ lower than that at cores UV1, V37 and BVC. rose further and reached the present height around 7000 cal. During the late Holocene, the deltaic shoreline advanced years BP, the deltaic basin was formed with river mouths rapidly due to human activity (Zong et al., 2009a, 2010). The retreated to their most landwards positions (Zong et al., decrease in water salinity in core JT81 about 2000 years ago 2009a). Coincidently, this is a period of strong monsoon (Figure 6) suggests the site had changed from a delta front (Hodell et al., 1999; An et al., 2004) and high freshwater environment to deltaic distributaries. During the same discharge (Zong et al., 2006), and resulted in water salinity period, however, water salinity in the area of cores UV1,

Core UV1 Core V37 Salinity (‰) Salinity (‰) 20 22 24 26 28 30 20 24 28 32 0

1000

2000

3000

4000 Age (cal. yr BP) 5000

6000

7000

8000

9000

Figure 7. Reconstructed paleo-salinities for cores UV1 and V37 plotted with error ranges with boxes indicating common errors estimated by the WA-PLS model and thin solid bars showing additional errors due to autocorrelation. The age model of the Holocene section of these cores is established using the method suggested by Heegaard et al. (2005) and Parnell et al. (2008).

V37 and BVC remained high and even increased (Figures 6 BVC. During the period of stable sea level since the middle and 7). Holocene, water salinity at these sites has gradually increased, The above mentioned results suggest that shoreline move- despite the advance of the deltaic shoreline. This increased ment of the deltaic plain seems to have little impact on water salinity is likely the result of a reduction in summer monsoon salinity at sites distal to freshwater sources, e.g. UV1, V37 and precipitation (Wang et al., 2005) and a consequent decrease

Copyright © 2010 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms (2010) Y.Q. ZONG ET AL. in freshwater discharge from the Pearl River (Zong et al., deltaic shoreline movement. Such quantitative reconstruction 2006). It also suggests the effect of shoreline advance was has great advantages over the traditional qualitative recon- weaker than that of the decreasing freshwater discharge. struction, and makes direct comparisons between the salinity Because the salinity of these distal sites is sensitive to changes data derived from fossil data and the instrumentally observed of monsoon-driven freshwater discharge, the small peak in values of salinity possible. water salinity around 8200–8000 cal. years BP recorded by core V37 (Figure 7) may refl ect the effects of a short period of Acknowledgements—This research is supported by a research grant rapid rise in sea level (Yim et al., 2006) or a period of climate from the National Science Foundation of China (No. 40771218) to cooling and thus a weakened summer monsoon (Wang et al., Huang and Zong, two research grants from the Research Grants 2005). For the same reason, water salinity around 4300– Council of the Hong Kong SAR, China (No. HKU7058/06P and HKU7052/08P) to Yim and a NERC/EPSRC 05-08 (UK) PhD student- 4000 cal. years BP from UV1 appeared a little high, and it is ship from the Dorothy Hodgkin Postgraduate Award to Yu. This possibly associated with the short period of climate cooling research is also supported partially by four radiocarbon dates awarded revealed by Wang et al. (2005). by the Natural Environment Research Council (UK) Radiocarbon Laboratory Steering Committee (No. 1150.1005). The authors thank the director of the Environmental Protection Department, Hong Kong Qualitative and quantitative reconstructions SAR for the collection of surface sediment samples and water salinity in the Hong Kong area. The majority of samples from the sediment cores have good modern analogues, indicating that the diatom-based salinity transfer function produces reliable quantitative reconstructions. Such successful quantitative reconstruction provides a References great advantage in understanding the history of environmental An C, Feng Z, Tang L. 2004. Environmental change and cultural change in the Pearl River estuary, paving the way towards responses between 8000 and 4000 cal. yr BP in the western Loess making comparisons with environmental changes for the past Plateau, northwest China. Journal of Quaternary Science 19: century as recorded by instruments. Such quantifi cation of 529–535. environmental parameters, with the available monsoonal Birks HJB. 1995. Quantitative palaeoenvironmental reconstructions. climate data, will help also the development of numerical In Statistical Modelling of Quaternary Science Data, Maddy D, Brew models for forecasting future changes in the estuarine environ- JS (eds). Technical Guide No. 5, Quaternary Research Association: ment. 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