Seasonal and interannual variability of the Mid-Holocene East Asian monsoon in coral δ18O records...

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Earth and Planetary Science L

Seasonal and interannual variability of the Mid-Holocene East

Asian monsoon in coral d18O records from the South China Sea

Donghuai Sun a,b, Michael K. Gagan c,*, Hai Cheng d, Heather Scott-Gagan c,

Carolyn A. Dykoski d, R. Lawrence Edwards d, Ruixia Su a

aSouth China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, ChinabState Key Laboratory of Loess and Quaternary Geology, Earth Environmental Institute, Chinese Academy of Sciences,

Xi’an, 710075, ChinacResearch School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

dMinnesota Isotope Laboratory, Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA

Received 15 November 2004; received in revised form 19 April 2005; accepted 7 June 2005

Available online 21 July 2005

Editor: E. Boyle

Abstract

Understanding the full range of past monsoon variability, with reference to specific monsoon seasons, is essential to test

coupled climate models and improve their predictive capabilities. We present a 54-year long, high-resolution skeletal oxygen

isotope (d18O) record extracted from a well-preserved, massive Porites sp. coral at Hainan Island, South China Sea, to

investigate East Asian monsoon variability during summer and winter ~4400 calendar yr ago. Analysis of modern coral d18O

confirms that Porites from Hainan Island are well positioned to record winter monsoon forcing of sea surface temperature

(SST), as well as the influence of summer monsoon rainfall on sea surface salinity (SSS).

The coral record for ~4400 yr ago shows ~9% amplification of the annual cycle of d18O, in good agreement with coupled

ocean–atmosphere models showing higher summer rainfall (lower coral d18O) and cooler winter SSTs (higher coral d18O) in

response to greater Northern Hemisphere insolation seasonality during the Middle Holocene. Mean SSTs in the South China

Sea during the Mid-Holocene were within 0.5 8C of modern values, yet the mean d18O for the fossil coral is ~0.6x higher than

that for the modern coral, suggesting that the d18O of surface seawater was higher by at least ~0.5x, relative to modern values.

The 18O-enrichment 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 South China Sea. The 18O-enrichment of the northern South China Sea ~4400 yr ago contributes to mounting

evidence for recent freshening of the tropical Western Pacific.

Today, winter SST and summer SSS variability in the South China Sea reflect the interannual influence of ENSO and the

biennial variability inherent to monsoon precipitation. Spectral analysis of winter SSTs ~4400 yr ago reveals a strong ENSO

cycle at 6.7 y, which is significantly longer than the average 3.6 y cycle observed since 1970. The results suggest that the

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ng author. Tel.: +61 2 6125 5926; fax: +61 2 6125 0738.

ss: [email protected] (M.K. Gagan).

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D. Sun et al. / Earth and Planetary Science Letters 237 (2005) 69–8470

influence of ENSO on winter SSTs in the South China Sea was well established by ~4400 yr ago. However, spectral analysis of

summer SSS ~4400 yr ago shows no significant ENSO cycle, suggesting that teleconnections between ENSO and summer

monsoon rainfall were restricted. Taken together, the results indicate marked differences in ENSO–monsoon interactions during

the winter and summer monsoon seasons in the past. The fossil coral d18O record also shows that the amplitude of interannual

SST and SSS variability was stronger ~4400 yr ago, despite ENSO variability being significantly weaker in the Pacific region.

Thus it appears that the strengthened Mid-Holocene monsoon was sensitive to forces, other than ENSO, that acted as alternative

drivers of interannual monsoon variability. If this is the case, greater interannual climate variability could accompany the

strengthening of the Asian monsoon predicted to occur during the 21st century as transient greenhouse warming preferentially

warms Eurasia, even if ENSO perturbations remain relatively stable.

D 2005 Elsevier B.V. All rights reserved.

Keywords: coral; oxygen isotopes; East Asian monsoon; ENSO; South China Sea

1. Introduction

The East Asian monsoon is a prominent feature of

the tropical general circulation that impacts the lives

of ~25% of the world’s population, yet its year-to-year

variability is still difficult to predict [1,2]. Recent

terrestrial paleoclimate records show that the East

Asian monsoon changed dramatically at millennial

to century time-scales during the last deglaciation

[3,4], and was stronger during the Early to Middle

Holocene [5,6]. In contrast, while the Mid-Holocene

East Asian monsoon was relatively strong, El Nino-

Southern Oscillation (ENSO) variability in the Pacific

was weak [7–10]. Recent paleoclimate models of the

Holocene evolution of the Asian monsoon and ENSO

are now generally in good agreement with paleo-

records in showing opposing Holocene trends toward

a weaker Asian monsoon and stronger ENSO (e.g.,

[11,12]; among others). However, given that mon-

soons are characterized by a summer precipitation

maximum and a winter precipitation minimum caused

by thermally-driven seasonal reversals of the winds,

understanding the processes controlling Holocene

monsoon behavior requires reconstructions capable

of resolving specific monsoon seasons. In particular,

there is a need to reconstruct the ocean–atmosphere

feed-backs on seasonal time-scales that enhance or

mitigate the response of monsoons to changes in

insolation forcing during the Holocene [13].

Precisely dated, long-lived Porites sp. corals are

well suited for defining paleomonsoon variability be-

cause they have the ability to track changes in sea-

surface temperature (SST) and salinity (SSS) within

the annual cycle [14]. Recently, oxygen isotope ratios

(d18O) in the skeletal aragonite of well-preserved

fossil Porites from the tropical Pacific have been

used to document ENSO variability during the Holo-

cene [8,10,15,16]. However, paleomonsoon recon-

structions based on Porites corals from the South

China Sea, which is closely linked to the East Asian

monsoon, are limited to investigations of 20th century

climate variability [17–19]. A recent study of skeletal

Sr /Ca ratios in fossil specimens of the branching

coral, Goniopora, revealed strong cooling of SSTs

during winter in the northern South China Sea 7500

to 7000 yr ago [20].

Abundant Holocene coral reefs are preserved along

the east coast of Hainan Island, in the northern South

China Sea (Fig. 1). The climate of the subtropical

northern South China Sea region is dominated by

the East Asian monsoon, which includes prominent

seasonal changes in SST, precipitation, and wind di-

rection [21,22]. We present a continuous 54-year-

long, high-resolution skeletal d18O record of seasonal

to interannual paleomonsoon variability extracted

from a high-quality Porites specimen dating to 4400

calendar yr ago. This time-slice of the Mid-Holocene

falls within the period of stronger monsoon [5,6] and

weaker ENSO variability [7–10].

Reconstructing monsoon variability during this

period is important because recent studies indicate

that the East Asian monsoon may not be a passive

component of the broader tropical climate system.

For example, statistical analysis of instrumental cli-

mate records indicates that the East Asian monsoon

may influence the evolution of ENSO events [23,24].

It has been proposed that a stronger East Asian

monsoon could reduce ENSO variability through

Fig. 1. Summary of SST, SSS, and surface-ocean circulation pat-

terns in the tropical western Pacific and South China Sea. (A)

Distribution of mean annual SST and location of coral sampling

site (white circle) on the northern subtropical periphery of the

Indo–Pacific Warm Pool (SSTN28 8C). (B) Distribution of mean

annual SSS showing relatively low SSS of the South China Sea.

(C, D) Surface-ocean circulation patterns in the South China Sea

during the boreal summer East Asian monsoon (C) and boreal

winter East Asian monsoon (D), from Ref. [73]. SST and SSS

data are from the NOAA NODC World Ocean Atlas 1998, http://

iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NODC/.WOA98/.

D. Sun et al. / Earth and Planetary Science Letters 237 (2005) 69–84 71

its increased influence on the Pacific tradewinds

[12,23]. Given that the strength of the Asian mon-

soon has been predicted to increase during the 21st

century as transient greenhouse warming preferen-

tially warms the Eurasian landmass [25,26], proxy

records of altered monsoon–ENSO interactions of

the past are essential for understanding future climate

change.

2. South China Sea and East Asian Monsoon

The oceanography and climatology of the northern

South China Sea are closely linked with the Indo–

Pacific Warm Pool, where mean annual SSTs exceed

28 8C [27]. Hainan Island is located on the subtropical

northern periphery of the Indo–Pacific Warm Pool

(Fig. 1) where the annual mean SST was 26.0 8Cfor the period 1970–2002, with an average seasonal

range of 12.1 8C. Annual SST maxima (ave. 30.8 8C)and minima (ave. 18.7 8C) typically arrive in July and

January, respectively. Winter SSTs at Hainan Island

are significantly colder than those at ~198N elsewhere

in the South China Sea because of dry, cold air flow-

ing from the Asian landmass and greater wind-in-

duced mixing of the coastal water column by strong

northeasterly winds [28].

The northern South China Sea is subject to prom-

inent seasonal changes in precipitation and wind

direction driven by the East Asian monsoon

[21,22]. Mean annual rainfall at Hainan Island is

1953 mm (1960–2002), with distinctly wet summers

(ave. 863 mm) and dry winters (ave. 151 mm).

Southwest monsoon winds bring humid air masses

from low latitudes during summer (May–October)

and cold, dry northeasterly winds result in a winter

dry season (November–April). Sea surface salinity

(SSS) in the South China Sea, as a whole, fluctuates

between 33.3–34.0 psu (ave. 33.7 psu, Ref. [29]).

However, SSS is strongly seasonal near the coast of

Hainan Island, falling to a mean value of 26.5 psu

due to runoff and direct in-mixing of freshwater

during the summer wet season.

The mean SSS of the South China Sea is a sensi-

tive indicator of the balance between the amount of

summer monsoon precipitation, evaporation, and the

rate of exchange of marginal South China Sea water

with the western Pacific Ocean [28]. There is a strong

SSS gradient between the South China Sea (relatively

fresh) and the saltier subtropical Pacific waters (Fig.

1B). The SSS gradient is maintained by the balance

between evaporation, excess monsoon precipitation,

and monsoon wind velocity, which controls the

amount of water exchange between the South China

Sea and the Pacific [28,29].

Ocean circulation patterns in the South China Sea

are clearly modulated by seasonal changes in the East

Asian monsoon winds (Fig. 1C,D). During the south-

west summer monsoon, surface currents flow broadly

northeast, bringing warm tropical water into the South

China Sea [28]. By contrast, a counter-clockwise gyre

circulation is set-up during winter. This general circu-

lation pattern, together with the northeast winter mon-

soon winds, draws cold extra-tropical water into the

http:\\iridl.ldeo.columbia.edu\SOURCES\.NOAA\.NODC\.WOA98\


South China Sea through the Taiwan Strait. The win-

ter surface-ocean circulation tends to be stronger than

the summer circulation because the winter monsoon

winds are reinforced by cold air from the nearby

Asian high-pressure system [21].

Fig. 2. Summary of coral sample locations and Holocene stratigra-

phy of the Qionghai raised reef, eastern Hainan Island. (A) Map

showing Hainan Island (inset) and local topography adjacent to

modern coral QG5 and fossil coral OC2. SST and SSS were

measured daily from 1961 to 2002 at Qinglan Station (19.58N110.88E), located ~40 km north of the modern coral sampling

site. Precipitation was measured daily at Qionghai meteorologica

station (19.258N, 100.58E). Topographic and bathymetric contours

are in meters. (B) Stratigraphic cross-section and topography of the

Mid-Holocene Qionghai raised reef showing +1–4 m horizon where

in situ Porites specimen OC2 was collected.

3. Methodology

3.1. Coral sampling

Well-preserved fossil Porites sp. corals were col-

lected from an emergent Holocene coastal terrace 7

km from Qionghai (19.38N, 110.678E), eastern Hai-

nan Island (Fig. 2A). Massive in situ colonies of

Porites are concentrated within a single fringing

reef stratigraphic horizon (Fig. 2B). Cross-sections

of the Porites were sampled by cutting 30�30 cm2

columns oriented parallel to their main growth axes.

Relative sea level stood ~1–2 m above the present

level at Hainan Island during the Mid-Holocene and

topographical analysis indicates that the paleoshore-

line would have been several kilometres from the

paleo-reef.

The study was designed specifically to compare

proxy climate records extracted from fossil and mod-

ern corals growing in similar reef environmental set-

tings. Therefore, we took particular care to analyse a

modern Porites lutea specimen (QG5) that was col-

lected from an offshore reef platform, ~5 km south of

the fossil coral paleo-reef, that provides an accurate

analogue for the Hainan paleo-reef during the Mid-

Holocene sea-level high-stand. The top of modern

coral colony QG5 (0.7 m high) grew at 2 m water

depth and was sampled on 6 May, 2002, by cutting a

30�30 cm2 column oriented parallel to its main

growth axis.

Initial screening of the fossil Porites corals

showed that specimen OC2 was particularly well

suited for paleoclimate reconstruction in terms of

its size (2.8 m high), straightforward growth habit,

and preservation. X-ray diffraction analysis of the

fossil specimen, and the modern coral, by the Anal-

ysis Center of Northwest University, China, showed

that their skeletons are both 100% aragonite. No

secondary aragonite over-growths were detected dur-

ing petrographic analysis of thin sections of the coral

skeletons.

,

l

3.2. Age determination and chronology

A sample was collected (OC2T-1) from the upper

portion of fossil coral OC2 (Fig. 3) for 230Th dating

using a Thermo-Finnigan MAT Element I magnetic

sector inductively coupled plasma mass spectrometer

(MS-ICP-MS) at the University of Minnesota [30].

Analysis of two aliquots of the sample yielded an

average initial d234U value of 147F1.5x (2r). Thisvalue agrees with the d234U values reported for

modern seawater and corals [145.8F1.7x, Ref.

[31], and confirms the good preservation of coral

OC2 (Table 1). 230Th analysis of the two aliquots of

Fig. 3. X-radiograph positive images of coral density banding in

modern coral QG5 and Mid-Holocene coral OC2. (A1) Modern

coral QG5 and close-up (A2) showing micro-sampling transect.

(B1) Mid-Holocene coral OC2 and close-up (B2) showing micro-

sampling transect. Black dot indicates position of sample used for230Th dating of OC2. Couplets of high skeletal density (dark bands)

and low skeletal density (light bands) represent one annual growth

increment. Density-band counts indicate that mid-Holocene coral

OC2 spans ~250 yr.


the sample yielded an average 230Th age of

4,379F32 y for the 54-year section of core OC2

(2.8 m long in total) analysed for d18O in this study.

In subtropical oceanic settings, such as that for

Hainan Island, seasonal variations in the skeletal den-

sity of Porites corals revealed by x-radiography pro-

vide annual time-markers for the development of

continuous chronologies for paleoclimate reconstruc-

tions. In preparation for coral x-radiography, 5-mm

thick slabs were cut parallel to the major growth axes

of the fossil and modern Porites using an automatic

stonecutter. After air-drying, the coral slabs were

photographed using a medical HITACHI X-ray ma-

chine operating at 50 kV voltage, 50 mA current, and

0.04 s exposure time. The X-ray negative images were

converted to digital images using a UMAX scanner.

Annual density band couplets, composed of high and

low-density bands, can be clearly observed in the X-

ray positive images of the fossil and modern Porites

(Fig. 3). The average annual extension rate for modern

coral QG5 is 13 mm/y, while that for fossil coral OC2

is 7 mm/y. Both extension rates fall well within the

range for Porites sp. corals (4–24 mm/y) growing in

shallow water (b10 m, Ref. [32]).

3.3. Coral sampling and isotopic analysis

Section lines for coral sampling were aligned with

the major growth axis of each coral slab using density

bands in the x-radiograph images as a guide (Fig. 3).

Slices of ~3 mm width were then carefully cut along

the section lines, washed with water, and vigorously

cleaned in an ultrasonic bath for 20 min. The ultra-

sonic cleaning was repeated two more times to com-

pletely remove any sawing residue and other adhering

contaminants. The slices were then dried at b40 8C.To obtain high-resolution powdered sub-samples, we

employed a coral slicing technique analogous to the

method described by Gagan et al. [33]. Samples of

~3�3 mm cross-sectional area were cut continuously

at an average thickness of 0.7 mm for modern coral

QG5 and 0.4 mm for fossil coral OC2 along the

growth-direction of the corallites, and transferred to

individual holders in preparation for isotope analysis.

High-pressure air cleaning was applied between each

sub-sample to avoid cross-contamination. This sam-

pling resolution is equivalent to ~18 samples per

annual growth increment for the modern coral, and

Table 1

MS-ICP-MS U/Th results for fossil coral OC2

Samplea

number

[238U]b (ppb) [232Th] (ppt) d234Umeas.c (x) d234Uinitial

d (x) (230Th / 238U)act�103 230Th agee

(uncorr. y)

230Th agef

(correct. y)

OC2T-1 (I) 2659F3 340F11 146.2F1.5 148.0F1.5 45.21F0.16 4391F17 4386F17

OC2T-1 (II) 2533F6 513F50 144.9F1.5 146.7F1.6 45.01F0.26 4376F27 4371F27

a I and II are replicate samples obtained by splitting a ~1 g coral sample into multiple fragments.b All errors reported in this table are quoted as 2r.c d234U={[234U/ 238U]activity�1}�103.d d234Uinitial was calculated based on 230Th age (T), where d234Uinitial=d234Umeasured�ek234xT and k234=2.8263�10�6 y�1.e Decay constant values are: k234=2.8263�10�6 y�1 , k230=9.1577�10�6 y�1 , k238=1.55125�10�10 y�1 (Ref. [31]).f Corrected 230Th ages assume an initial 230Th / 232Th atomic ratio of 4.4F2.2�10�6. This is the value for a material at secular equilibrium,

assuming a crustal 232Th/ 238U value of 3.8.


~15 samples per annual growth increment for the

fossil coral.

Analyses of d18O in 273 modern coral samples and

799 fossil coral samples were performed in the State

Key Laboratory of Loess and Quaternary Geology,

China, using a Finnigan MAT-252 mass spectrometer

coupled with a Kiel auto-carbonate device. Individual

aliquots of carbonate powder were reacted with 5

drops of H3PO4 under vacuum at 75 8C to liberate

CO2 for isotope analysis. The d18O values are pre-

sented in standard delta notation (x units) relative to

Vienna Peedee Belemnite (V-PDB). Measurements of

the National Bureau of Standards NBS-19

(d18O=�2.20x) were used to calibrate the results

to the V-PDB scale. Analytical precision for replicate

measurements of d18O in NBS-19 (n =119)

wasF0.07x (1r).

3.4. Oceanographic and climatologic data

Freshwater inputs from rainfall and river discharge

are the main factors affecting SSS and the d18O of

seawater near Hainan Island. Therefore, on 8 May

2002 samples of rainfall and seawater were collected

offshore of Boao, near the Wanquian River mouth, to

determine the local relationship between seawater

d18O and SSS. Salinity was measured in the labora-

tory using an SYA-2 instrument produced in Sandong,

China, with an accuracy of 0.01 psu. Analysis of

seawater d18O followed the H2O–CO2 equilibration

technique of Epstein and Mayeda [34]. 3-ml water

samples were equilibrated with 25 mb of CO2 gas and

shaken for 4 h in glass vials immersed in a water bath

at 25.0F0.1 8C. The equilibrated H2O–CO2 gas was

then injected into a vacuum line for CO2 extraction.

The d18O value of the purified CO2 was measured on

a Finnigan MAT-252 mass spectrometer and

expressed relative to Vienna Standard Mean Ocean

water (V-SMOW).

SST and SSS are measured daily at 1 m water

depth at Qinglan station (19.58N 110.88E), located~40 km north of the modern coral sampling site

(Fig. 2). SST is measured manually (in situ) and

SSS is measured in the laboratory with the SYA-2

salinity meter. We determined the relationship be-

tween SSS and local precipitation using daily precip-

itation recorded at Qionghai meteorological station

(19.258N 100.58E), located 8 km west of the coral

sampling sites. It is important to note, however, that

seasonal changes in SSS measured at Qinglan Station,

located near a large estuary, are much greater than

those experienced by modern coral QG5, which is

located in an open fringing reef setting. However,

SSS at Qinglan Station does provide a reasonable

estimate of the interannual variability in SSS experi-

enced by coral QG5.

4. Results

4.1. Modern coral d18O record

4.1.1. Reconstruction of winter SST

The 15-year d18O record for modern coral QG5

shows clear annual cycles with an average amplitude

of 1.6x (Fig. 4A). The seasonal oscillations in coral

d18O are primarily driven by the large seasonal range

in SST (ave. 12 8C; Fig. 4B). However, while the

-6.5

-5.5

-4.5

024681012141618Sample depth (cm)

A W/W

C/D

-6.5

-5.5

-4.5

15

20

25

30

35

18Oδ SST

B

1988 1990 1992 1994 1996 1998 2000 2002

-1

-0.5

00.5

1

0

10

20

30 Sal

inity

(ps

u)

Year

-0.6-0.4-0.2

00.2

0.2

0.4

0.4

C 2

0

-2

SS

T (

oC

)S

ST

(oC

)∆

Winter SST

∆SST∆δ18O

1988 1990 1992 1994 1996 1998 2000 2002

-0.4-0.2

0

0.6

∆δ18

O (

)

‰∆δ

18O

(

)‰

δ18 O

(

)‰

δ18 O

(

)‰

∆δ18

O (

)

‰

∆SS

S (

psu)

Summer SSS

Year

-2

0

2∆SSS∆δ18O

Fig. 4. Modern coral (QG5) skeletal d18O record compared with

instrumental SST and SSS records for the period 1988–2002. (A)

d18O versus depth from the upper surface of coral QG5. d18O axis is

reversed to show wetter/warmer (W/W) conditions upward. (B)

d18O versus time compared with 10-day average SST calculated

from daily readings at Qinglan Station (Fig. 2). Middle and lower

curves show d18O deviation (Dd18O) and 10-day average SSS

calculated from daily readings at Qinglan Station. The contribution

of SST to the coral d18O signal was removed by subtracting the

average seasonal cycle from the coral d18O record, thereby provid-

ing an estimate of changes in d18O of seawater, and SSS. Note that

the seasonal changes in SSS measured at Qinglan Station, located

near a large estuary ~40 km north of coral QG5, are much greater

than those experienced by coral QG5, which is located in an open

fringing reef setting. However, SSS at Qinglan Station provides a

reasonable estimate of the interannual variability in SSS experi-

enced by coral QG5 (see Methods section). (C) Comparison of

interannual variability in winter (December–February) d18O

(Dd18O) and winter SST (DSST), after subtracting the average

winter d18O and SST values for the records. Comparison of inter-

annual variability in summer (June–August) d18O (Dd18O) and

summer SSS (DSSS), after subtracting the average summer d18O

and summer SSS for the records.


major features of the d18O record resemble those of

the SST records, the shapes of the annual cycles in

coral d18O are not the same as those for SST when the

coral data are simply plotted against distance along

the sampling track. A slight widening of the summer

portion of the annual cycle of coral d18O could be due

to the combined influence of higher SST and lower

d18O of seawater (lower SSS) during the summer wet

season (Fig. 4B). Also, skeletal extension in Porites is

usually slightly faster in summer, relative to winter, in

subtropical oceanic settings with seasonal extremes in

SST, such as Hainan Island [32]. Therefore, we res-

trict our comparisons of coral and instrumental data to

the winter dry seasons (higher d18O values) and sum-

mer wet seasons (lower d18O values), when the effect

of any uncertainties in the coral chronology will be

negligible.

Chronologies were established for the modern and

fossil coral records using the annual arrival-times of

the winter SST minima at Hainan Island. SST minima

are well defined by modern coral d18O maxima during

the dry seasons, when the influence of changes in

seawater d18O on the coral record is small, and pro-

vide exact annual anchor points to establish chrono-

logies for the coral records. Analysis of instrumental

SSTs for 1987–2002 shows that the SST minima

arrive near 23 January (F25 days, 2r) at Hainan

Island. Therefore, the coral time-series are simply

plotted by assigning this date to each coral d18Omaximum, and then allocating equal time spans to

the data points between adjacent d18O maxima using

linear interpolation [35].

The resulting chronology is sufficiently accurate

for the season-specific analysis of modern and Mid-

Holocene SSTs during winter, and rainfall during

summer. In order to examine reconstructed winter

SSTs, we use the average of six skeletal d18O-tem-

perature relationships established for Porites else-

where in the tropical Pacific [33,36–41]. These

high-resolution coral d18O records from the relatively

dry eastern equatorial Pacific and subtropical western

Pacific are well positioned to record SSTs because the

influence of changes in seawater d18O on coral d18O

is small, relative to that brought about by changes in

SST. The slopes of the d18O-temperature relationships

are �0.216x 8C�1 for Galapagos [36,37], �0.189x8C�1 for New Caledonia [38], �0.179x 8C�1 for the

Great Barrier Reef [33,39], and �0.153x 8C�1 for

0

10

20

30

40

0

200

400

600

800

1000

1988 1990 1992 1994 1996 1998 2000 2002Year

Sal

inity

(ps

u)

Pre

cipi

tatio

n (m

m)

A

5

10

15

20

25

30

35

0 200 400 600 800S

alin

ity (

psu)

B

Precipitation (mm)

S = -0.029*P+ 30.45

(r = 0.75)

0 5 10 15 20 25 30 35-7

-6

-5

-4

-3

-2

-1

0

Salinity (psu)

18O

wδ

(‰,

)S

MO

W

C

18O

wδ = 0.175*S - 6.13

(r = 0.96)

Fig. 5. Relationship between precipitation, salinity, and d18O of

seawater at Hainan Island. (A) Comparison of 10-day average

precipitation and salinity for the period 1988–2002. Averages

were calculated using daily precipitation at Qionghai meteorologica

station and daily salinity at Qinglan Station. (B) Linear regression

analysis of precipitation and salinity. (C) Linear regression analysis

of salinity and d18O of seawater.


Japan [40,41]. The area-averaged mean slope is

�0.183x 8C�1, which is close to the slope of

�0.18x 8C�1 established for P. lutea using coral

d18O data restricted to drought years in the central

Great Barrier Reef [33,39].

Based on this d18O-temperature sensitivity, we

compare the magnitude of interannual variability of

coral d18O and instrumental SSTs during the winter

dry seasons (Fig. 4C). Least squares regression anal-

ysis of reconstructed and instrumental winter (Decem-

ber–February) SST variability yields a good best-fit

correlation (r) of 0.65. The range of interannual var-

iability in reconstructed winter SSTs is 1.2 8C (1r)over the period 1987–2002, in good agreement with

the 1.5 8C (1r) range shown by the instrumental SST

record. There is a good correlation between the warm-

est winter SSTs (1991, 1998, 2001) and recorded

moderate–strong El Nino events. Previous studies

have shown that the Asian winter monsoon winds

are weaker during the mature phase of an El Nino

[42–44]. Weaker northeast winds in winter create

warmer winter SSTs during El Nino events because

of reduced advection of relatively cool extra-tropical

water into the South China Sea [28].

4.1.2. Reconstruction of summer rainfall

There is a clear relationship between summer

rainfall, SSS, and d18O of seawater at Hainan Island

(Fig. 5), indicating that coral d18O should be a

sensitive recorder of changes in summer rainfall

amount. SSS variations offshore from Qionghai me-

teorological station, which reflect the combined in-

fluence of direct precipitation and nearby river

runoff, correlate strongly (r =0.75) with precipitation

amount for the period 1987–2002 (Fig. 5B). More-

over, SSS and seawater d18O at Hainan Island are

highly correlated (r=0.96) because summer mon-

soon rainfall is depleted in 18O, relative to ambient

seawater. The average d18O value of summer rainfall

analyzed in this study is �6.1xSMOW, which is in

good agreement with the typical d18O values of

rainfall in the East Asian monsoon domain [�5x to

�7xSMOW; Ref. 45]. Regression analysis of the mix-

ing-line for 18O-depleted rainfall and seawater yields a

d18O /SSS slope of 0.175x psu�1 (Fig. 5C).

In order to reconstruct interannual variability in

summer SSS from coral d18O, we subtracted the

average summer d18O value for the coral record

l

from the mean value for June–August in each year

(Fig. 4C). The maximum interannual variation in

summer SST at Hainan Island is only ~1 8C, and

often much less, so the potential effect of SST on

coral d18O during summer is b0.2x. The range of

interannual variations of summer d18O recorded by

modern coral QG5 is 1x, which is five times larger

than that which could be due to interannual differ-

ences in summer SST. While we do not have in situ

time-series of SSS or d18O of seawater for the

modern coral site, regression analysis of interannual

variations of summer SSS at Qinglan station and

coral d18O during summer yields a strong correlation

(r =0.87). The results show that the coral d18Ovalues during summer are significantly affected by

interannual variations of summer rainfall, coastal

SSS, and d18O of seawater.

The interannual variability in reconstructed sum-

mer rainfall reflects wider ocean–atmosphere interac-

tions, including the influence of ENSO events on East

Asian summer monsoon rainfall. It has been shown

that La Nina years are associated with greater typhoon

activity in the northern sector of the South China Sea


[46]. Negative d18O residuals in Fig. 4C indicate

summers with above average in-mixing of 18O-deplet-

ed monsoon rainfall, and lower SSS. The summer of

1995 was particularly wet, and corresponds with a

well-developed La Nina event. In contrast, positive

d18O residuals in summer correspond to lower rain-

fall, and higher SSS. With the exception of the sum-

mers of 1988 and 1989, relatively dry summers reflect

generally reduced rainfall over the northern South

China Sea [44,47] during El Nino events, and the

reduced incidence of typhoons.

-6

-5

-4

18202224262830323436Sample de

δ18 O

(

)‰

A

-6

-5

-4

δ18O

(

)‰

B

-1-0.5

00.5

110 12 14 16 18 20 22 24 26 28

Nominal

∆δ18

O (

)

‰

-0.6-0.4-0.2

00.20.4

∆δ18

O (

)

‰ C Winter SST

-0.4-0.2

00.20.40.6

282624222018161412108642Nominal

∆δ18

O (

)

‰

Summer SSS

8642

Fig. 6. Mid-Holocene coral (OC2) skeletal d18O record spanning 54 yr. (A)

reversed to show wetter/warmer (W/W) conditions upward. (B) d18O with

to modern coral QG5. d18O deviation (Dd18O) after removing the SST cont

thereby providing an estimate of changes in d18O of seawater, and SSS. (C

the average winter d18O value for the record, as an indicator of interann

summer (June–August) d18O deviation, after subtracting the average summ

SSS during the southwest monsoon.

4.2. Mid-Holocene coral d18O record

Fig. 6 shows the 54-year d18O record for fossil

coral OC2 converted to time and divided into winter

(SST) and summer (SSS) components using the

same protocols applied to the modern coral d18O

record. The average seasonal amplitude in d18O for

the fossil coral (1.77x) is slightly larger than that

recorded by the modern coral (1.62x). The result

suggests that winter–summer seasonality was en-

hanced by ~9% during the Mid-Holocene, relative

0246810121416pth (cm)

W/W

C/D

W/W

C/D

30 32 34 36 38 40 42 44 46 48 50 52 54year

W

D

SS

T (

o C)

∆3210

-1-2

54525048464442403836343230 year

SS

S (

psu)

∆

-3-2-101234

d18O versus depth from the upper surface of coral OC2. d18O axis is

sample depth converted to time using chronological method applied

ribution to coral d18O (using method applied to modern coral QG5),

) Variability in winter (December–February) d18O, after subtracting

ual variability in SST during the northeast monsoon. Variability in

er d18O for the record, as an indicator of interannual variability in


to the present. The standard deviation of the winter

d18O maxima in the Mid-Holocene coral (0.22x), as

an indicator of interannual variability, is 5% larger

than the 0.21x standard deviation recorded by the

modern coral. In addition, the standard deviation of

the Mid-Holocene summer d18O minima is 0.29x,

which is 12% larger than that recorded by the mod-

ern coral (0.26x). Taken together, the results show

enhanced seasonality, slightly stronger interannual

SST variability for winter dry seasons, and substan-

tially stronger interannual SSS variability for summer

wet seasons.

The mean value for the summer d18O minima in

the 54-year fossil coral record is �5.76x (Fig. 7),

which is 0.56x higher than the mean d18O value of

�6.32x for summer minima in the modern coral.

Similarly, the mean value for the winter d18O maxima

in the fossil coral record (�3.99x) is 0.71x higher

than the mean d18O value (�4.70x) for the winter

maxima in the modern coral. The potential influence

of changes in mean SST and SSS on this ~0.6x shift

in mean coral d18O during the Mid-Holocene is dis-

cussed in the next section.

-7

-6

-5

-4

1987 1997

δ18 O

(

)‰

W/m2154

Year

Modern

W/W

C/D

W/m2171

0 10 20 4030 50Nominal year

4,400 yBP

Fig. 7. Comparison of difference between modern coral (QG5) and

Mid-Holocene coral (OC2) d18O and difference between modern

and Mid-Holocene insolation seasonality. Horizontal lines indicate

mean d18O values for winter and summer recorded by the modern

and fossil corals, as an estimate of the difference between modern

and Mid-Holocene d18O of seawater (see text). Vertical bars show

insolation seasonality at 198N for the present (154 W/m2) and 4400

yr ago (171 W/m2) calculated using the method of Berger [65]. The

increase in insolation seasonality 4400 yr ago (11%) is consistent

with the ~9% increase in the amplitude of the annual cycle of fossil

coral d18O at Hainan Island.

5. Discussion

5.1. SST and SSS during the Middle Holocene

The ~0.6x 18O enrichment of the fossil coral

skeleton may signal a significant shift toward cooler

SSTs and/or higher SSS at Hainan Island during the

Middle Holocene. However, other potential factors

must be considered before the increase in coral skel-

etal 18O can be attributed to regional climate change.

These include potential biological controls on the

fractionation of 18O / 16O in coral skeletons, diagenetic

alteration, and changes in SST and/or SSS restricted to

the local reef environment.

Light-enhanced calcification in hermatypic corals

and non-equilibrium fractionation of skeletal 18O / 16O

leads to depletion of the heavy isotope 18O during the

CO2 hydration and hydroxylation reactions involved

in calcification [36,48]. It has been shown that the

disequilibrium fractionation of skeletal 18O / 16O is not

necessarily constant within a coral colony [36,49,50],

and among coral colonies [51,52]. However, recent

studies have shown good replication of skeletal d18O

records (bF0.15x, 2r) extracted from modern and

fossil Porites colonies when rigorous coral cleaning

and microsampling protocols are applied [15,39,53–

55]. Given the high quality of fossil coral OC2, and

careful attention to microsampling, it is likely that the

~0.6x 18O enrichment recorded by coral OC2 is well

beyond the scope of analytical uncertainty.

Vadose-zone diagenesis of subaerially exposed cor-

als and early marine diagenesis both have the potential

to alter coral d18O values. In the vadose-zone envi-

ronment, the d18O of secondary calcite is determined

by the mix of oxygen derived from dissolving coral

aragonite and 18O-depleted meteoric water [56]. In the

East Asian monsoon domain, where the d18O of

meteoric water is particularly low [45], the d18Ovalue of calcite precipitated in equilibrium will be

close to that of coral aragonite. Thus the effect of

any secondary calcite not detected through X-ray

diffraction analysis (b1%) will be negligible for Hai-

nan Island corals. On the other hand, studies of early

marine diagenesis in coral skeletons demonstrate that

inorganic aragonite cements, which are enriched in18O relative to coral aragonite, do have the potential to

artificially depress reconstructed SSTs [57,58]. How-

ever, given the relatively small isotopic contrast be-


tween the aragonite of fossil coral OC2 (ave. �5x)

and marine aragonite (~�1x), ~15% of the analysed

sample mass would have to be marine aragonite to

produce the observed 0.6x increase in d18O. Such a

mass of secondary aragonite would be clearly visible

in x-radiographs, let alone in the thin sections ana-

lysed in this study.

It is unlikely that the ~0.6x 18O-enrichment of

fossil coral OC2 reflects a large drop in mean SST

(equivalent to ~3 8C) because paleoclimate records

and models both show that SSTs in the South China

Sea were near modern temperatures by ~4400 yr ago.

Studies of alkenone unsaturation ratios and foraminif-

eral Mg/Ca in deep-sea sediment cores from the

South China Sea [59,60] and adjacent Sulu Sea

[61,62] all show that SSTs were within 0.5 8C of

modern values during the Middle Holocene. The

paleo-SST estimates are in good agreement with the

subtle cooling of mean SSTs in the South China Sea

(~0.5 8C) given by coupled ocean–atmosphere models

of direct orbital forcing during the Middle Holocene

[13,63].

Estimates of Mid-Holocene SSTs from coral Sr /Ca

paleothermometry in the South China Sea are limited

to short records (b10 yr) over the period 7500–7000

calendar yr ago, based on analyses of the branching

coral, Goniopora, from the raised reefs of Leizhou

Peninsula, located ~120 km north of our study site

[20]. These records indicate that average summer

SSTs were similar to those for the 1990s, yet winter

SSTs were 1.5–3 8C cooler. While the result agrees

with the strengthening of winter monsoon-induced

cooling of SSTs given by coupled models, the exact

magnitude of winter cooling awaits verification be-

cause little is known about skeletal Sr /Ca systematics

in Goniopora.

Based on these observations, it is probable that no

more than ~0.1x of the ~0.6x 18O enrichment of the

fossil coral skeleton can be explained by the potential

effect of Mid-Holocene cooling of the South China

Sea. Therefore, the remaining ~0.5x 18O enrichment

of the fossil coral must reflect higher mean d18O of

seawater at Hainan Island ~4400 yr ago. We showed

earlier (Fig. 5) that modern coral QG5 was drilled

offshore to mimic the positioning of coral OC2 rela-

tive to the paleo-shoreline ~4400 yr ago, and thus

minimize the potential effect of local changes in

d18O of coastal seawater. Moreover, the fact that the

~0.5x 18O enrichment occurs consistently during the

winter dry seasons, when coastal seawater d18O is

essentially the same as that offshore, suggests that

the 18O enrichment is a regional signal.

Today, the relatively low SSS of the South China

Sea (Fig. 1) reflects the balance between high precip-

itation, evaporation, and the somewhat restricted ex-

change of South China Sea water with the more open

Pacific. Paleoclimate records and models agree that

summer monsoon rainfall was generally higher over

East Asia ~4400 yr ago [5,6], so it is possible that

greater convergence of 18O-depleted monsoon rainfall

on land could have left the surface-ocean enriched in18O. However, coupled model simulations indicate

that rainfall also increased over the South China Sea

[13,64], so it is difficult to simply invoke a reduction

in 18O-depleted rainfall to drive higher SSS and 18O

of surface seawater. A significant 18O-enrichment of

East Asian summer monsoon rainfall during the Mid-

Holocene is also unlikely because the d18O of dated

groundwater from the East Asian monsoon domain

has not changed during the Holocene [45].

5.2. Enhanced monsoon seasonality and mean SSS

We propose that the increase in seawater d18O in

the South China Sea during the Mid-Holocene is

due, at least in part, to enhanced wind-induced

evaporation, vertical mixing, and/or greater seawater

exchange rates with the open Pacific associated

with the ~9% increase in winter–summer monsoon

seasonality, observed in the Hainan fossil coral. The

effect of orbital forcing on insolation seasonality

and the East Asian monsoon during the Holocene

is well documented [e.g., Refs. [13,64], among

others]. The difference in insolation seasonality at

the study site (198N) 4400 yr ago was calculated

using the method of Berger (Ref. [65]; Fig. 7).

Today, the seasonal amplitude of insolation at

198N (154 W/m2) ranges from a minimum of 298

W/m2 in December to a maximum of 452 W/m2 in

June. 4400 yr ago, the seasonal range was 17 W/

m2 larger. This 11% increase in insolation season-

ality 4400 yr ago, relative to the present, is con-

sistent with the ~9% increase in the seasonal

amplitude of the fossil coral d18O at Hainan Island

(Fig. 7). While we note that the good match may

be fortuitous, nevertheless it is generally accepted

0

5

10

15

20

25

30

Spe

ctra

l den

sity

Frequency

Inst. SST

14.2 y

3.6 y

AS

pect

ral d

ensi

ty (

x103 )

Frequency

Inst. SSS

19 y

5.7 y

3.1 y

2 y

B2.0

1.0

1.5

0.5

0.5

0.4

0.3

0.2

0.1

0

0

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 10 0.2 0.4 0.6 0.8 1

Spe

ctra

l den

sity

Frequency

Winter SST6.7 y

2.3 y

C

Spe

ctra

l den

sity

Frequency

Summer SSS

4.2 y6.7 y

2.3 y

D

Fig. 8. Results of spectral analysis of SST, SSS, and Mid-Holocene

coral (OC2) d18O records. Frequency spectra for 10-day average

SST (A) and SSS (B) at Qinglan Station for the period 1970–2002

Frequency spectra for winter d18O (SST, panel C) and summer d18O

(SSS, panel D) for Mid-Holocene coral OC2. Lines indicate spectra

peaks significant at the 95% confidence level.


that the increase in insolation seasonality served to

increase the land–sea temperature contrast, and en-

hance the East Asian monsoon during the Middle

Holocene.

Given that the rate of surface-ocean evaporation is

strongly linked to wind speed [66], any increase in

monsoon wind velocity over the South China Sea

during the Mid-Holocene, particularly during the win-

ter dry season, would serve to increase SSS. More-

over, wind-induced entrainment and mixing of

relatively high salinity sub-surface water with low

salinity surface water would reduce water column

stratification, and help sustain the high SSS. Also,

the invigorated seasonal changes in East Asian mon-

soon winds would enhance exchange of 18O-enriched

subtropical Pacific water with the South China Sea,

and further increase SSS.

The 18O-enrichment of the northern South China

Sea ~4400 yr ago indicated by the Hainan Island coral

record contributes to mounting evidence for sustained

freshening of the tropical Western Pacific throughout

the Holocene. Recently, tandem measurements of

Mg/Ca and d18O in planktonic foraminifera, and

Sr /Ca and d18O in corals, from the tropical western

Pacific have been used to reconstruct changes in SST

and d18O of surface seawater over the last glacial/

Holocene transition [39,59,60,67]. These high-resolu-

tion records show a ~0.4–0.6x 18O-enrichment of

western Pacific surface seawater during the Early–

Middle Holocene (~11,000–4000 yr ago) and a pro-

gressive freshening towards the present. Therefore,

the 18O-enrichment of the South China Sea during

the Mid-Holocene may be driven, in part, by a wide-

spread salinity anomaly related to regional changes in

the balance between precipitation, evaporation, and

ocean mixing.

5.3. Interannual variability of the East Asian monsoon

5.3.1. Monsoon cyclicity

In order to identify the dominant cycles of mon-

soon climate variability, spectral analysis was per-

formed using the maximum entropy method of the

SSA-MTM toolkit [68]. We analyzed the instrumental

SST and SSS data from Qinglan station, which is

available for the period 1970–2002. Prior to analysis,

the time-series were interpolated to monthly intervals

and filtered to remove the annual cycle. These results

are compared with the analysis of winter SST and

summer SSS for the 54-year fossil coral d18O record

(Fig. 8). For each maximum entropy series, different

numbers of the auto-regression component were test-

ed to maximize the stability and sensitivity of the

resulting spectrum.

The spectral analysis of SST for the period 1970–

2002 shows ENSO band variance, with a significant

cycle at 3.6 y (Fig. 8A). Given the influence of ENSO

on winter SSTs in the South China Sea [42,44],

ENSO-band variance is expected. In contrast, analysis

of SSS for the same period shows a strong biennial

cycle (2 y) and weak modulation in the 3–8 y ENSO

band (Fig. 8B), reflecting the complex interplay be-

tween ENSO and the biennial variability inherent to

monsoon precipitation [23,69,70].

In contrast to the modern situation, the proxy

monsoon index provided by the Mid-Holocene coral

d18O record displays a different spectrum. The anal-

ysis of winter SST reveals a strong cycle at 6.7 y (Fig.

8C), which is still within the ENSO band (3–8 y), but

significantly longer than the 3.6 y cycle observed

since 1970. However, analysis of summer SSS

.

l

-0.6

-0.4

-0.2

0

0.2

0.4

18O

(

)∆δ

‰

Winter SST3

2

1

0

-1

-2

∆S

ST

(o C

)

A

-0.4

-0.2

0

0.2

0.4

0.6 Modern 4,400 yBP

Summer SSS

1987 1997 0 10 20 30 40 50Year Nominal year

18O

(

)∆δ

‰

-3

-2

0

1

-1

3

2

4

∆S

SS

(ps

u)

B

Fig. 9. Comparison of interannual climate variability recorded by

modern coral (QG5) and Mid-Holocene coral (OC2) d18O. (A

Comparison of winter (December–February) d18O variability, afte

subtracting the average winter d18O values for the records. (B

Comparison of summer (June–August) d18O variability, after sub

tracting the average summer d18O for the records. Shaded areas

show standard deviations as an indicator of the amplitude o

interannual variability in winter SST (mainly ENSO band during

the northeast monsoon).


shows no significant ENSO variance ~4400 yr ago,

with the biennial cycle (2.3 y) of monsoon precipita-

tion still dominant (Fig. 8D).

The results indicate that there were marked differ-

ences in ENSO-monsoon interactions during the win-

ter and summer monsoon seasons in the past. While

the influence of ENSO on winter SSTs in the South

China Sea was well established by ~4400 yr ago, it

appears that atmospheric teleconnections between

ENSO and summer monsoon rainfall may have

been more restricted. High-resolution paleoclimate

records and model studies agree that ENSO variabil-

ity was weaker during the Mid-Holocene [7–12,71]

and that the mean climate of the tropical Pacific was

in a La Nina-like state [61,72]. Thus the influence of

ENSO variability on summer monsoon rainfall may

have been limited in the Middle Holocene.

Also, recent analysis of instrumental records and

climate models suggest that the East Asian monsoon

may influence the evolution of ENSO by affecting the

western Pacific winds [12,23,24]. However, our

results show that despite biennial variability of the

East Asian monsoon being strong ~4400 yr ago, it did

not force strong biennial variability in high-resolution

paleo-ENSO records [7–10].

5.3.2. Monsoon variability

The Hainan Island fossil coral d18O record pro-

vides the first opportunity to examine the strength

of interannual monsoon variability during the Mid-

Holocene to gain a better understanding of ENSO-

monsoon interactions. Fig. 9 shows the 54-year

fossil coral d18O record divided into winter SST

and summer SSS for comparison of their interan-

nual variability with that in the modern coral re-

cord. The standard deviation of the winter d18O

maxima is 0.22x in the Mid-Holocene, which is

5% larger than the 0.21x standard deviation

recorded by the modern coral. Likewise, the stan-

dard deviation of the summer d18O minima in the

Mid-Holocene is 0.29x, which is 12% larger than

that recorded by the modern coral (0.26x). Taken

together, the results show stronger interannual var-

iability in winter SST and summer SSS ~4400 yr

ago, despite ENSO ocean–atmosphere variability

being significantly weaker in the Pacific region at

that time. The results suggest that the monsoon was

sensitive to forces, other than ENSO, that signifi-

)

r

)

-

f

cantly influenced interannual variability in the mon-

soon system.

6. Conclusions

(1) Massive Porites sp. corals are abundant in an

emergent Holocene coastal terrace located 7 km

from Qionghai (19.38N, 110.678E), eastern Hai-

nan Island, in the subtropical northern South

China Sea. Oxygen isotope ratios (d18O) in

the skeletons of long-lived Porites were used

to reconstruct the dynamics of the East Asian

monsoon, which includes prominent seasonal

changes in SST, rainfall, and wind direction.

We produced a 54-year long, high-resolution

skeletal d18O record by analysing a particularly

well preserved, high-quality Porites specimen

to investigate East Asian monsoon variability

during summer and winter ~4400 calendar yr

ago. The coral record for ~4400 yr ago shows

~9% amplification of the annual cycle of d18O,in good agreement with coupled ocean–atmo-

sphere models showing higher summer rainfall


(lower coral d18O) and cooler winter SSTs

(higher coral d18O) in response to greater North-ern Hemisphere insolation seasonality during

the Middle Holocene.

(2) Previously published records show that mean

SSTs in the South China Sea were within 0.5

8C of modern values during the Mid-Holocene,

yet the mean d18O for the fossil coral is ~0.6xhigher than that for a modern coral nearby,

suggesting that the d18O of seawater was higher

by at least ~0.5x, relative to modern values.

The 18O-enrichment ~4400 yr ago cannot be

explained by changes in monsoon rainfall

amount alone, and appears to be driven, in

part, by enhanced monsoon wind-induced evap-

oration and vertical mixing, and/or invigorated

advection of saltier 18O-enriched Pacific water

into the relatively fresh South China Sea. The18O-enrichment of the northern South China Sea

~4400 yr ago indicated by the Hainan Island

coral record contributes to mounting evidence

for sustained freshening of the tropical Western

Pacific throughout the Holocene.

(3) Winter SST and summer SSS variability in the

South China Sea reflect the interannual influ-

ence of ENSO and the biennial variability in-

herent to monsoon precipitation. Spectral

analysis of winter SSTs reconstructed for

~4400 yr ago reveals a strong ENSO cycle at

6.7 y, which is significantly longer than the

average 3.6 y cycle observed since 1970. How-

ever, spectral analysis of summer SSS ~4400 yr

ago shows no significant ENSO cycle, with the

biennial cycle (2.3 y) of monsoon rainfall still

dominant. The fossil coral d18O record also

shows that the amplitude of interannual variabil-

ity in winter SST and summer SSS was stronger

~4400 yr ago, despite ENSO variability being

significantly weaker in the Pacific region.

(4) The results indicate marked differences in

ENSO–monsoon interactions during the winter

and summer monsoon seasons in the past.

While the influence of ENSO on winter SSTs

was well established by ~4400 yr ago, the lack

of ENSO-band variance in summer SSS sug-

gests that atmospheric ENSO–monsoon tele-

connections may have been more restricted.

However, given that the amplitude of interan-

nual SST and SSS variability was stronger

~4400 yr ago, it appears that the monsoon

was influenced by forces, other than ENSO,

that acted as alternative drivers of monsoon

variability. If this is the case, greater interannu-

al variability in monsoon rainfall could accom-

pany the strengthening of the Asian monsoon

predicted to occur during the 21st century as

transient greenhouse warming preferentially

warms Eurasia, even if ENSO perturbations

remain relatively stable.

Acknowledgements

Financial support for this research was provided

by Chinese National Science Foundation of China

grants 40272075 and 90411012 (to SD), US NSF

grant 0214041 (to RLE), and Gary Comer Science

and Education Foundation grant CC8 (to RLE). We

thank Dr. J.-x. Zhao and an anonymous referee for

their helpful reviews, and Prof. H. Kawahata, Dr.

A. Suzuki, Prof. Z. An, Prof. Z. Peng, Prof. W.

Liu, and Dr. S. Song for valuable discussions on

coral geochemistry.

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Seasonal and interannual variability of the Mid-Holocene East Asian monsoon in coral δ18O records...

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