A high resolution record of sea surface temperature in southern Okinawa Trough for the past 15,000 years

    A high resolution record of sea surface temperature in southern Okinawa Trough for past 15000 years Jiaping Ruan, Yunping Xu, Su Ding, Yinghui Wang, Xinyu Zhang PII: DOI: Reference:

S0031-0182(15)00129-7 doi: 10.1016/j.palaeo.2015.03.007 PALAEO 7199

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

20 June 2014 4 March 2015 6 March 2015

Please cite this article as: Ruan, Jiaping, Xu, Yunping, Ding, Su, Wang, Yinghui, Zhang, Xinyu, A high resolution record of sea surface temperature in southern Okinawa Trough for past 15000 years, Palaeogeography, Palaeoclimatology, Palaeoecology (2015), doi: 10.1016/j.palaeo.2015.03.007

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ACCEPTED MANUSCRIPT A high resolution record of sea surface temperature in southern Okinawa Trough for past 15000 years

MOE Key Laboratory for Earth Surface Process, College of Urban and

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Jiaping Ruan a, Yunping Xu a,b*, Su Ding a, Yinghui Wang a, Xinyu Zhang a

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Environmental Sciences, Peking University, Beijing 100871, China

Qingdao Collaborative Innovation Center of Marine Science and Technology,

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Qingdao 266003, China

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*Corresponding author. E-mail: [email protected]

ABSTRACT

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The study of long-chain alkenones in the Ocean Drilling Program (ODP) core 1202B

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reveals a sub-centennial resolution record of sea surface temperature (SST) in the

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southern Okinawa Trough for past 15 thousand years (kyr). From the Deglaciation to Holocene, SST varied from 21.1 to 26.5 oC. The presence of Bølling (15.0~14.2 kyr BP) and Allerød (13.7~13.4 kyr BP) warming phases, Older Dryas (14.2~13.7 kyr BP)

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and Younger Dryas (12.8~11.6 kyr BP) cold periods reflects a tight teleconnection of climate between the Okinawa Trough and the North Atlantic region in the last Deglaciation. After rapidly increased and reached the maximum at ~7.4 kyr BP, SST in the southern OT gradually decreased, corresponding with the lowering of northern hemisphere summer solar insolation. However, a series of abrupt SST drops were identified at ca. 8.6~8.1, 5.8~4.8, 4.1~3.9, 3.0~2.5, 1.6~1.3 and 0.6~0.5 kyr BP, which cannot be explained by solar insolation changes alone, and instead are mediated by a complex sun-ocean-atmosphere coupling. Keywords: Alkenone; Sea surface temperature; Okinawa Trough; Kuroshio Current; Monsoon; Holocene 1

ACCEPTED MANUSCRIPT 1. Introduction Okinawa Trough (OT), the back-arc basin of the Ryukyu subduction system, has

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been regarded as an ideal location for paleoclimatic, paleohydrogeological and

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paleoceanographic studies because of several reasons (Jian et al., 2000; Li et al.,

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2001). Firstly, OT has a mean water depth of over 1000 meters and therefore has continuous sedimentation in the late Quaternary, whereas most of the East China Sea (ECS) was exposed during the last glacial maximum (LGM) when the sea level

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was >100 meters lower than present (Peltier & Fairbanks, 2006). Secondly, the OT

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has extremely high sedimentation rate due to enormous fluvial material inputs (Diekmann et al., 2008; Li et al., 2009), and thus the high resolution environmental

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information is preserved in its sediments. Thirdly, the hydrology of OT is subject to

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the strong influence of East Asian Monsoon (EAM) and Kuroshio Current, both playing important roles in regulating heat and precipitation in densely inhabited East

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Asia (Salisbury et al., 2002; Wu et al., 2012). Previous studies have demonstrated a link of climates between the Eastern Asia and the North Atlantic region such as the

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coeval occurrence of Bølling and Allerød warm phase (B/A), Younger Dryas cold period (YD) and 8.2 kyr-BP cold event (e.g., Gupta et al., 2003; Ijiri et al., 2005; Li et al., 2001; Porter & An, 1995; Wang et al., 2001). Given these facts, the paleoenvironment of the OT has attracted considerable attentions (e.g., Diekmann et al., 2008; Dou et al., 2012; Jian et al., 2000; Kao et al., 2008; Li et al., 2001; Sun et al., 2005; Ujiie et al., 2001; Xiang et al., 2007; Zhao et al., 2005). The majority of those studies, however, are either the non-quantitative manner of

proxy

reconstructions

or

low

resolution

records.

Among

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paleoenvironmental parameters, SST is an important one in oceanography and meteorology due to its major influence on the exchange processes at the air-sea 2

ACCEPTED MANUSCRIPT interface. In the OT, SST is strongly influenced by the Kuroshio Current and EAM. Presently, three geochemical proxies, namely

(Brassell et al., 1986), Mg/Ca ratio

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(Elderfield & Ganssen, 2000) and TEX86 (Schouten et al., 2002), are commonly used is defined as relative abundance of C37:2 alkenone

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for SST reconstruction. The

, where C37:2

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over the sum of C37:2 and C37:3 alkenones:

and C37:3 denote the abundance of C37 alkenones with two and three double bonds, respectively. In marine environments, long-chain alkenones are produced by some

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planktonic Haptophyceae algae such as Emiliania huxleyi and Gephyrocapsa

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oceanica, and the number of their double bonds is primarily controlled by ambient temperature of Haptophyceae algae (Prahl & Wakeham, 1987; Volkman et al., 1995;

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Volkman et al., 1980). Based on a global core-top calibration, Muller et al. (1998)

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established a linear relationship between the unsaturated degree of alkenones ( and annual mean SST, expressed as:

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index has been used for a variety of environments (e.g., Calvo et al.,

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The

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2002; Herbert, 2001; Rosell-Melé, 1998 and references therein). By analyzing

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long-chain alkenones in the ODP core 1202B, Zhao et al. (2005) reconstructed SST over the past 28 kyr, showing a 5 oC warming from the last Deglaciation to Holocene. However, the B/A, YD and 8.2 kyr BP events were not identified in their study. The mineral compositions of the ODP core 1202B also suggest no strong impacts of the Heinrich 1, B/A and YD events on the environment in the southern OT (Diekmann et al., 2008). These abrupt deglacial climate events, however, were unambiguously detected in the sediment records from the middle and northern OT (Chang et al., 2009; Li et al., 2001; Liu et al., 1999; Sun et al., 2005; Xiang et al., 2007). By compiling available records of past SST in the Pacific Ocean, Kiefer and Kienast (2005) proposed that the deglacial SST variability in marginal seas was consistent with that 3

ACCEPTED MANUSCRIPT of the North Atlantic region. Therefore, the lack of the evidence for those abrupt climate events in the southern OT is probably attributed to the low resolution SST

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used (such as grain size distributions, Diekmann et al., 2008).

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records (ca. 200 to 1000 years per sample) (Zhao et al., 2005) or insensitive proxies

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Here, we analyzed long-chain alkenones in the uppermost 75 meters of the ODP site 1202B at 5 to 20 cm intervals, corresponding to ca. 20 to 100 years per sample (Wei et al., 2005). Compared to Zhao et al. (2005), our study has one order of

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magnitude higher resolution for the SST record. Our main objectives are to: 1)

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reconstruct a bidecadal-centennial resolution record of SST in the southern OT since late last Deglaciation; 2) test whether the rapid climate changes such as YD, B/A and

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8.2 kyr BP events exist in the southern OT; and 3) evaluate the climate linkage of the

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southern OT to the low and high latitudes.

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2. Oceanographic setting

The OT is a curved backarc basin between the northwestern Pacific Ocean and the

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southeastern ECS (Fig. 1). It is about 1200 km long and 100-230 km wide. The mean water depth of the OT is ~500 m in the north and over 1000 m in the middle and southern parts. The surface hydrology of the OT is strongly influenced by the EAM and Kuroshio Current. The EAM is a result of the thermal contrast between the Asian landmass and the tropical Pacific (Wang, 2009), whereas Kuroshio Current is the biggest warm current in the western Pacific with the depth of up to 1 km and the width of 150~200 km. It originates from the western Pacific warm pool and brings a large volume of warm, salty water to the high latitudes in the north Pacific (Salisbury et al., 2002). The Kuroshio Current in late Pleistocene was generally weaker than today and may be displaced to the east of the Ryukyu Islands during the LGM. 4

ACCEPTED MANUSCRIPT However, the timing that the Kuroshio Current reentered the OT is still under debate,

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probably varying from 7.3 kyr BP (Jian et al., 2000) to 16 kyr BP (Li et al., 2001).

Fig. 1. Map of the Western North Pacific showing the locations of the ODP site 1202B (this study), and selected paleoenvironmental settings in relation to this study, including: central part of Greenland ice sheet (GISP2; 72.58°N, 38.48°W) (Stuiver & Grootes, 2000), the core A7 (27°49.2′N, 126°58.7′E) in the middle OT (Sun et al., 2005), the core DOC082 (29°13.93′N, 128°08.53′E) in the northern OT (Chang et al., 2008), the core MD9821-81 (or MD81; 6°27′N, 125°50′E) in Mindananao Sea, the Western Pacific Ocean (Stott et al., 2004) and the Dongge Cave (25°17′N, 108°5′E) in the southern China. Grey lines denote the Kuroshio Current and its branches (the fine grey lines), and blue lines represents the China coastal current (CCC).

3. Materials and methods 5

ACCEPTED MANUSCRIPT 3.1. Materials and age model The site 1202B of the ODP Leg 195 (24o48.24’N, 122o30’E; water depth 1274 m)

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is located in offshore of northeastern Taiwan Island in the southern OT (Fig. 1). The

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total length of the core recovery is 140.5 m (Salisbury et al., 2002). There is a

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continuous deposition for the upper 110 m, below which several turbid layers were present (Huang et al., 2005). In our study, the upper 75 m sediments were investigated. A total of 435 samples (1 or 2 cm section) were taken at 5~20 cm intervals. The

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sediments were predominantly composed of hemipelagic mud.

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The age model was established based on 10 accelerator mass spectrometer (AMS) 14

C dates for planktonic foraminifera (Table 1; Wei et al., 2005; Wu et al., 2012). The

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raw 14C dates were converted into calendar age before present (BP = 1950 AD) using

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the CALIB program (Stuiver et al., 1998). A 400-year reservoir correction was used (Hideshima et al., 2001). The AMS 14C data showed no age reversal for the upper 75

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m core, spanning the past ca. 15000 years. A linear interpolation showed that the sedimentation rate varied from 2.7 m/kyr (Holocene) to 9.0 m/kyr (Deglaciation),

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resulting in our SST resolution varying from 20 to 100 years per sample. Table 1: AMS radiocarbon ages of mixed planktonic foraminifera in sediments of the ODP core 1202B (southern Okinawa Trough). Sample ID Depth ODP 1202B/mbsf* 1H-1W-95-97+100-102cm 0.95 2H-1W-20-22+25-27cm 2.90 2H-3W-5-7+10-12cm 5.15 2H-3W-124-125+130-132cm7.00 2H-4W 135-137+140-142cm9.10 2H6W-34-46+50-56cm 10.88 3H6W-34-45.5+50-56cm 20.48 4HCC 31.35 5H6W-40-56cm 39.35 9H6W-40-56cm 77.35

Dating material Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams Planktonic forams

*mbsf: meters below sea floor 6

Conventional age**Calendar /14C year BP year BP 580±90 180 1150±90 750 1880±60 1500 2090±120 1700 2865±25 2500 3153±40 2890 5135±45 5490 8554±70 8970 10205±55 11074 13340±95 15100

ACCEPTED MANUSCRIPT **14C data for sediments 0-10 mbsf are from Wu et al. (2012), whereas those for the sediments 10-80 mbsf are from Wei et al. (2005).

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3.2. Alkenone analysis

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Sediment samples were freeze-dried and ground for homogeneity. Dry sediments

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(ca. 5 g) were ultrasonically extracted three times, each with 20 mL dichloromethane:methanol (3:1 v:v) for 15 min. After centrifugation (3,000 rpm for 10 min) and rotary evaporation, the supernatants were decanted into clean flasks. The

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extracts were separated into three fractions by silica gel columns, namely

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hydrocarbons (hexane:dichloromethane; 9:1 v:v), alkenones (hexane:dichloromethane; 1:1 v:v) and polar fractions (dichloromethane:methanol; 1:1 v:v).

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Long-chain alkenones were analyzed on an Agilent 7890A gas chromatography

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coupled to flame ionization detection (GC-FID). Two type GC columns, an Agilent DB-5 ms column (30 m × 250 μm × 0.25 μm) and an Agilent VF-200 ms column (60

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m × 250 μm × 0.25 μm), were used to separate alkenones. The GC oven was programed from 70 oC (held 1 min) to 250 oC at a rate of 20 oC per min, further

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increased to 310 oC at a rate of 2 oC per min and was held at 310 oC for 10 min. The index was converted into SST according to the equation of Muller et al. (1998): = 0.033 × SST + 0.044. The duplication analyses showed that instrumental errors were 0.01, corresponding to the SST deviation of ±0.3 oC. Selected samples were analyzed on an Agilent 7890 gas chromatography coupled to an Agilent 5975C mass spectrometry (GC-MS) for the compound identification.

4. Results and discussion 4.1. General SST pattern over past 15 kyr Among a total of 435 samples analyzed, 394 sediments contain sufficient amount 7

ACCEPTED MANUSCRIPT of long-chain alkenones for calculation of

. Throughout the core 1202B, C37:2 and

C37:3, are constantly dominant alkenones, whereas other homologues such as C38:2 and

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C38:3 are minor and even below detection limit (Fig. 2). The comparison of mass

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spectra showed different resolution of alkenones with the DB-5 ms column and

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VF-200 ms column. Although the former with non-polar poly(dimethylsiloxane) (DMPS) stationary phase has been widely used for alkenone analyses (Herbert, 2003 and references therein), it is unable to completely separate alkenones from impurities

(Holocene). In contrast, no coelution of long chain

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-derived SST in Deglaciation

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in some sediments of the ODP core 1202B, resulting in abnormally high (low)

alkenones was observed for the VF-200 ms column. Our result agrees with the recent

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finding by Longo et al. (2013) who achieved unprecedented selectivity and separation

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of long chain alkenones with the VF-200 ms column for different samples from open ocean, coastal and lacustrine sediments, haptophyte cultures and water column

-SST (Table S1).

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calculate the

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particulates. Therefore, we used the VF-200 ms column to analyze alkenones and

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ACCEPTED MANUSCRIPT Fig. 2. A partial GC-MS total ion current (TIC) chromatogram for the alkenone fraction of the ODP core 1202B-5H-3W 140-141 cm. For the past 15 kyr, the index in the ODP core 1202B ranged from 0.71 to

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0.94, corresponding to SST from 20.3 to 27.3 oC according to the calibration of

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Muller et al. (1998). Among them, over 90% fell in a range of 21.5 to 26.5 oC (Fig. 3),

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which is consistent with the previous study for the same core but much lower resolution (200 to 1000 years per sample) (Zhao et al., 2005). For the uppermost sediments, our alkenone-derived SST is ~25 oC, close to the satellite derived annual

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mean SST for the southern OT (ca. 26 oC; http://www.noaa.gov). Thus, we interpreted

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-derived SST as annual mean SST in the southern OT. Recently, Wu et al. (2012) analyzed glycerol dialkyl glycerol tetraether lipids, a biomarker for marine planktonic archaea (Schouten et al., 2013), in the top 10 meters of the ODP core 1202B. Their

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reconstructed SST varied from 24.9 to 27.5 oC with a mean of 25.9 oC for the past ~2700 years, close to our

-derived SST (23.6 to 27.3 oC with a mean of 25.3 oC).

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Such consistence between two molecular paleotemperature proxies suggests the applicability of

to infer past SST in the southern OT. From the Deglaciation to

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the Holocene, a ~5 oC warming in the southern OT is substantially larger than that in tropical oceans (ca. 1~3 oC) (Lea et al., 2000; Pelejero et al., 1999; Rosenthal et al., 2003; Thunell et al., 1994), but is comparable with that in the middle OT (Li et al., 2001; Zhou et al., 2007) and northern OT (Ijiri et al., 2005). The amplification of the SST oscillations in the OT is attributed to the complex interaction of the EAM, Kuroshio Current and solar insolation (Kiefer & Kienast, 2005; Zhao et al., 2005).

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Fig. 3. Alkenone-based sea surface temperature (SST) for the southern OT using the top 75 meters from the ODP 1202B. Red solid line represents three-point running mean of SST.

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4.2. Abrupt climate changes in Deglaciation For the period of 15 to 11.6 kyr BP, the SST pattern of the core 1202B is

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composed of a warm phase (15~13 kyr BP) and a cooling phase (13~11.6 kyr BP) (Fig. 4D). Within the dating uncertainty, the cooling phase varying from 21.0 to 22.6 o

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C and centered at ~12.1 kyr BP is synchronous with the North Atlantic YD event

(12.8~11.6 kyr BP). The YD event was thought to be triggered by the abrupt collapse of the Laurentide ice sheet (Alley et al., 1993; Muscheler et al., 2008). The amplitude of the SST drop in the southern OT (1.6 oC) is comparable with that in the middle OT (1~2 oC) (Zhou et al., 2007), suggesting a similar mechanism controlling their SST. Besides the YD cold event, the warm period from 15.0 to 13.0 kyr BP is in phase with the North Atlantic B/A warming event (14.7~12.7 kyr BP) (Pedro et al., 2011). A number of studies have demonstrated that the YD and B/A events have wide effects in northern hemisphere including the middle OT (Chen et al., 2010; Li et al., 2001; Sun et al., 2005; Xiang et al., 2007) and the north OT (Ijiri et al., 2005), but not in the 10

ACCEPTED MANUSCRIPT southern OT (Diekmann et al., 2008; Zhao et al., 2005). Thus, our study confirms the presence of the YD and B/A imprints in sediment environments of the southern OT.

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The similarity in the pattern of deglacial climate suggests a strong teleconnection

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between high-latitude North Atlantic and the OT. The mechanism controlling this

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teleconnection is likely related to variations of the EAM (Sun et al., 2005) and the

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meandering of the westerlies in the Northern Hemisphere (Ijiri et al., 2005).

Fig. 4. Comparison of the Deglacial-Holocene mean annual SST variations from core 11

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1202B (the southern OT) with selected paleoclimate data. (A) June solar insolation (W/m2) at 30o N latitudes (Berger et al., 1991); (B) δ18O variability in stalagmites from the Dongge Cave (China) (Wang et al., 2005); (C) δ18O records from GISP2 (Stuiver & Grootes, 2000); (D) reconstructed SST in 1202B core (three-points running mean; this study); (E) annual mean SST based on radiolarian transfer function in DOC082 core (Chang et al., 2008); (F) Mg/Ca-based SST in the A7 core (Sun et al., 2005), the red line is 5-point averaging smoothing; (G) Mg/Ca-based SST in MD81 core (Stott et al., 2004). The dash lines show the timing of abrupt cold events, including Bond events 0 to 8 in North Atlantic, the Younger Dryas (YD) cold period and Older Dryas (OD) cold event. The BA represent the Bølling and Allerød warm phase.

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Based on the Mg/Ca ratio of planktonic foraminifera, Sun et al. (2005) found different temporal patterns between the GISP2 and the middle OT (site A7) that the

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former reached the highest temperature at 14.5 kyr BP (Fig. 4C), whereas the latter continued to rise and peaked 500~000 years later (Fig. 4F). Our alkenone-derived SST

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record shows a maximum at ca. 14.8 kyr BP, without significant lag compared to the

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GISP2. The asynchronous SST maxima between the sites ODP 1202 and A7 can be

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explained by different timing that the Kuroshio Current flew across the southern and middle OT in the Deglaciation. A numerical model suggests significant changes in the path of the Kuroshio Current since LGM (Kao et al., 2006). The meander of the

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Kuroshio Current was reduced with sea level (s.l.) rose, and its main outlet shifted from the Kerama Gap in the mid-Ryukyu Islands (-135 to -80 m s.l.) to the Tokara Strait in northern OT (-40 to 0 m s.l.) (Fig. 1; Kao et al., 2006). Therefore, the southern OT is influenced by the Kuroshio Current earlier than the middle and northern OT, resulting in earlier warming at the site 1202B than the site A7. However, the timing that the Kuroshio Current reentered the OT is still under debate. Some study argued that even at the LGM, the Kuroshio Current still flew into the OT despite much weaker influence (e.g., Ijiri et al., 2005), whereas others suggested that the Kuroshio Current was displaced out of the OT during the LGM (e.g., Jian et al., 2000; Li et al., 2001). More studies with the high precision dating are needed to resolve this 12

ACCEPTED MANUSCRIPT controversy. Besides the B/A warming and YD cold phases, our high resolution SST record

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reveals decadal to centennial scale climate changes in the southern OT. Most

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remarkably, large SST oscillation occurred during the B/A warm period, varying from

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21.3 to 23.6 oC for three-point running mean SST (Fig. 4D). A cooling stage at 14.3 to 13.7 kyr BP was observed between Bølling and Allerød warm phases, which is coeval with so-called Older Dryas event (~14 kyr BP) (Pedro et al., 2011). This cold event

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was not observed as widely as the YD cold event, but has been identified in some

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terrestrial and marine archives such as the GISP 2 (Stuiver & Grootes, 2000). In sum, the presence of the YD cold period, B/A warming period and Older Dryas

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cold event in the sediment record of the ODP 1202B supports a predominant influence

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of high-latitude North Atlantic on the climate of the southern OT in the late last Deglaciation. This teleconnection is likely established via the EAM, which has a

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strong linkage with the North Atlantic region by atmospheric circulation (Porter & An, 1995; Xiang et al., 2007). When the east Asian winter monsoon becomes stronger, the

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bifurcation point of Kuroshio Current shifted southward and less volume of warm water flew across the OT, causing lower SST there (Sun et al., 2005).

4.3. Holocene SST variations in southern OT In the Holocene, our three-point running mean SST varied from 24.4 to 26.6 oC with a range of 2.2 oC (Fig. 4D). From 12 kyr BP, SST sharply increased, stayed at a relatively constant level (~24 oC) from 11.4 to 10 kyr BP, and further increased to the maximum at 7.2 kyr BP (26.6 oC). Afterwards, a slightly declining trend was observed (Fig. 3). Such temporal pattern is similar to the δ18O record of stalagmites in eastern Asia, which is a robust proxy for the intensity of Asian summer monsoon (e.g., Wang 13

ACCEPTED MANUSCRIPT et al., 2001; Zhang et al., 2008). For example, the δ18O of the stalagmites in the Dongge Cave (southern China) (Fig. 4B; Wang et al., 2005) and the Heshang Cave in

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the mid-reach Yangtze River (Hu et al., 2008) showed a long-term weakening of EAM

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after 7 kyr BP, corresponding with orbitally induced lowering of the northern

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hemisphere summer solar insolation (Fig. 4A; Berger & Loutre, 1991). These covariations suggest that solar insolation is a driving forcing controlling the long-term variability in Holocene SST in the OT.

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However, solar insolation cannot explain several rapid SST changes at the

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centennial scale in our record. Unlike a smooth decline in solar insolation, SST in the Holocene, despite constantly higher than in the Deglaciation, is punctuated by a series

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of abrupt drops, each lasting 200 to 1000 years and centered at ca. 8.3, 7.3, 4.9, 4.0,

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3.4, 2.5, 1.5 and 0.5 kyr BP (Fig. 4D). The cooling phases at 8.6~8.1, 5.8~4.8, 4.1~3.9, 1.6~1.3 and 0.6~0.5 kyr BP are synchronous with the well-established ice-rafting

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events in the North Atlantic (Bond Event; Fig. 4) (Bond et al., 1997), reflecting teleconnection between the OT and the high latitude northern Hemisphere. Other

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cooling phases at ~7.3, 3.4 and 2.5 kyr BP are not identified in the sediment record of the North Atlantic (Bond et al., 1997), suggesting influences from other factors such as variations in path and intensity of Kuroshio Current (Jian et al., 2000). The SST drop at 8.3 kyr BP in our record is coeval with an abrupt δ18O decrease of the GISP2 (Fig. 4C; Alley et al., 1997; Stuiver & Grootes, 2000), which is also reflected by radiolarian assemblages in sediments of northern OT (Fig. 4E)(Chang et al., 2008). This well-known “8.2-kyr event” is induced by the abrupt outburst of freshwater from Laurentide ice margin lakes to Hudson Bay (Alley et al., 1997; Bond et al., 1997; Spooner et al., 2002). However, its amplitude in our record is significantly smaller than that in the high latitude northern Atlantic where the 8.2-kyr 14

ACCEPTED MANUSCRIPT BP event is the strongest Holocene cooling event (e.g., Alley et al., 1997; Bond et al., 1997). The comparison of our record with that from tropical western Pacific (MD81;

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Fig. 4G; Stott et al., 2004) shows asynchronous SST maxima (7.2 kyr BP vs. 10.3 kyr

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BP). These differences between the OT and high/low latitude regions are attributed to

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regional influence of Kuroshio Current since foraminiferal distributions suggested that the main axis of Kuroshio Current reentered the OT at ~7.3 kyr BP and stronger East Asian Summer Monsoon in early Holocene strengthened Kuroshio Current (Jian et al.,

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2000; Wang et al., 2001), which alleviate the 8.2-kyr cooling event in southern OT.

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The SST drop (ca.1 oC) at 5.8~4.8 kyr BP is contemporaneous with the rapid climate change in mid-Holocene (ca. 6~5 kyr BP) reflected by the increase of ice rafting debris abundance in the North Atlantic sediments (Bond et al., 1997),

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enhanced K+ concentration in the Greenland Ice Core (Mayewski et al., 1997), termination of the African humid period (Gasse, 2001) and the lake desiccation in

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northwestern India (Enzel et al., 1999). For the OT surrounding regions, Selvaraj et al. (2007) found a dramatic decrease in organic carbon contents in lake sediments at 5.4

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kyr BP. The Mg/Ca ratio of planktonic foraminifera also suggested relatively lower SST in tropical Pacific in mid-Holocene (Fig. 4G), suggesting a teleconnection of climate between Western Pacific Warming Pool (WPMP) and the OT (Stott et al., 2004). Furthermore, the cooling phase centering at 4.0 kyr BP (Fig. 4D) is coeval with a sharp decrease in the abundance of planktonic foraminifer Pulleniatina obliquiloculata, a Kuroshio Current indicator species, in sediment records from the southern and northern OT (Jian et al., 2000; Ujiie et al., 2003). The SST drop at 2.6 kyr BP in our record, within dating uncertainty, is synchronous with a drier and colder climate in China around 2.8 kyr BP, known as Zhou-Han Dynast cold period (Chu, 1973; Shi et al., 1993), suggesting environmental 15

ACCEPTED MANUSCRIPT deteriorations occurring in both eastern Asian continent as well as its marginal sea. The most two recent cooling events centered at 1.5 and 0.5 kyr BP are in phase with

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increased δ18O values of the stalagmite in the Dongge Cave (southern China),

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implying a weakened Asian summer monsoon (Wang et al., 2005). Particularly, the

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cooling at 0.5 kyr BP is consistent with the centennial-scale cold, dry conditions between the 15th and 19th centuries, well-known as “Little Ice Age” (LIA), which probably relates to low solar radiation, heightened volcanic activity or changes in the

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ocean circulation (Mann et al., 2009; Matthews & Briffa, 2005).

5. Conclusions

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By analyzing long-chain alkenones in the ODP core 1202B, we have reconstructed

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a continuous, high resolution SST record in the southern OT over the past 15,000 years, from which three conclusions were drawn.

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1) The SST in the southern OT increased by ca. 5 oC (21.1 to 26.5 oC) from the late last Deglaciation to Holocene, which is in the same amplitude as marginal sea of

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northern Pacific, but larger than open oceans. 2) The abrupt SST changes during the B/A warming, Older Dryas and YD cold phases are unambiguously identified in our SST record, confirming a predominant influence of the high-latitude North Atlantic on the southern OT during the late last Deglaciation. This teleconnection is likely established via the variations of EAM and Westerlies when the Kuroshio Current was weak. 3) In the Holocene, the SST temporal pattern in the OT follows a declining trend of northern hemisphere solar insolation. However, several rapid climate changes at ca. 8.6~8.1, 5.8~4.8, 4.1~3.9, 3.0~2.5, 1.6~1.3 and 0.6~0.5 kyr BP cannot be explained by solar forcing alone and instead are mediated by a complex interaction of sun, 16

ACCEPTED MANUSCRIPT tropical Pacific and high latitude northern Atlantic via variations of EAM and

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Kuroshio Current.

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Acknowledgments. This project was financially supported by NSFC (41176164;

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41476062). This research used samples provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI),

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Inc.

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High resolution sea surface temperature in Okinawa Trough was reconstructed for past 15000 years Abrupt climate changes such as the B/A warming, Older Dryas, YD and 8.2 ka events were detected in Okinawa Trough Holocene sea surface temperature in Okinawa Trough was mediated by a complex sun-ocean-atmosphere coupling.

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