Subtropical sea-level history from coral microatolls in the Southern Cook Islands, since 300 AD

Marine Geology 253 (2008) 14–25

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Subtropical sea-level history from coral microatolls in the Southern Cook Islands, since 300 AD Ian D. Goodwin a,⁎, Nick Harvey b a b

University of Newcastle, Callaghan, NSW 2308, Australia University of Adelaide, South Australia 5005, Australia

A R T I C L E

I N F O

Article history: Received 5 December 2007 Received in revised form 3 April 2008 Accepted 21 April 2008 Keywords: sea-level history climate variability coral microatoll south-west Pacific fringing reef Porites sp.

A B S T R A C T Relative sea-level (RSL) history is reconstructed for the Southern Cook Islands since 300 AD using Porites sp. coral microatolls. The upper surface or height of living coral of the modern microatolls on Rarotonga was rigorously determined by 400 laser survey measurements to be constrained by the sea-level −0.36 m (below Mean Sea Level) with a standard deviation of ±0.008 m, that is equivalent to midway between Mean Low Water Neaps and Mean Low Water Springs tidal datums. Fossil microatoll history on the fringing reefs is fragmentary due to multi-decadal to centennial windows of microatoll growth, cyclone destruction, and preservation through coastal progradation. The mid-late Holocene highstand ceased prior to 500 BC and RSL fell from +1.3 ± 0.1 m to 0.45 ± 0.15 m by 1000 AD at a rate of −0.5 mm/yr. The RSL minimum occurred during the late 1700s to 1800s AD at − 0.2 m below present, before rising during the 1900s. During the past 500 yr RSL fluctuated, including an abrupt fall at ~ 1750 AD following a sustained multi-decadal high anomaly. Similarly, low RSL anomalies and slow coral growth rates during the 950 to 1000 AD period are indicative of Pacific Basin-wide climate variability and are consistent with extreme La Niña-like climate anomalies previously reported around the Pacific basin. The fall in RSL during the first millennium AD crossed-over tidal thresholds for rapid coastal progradation, when ambient Mean High Water Neaps and Mean High Water Springs fell below the low tide level of the mid-late Holocene high stand. This coastal response is inversely analogous to the projected response to sea-level rise during the latter part of the modern century. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Sea-level change together with wave climate, sediment supply and lithology are the most important variables determining shoreline configuration and position on both continental and island coasts. Recent and future sea-level rise due to ocean warming associated with the ‘accelerated greenhouse effect’ is of great concern to coastal managers, particularly on low-lying islands and atolls. This sea-level rise comprises: (i) steric change in relative sea-level controlled by temperature and salinity changes in the surface mixed ocean, together with, (ii) eustatic sea-level change due to ocean mass changes in association with ice meltwater inflows. Other secular, climate-driven sea-level changes are forced by regional changes in wind stress and ocean currents on seasonal to centennial time scales (Goodwin, 2003). In addition to the above, coastlines also experience non-climatic driven sea-level change due to the global glacio-isostatic adjustment (GIA) process, thermotectonic subsidence and volcanic uplift (Pir-

⁎ Corresponding author. Now at Climate Risk CoRE and Department of Physical Geography, Macquarie University, NSW 2109, Australia. Tel.: +61 2 9850 8354; fax: +61 2 9850 8420. E-mail address: [email protected] (I.D. Goodwin). 0025-3227/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.04.012

azzoli, 1996). The GIA is the response of the earth's lithosphere and mantle to the redistribution of post-glacial meltwater around the globe, and the associated changes in water and ice loads (Mitrovica and Peltier, 1991). Hence, applying estimates of future sea-level rise such as those in IPCC (2007) to forecast relative sea-level change for an oceanic island is difficult, without knowledge of the magnitude of the GIA adjustment, thermotectonic trends on geological timescales, and the magnitude of former relative sea-level changes due to ‘climaticdriven’ factors such as trends or fluctuations in steric and eustatic sea-level. The imperative in sea-level research is to resolve these questions particularly for the multitude of low-lying atolls and islands. This study is focused on resolving the relative sea-level history for the subtropical south-west Pacific region, and to investigate the sensitivity to climate-driven sea-level variability. The latter is a reliable proxy for Pacific basin west–east thermocline behaviour. Many former sea-level studies in the south-west Pacific region have suffered from the use of mixed or low resolution proxy sealevel indicators. In this study, we have developed a sea-level history based exclusively on coral microatolls that are fixed biological indicators of former sea-level and define the same sea-level datum throughout time. Coral microatolls have previously been used to

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resolve high-resolution sea-level history on reef flats in the tropics (Woodroffe and McLean, 1990; Spencer et al., 1997; Smithers and Woodroffe, 2000, 2001 etc) but not an age survey and high-resolution sea-level history using multiple microatolls in the subtropics as presented in this paper. The previous studies established that Porites sp. coral microatoll upward growth (height of living coral HLC) is constrained by water level (typically between Mean Low Water Neaps (MLWN) and Mean Low Water Springs (MLWS)), and hence, the upper surface of the microatoll potentially preserves the fluctuations or changes in the constraining water level. Coral microatoll growth is also a function of sea-surface temperature, sea-surface salinity, water turbidity, reef flat environment, sedimentation rates, substrate hardness, tidal circulation, reef flat drainage and residual ponding, and nutrients (Scoffin et al., 1978). On some reef flat environments where tidal flow is partially obstructed by cemented ramparts and the reef flat is moated, microatoll HLC can be artificially maintained above HLC for free draining reef flats. Subsequent erosion of ramparts can result in emergent microatolls and an incorrect interpretation of RSL fall. Woodroffe and Gagan (2000) and Woodroffe et al. (2003) have demonstrated that stable isotope and growth rate analysis of Porites sp. microatolls can also be used to reconstruct sea-surface temperature and rainfall variability associated with the El Niño-Southern Oscillation (ENSO). This paper investigates the growth and decay episodes of Porites sp. microatoll colonies on microtidal (~ 0.9 m range for springs, 0.4 m range for neaps), fringing reef flats at Rarotonga and Aitutaki in the Southern Cook Islands, South Pacific Ocean. Coral growth rates are presented for both modern and fossil microatolls and are investigated with respect to principally, the constraining sea-level and sea-surface temperature, together with the above listed parameters. A relative sea-level curve is reconstructed for the past 2000 yr and is applied as the basis for understanding climatic fluctuations and the evolution of the reef flat environment.

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2. Regional setting and former sea-level studies 2.1. Geodynamics and island morphology The Southern Cook Islands (SCI) are located (see Fig. 1) in farfield intraplate sites where theoretically the isostatic and tectonic histories during the mid-late Holocene are not complex and afford the most potential for resolving the climate signal in sea-level change. Rarotonga is a highly dissected, volcanic island surrounded by a coastal plain developed over an elevated former remnant Pleistocene and mid-Holocene-age fringing reef, and encircled by a modern fringing reef (shown in Fig. 2A). The latter is narrow (~ 200 m wide) along the northern and western sides of the island, whilst in the east and south a reef flat and lagoon morphology (~ 800 m wide) has evolved. Rarotonga is the youngest of the SCI and experienced thermal rejuvenation by hotspot volcanism 1 to 2 Ma BP (Dickinson, 1998). As a result of this volcanism the surrounding islands, including Mangaia, Mauke, Mitiaro, Atiu experienced uplift to form makatea (annular fringing plateaux of uplifted reef limestone) due to isostatic flexure of the lithosphere (Dickinson, 1998). Aitutaki is an ‘almost atoll’ with a remnant volcanic island at the northern rim of an atoll lagoon and reef flat morphology (Stoddart, 1975) and is shown in Fig. 2B. Whilst Aitutaki lies within the zone of flexural uplift of Rarotonga, no emergent makatea have formed. This is probably due to renewed volcanism on Aitutaki contemporaneous with Rarotonga between 1.9 to 0.7 Ma BP (Dickinson, 1998). The latter research estimated that both Rarotonga and Aitutaki have experienced ~ 0.55 m and 1.6 m of thermotectonic subsidence since the mid-Holocene and Last Interglacial, respectively. These rates are consistent with the 2.5 ± 1 m elevation of the emergent Late Pleistocene reef at Ngatangiia and on the north coast (presumed to be Last Interglacial age, and certainly pre Holocene age, as determined by Schofield, 1970; Dickinson, 1998 and remeasured in this study). No evidence for Holocene co-seismic uplift or subsidence has been reported

Fig. 1. Location map of the South-West Pacific Ocean region showing the Southern Cook Islands in the subtropical sector.

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Fig. 2. (A) Geomorphic map of Rarotonga and its fringing reef; (B) geomorphic map of Aitutaki, its lagoon and fringing reefs.

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for either island. Hence, the islands are tectonically stable with thermotectonic subsidence rates of ~0.01 mm/yr. 2.2. Climate and sea-level variability Climatically, the SCI are located at the southern fringe of the persistent tradewind zone, in the central South Pacific Ocean, and lie beneath the seasonal path of the South Pacific Convergence Zone (SPCZ). The SPCZ generally lies over the SCI during summer and to the north during winter. As a consequence, the SCI experience persistent easterly and south-easterly tradewinds during the May to October dry season, and easterly and north-easterly tradewinds from November to April wet season (Thompson, 1986). The islands lie within the path of tropical cyclones tracking to the south-east, and are more prevalent during La Niña climate phases. Modern interannual sea-surface temperature (SST) variability in the SCI region has a moderate +ve correlation (R N 0.5) with the Southern Oscillation Index (SOI), and a high −ve correlation (R N −0.7) with the Pacific Decadal Oscillation (PDO) Index, such that warm (cool) SST anomalies occur during La Niña (El Niño) years, and/or La Niña-like (El Niño-like) decades. Continuous measurement of relative sea-level has been conducted since 1977, using a tide gauge established at Avatiu Harbour, Avarua, on the north coast of Rarotonga (21° 12.4′S, 159° 46.5′E, GLOSS # 139, NTF). The latest ‘Seaframe’ tide gauge was installed by the Australian National Tidal facility, as part of the South Pacific Sea Level and Climate Monitoring Project in January, 1993. The monthly RSL and SST record are shown in Fig. 3. Sea-level experiences a distinct annual cycle, with sea-levels from late Autumn to Spring (May to October) some 50 mm lower, than in the summer months. Interannual sea-level varies by ±90 mm, and the maximum monthly anomalies from the mean for 1977 to 2004 are ±200 mm, and lag the tropical western Pacific sea-level anomalies and the Southern Oscillation Index and the Niño 3 SST index by 3 to 6 months, due to eastward propagating Kelvin waves (Graham and White, 1988). 2.3. Previous Holocene coastal evolution and paleo-sea-level studies 2.3.1. Rarotonga Schofield (1970) reported the results of a reconnaissance survey that included some radiocarbon ages of elevated reef remnants. He reported an age of 2030 ± 60 yr BP (uncorrected) for emergent coral 3 ft above height of living coral (HLC) at Avarua, on the north coast, together with raised beachrock at a similar elevation near Titkaveka.

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Schofield (1970) also surveyed the coastal plain, formed from raised beach ridges of cemented and uncemented coral sands, later referred to by Wood and Hay (1970) as the Aroa Sands. Traverses at Kavera, Black Rock and Matavera showed a similar storm ridge prograded plain, with the uppermost elevations between 4.5 to 7.5 m above MSL, (some 2 to 3.5 m above modern storm ridges). Coral in the surface sediments was radiocarbon dated between 1230 and 3500 yr BP (uncorrected radiocarbon (14C) years), and indicated a late Holocene age for the progradation. In addition, to storm-wave overtopping, some of the coral sediments may have been deposited by tsunami with typical setup of ~ 1.5 m, as reported by Keys (1963). However, Wood (1967) observed that the Aroa Sands had formed over an elevated reef flat, which indicated the role of RSL fall in the progradation phase. Chikamori (1995) and Moriwaki et al. (2006) have conducted extensive excavations of the Aroa Sands in the Ngatangiia district. Chikamori (1995) obtained radiocarbon ages of between 3000 and 4000 yr BP on in situ reef flat microatolls buried 1 m below the modern coastal plain surface, at an elevation 0.9 m above modern low tide level. Moriwaki et al. (2006) obtained radiocarbon ages of between 1420 and 7250 cal yr BP on shell, coral, wood and peat sampled from the Aroa Sands. Yonekura et al. (1984) drilled the modern reef crest at two sites along the north coast of Rarotonga. The coral heads in growth position and the species of coral in these cores indicated that the reef crest was ~1.4 m higher and that RSL was higher by an equivalent amount than the present RSL at 6000 yr BP (uncorrected radiocarbon (14C) years). This previous work identifies that a relative sea-level highstand occurred during the midHolocene at Rarotonga. This is consistent with both the GIA theory and observations on many islands in the south-west Pacific (Grossman et al., 1998; Dickinson, 2004). 2.3.2. Aitutaki Hein et al. (1997) summarised the Holocene evolution of the Aitutaki reef platform and lagoon sedimentation. They concluded that relative sea-level was within 1 m of the modern level by 4700 yr BP (uncorrected, after Yonekura et al., 1984). Stoddart (1975) did not report any emergent reef on Aitutaki, although he made the conclusion that the reef islands, comprising coral sand sands and shingle, post date the formation of conglomerate platforms that are ubiquitous around the reef flat of Aitutaki. This is consistent with late Holocene RSL lowering. Along the east coast of Aitutaki, Stoddart (1975) observed modern Porites lutea microatolls on the inner reef flat, and Yonekura et al. (1988) observed emergent fossil Porites sp.

Fig. 3. Combined monthly relative sea-level time series from the Avatiu Harbour (Avarua) tide gauge on Rarotonga shown in the bold black line, and monthly sea-surface temperature time series from the IGOSS grid box 21.5° S, 159.5° W, shown in the grey line, spanning 1980 to 2007.

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microatolls at 0.4 to 0.5 m above present (although the datum was not given), with a radiocarbon age of 1530 ± 210 yr BP (uncorrected radiocarbon (14C) years). Allen (1998) reported the results of archaeological evidence for human colonisation of Aitutaki and the role of coastal evolution. At Amuri on the west coast, a coastal barrier or beach ridge plain was found to overly emergent reef flat, with the overlying foreshore and beach ridge carbonate sands generally fining upwards, together with coarser storm deposits, prograding from ~1200 yr BP. Allen (1998) concluded that the RSL fall together with storm-wave events transporting sediment and coral rubble over the fringing reef were the driving forces of the progradation. The previous work on Rarotonga and Aitutaki describes a regional RSL fall after ~ 1000 to 1500 yr BP which is consistent with the trend modelled for the GIA during late Holocene using the ICE-5G model (Peltier, 2004). In the following sections we investigate the character of the RSL fall to determine whether sea-level fell monotonically or whether there is evidence for fluctuations in RSL due to climatic forcing.

over the reef flat at MLWS is less than 0.3 m. These locations are located on leeward sides of the island and are exposed to a reduced South Pacific wave power, since the incident mean wave climate is refracted from the south and south-east (Barstow and Haug, 1994). However, the microatoll locations are exposed to cyclone storm waves and to long-period swell from the North Pacific. The Black Rock reef flat is the most leeward location on the island to tradewind-generated wind waves and South Pacific generated swell. Similar fringing reef surveys were conducted on Aitutaki during May 2003. Our surveys were guided by the earlier work of Stoddart (1975) and we located fields of modern Porites sp. microatolls along the 100 m wide inner fringing reef flat in the Anaunga District near the northern apex of the island, and fossil Porites sp. microatolls along the 80–120 m wide reef flat on the north-eastern coast, near the airstrip and south-east to Ootu and Akitua. Modern Porites sp. microatolls were observed growing in the Ootu Passage. 3.2. Modern microatoll elevation surveys and tidal datums

3. Methods 3.1. Fringing reef geomorphology and Porites sp. microatoll habitat surveys Field surveys of the fringing reef environments on Rarotonga were made on 2 visits in October 2002 and May, 2003. Porites sp. microatolls are not abundant along the fringing reefs of the Southern Cook Islands because of the high wave power and strong tradewinds, and the variability in maturity of the annular fringing reefs. However, fields of modern and fossil Porites lutea microatolls were found in two localities on Rarotonga: Black Rock on the north west coast; and modern Porites sp. at Kikii, along the north coast. The ~200 m wide Black Rock fringing reef profile extends from an algal rim covered reef crest, a zone of dense branching Acropora sp. corals, a middle to inner moat and reef flat, a carbonate sand beach and outcropping beachrock (Fig. 4). The moat is free draining and is between 0.3 and 1.0 m deep on Mean Low Water Springs (MLWS). The Kikii fringing reef is narrower at 60–80 m and comprises a continuous reef flat extending from the algal rim and reef crest to a steep carbonate boulder, storm beach. The water depth

The mean tidal range on Rarotonga and Aitutaki is microtidal (~ 0.9 m for springs, 0.4 m for neaps). The elevation of Mean Low Water Neaps (MLWN), Mean Low Water Springs (MLWS), and Lowest Astronomical Tide (LAT) are 0.228 m below MSL, 0.424 m below MSL, and 0.542 m below MSL respectively. The HLC or the constraining water level for upward growth of Porites sp. microatolls (shown in Fig. 4) was surveyed using a Spectra-Physics laser level. The microatoll levelling surveys were tied by a levelling traverse to benchmarks of the Rarotonga Level Survey Network, and all microatoll HLC measurements were calculated with respect to Mean Sea Level (MSL) and to Lowest Astronomical Tide (LAT). The four cardinal points and the centre of each microatoll were surveyed to 0.001 m accuracy. The spatial variability of microatoll HLC was determined from the five measurements on each microatoll. A total of 80 modern microatolls were surveyed on Rarotonga. The elevations of fossil microatolls were also surveyed using the same methods. On Aitutaki both modern and fossil microatolls were surveyed at 0.01 m accuracy to determine the difference in the modern and fossil HLC for the same Porites sp., as no absolute MSL benchmarks were available.

Fig. 4. Cross-section of the Rarotongan fringing reef near Black Rock, showing the morphology of the reef and the height of living coral (HLC) on microatolls with respect to the constraining tidal planes.

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3.3. Microatoll microtopography and geochemistry Both modern and fossil microatolls were cut in the field using handsaws and cold chisels. In addition the fossil microatolls were cored using an electric drill and a 2 inch diameter core barrel. The cores were taken at the centre and at 0.1 to 0.15 m intervals across the upper surface. These slabs and cores were returned to Newcastle for laboratory analysis. A detailed study of the Black Rock microatoll growth and microtopography was made. Each microatoll slab was reassembled and imaged using UV fluorescence techniques, to enable the delineation of the annual growth density bands (Fig. 5), and the measurement of the microtopography of the upper microatoll surface as a proxy for sea-level variability. In addition, the cores were sectioned and imaged to show the growth bands. Annual growth rates for each of the microatolls was also measured. The microatoll sections and cores were sampled and analysed for stable isotope geochemistry and trace element chemistry using an ICP-AES. These detailed analyses are the subject of another paper but aspects relevant to the current paper will be referred to later in the results and discussion sections. 3.4. Fossil microatoll age determination Samples from fossil microatolls were analysed by accelerated mass spectrometry methods for radiocarbon age determination at the University of Waikato Laboratory, and at the Australian Nuclear Science and Technology Organisation (ANSTO). All dates are listed in Table 1 and show calibrated ages BP, using a delta R value of 57.0 ± 23.0 yr. All previously published SCI radiocarbon ages were calibrated using Calib version 4.4, (Stuiver and Reimer, 1993). 4. Results 4.1. Modern microatoll growth and sea-level fluctuations The mean HLC for Porites sp. on Rarotonga is −0.36 with a standard deviation of ±0.008 m below MSL (400 observations), which is midway between MLWN (−0.23 m below MSL) and MLWS (−0.43 m below MSL). The relationship of the HLC to the fringing reef crosssection and tidal heights is shown in Fig. 4. The mean HLC for the two sites on Rarotonga varied by b0.02 m ± 0.008 m. This indicates the robustness of the HLC on Porites sp. microatolls as fixed biological proxy sea-level recorders with a defined datum. The spatial variability of HLC across individual microatolls ranges from 0.002 to 0.019 m at the Black Rock site, and ranges from 0.006 to 0.036 m at the Kikii site. The mean variability in HLC between microatolls (raw measurements)

Fig. 5. Ultra-Violet (UV) photograph showing the annual light and dark density growth bands in a modern Porites sp. microatoll slab (Microatoll D) from the Black Rock reef on Rarotonga. The mean annual growth rate is 13 mm/yr.

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Table 1 14 C ages of coral microatoll sea-level indicators from Aitutaki and Rarotonga Lab code

Description⁎

Aitutaki, Southern Cook Islands (18° 50.74′ S, 159° 45.5′ W) OZG828 Porites sp. Ootu 1 OZG829 Porites lutea Ootu 3 OZG830 Porites sp. Ootu 4 JGS219 Microatoll AITU-2 (Yonekura, 1988) Rarotonga, Southern Cook Islands (21° 12.00′ S, 159° 49.51′ W) OZG706 Porites lutea MaP3 OZG707 Porites lutea MaP6 OZG708 Porites lutea MaS1 OZG831 Porites lutea MaJ WK12087 Porites lutea MaI (centre) WK12088 Porites lutea MaI (outer) WK12089 Porites lutea MaH (centre) WK12090 Porites lutea MaH (outer) †

Elevation δ13C (m, above HLC)

0.8 0.6 0.8 0.45

C ± (1σ)†

14

Cal age (2σ range) (yr AD)

−0.3 2080 ± 40 −0.6 2030 ± 40 +0.3 2090 ± 40 −0.93 1530 ± 210

359 (252–466) 430 (313–546) 349 (245–453) 918 (514–1321)

0.15 0.25 −0.2 0.35 0.06

−3.0 −2.6 −2.4 −2.8 −0.3

635 ± 35 585 ± 35 625 ± 35 885 ± 40 1553 ± 43

1690 (1648–1823) 1750 (1679–1888) 1695 (1653–1834) 1480 (1422–1583) 950 (790–1010)

0.06

0.4

1505 ± 41

990 (850–1050)

0.15

−1.3

690 ± 46

1674 (1540–1810)

0.15

−0.9

551 ± 45

1800 (1713–1950)

Conventional 14C date. All ages were calibrated using CALIB 4.4 and using ΔR = 57 ± 23.0.

is 0.056 m and 0.06 m for the two sites. These variations in observed HLC across the reef flats is related to distance from the reef crest, and exposure to wave action. These statistics, including both the internal survey accuracy and the small spatial variability of HLC, confirm that the HLC on microatolls can be of high enough accuracy and reliability to extend the time series of instrumental tide gauge measurements. At the Black Rock (Rarotonga) locality, the diameter of individual modern Porites sp. microatolls ranged from 0.7 to 2.2 m, with multiple or coalescent microatoll colonies having diameters up to 10.2 m. The modern growth rates range from 0.009 to 0.02 m/yr, with an average of 0.013 ± 0.0035 m/yr over the period from 1982 to 2002. At these growth rates, the onset of the phase of renewed continuous microatoll growth commenced about ~1950 AD. The microtopography of the modern microatolls over the period from ~1970 to 2002 was measured on 3 microatolls, (MaD, MaE and MaG) at biannual resolution. High correlation between the microatoll microtopography fluctuations and the summer average RSL exists for 1982 to 2002, as shown in Fig. 6 for MaD). The higher correlation with summer sea-level is due to the enhanced coral growth during warmer SSTs in summer. Fig. 6 describes fluctuations in HLC of 0.02 to 0.03 m on 3 to 5 year frequency, that are in phase with ENSO variability, when compared to the SOI and Niño 3 indices. The microatoll record of biannual RSL variability is dampened (~20%) with respect to the interannual RSL fluctuations of ±0.05 to 0.1 m recorded in the nearby tide gauge, since the slow growth rate of the coral cannot keep pace with the magnitude of the sea-level variations. Hence, the interannual and intradecadal HLC record provides a filtered RSL record. However, on interdecadal to multi-decadal periods of sustained RSL rise or fall, the coral growth catches up to long-term changes in the constraining HLC, and the microatolls record the full range of multidecadal RSL variability, also shown in previous studies (Smithers and Woodroffe, 2001). The coalescent microatoll colonies with diameters up to 10.2 m show renewed upward growth of 0.1 to 0.15 m on top of an eroded surface, whilst the lateral growth of the microatolls appears to have been continuous. Microatoll S (Fig. 7) yielded an age of 1695 AD (1653 to 1834 AD) for the base of a 0.25 m core sample taken at the centre, below the eroded surface. The growth rate was 0.015–0.020 m/yr and the growth axis is vertical, indicating that at this time the Porites sp. coral had not reached its constraining HLC to form a microatoll.

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Fig. 6. Plot showing the mean summer (DJFMA) RSL at Avatiu Harbour (Avarua) tide gauge on Rarotonga (solid black line), together with the corresponding HLC for Microatoll D (MaD) from the Black Rock fringing reef on Rarotonga (dashed grey line).

Subsequently, after the microatoll formed, the upper surface was planed by erosion under RSL fall. The latter low RSL existed until the past ~ 50 yr when RSL rise has accommodated renewed microatoll growth. The 0.1 to 0.15 m rise in the HLC is consistent with estimates of sea-level rise in the South Pacific of 0.1 to 0.2 m over the past century. Hence, Microatoll S has recorded a fall in RSL during the late 1700s and 1800s AD and a subsequent rise in RSL during the 1900s AD. 4.2. Fossil microatoll clusters The environmental window for modern Porites sp. on the fringing reefs is limited. These microatolls are not ubiquitous along the fringing reefs with the modern wave climate and SSTs. Our observations also revealed that fossil Porites sp. microatolls are rare, and are shown below to have grown in defined temporal windows in the past few hundred to thousand years. These microatolls are sensitive to SST variability, particularly in this subtropical location. In addition to the environmental sensitivity of microatoll growth, disintegration and destruction of living microatolls during cyclones can severely constrain the number of fossil microatolls to survive as fixed biological proxy sea-level indicators. Hence, we observed spatial and temporal population clusters of fossil Porites sp. microatolls, either in situ, or destroyed. On Aitutaki and Rarotonga, the only microatolls older than a few hundred years to have survived are those that have been buried by foreshore and strandplain progradation, and are now accessible through exhumation by shoreline recession or by excavation.

Fig. 7. Photograph of living Microatoll S (MaS) and the Black Rock fringing reef, Rarotonga.

4.2.1. Rarotonga The inner moat on the Black Rock fringing reef flat is littered with clusters of fossil microatolls, some preserved in growth position, others fractured and transported shorewards. The radiocarbon ages range from 950 to 990 AD (Microatoll I) within the beachrock zone, to ages clustered around 1500 to 1750 AD (e.g. Microatoll J, P and H). These four fossil microatolls were cored and/or sectioned for laboratory analysis. Microatoll I (MaI) is located near the present shoreline and is currently being exhumed by wave erosion of an adjacent beachrock layer, in conjunction with shoreline recession (shown in Fig. 8A). Cores taken at 0.1 m intervals across the 2.4 m diameter of MaI have shown that the microatoll is in growth position and that all cores from the centre to the edges show a vertical growth axis. This demonstrates that the modern exposure is a basal remnant of the former MaI. The mean growth rate is 0.008 m/yr, confirming that MaI grew continuously for ~ 100–125 yr. The elevation of the eroded surface is + 0.06 ± 0.05 m higher than the modern HLC. Field examination of the beachrock morphology shown in Fig. 8A, indicates that beachrock did not form over MaI and the outline of MaI exists in the outline of the beachrock. Hence, we interpret that the former HLC on MaI was at least to the top of the adjacent beachrock at + 0.36 m above modern HLC ( equivalent to modern MSL) at the time of beachrock formation. Adjacent to MaI is the fossil Microatoll J (MaJ) that has been transported shoreward from the moat to its present location during a storm event. We observed that MaJ had undergone renewed coral growth in the new location (shown in Fig. 8A). We sampled this renewed coral growth and the age is 1480 AD (1422 to 1583 AD), with a growth rate of 0.015 to 0.018 m/yr. The HLC for the renewed growth is equivalent to +0.35 ± 0.1 m above modern MSL. Following mortality of this coral, beach progradation partially burying the microatolls, was followed by a phase of beachrock formation. On the inner reef flat and moat, two microatolls were chosen from the cluster. Microatoll H (MaH) is an eroded remnant, 3.9 m diameter, with the coral growth axis vertical in the centre, and lateral in the outer half (Fig. 8B). This indicates that the eroded surface is within ±0.1 m of the original surface. MaH has a central age of 1674 AD (1540 to 1810 AD) and its HLC is between +0.15 m and +0.2 m above modern HLC (equivalent to −0.21 m and −0.16 m below modern MSL). The growth rate over the estimated lifespan of ~ 150 yr is 0.012 m/yr. Nearby, Microatoll P (MaP) has experienced a similar history to MaJ, where it was transported shorewards a short distance in the growth position, followed by renewed in situ growth (Fig. 8C). The renewed growth had a HLC of +0.25 ± 0.05 m higher than modern HLC, and a sample taken from a core through this second growth phase yielded

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4.2.2. Aitutaki Fossil Porites sp. microatolls were sampled over a 100 m long section of the inner reef flat at Ootu. These microatolls have been exposed in the past 10 yr as the barrier spit has receded, in response to increased tidal currents in the recently dredged, Ootu Passage. The spit recession is confirmed by the recent slumping of World War 2 age field gun debris now strewn on the reef flat. Six of the microatolls were cored and we obtained a radiocarbon age on three. The microatolls ranged from 1.6 to 3.9 m in diameter (Fig. 9). The radiocarbon ages were tightly clustered between 349 ± 110 AD and 430 ± 110 AD. The HLC of the fossil Ootu microatolls is 0.8 ± 0.1 m above the modern HLC of Porites sp. microatolls in the nearby Ootu passage. The growth rates of the fossil corals is ~ 0.010 m/yr. There were no modern Porites sp. microatolls observed on the reef flat. 4.3. Reconstruction of relative sea-level changes The above fossil microatoll record provides a time series of vertical constraints to sea-level variability over the past 1650 yr. These data provide an internally reliable data set because the same fixed biological sea-level proxy is used, and provides a record of the changes in HLC (−0.36 m MSL, or between MLWS and MLWN). In addition, we have used all previously published fossil microatoll HLC (where enough information was published to allow the calculation of HLC with respect to MSL) together with our new data in the following RSL reconstructions. Fig. 10 shows the relative sea-level envelope constructed for Aitutaki. The envelope is based on the Ootu fossil microatoll data, the fossil microatoll data reported in Yonekura et al., 1988 (adjusted to microatoll HLC datum) and regional observations of RSL rise over the past 100 yr (White et al., 2005). The RSL envelope shows a faster rate of RSL lowering since 1000 AD (−0.56 mm/yr) compared to GIA estimate for the past 100 yr (−0.3 mm/yr) by Peltier (2004). The reconstructed RSL envelope for Rarotonga is shown in Fig. 11. The grey shaded envelope defines the locus of uncertainty in the age determination and in the geomorphic determination of the former HLC, as a result of post-mortality erosion of the microatolls. The tie points for the envelope include: (i) the mid to late Holocene highstand limit defined by the fossil (3500 to 4200 yr BP) microatoll elevations reported by Chikamori (1995) at Avana corrected to height above modern HLC; (ii) the Black Rock fossil microatoll elevations from this study; (iii) the emergent reef elevation at Avarua reported by Schofield (1970), and (iv) the fossil microatoll elevations for Aitutaki (replotted from Fig. 11). The Aitutaki RSL constraints are included because there

Fig. 8. (A) Photograph of fossil Microatoll I (MaI) and fossil Microatoll J (MaJ), the Black Rock fringing reef, shoreline, and beachrock deposits, Rarotonga. (B) Photograph of fossil Microatoll H (MaH), and the Black Rock fringing reef, Rarotonga. (C) Photograph of fossil Microatoll P (MaP) and the Black Rock fringing reef, Rarotonga.

an age of 1750 AD (1679 to 1888 AD). The growth rate was 0.015 to 0.018 m/yr. The fossil HLC provide limits on RSL variability during the past millennium, and together with the microatoll growth rates provide constraints on the evolution of the fringing reef environment. Microatoll diameter defines the lifespan of the microatolls and as such, is an indicator of the SST window for growth, low turbidity, low carbonate sediment volumes, and adequate water levels over the reef flat and moat, and time interval between major cyclone storm events, that cause instant mortality.

Fig. 9. Photograph of fossil microatolls and the fringing reef and shoreline, at Ootu on Aitutaki.

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Fig. 10. Plot showing the reconstruction envelope of solutions for the RSL history on Aitutaki, developed from the former HLC of microatolls. Also shown is the modelled hindcast RSL change due to the GIA for the past millennium from ICE-5G (Peltier, 2004).

is no evidence to indicate that the two islands have experienced different thermotectonic or GIA histories during the late Holocene (Dickinson, 1998). RSL fall from the highstand had commenced by 500 BC and fallen at a rate of 0.45 to 0.5 mm/yr over the past 2000 yr, RSL has fallen in the Southern Cook Islands by ~ 0.3 mm/yr. equivalent to the rate modelled for the GIA over the past 100 yr by Peltier (2004). The reconstructed HLC for microatoll I at +0.35 m above modern MSL is overlapping with the microatoll height of +0.4 to +0.45 m above modern MSL for the Aitutaki microatolls reported by Yonekura et al. (1988). This is also confirmed by our reworking of the Moriwaki et al. (2006) RSL data for Ngatangiia, discussed in the next section. Hence, we interpret a 0.25 to 0.6 m fall in RSL from 0 to ~1000 AD. After 1000 AD RSL has continued to fall at a rate close to that modelled for

the GIA by Peltier (2004) but with some discernable fluctuations. Taking into account the elevation uncertainties, the Black Rock (Rarotonga) microatoll elevations display fluctuations in RSL on the order of 0.1 to 0.2 m between 1500 to 1750 AD, followed by an abrupt fall of between −0.35 and −0.4 m after 1750 AD. In the following section, we test whether this RSL variability is signal or noise in the reconstruction, by comparing the interpreted RSL history with proxy SST and climate data from Rarotonga and Fiji. 5. Discussion Our reconstruction of RSL allows for discussion on the cessation of the highstand, and the nature of the subsequent RSL fall, whether monotonic or punctuated by climate-forced variability.

Fig. 11. Plot showing the reconstruction envelope of solutions for the RSL history on Rarotonga and Aitutaki, developed from the former HLC of microatolls. Also shown is the modelled hindcast RSL change due to the GIA for the past millennium from ICE-5G (Peltier, 2004).

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5.1. Cessation of the sea-level highstand The GIA theory predicts that sea-level highstand conditions persisted at global locations far from the direct influence of post Last Glacial Maximum deglaciations, (defined by Clark et al., 1978 as their Zone V), until there is a cessation of ice meltwater from disintegrating ice masses entering the oceans (Lambeck, 2002). The highstand is maintained by the exceedance of the meltwater rate over the hydro-isostatic lowering rate due to the equatorial oceansyphoning effect (Mitrovica and Peltier, 1991). There is agreement amongst glaciologists and geophysicists that the timing of the meltwater cessation (or rates less than 0.5 mm/yr) was post 3000 yr BP (Lambeck, 2002). Dickinson (2003) discussed the regional highstand duration throughout the Pacific and concluded that in the region of the central South Pacific, including the Cook Islands, Society Islands, Tuamotu Archipelago and Gambier Islands, the highstand had ceased by the first millennium AD. Our RSL envelope for the Southern Cook Islands supports both the melting theory and conforms to field evidence from other related island groups that the highstand ceased by 0 to 500 BC (2000 to 2500 yr BP). However, our RSL envelope is contrary to that recently published by Moriwaki et al. (2006) with respect to the timing of the cessation of the highstand. Moriwaki et al. (2006) reported that the highstand had continued until at least 1150 to 1450 AD (800–500 yr BP). Their evidence for the latter is based on radiocarbon dated fossil emergent corals near Ngatangiia on the Rarotonga east coast, that they interpreted to be an indicator of freedraining HLC. We have inspected these corals and have concluded that these are encrusting corals that did not grow in a free draining reef flat environment with an HLC between MLWN and MLWS, but instead grew in micro-moat pools within karstified Pleistocene emergent reef, on an energetic wave impact environment. The uppermost coral samples at 1.35 m above modern MSL may also be transported clasts, located near the modern beach berm, and hence may not be fixed former sea-level indicators. The Moriwaki et al. (2006) interpretation of these corals places RSL on Rarotonga approximately 1 to 1.3 m higher than our reconstruction at around 1200 to 1500 yr AD. Since, there is no evidence for neotectonic tilting across Rarotonga in the past millennium, we propose that the encrusting corals reported by Moriwaki et al. (2006) are not fixed-indicators of changes in mean sealevel or the microatoll HLC datum but that the two lower samples described by Moriwaki et al. (2006) at +0.2 to +0.5 m above modern MSL are fixed-indicators of changes in MHWN, since this is the maximum constraining water level for moated coral growth observed in our levelling surveys. Therefore, the sample at +0.5 m above modern MSL is equivalent to +0.39 m above modern MHWN, and indicates that the respective fossil tidal datums on the east coast of Rarotonga were ~ +0.39 m above present at 800–500 yr BP. This agrees well with our estimated height of the top of Microatoll I (+0.36 m above modern HLC), that has an overlapping age range with the Ngatangiia emergent corals, and the lower estimate of the fossil microatoll height on Aitutaki (with respect to modern MSL), reported by Yonekura et al. (1988). Hence, we have included the reinterpreted RSL evidence from the Moriwaki et al. (2006) study in our RSL envelope. Further evidence to support our conclusions on the onset of RSL fall is the coastal progradation and aggradational history and the evolution of the Ngatangiia reef islands (Motutapu, Oneroa and Koromiri). Dickinson (2004), introduced the concept of a tide level ‘cross-over date’ as the timing for the development of reef islands on emergent reef flat substrate. The cross-over date is the time when the declining ambient high tide level falls below the mid-Holocene highstand low tide level (MHHLTL). At this point the mid-Holocene highstand reef flat transitions from being intertidal to supratidal, washed only during storm surges. Dickinson (2003) had estimated from previously published data that the cross-over date was ~ 800 AD for the Southern Cook Islands, and around 1250 AD for most Pacific

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islands (Dickinson, 2004). We have calculated from Fig. 11 that the cross-over date for both the decline of MHWN and MLWS, is between 500 BC and 300 AD, and 500 and 1300 AD respectively, using the modern tidal ranges. Considerable progradation would have commenced when the ambient MHWN crossed-over the MHHLTL, with rapid progradation after the cross-over with MSLW. We concur with Stoddart (1971) that the Ngatangiia reef islands have an aggradational history of storm-deposited rubble with infill, beach and spit growth of fine sands and gravels. The deposition of the latter is a direct result of RSL falling below the cross-over point. Okajima (1999) cited in Moriwaki et al. (2006) reports that human settlement commenced by 720 ± 50 14C yr BP on Motutapu Island. A contemporaneous history of coastal progradation occurs on Aitutaki. Allen (1998) reported that coastal progradation near the present shoreline, or transition from beach deposits to overlying backshore and strandplain deposits had commenced by 900 to 1040 AD, as a result of aggradation of stormdeposited rubble and infilling of fine sands to gravels. This coastal progradation on Aitutaki and Rarotonga has persisted up until the RSL rise during the past century. 5.2. Comments on the RSL reconstruction and Paleoclimatic indicators Previous papers by Nunn (1998, 2000) have hypothesised a rapid RSL fall of ~ 1 m throughout the south-west Pacific region at or after 1300 AD, based on mixed geomorphological and archeological evidence for sea-level change. He attributed the rapid fall in SL to be associated with previously published data on the onset of the Little Ice Age, and an increase in El Niño frequency. However, the evidence he reports may be symptomatic of the rapidity of the progradational phase following the MHHLTL cross-over. Our RSL reconstruction indicates that the rate of RSL fall during the last millennium was ~0.4 to 0.8 mm/yr, with multi-decadal to centennial variability of 2.5 to 4 mm/yr, and reaching the lowest stand during the 1800s AD. There is a significant body of evidence to show that a Pacific basin transition from multi-centennial La Niña-like to El Niño-like climate occurred after 1000 to 1200 AD (Clement et al., 1996; MacDonald and Case, 2005; Goodwin and Mayewski, 2007). Such Pacific basin-wide climate variability may explain the interpreted RSL, coastal and reef morphological changes and evolution over the past millennium. Since the modern sea-level variability is coupled to Pacific Basin wind stress and SST Wyrtki (1975) we explore the variability within the RSL reconstruction with derived paleo-SST data. Linsley et al. (2000) reconstructed SST from a Porites lutea coral head on the forereef slope, approximately 1 km from the Black Rock fringing reef location on Rarotonga. Their proxy SST record was determined from high-resolution Sr/Ca measurements on the coral core and span the period from 1726 to 1997 AD. Their SST record is consistent with the modern record of the past 2 decades (IGOSS SST shown in Fig. 3) with a strong +ve correlation with La Niña-like climate. The proxy SST record shows a strong +ve SST anomaly of 1.5 to 2 °C warmer than the past few decades between the start of the record at 1726 and 1760 AD. This is contemporaneous with the interpreted microatoll growth window defined by MaS, MaH and MaP. The high RSL fluctuation defined by MaP is synchronous with the warmest SST in the past 300 yr. Similarly, the annual growth rate for MaP is 0.015 to 0.018 m/yr compared to the 0.013 m/yr average for the past 20 yr. The subsequent RSL fall (0.35 to 0.4 m) since ~ 1760 AD is synchronous with a period of 2 °C cooling in SSTs. The magnitude of these multi-decadal to centennial changes is larger than the magnitude of modern seasonal summer to winter RSL and SST changes of 0.2 m for 4 °C, respectively. The changes are consistent with the magnitude of modern interannual RSL and SST variability associated with ENSO, and imply a shift towards strengthened El Niño-like climate during the late 1700s AD and the associated westward shift in the South Pacific Convergence Zone (SPCZ) during this period and a reduced east–west SST gradient across the tropical Pacific Ocean

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Linsley et al. (2006). The subsequent increase in the east–west SST gradient during the past 100 yr (Linsley et al., 2006) is synchronous with the RSL rise in the SCI. The RSL record since ~1760 AD is supported by the stratigraphic evidence for beachrock formation after ~ 1600 AD, since MaJ was buried by beach accretion and subsequent beachrock formation. The sustained period of beach accretion and shoreline progradation was in response to sub-aerial erosion of the reef crest and flat due to low RSL. Living microatolls were constrained during this period to only the MaS age population that were living in the deepest part of the fringing reef moat. Modern exhumation of beachrock is ubiquitous on south-west Pacific island coasts as shorelines recede in response to RSL rise. Age determination of beachrock in Samoa revealed contemporaneous evolution after 1500 or 1600 AD (Goodwin and Grossman, 2003), and further supports the interpretation for significantly lower SL during the 1700s and 1800s AD. SL in the Central South Pacific is also controlled by the basin-wide wind stress field, and in off-equatorial locations like the SCI, SL anomalies are propagated westward from the eastern Pacific by Rossby waves (Graham and White, 1988). Strengthened (weakened) tradewinds during La Niña (El Niño) cause elevated (lowered) SL in the western Pacific and this also impacts the SCI region, albeit subdued (e.g. SL anomalies of ±0.1 m over the past 15 yr, National Tidal Centre, 2006). Our multi-decadal to centennial resolution of RSL change at the SCI over the past 500 yr robustly conforms to the proxy SST variability and corresponding magnitude of the sea-level response to ENSO. The second time period where we have some insight into centennial scale SL behaviour is during the 900s AD, by comparing MaI with the microatoll observations of Yonekura et al. (1988) on Aitutaki, and emergent encrusting corals at Ngatangiia (Moriwaki et al., 2006). Our RSL reconstruction includes a 0.35 m uncertainty in elevation between the two island tiepoints, so the elevations alone are not sufficient to provide an insight into climate variability. Instead, the average growth rate provides some information on climatic conditions. The average (100 to 150 yr) growth rate for MaI is 0.006 m/yr which is the minimum growth rate determined for the fossil microatolls and is equivalent to the minimum growth rate in the past 20 yr during 2000. Growth rate is positively correlated with RSL variability. Hence, the period when MaI was living, −ve RSL and SST anomalies are implied. Using the modern instrumental record as an analogue suggests that a multi-decadal El Niño-like climate persisted during the 900s AD. However, this is contrary to other proxy climate evidence that emphatically describes an extreme low frequency La Niña-like climate anomaly observed throughout the Americas (Cobb et al., 2003; Rein et al., 2004; MacDonald and Case, 2005), Antarctica and eastern Australia (Goodwin and Mayewski, 2007) at this time. The occurrence of cool SST anomalies and low RSL anomalies in the subtropical Central South Pacific during the 900 to 1000 AD period requires a westward expansion of the eastern Pacific cold tongue into the Central Pacific, (e.g. during the extended La Niña in 1974–75) and a high sea-level pressure anomaly over the subtropical to extratropical central South Pacific as interpreted by Goodwin and Mayewski (2007). 6. Conclusions The RSL reconstructions for the SCI: refine the timing of the decline of SL during the first millennium AD in the Central South Pacific; and provide constraint of the timing of the cross-over between ambient MNHW and MSHW below the mid-Holocene Low Tide Level, to between 400 and 600 AD, and 750 and 1250 AD respectively. These RSL cross-over events initiated rapid coastal progradation, and provide an insight into the thresholds for both progradation and recession on island coasts, that are important analogues for predicting coastal response to the present exponential trend in SL rise (White et al., 2005). Our RSL reconstruction is rigorously based on 400 laser survey measurements of the constraining water level for modern microatoll growth with an inter site accuracy

of 0.02 ± 0.008 m. The RSL reconstruction shows that a sustained SL rise over the modern century of between +0.5 and +1.0 m will place RSL at comparative positions to that between 0 and 1000 AD. Hence, SL rise this century will move above the mid-Holocene Low Tide Level threshold and cause coastline recession, on islands where sediment supply is limited, and fringing reef growth rates are restricted by subtropical SSTs. The methods used in this study demonstrate a successful reconstruction at a decimetre scale of RSL variability on multi-decadal to centennial timescales using microatolls. The major limitations are the temporal distribution of coral growth windows, cyclone destruction and preservation of microatolls as RSL indicators. Whilst uncertainties will always exist in the elevation of fossil microatolls due to surface erosion, their growth rate and geochemistry provide corroborative information on SST. Our results indicate that RSL during the past 500 yr has fluctuated about a monotonic fall due to the GIA process. The faster rate of RSL lowering over the past millennium (−0.45 to −0.56 mm/yr) than that modelled (ICE5-G) for the GIA by Peltier (2004) of −0.3 mm/yr, may be indicative of the climatic influence on regional sea-levels, or be the more accurate value for the combined GIA and eustatic SL trends in the SCI. The reconstructed RSL anomalies from ~ 1700 AD are at a higher temporal resolution and provide some information on RSL variability of ±0.2 m due to climatic influences. High RSL anomalies of +0.15 m occurred during the 1700 to 1750s AD and were associated with +ve SST anomalies of 1.5 to 2 °C. In the subsequent period RSL fell by 0.35 to 0.4 m in conjunction with a 2 °C SST cooling until the late 1800s before RSL and SST increased. These RSL fluctuations define the minimum in SL to have occurred during the 1800s AD before the 20th century rising trend. The sensitivity of RSL in the Central South Pacific to combined SST and wind stress forcing is 0.17 to 0.2 m of SL change per 1 °C change in SST, and is consistent with modern fluctuations associated with low frequency ENSO behaviour, except for the period 950 to 990 AD where the Central Pacific Ocean region probably experienced an extreme expansion of the Eastern Pacific Cold Tongue associated with the La Niña-like climate anomaly (Rein et al., 2004). Acknowledgments We thank Mr. Arthur Taripo (Chairman, Foundation for National Research, Cook Islands) for supporting this research project, and Ms. Pasha Carruthers, PICCAP, Environment Service, Cook Islands who coordinated our field activities in the Cook Islands, and Mr. Bobby Bishop (Aitutaki Climate Change Coordinator) for providing local assistance on Aitutaki. The field work was conducted with the assistance of Ms Kris James, Mr. Darren Dean, Mr. Sean Phillipson, and Ms. Deborah Goodwin. Laboratory analyses of the coral microatolls at the University of Newcastle were conducted by Dr. Janece MacDonald and Ms. Rachel Skelley-Vazey. The authors also thank Ms. Christine Crothers for cartographic assistance. The research was funded by internal University of Newcastle research project grants, the Asia-Pacific Network, and the radiocarbon dating of coral was funded by AINSE grant 03047P. We thank Dr. Ugo Zoppi from the Australian Nuclear Science Technology Organisation (ANSTO) for conducting the radiocarbon analyses. References Allen, M.S., 1998. Holocene sea-level change on Aitutaki, Cook Islands: landscape change and human response. J. Coast. Res. 14 (1), 10–22. Barstow, S. and Haug, O. 1994. The wave climate of the Cook Islands. SOPAC Technical Report 200. South Pacific Applied Geoscience Commission, Fiji.33 pages. Chikamori, M., 1995. Development of coral reefs and human settlement: archaeological research in the Northern Cook Islands and Rarotonga. In: Chikamori, M., Yoshida, S., Yamaguchi, T. (Eds.), Archaeological studies on the Cook Islands. Series 1., Department of Archaeology and Ethnology, Keio University, pp. 3–14. Clark, J.A., Farrell, W.E., Peltier, W.R., 1978. Global changes in postglacial sea level: a numerical calculation. Quat. Res. 9, 265–278. Clement, A.C., Seager, R., Cane, M.A., Zebiak, S.E., 1996. An ocean dynamical thermostat. J. Climate 9, 2190–2196.

I.D. Goodwin, N. Harvey / Marine Geology 253 (2008) 14–25 Cobb, K.M., Charles, C.B., Cheng, H., Edwards, R.L., 2003. El Niño/Southern Oscillation and tropical Pacific climate during the last millennium. Nature 424, 271–276. Dickinson, W.R., 1998. Geomorphology and geodynamics of the Cook–Austral Island seamount chain in the South Pacific Ocean: implications for hotspots and plumes. Int. Geol. Rev. 40, 1039–1075. Dickinson, W.R., 2003. Impact of mid-Holocene highstand in regional sea level on habitability of islands in Pacific Oceania. J. Coast. Res. 19 (3), 489–502. Dickinson, W.R., 2004. Impacts of eustasy and hydro-isostasy on the evolution and landforms of Pacific atolls. Palaeogeogr., Palaeoclimatol. Palaeoecol. 213, 251–269. Goodwin, I.D., 2003. Unravelling climate influences on late Holocene sea-level and coastal evolution. In: Mackay, A., Battarbee, R., Birks, J., Oldfield, F. (Eds.), Global Change in the Holocene. Edward Arnold, London, pp. 406–421. Goodwin, I.D., Grossman, E.E., 2003. Middle to Late Holocene coastal evolution along the south coast of Upolu Island, Samoa. Mar. Geol. 202 (1–16). doi:10.1016/S00253227(03)00284-6. Goodwin, I.D., Mayewski, P.A., 2007. Multi-decadal climate variability in the western Pacific and Antarctic regions during the past 1500 years. Quat. Int. 167–168 supplement, page 145. Graham, N.E., White, W.B., 1988. The El Niño cycle: a natural oscillator of the Pacific Ocean — atmosphere system. Science 240, 1293–1302. Grossman, E.E., Fletcher, C.F., Richmond, B.M., 1998. The Holocene sea-level highstand in the equatorial Pacific: analysis of the insular paleosea-level database. Coral Reefs 17, 309–327. Hein, J.R., Gray, S.C., Richmond, B.M., 1997. Geology and hydrogeology of the Cook Islands. In: Vacher, H.L., Quinn, T. (Eds.), Geology and Hydrogeology of Carbonate Islands. Developments in Sedimentology, vol. 54, pp. 503–535. IPCC, (2007). Climate Change 2007 — The Physical Science Basis. Working Group 1 Contribution to the Fourth Assessment Report of the IPCC. Edited by Solomon, S., Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M., and Miller, H.R. Cambridge University Press, pp 1009. Keys, J.E., 1963. The tsunami of 22 May 1960 in the Samoa and the Cook Islands. Bull. Seis. Soc. Am. 53, 1211–1227. Lambeck, K. 2002. Sea-level change from mid Holocene to recent time: an Australian example with global implications. In: Mitrovica, J.X. and Vermeersen, B.L.A. 2002. Ice Sheets, Sea Level and the Dynamic Earth. Geodynamics Series 29, American Geophysical Union, 33–50. Linsley, B.K., Wellington, G.M., Schrag, D.P., 2000. Decadal sea surface temperature variability in the subtropical south Pacific from 1726 to 1997 A.D. Science 290, 1145–1148. Linsley, B.K., Kaplan, A., Gouriou, Y., Salinger, J., deMenocal, P.B., Wellington, G.M., Howe, S.S., 2006. Tracking the extent of the South Pacific Convergence Zone since the early 1600s. Geochem. Geophys. Geosyst. 7 (4). doi:10.1029/2005GC001115 3rd May 2006, Q05003. MacDonald, G.M., Case, R.A., 2005. Variations in the Pacific Decadal Oscillation over the past millennium. Geophys. Res. Lett. 32 (32). doi:10.1029/2005GL022478 L08703. Mitrovica, J.X., Peltier, W.R., 1991. On postglacial geoid subsidence over the equatorial oceans. J. Geophys. Res. 96 B12, 20,053–20.071. Moriwaki, H., Chikamori, M., Okuno, M., Nakamura, T., 2006. Holocene changes in sea level and coastal environments on Rarotonga, Cook Islands, South Pacific Ocean. Holocene 16 (6), 839–848. National Tidal Centre. 2006. Sea level data summary report November 2005–June 2006. The South Pacific Sea Level and Climate Monitoring Project. National Tidal Centre, Australian Bureau of Meteorology, South Australia, 39 pp.

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Nunn, P.D., 1998. Sea-level change over the past 1000 years in the Pacific. J. Coast. Res. 14 (1), 23–30. Nunn, P.D., 2000. Environmental catastrophe in the Pacific Islands around A.D. 1300. Geoarchaeology 15 (7), 715–740. Okajima, T. 1999. Cannibalism of Rarotonga, Cook Islands in AD 13 c. Abstract for 33rd Congress of Ethnology of Japan 59 (in Japanese). Peltier, W.R., 2004. Global glacial isostasy and the surface of the ice-age Earth: the ICE5G (VM2) model and GRACE. Ann. Rev. Earth Planet. Sci. 32, 111–149 May 2004. Pirazzoli, P.A., 1996. Sea-Level Changes. The Last 20,000 Years. Wiley, Chichester, England, U.K. Rein, B., Lückge, A., Sirocko, F., 2004. A major ENSO anomaly during the Medieval period. Geophys. Res. Lett. 31, L17211. doi:10.1029/2004GL020161. Schofield, J.C., 1970. Notes on late Quaternary sea levels, Fiji and Rarotonga. N. Z. J. Geol. Geophys. 13, 199–206. Scoffin, T.P., Stoddart, D.R., Rosen, B.R., 1978. The nature and significance of microatolls. Philos. Trans. R. Soc. Lond., Series B, Biol. Sci. 284 (999), 99–122 The Northern Great Barrier Reef. Smithers, S.G., Woodroffe, C.D., 2000. Microatolls as sea-level indicators on a mid-ocean atoll. Mar. Geol. 168, 61–78. Smithers, S.G., Woodroffe, C.D., 2001. Coral microatolls and 20th century sea level in the eastern Indian Ocean. Earth Planet. Sci. Lett. 191, 173–184. Spencer, T., Tudhope, A.W., French, J.R., Scoffin, T.P., Utanga, A., 1997. Reconstructing sea level change from coral microatolls, Tongareva (Penryn) Atoll, Northern Cook Islands. Proc. 8th International Coral Reef Symposium 1, 489–494. Stoddart, D.R., 1971. Reef islands of Rarotonga. Atoll Res. Bull. 160, 1–14. Stoddart, D.R., 1975. Almost-atoll of Aitutaki geomorphology of reefs and islands. Atoll Res. Bull. 190, 31–57. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230. Thompson, C.S., 1986. The climate and weather of the Southern Cook Islands. N. Z. Meteorol. Serv. Misc. Publ. 188 (2) Wellington, New Zealand. White, N.J., Church, J.A., Gregory, J.M., 2005. Coastal and global averaged sea level rise for 1950 to 2000. Geophys. Res. Lett. 32, L01601. doi:10.1029/2004GL021391. Wood, B.L., 1967. Geology of the Cook Islands. N. Z. J. Geol. Geophys. 10, 1429–1445. Wood, B.L., Hay, R.F., 1970. Geology of the Cook Islands. N. Z. Geol. Surv. Bull., New Ser. 82, 1–103. Woodroffe, C., McLean, R., 1990. Microatolls and recent sea-level change on coral atolls. Nature 344, 531–534. Woodroffe, C.D., Gagan, M.K., 2000. Coral microatolls from the central Pacific record late Holocene El Niño. Geophys. Res. Lett. 27 (10), 1511–1514. Woodroffe, C.D., Beech, M.R., Gagan, M.K., 2003. Mid-late Holocene El Niño variability in the equatorial Pacific from coral microatolls. Geophys. Res. Lett. 30 (7), 1358. doi:10.1029/2002GL015868. Wyrtki, K., 1975. El Niño, the dynamical response of the equatorial Pacific Ocean to atmospheric forcing. J. Phys. Oceanogr. 5, 572–584. Yonekura, N., Matsushima, Y., Maeda, Y., Kayanne, H., 1984. Holocene sea-level changes in the Southern Cook Islands. In: Sugimura, A. (Ed.), Sea Level Changes and Tectonics in the Middle Pacific. Report of the HIPAC Project in 1981, 1982 and 1983. Department of Earth Sciences, Kobe University, Nada, Kobe, Japan, pp. 113–136. Yonekura, N., Ishii, T., Saito, Y., Maeda, Y., Matsushima, Y., Matsumoto, E., Kayanne, H., 1988. Holocene fringing reefs and sea-level change in Mangaiia Island, Southern Cook Islands. Palaeogeogr. Palaeoclimatol. Palaeoecol. 68, 177–188.