Late Quaternary palaeoenvironmental changes documented by microfaunas and shell stable isotopes in the southern Pearl River Delta plain,South China

Liu Chunlian, Franz T.Fürsich , Wu Jie, , Dong Yixin, Yang Tingting, Yin Jian

1.Department of Earth Sciences, Sun Yat-Sen University, Guangzhou 510275, China

2.Geozentrum Nordbayern, FG Pal?oumwelt der Friedrich-Alexander-Universit?t Erlangen, Loewenichstr.28, D-91954 Erlangen, Germany

3.Guangdong Provincial Key Laboratory of Mineral Resources and Geological Processes, Guangzhou 510275, China

1 lntroduction

The estuary area is a transitional zone between land and sea, with highly variable water conditions (especially salinity f l uctuations), to which most marine and freshwater organisms cannot adapt.Among the microfauna, ostracods and, to a far lesser extent, foraminifers are the most important representatives in estuarine environments due to their wide ecological tolerances.These two groups have proven to be essential for reconstructing Quaternary brackishwater environments (e.g.,Ingram, 1998; Mazziniet al.,1999; Anadónet al., 2002; Boomer and Eisenhauer, 2002;Boomeret al., 2003, 2005; Frenzel and Boomer, 2005).The analysis of the composition, species distribution and geochemical characteristics of ostracod and foraminiferal assemblages can provide valuable information on the physical and chemical changes of the water body which should be affected by sea level variations and climatic factors.

Over the past years, some studies have investigated sea-level and environmental changes during the Late Quaternary in the Pearl River Delta, and in some of them microfossils were mentioned as supportive evidence (Huanget al., 1982; Xuet al., 1986; Maet al., 1988; Fanget al.,1991; Lan, 1991, 1996; Liet al., 1991; Zonget al., 2009).However, only sporadic studies on Quaternary microfaunas from the Pearl River Delta plain and nearby coastal area have been carried out (Zhaoet al., 1987; Yim and He,1991; Cao, 1998; Huang and Yim, 1998; Wang, 1998; Liuet al., 2008).As most of these studies were based on very sparse sampling and chronostratigraphically are poorly constrained with sporadic dates, they could not provide evidence for recognizing short-term environmental f l uctuations.Very little attention has been paid to detailed quantitative microfossil analysis, and stable isotopic studies on shells have never been carried out before.

In the present study, a high-resolution microfaunal analysis, together with stable isotopic analysis of ostracod shells, is performed on core samples collected from the southern plain of the Pearl River Delta.The aim is to reconstruct palaeoenvironmental evolution and its relationship to sea level and climate changes in the southern plain during the Late Quaternary.Special attention has been paid to the timing of the initial postglacial sea level rise,short-term sea-level f l uctuations, and to the timing of peak transgression during the Holocene.

2 Study area

The Pearl River, the largest river in South China and the third largest in China, is primarily formed by the con-fl uence of the West-, North-, East-, and Liuxi rivers (Fig.1), of which the West River is the longest with a channel length of 2214 km (Liet al., 1991).In southern Guangdong Province, it discharges into the South China Sea through eight main outlets: four outlets in the east (Humen, Jiaomen, Hongqili, and Hengmen)and four in the west (Modaomen, Jitimen, Hutiaomen, and Yamen)(Wu and Zhou, 2001; Wuet al., 2006).The Pearl River Delta is a low-lying plain with many small rocky hillocks, descending towards the South China Sea and delimited by hills in the east, west, and north.The con fi guration of the Pearl River Delta is controlled by EW, NW and NE-striking faults (Chenet al., 1995, 2001), which are all covered by Quaternary sediments.The delta formed since the Late Pleistocene (Huanget al., 1982; Chenet al., 1994; Li,2004).Its Late Quaternary sediments, overlying Pre-Neogene bedrock (sandstones or granites), are generally 25-40 m thick (Huanget al., 1982).Similar to other deltaic basins in East and Southeast Asia, the sedimentary succession generally records two marine transgression cycles,which occurred in the Late Pleistocene and Holocene, respectively (Xuet al., 1986; Zhaoet al., 1987; Chenet al.,1994; Lan, 1996; Zonget al., 2009).The Late Pleistocene transgression was followed by the Last Glacial, when sea level dropped to -131 m in the South China Sea (Chenet al., 1990).During this period of low sea level, the Pearl River Delta area underwent subaerial weathering, indicated by a layer of widely-distributed mottled clay or fl uvial sandy gravel.The Holocene transgression was considered to be more extensive than the Late Pleistocene one, with the coastline shifting northwards far from the present-day coast (Fanget al., 1991).During this transgressive period,most parts of the southern Pearl River Delta were submerged and formed a semi-enclosed estuary (Wuet al.,2006).However, due to topographic differences, the timing of marine ingression and regression is not identical in different parts of the delta.

The studied core is located on the present-day southern plain of the Pearl River Delta, close to the Modaomen outlet (Fig.1), the major outlet of the West River.During the Late Quaternary, the southern plain received over 30 m of sediments, more than the other parts of the delta.

3 Material and methods

Two cores (PRD04 and PRD05)were drilled on the Da’ao Sand of southern plain of the Pearl River Delta, near the Modaomen outlet.Only Core PRD05 (113°11′02″E,22°31′24″N), 35 m in depth, covers the succession from the Late Pleistocene to the Holocene and therefore was selected for this study.For the microfossil analyses, 269 samples were collected from 2.51 m to 31.38 m core depth,generally at approximately 10 cm intervals, on average corresponding to ～50 years in the case of Holocene sediments and ～500 years in the case of Pleistocene sediments.No samples were collected from the top and bottom of the core, which have been disturbed by human activities or are composed of gravels.

Dried samples (about 100 g)were disaggregated with tap water and left for 24-48 h, then were boiled in a beaker for about 10 minutes on a hot plate.The fraction larger than 63 mm was separated by wet sieving and dried.A further separation was performed with a 125 mm-mesh sieve.Only the sample fractions larger than 125 mm were used for microscopic observation.Ostracods and foraminifers were handpicked, identif i ed, and counted.The microfossil frequencies have been calculated for 100 g of dried sediment.

The oxygen and carbon isotopes were determined onBicornucythere leizhouensis.This species has been chosen because it is one of the most abundant ostracod species with a wide ecological range, occurring throughout the core section from 24.7 m to 12.4 m.The adult valves ofBicornucythere leizhouensisare bigger than those of other species, so that about 10 specimens were enough for analysis.The selected adult valves were cleaned with deionized water, and then treated with anhydrous phosphoric acid at 75℃ and analyzed utilizing a Kiel III device connected to a Finnigan MAT 252 isotope ratio mass spectrometer in the Geozentrum Nordbayern at the University of Erlangen, Germany.Isotopic results are reported with respect to PDB via the NBS19 standard.The precision is better than 1σ.

For the method of grain size analysis see Heet al.(2007).

4 Results

4.1 Lithology and radiocarbon dates

Combining detailed core observation and grain size analysis, Core PRD05 can be divided into 11 lithological units from 34.56 m to the top of the core (bedrock is below 34.56 m)(Fig.2; Table 1).

Eighteen radiocarbon dates were obtained from organic-rich sediments of the core with conventional14C dating methods at the Isotope Laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.Two dates (6640±270 yr B.P.at 23.19-23.29 m core depth and 8640±240 yr B.P.at 20.1-20.20 m core depth)suggest unreasonable ages and were not used.The oldest14C date is 26,355±550 yr B.P.(30,900±340 cal yr B.P.), obtained from argillaceous silt at 28.97-29.07 m core depth.

Table 1 Lithological units of Core PRD05

However, it has been argued (Yimet al., 1990; Yim,1999; Zonget al., 2009)that radiocarbon ages from this and similar levels do not represent true ages due to subaerial weathering during the last glacial period, but that these levels were actually formed under marine conditions during the last interglacial stage.The gravels at the bottom of the core, the earliest Quaternary sediments at the study site,should then be even older.The youngest14C date obtained at 4.69-4.81 m of core depth gave an age of 1915±200 yr B.P.(1880±240 cal yr B.P.).The conventional dates and calibrated dates are presented in Figure 2.The calibrated dates are calculated using the14C-age calibration program CalPal (www.calpal.de).The ages of the lithological unit boundaries and those in the discussion of environmental stages were estimated by extrapolation of sedimentation rates and given as calibrated years.

4.2 Microfossils

The distribution patterns of ostracods and foraminifers are generally similar in Core PRD05 (Figs.3, 4).Microfaunas are absent in the lower part of the borehole.The first microfauna are found at 24.7 m core depth, the main taxa include the ostracodsBicorncythere leizhouensis,Neomonoceratina delicate, andSinocytheridea impressaand the foraminiferaAmmonia beccariivar.andQuin?queloculina seminula.Microfaunal abundance increases gradually from 24.7 m to 21.7 m, and then decreases from 21.7 m to 20.1 m.The most pronounced peaks occur from 20.1 m to 17.4 m and from 16.9 m to 16.3 m.The abundance decreases again from 16.3 m to 12.4 m.From 12.4 m towards the top of the core only scarce individuals have been collected.Both ostracods and foraminifers are dominated by euryhaline coastal species.Figures 3 and 4 show the distribution of key ostracod and foraminiferal taxa which represent more than 1% of the total specimens in Core PRD05.

A useful method for reconstructing marginal marine environments is to divide microfaunas into different ecological groups (e.g.,Mazziniet al., 1999; Taet al., 2001;Anadónet al., 2002).We also employ this method in this study.

4.2.1Ostracods

Over 40 ostracod species were identif i ed in Core PRD05.Most species are well known in coastal marine and estuarine settings of the present-day South and East China Sea.With respect to their ecological ranges and co-occurrence,the ostracod taxa found in the core can be grouped into four types (Table 2).Their ecological distributions are discussed as follows:

Table 2 Ostracod and foraminiferal taxa from Core PRD05 arranged in ecological groups.In each group the taxa are ranked in decreasing relative abundance

Group 1 includes marine ostracod species.They are present in PRD05 in very low abundance, comprising only 2.6% of all individuals.Among them,Macrocypris decora,Mutilussp., andStigmatocythere bonaare comparatively common, comprising 0.5% to 0.7% of the total ostracods,respectively.The other marine species of the core account for less than 50 individuals per 100 g sediment.These species are typically found in the middle and outer shelf area in the present-day East China Sea and South China Sea in euryhaline waters (Zhaoet al., 1986; Cai, 1988; Wanget al., 1988; Zhao and Wang, 1988; Liuet al., 2002).The occurrence of most of them at the core site might result from transportation by marine currents.

Group 2 consists of those ostracod species being able to withstand a wider range of salinity, from polyhaline to poly-euryhaline.They are moderately abundant in PRD05,comprising 22.9% of the total ostracods.This group is dominated byPistocythereis bradyformis(6.2% of the total ostracods),Neosinocythere elongata(4.7%),Spinileb?eris quadriaculeata(3.3%),Sinocythere sinensis(2.8%),Alocopocythere kendengensis(1.7%),Bicorncythere bisanensis(1.6%), andKejella hodgii(1.3%).They usually prefer salinity values above 25‰ and water depth from 20 to 50 m, although some of them can also live under coastal conditions (Zhaoet al., 1986; Wanget al., 1988; Zhao and Wang, 1988; 1990; Cai, 1993).

Group 3 includes the most strongly euryhaline coastal species, which can tolerate a wide range of salinity and very shallow water depth.They are the major ostracod elements of the core and comprise 74.3% of the total individuals.Strikingly predominant species includeSino?cytheridea impressa(26.7%),Bicorncythere leizhouensis(23.6%), andNeomonoceratina delicata(23.5%).These three species occur throughout the core section from 24.7 to 12.4 m.Sinocytheridea impressais a very well-known euryhaline species, widely distributed in recent and Quaternary sediments along the coast of China (Zhao and Han,1980; Zhao, 1987; Wanget al., 1988; Zhao and Wang,1990; Cao, 1998; Liuet al., 2002).It can tolerate salinity ranges from 2‰ to euryhaline and a water depth of less than 20 m.Neomonoceratina delicatais also a typical shallow-water species off coastal area in the South China Sea(Cai, 1988; Zhao and Wang, 1988, 1990).There are only a few studies on the distribution ofBicorncythere leizhouen?sis.It is here considered as a coastal species because of its common co-occurrence withSinocytheridea impressaandNeomonoceratina delicatain the core.The remaining species of this group are only sporadically found in the core.

Group 4 consists of freshwater ostracods.Elements of this group occur sporadically and as very few valves in the studied core, comprising about 0.2% of the total ostracods.They are only found in samples from the upper part of the core, where few other ostracods have been collected.These genera are adapted to live in freshwater environments but also tolerate slightly brackish conditions(De Deckker, 1981; Houet al., 1982, 2002; Anadónet al.,1986; Zhao, 1987; Neale, 1988; Mazziniet al., 1999).

4.2.2Benthic foraminifers

The foraminiferal fauna consist of a total of 30 species.No planktic foraminifers were found.In modern sediments of the South and East China Sea, the distribution of many benthic species shows a strong relationship with water depth and salinity (e.g.,Wanget al., 1980, 1988; Li, 1985;Li and Bian, 1989).According to their ecological preferences and co-occurrence, the encountered benthic species can be included in three groups (Table 1), which approximately correspond to Group 1, 2, and 3 of the ostracods.

Group 1 consists of marine species, which occur sporadically in very low abundance, accounting for about 0.8% of the total foraminifers in the core.OnlyMassilina penglaiensisshows a higher abundance, reaching a maximum of 116 individuals per 100 g sediments at 18.92 m core depth.Similar to the marine ostracod species in the core, they are, based on their ecological requirements, interpreted to be mostly allochthonous elements.

Group 2 is composed of foraminiferal species, whose ecological ranges correlate with those of ostracod Group 2.This group comprises about 32.4% of the total foraminifers in the core.The dominant species includeQuinque?loculina seminula(10.1% of all individuals of foraminifers),Elphidium advenum(6.5%),Cavarotalia annectens(4.2%),Ammonia pauciloculata(3.6%),Protelphidium granosum(3.1%),Cribrononion asiaticum(2.5%), andMassilina laevigata(1.2%).They occur in most samples from 24.7 to 12.4 m core depth.They are also common species along the present-day Southeast China Sea, mostly distributed in outer parts of the inner shelf to the middle shelf with water depth ranging from 20 m to 50 m and a salinity varying between polyhaline and eupolyhaline(Wanget al., 1988).

Group 3 includes coastal marine species, mainly limited to shallow water depths of 20 m or less.As euryhaline taxa they can tolerate waters of low salinity.This group accounts for 66.9% of the total foraminifers.Ammonia beccariivar.(48.7%),A.tepida(6.5%), andQuinquelo?culina akneriana(10.5%)are the predominant species.These species, particularlyAmmonia beccariivar., are very broadly distributed in modern coastal brackish environments of the South China Sea, East China Sea, Yellow Sea, and Bohai (Wanget al., 1980, 1988; Li, 1985, 1994;Li and Bian, 1989; Zhuet al., 1998).They live in water depths of 0 to 50 m, but largely occur at a depth of less than 20 m.Ammonia beccariivar.is the most abundant species and occurs in all samples from 24.7 m to 12.4 m core depth.The highest abundance is 3180 specimens per 100 g sediment (at 18.82 m core depth).A.tepidaandQ.aknerianaare found in most samples.The remaining species of this group are rare.

4.3 Stable isotope data of ostracod valves

In this study, δ13C and δ18O isotope analyses were per-formed on valves ofBicornucythere leizhouensis, which is one of the most abundant species in PRD05.By measuring a single species, possible interspecif i c differences in isotope composition resulting from vital effects can be avoided.The isotope results of 64 samples are shown in Figure 5.δ18O shows a comparatively small range of values between -7.87‰ and -3.89‰, with an average value of -5.79‰.The δ13C values f l uctuate between -13.34‰and -8.54‰, with an average value of -10.34‰, corresponding to a relative enrichment in12C.δ13C and δ18O show a co-variant trend for most parts of the core (r=0.59),particularly for the core depth below 15.5 m (r=0.71).This co-variation between δ13C and δ18O suggests that the variation in isotopic compositions corresponds to variations in salinity (Ingramet al., 1996).

5 Palaeoenvironmental reconstruction

By combining lithological, microfauna, and stable isotopic data of ostracod shells from Core PRD05, palaeoenvironmental changes largely related to sea-level f l uctuations during the Late Pleistocene to Holocene can be reconstructed.The periods of the Holocene transgression are highlighted.

5.1 Stage I: Pre-Holocene (core depth: 34.56-26.81 m; before ～16,700 cal yr B.P.)

No microfossils were found in this core interval.Based on lithological characteristics, it can be divided into three phases.

5.1.1Substage I1:34.56-29.12m core depth(before～31,000cal yr B.P.)

The basal coarse sediments, overlying bedrock, represent the earliest Quaternary record in the study area.This fl uvial sand-gravel interval is usually 2-7 m thick, reaching a maximum of 11 m in the Pearl River Delta area (Xuet al., 1986).In the studied core, its thickness of 5.44 m is moderate, corresponding to the older terrestrial unit (T2)of Zonget al.(2009).The sediments were deposited in a palaeo-valley, with initially very high energy conditions,which diminished later as is demonstrated by the reduction in grain size.No radiocarbon dates were obtained from this core interval, but a relative age of older than ～31,000 cal yr B.P.can be assumed by extrapolation from the overlying sediments.In previous work, ages between 30,000 and 40,000 yr B.P.were obtained for this sand-gravel interval from other parts of the delta and it is usually considered to have been deposited during the early phase of the last interglacial stage (Xuet al.,1986; Wenet al., 1997).Zonget al.(2009)considered this terrestrial unit as having been deposited prior to MIS 5.

5.1.2Substage I2: 29.12-27.63m core depth (from～31,000to～21,700cal yr B.P.)

This core interval is still unfossiliferous, but lithologies change to argillaceous silt.This suggests a change of depositional conditions.Rare earth elements (REE)data for this fine-grained interval indicate an estuary environment, with a relatively high ?REE (average value: 234.2 ppm)and a clear enrichment of light rare earth elements(LREE)relative to heavy rare earth elements (HREE)(Liuet al., 2011).We deduce that this core interval formed under at least brackish conditions during the last interglacial stage and it should correspond to the older marine unit(M2)of Zonget al.(2009).The lack of any fossils might be an artifact produced by dissolution of any carbonate skeletal grains during subsequent subaerial exposure.

Three organic-rich samples were taken at 27.36 m,28.39 m and 29.02 m core depth, respectively from this interval of PRD05.They provided conventional radiocarbon ages of 17,968±430 yr B.P.(21,460±630 cal yr B.P.), 18,635±450 yr B.P.(22,350±560 cal yr B.P.)and 26,355±550 yr B.P.(30,900±340 cal yr B.P.), respectively.Similar ages have been obtained in this marine unit from other sites of the Pearl River Delta (Huanget al.,1982; Liet al., 1991; Lan, 1996; Zonget al., 2009).The age of this marine sequence is under debate.Some authors dated it as belonging to MIS 3 (e.g.,Xuet al., 1986; Zhaoet al., 1987; Wenet al., 1997).The equivalent marine sediment, the Gehu marine sequence, with an age between 20,000 and 40,000 yr B.P., occurring along the eastern coast of China, has also been placed in MIS 3 (Chen, 1992;Yuet al., 2005; Wanget al., 2008, 2008).Other authors argued that these dates cannot ref l ect true ages due to subaerial weathering during the last glacial period and that this marine unit should be assigned to MIS 5, a period with a sea-level highstand comparable to today (Yimet al., 1990;Yim, 1999; Zonget al., 2009).There exists a discrepancy of up to tens of meters in estimates of sea level during MIS 3.Global sea level in MIS 3 inferred from deep-sea oxygen isotope series and coral reef records was as low as around ?80 m (e.g.,Chappellet al., 2002; Yokoyamaet al., 2007).However, marine records likely formed during MIS 3 have been also reported from other coastal areas of the world (e.g.,Belluominiet al., 2002; Hanebuthet al.,2006; Graciaet al., 2008)and it has been proposed that the sea level during MIS 3 might have been much higher(e.g.,Liu, 1997; Belluominiet al., 2002; Yanget al., 2004;Huang and Cai, 2007; Graciaet al., 2008).After studying numerous relic records of sea-level stands along the coasts from the Bohai to the South China Sea, Liu (1997)suggested that sea level between 20,000 and 40,000 yr B.P.was very likely close to its present position.According to Shi and Yu (2003), precipitation and temperature during late MIS 3 were higher than today in China and, correspondingly, a large transgression occurred in delta areas of the Yangtze River and the Pearl River.

If the dates obtained from PRD05 were acceptable, this interval would belong to MIS 3.However, it is possible that these ages are too young as argued by Zonget al.(2009).In our opinion additional independent chronological controls are required to constrain the exact ages of this marine sequence.

We estimate that the marine waters intruded the core site mainly through the easterly outlets, because the Modaomen channel probably had not yet formed at this time.The oldest14C date for the f l uvial sediments of the Modaomen channel is ～20,300 yr B.P.(Chenet al., 1994).

5.1.3Substage I3: 27.63-26.81m core depth(from～21,700to～16,700cal yr B.P.)

This substage is characterized by a mottled clay layer,which is widely distributed in the Pearl River Delta and most likely formed by subaerial oxidation of the sediments (Lan, 1991, 1996; Huanget al., 1997; Wenet al.,1997; Donget al., 2007; Huang and Cai, 2007; Zonget al.,2009).Along palaeo-river channels in the Pearl River Delta area, a layer of sand and gravel was deposited instead of this weathered clay, which corresponds to the younger terrestrial unit (T1)of Zonget al.(2009).This layer has been usually considered to have been formed during the last glacial maximum (Xuet al., 1986; Huang and Cai, 2007).It approximately corresponds to MIS 2 and can be correlated with the first “hard mud” unit in the Yangtze River Delta (Wanget al., 2008).It was estimated that during this period the sea level fell to around -130 m in the South China Sea, where palaeoshoreline records were found(Chenet al., 1990).However, Huanget al.(1995)argued that the lowest sea level of the last glacial maximum might have been at about -80 m, comprehensively considering various records, including tectonic subsidence.In the East China Sea, Feng (1983)suggested the lowest sea level during the last glacial maximum was 140-160 m below present level.

During this time of sea-level lowstand, the region from the Pearl River mouth to the northern South China Sea was exposed and experienced continental weathering.The weathered sediments usually have a thickness of 1-3 m,with a maximum of 5 m in the Pearl River Delta.The mottled clay layer in the core site is ＜ 1 m thick, probably due to the topographically lower position compared to other parts of the delta.

5.2 Stage II: Early Holocene (core depth: 26.81-24.7 m; from ～16,700 to ～10,100 cal yr B.P.)

This core interval is characterized by a 17 cm-thick peat layer at the base and by dark-grey argillaceous silt with abundant plant remains higher in the interval.No microfossils were observed.This facies can be interpreted as a swamp deposit that was formed at the onset of the postglacial sea-level rise.The beginning of the postglacial transgression in the Pearl River estuary area has been previously estimated at about 12,000 yr B.P.by some authors(e.g., Xuet al., 1986; Liet al., 1991), but with no reliable evidence.In this study, a14C age of 13,380±220 yr B.P.(16,450±560 cal yr B.P.)was obtained from the peat sample close to the base of this interval, at 26.72 m core depth.The peat deposition has been taken as reliable evidence of a warmer climate (Kim and Kennett, 1998).This suggests that the postglacial sea-level rise might correspond to the formation of the peat sediments, at ca.16,700 cal yr B.P.(there are about 10 cm peat below the14C measurement point).These results approximately agree with that obtained by Chenet al.(1990), who demonstrated that the last glacial phase should have ended in the South China Sea at 13,700±600 yr B.P.(non-calibrated age).A similar date (13,000 to 14,000 yr B.P., non-calibrated age)for the onset of the postglacial transgression has also been obtained for the Yangtze River Delta area (Li and Zhang,1996).

For the Sunda shelf, a slightly older age (14,600 yr B.P.)is given for the onset of rapid sea-level rise (Hanebuthet al., 2000).Kienastet al.(2003)arrive at a similar f i gure(14,700 yr B.P.)for the northern South China Sea.

5.3 Stage III: Holocene transgression (core depth:24.7-12.4 m; from ～10,100 to 5560 cal yr B.P.)

At around 10,100 cal yr B.P., the rate of sea-level rise rapidly increased and a brackish estuary with abundant microfaunas, strikingly dominated by euryhaline species,developed and persisted at the core site until 5560 cal yr B.P.During this transgressive interval, several substages corresponding to short-term environmental f l uctuations can be recognized based on the composition and abundance of microfossils and stable isotope data (Fig.5).These changes in the microfauna record are thought to primarily represent changes in relative sea level and salinity, although other environmental factors such as shifting current systems, f l uctuations in river discharge, and changing topography might have also played some role.

5.3.1Substage III1: 24.7-21.7m core depth(from～10,100to8630cal yr B.P.)

In the interval immediately above the freshwater swamp sediments, microfaunas record a rapid rise in sea-level.At about 10,100 cal yr B.P., marine waters began to intrude the Da’ao area, where the core site is located, through the Modaomen channel (Heet al., 2007).The first evidence of microfaunas within the core occur at 24.7 m core depth.From 24.7 to 23.7 m, the microfaunal abundance is very low, with 192 specimens per 100 g sediments on average for foraminifers and 251 for ostracods.Foraminifers of Group 2 are more abundant (59.5%)than those of Group 3 (33.3%).Ostracods of Group 2 show the highest relative abundance within the core (34.5%), although lower than that of Group 3 (60.1%).Marine species, both of foraminifers and ostracods, are in low abundance, usually less than 50 specimens per 100 g sediments, but their relative abundance is the highest within the core, with 7.2% and 5.4%, respectively.This interval marks the first stage of the Holocene transgression, when the study area began to develop into a shallow brackish estuary, and became colonized by euryhaline and marine species.

From 23.7 m upwards, microfaunal abundance gradually increases with the expanding transgression.Foraminiferal abundance peaks at 1571 specimens per 100 g sediment (average: 613 specimens); ostracod abundance peaks at 2462 specimens (average: 915 specimens).The increase in the percentage of taxa of coastal groups in both ostracods and foraminifers is much higher than that of taxa from Group 2, which, together with marine species, show a decrease in relative abundance.The high-stress, brackish, shallow-water environment limited their development.The δ18O and δ13C values generally show an increase during this interval, both reaching peaks at 21.7 m, with-3.89‰ and -8.71‰, respectively.This indicates increasing salinity due to intrusion of marine waters.

These data can be compared with the curves of the Holocene relative sea-level change proposed by several authors for the Pearl River Delta area (Fig.6)(Fanget al.1991; Liet al., 1994; Zong, 2004).It has been suggested that a rapid rise in relative sea level occurred during the Early Holocene, when the sea level rose at a rate of about 21.6 mm/yr (Fanget al., 1991).At other localities along the southeast coast of China, a similar trend in Early Holocene sea level rise has been recorded (Zong, 2004).

5.3.2Substage III2: 21.7-20.1m core depth(from～8630to8520cal yr B.P.)

During this interval, microfaunas, particularly ostracods, decrease to an average abundance of 537 specimens(ostracods)and 604 specimens (foraminifers)per 100 g sediments.Members of Group 2 decrease more obviously in abundance than members of the coastal group, which become strongly dominant with relative abundances of up to 85.3% and 74.1% for foraminifers and ostracods, respectively.Among coastal foraminifers,Ammonia becca?riivar.strongly dominates, reaching an average of 67.4%.Such changes may be linked to decreasing water depth,accompanied by a decrease in salinity.This is supported by a sharp decrease of the δ18O and δ13C values, from -3.89 to -8.92‰ and from -8.71 to -12.33‰, respectively.Fanget al.(1991)reported that there was a short episode of seal-level standstill or fall between 7400 and 7100 yr B.P.(non-calibrated age), to which this substage of Core PRD05 is thought to be comparable.

5.3.3Substage III3: 20.1-17.4m core depth(from～8520to8200cal yr B.P.)

From 20.1 m to 17.4 m, the microfauna is very abundant.Three phases can be distinguished: From 20.1 m to 19.6 m, a sharp increase in microfaunal abundance is observed; 1626 specimens in the case of foraminifers and 1856 specimens in the case of ostracods per 100 g sediment on average.From 19.6 m to 18.5 m, microfaunal abundance reaches the highest value.Foraminifers reach a peak value of 5029 specimens, with an average value of 4123 specimens per 100 g sediments; ostracods reach a peak value of 5486 specimens, with an average value of 2753 specimens per 100 g sediments.Among foraminifers,species of Group 2 show an obvious increase in relative abundance, while coastal species, particularlyAmmonia beccarii, decrease in relative abundance.Marine foraminifers show the highest abundance within the core (up to 116 specimens per 100 g sediments), although their relative abundance is low.From 18.5 m to 17.4 m, the microfauna is still abundant: 2374 specimens (foraminifers)and 2422 specimens (ostracods)per 100 g sediments.Although the abundance values are smaller than those during the previous phase, the relative abundance of foraminifers and ostracods of Group 2, particularly the foraminifera, distinctly increase.

These microfaunal results suggest that salinity and depth increased with a renewed sea-level rise at about 8520 cal yr B.P., which favoured the development of an abundant microfauna.This environment persisted until～8200 cal yr B.P.Isotopic data correlate well with the microfaunal results.δ18O and δ13C show a positive shift and then stay at comparatively high values until 17.4 m core depth, implying an increase in salinity due to an enhanced marine inf l uence.This interval records the peak marine intrusion in the study area.

5.3.4Substage III4: 17.4-16.9m of core depth(from～8200to8080cal yr B.P.)

A short interval of reduced water depth is evidenced by sharply decreased microfaunal abundance, with an average of 945 foraminifer specimens and 626 ostracod specimens per 100 g sediments.Among foraminifers,Ammonia beccariivar.shows a prominent increase in relative abundance.This environmental change seems to have had the same effect on ostracods of Group 2 and 3, which decrease in a similar way, with their relative abundance showing little variation.In contrast to the trend of the microfauna, stable isotope values show a positive shift, reaching -4.35‰for δ18O and -8.54‰ for δ13C, which indicates an increase in salinity.This might be related to the dry climate, which prevailed during this period of regression and resulted in high evaporation rates.

5.3.5Substage III5: 16.9-16.3m core depth(8080to7900cal yr B.P.)

Microfaunal abundance shows peak values again in this interval.Foraminifers increase up to 2176 specimens(maximal value: 3968)and ostracods increase up to 2220 specimens (maximal value: 5742)per 100 g sediments.Species of Group 2 increase again both in the case of foraminifers and ostracods.Particularly in foraminifers, relative abundance of Group 2 taxa (54%)is higher than that of coastal species (45%);Ammonia beccariivar.shows a sharp decrease in abundance, accompanied by an increase in species of Group 2.Marine species are more common,even though they are allochthonous.This suggests an increase in water depth, possibly related to a new marine transgression following the previous short regressive period (see also Weiet al., 2011).The δ18O and δ13C values,although decreasing, are comparable to those of the previous transgression.This means that the climate turned humid again and water salinity was mainly inf l uenced by the extent to which marine and f l uvial waters became mixed.

As suggested by both the microfauna and stable isotopic data, the maximum marine transgression in the study area took place from 8520 to 7900 cal yr B.P., although a shortterm regression occurred between ～8200 to 8080 cal yr B.P.The striking dominance of euryhaline taxa suggests a shallow, brackish estuary with a water depth of less than 20 m.This interval most likely corresponds to the Mid-Holocene sea-level highstand, which can be identif i ed along the southeastern coast of China (Zong, 2004), although differing in height and timing at different localities.The occurrence of the sea-level highstand seems to be earlier at the core site than in other parts of the Pearl River Delta.Thus, Weiet al.(2011)assumed maximum transgression around 6000 yr B.P.(non-calibrated)for the Pearl River delta as a whole.

5.3.6Substage III6: 16.3-12.4m core depth(from～7900to5560cal yr B.P.)

During this time interval, other parts of the Pearl River Delta were marked by the peak Holocene marine transgression (e.g., Xuet al., 1986; Fanget al.1991; Liet al.,1991; 1994; Weiet al., 2011).In contrast, a general decrease in microfaunas is observed at the core site, although there is a small increase in the middle part (from 15.6 m to 14.3 m).It seems that an unstable environment developed at the core site.From 16.3 m to 15.6 m there is a sharp decrease in microfaunal abundance (714 foraminifer and 395 ostracod specimens per 100 g sediments).Both δ18O and δ13C show a negative shift and δ18O reaches the lowest value (-7.87‰)within the core, implying reduced salinity.From 15.6 m to 14.3 m, there is a small increase in abundance, particularly of foraminifers (up to 1332 specimens per 100 g sediments).Taxa of Groups 2 and 3 increase, and in the case of the latter more strongly.Ostracods do not show an obvious increase in abundance, but elements of Group 2 are more abundant than before.The isotopic data show higher values than those in the previous interval, indicating an increase in salinity, which should be linked to an enhanced marine inf l uence.From 14.3 m upwards, microfossils clearly decrease.Ostracods are low in abundance with 191 specimens per 100 g sediment.Most species are represented only by a few reworked valves.Foraminifers are overwhelmingly dominated by coastal species (reaching 92%), especially byAmmonia beccariivar.andA.tepida(78% and 11.2%, respectively).δ18O and δ13C show little positive correlation.δ18O values exhibit small variations, whereas δ13C values f l uctuate strongly and show a large negative shift to -13.34‰ (at core depth 12.87 m).This may correspond to a period when an enhanced freshwater inf l ux with increased organic matter resulted in the enrichment of12C and a distinctly low salinity.

According to Heet al.(2007), the southern part of the Da’ao plain, where the studied core is located, experienced a higher sedimentation rate with an average value of ～0.9 cm/yr from ～9000 to 7000 yr B.P.(～10,100 to 7850 cal yr B.P.).They suggested the existence of a turbid water mass with dense muddy material, carried into this area by tidal currents.This rapid sedimentation might have altered the topography at the core site.From ～7900 cal yr B.P.onwards, marine inf l uence gradually decreased and run off inf l uence increased.This is further supported by the increase in grain size of sediments from 14.3 m upwards.Probably starting at that time, the number of small tributaries joining the West River increased, which enhanced water energy and river inf l uence.

5.4 Stage IV: Mid- to Late Holocene (core depth:12.4-0 m; since ～5560 cal yr B.P.)

From 12.4 m to 3.3 m core depth, barren sands are present, with sporadic small pebbles and abundant fragments of molluscs and barnacles at the bottom.From 3.3 m onwards, the sediment changes to argillaceous silt.Scattered reworked valves of euryhaline ostracods are present in a few samples.Fresh- to brackish-water ostracods, which rarely occur elsewhere in the core, are sporadically present from 4.7 m to the core top, but occur in low abundance.Some characean oogonia occur at the core top.A few reworked tests ofAmmonia beccariiare also present.Such deposits indicate that, starting from ～5560 cal yr B.P., the study area was predominantly affected by runoff input,but with occasional tidal inf l uence.A sandy f l uvial depositional environment developed from ～5560 to 3100 cal yr B.P., then changed into an alluvial plain setting, with a very low salinity and little marine inf l uence.This development differs from that of most parts of the Pearl River Delta, which were still submerged and inf l uenced by marine waters (Fanget al., 1991; Liet al., 1994).It has even been considered that the sea level reached its peak at 3000 cal yr B.P.(Zong, 2004).This discrepancy possibly results from different topographic conditions; the core site was strongly affected by freshwater from the West River during this time.

6 Conclusions

This integrated study of the lithological, microfaunal and ostracod stable isotopic records allows us to reconstruct, in detail, palaeoenvironmental evolution during the late Quaternary at the site of Core PRD05 in the Pearl River Delta.

Gravel deposits at the bottom of the core most likely were formed prior to the transgression of the last interglacial stage.They represent a fl uvial environment with water energy decreasing upwards and are the earliest evidence of Quaternary sedimentation at the core site.Subsequently marginal marine conditions became established.The finegrained sediment became decalci fi ed during the last glacial episode when it was subaerially exposed, which is shown by a mottled clay layer in the core.The onset of the postglacial sea-level rise in the South China Sea was no later than ～16,700 cal yr B.P.During the first phase of sea-level rise (from ～16,700 to 10,100 cal yr B.P.), a swamp environment developed at the core site.At around 10,100 cal yr B.P., when sea-level rapidly increased, marine waters intruded the Da’ao plain and reached the core site through the Modaomen channel.This occurred much earlier than in other parts of the Pearl River Delta.Since then, a semi-enclosed estuary developed, with a dominance of euryhaline microfaunas, which persisted until ～5560 cal yr B.P.During this transgressive interval, short-term environmental fl uctuations can be recognized based on microfaunal and stable isotope data.The rise in sea-level from ～10,100 to 8630 cal yr B.P.was followed by a sea-level fall accompanied by a decrease in salinity and water depth from ～8630 to 8520 cal yr B.P.An extended transgression occurred between ～8520 and 7900 cal yr B.P.The time intervals from ～8520 to 8200 cal yr B.P.and from ～8080 to 7900 cal yr B.P.mark the peak transgressions, during which the microfauna reached the greatest abundance.From ～7900 to 5560 cal yr B.P., the core site generally showed a reduced marine in fl uence and enhanced freshwater input, in contrast to other parts of the Pearl River Delta.A fl uvial environment developed from ～5560 to 3100 cal yr B.P.and was succeeded by an alluvial plain setting.

Changes in salinity as evidenced by microfaunal composition as well as by stable oxygen isotopes were not necessarily connected only to relative changes in sea level and/or transgressions/regressions, but might have also been inf l uenced by changing current patterns, shifting depocenters, and f l uctuations in f l uvial discharge.Still, at the time scales considered here (hundreds to thousands of years)relative changes in sea level should have been of overriding importance.

Acknowledgements

This research was supported by the Natural Science Foundation of China (Grant No.40872024 and No.40331007)and by travel grants within the framework of the Project-based Personnel Exchange Programme of the DAAD (Deutscher Akademischer Austauschdienst)and CSC (China Scholarship Council).We would like to thank Prof.Dr.Wu Chaoyu at the Center for Coastal Ocean Science and Technology Research, Sun Yat-Sen University,for supporting the drilling of the core and numerous discussions, and Manja Hethke and Matthias Alberti, Erlangen University, for critical reading of the manuscript.We also acknowledge the constructive criticism of several anonymous reviewers.

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