Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait

Accepted Manuscript Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait

Peng Zhang, Jian Xu, Jan F. Schröder, Ann Holbourn, Wolfgang Kuhnt, Karlos G.D. Kochhann, Fei Ke, Zhen Wang, Hanning Wu PII: DOI: Reference:

S0921-8181(18)30009-2 doi:10.1016/j.gloplacha.2018.08.003 GLOBAL 2814

To appear in:

Global and Planetary Change

Received date: Revised date: Accepted date:

5 January 2018 25 June 2018 3 August 2018

Please cite this article as: Peng Zhang, Jian Xu, Jan F. Schröder, Ann Holbourn, Wolfgang Kuhnt, Karlos G.D. Kochhann, Fei Ke, Zhen Wang, Hanning Wu , Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait. Global (2018), doi:10.1016/j.gloplacha.2018.08.003

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ACCEPTED MANUSCRIPT Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait

Peng Zhang1, Jian Xu1,* [email protected], Jan F. Schröder2, Ann Holbourn2, Wolfgang

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Kuhnt2, Karlos G. D. Kochhann2,3, Fei Ke1, Zhen Wang1, Hanning Wu1

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Institute of Cenozoic Geology and Environment, State Key Laboratory of Continental Dynamics and

Department of Geology, Northwest University, Xi’an 710069, China 2

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Institute of Geosciences, Christian-Albrechts-University, Ludewig-Meyn-Str. 10, D-24118 Kiel,

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Germany 3

Technological Institute of Micropaleontology, Unisinos University, Av. Unisinos 950, 93022-750 São

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Leopoldo, Brazil *

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Corresponding author.

ACCEPTED MANUSCRIPT Abstract The Indonesian Throughflow (ITF), the sole conduit between the Pacific and Indian Oceans, regulates the heat and fresh water budgets between these oceans and plays a key role as the ‘warm water pathway’ in the global thermohaline circulation. In this

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study, we reconstructed a 25-kyr history of the ITF thermocline, based on well-dated,

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high-resolution (~200 yrs) records of Pulleniatina obliquiloculata δ18O and Mg/Ca

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from Core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m), retrieved from the southern Makassar Strait. We integrated these new records with data from offshore

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Luzon (MD98-2188; 14.82°N, 123.49°E; water depth: 730 m), from the center of the

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Western Pacific Warm Pool (WPWP) (3cBX; 8.02°N, 139.6°E; water depth: 2829 m) and from the Timor Sea (MD01-2378; 13.08°S, 121.79°E; water depth: 1783 m). Both

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sea surface and thermocline temperatures decreased and their difference (ΔT)

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increased in the southern Makassar Strait and Timor Sea between 11 to 2.5 ka, corresponding to enhanced El Niño activity and dominance of East Asian winter

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monsoon over summer monsoon. In contrast, sea surface and thermocline

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temperatures increased and ΔT decreased during the last ~2.5 kyrs concomitantly with declining El Niño activity and relative weakening of the East Asian winter monsoon. Our results additionally show that ice volume corrected seawater 18O of both sea surface and thermocline in Cores SO217-18515 and MD01-2378 co-vary with Borneo stalagmite δ18O, a proxy of regional precipitation in the WPWP, thus indicating vertical mixing of upper ocean waters along the ITF pathway. We propose that changes in the state of the El Niño-Southern Oscillation and in East Asian monsoon

ACCEPTED MANUSCRIPT related to regional precipitation exerted a major influence on the ITF thermocline profile during the Holocene and largely controlled changes in ITF thermocline temperature throughout the last 25 kyrs.

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Seawater temperature, Ice volume corrected seawater 18O

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Keywords: Indonesian Throughflow, Thermocline, Southern Makassar Strait,

ACCEPTED MANUSCRIPT 1. Introduction Today, about 10-15 Sv of water that mainly stems from North Pacific thermocline waters meanders through the Indonesian archipelago and flows into the Indian Ocean, forming the Indonesian Throughflow (ITF) (Gordon, 2005; Sprintall et al., 2009). The

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ITF is not only the sole tropical pathway of the global thermohaline circulation

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(Gordon, 1986), but also a unique low-latitude conduit between the Pacific and Indian

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Oceans (Gordon, 2005), and thus plays a key role in regional and global climate. Modeling studies suggested that blockage of the ITF would result in a warmer sea

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surface in the tropical Pacific Ocean and a cooler sea surface in the Indian Ocean

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(Song et al., 2007). Moreover, upper ocean stratification of the tropical Pacific Ocean would be weakened and the sea surface temperature (SST) gradient between the

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Western Pacific Warm Pool (WPWP) and the eastern tropical Pacific cold tongue

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would be diminished if ITF passages are closed (Lee et al., 2002; Song et al., 2007). The ITF crosses a wide range of latitudes from ~15N to ~15S, a vast area strongly

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influenced by both seasonal and inter-annual climate features and characterized by

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complex topographic barriers and tidal and wind driven vertical mixing, further leading to unusual hydrological conditions along the ITF route (Ffield and Gordon, 1996; Koch-Larrouy et al., 2008; Gordon et al., 2010; Sprintall et al., 2014). Previous studies suggested a close link between the evolution of the ITF thermocline and the dynamics of the East Asian monsoon and El Niño-Southern Oscillation (ENSO) (Bray et al., 1996; Meyers, 1996; Potemra et al., 1997; Ffield et al., 2000; Lee et al., 2002; Wajsowicz et al., 2003; Gordon et al., 2003; Sprintall et al., 2009; Gordon, 2005, 2012;

ACCEPTED MANUSCRIPT Susanto et al., 2012). During boreal winter, wind drives low-salinity Java Sea and South China Sea (SCS) surface waters, related to heavy precipitation and large river runoff from Southeast Asia into the southern Makassar Strait (Qu et al., 2005), inhibiting the ITF surface flow and enhancing the subsurface flow. As a result, the ITF

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thermocline temperature (TT) declines. During boreal summer, the flow of warm and

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fresh surface waters from the Western Pacific increases and ITF surface flow is

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advected through the Makassar Strait (Gordon et al., 2003; Gordon, 2005), leading to an increase in TT. Both modeling studies and modern observations indicated that the

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ITF transport is weaker and the TT is lower during El Niño conditions (Bray et al.,

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1996; Meyers, 1996; Potemra et al., 1997; Gordon et al., 1999; Ffield et al., 2000; Lee et al., 2002; Wajsowicz et al., 2003; Wainwright et al., 2008; Sprintall et al., 2009;

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Susanto et al., 2012).

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The Makassar Strait provides the main passage of the ITF and is, thus, an ideal area to monitor variations in the ITF thermal profile associated with the dynamics of the East

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Asian monsoon and ENSO. Previous studies have investigated the response of the ITF

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thermocline to the East Asian monsoon and ENSO since the last glacial in the Timor Sea (e.g., Xu et al., 2006, 2008; Xu, 2014; Cappelli et al., 2016). But so far, few comparable thermocline temperature records are available from the Makassar Strait in the center of the ITF pathway. Here, we present high-resolution (~200 yrs) stable isotope and Mg/Ca records from Core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m), retrieved from the southern Makassar Strait (Fig. 1). We compare these new thermocline records with published data from nearby regions, including Core

ACCEPTED MANUSCRIPT MD98-2188 (14.82°N, 123.49°E; water depth: 730 m) from the ITF inflow area offshore Luzon (Dang et al., 2012), Core 3cBX (8.02°N, 139.6°E; water depth: 2829 m) from the center of the WPWP (Sagawa et al., 2012) and Core MD01-2378 (13.08°S, 121.79°E; water depth: 1783 m) from the ITF outflow area in the Timor Sea

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(Xu et al., 2008; Sarnthein et al., 2011), to investigate variations in ITF thermocline

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waters in response to the dynamics of the East Asian monsoon and ENSO over the last

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25 kyrs.

2. Modern hydrological settings

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Annual mean TT (at a water depth of 112 m) at the location of Core SO217-18515

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exhibits pronounced inter-annual variability with TT declines during El Niño episodes, correlating well with the Nino3.4 index (Fig. 2a). For instance, TT significantly

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decreased during the major 1982/83 and 1997/98 El Niño events. Moreover, the

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temperature difference between sea surface and thermocline waters (ΔT) is also closely tied to the Nino3.4 index, with larger ΔT values during El Niño events (Fig.

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2b).

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In addition, the thermocline temperature and salinity within the Indonesian Archipelago are in general distinct from those in the eastern Indian Ocean and the western Pacific, especially in the Mindanao and Java-Sumatra upwelling areas. It is also worth noting that the warm and saline thermocline in the WPWP is split in half by a zonal area of cold and fresh seawaters corresponding to the equatorial upwelling associated with the Mindanao dome (Fig. 2c and 2d).

3. Materials and methods

ACCEPTED MANUSCRIPT Sediment samples were taken at 10 cm intervals over the upper 1076 cm of piston core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m) recovered from the southern Makassar Strait during the SONNE cruise SO217 MAJA in 2011 (Kuhnt et al., 2011). Samples were oven-dried below 40 °C, and disaggregated by soaking in

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water, and then wet sieved over a 63 µm screen. Residues were dried on a sheet of

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filter paper below 40 °C, and sieved into four size fractions: 63-150 μm, 150-250 μm,

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250-315 μm and >315 μm.

Approximately 30 tests of the upper thermocline species Pulleniatina obliquiloculata

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were picked from the size fractions 250-315 μm, then weighed. All tests were checked

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for cement encrustations and infillings before being crushed into fragments. Crushed tests were then mixed homogeneously and visually split into three aliquot parts. One

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part was used for stable isotope analysis and the other two parts for Mg/Ca analysis.

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The age model of Core SO217-18515 is constrained by 17 AMS radiocarbon ages, with a surface ocean reservoir age correction based on paired wood and foraminiferal

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samples (Schröder et al., 2016). As a result, our samples track the past 25 kyr history

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of the ITF with a temporal resolution of ∼200 years. The δ18O and Mg/Ca records of the surface species Globigerinoides ruber from Core SO217-18515 were published and discussed previously by Schröder et al. (2016). 3.1 Stable oxygen isotopes For stable isotopes analysis, crushed tests were cleaned in alcohol in an ultrasonic bath and dried at 40°C. Stable carbon and oxygen isotope measurements were made with the Finnigan MAT 251 mass spectrometer at the Leibniz Laboratory, Kiel

ACCEPTED MANUSCRIPT University. The average reproducibility of δ18O for P. obliquiloculata (10 replicates) is ±0.1‰. 3.2 Mg/Ca paleothermometry For Mg/Ca analysis, crushed foraminiferal tests were cleaned of contaminant phases

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using the standard cleaning procedure with a reductive step (Rosenthal et al., 1997;

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Martin and Lea, 2002; Barker et al., 2003). Samples were analyzed on an ICP-OES

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(Spectro Ciros SOP) at the Institute of Geosciences, Kiel University. The average reproducibility of Mg/Ca for P. obliquiloculata (11 replicates) is ±0.1 mmol/mol.

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Mg/Ca ratios of P. obliquiloculata were then converted into TT using the equation T=

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ln(Mg/Ca/0.328)/0.09 (Anand et al., 2003) (Fig. 3). Zhang et al. (2016) suggested that the habitat depth range of P. obliquiloculata is 75-125 m in the ITF region. The

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estimated TT for the core-top sample is 22.5°C, comparable with the modern

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temperature of ~22.0°C at a water depth of 112 m in the upper thermocline (Fig. 2c) (Locarnini et al., 2013).

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3.3 Seawater 18O Estimates

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Seawater 18O (18Osw) of thermocline for Core SO217-18515 was calculated from paired Mg/Ca-derived temperature and shell oxygen isotope using the equation: 18Osw(VSMOW)=0.27+(T16.5+4.8×18Ocaicite(VPDB))/4.8 (Bemis et al., 1998). The reconstructed thermocline 18Osw was corrected for the effect of temperature on the oxygen isotope fractionation between foraminiferal tests and surrounding seawaters. We then corrected 18Osw for the effect of ice volume following Waelbroeck et al. (2002) to obtain an ice-volume corrected 18Osw (18Osw-c).

ACCEPTED MANUSCRIPT 4. Results P. obliquiloculata 18O exhibits a glacial-interglacial difference of ~1.6 ‰. Values remain relatively stable around a mean of 0.14±0.11 ‰ between 25 and 16 ka, show a marked depletion from 0.14 to -1.32 ‰ at 11 ka and then exhibits low amplitude

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fluctuations between -1.32 to -1.86 ‰ until the present (Fig. 3b). When comparing G.

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ruber and P. obliquiloculata 18O records in Core SO217-18515, we note that the

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timing for the onset of the deglacial decrease is markedly different (Fig. 3a and 3b). In the P. obliquiloculata 18O records, the decrease takes places at 16 ka (Fig. 3b),

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whereas the onset of the G. ruber 18O decrease occurs ~3 kyrs earlier at 19 ka (Fig.

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3a).

The TT of the ITF, based on Mg/Ca analysis of P. obliquiloculata, remains relatively

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constant between 23 and 16 ka with a mean value of 21.3±0.5°C. It then increases

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from 16 to 11 ka, showing an overall deglacial warming of 2-3°C. The TT shows a progressive decreases from 24.2 to 21.5°C between 11 and 2.5 ka, followed by a

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marked increase from 21.5 to 22.5°C after 2.5 ka (Fig. 3d). Compared with SST

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estimates derived from G. ruber Mg/Ca, TT displays a later onset of deglacial warming by ~ 3 kyr (Fig. 3c and 3d). , the gradient of sea surface and thermocline temperatures, exhibits an overall increase of ~3.0°C from 23 to 16 ka, followed by a decreases of ~1.8°C between 16 and 11 ka. A further increase of ~3°C occurs from 11 to 2.5 ka followed by a decreases of 1.4°C from 2.5 ka to the present (Fig. 3e). Ice volume corrected 18Osw (18Osw-c) of the thermocline shows an increase of

ACCEPTED MANUSCRIPT ~0.2 ‰ from 23 to 16 ka. It then progressively decreases from 0.6 ‰ at 16 ka towards -0.45 ‰ at 2.1 ka, followed by an increase of ~0.12 ‰ over the last 2.1 kyr (Fig. 3g). The long-term trends of thermocline 18Osw-c are similar to those of sea surface 18Osw-c, indicating a close relation of sea surface and thermocline 18Osw-c (Fig. 3f

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and 3g).

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5.1 Effect of regional precipitation on ITF 18Osw-c

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5. Discussion

Previous studies suggested that in addition to variable freshwater input by

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precipitation and river runoff (Rosenthal et al., 2003; Lo et al., 2014; Mohtadi et al.,

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2014), 18Osw-c could be affected by variability in the isotopic signal of regional precipitation due to changes in seasonality and provenance of rainwater (Konecky et

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al., 2016; Schröder et al., 2016) and by lateral advection of surface waters with

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different 18Osw (Dang et al., 2012; Gibbons et al., 2014; Setiawan et al., 2015). To better understand how far 18Osw-c reflects freshwater input and seawater salinity in

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the ITF region, we compared the 18Osw-c curve of Core SO217-18515 with recently

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published δDprecip and δ13Cwax data in the same core (Wicaksono et al., 2017) and with the Borneo stalagmite δ18O profile (Partin et al., 2007) (Fig. 4). This comparison shows that both sea surface and thermocline 18Osw-c differ from δDprecip during the Last Glacial Maximum (LGM) and the last deglaciation (Fig. 4a, 4c and 4e). Wicaksono et al. (2017) proposed that δDprecip in Core SO217-18515 reflects rainwater isotope variability during the LGM and the last deglaciation, whereas it is more closely related to precipitation amounts during the Holocene. However, 18Osw-c

ACCEPTED MANUSCRIPT exhibits a reverse trend during the last glacial compared to δ13Cwax, a proxy for changes in the relative abundance of C3 versus C4 plants, largely related to the predominance of humid versus arid environments (Fig. 4a, 4d and 4e). This suggests that changes in local rainwater isotope composition are not the main factor controlling

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changes in 18Osw-c at the location of Core SO217-18515. In contrast, both sea surface

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and thermocline 18Osw-c closely track Borneo stalagmite δ18O during the last glacial

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(Fig. 4a, 4b and 4e), indicating that glacial 18Osw-c in the southern Makassar Strait is influenced by precipitation and runoff from Borneo into the Celebes Sea and at the

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northern end of the Makassar Strait. These isotope signals then become advected to

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the southern part of the Makassar Strait by the ITF near-surface flow, which was likely strengthened during the LGM (Kuhnt et al., 2004).

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Comparison of 18Osw-c in Cores MD98-2188, MD01-2378 and SO217-18515 with

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Borneo stalagmite δ18O (Fig. 4) indicates, that all isotope records are associated with regional precipitation variability in the central part of the Indonesian archipelago and

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along the ITF path. In contrast, the central WPWP 18Osw-c in Core 3cBX exhibits a

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different pattern with lighter 18Osw-c during the last glacial than during the Holocene (Fig. 4a and 4e), which may be due to relatively reduced evaporation in the heart of the WPWP during the last glacial (Sagawa et al., 2012). Sea surface 18Osw-c in the West Pacific Ocean and Indonesian archipelago started to diverge at ~16 ka, indicating more pronounced freshening of surface seawaters along the ITF pathway than in the central WPWP (Fig. 4a). This freshening in the southern Makassar Strait may be ascribed to precipitation-driven fresh water input by local rivers, located in

ACCEPTED MANUSCRIPT southwestern Sulawesi (Fig. 1). Additionally, advection of SCS surface flow may have contributed fresh waters through the Sibutu Strait into the Celebes Sea with an enhanced boreal East Asian winter monsoon，following the opening of the Kalimata Strait into the Java Sea, when sea level rose above the sill depth (Gordon et al., 2003;

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Qu et al., 2009; Griffiths et al., 2013). Similarly, freshening of the sea surface offshore

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Luzon was possibly caused by freshwater runoff from surrounding islands and

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along-shore currents under the background of strengthening regional precipitation. Interestingly both sea surface and thermocline 18Osw-c in the ITF region (Cores

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SO217-18515 and MD01-2378) show similar trends and amplitudes of change,

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reflecting regional precipitation change over the entire deglaciation (Fig.4a, 4b and 4e). Intense vertical mixing of upper ocean waters by vigorous tides and strong

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interactions between monsoonal winds and the sea surface within the Indonesian seas

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(Sprintall et al., 2014) probably caused this transmission of a precipitation signal from the sea surface into the upper thermocline.

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dynamics

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5.2 Changes in ITF thermal structure in relation to ENSO and monsoonal

5.2.1 Impact of ENSO on the ITF thermal structure At the location of Core SO217-18515, the modern seawater temperature at a depth of 112 m, approximately corresponding to the depth of the thermocline, and the modern temperature difference between 5 and 112 m co-vary with the Nino3.4 index, in particular during major La Niña and El Niño events (Fig. 2a and 2b). It is, thus, highly likely that ENSO-like states significantly influenced the upper thermal structure of the

ACCEPTED MANUSCRIPT ITF on longer timescales. We speculate that thermocline cooling and intensification of the upper thermal gradient occurred during El Niño-like conditions, and vice versa during La Niña-like conditions. From 23 to 16 ka, the long-term increase in ΔT at the location of Core SO217-18515

(Fig. 5b and 5d). According to the

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similar to the records from Core MD01-2378

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concurred with TT oscillating within a narrow range of 20.5°C to 22°C, which is

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aforementioned relationship of the modern profiles at coring site of Core SO217-18515, it seems that this scenario is not a result of an ENSO-like state. The

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long-term increase of ΔT in this duration is, however, mainly resulted from rising of

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SST (Fig. 5a).

From 16 to 11 ka, TT increased by ~3°C, concomitantly with a decrease of ΔT by

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~1°C and decreasing ENSO variability (Fig. 5b, 5d and 5f), supporting a transition to

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a La Niña-like mean state, as suggested by Fan et al. (2018). From 11 to 2.5 ka, the records from Core SO217-18515 indicate a long-term cooling of the ITF thermocline

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and sea surface, resulting in an overall increase in ΔT, also evident in Core

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MD01-2378 (Fig. 5a, 5b and 5d). This long-term trend does not indicate La Niña-like conditions, as suggested by Fan et al. (2018), but rather point to increasing El Niño-like activity, in agreement with modern profiles at the location of Core SO217-18515 (Fig. 2). During this interval, sedimentary records from Laguna Pallcacocha and El Junco Crater Lake indicate a gradual increase in El Niño activity (Moy et al., 2002; Conroy et al., 2008) (Fig. 5e). Moreover, a model study by Liu et al. (2014) also suggested that ENSO variability gradually strengthened between 11 and

ACCEPTED MANUSCRIPT 2.5 ka (Fig. 5f). The development of El Niño conditions would promote a decline in SST and TT and shoaling of the thermocline, thus enhancing ΔT in the western Pacific Ocean (Fig. 2a and 2b), since weakened trade winds would favor a shift of warm surface waters towards the eastern Pacific Ocean (Lee et al., 2002; Song et al., 2007).

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Thus, the decline in SST and TT and the increase of ΔT along the ITF and nearby area

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between 11 and 2.5 ka suggest increasing El Niño activity. In contrast, the gradual

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increase in SST and TT, together with the decrease of ΔT after 2.5 ka (Fig. 5a, 5b and 5d), indicate reduced El Niño activity and ENSO variability (Fig. 5e and 5f),

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highlighting the influence of ENSO on seawater temperatures and upper ocean

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thermal structure in the western Pacific and the ITF region. Recently, Fan et al. (2018) investigated ITF variability over the last 30 kyrs based on

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records of Cores MD98-2161 and MD98-2178 respectively within the ITF inflow and

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outflow areas. The long-term trend in both cores is consistent with that in Core SO217-18515 (Fig. S1). Fan et al. (2018) suggested that a La Niña-like climate mean

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state prevailed during the Holocene and an El Niño-like climate mean state during the

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last glacial, which controlled the strength of the ITF thermocline current. Fan et al. (2013) additionally suggested that ENSO variability was high during the Holocene, as El Niño-like conditions prevailed at 1.2–3, 3.8–4.6, 5.2–7, 8–9.6 and 11.2–12 ka, whereas La Niña-like conditions dominated at 3–3.8, 4.6–5.2, 7–8 and 9.6–11.2 ka. ENSO variability during the late Pleistocene has been the topic of intense debate. Lea et al. (2000) proposed that La Niña-like conditions prevailed during glacial periods, based on more intense sea surface cooling in the equatorial Eastern Pacific than in the

ACCEPTED MANUSCRIPT center of the WPWP. This scenario was later challenged by a comparison of SST records from the Galapagos Islands and South China Sea that indicated El Niño-like conditions during the last glacial (Koutavas et al., 2002). More recently, a modeling study indicated a long-term decrease in ENSO variability between the last glacial and

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~16 ka (Fig. 5f; Liu et al., 2014), in contrast to the El Niño-like climate mean state

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proposed by Fan et al. (2018) based on reconstructed zonal Pacific SST gradient

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anomaly of Koutavas and Joanides (2012). Besides, there are also other views indicating that ENSO-like climate has been always existing in the last 150 kyrs, even

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during glacial periods (Rodbell et al., 1999; Tudhope et al., 2001). With regard to

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ENSO variability in the Holocene, opposite views also exist. Archeological records and bivalve oxygen isotopes from Peru revealed that ENSO-like events happened

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frequently during the early to mid-Holocene (Keefer et al., 1998; Carre et al., 2005,),

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whereas other studies indicated that ENSO-like events started to occur or to strengthen 5 to 7 kyrs ago (Wells, 1990; Sandweiss et al., 2001; Moy et al., 2002).

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Overall, the intrinsic irregularity of ENSO and its complicated relationship with other

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climate phenomena, leading to reconstructions and model simulations of ENSO variability are subject to inherent difficulties in interpretations and biases (Lu et al., 2018). Finally, there is also the possibility of a uniform continuous mid-late Holocene weakening of the trade winds, which would have a similar impact on our marine proxy records as an increase in the frequency of El Niño events and respectively a change in El Niño mean state. 5.2.2 Contribution of the East Asian Monsoon to variations in ITF hydrology

ACCEPTED MANUSCRIPT Modern observations indicate that the ITF surface flow is depressed in boreal winter due to fresh SCS surface seawaters driven by winter monsoon winds into the southern Makassar Strait, thus leading to a colder ITF and intensification of the ITF subsurface flow (Gordon et al., 2003). Moreover, the sea surface is slightly warmer and much

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fresher in the south than in the north of the Makassar Strait during the boreal winter

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monsoon (Fig. 3 in Gordon, 2005). The situation is reversed during the boreal summer

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monsoon, with relatively enhanced ITF surface flow, a warmer ITF, slightly cooler sea surface in the south than in the north and reduced south-north salinity gradient within

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the Makassar Strait due to input of saline Banda Sea surface water (Gordon et al.,

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2003; Fig. 3 in Gordon, 2005; Fig. 1 in Fan et al., 2018). Following deglacial warming, the records from the ITF region, including Cores

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SO217-18515, MD01-2378, MD98-2161 and MD98-2178, show a long-term cooling

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of both the thermocline and sea surface from 11 ka to 2.5 ka, followed by slight warming to the present (Fig.5 and Fig. S1). The East Asian winter and summer

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monsoons were previously considered to be in anti-phase over longer timescales (e.g.,

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Yancheva et al., 2007). However, recent modeling study (Wen et al., 2016) and proxy records (Huang et al., 2007; Tian et al., 2010; Wang et al., 2012; Jia et al., 2015) suggested that the winter monsoon was in-phase with the summer monsoon during the Holocene. Both summer and winter monsoon gradually weakened from 11 to 2.5 ka and then slightly intensified towards the present (Fig. 5g and 5h), as suggested by the composite China stalagmite record (Wang et al., 2001; Yuan et al., 2004), the SST gradient in the SCS (Huang et al., 2011) and simulation results (Wen et al., 2016).

ACCEPTED MANUSCRIPT Records from the ITF region show that the ITF was cooling in both its surface and thermocline from 11 to 2.5 ka (Fig. 5 and Fig. S1). Moreover, the sea surface was slightly warmer and fresher in the south (Core MD98-2161) than in the north (MD98-2178) over this period (Figs. 3 and 4 in Fan et al., 2018). This suggests that

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the East Asian winter monsoon was relative intensifying from 11 to 2.5 ka, since

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modern observations show a colder ITF, as well as warmer and fresher sea surface in

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the south than in the north of the Makassar Strait during the winter monsoon season (Gordon et al., 2003; Gordon, 2005). This interpretation is supported by model

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simulations, indicating higher wind speeds during winter (Wen et al., 2016). The

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dominance of the winter monsoon over the summer monsoon would also have favored the flow of fresh SCS surface waters entering the southern Makassar Strait (Gordon et

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al., 2003; Qu et al., 2009), promoting the ITF subsurface flow over the surface flow

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and leading to an increase in ΔT (Fig. 5d). Conversely, warming of both the ITF thermocline and sea surface and decreasing of ΔT over the last 2.5 kyrs may have

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been caused by relative weakening of the winter monsoon.

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5.3 Fingerprint of boreal summer insolation on thermocline temperature conveyed by North Pacific Tropical Water All deglacial TT records in cores from offshore Luzon, the southern Makassar Strait, the central WPWP and the Timor Sea show a gradual temperature increase with the highest temperatures occurring at ~11 ka. TT then gradually declined during the Holocene with the smallest amplitude evident in the central WPWP (Fig. 5b). In fact, TT in all cores closely followed boreal summer insolation (Laskar et al., 2004) (Fig.

ACCEPTED MANUSCRIPT 5b and 5c). Records from Cores MD98-2188 and 3cBX from offshore Luzon and the central WPWP, which are not influenced by the ITF (Fig. 1), exhibit similar trends as Cores SO217-18515 and MD01-2378 from the ITF region (Fig. 5b). This close regional similarity may be related to a common North Pacific source area of

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thermocline water masses. North Pacific Tropical Water (NPTW) forms in the center

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of the North Pacific (10°-25°N, 140°E-160°W) and is found at depths of 50-300 m

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(Cannon, 1966; Tsuchiya, 1968). The most remarkable feature of the NPTW is that its salinity is above 34.75 psu (Qu et al., 1999). The formation of NPTW occurs due to

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excessive evaporation over precipitation, resulting in increased SSS, which leads to

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the sinking of these surface water (Fine et al., 1994, 2001). Thus, the temperature of NPTW is effectively controlled by summer insolation at 10°-25°N. NPTW is carried

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to the Philippine Sea (PS) by the westward flowing North Equatorial Current (NEC),

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then splits owing to the bifurcation of the NEC in the Philippine Sea (Fig. 1). Part of the NPTW spreads into the SCS (Qu et al. 2000) or the East China Sea (Yang et al.

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2012) along the flow path of the Kuroshio Current (Fig. 1). The other part moves

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towards the equator with the Mindanao Current, providing a considerable amount of waters into the Celebes Sea and ultimately feeding the ITF (Gordon 1986; Fine et al. 1994), leaving the remnant waters to turn east and join the North Equatorial Counter Current (NECC) (3°-10°N) and ultimately contributing to the thermocline of the tropical Pacific (Fig. 1) (Goodman et al. 2005). All thermocline waters at these coring sites therefore carry signals of the NPTW, which would explain the close relationship between their TT and boreal summer insolation.

ACCEPTED MANUSCRIPT Rosenthal et al. (2013) reconstructed changes in intermediate water temperatures (IWT) based on benthic foraminifer Hyalinea balthica from the Indonesian bathymetric transect during the Holocene. Comparison of IWT records from the ITF region and SST and TT records from the WPWP, Rosenthal et al. (2013) found that

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their changing trends are very much alike, i.e., all of them gradually declined during

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the Holocene. Since the intermediate water in the ITF region carries a substantial

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contribution of high-latitude intermediate waters (Zenk et al., 2005), these authors suggested that cooling of the thermocline and sea surface in the WPWP could be due

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to high-latitude cooling during the Holocene. As the records of Rosenthal et al. (2013)

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are derived from the lower thermocline, the Holocene cooling trend may have affected the entire subsurface ITF.

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6. Conclusion

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In this study, we investigated the history of the ITF thermocline over the past 25-kyr, based on records of P. obliquiloculata δ18O and Mg/Ca from Core SO217-18515

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retrieved from the southern Makassar Strait. We integrated these new records with

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published data from Cores MD98-2188 (offshore Luzon), 3cBX (the center of the WPWP) and MD01-2378 (the Timor Sea). Our results show that themocline 18Osw-c in the Makassar Strait and Timor Sea closely followed the major trends of the Borneo stalagmite δ18O, reflecting the influence of regional precipitation. We infer that the precipitation signal was transmitted to the thermocline by vertical mixing of the upper ocean water column. The decline of TT and intensification of ΔT between 11 ka and 2.5 ka in the ITF region can be attributed to increasing El Niño activity and

ACCEPTED MANUSCRIPT dominance of the East Asian winter monsoon. In contrast, the increase in TT and decrease in ΔT over the past 2.5 kyr probably reflect declining El Niño activity and relative weakening of the East Asian winter monsoon. Over the past 25 kyr, thermocline temperatures in the WPWP and the ITF region closely followed boreal

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summer insolation. This signal may have been conveyed to the studied sediment cores

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by NPTW.

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Acknowledgments

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We are grateful to Dieter Garbe-Schönberg (Kiel University) for ICP-OES measurements and Nils Andersen (Leibniz Laboratory Kiel) for stable isotopes

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analyses. We appreciate the shipboard members and crews of the cruise of SO217 MAJA for their support in retrieving sediment cores. Our sincere thank goes to the

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editor and two anonymous reviewers for their constructive comments that greatly improved the manuscript. This work was supported by the National Natural Science

Foundation

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Foundation of China [grant number 41576045], China Postdoctoral Science [grant

number

2017M623220],

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Fund for Fostering Talents in Basic Science [grant number XDCX2017-01] and Undergraduate Training Program for Innovation and Entrepreneurship of Northwest University [grant number 2018242] and Shaanxi province [grant number 201807039].

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https://doi.org/10.1016/j.pocean.2005.05.003. Zhang, P., Zuraida, R., Xu, J., Yang, C., 2016. Stable carbon and oxygen isotopes of four

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significance.

Acta

Oceanologica

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35(10),

63-75.

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planktonic foraminiferal species from core-top sediments of the Indonesian throughflow region

https://doi.org/10.1007/s13131-016-0890-1. Zweng, M.M, Reagan, J.R., Antonov, J.I., Locarnini, R.A., Mishonov, A.V., Boyer, T.P., Garcia, H.E., Baranova, O.K., Johnson, D.R., Seidov, D., Biddle, M.M., 2013. World Ocean Atlas 2013, Volume 2: Salinity. Levitus, S., Ed.; Mishonov, A., Technical Ed.; NOAA Atlas NESDIS 74, 39 pp.

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Figure 1. Map of sites involved in this study. (Right panel) The western Pacific and its adjacent seas showing study sites and surface water currents; (Left panel) Zoom of

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the Makassar Strait showing approximate extensions of main rivers from islands (blue

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dashed lines). Red star represents the coring site of Core SO217-18515. Black dots represent stalagmite sites of Hulu, Dongge and Gunung Buda (Wang et al., 2001;

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Yuan et al., 2004; Partin et al., 2007), and coring sites of deep-sea Cores 3cBX

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(Sagawa et al., 2012), MD98-2188 (Dang et al., 2012) and MD01-2378 (Xu et al., 2008). NEC: North Equatorial Current; KC: Kuroshio Current; MC: Mindanao Current; NECC: North Equatorial Countercurrent; SCSTF: South China Sea Throughflow; ITF: Indonesian Throughflow. Map created with Ocean Data View (Schlitzer, 2018).

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Figure 2. Relationship of thermocline (at 112 m of water depth) temperature (a) and

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temperature difference between sea surface (at 5 m) and thermocline (b) at the coring site of Core SO217-18515 (Carton and Giese, 2008) against Nino3.4 index during

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1982-2009, and distribution of annual mean thermocline temperature (c) and salinity (d) in the WPWP. Red star represents the coring site of Core SO217-18515. Annual mean temperature and salinity data are from the World Ocean Atlas 2013 (Locarnini et al., 2013; Zweng et al., 2013). Temperature profiles during 1982-2009 from http://iridl.ldeo.columbia.edu/SOURCES/.CARTON-GIESE/.SODA/.v2p0p2-4/

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Nino3.4 index from http://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices. Map created with Ocean Data View (Schlitzer, 2018).

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Figure 3. Temperature and δ18O profiles in Core SO18515 over the last 25 kyr. Planktonic δ18Ocalcite from shells of G. ruber (a) and P. obliquiloculata (b);

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Mg/Ca-derived sea surface temperature (SST) (c) and thermocline temperature (TT) (d); difference between SST and TT (e); ice-volume corrected seawater δ18O (δ18Osw-c) of surface (f) and thermocline (g); G. ruber datasets are from Schröder et al. (2016). The blue arrow marks the onset of deglacial increasing in TT and depleting in P. obliquiloculata 18O at ~16 ka, respectively. The green arrows denote possible onsets of deglacial change for both G. ruber 18O and SST at ~19 ka. Red arrows indicate long-term trends.

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Figure 4. Comparison of seawater δ18O and precipitation indices. Ice-volume

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corrected seawater δ18O (δ18Osw-c) of surface (G. ruber s.s.) (a) and thermocline (P. obliquiloculata) (e) from Cores SO217-18515, MD98-2188 (Dang et al., 2012), 3cBX (Sagawa et al., 2012) and MD01-2378 (Xu et al., 2008; Sarnthein et al., 2011); Borneo stalagmite δ18O (b) (Partin et al., 2007); δDprecip (c) and δ13Cwax (d) from Core SO217-18515 (Wicaksono et al., 2017). Pink-colored shading marks repercussions before relatively continuous increasing in surface and thermocline 18Osw-c. Red arrows indicate long-term trends.

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Figure 5. Comparison of temperature profiles with paleoclimatic records. (a) SST and

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(b) TT from Cores SO217-18515, MD98-2188 (Dang et al., 2012), 3cBX (Sagawa et al., 2012) and MD01-2378 (Xu et al., 2008; Sarnthein et al., 2011); (c) summer (June)

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insolation at 15°N (Laskar et al., 2004); (d)  from Cores SO217-18515, MD98-2188, 3cBX and MD01-2378 (references as above); (e) Frequency of ENSO events (Moy et al., 2002; Conroy et al., 2008) and (f) ENSO Variability (Liu et al., 2014); (g) Composite China cave stalagmite δ18O as a proxy of East Asian summer monsoon (EASM) (Wang et al., 2001; Yuan et al., 2004); (h) SCS SST gradient as a proxy of East Asian winter monsoon (EAWM) (Huang et al., 2011). Red arrows indicate long-term trends.

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Themocline 18Osw-c in the Makassar Strait and Timor Sea co-vary with Borneo stalagmite δ18O, a proxy of regional precipitation in the western Pacific warm pool (WPWP), thus indicating vertical mixing of upper ocean

The decline of TT and intensification of ΔT between 11 ka and 2.5 ka in the

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waters along the Indonesian Throughflow (ITF) pathway.

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ITF region can be attributed to increasing El Niño activity and dominance of East Asian winter monsoon over summer monsoon. In contrast, the increase in

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TT and decrease in ΔT over the past 2.5 kyrs probably reflect declining El

Over the past 25 kyrs, thermocline temperatures in the WPWP and the ITF

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region closely followed boreal summer insolation, the signal of which may

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have been conveyed to the studied sites by North Pacific Tropical Water.

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Niño activity and relative weakening of the East Asian winter monsoon.