Evolution of Asian monsoon variability revealed by oxygen isotopic record of middle Holocene massive coral in the northern South China Sea

Quaternary International 213 (2010) 56–68

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Evolution of Asian monsoon variability revealed by oxygen isotopic record of middle Holocene massive coral in the northern South China Sea Ruixia Su a, *, Donghuai Sun b, Hai Chen c, Xiaoming Chen d, Zaijun Li b a

Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, 164 West Xinguang Road, Guangzhou, Guangdong 510301, China Key Laboratory of West Chinese Environmental Systems (Ministry of Education), College of Earth and Environment Sciences, Lanzhou University, Lanzhou 730000, China c Minnesota Isotope Laboratory, Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA d Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 20 May 2009

A 40-year d18O record of the w5.4 ka BP Porites lutea from the east coast of Hainan Island in the northern South China Sea (SCS) was analyzed to investigate evolution of Asian monsoon variability during the middle Holocene. The mean, maximum and minimum of the coral skeletal d18O were 0.49&, 0.69& and 0.14& higher than those of modern corals respectively. Thus, the sea surface temperature (SST) for mean annual, winter and summer at w5.4 ka BP was 2.2  C, 3.1  C and 0.6  C lower, and accordingly the surface salinity (SSS) was 1.5&, 2& and 1& higher than today. The d18O amplitude of the w5.4 ka coral for winter–summer (w26% higher than modern values), winter (w23% higher) and summer (w18% lower than that of the modern corals) implied enhanced seasonality in winter, but decreased seasonality in summer for SST, SSS and Asian monsoon conditions at w5.4 ka BP relative to today. Thus, seasonal increment of coral d18O mainly resulted from decrease of winter SST and increase of winter seawater d18O during the middle Holocene, which possibly correlated with seasonal changes of insolation. The w5.4 ka coral d18O values indicate that interannual variability of the Asian monsoon was much stronger in winter–summer and winter, although much weaker in summer during the middle Holocene relative to today. Spectral analysis revealed a significant variance with a period of 18.7 years and subsignificant variances at interannual periods of 2.3–2.6 years, which implies that ENSO variance and a teleconnection between the atmospheric ENSO and monsoonal rainfall during the middle Holocene existed, although they were much weaker than today. The long-term decrease trends of the coral d18O minima, mean and maxima are controlled by the long-term trends of SST increase, and SSS and local insolation decreases from the middle Holocene to the present. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The variability from seasonal to interannual scale is important for researching climatic variance and variational mechanisms. ENSO abnormalities in the tropical ocean and the Arctic Oscillation are prominent problems. The Asian monsoonal system closely correlates with both ENSO and the Arctic Oscillation. Research results have indicated that the tropical oceanic circulation abnormality aroused by ENSO evidently influenced summer monsoonal rainfall (Li, 1988) and winter monsoonal temperature (Li, 1990) in Chinese monsoonal areas, and statistical analysis has revealed that the Asian monsoon obviously influences ENSO circumfluence (Li,

* Corresponding author. Tel.: þ86 20 8910 5467; fax: þ86 20 8445 1672. E-mail address: [email protected] (R. Su). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.04.006

1990; Kim and Lau, 2001; Lau and Wu, 2001). The Arctic Oscillation through Asian monsoonal circumfluence influences oceanic and atmospheric circumfluence in the middle latitude areas (Gong and Ho, 2003). However, Asian monsoonal variability from seasonal to interannual scale is still very difficult to forecast because of the complexities of its system and driving mechanism (Webster et al., 1998; Lau et al., 2000; Tudhope et al., 2001). Nevertheless, researchers have widely considered that characteristics and variability at seasonal, interannual and interdecadal scales would provide useful data and clues for comprehending monsoonal variational mechanisms, especially the connection with ENSO in various boundary conditions, and serve as a reference for forecasting monsoonal climate. Banded corals from the modern and fossil reefs of the tropical and subtropical oceans provide an important archive to reconstruct modern and past climatic variability. The oxygen isotopic ratios

R. Su et al. / Quaternary International 213 (2010) 56–68

(18O/16O) are primarily influenced both by temperature and ambient seawater d18O (Keith and Weber, 1965; Omata et al., 2005). In localities where one of these two environmental factors dominates, coral d18O records can therefore provide information either on SST or seawater d18O variations with the latter being related to the hydrologic balance (Felis et al., 2004). As temperature increases, there is a decrease in the d18O of the coral skeleton (Fairbanks and Dodge, 1979; Aharon, 1991). Coral-based palaeoclimatic research has provided important implications on past variability of the ENSO phenomenon and decadal tropical climatic variability in the tropics (Cole et al., 1993, 2000; Charles et al., 1997; Urban et al., 2000; Wei et al., 2000; Cobb et al., 2001; Peng et al., 2003; Sun et al., 2003; Su et al., 2006; Liu et al., 2008), southeastern Indian Ocean (Kuhnert et al., 1999); North Atlantic (Pa¨tzold and Wefer, 1992) and northern Red Sea (Felis et al., 2000, 2004; Rimbu et al., 2003). These palaeoclimatic records were generated from living corals covering the past centuries. In contrast, fossil corals have revealed important aspects on climate variability during time-windows to several decades throughout the last millennium (Kuhnert et al., 2002; Cobb et al., 2003), the Holocene (Beck et al., 1997; Gagan et al., 1998; Corre`ge et al., 2000; Moustafa et al., 2000; Abram et al., 2001; Tudhope et al., 2001; Wei et al., 2004; Yu et al., 2004, 2005; Sun et al., 2005; Morimoto et al., 2007), and the last interglacial warm period (Hughen et al., 1999; Suzuki et al., 2001; Tudhope et al., 2001; Felis et al., 2004). However, relatively few high-resolution records for fossil massive corals have been used for the reconstructions of the Holocene monsoonal climatic variability in the South China Sea (SCS) (Wei et al., 2004; Yu et al., 2004, 2005; Sun et al., 2005). Wei et al. (2004) using high-resolution Sr/Ca ratios of two fossil Porites corals from Leizhou Peninsula along the northern SCS, revealed that sea surface temperatures (SST) during 539–530 BC were roughly the same as those during 1990–2000 AD, but the period of 489–500 AD was significantly cooler, with annual mean, minimal winter and maximal summer SSTs being w2  C, 2.9  C and 1  C lower than today. Yu et al. (2004, 2005), using Sr/Ca-d18O combined data from Leizhou Peninsula fossil corals in the northern SCS, showed that the Sr/Ca-SST at 7.0–7.5 ka BP was w1.5–5  C lower than today, and that the mean annual, maximal and minimal Sr/Ca-SST, at 6.789– 5.011 ka BP was w0–1.6  C higher than today. At 2.541–1.5513 ka BP, SST was w0.1–3.8  C lower than today, and correspondingly the mean annual d18O at 6.789–5.011 ka BP was 0.2–0.32& higher than that of modern corals. At 2.541–1.513 ka BP, sea surface salinity (SSS) was 0.3–0.4& lower than that of modern corals, and seawater d18O at 6.789 ka BP and 5.906 ka BP was 0.2–0.3& higher than today. Sun et al. (2005), using one 54-year high-resolution d18O record of the w4.4 ka BP coral from Hainan Island in the northern SCS, have revealed that the 18O increment is likely to be driven by greater advection of moisture towards the Asian landmass, enhanced monsoon wind-induced evaporation and vertical mixing, and/or invigorated advection of saltier 18O-enriched Pacific water into the relatively fresh SCS. Surface seawater d18O was at least w0.5& higher than today, and the interannual variability in SST and SSS at w4.4 ka BP was stronger than today. This study measured the oxygen isotope composition of the w5.4 ka BP coral OC1 at a monthly resolution for 40 years. Based on the connection between coral oxygen isotope composition and winter temperature and summer monsoonal rainfall using the d18O of the modern coral OC03 (Su et al., 2006), the seasonality and interannual variability of the Asian monsoonal climate were reconstructed. Consequently, the evolution of monsoonal seasonality, interannual variability and interdecadal trends during the middle Holocene are discussed. Spectral analysis examined the interannual variability recorded by the d18O in the w5.4 ka BP coral OC1, and the results compared with that of the published

57

w4.4 ka BP coral OC2 (Sun et al., 2005) and modern coral OC03 records from Hainan Island in the northern SCS. 2. Methods 2.1. Study site Hainan Island (w18 100 to w20100 N and w108 370 to w111050 E) is located in the northern SCS, South China (Figs. 1A,B). The SCS is a pivotal area between the low and middle–high latitudes for material and energy transmissions associated with the Asian monsoon. This area is strongly affected and climatically controlled by the Asian monsoon, bi-directionally adjusted by the low latitude tropical and middle latitude subtropical northwestern Pacific. Therefore, there is a higher temperature (25–29  C), lower salinity (4.1–33.7&), more plentiful rainfall (2000–2400 mm) and wetter climate during the summer monsoonal seasons (from May to October). In contrast, lower temperature (16–24  C), higher salinity (20.5–35.5&), lower rainfall (1000–1200 mm) and drier climate occur during winter seasons (from November to April of next year). Annual total insolation is about 110–140 therm/cm2, the sun annually shines from 1750 h to 2650 h, and insolation rate is 50–60% in this area. Hainan Island was appropriate for investigating detailed millennial climate changes in geomorphology during the Holocene because periodic uplifts elevated a succession of fossil reefs above present sea level. Abundant Holocene coral formations have developed in the SCS, along the coastal shallow sea area around Hainan Island (Figs. 1C,D). Therefore, marine environmental conditions for coral growth are controlled by the Asian monsoon, and stable isotope composition of coral skeletons may provide a high-resolution record of Asian monsoon history. 2.2. Coral sampling In February 2002, samples of the well-preserved fossil Porites lutea OC1 (1.8 m high and 1.6 m diameter) and OC2 (1.8 m high and 1.5 m diameter, Sun et al., 2005) were collected from an emergent Holocene coastal terrace at the Oucun coast, w10 km from Qionghuai City (19.3 N, 110.67 E), east Hainan Island (Figs. 1B,C). Massive in situ colonies of Porites are concentrated within a single fringing reef stratigraphic horizon (Fig. 1D). Relative sea level stood w1–2 m above the present level at Hainan Island during the middle Holocene and topographical analysis indicates that the palaeoshoreline would have been several kilometres from the palaeo-reef. Crosssections of the Porites were sampled by cutting 30  30 cm2 columns oriented parallel to the main growth axes. The study is designed specifically to compare proxy climate records extracted from fossil and modern corals growing in similar reef environmental settings. Therefore, two modern P. lutea specimens were used: core OC03 (1.715 m high and 5.5 cm diameter), and specimen QG5 (0.6 m high and 0.5 m diameter), collected from an offshore reef platform on 6 September, 2003 and 5 May, 2002, w5 km southeast and w5 km southwest of the palaeo-reef respectively (Fig. 1C, Su et al., 2006). These provide an accurate analogue for the Hainan palaeo-reef during Holocene sealevel highstand. 2.3. Age determination and chronology In the laboratory, the sample OC1 was cut into 5-mm-thick slabs paralleling the main growth axis using an automatic stonecutter as for the modern corals (Su et al., 2006). After air-drying, the coral slabs were photographed using a medical Hitachi X-ray machine operating at 50 kV voltage and 50 mA current with 0.04 s exposure

58

R. Su et al. / Quaternary International 213 (2010) 56–68

A

B

Taiwan

Qinlan

Sampling site Hainan Island Indochina peninsula

Sample site

Qionghai

Boao

Philippines

Sumatra Malaya

Indian Peninsula

China

Haikou

Sanya

Kilimantan

C

20

7

OC1

Height (m)

10

D

90°E

5

5 OC2

OC03

3 1 -1 -3 -5

QG5

1

-2

6

11

16

21

26

31

36

41

46

51

Horizatal distance (*10m)

-5 -10 1 km

Fig. 1. Regional map of the SCS, sampling site and Holocene stratigraphy of the Qionghai raised reef, eastern Hainan Island. A. Map showing the SCS (blank area), Hainan Island (dark area) and sampling site (indicated with black dot). B. Map showing Hainan Island (main cities, meteorological and oceanic stations and sampling sites are indicated by black dots; Sun et al., 2005). C. Local topography adjacent to sampling sites for modern coral OC03 and QG5 and fossil coral OC1 and OC2 indicated by black dots (Sun et al., 2005). SST and SSS were measured daily from 1961 to 2003 at Qinglan oceanic station (19.5 N 110.8 E), located w40 km north of sampling sites. Rainfall was measured daily at Qionghai meteorological station (19.25 N 100.5 E, Sun et al., 2005). Topographic and bathymetric contours are in meters. D. Stratigraphic cross-section and topography of the mid-Holocene uplifted coast terrace at Oucun showing þ1–4 m horizon where in situ the fossil P. lutea OC1 and OC2 were collected, w10 km northeast of Qionghai City (19.3 N, 110.67 E, Sun et al., 2005).

time. The negative X-ray images were converted to the digital images using a UMAX scanner. The annual density band couplets, composed of high- and low-density bands, can be clearly observed in the X-ray positive images of the fossil P. lutea OC1 (Figs. 2A,B). The average annual extension rate of the fossil coral OC1 is w0.90 cm/a, slightly higher than that of the fossil coral OC2 (w0.72 cm/a, Sun et al., 2005), but lower than those of the modern coral OC03 (w1.51 cm/a) and QG5 (w1.3 cm/a), respectively, which fall well within the range for Porites sp. corals (0.4–2.4 cm/a) growing in shallow water (<10 m; Lough and Barnes, 1997). Initial screening showed that the fossil P. lutea OC1 was particularly well suited for palaeoclimate reconstruction in terms of its size, straight-forward growth habit, and good preservation. Its mineral composition was analyzed with X-ray diffraction analysis in the Analysis Center of Northwest University of China. The result showed that its skeleton is composed almost of pure aragonites (w100%), and during petrographic analysis of thin sections no secondary aragonite over-growths were detected. Samples OC1T-1 and OC1T-2 were collected from the upper and central parts of the fossil coral OC1 (Fig. 2B) for 230Th dating using a Thermo-Finnigan MAT Element I magnetic sector inductively

coupled plasma mass spectrometer (MS-ICP-MS) in the University of Minnesota (Shen et al., 2002). Two 230Th dates obtained were 5308  21 a and 5406  19 a respectively. Annual growth bands detected by X-ray image analysis indicate 95 years between the two 230 Th sampling points, approximately consistent with the 230Th dating measured results, and indicating continuous growth for the fossil coral OC1. The average initial d234U values of the two analysis samples is 144.8  1.4& and 145.1  1.4& respectively (Table 1), similar to that of modern corals and seawater (145.8  1.7&, Cheng et al., 2000), which indicates good preservation in the fossil coral OC1. 2.4. Coral micro-subsampling and analysis Based on a detailed observation of the density image, a section line was selected along a typical major axis of each slab of the fossil coral OC1 (Fig. 2B). Slices of 3 mm diameter coral subsamples were carefully cut along the selected section line. The subsamples were washed first with water and then with distilled water in an ultrasonic instrument. This process was repeated three times for 20 min each to completely remove any sawing residues and other possible

R. Su et al. / Quaternary International 213 (2010) 56–68

59

Fig. 2. X-radiograph positive images of density bandings and section lines of isotope micro-subsampling in mid-Holocene fossil coral OC1 for A. density bandings. B. Oxygen isotopic micro-subsampling transect. Couplets of a high (dark) and low (light) skeletal density banding represent one annual growth increment, and density banding counts indicate that the fossil coral OC1 spans w200 years. Black dots indicate sample positions used for 230Th dating; the lines indicate oxygen isotopic micro-subsampling.

contaminants. After cleaning, the slices were dried at 40  C. Finally, the high-resolution powder micro-subsamples were obtained by the cutting method for micro-subsampling (Sun et al., 2005; Su et al., 2006). The micro-subsamples were discretely cut perpendicular to a single coralline vessel and at equal intervals with a resolution of w25–28 micro-subsamples per centimeter along the coral skeletal growth axes. This resolution is equivalent to w25–28 micro-subsamples per year defined by the annual density band. Finally, the micro-subsamples were transferred into the sample holders of a MAT252 mass spectrometer coupled with an autocarbonate system for the analysis of the stable oxygen and carbon isotopes in the Isotopic Laboratory of Nanjing University of China. Oxygen isotope ratios were presented in standard per mille notation relative to PDB. The average analytical error for the isotope measurement of 104 standard samples was 0.063&.

2.5. Oceanographic and climatologic data SST and SSS are measured daily at 1 m water depth at Qinglan oceanic station (19.5 N 110.8 E), located w40 km north of the sampling site (Fig. 1B, Sun et al., 2005; Su et al., 2006). The relationship between SSS and local rainfall was determined using daily rainfall recorded at Qionghai meteorological station (19.25 N 100.5 E), located w10 km southwest of the coral sampling site. It is important to note that seasonal changes in SSS measured at Qinglan oceanic station, located near a large estuary, are much greater than those experienced by the modern corals, located in an open fringing reef setting. However, SSS at Qinglan oceanic station does provide a reasonable estimate of the interannual variability experienced by the modern corals (Su et al., 2006).

3. Results The 40-year d18O depth record of the w5.4 ka BP coral OC1 show obvious annual cycles (Fig. 3A) which have been converted to time series (Fig. 3B). The d18O deviation (Dd18O) is calculated removing SST contribution to the coral d18O (Fig. 3C), and divided into winter (SST, Fig. 3D) and summer (SSS, Fig. 3E) components using the same protocols applied to the modern corals (Su et al., 2006). Its d18O variance is similar to that of the w4.4 ka BP coral OC2 (Sun et al., 2005) but different from that of the modern corals on an annual cycle (Su et al., 2006). The winter concave valleys are sharper, and summer convex apices are much narrow, and the variation rates from winter to summer show a step in spring for the fossil coral d18O curves, which possibly show a larger growth rate than that from summer to winter. The d18O mean (5.04&), maximum (3.97&) and minimum (6.12&) of the w5.4 ka BP coral OC1 were 0.49&, 0.69& and 0.14& higher than that of the modern corals (5.53&, 4.66& and 6.26&; Su et al., 2006), and were respectively 0.26& lower, 0.02& higher and 0.36& lower than that of the w4.4 ka BP coral (4.78&, 4& and 5.76&; Sun et al., 2005). This shows a significant SST cooling and/or SSS (sea water d18O) increment and/or rainfall decrease during the middle Holocene relative to today, a significant summer SST increase and/or SSS (sea water d18O) decrease and/or rainfall increase, and a very slight winter SST decrease and/or SSS increase at w5.4 ka BP ago relative to w4.4 ka BP. The seasonal d18O mean, maximal and minimal amplitudes (2.15&, 1.11& and 0.95&) of the w5.4 ka BP coral OC1 were respectively 0.55& and 0.26& higher and 0.17& lower than the values of the modern corals (1.60&, 0.85& and 1.12&, Su et al., 2006). Compared to those of the w4.4 ka BP coral OC2 (1.81&,

Table 1 MS-ICP-MS U/Th results for the fossil coral OC1. Samplea number

238

Ub (ppb)

OC1T-1 OC1T-2

2538  3 2636  3

232

Th (ppb)

1203  13 949  10

230

Th/232Th (ppm, atomic)

d234Uc

230

Th/238U (activity)

230

Th Aged (year) (uncorrected)

230

Th Agee (year) (corrected)

234

(measured)

Uf (initial)

1898  21 2544  27

144.8  1.4 145.1  1.4

0.05449  0.00019 0.05545  0.00017

5320  20 5415  18

5308  21 5406  19

147.0  1.5 147.3  1.4

a

1 and 2 are replicate samples obtained by splitting a w1 g coral sample into multiple fragments. All errors reported in this table are quoted as 2s. c d234U ¼ {[234U/238U]activity  1}  103. d Decay constant values are: l234 ¼ 2.8263  106 a1, l230 ¼ 9.1577  106 a1, l238 ¼ 1.55125  106 a1 (Cheng et al., 2000). e Corrected 230Th ages assume an initial 230Th/232Th atomic ratio of 4.4  2.2  106. This is the value for a material at secular equilibrium, assuming a crustal value of 3.8. f d234 Uinitial was calculated based on 230Th age (T), where d234Uinitial ¼ d234Umeasured  el234  T and l234 ¼ 2.8263  106 a1. b

232

Th/238U

60

R. Su et al. / Quaternary International 213 (2010) 56–68

A

-7

-5 37.11

35.11

33.11

31.11

29.11

27.11

25.11

23.11

21.11

19.11

17.11

15.11

13.11

δ18O (‰)

-6

-4 11.11

Depth (cm)

B

150

152

154

156

158

160

162

164

166

168

170

172

174

176

178

180

182

184

186

188 -7

-5

δ18O (‰)

-6

-4 1.0 0.0 150

152

154

156

158

160

162

164

166

168

170

172

174

176

178

180

182

184

186

188

-1.0

Δδ18O (‰)

C

Relative time (yr)

151 153 155 157 159 161 163 165 167 169 171 173 175 177 179 181 183 185 187 189

D SST (°C)

-0.2

0 -2

0.2

-4

0.6

-8

-0.6

Summer SSS

-4

-0.2

0 0.2

4 8

151 153 155 157 159 161 163 165 167 169 171 173 175 177 179 181 183 185 187 189 Relative time (yr)

Δδ18O (‰)

SSS (‰)

E

-0.6

Winter SST

2

Δδ18O (‰)

4

0.6

Fig. 3. Oxygen isotopic record of the w5.4 ka BP coral OC1 for A. d18O versus depth from the upper surface. B. d18O time series using chronological method applied to the modern coral converted sample depth to time (Su et al., 2006). C. d18O deviation (Dd18O) after removing the SST contribution to coral d18O using method applied to the modern corals (Su et al., 2006), thereby providing an estimate of changes in d18O of seawater and SSS. D. Winter (December–February) d18O deviation (Dd18O), after subtracting the average winter d18O value from the record, as an indicator of SST interannual variability during the northeast monsoon. E. Summer (June–August) d18O deviation (Dd18O), after subtracting the average summer d18O deviation from the record, as an indicator of SSS interannual variability during the southwest monsoon.

0.98& and 1.25&, Sun et al., 2005), the respective values were 0.34& and 0.13& higher and 0.3& lower. These data suggest that winter–summer and winter d18O seasonality of the w5.4 ka BP coral would be w26% and w23% higher than that of the modern corals, whereas the summer seasonality was 18% lower. Compared to the w4.4 ka BP coral, the respective values were w16% and w12% higher and 32% lower. In addition, as an indicator of interannual variability, the d18O standard deviation for the w5.4 ka BP coral OC1 mean (0.68&), maxima (0.23&) and minima (0.21&) were w23% and w22% higher and w24% lower than the respective values of the modern corals (0.525&, 0.18& and 0.26&, Su et al., 2006). The respective values were w19% and w4% higher and w38% lower than those of the w4.4 ka BP coral OC2 (0.55&, 0.22& and 0.29&, Sun et al., 2005). Together, the results imply enhanced winter–summer and winter seasonality, and much stronger interannual variability for SST, SSS and Asian monsoon during the middle Holocene relative to today. Decreased seasonality and weakened interannual variability marked in summer at w5.4 ka BP relative to w4.4 ka BP and to today.

4. Discussion 4.1. Reconstructions of winter SST and summer rainfall The d18O data for the modern coral OC03 (Figs. 4A1,A2) and QG5 (Figs. 4B1,B2) contain 36 and 17 annual cycles respectively, which show an excellent correlation between d18O and SST, especially over the winter half year (Su et al., 2006). Regression analysis demonstrates a strong correlation between the d18O of the modern coral OC03 and QG5 and SST for the entire dataset (R ¼ 0.75 and 0.88), winter extremes (R ¼ 0.88 and 0.65, Fig. 4C), and annual means (R ¼ 0.42 and 0.57). This shows seasonal-annual variability of SST control on that of coral d18O especially in winter (Su et al., 2006). The slopes of the d18O-temperature relationship, 0.183&  C1 (Sun et al., 2005) and 0.17&  C1 in this study for the northern SCS, are within the range of 0.216&  C1 to 0.153&  C1 for the Pacific Ocean (McConnaughey, 1989; Suzuki et al., 1999; Wellington and Dunbar, 1995; Mitsuguchi et al., 1996; Quinn et al., 1996; Gagan et al., 1998). Thus, high-resolution coral

R. Su et al. / Quaternary International 213 (2010) 56–68

SST (°C)

-5.8 -4.8 -3.8 60

40

20

SST

35

-5.8 -4.8

20

-3.8 15 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004

0

Time (yr)

B2

)

16

14

12

10

8

6

4

2

O( 18

-4.8

20 15 1987

0

-5.8

25

)

-4

1990

D

24

1999

2002

-3.8

)

24

SSS (

SST (°C)

20

16 SST=-5.8925 18Ocoral-7.577 R= -0.88

-5.4

1996

32

22

18

1993

Time (yr)

Depth (cm)

C

-6.8

O(

-5

Oxygen Isotope

30

18

-6

SST

35

SST (°C)

-7

18

-6.8

25

Depth (cm)

B1

Oxygen Isotope

30

O( )

18

A2

-6.8

18

O( )

A1

61

-5.2

-5.0 18

-4.8

O(

SSS=12.489 18Ocoral + 95.89 R = 0.66

-4.6

-4.4

)

8 -6.4

-6.2

-6.0

-5.8 18

O(

-5.6

-5.4

)

Fig. 4. Comparison between modern coral d18O and instrumental SST and linear regression analysis between the coral OC03 d18O and winter SST and summer SSS extremes for A1. d18O versus depth from the upper surface of the coral OC03 (Su et al., 2006). A2. d18O versus time of the coral OC03 compared with 10-day average SST calculated from daily records for the period of 1968–2003 at Qinglan oceanic station (Su et al., 2006). B1. d18O versus depth from the upper surface of the coral QG5 (Su et al., 2006). B2. d18O versus time of the coral QG5 compared with 10-day average SST calculated from daily records for the period of 1987–2002 at Qinglan oceanic station (Su et al., 2006). C. Linear regression analysis between the modern coral OC03 d18O maxima and winter SST extremes from 1968 to 2003. D. Linear regression analysis between the modern coral OC03 d18O minima and summer SSS extremes from 1968 to 2003.

d18O records from the relatively dry eastern equatorial Pacific and subtropical western Pacific are good recorders of SSTs because seawater d18O influence on coral d18O is small, relative to that brought about by SST. Linear regression analysis of instrumental and reconstructed winter SST yielded good best-fit correlations (r) of 0.65 (Sun et al., 2005) and 0.88 in this study. The range of interannual variability in reconstructed winter SSTs is 1.0  C (1s) from the coral OC03 d18O over the period of 1968–2002 and 1.2  C (1s) from the coral QG5 d18O over the period of 1987–2002, in good agreement with the 1.0  C and 1.5  C (1s) range shown by the instrumental SST record respectively. Moreover, there is a good correlation between the warmest winter SSTs and recorded ˜ o events (such as 1969, 1972, 1975, 1977, moderate–strong El Nin 1982, 1987, 1991, 1993, 1998, 2002; Figs. 4A2,B2). Previous studies have shown that Asian winter monsoonal winds are weaker during ˜ o (Hanawa et al., 1989; Li, 1990; Wang the mature phase of an El Nin et al., 2000). Weaker northeast winds in winter create warmer ˜ o events because of reduced advection of winter SSTs during El Nin relatively cool extra-tropical water into the SCS (Bogdanov and Moroz, 1995). In addition, the deviations of the modern coral d18O with respect to SST mostly occur in annual summer extremes when SSSs decrease to annual minima (Figs. 4A2,B2). Seawater composition, represented by SSS, significantly influences coral d18O in summer, resulting in a deviation from the expected SST effect, particularly when the salinity reaches the annual minima revealed by linear

regression analysis between coral d18O deviation from SST and SSS (Sun et al., 2005; Su et al., 2006). Statistical analysis of meteorological data indicates that the local summer SSTs show a range of 1.6  C over the last 36 years, which only induces 0.27& of coral d18O variance. However, the range of the summer d18O minima of the coral OC03 is 1.13&. It is clear that 76% of the coral OC03 d18O is induced by seawater composition, and only 24% is induced by SST. Thus, interannual variability for coral d18O summer extremes mainly is associated with seawater composition, indicating that coral d18O should be a sensitive recorder of summer rainfall (Su et al., 2006). Linear regression and covariance analyses also demonstrate annual mean SSSs from the Qinglan oceanic station, which reflect combined influence of direct rainfall and nearby river runoff, and more significantly correlate with annual mean rainfalls from the Qionghai meteorological station for the periods of 1987– 2002 (R ¼ 0.91 and COV(SSS, rainfall) ¼ 95.5) and 1968–2002 (R ¼ 0.51 and COV(SSS, rainfall) ¼ 53.2). A significant correlation exists between SSS and seawater d18O (R ¼ 0.96, Sun et al., 2005), and coral d18O and SSS extremes in summer (R ¼ 0.66, Fig. 4D) over the last 36 years at Hainan Island. Linear regression analysis of instrumental and reconstructed summer SSS yield a good best-fit correlation (r) of 0.65. The maximal interannual variability in summer SST at Hainan Island is only w1.6  C, and is often much less, so the SST potential effect on coral d18O during summer is less than 0.3&. The interannual variational range of summer d18O recorded by the modern coral OC03 and QG5 is 1.13&

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and 1.1& respectively, which is three to four times larger than that induced by summer SST interannual differences. Thus, coral d18O during summer is significantly affected by interannual variability in summer rainfall, coastal SSS and seawater d18O. ˜ a years are associated with greater typhoon activity in the La Nin SCS (Elsner and Liu, 2003). Some summers, being particularly wet, ˜ a event (such as 1970, correspond with a well-developed La Nin 1972, 1974, 1975, 1986, 1996 and 2001; Figs. 4A2,B2). Relatively dry summers generally reflect rainfall decreases over the SCS (Wang ˜ o events (such as et al., 2000; Dai and Wigley, 2000) during El Nin 1969, 1980, 1981, 1988, 1989 and 1992; Figs. 4A2,B2), and reduced incidence of typhoons. 4.2. Evolution of SST and SSS The d18O mean, maximum and minimum values show a significantly cooler SST and/or higher SSS and seawater d18O and/or lower rainfall both at w5.4 ka BP and w4.4 ka BP, especially in winter, relative to today, and a significant warmer SST and/or lower SSS and seawater d18O and/or higher rainfall in summer at w5.4 ka BP relative to w4.4 ka BP. Thus, significant factors affecting coral d18O changes were SST or SSS variance or a joint effect of both SST and SSS variance, mainly influenced by seawater d18O and/or rainfall and/or evaporation, vertical mixing and seawater exchange between the SCS and the northwestern Pacific. Coral d18O increment would be mainly induced by a joint effect of SST cooling and SSS (seawater d18O) increment during the middle Holocene. According to the reconstructed SST- and SSS-d18Ocoral formulas (Figs. 4C,D), if the respective w0.49& and w0.75& mean d18O increments for the w5.4 ka BP and w4.4 ka BP coral was solely affected by SST cooling or SSS increments, this might imply a much more significant mean SST cooling in the past (equal to w2.9  C and w4.4  C) or SSS increment (equal to w6.1& and w9.4&) relative to today. It is unlikely that such significant SST cooling or SSS increments occurred, as palaeoclimatic records and models both show that SSTs and SSSs during the middle Holocene were similar to the modern SSTs and SSSs in the SCS and the northwestern Pacific. Many reconstructed palaeo-SSTs show a cooling with deviations of w4–0  C. Comparatively, only a few showed warming with deviations of w0–1.6  C during the middle Holocene in the SCS and northwestern Pacific relative to today. Wei et al. (2004) from coral Sr/Ca ratios, revealed that SSTs during 539–530 BC were roughly the same as those during 1990–2000 AD, but the period of 489–500 AD was significantly cooler, with annual mean, minimal month winter and maximal month summer SSTs being 2  C, 2.9  C and 1  C lower than today. Sun et al. (2005) indicated that the mean d18O-SST drop was less than 0.5  C of modern values during the middle Holocene in the northern SCS. Morimoto et al. (2007) reconstructed SSTs from coral d18O-Sr/Ca ratios at Kikai Island in the subtropical northwestern Pacific at 6.18 ka BP that were approximately the same as those at present, and at 7.01 ka BP were slightly cooler than today by 0.2–0.8  C. Reconstructed SSTs from many deep sea sediment cores in the SCS (Pelejero et al., 1999a, b; Wang et al., 1999; Kienast et al., 2001; Steinke et al., 2001; Guodong et al., 2006; Yu et al., 2006) and adjacent Sulu Sea (Linsley, 1996; Rosenthal et al., 2002; Stott et al., 2002; Oppo et al., 2003), the Ontong Java Plateau (Gagan et al., 2004), Okinava Trough (Zhao et al., 2005), tropical Pacific (Koutavas et al., 2006; Brijker et al., 2007), all show a SST cooling with deviations within 1.5–0.5  C during the middle Holocene relative to today. Furthermore, compiled alkenone-derived annual mean SST records from marine sediments indicate a gradual temperature increase over the entire northwestern Pacific from 7.0 ka BP to the present (Kim et al., 2004). However, Yu et al. (2004, 2005) showed that the mean coral

Sr/Ca-SST at 7.0–7.5 ka BP would be w1.5–4  C lower than today, and the mean annual, maximal and minimal Sr/Ca-SSTs, at 6.789 ka BP, 5.906 ka BP and 5.011 ka BP were w0–1.6  C higher than today, and at 2.541 ka BP and 1.513 ka BP were w3.8–0  C lower than today. Lin et al. (2006) showed warmer conditions in the 4–6 ka BP relative to 6–10 ka BP and 0–4 ka BP with w0.3–0.8  C oscillations in the northern SCS. Moreover, SSTs reconstructed by foraminiferal Mg/Ca showed a decreasing trend over the last w7.3 ka in the tropics (Stott et al., 2004) and offshore southeast of Taiwan (Wang et al., 1994). In addition, many reconstructed palaeo-SSS have shown an increment with deviations of 0.5–2& (0.2–0.5& for seawater d18O). Comparatively, some show a drop with deviations of w0.5– 0& during the middle Holocene relative to today in the SCS and the northwestern Pacific. Coral Mg/Ca-d18O combined reconstructed SSS at 7.01 ka BP were w1& higher than today throughout the year, and w2& higher in winter than today at Kikai Island in the subtropical Pacific (Morimoto et al., 2007). Coral d18O reconstructed palaeo-SSS showed that seawater d18O during the middle Holocene was at least w0.5& higher than today (Sun et al., 2005), and coral Sr/Ca-d18O combined data implied 0.2–0.3& higher seawater d18O at 6.79 ka BP and 5.91 ka BP in the northern SCS (Yu et al., 2005). Foraminiferal Mg/Ca-d18O combined proxy analysis indicated a 0.5& increase in seawater d18O (1–1.5& higher in SSS) in the tropical western Pacific (Stott et al., 2004), and a w0.8–1.7& increase in SSS offshore southeast of Taiwan Island in the subtropical Pacific (Wang et al., 1994) during the middle Holocene relative to the late Holocene. However, Lin et al. (2006) reconstructed palaeo-SSS that showed fresher conditions in 4–6 ka BP with SSS decrease of w0.5& and more saline conditions in 6–10 ka BP and 0–4 ka BP characterized by larger oscillations of w0.5–0.6& in the northern SCS. Based on the above reconstructed palaeo-SSTs and SSS, relative to today, if the winter DSSS increased at most by 2& (equal to w0.16& for coral Dd18O increment; Morimoto et al., 2007), the remainder of the w0.53& and w0.51& d18O maximal increment for the w5.4 ka BP and w4.4 ka BP coral would signal a 3.1  C and 3  C SST drop. If summer DSSS increased no more than 0.5& (equal to w0.04& for coral Dd18O increment; Yu et al., 2005), the remainder of the 0.1& and 0.45& Dd18O minimal increment for the two corals would signal a 0.6  C and 2.7  C SST drop. Similarly, if the mean SSS increased at most by w1.5& (equal to w0.12& for coral Dd18O increment; Stott et al., 2004; Sun et al., 2005), the remainder of the 0.37& and 0.63& Dd18O mean increment for the two corals would imply 2.2  C and 3.7  C mean SST drops, respectively. The results show that a larger part of coral d18O increment was induced by SST drop. Comparatively, only a tiny part was induced by SSS increment during the middle Holocene, particularly in winter relative to today. In addition, the 0.02& and 0.36& differences between the two fossil coral d18O maxima and minima might indicate significant SST and SSS changes in summer but almost no changes in winter between w5.4 ka BP and w4.4 ka BP. Similarly, if the summer DSST increased less than 1.6  C (Yu et al., 2005), DSSS might decrease more than 1.1&, indicating warmer and less saline conditions in summer at w5.4 ka BP relative to w4.4 ka BP. These results appear to show that DSST drops and DSSS increments appropriately were within the reconstructed palaeo-SST decreased and DSSS increased deviations of w4–0  C and 0.5–2&, but summer DSSS decrease at w5.4 ka BP ago significantly exceed the reconstructed palaeo-SSS decrease deviations of w0.5–0&, because they all were derived from the suppositions based on reconstructed palaeo-DSSS and DSST in different study sites, which might restrict SST and SSS changes at the study site. Moreover, if SST decreased on average by w0.5  C (Sun et al., 2005), only w0.09& of coral d18O increment would be affected by

R. Su et al. / Quaternary International 213 (2010) 56–68

middle Holocene cooling, and the remaining w0.40& and w0.66& for the d18O mean increments of the two fossil corals might reflect a much higher seawater d18O (or SSS) increment (equal to w5& and w8.2& for SSS increment respectively) relative to today. Apparently, it is impossible for such high SSS mean increments to occur. Similarly, if SST average decreases exceed w4  C (Yu et al., 2004), there would be at least w0.68& of coral d18O increment affected by middle Holocene cooling, higher than the 0.49& value for the w5.4 ka BP coral. Only w0.07& would remain from the 0.75& for the w4.4 ka BP coral, reflecting w0.9& SSS increment. It is obvious that SST cooling would be within w4–0.5  C during the middle Holocene. Therefore, at w5.4 ka BP and w4.4 ka BP, the mean, winter and summer SST would be at most 2.2  C, 3.1  C and 0.6  C lower, and 3.7  C, 3  C and 2.6  C lower than today, respectively. The summer SST at w5.4 ka BP would be at least 1.6  C higher than w4.4 ka BP. Accordingly, the mean, winter and summer SSS during middle Holocene would be at least 1.5&, 2& and 1& higher than today, and summer SSS at w5.4 ka BP would be less by 1.1& relative to w4.4 ka BP. Today, relatively low SSS in the SCS reflects the balance between high rainfall, evaporation, and the somewhat restricted seawater exchange between the SCS and Pacific. Palaeoclimatic records and models agree that summer monsoonal rainfall was generally higher over East Asia (Morrill et al., 2003; Wang et al., 2005) during the middle Holocene, so it was possible that greater convergence of 18 O-depleted monsoon rainfall on land could have left the surface ocean enriched in 18O (Sun et al., 2005). However, palaeoclimate records (Penner et al., 1997; Wang et al., 1999; Rujian and Jian, 2003) and coupled model simulations (Liu et al., 2003, 2004) indicated that rainfall also increased over the SCS, so it is difficult to simply invoke a reduction in 18O-depleted rainfall to drive higher SSS and surface seawater d18O. A significant 18O increment of Asian summer monsoon rainfall during the mid-Holocene was also unlikely because the d18O of dated groundwater from the Asian monsoon domain had not changed during the Holocene (Aggarwal et al., 2004). Thus, SST cooling and SSS increment might correlate with a more stable and intensified Asian monsoon during the middle Holocene, especially in winter. 4.3. Seasonal evolution of Asian monsoon and SSS Comparing the 40-year and 54-year middle Holocene coral OC1 (Figs. 3A,B) and OC2 (Sun et al., 2005) d18O records with the 36-year and 17-year modern coral OC03 (Figs. 4A1,A2) and QG5 d18O records (Fig. 4B1,B2, Su et al., 2006) shows that the winter–summer and winter seasonality of the coral d18O at w5.4 ka BP increased by w26% and w16%, and w23% and w12% relative to the 4.4 ka BP and modern corals, respectively. The summer seasonality decreased by w18% and w32% respectively. These results imply that winter– summer and winter seasonality of Asian monsoon during the middle Holocene was enhanced compared to today, but summer seasonality at w5.4 ka BP was less than that at w4.4 ka BP ago and today in the northern SCS. The SCS seawater d18O increase during the middle Holocene was due, at least in part, to enhanced seasonality of insolation which strengthened the Asian monsoon, wind-induced evaporation, stronger vertical mixing, and/or greater seawater exchange rates with the open Pacific. Research has shown that insolation variability was partly responsible for changes of the Holocene Asian monsoonal intensification (Shindell et al., 2001). The seasonal amplitude of insolation was 193 W/m2 at 5.4 ka BP, 171 W/m2 at 4.4 ka BP, and 154 W/m2 at the present study site (19 N, Sun et al., 2005), as calculated by the method of Berger (1978). The winter– summer seasonality of insolation at 5.4 ka BP and 4.4 ka BP ago was w20% and w11% (Sun et al., 2005) higher than today, which almost

63

corresponds to the w26% and w12% values of the winter–summer seasonal d18O increment of the w5.4 ka BP and w4.4 ka BP corals respectively. Therefore, seasonal characteristic and amplitude of middle Holocene coral d18O could be explained by insolation seasonal changes. Seasonal increments in coral d18O were mainly caused by SST cooling. The Asian monsoon intensified and was sustained during the middle Holocene (Wajsowicz and Schopf 2001) particularly during winter dry seasons, which transported cooler air masses over the SCS and northwestern Pacific, which resulted in air temperature and SST decreases (Gong et al., 2001; Hsu et al., 2001). At the same time, evaporation from the sea surface, vertical entrainment and mixing, and seawater exchange between the SCS and northwestern Pacific all would be enhanced, which further induced SST decrease and SSS increase (Shindell et al., 2001). This would directly correlate with the orbitally induced insolation seasonal changes (Liu et al., 2003; Wang et al., 2005) and/or northward-displaced marine intertropical convergence zone (ITCZ) favoring more permanent southeast trades and promoting cool upwelling (Koutavas et al., 2006). In addition, seawater d18O increase in winter and decrease in summer both probably produce coral d18O seasonal increment. SSS is close to the local SSS in winter. The seasonal variability of modern SSS is mainly caused by summer rainfall, and values remain at the winter high level over most of the year (Sun et al., 2005). Moreover, SSS only decreases while rainfall is obviously enhanced, so SSS variance is close to the winter SSS average state in a large area of the SCS. Seawater d18O and SSS in winter dry seasons during the middle Holocene would be significantly higher than today in the SCS. Summer monsoonal seasonality was relatively unstable and much weaker at w5.4 ka BP ago relative to today and w4.4 ka BP ago. As monsoonal rainfall could not significantly increase, the seasonal increment of coral d18O mainly resulted from decrease of winter SST and increase of winter seawater d18O.

4.4. Evolution of interannual variability of Asian monsoon The interannual variability showed stronger interannual variability for winter–summer and winter Asian monsoon during the middle Holocene. Weaker interannual variability for summer Asian monsoon at w5.4 ka BP ago is consistent with significantly weaker ENSO ocean-atmosphere variability (Clement et al., 2000; Tudhope et al., 2001; Moy et al., 2002; Koutavas et al., 2006). The interannual variability of the Asian Monsoonal system is directly correlated with orbitally induced insolation interannual variance (Liu et al., 2003; Wang et al., 2005) during the middle Holocene in the northern SCS. In addition, the spectral analyzed results, using the Multitaper Method offered by the SSA-MTM software (Fig. 5; Ghil et al., 2002), reveal that interannual variances are much stronger at 3.5 years, 4 years, 4.7 years, 8 years and 11 years, stronger at 3.1 years and 5.6 years and sub-stronger at 2.6 years in the instrumental SST time series (Fig. 5A), are much stronger at 14 years and 2 years and substronger at 3.1 years and 4.7 years in the instrumental SST and SSS time series (Fig. 5B). They are also stronger at 14 years and substronger at 2.2 years, 3.5 years and 4 years in the modern coral OC03 d18O time series (Fig. 5C). The cycles of 3.1 years, 3.5 years, 4 years, 4.7 years and 8 years all are within the period of ENSO variance from 3 years to 8 years. This indicates that ENSO significantly influences SST and subsignificantly affects on SSS and coral d18O (Hanawa et al., 1989; Wang et al., 2000). The 14 year, 11 year and 2 year cycles possibly reflect a complicated correlation among ENSO, rainfall, SSS, SST and coral d18O today in the northern SCS.

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Frequency (cycles yr-1) Fig. 5. Results of Multitaper Method of spectral analysis with F-test (number of tapers, 3; resolution, 2; Ghil et al., 2002) for A. the instrumental SST time series for the period of 1961–2003. B. The instrumental SSS time series for the period of 1961–2002. C. The modern coral OC03 d18O time series from 1968 to 2003 (Su et al., 2006). D. The w4.4 ka BP coral OC2 d18O time series (Sun et al., 2005). E. The w5.4 ka BP coral OC1 d18O time series. 99%, 95% and 90% significant level were indicated by broken lines.

In contrast, the interannual variances of the middle Holocene coral d18O records display significant different spectral cycles. The sub-significant variances at interannual periods of 2.6–3 years and 2.3–2.6 years was revealed in the w4.4 ka BP (Fig. 5D) and w5.4 ka BP coral d18O records (Fig. 5E). They are similar but shorter than the 3–8 year ENSO variation band, which showed that a biennial cycle of monsoon rainfall would be still dominant although no significant ENSO variance occurred during the middle Holocene. At the same time, a much more significant cycle at 18.7 years was revealed in the w5.4 ka BP coral d18O record (Fig. 5D), which might show an internal correlation of interannual variability among summer and winter monsoon, SST, SSS and coral d18O during the middle Holocene. The above results suggest that ENSO influence on SST and a teleconnection between the atmospheric ENSO and monsoon rainfall during the middle Holocene existed, although they are much weaker than today in the northern SCS. The recent analytical results of instrumental records and climatic models also indicate

that the Asian monsoon can influence ENSO evolution through circumfluence over the northwestern Pacific (Rodbell et al., 1999; Kim et al., 2001; Lau, 2001). Monsoonal intensification would have restrained developments of warm ENSO events through intensification of trade winds in the equatorial Pacific (Liu et al., 2000). Modeling (Clement et al., 2000), coral records from Papua New Guinea (Tudhope et al., 2001), lake sediments from Ecuador (Moy et al., 2002), and foraminifer d18O-Mg/Ca data from deep-sea sediments in the eastern tropical Pacific (Koutavas et al., 2006) have revealed a drastic reduction of mid-Holocene ENSO activity. This was the result of more sustained southeast trades and a steadier equatorial upwelling regime coinciding with a northward-displaced ITCZ, the position of which was an important factor in the low-frequency modulation of ENSO and could influence its future evolution (Koutavas et al., 2006). The variations most likely had an orbital source (Clement et al., 2000; Haug et al., 2001. However, Brijker et al. (2007) suggested that the present-day ENSO activity started around 5.5 ka BP in the Indo-Pacific Warm Pool, which was

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attributed to precessionally-driven changes in the seasonal distribution of solar radiation. Apparently, these results show no significant ENSO variance at the interannual periods of 3–8 years, but only sub-significant variances at the interannual periods of 2.3–2.6 years and 2.6– 3 years at w5.4 ka BP and w4.4 ka BP respectively. Do they show weaker palaeo-ENSO variance or biennial variance of Asian monsoon rainfall? Biennial variance of the Asian monsoon did not force strong biennial variance of ENSO (Rodbell et al., 1999; Tudhope et al., 2001; Moy et al., 2002; McGregor and Gagan, 2004). Thus, ENSO variance affecting Asian monsoonal rainfall and biennial rainfall might have been restricted during the middle Holocene in the northern SCS. 4.5. Evolution of decade trend The means, winter maxima and summer minima for the modern coral OC03 d18O from 1968 to 2003 (Fig. 6A) and QG5 from 1987 to 2002 (Fig. 6B) all show a long-term decreasing trend, which are consistent with that for SSTs showing a long-term increasing trend (Fig. 6C) and that for SSSs showing a long-term decreasing trend (Fig. 6D) from 1967 to 2003 at the Qinglan oceanic station. The means and summer maxima for rainfalls show a long-term increase trend from 1967 to 2003 at the Qionghai meteorological station (Fig. 6E). These results suggest that long-term mean, maximal and

minimal trends of modern coral d18O decreases are controlled by SST increases, SSS decreases and long-term mean and summer trends of rainfall increases. However, this is not consistent with the long-term winter trend of rainfall decrease differing from that of SSS decrease, which shows a decadal decreasing trend of modern winter rainfall with no effect on modern winter coral d18O and SSS. The d18O means, winter maxima and summer minima for both the w5.4 ka BP and w4.4 ka BP coral OC1 (Fig. 6F) and OC2 (Fig. 6G) all show long-term decreasing trends, consistent with that of the modern corals OC03 (Fig. 6A) and QG5 (Fig. 6B). However, the long-term trends of middle Holocene coral d18O decreases should be similar to that of the modern coral d18O decrease controlled by SST increase, SSS decrease and/or summer rainfall increase. Previous studies of Holocene coral d18O records (Abram et al., 2001; Yu et al., 2005; Morimoto et al., 2007) showed similar increases to the 0.49& and 0.75& mid-Holocene mean increments relative to that of modern corals at Hainan Island. Combining these results, d18O values of middle Holocene corals were consistently higher than those of modern corals in the subtropical and tropical northwestern Pacific, and similar to the trends shown by middle Holocene foraminifers in the northern SCS (Wang et al., 1999) and tropical west Pacific (Stott et al., 2004) over the last w7.3 ka BP. The above discussion suggests a long-term SST increase trend (Linsley 1996; Pelejeri et al., 1999a,b; Wang et al., 1999; Kienast

B )

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Fig. 6. Comparison among long-term trends of maximal, mean and minimal time series of the modern and middle Holocene coral d18O, instrumental SST, SSS and rainfall, and local insolation in the northern SCS for A. the modern coral OC03 d18O time series. B. The modern coral QG5 d18O time series. C. The instrumental SST time series at Qinglan oceanic station. D. The instrumental SSS time series at Qinglan oceanic station. E. The instrumental rainfall time series at Qionghan meteorological station. F. The w4.4 ka BP coral OC2 d18O relative time series. G. The w5.4 ka BP coral OC1 d18O relative time series. H. Variability of the local insolation from w7.3 ka BP to today at the east coast of Hainan Island (calculated using the method provided by Berger, 1978). Perpendicular oblong bars express the fossil coral OC1 and OC2 and modern corals.

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et al., 2001; Kim et al., 2004) and a long-term SSS decrease trend over the last w7.3 ka BP in the SCS and northwestern Pacific (Wang et al., 1994; Lin et al., 2006; Stott et al., 2004; Morimoto et al., 2007), which appear to be consistent with the 5.4 ka BP and 4.4 ka BP coral d18O decrease trends. Moreover, a long-term decreasing trend of the Qingge insolation (Fig. 6H) was significantly consistent with that of the middle Holocene coral OC1 (Fig. 6G) and OC2 (Fig. 6F) and modern coral OC03 (Fig. 6A) and QG5 (Fig. 6B), which reflects a close connection between the long-term decreasing trend of coral d18O and local insolation on the east coast of Hainan Island. Therefore, the long-term trends of the 5.4 ka BP and 4.4 ka BP coral d18O decreases would be controlled by SST increases, and SSS and local insolation decreases from the middle Holocene to the present. 5. Conclusions The d18O mean, maximum and minimum of the 5.4 ka BP coral were 0.49&, 0.69& and 14& higher than that of the modern corals, and were 0.26& lower, 0.02& higher and 0.36& lower than that of 4.4 ka BP coral. The data imply that the mean, winter and summer SST at w5.4 ka BP, would be at most 2.2  C, 3.1  C and 0.6  C lower, and at w4.4 ka BP ago, would be at most 3.7  C, 3  C and 2.6  C lower than today respectively. The summer SST warming would exceed 1.6  C at w5.4 ka BP relative to w4.4 ka BP. Accordingly, the mean, winter and summer SSS during the middle Holocene would be at least 1.5&, 2& and 1& higher than today, and summer SSS at w5.4 ka BP would be less than 1.1& lower than at w4.4 ka BP. At the same time, there would be similar conditions in winter both at w5.4 ka BP and w4.4 ka BP. Thus, SST cooling and SSS increments might correlate with the more stable and intensified Asian monsoon during the middle Holocene, especially in winter. There was much stronger and enhanced winter–summer and winter, and weaker and decreased summer seasonality and interannual variability for SST, SSS and Asian monsoon at w5.4 ka BP, as revealed by the enhanced mean and maximal seasonal amplitudes and standard deviations. Decreased minimal seasonal amplitude and standard deviation is apparent in the w5.4 ka BP coral d18O record relative to w4.4 ka BP and today, which might be controlled by the orbitally induced insolation seasonal and interannual changes during the middle Holocene. The spectral analysis results indicate that ENSO variance significantly influences interannual variability of SST and sub-significantly influences that of SSS and coral d18O. Biennial rainfall significantly influences that of SSS and sub-significantly influences that of SST and coral d18O. There is a complicated correlation in interannual variability among ENSO, SST, rainfall, SSS and coral d18O at present. In contrast, no significant ENSO variance exists at the interannual periods of 3–8 years, and only sub-significant variances are present at the interannual periods of 2.3–2.6 years and 2.6–3 years at w5.4 ka BP and w4.4 ka BP. The influence of ENSO variance on Asian monsoonal rainfall and biennial rainfall might have been restricted during the middle Holocene. A significant variance at 18.7 years was revealed in the w5.4 ka BP coral, which would reflect an internal correlation of interannual variability among summer and winter monsoon, SST, SSS and coral d18O during the middle Holocene in the northern SCS. The long-term trend of the w5.4 ka BP coral d18O decrease, similar to that of the modern and w4.4 ka BP coral d18O decreases, would be controlled by SST increases, and SSS and local insolation decreases from the middle Holocene to the present. Acknowledgments This research was financially supported by the National Science Foundation of China grants 40876025 and 40625009. We thank

Editor-in-Chief Norm Catto for earnestly taking charge of our manuscript review, two reviewers for strictly and finely commenting on our manuscript and putting forward many good recommendations.

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