Oscillations in the southern extent of the Indo-Pacific Warm Pool during the mid-Holocene

Quaternary Science Reviews 28 (2009) 2794–2803

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Oscillations in the southern extent of the Indo-Pacific Warm Pool during the mid-Holocene Nerilie J. Abram a, b, *, Helen V. McGregor a, c, Michael K. Gagan a, Wahyoe S. Hantoro d, Bambang W. Suwargadi d a

Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, UK School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia d Research and Development Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 40135, Indonesia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 November 2008 Received in revised form 2 July 2009 Accepted 14 July 2009

The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between w5500 and 4300 years BP (w1.2  C  0.3  C) and were similarly cool before w6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between w6600 and 6300 years BP (w1.3  C  0.3  C), while similarly warm conditions may have also existed after w4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesting that the Asian monsoon–IOD interaction that exists at interannual time scales also persists over centennial to millennial scales. Associated mean changes in the Pacific ENSO modes may have also occurred during the mid-Holocene. The dynamic and inter-connected behaviour of the IPWP with tropical climate systems during the mid-Holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics and beyond. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The Indo-Pacific Warm Pool (IPWP) is the warmest body of open-ocean water on Earth. Defined as the region where mean sea surface temperature (SST) is greater than 28  C (Yan et al., 1992),

* Corresponding author. British Antarctic Survey, Natural Environment Research Council, High Cross, Cambridge CB3 0ET, UK. Tel.: þ44 1223 221539; fax: þ44 1223 221279. E-mail address: [email protected] (N.J. Abram). 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.07.006

the IPWP spans from the western tropical Pacific Ocean, through the Indonesian archipelago, and across the eastern tropical Indian Ocean (Fig. 1a). The vigorous atmospheric convection that occurs over the warm pool influences the global distribution of heat and water vapour. Because of this, changes in the temperature, size and position of the IPWP have a profound effect on global climate (Gagan et al., 2004). Climate variability in the IPWP region is influenced by three ˜ o-Southern Oscillation (ENSO) major climate systems: the El Nin (Rasmusson and Wallace, 1983; Trenberth and Shea, 1987), the

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reconstruct mean SST since the mid-Holocene. The collection of corals is analyzed at low-resolution to efficiently provide mean SST estimates from a large number of individual colonies, allowing the error in reconstructing secular SST changes to be significantly reduced. The resulting SST reconstruction is used to examine changes in the extent of the southern IPWP and how this relates to the interplay between the Asian monsoon, IOD and ENSO systems since the mid-Holocene. 2. Regional setting

Fig. 1. (a) Mean annual SST map for the tropical Indian and Pacific Oceans (Reynolds et al., 2002). Contour interval is 2  C, and shading where mean annual SST exceeds 28  C defines the Indo-Pacific Warm Pool that spans from the western Pacific to the eastern Indian Ocean. The location of the Mentawai and Muschu/Koil study sites near the southern margin of the IPWP are shown. (b) Sr/Ca estimates of mean SST anomalies at the Mentawai (solid circles) and Muschu/Koil (open circles) sites since the midHolocene. Error bars show 2s ranges on age determinations, and the combined error on mean SST estimates (Table 1). Grey lines show weighted mean SST (and 2SE in grey shading) at the Mentawai site over intervals where a large number of Mentawai corals provide independent SST estimates.

Asian–Australian monsoon (Webster et al., 1998), and the Indian Ocean Dipole (IOD) (Saji et al., 1999; Webster et al., 1999). These three systems influence rainfall variability throughout the tropics and subtropics, and understanding their behaviour and interplay is crucial for improving rainfall forecasts. This is particularly relevant for predictions of 21st century changes in the distribution and variability of rainfall, as there is still considerable uncertainty in climate model assessments as to how these tropical climate systems, and the IPWP, will respond to anthropogenic greenhouse warming (Conway et al., 2007; Meehl et al., 2007; Vecchi and Soden, 2007). An essential task for improved modelling of future climate change involves the validation of palaeoclimate models with proxy records (Brown et al., 2008b). Proxy records from the mid-Holocene, when the seasonality of insolation was distinctly different to present day, are particularly useful for examining the response of the climate system to changing boundary conditions. In the tropical regions, the skeletal chemistry of fossil corals provides a powerful proxy for reconstructing Holocene environmental changes on seasonal to interannual time scales (Gagan et al., 1998; Tudhope et al., 2001; Cobb et al., 2003; Felis et al., 2004; McGregor and Gagan, 2004; Abram et al., 2007). However, early attempts to reconstruct mean changes in Holocene SST using fossil corals (e.g. Beck et al., 1992, 1997; McCulloch et al., 1996) have often produced results that are significantly different from estimates of SST based on sediment core proxies (Gagan et al., 2004). Studies of modern and young fossil coral records have shown that mean SST estimates from individual coral specimens can involve large uncertainties (Evans et al., 2000; Cobb et al., 2003; Linsley et al., 2006; Wilson et al., 2006) that may be related to species effects, micro-environment or other as-yet unresolved influences. This suggests that analysis of multiple coral specimens is needed to converge upon the true mean SST when using reconstructions from fossil corals (Cobb et al., 2003). In this study, we use an extensive suite of fossil corals from uplifted reefs in two regions of the southern IPWP (Fig. 1a) to

The first of the two study sites is the Mentawai Island chain, which lies approximately 200 km offshore of Sumatra in the tropical eastern Indian Ocean (Abram et al., 2003). Modern-day mean SST around the Mentawai Islands is 29.2  C, with an average maximum of 29.6  C in May and minimum of 28.6  C in November (Reynolds et al., 2002). The Asian–Australian monsoon imparts a strong seasonality on the wind fields in the Mentawai region. During the Asian summer monsoon season southeasterly tradewinds occur along the Sumatran coast. These winds reverse to northwesterlies during the Australian summer monsoon. The associated migration of the Inter-tropical Convergence Zone (ITCZ) over the Mentawai region produces maxima in rainfall during April and November, although all months receive high amounts of rainfall resulting in an annual average rainfall of 4200 mm at nearby Padang station (Vose et al., 1992). The Mentawai Islands lie in the eastern upwelling region of the IOD system (Saji et al., 1999; Webster et al., 1999; Saji and Yamagata, 2003). Here, IOD upwelling events can produce cool SST anomalies in excess of 4  C (Abram et al., 2003, 2007). IOD upwelling along the Sumatran coast is inextricably linked to the strength of the southeasterly tradewinds that drive the Ekman upwelling process, and this constrains the development of IOD events to the Asian summer monsoon season. Recent studies have found that positive feedbacks exist between strengthened Asian monsoon rainfall and IOD events (Ashok et al., 2004; Gadgil et al., 2004; Abram et al., 2007, 2008; Swapna and Krishnan, 2008). IOD variability is also influenced by the ENSO system (Saji and Yamagata, 2003; Meyers et al., 2007; ˜ o conditions causes Abram et al., 2008). The development of El Nin the atmospheric convection associated with Pacific Walker circulation to shift eastward, and the decreased convection (increased atmospheric pressure) over Indonesia and Papua New Guinea (PNG) allows anomalous surface easterly winds to develop across the equatorial Indian Ocean (Fischer et al., 2005). Shoaling of the ˜ o events also allows thermocline in the western Pacific during El Nin subsurface cool anomalies to be transported to the tropical eastern Indian Ocean via the Indonesian throughflow (Alory et al., 2007). ˜ o events promote These ocean–atmosphere processes during El Nin the development of IOD upwelling along the Sumatran coast. The tropical climate interactions that occur in the Mentawai Island region make this a particularly sensitive site for studying changes in tropical climate behaviour (Abram et al., 2008). The second location used in this study is the Muschu and Koil Islands, which lie 73 km and 115 km, respectively, offshore of the Sepik River mouth in northeastern PNG (McGregor and Gagan, 2004). Mean SST in this region is 29.0  C, with a May maximum of 29.4  C and August minimum of 28.6  C (Reynolds et al., 2002). The Muschu/Koil Islands also lie within the region influenced by the ITCZ, resulting in a high average annual rainfall of 2000 mm at nearby Wewak station that is distributed fairly evenly throughout the year (McAlpine et al., 1983). Surface ocean temperatures and salinity around Muschu/Koil Islands are strongly affected by ENSO variability (McGregor and ˜ o event SSTs here cool by up to 1  C, Gagan, 2004). During an El Nin ˜ o years, due to the eastward migration of compared to non-El Nin

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the warm surface waters of the western tropical Pacific and shoaling of the thermocline that induces mixing with cooler ˜ o events also subsurface waters (Tourre and White, 1997). El Nin cause sea surface salinity to increase in this region, typically by 0.5– 1 psu. This is because as the warm pool waters move eastward the ascending branch of the Walker circulation cell is also displaced, which locally suppresses the rainfall that contributes to warm pool freshness. The thermocline waters that are mixed into the surface ˜ o events are also more saline than the charwaters during El Nin acteristically ‘‘fresh’’ waters of the IPWP. Together, the two sites used in this study provide information about the Holocene history of the southeastern and southwestern margins of the IPWP. The sites are both influenced by the meridional (north–south) migrations of the ITCZ associated with the seasonally reversing Asian–Australian monsoon. They are also sensitive to zonal (east–west) climate variability of the ENSO and IOD systems that act across the tropical Pacific and Indian Oceans, respectively. 3. Materials and methods Cores of modern and fossil Porites sp. corals were collected around Muschu/Koil Islands during leg 6b of Project TROPICS in 1998. The modern and fossil Porites coral cores from the Mentawai Islands were collected during field trips in 1999 and 2001. To address the large uncertainties associated with mean SST estimates from single coral colonies, we utilise a large number of modern and fossil corals from both localities. In total, this study uses results for six modern and thirty-one fossil corals from the Mentawai Islands, and three modern and eight fossil corals from Muschu/Koil Islands. Each of the coral cores was slabbed parallel to the major growth axis and x-rayed to reveal the annual coral density bands. The coral slabs were cleaned in milli-Q H2O using an ultrasonic probe, then dried in an oven at 40  C (Gagan et al., 1994). X-ray diffraction and petrographic analysis were used to ensure that only well-preserved coral specimens were selected for geochemical analysis (McGregor and Gagan, 2003; McGregor and Abram, 2008). Uranium-series ages for the Muschu/Koil corals were determined by both isotope dilution thermal ionisation mass spectrometry and by multicollector inductively coupled plasma mass spectrometry. Initial fossil coral d234U was constrained to 146–151& (McGregor et al., 2008). The fossil coral samples from the Mentawai Islands were dated using 14C measurements made at the Australian National University (ANU) Radiocarbon Dating Laboratory. Conventional radiocarbon measurements were corrected using measured d13C compositions and adjusted for a local reservoir effect of 85  65 years (Abram et al., 2003) before being calibrated using the IntCal04 marine database (Hughen et al., 2004). Fossil coral ages are expressed in years before present (years BP), where present is 1950 AD. To efficiently estimate mean skeletal Sr/Ca the Mentawai and Muschu/Koil corals were subsampled continuously at low-resolution (annual or mulit-year) using a low-speed milling system (Hendy et al., 2002). The Muschu/Koil corals were milled at annual resolution, using density bands in the corals to guide the sampling. In order to efficiently estimate mean Sr/Ca, precise aliquots of coral powder from each annual sample were weighed and combined to produce 5 composite samples for each coral. The mean Sr/Ca determinations for the Muschu/Koil corals represent between 9 and 89 years of coral growth (Table 1). Subsampling of the Mentawai corals was performed at either annual resolution based on coral density banding, or as single bulk samples spanning several years. Mean coral Sr/Ca was determined using either the average of single measurements on the annual

resolution samples, or the mean of triplicate measurements on bulk coral powders. Comparison of the geochemical results produced using both the annual and bulk sampling methods in three of the Mentawai fossil corals confirms that there is no bias in the results produced by the different low-resolution sampling techniques. To minimise the influence of interannual climate variability on the results, more than 7 years of coral growth was sampled in all but one of the Mentawai coral cores (range of 4–15 years; Table 1). Skeletal Sr/Ca was measured at the Research School of Earth Sciences, ANU, using a MAT-261 thermal ionisation mass spectrometer. The analytical precision of coral Sr/Ca determinations using this instrument is 0.03 mmol mol1 (2s, n ¼ 14 pairs). The mean Sr/Ca values of the fossil corals were converted to SST anomalies relative to the mean Sr/Ca value obtained from the modern corals for each locality using a Sr/Ca-SST dependence of 0.061 mmol mol1  C1 (Abram et al., 2007). This is the mean dependence produced by 38 published calibrations for Porites sp. corals (Correge, 2006). The error assigned to each fossil coral Sr/Ca-SST estimate is derived from the combination (root of the sum of the squares) of: 1) The standard deviation (2s) around the reconstructed modern mean SST based on Sr/Ca analysis of the six modern corals from the Mentawai Islands, or the three modern corals from Muschu/Koil Islands (i.e. an estimate of the likely range of reconstructed SSTs given by multiple corals from a single location), and 2) The standard error (2SE) of the mean of multiple Sr/Ca measurements for each fossil coral (i.e. the uncertainty associated with the determination of mean Sr/Ca in an individual coral). For time intervals where a group of similar age fossil corals from the Mentawai site give independent realizations for the mean SST anomaly, a random effects model (Galbraith and Laslett, 1993; Galbraith et al., 1999) was used to calculate a weighted mean SST and standard error. This weighting and standard error calculation takes into account the errors on each individual SST estimate within the interval. 4. Results The Holocene coral SST reconstructions are shown in Fig. 1b. Since w700 years BP the fossil corals from the Mentawai Islands show mean SSTs that are consistent with present-day conditions (0.1  0.3  C; based on the seven coral SST estimates around this time). A cluster of five coral samples from the Mentawai Islands dated at w2000 years BP return a mean SST that is slightly cooler than present day, although with much greater variability (0.9  0.6  C). It is possible that the increased variability of the mean Sr/Ca-SST during this interval is indicative of the heightened ENSO activity at this time (Moy et al., 2002; McGregor and Gagan, 2004; Conroy et al., 2008), or it may instead reflect a reduction in the quality of the coral reconstruction of mean SST during this interval. During the mid-Holocene, the large array of Mentawai corals suggests that the mean SST of the tropical eastern Indian Ocean was predominantly cooler than present day. Between w5500 years BP and w4300 years BP a collection of seven Mentawai corals defines mean SSTs of 1.2  0.3  C. Similarly cool conditions also appear to have existed here before w6800 years BP. This mid-Holocene cooling in the eastern Indian Ocean was punctuated by an abrupt, short-lived shift to conditions that were 1.3  0.3  C warmer than present between w6600 years BP and w6300 years BP (defined by

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Table 1 Details of the coral Sr/Ca-SST reconstructions used in this study. Record length

Analysis resolution

Mean Sr/Ca (2SE)b (mmol mol1)

Reconstructed SST anomaly (error)c ( C)

Mentawai Islands coral samples PG01-A-2 Modern PG01-A-3 Modern TM01-A-10 Modern TN99-A-4 Modern LB99-A-3 Modern SMG01-A-7 Modern B01-A-1 Y83 2–393 years BP B01-A-1 Y95 14–405 years BP B01-A-1 Y145 64–455 years BP SMG01-A-10 127–532 years BP P01-C-1a 304–606 years BP SMG01-A-5b 381–700 years BP SMG01-A-1 541–892 years BP TF99-A-3 1492–1884 years BP TF01-A-8 1569–2017 years BP TF01-A-7 1636–2058 years BP P01-B-2 1632–2113 years BP P01-B-1a 1719–2183 years BP TF01-A-5c 3379–3818 years BP TF99-A-4 3641–4129 years BP TF01-A-3 3639–4173 years BP LB99-A-5 4130–4724 years BP TF99-A-7b 4058–4608 years BP TF99-A-5 4156–4698 years BP LB99-A-7 4220–4778 years BP P01-A-4a 4847–5325 years BP P01-A-2 5321–5790 years BP P01-A-3 5298–5676 years BP TN01-A-7 5650–6149 years BP TN01-A-6 6002–6445 years BP TN99-A-1 Y35 6133–6517 years BP TN99-A-1 Y85 6183–6567 years BP TM01-A-9 6160–6642 years BP SK01-A-1 6334–6854 years BP TM01-A-3b 6334–6854 years BP TM01-A-4 6646–7126 years BP TM01-A-2 6642–7143 years BP TM01-A-1 6794–7244 years BP

1982–1993 1983–1990 1993.5–1998 1993.0–1997.2 1989–1995 1993.8–2001.4 9 years 12 years 13 years 11 years 8 years 9 years 9 years 14 years 11 years 12 years 11 years 12 years 10 years 7 years 13 years 7 years 14 years 15 years 12 years 4 years 9 years 7 years 15 years 10 years 8 years 12 years 9 years 11 years 9 years 11 years 9 years 7 years

Yearly Yearly Monthly Monthly Yearly Monthly Yearly Yearly Yearly Bulk Yearly Bulk Bulk Yearly Yearly Yearly Yearly Yearly Bulk Yearly Yearly Bulk Yearly & bulk Yearly Yearly Yearly & bulk Bulk Bulk Bulk Bulk Yearly Yearly Bulk Bulk Yearly & bulk Yearly Bulk Bulk

8.755 8.772 8.807 8.890 8.817 8.855 8.797 8.794 8.802 8.844 8.852 8.834 8.793 8.874 8.967 8.764 8.800 8.980 8.692 8.664 8.769 8.961 8.866 8.866 8.926 8.877 8.911 8.873 8.844 8.782 8.762 8.685 8.684 8.800 8.745 8.730 8.904 8.897

0.85 0.55 0.40 1.13 0.04 0.62 0.41 (1.52) 0.47 (1.52) 0.33 (1.52) 0.35 (1.62) 0.49 (1.53) 0.19 (1.58) 0.48 (1.51) 0.85 (1.55) 2.37 (1.55) 0.96 (1.52) 0.37 (1.54) 2.58 (1.57) 2.14 (1.52) 2.60 (1.53) 0.87 (1.54) 2.27 (1.55) 0.72 (1.51) 0.72 (1.53) 1.70 (1.61) 0.90 (1.51) 1.45 (1.51) 0.83 (1.52) 0.35 (1.52) 0.66 (1.51) 0.99 (1.55) 2.25 (1.55) 2.27 (1.52) 0.37 (1.51) 1.27 (1.51) 1.51 (1.54) 1.34 (1.51) 1.22 (1.59)

Muschu/Koil Island coral samples MS01 Modern MS04 Modern KL03 Modern FM09 2065  85 years FM22 5420  35 years FM07 5855  35 years FM21 5830  25 years FM15 6025  30 years FM23 7030  35 years FM24 7215  35 years FK05 7550  35 years

1984–1997 1984–1997 1984–1997 19 years 25 years 9 years 24 years 89 years 18 years 18 years 33 years

Yearly Yearly 4-Year composites 4-Year composites 5-Year composites 2-Year composites 5-Year composites 18-Year composites 4-Year composites 4-Year composites 7-Year composites

8.77 8.72 8.82 8.79 8.89 8.79 8.82 8.74 8.85 8.79 8.97

Coral ID

Agea

BP BP BP BP BP BP BP BP

(0.008) (0.009) (0.012) (0.035) (0.015) (0.028) (0.004) (0.020) (0.020) (0.009) (0.017) (0.025) (0.009) (0.016) (0.019) (0.021) (0.002) (0.014) (0.033) (0.002) (0.003) (0.011) (0.009) (0.005) (0.021) (0.022) (0.010) (0.005) (0.005) (0.019) (0.006) (0.031)

(0.022) (0.060) (0.032) (0.118) (0.028) (0.042) (0.032) (0.098)

0.00 0.82 0.82 0.32 (1.68) 1.96 (1.91) 0.32 (1.72) 0.81 (2.55) 0.50 (1.70) 1.30 (1.78) 0.32 (1.73) 3.27 (2.29)

a Fossil coral age determinations made using calibrated radiocarbon measurements (2s age range) for the Mentawai corals and U/Th measurements for the Muschu/Koil corals (2s error). All fossil coral ages given in years before present (BP) notation, where present is 1950 AD. b Standard error of the average of yearly or composite resolution Sr/Ca measurements, or of the average of triplicate Sr/Ca measurements of bulk coral powders. c Combined error (root of the sum of the squares) of the standard deviation (2s) around coral Sr/Ca reconstructions of modern mean SST at each site (1.51  C for the Mentawais; 1.64  C for Muschu/Koil), and the standard error (2SE) of the averaged Sr/Ca measurements for each individual coral.

six coral Sr/Ca estimates). A similar warming event may have also taken place after 4300 years BP, although this temperature excursion (1.9  0.5  C) is currently only defined by three coral specimens that date between w3900 and w3600 years BP. The smaller collection of fossil corals from Muschu/Koil Islands defines a similar progression of SST change during the mid-Holocene (Fig. 1b). These corals support cooler mean SSTs before w6800 years BP, and also document the transition from warmer to cooler than present conditions between w6300 years BP and w5500 years BP. The remarkable consistency between coral records from distant sites in the southern IPWP suggests that these oscillations in mean SST during the mid-Holocene are part of a broad-scale change in tropical climate.

5. Discussion 5.1. Comparison with Indo-Pacific Warm Pool records To assess how the coral records of mid-Holocene SST excursions relate to the temperature, size and location of the IPWP we examined our results alongside other coral and sediment core records of Holocene SST from nearby sites in the warm pool (Fig. 2). High-resolution Mg/Ca records from planktonic foraminifera in sediment cores collected in the central IPWP (MD76, MD81 and MD62) indicate that temperatures were approximately w0.5– 1.0  C warmer than at present over a broad interval during the early to mid-Holocene (Visser et al., 2003; Stott et al., 2004). This is

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sediment core RC12-344 located in the Andaman Sea in the northeastern Indian Ocean (Rashid et al., 2007). This level of cooling is in agreement with the mid-Holocene cooling observed at the Mentawai Island site. It is possible (and warrants further investigation) that the mid-Holocene cooling we observe at the Mentawai Islands was part of a broad-scale contraction along the western margin of the IPWP that generally restricted the western extent of the warm pool to the Indonesian maritime continent during the mid-Holocene. 5.2. Comparison with monsoon records

Fig. 2. Holocene SST records from the central and southern Indo-Pacific Warm Pool. (a) Sample locations are shown for the MD70, MD76, MD81, MD62 and RC12-344 sediment cores where foraminiferal Mg/Ca has been used to reconstruct Holocene SST (Visser et al., 2003; Stott et al., 2004; Rashid et al., 2007). Estimates of mean SST for Sumba and Alor are from fossil coral Sr/Ca records (Gagan et al., 2004). (b) SST anomaly records for the Mentawai (filled circles; grey lines/shading) and Muschu/Koil (open circles) sites are shown (details as in Fig. 1b). Upper plot also shows SST anomaly records for sites MD81 (filled triangles) and MD76 (open triangles) from the central IPWP. Lower plot also shows SST anomaly records for the Sumba (open squares), Alor (open diamonds) and MD70 (grey triangles) sites from the southern-central IPWP. Warm intervals (w6600–6300 years BP and w3900–3600 years BP) at the Mentawai and Muschu/Koil sites are broadly consistent with the level of mid-Holocene warming recorded in central IPWP sediment cores. However, the cool intervals (before w6800 years BP and w5500–4300 years BP) at the Mentawai and Muschu/Koil sites are not warm pool-wide features, and likely reflect contractions in the extent of the southeastern and southwestern margins of the IPWP during the mid-Holocene.

consistent with the level of warming we observe in our coral records during the w6600–6300 years BP interval suggesting that during this interval the warm pool extended beyond our coral sites. However, there is no evidence from the central IPWP for extended periods of cooler SSTs during the mid-Holocene. This indicates that the predominantly cooler SSTs we observe at the Mentawai and Muschu/Koil sites during the mid-Holocene were not associated with a mean cooling of the entire warm pool, but were instead related to changes in the extent and/or position of the IPWP. A small number of mid-Holocene coral records from Sumba and Alor suggest that SSTs near the southern-central margin of the IPWP were close to modern levels during the mid-Holocene (Gagan et al., 2004). A nearby Mg/Ca record from sediment core MD70 (Stott et al., 2004) does show a slight cooling during the 5500–4300 years BP interval, however the low-resolution of the core means that this cooling is defined by a single data point. While it seems that the mid-Holocene warming observed in the central IPWP was not experienced at the southern-central margin, there is no clear evidence, as-yet, that the cool mid-Holocene SST anomalies we observe at the Mentawai and Muschu/Koil sites were related to a migration of the entire southern margin of the IPWP. Instead, the southeastern and southwestern margins of the IPWP may have contracted as a common response to changes in the tropical climate system. Mean SSTs that were consistently w1–2  C cooler during the mid-Holocene are recorded in a low-resolution Mg/Ca record from

During the mid-Holocene the northern hemisphere summer monsoons were stronger than at present day (Fig. 3) due to changes in insolation seasonality which caused the ITCZ to be located in a more northerly position (Haug et al., 2001; Fleitmann et al., 2003, 2007; Wang et al., 2005; Partin et al., 2007; Hu et al., 2008). In the present-day climate, the seasonal north–south migration of the ITCZ is matched by migrations of tropical SST patterns (Fig. 4). Using this association, the more northerly position of the ITCZ during the mid-Holocene is likely to have been accompanied by a northward shift in the position of the IPWP (i.e. August-like in Fig. 4). Such a shift in the location of the southern boundary of the warm pool could account for the coherent mid-Holocene cooling observed at the Mentawai and Muschu/Koil sites near the southwestern and southeastern boundaries of the warm pool. It was, however, noted earlier that there is no clear evidence for midHolocene cooling (i.e. northward migration) at the southerncentral margin of the IPWP. One possibility is that this reflects geographical constraints, with the Indonesian throughflow continuing to direct the warmer central IPWP waters through the southern-central region and masking the influence of the more northerly position of the ITCZ during the mid-Holocene. A connection between the position of the IPWP and migrations of the ITCZ during the mid-Holocene may have also existed at the

Fig. 3. Comparison of (a), mid-Holocene mean SST from the Mentawai (filled circles, grey lines/shading) and Muschu/Koil (open circles) coral records (details as in Fig. 1b) with (b), a speleothem d18O record (dark grey curve; dashed vertical line and shading shows approximate modern-day mean and variability) of Asian summer monsoon strength from Dongge Cave, southern China (Wang et al., 2005). The predominantly cooler SSTs at the Mentawai and Muschu/Koil sites during the mid-Holocene coincide with strengthened Asian/Indian summer monsoons (Wang et al., 2005; Fleitmann et al., 2007). Black boxes on the Dongge d18O record indicate prominent episodes of centennial-scale weakening in the Asian monsoon identified by Wang et al. (2005). Within the dating errors on our coral records, the timing of these monsoon weakening events during the mid-Holocene appears to correspond with excursions to warmer SSTs at the Mentawai and Muschu/Koil sites.

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Fig. 4. Climatology maps of SST (left) and rainfall (right) showing the seasonal migration of the Indo-Pacific Warm Pool and the Inter-tropical Convergence Zone (Janowiak and Xie, 1999; Reynolds et al., 2002). Upper panels are for August, lower panels for February. SST contours are at 1  C intervals, with shading above 28  C. Mid-Holocene cooling at the Mentawai and Muschu/Koil sites may reflect a northward shift in the margin of the IPWP associated with the more northerly position of the ITCZ (i.e. more August-like).

centennial-scale (Fig. 3). Superimposed upon the mean strengthening of the Asian/Indian monsoons during the mid-Holocene, a number of centennial-scale episodes of abrupt monsoon weakening have been detected in high-resolution speleothem records (Fleitmann et al., 2003; Wang et al., 2005; Fleitmann et al., 2007). It has been proposed that these weak monsoon episodes are related to southward excursions of the ITCZ. One of the most prominent multi-century monsoon weakening events was centred at 6300 years BP, and is evident in widely separated speleothem records from China and Oman (Fleitmann et al., 2003, 2007; Wang et al., 2005). Within the dating errors on our coral records, the timing of this monsoon weakening event is consistent with the abrupt shift to warmer SSTs between w6600 and 6300 years BP at the Mentawai and Muschu/Koil sites. During this interval, the warm SST anomalies at our sites matched those of the central IPWP (Stott et al., 2004) suggesting that this short-lived episode of monsoon weakening (and southward migration of the ITCZ) caused the margins of the IPWP to extend beyond our coral sites. An abrupt and prolonged weakening of the northern hemisphere monsoons that began at 4400 years BP (Wang et al., 2005; Fleitmann et al., 2007) is also consistent with the warming of SSTs (i.e. warm pool expansion) in the Mentawai region after w4300 years BP. This episode of monsoon weakening was part of a widescale change in tropical climate throughout Asia, the middle-east and eastern Africa and has been implicated in the collapse of a number of civilizations (Cullen et al., 2000; Thompson et al., 2002; Stubwasser et al., 2003; Wang et al., 2005). 5.3. Comparison with tropical Indian Ocean records Mid-Holocene changes in the extent of the IPWP in the region offshore of Sumatra may have influenced the zonal ocean–atmosphere structure across the equatorial Indian Ocean. The coral records from the Mentawai Islands lie in the key eastern region of the IOD climate mode, where IOD events result in cool and dry anomalies (Abram et al., 2007, 2008). At the same time, IOD events produce warmer temperatures and enhanced rainfall in the equatorial western Indian Ocean and eastern Africa (Saji et al., 1999; Webster et al., 1999; Saji and Yamagata, 2003). To assess whether the mid-Holocene SST changes at the Mentawai Island site were associated with changes in the mean configuration across the tropical Indian Ocean we compared our coral SST reconstruction with a record of Holocene air temperature from the Mount Kilimanjaro ice core, and with lake level records from equatorial eastern Africa (Fig. 5).

During the mid-Holocene, when the eastern Indian Ocean corals record temperatures that were predominantly cooler than at present, Kilimanjaro ice core d18O values were generally high indicating that east African temperatures were warmer than at present (Thompson et al., 2002). Lake level records from lakes Abhe and Ziway–Shala in tropical east Africa preserve evidence of water levels that were higher than at present during the mid-Holocene, suggesting that rainfall was also generally more abundant then (Gasse, 2000). This mid-Holocene climatic configuration of generally cooler SSTs in the eastern Indian Ocean and warmer and wetter conditions in eastern Africa, represents a more positive IOD mean state (i.e. more IOD event-like). This suggests that the mean zonal SST gradient across the equatorial Indian Ocean was reduced, or reversed, at this time. Modelling of insolation-driven strengthening of the Asian monsoon during the mid-Holocene has been shown to produce IOD-like SST anomalies across the equatorial Indian Ocean (Liu et al., 2003; Abram et al., 2007). In model simulations, this positive IOD mean state is caused by strengthened monsoon surface winds that enhance upwelling in the eastern Indian Ocean by producing a more easterly zonal wind component along the equator. A millennial-scale association between a strengthened Asian monsoon and a more IOD-like mean state is also consistent with the positive feedbacks observed between these systems on interannual time scales (Ashok et al., 2004; Gadgil et al., 2004; Abram et al., 2008; Swapna and Krishnan, 2008). Together, the evidence suggests that the strengthened Asian monsoon of the mid-Holocene also generated a more positive IOD mean state across the equatorial Indian Ocean. The centennial-scale excursions to warmer conditions in the eastern Indian Ocean (reflecting IPWP expansion during episodes of monsoon weakening) also appear to follow this IOD–monsoon relationship (Fig. 5). During the w6600–6300 years BP warm interval at the Mentawai site, the Kilimanjaro ice core records an abrupt transition to cooler conditions (Thompson et al., 2002). There is also evidence from Ethiopian lake level records for a shortlived dry interval at w6500 years BP (Gasse, 2000). Together these proxy records suggest that the Indian Ocean underwent a brief transition to a more negative mean IOD state (i.e. less IOD eventlike) during this interval. A similar, but more prolonged, shift in Indian Ocean climate to a more negative mean IOD state after w4300 years BP is again evident by opposing temperature changes across the equatorial Indian Ocean as well as intense drought in eastern Africa (Gasse, 2000; Thompson et al., 2002). The cooccurrence of more negative IOD mean states with centennial-scale

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Fig. 5. Composite anomaly maps for (a), SST and (b), rainfall during July–November of the 1994, 1997 and 2006 positive IOD events (Janowiak and Xie, 1999; Reynolds et al., 2002). IOD events are characterised by a reversal of the zonal SST gradient across the Indian Ocean (warmer in the west, cooler in the east), which produces drought in western Indonesia and heavy rain in eastern Africa. The locations of the Mount Kilimanjaro ice core (Thompson et al., 2002), and Ethiopian lake records (Gasse, 2000) are shown. (c), The Kilimanjaro d18O record (solid grey line) is a proxy for east African air temperature (Thompson et al., 2002) and is inverted (cooler temperatures upward) for comparison with the IPWP coral records (filled circles, Mentawai Islands; open circles, Muschu/Koil Islands). Intervals of wet (dry) conditions in east African lake level records are shown by green hashed (brown) shading (Gasse, 2000). Changes in mid-Holocene temperature and rainfall appear to be linked (and opposite) across the equatorial Indian Ocean during the mid-Holocene, suggesting that they are associated with zonal changes in the mean state of the IOD system. A more positive IOD-like mean state appears to have existed during much of the midHolocene (coinciding with cool intervals before w6800 years BP and between w5500 and 4300 years BP at the Mentawai site) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

episodes of monsoon weakening further supports a strong coupling between the IOD and Asian monsoon systems that persists at interannual, centennial and millennial time scales. 5.4. Comparison with tropical Pacific Ocean records The similarity of the Mentawai and Muschu/Koil SST records during the mid-Holocene suggests that changes in tropical Pacific Ocean climate also accompanied the mid-Holocene changes in the IOD and monsoon systems discussed above. Proxy records from around the Pacific consistently suggest that there was a reduction ˜ o events during the midin the frequency and amplitude of El Nin Holocene (Rodbell et al., 1999; Tudhope et al., 2001; Moy et al., 2002; McGregor and Gagan, 2004; Conroy et al., 2008; Donders et al., 2008). However, changes in the configuration of the mean climatic state across the tropical Pacific during the mid-Holocene are still debated. Comparison of sediment cores from the eastern and far western equatorial Pacific have been used to suggest that the mean SST gradient across the Pacific was enhanced (i.e. more La ˜ a-like) during the mid-Holocene due primarily to cooling in the Nin eastern equatorial Pacific (Koutavas et al., 2002). However, other marine records from the eastern Pacific suggest warming accompanied reduced ENSO variability during the mid-Holocene (Donders et al., 2008). Some models simulations of the mid-Holocene ˜ a-like mean state across the Pacific climate produce a more La Nin that is attributed to strengthening of the Pacific Ocean tradewinds and teleconnections with the strengthened mid-Holocene Asian monsoon (Liu et al., 2000; Otto-Bliesner et al., 2003). However,

˜ aother modelling studies have questioned the analogy of a La Nin like mean state, and instead produce no clear east–west change in the mean equatorial SST gradient or thermocline depth during the mid-Holocene (Brown et al., 2008a). Interpreting the mid-Holocene changes in mean SST at Muschu/ Koil Islands in relation to changes in the mean zonal configuration across the Pacific is hampered by the scarcity of similar resolution SST reconstructions for the key ENSO regions in the central and eastern Pacific. Three possible explanations are discussed below, although these scenarios remain speculative and require testing ˜ awith additional proxy records. The scenarios are 1) a mean La Nin ˜ o–Modoki-like state, and 3) meridional like state, 2) a mean El Nin changes related to the ITCZ. ˜a-like state. La Nin ˜ a events are assoScenario 1: a mean La Nin ciated with strengthened upwelling in the eastern Pacific, and extension of the cold-water tongue along the equator, while the ˜ a-like mean state did IPWP contracts westward (Fig. 6a). If a La Nin exist across the Pacific during the mid-Holocene, then the cooling observed at Muschu/Koil Islands may imply that at times the cold˜ a conditions water tongue associated with enhanced La Nin extended across the Pacific as far as PNG. In this scenario, the coherence of mid-Holocene cooling at Muschu/Koil Islands with times of a strengthened Asian monsoon (Section 5.2) would imply that the tight monsoon–ENSO coupling observed in interannual variability (Webster et al., 1998; Kumar et al., 2006) also applies to changes in these systems over centennial to millennial scales. In the present-day climate, however, the enhanced atmospheric convection over the IPWP and deepened western Pacific thermocline that

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Fig. 6. Composite maps of the SST and rainfall anomalies associated with La Nin˜a events and El Nin˜o–Modoki events (Janowiak and Xie, 1999; Reynolds et al., 2002). (a) SST and (b) rainfall anomalies during November–February of the 1988, 1998, 1999 and 2007 La Nin˜a events. (c) SST and (d) rainfall anomalies during November–February of the 1994, 2002 and 2004 El Nin˜o–Modoki events (Ashok et al., 2007; Weng et al., 2007). Mid-Holocene cooling in the Muschu/Koil Islands could reflect a mean La Nin˜a-like state if the cool tongue extended as far west as PNG, or could represent a more El Nin˜o–Modoki-like configuration across the equatorial Pacific Ocean. Alternately, cooling at Muschu/Koil Islands may represent meridional (Fig. 4) rather than zonal changes in Pacific mean SST patterns during the mid-Holocene.

˜ a events do not favour the development of IOD accompany La Nin events (Webster et al., 1999; Meyers et al., 2007; Abram et al., 2008). Taken in conjunction with our evidence for a more IOD-like mean state during the mid-Holocene (Section 5.3), this scenario would suggest that the strengthened mid-Holocene Asian monsoon exerted the dominant influence on Indian Ocean climate, ˜ a-like mean state in with opposing teleconnections from a La Nin the Pacific being less significant. ˜o–Modoki-like state. The recently Scenario 2: a mean El Nin ˜ o–Modoki’’ identified zonal Pacific climate feature termed ‘‘El Nin ˜ o) has emerged as the second mode of Pacific (or pseudo-El Nin Ocean variability since 1979 AD (Ashok et al., 2007; Weng et al., 2007), perhaps associated with weakening of the Walker overturning circulation driven by greenhouse warming (Vecchi et al., ˜ o–Modoki events are characterised 2006; Ashok et al., 2007). El Nin by a tripolar pattern in SST anomalies across the equatorial Pacific Ocean, with warming of the central Pacific flanked by cooling in the eastern equatorial Pacific and the western equatorial Pacific near PNG (Fig. 6c). Convection over the central Pacific warm anomaly allows a twin Walker cell configuration to develop in Pacific atmospheric overturning circulation. Some researchers argue for El ˜ o–Modoki being a different climate mode to El Nin ˜ o (Ashok Nin et al., 2007), while others describe the warming pattern in the ˜ o event central Pacific Ocean as a different ‘‘flavour’’ of El Nin (Kumar et al., 2006). ˜ o–Modoki events involve simultaneous cooling Given that El Nin ˜ o– in the eastern and western equatorial Pacific Ocean, an El Nin Modoki-like mean state in the mid-Holocene could account for proxy evidence of cooling in eastern Pacific sediment cores (Koutavas et al., 2002) and in the Muschu/Koil coral records without ˜ a-like cold tongue to extend across to northrequiring a La Nin eastern PNG (Fig. 6c). Atmospheric subsidence over the western ˜ o–Modoki-like state tropical Pacific associated with a mean El Nin (Weng et al., 2007) would also favour the positive IOD mean state in the Indian Ocean during the mid-Holocene (although a relationship ˜ o–Modoki variability has yet to be between IOD and El Nin demonstrated in instrumental records). The simultaneous subsidence over the eastern and western tropical Pacific induced by an El ˜ o–Modoki-like mean state might also explain evidence for Nin increased salinity (i.e. drier conditions) in the central IPWP during the mid-Holocene (Stott et al., 2004), as well as reduced rainfall in Ecuador/Gala´pagos at that time (Rodbell et al., 1999; Moy et al., 2002; Conroy et al., 2008).

Scenario 3: meridional changes. Finally, the changes in mean SST at the Muschu/Koil site during the mid-Holocene may not be representative of changes in the zonal structure across the equatorial Pacific Ocean. Instead, as discussed earlier (Section 5.2), they may reflect purely meridional changes in mean SST patterns that tracked the migrations of the ITCZ during the mid-Holocene. It has been suggested that conflicting evidence from the eastern Pacific for mean mid-Holocene warming/cooling may be due to changes in the latitudinal position of the eastern upwelling zone, rather than being related to changes in the mean zonal structure across the Pacific (Donders et al., 2008). It has also been suggested that increased salinity in the central IPWP during the mid-Holocene, ˜ a-like state, might which is not consistent with a mean La Nin instead reflect basin-wide changes in salinity driven by changes in vapour flux to the Pacific Ocean due to the more northerly ITCZ (Stott et al., 2004). The mean state of Pacific climate during the mid-Holocene, and its interactions with other tropical climate systems, clearly needs to be further assessed using proxy data and climate models. In particular, mid-Holocene coral records or high-resolution sediment cores from the central Pacific should help to resolve whether equatorial cooling extended across the Pacific as far as PNG (i.e. La ˜ a-like), or eastern and western equatorial cooling flanked Nin ˜ o–Modoki-like), or SST a central Pacific warm anomaly (i.e. El Nin patterns followed meridional migrations the ITCZ, rather than changes in the mean zonal configuration across the Pacific. 6. Conclusions and implications This study has demonstrated the potential for reconstructing changes in the mean SST of the tropics using fossil corals. Traditionally, the reconstruction of mean SST from individual fossil corals has been associated with large uncertainties. However, our study demonstrates that the use of low-resolution Sr/Ca determinations from a large suite of fossil corals, along with careful screening for coral diagenesis, allows these uncertainties to be reduced so that coherent records of mean SST can be produced. This provides a powerful tool for tropical palaeoclimatology; low-resolution measurements of isotopic and trace element tracers from a large number of corals allow mean climatic conditions to be accurately reconstructed, while detailed analysis of selected samples can provide additional information on seasonal to interannual climate variability for key time intervals.

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Our coral-based reconstruction demonstrates that the IPWP was a dynamic component of the tropical climate system during the mid-Holocene. The coral sites near the southeastern and southwestern margins of the IPWP show that the extent of the warm pool was generally reduced in these regions during the midHolocene. We suggest that this was due to a more northerly position of the IPWP (and ITCZ) associated with the strengthened midHolocene Asian summer monsoon. The position of the IPWP was also sensitive to centennial-scale climate oscillations, with abrupt increases in the southeastern and southwestern extent of the warm pool coinciding with short-lived episodes of monsoon weakening and southward migrations of the ITCZ during the mid-Holocene. The interannual connection that exists between the Asian summer monsoon and the zonal IOD system is seen to also persist over centennial and millennial time scales, with the strengthened Asian monsoon of the mid-Holocene matched by a positive IOD-like mean configuration across the tropical Indian Ocean. With the proxy records currently available it is not possible to determine whether the similar changes in mean SST we observe in the southeastern IPWP were associated with zonal and/or meridional changes in the mean SST configuration of the Pacific Ocean. The dynamic and inter-connected behaviour of the IPWP, the Asian monsoon and the IOD systems during the mid-Holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics. The potential for oscillations in the extent of the IPWP has broader implications for the propagation and amplification of climate change through its influence on heat transfer to the extratropical regions. Thus, the mid-Holocene represents a useful target for the development of proxy records from key locations to better understand the full scope of changes in position/size of the IPWP. Testing the ability of climate models to reproduce mid-Holocene changes in the IPWP, and in the tropical climate interactions of this region, will improve confidence in predictions of the likely impacts of future climate change. Acknowledgements We thank D. Prayudi, I. Suprianto, K. Glenn, T. Watanabe, H. Scott-Gagan K. Sieh and the Indonesian Institute of Sciences (LIPI) for support with fieldwork in the Mentawai Islands, which was carried out under LIPI Research Permit numbers 3551/I/KS/ 1999 and 2889/II/KS/2001. We also thank G. Brunskill, I. Zagorskis, J. True and the crew of the rv Lady Basten for facilitating our fieldwork in PNG as part of Project TROPICS. We thank M. McCulloch, J. Chappell, H. Scott-Gagan, G. Mortimer, A. Alimanovic, D. Kelleher and E.-K. Potter for laboratory support. This study was supported by an Australian Postgraduate Award and RSES Jaeger Scholarship to N.J.A., an Australian Postgraduate Award to H.V.M., and Australian Research Council Discovery grant DP0663227 to M.K.G and W.S.H. References Abram, N.J., Gagan, M.K., McCulloch, M.T., Chappell, J., Hantoro, W.S., 2003. Coral reef death during the 1997 Indian Ocean Dipole linked to Indonesian wildfires. Science 301, 952–955. Abram, N.J., Gagan, M.K., Liu, Z., Hantoro, W.S., McCulloch, M.T., Suwargadi, B., 2007. Seasonal characteristics of the Indian Ocean Dipole during the Holocene epoch. Nature 445, 299–302. doi:10.1038/nature05477. Abram, N.J., Gagan, M.K., Cole, J.E., Hantoro, W.S., Mudelsee, M., 2008. Recent intensification of tropical climate variability in the Indian Ocean. Nature Geoscience 1 (12), 849–853. Alory, G., Wijffels, S.E., Meyers, G., 2007. Observed temperature trends in the Indian Ocean over 1960–1999 and associated mechanisms. Geophysical Research Letters 34, L02606. doi:10.1029/2006GL028044. Ashok, K., Guan, Z., Saji, N.H., Yamagata, T., 2004. Individual and combined influences of ENSO and the Indian Ocean Dipole on the Indian summer monsoon. Journal of Climate 17, 3141–3155.

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