The coral record of last interglacial sea levels and sea surface temperatures

Ž .Chemical Geology 169 2000 107–129www.elsevier.comrlocaterchemgeo

The coral record of last interglacial sea levels and seasurface temperatures

Malcolm T. McCulloch), Tezer Esat( )Research School of Earth Sciences, Australian National UniÕersity, G.P.O. Box 4 1 Mills Road , Canberra, A.C.T. 0200, Australia

Received 31 May 1999; accepted 15 March 2000

Abstract

Ž . Ž .The rise and fall of the Last Interglacial LI sea levels and sea surface temperatures SSTs are evaluated using U-seriesdating combined with SrrCa ratios in corals from both stable and tectonically uplifted sites. Along the stable coastal marginof Western Australia, an extensive series of LI coral reefs occur at heights of 2–3 m above present-day sea level. Thesecorals have a very tight cluster of 234U–230Th ages ranging from 129"1 to 119"1 ka, as well as a narrow range of initiald

234U values of 150"5, similar to modern seawater. Bahamas, which is also a stable site, has an essentially identicalpattern of U-series ages from 130"1 to 120"1 ka. Barbados and Huon Peninsula are tectonically active sites where the LIterraces are found at elevations of )50 and )200 m, respectively. U-series ages from corals exposed in the lower footwallof these uplifted reefs, allow better constraints to be placed on the rate of sea level rise which initiated the LI. Corals fromthe Huon Peninsula constrain sea level at y80"10 m at 131"2 ka, and from Barbados, at y30"5 m at 129"1 ka.Combined with constraints from stable sites, these observations require an exceedingly rapid rise in sea level of 30–50 m per1000 years at 130"1 ka. This indicates that large-scale catastrophic melting of the once massive continental ice sheets

Ž .occurred in phase with the rapidly increasing northern hemisphere NH summer insolation, consistent with the orbitalforcing being the main driver of glacial–interglacial climate change. There is also some evidence from Huon Peninsula,although still not conclusive, for a precursor oscillation in sea level during the penultimate deglaciation, that may have beenwithin ;y20 m of present-day levels at ;135 ka.

SSTs for the LI Porites corals from the Huon Peninsula and Western Australia have mean annual temperatures andseasonal ranges that are remarkably similar to present-day patterns. The tropical site of Huon Peninsula has SSTs of29"18C, which is indistinguishable from the SSTs given by modern corals. At Ningaloo Reef in Western Australia, similar

Ž .mean annual ;248C and summer maximum SSTs of 27–298C are found in both LI and modern corals. The onlysignificant difference is the ;18C cooler winter minimum SSTs of ;218C for the LI compared to present-day minimums

Ž .of ;228C. LI SSTs from these southern hemisphere SH sites were thus very similar, or at most, only slightly cooler thantoday, despite sea levels being up to 4 m higher. This maybe indicative of asymmetric warming of the Earth, with theincreased NH insolation during the LI period being responsible for the extensive melting of the mainly NH-based ice sheets,and hence, higher global sea levels. The observation of relatively high sea levels in the LI, together with the rapid pulses of

) Corresponding author. Fax: q61-6-24-95-443.Ž .E-mail address: [email protected] M.T. McCulloch .

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 00 00260-6

( )M.T. McCulloch, T. EsatrChemical Geology 169 2000 107–129108

sea level rise, indicates that the potential now exists for greenhouse warming to initiate increases in sea level of at leastŽ 2 .several metres on relatively short time-scales 10 years . q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Last Interglacial; Sea levels; Sea surface temperatures

1. Introduction

The last time that sea levels were at or slightlyabove present-day heights was during the Last Inter-

Ž .glacial LI period ;125 ka ago. During this periodŽ .isotope substage 5e , global climate was similar to,

Žor slightly warmer than today e.g. Montoya et al.,.1998 . With the possibility that the Earth is about to

enter into a new ‘super-interglacial’ phase due togreenhouse forcing, a better understanding of thefactors controlling interglacial climates is importantŽ .e.g. Crowley, 1990 . This is necessary not only toprovide a natural baseline against which future an-thropogenic-induced changes in climate regimes canbe evaluated, but also to obtain a better understand-ing of, for example, the relationship between globaltemperatures and sea level. The main driver ofglacial–interglacial cycles is generally considered tobe variations in the summer insolation received at

Ž .the high latitudes in the northern hemisphere NHresulting from changes in the Earth’s orbit. This iscommonly termed the Milankovitch theoryŽ .Milankovitch, 1941 of climate change, and is basedon the premise that the large ice sheets of the NHrespond directly to the changes in summer insolation,and in turn, control global glacial climates via ice-al-bedo feedbacks. The changes in the volume ofgrounded ice sheets is thus directly manifested bythe changes in sea levels, which, during for example,the Last Glacial Maximum were ;125–130 m lower

Žthan today Nakada and Lambeck, 1988; Fairbanks,.1989 .

It has long been recognised that corals, particu-larly those species that grow within a narrow depthrange, provide an excellent means to reconstruct past

Ž .sea level e.g. Goreau and Wells, 1967 . Althoughstrictly speaking, corals provide only a minimumestimate of sea level heights, some species such as

Ž .Acropora palmata Blanchon and Shaw, 1995 in theŽCaribbean and Acropora dania-robusta Bard et al.,

.1996 in the Pacific Ocean are almost exclusivelyrestricted to reef crests, implying water depths of,generally, -5 m. Corals have long been the focus of

sea level studies as they also contain appreciableŽ .amounts of U 2–3 ppm , negligible Th, and are

therefore amenable to 234 U–230 Th dating. U-seriesdating was first undertaken in the early 1960s using

Žalpha-counting methods e.g. Veeh, 1966; Broecker.et al., 1968 . With the advent of thermal ionization

Ž .mass spectrometry TIMS applied to U-series, aŽtechnique pioneered at Caltech Edwards et al., 1986,

.1987; Chen et al., 1986 , it is now possible to placemuch more precise and accurate chronologies oncorals. This increased temporal resolution is espe-cially important during the LI and penultimatedeglaciation as it allows a more direct test of the

Ž .Milankovitch theory e.g. Stirling et al., 1995, 1998 .Here, we report constraints on the height and

duration of the LI using combined field data andhigh precision TIMS U-series dating of LI corals.We will show that by using a judicious combinationof data from both stable and tectonically upliftedsites, it is possible to better constrain the relativeheight of LI sea levels, as well as to determine boththe onset and termination of the LI. Using informa-tion from the tectonically active sites, such as HuonPeninsula and Barbados, it is also possible to placesome constraints on the rate of rise of LI sea levels,as well as to identify a precursor sea level rise eventprior to the commencement of the LI. In addition tocorals providing an excellent recorder of past sea

Žlevel, their trace element constituents e.g. SrrCa,.UrCa provide quantitative estimates of past sea

Ž .surface temperatures SSTs . Hence, it is, in princi-ple, possible to determine the relationship betweenchanging sea level and ocean temperatures, the latterhaving a major and very direct control on bothmarine and continental climates.

2. Methods

2.1. TIMS 234U–230Th isotopic abundance measure-ments

The methods for U-series dating employed atANU are based on those first described by Chen et

( )M.T. McCulloch, T. EsatrChemical Geology 169 2000 107–129 109

Ž . Ž .al. 1986 and Edwards et al. 1986 . The maindifference at ANU, is the use of charge capacitancerather than high ohmic resisitors in the electrometresfor the multiple faraday cup array in the 61-cm massspectrometer used for 229,230,232 Th measurementsŽ .Esat, 1995 . The low noise charge capacitance sys-tem enables low intensity ion beams of 10y13–10y16

A to be measured simultaneously giving an enhancedaccuracy. Mass fractionation effects for 229 Thr230 Thstill do not appear to be a limiting factor. Foruranium isotopic composition measurementsŽ233,234,235,238 . 233,235,238U , combined Faraday U and

234,235 Ž .electron multiplier U analogue mode modesare employed using a MAT 261 with 235U beingmeasured simultaneously in both modes and used tocorrect for the multiplier gain. Compared to singlechannel peak jumping modes, these are relativelyefficient data collection schemes with the advantage

Fig. 1. Plot showing measurements of d234 U in the secular

equilibrium standard, Harwell Uraninite and SRM 960. With themulti-collector Faraday-SEM mode, the typical uncertainty is in"1‰.

of simultaneous ion collection and relatively highŽ .precision Fig. 1 .

2.2. SrrCa ratios in corals as recorders of seasurface temperature

The possibility of utilising variations of SrrCaratios in coralline aragonite as a proxy for SST was

Ž .first demonstrated by Smith et al. 1979 , but the fullpotential of the method was only realised with theapplication of high-precision isotope dilution analy-

Žses TIMS Beck et al., 1992; de Villiers et al., 1995;.McCulloch et al., 1994 . This enables higher preci-

sion measurements of the SrrCa ratio, which isimportant as a 18C change in temperature results in aless than 1% change in the SrrCa ratio. The calibra-tion of SrrCa ratio vs. temperature depends onseveral factors, the constancy of the SrrCa ratio inseawater, and variations in ‘vital’ effects that thecoral organism may superimpose upon theseawaterrcoral aragonite fractionation. In seawater,

Ž 7both Sr and Ca have long residence times )10.years , and although there is sometimes a weak

nutrient-like behaviour of Sr which can lead to higherŽSrrCa in regions of intense upwelling, e.g. west

.coast of South America , sampling across the GreatŽ . Ž .Barrier Reef GBR Alibert and McCulloch, 1997

has shown that the SrrCa ratio is remarkably uni-Ž .form SrrCa atomics0.00849"0.00001 . Massive

Porites coral has generally been employed in SSTstudies, due its relatively uniform geometry, in par-ticular, the small size and limited depth of the calyxrelative to the thickness of the annual growth layers.Sampling is usually undertaken at a temporal resolu-

Žtion of ;40 samples per year ;0.25"0.01 mm.slices . While finer sampling is possible, this is of

little practical use as the resolution of the coral isŽ .limited by the calyx depth 0.3–0.5 mm , as well as

by ongoing calcification of the coral skeleton thatoccurs in the upper portion of the tissue zone. SrrCais analysed using isotope dilution TIMS procedures,

Ž .similar to that described by McCulloch et al. 1994 ,but with modifications to the Ca isotope dilutionprocedure. A mixed 43Ca–84Sr spike is now usedwith mass fractionation being corrected with the42 Car44Ca ratio and the Ca concentration deter-mined using the 43Car44Ca ratio. This procedure has


the advantage of avoiding measurement of the highabundance 40Ca isotope and increased precision forthe correction of Ca isotope fractionation.

The robustness of the SrrCa calibration has beenŽ .evaluated by Alibert and McCulloch 1997 in the

central GBR using different species of Porites coralŽ .from Davies Reef Fig. 2 . An example of the excel-

Ž .lent temporal fit rs0.98 of the SrrCa ratios to thein-situ measured SSTs from Davies Reef is shown inFig. 3a for a Porites mayeri colony. This is a

Žmoderate size colony ;1.4 m high and ;1 m. Ž .diameter growing in ;3 m water depth low tide

which shows clearly defined and very regular annualgrowth bands of 13"3 mmryear. In order to fur-ther test the general validity of the SrrCa vs. SSTcalibration, Porites corals have also been analysed

Ž .from an outer-shelf site Myrmidon Reef and anŽ . Ž .inshore site Orpheus Island in the GBR Fig. 2 .

Myrmidon Reef protrudes into the Coral Sea and,thus, much more strongly influenced by the EastAustralian current than the mid-shelf site of Davies

Fig. 2. Map of the central GBR lagoon showing the location ofOrpheus Island, Davies and Myrmidon Reefs. Davies Reef is amid-shelf site typical of the central GBR and is flushed on a dailybasis by tidal action, as well as by the southeast trade winds.Mixing of waters between the Coral Sea and central GBR gener-ally occurs on time-scales of several weeks. Myrmidon Reef is onthe outer-shelf of the GBR and is much more strongly influencedby the East Australian Current and as a consequence, SSTs in boththe summer and winter are generally warmer at Myrmidon com-pared to Davies Reef. Orpheus Island is an inshore fringing reefwhere there are generally more turbid water conditions especiallyduring flooding from coastal rivers that occurs from cyclones andmonsoonal depressions.

Ž .Fig. 3. a Comparison of SrrCa ratios for Porites mayeri vs.instrumental SSTs from Davies Reef for the period betweenAugust 1990 and October 1993. The limited range in SrrCa ratiosŽ .0.00915–0.0087 requires high precision measurements of this

Ž .ratio -"0.1% to achieve a temperature resolution of -

"0.38C. The translation between distance and time is undertakenby linear interpolation between winter minima. There is generallyan excellent agreement between the instrumental and coral recordswith, for example, the narrow summer peak of 1990–1991 being

Ž .faithfully recorded. b Comparison of SSTs obtained from SrrCaratios in Porites coral at Myrmidon Reef vs. in-situ measuredinstrumental records. The SSTs calculated using SrrCa ratiosfrom Davies and Myrmidon Reefs give the same calibrations

Ž .within analytical error Fig. 4 and apart from the 1993–1994summer for which there is a 18C discrepancy, there is, overall, avery good agreement.

Reef. This is apparent in the much narrower range ofŽ .summer SSTs Fig. 3b due to the upwelling surges

of cooler shelf break water. In contrast, OrpheusIsland is located in the inshore region of the GBRŽ .Fig. 2 , ;20 km from the mouth of the HerbertRiver and subject to episodic flood plumes, particu-larly from the very large Burdekin River. The cali-brations derived from Myrmidon, Davies and Or-pheus Island reefs are compared with each other in


Fig. 4, a plot of SrrCa ratios against in-situ mea-sured SSTs. Within the uncertainty limits, calibra-tions from Myrmidon and Davies are indistinguish-able from each other and give a combined calibration

Ž .of: 10=SrrCa 1000s10.42–0.06T , where T istemperature and the SrrCa is an atomic ratio. Thiscalibration is also shown to apply to the modern

ŽNingaloo Reef of Western Australia see later dis-.cussion .

The calibration obtained for a coral from OrpheusŽ .Island Gagan et al., 1998 is, however, substantially

different, giving ;28C higher temperatures than themid-outer reef sites at the same SrrCa ratio. Aninitial concern was the use of satellite data to esti-mate SSTs for this inshore site, whereas in-situmeasured records were used at the mid- and outershelf sites. The large grid size of satellite data,combined with the greater seasonality in the inshore

Žregion of the GBR i.e. higher summer maximum.and lower winter minimums can readily lead to

biased estimates of SSTs. This effect can be compen-sated for by the comparison of satellite with in-situ

Fig. 4. In-situ measured SST vs. SrrCa ratios measured inPorites coral from the mid-outer reef sites of Davies and Myrmi-

Ž .don and the inshore site at Orpheus Island Gagan et al., 1998 .Davies and Myrmidon Reefs give the same well-defined calibra-tion, whereas the calibration for the coral from Orpheus Island isoffset to ;28C higher temperatures. The reasons for the offset arenot yet well understood although different environmental condi-tions in the inshore vs. mid-outer reef regions of the GBR may bea contributing factor. The calibration for modern corals fromHuon Peninsula corresponds most closely to the Orpheus IslandŽ .inshore , whereas the Westen Australian Ningaloo reef coral has

Žthe same calibration as the mid-outer GBR Alibert and McCul-.loch, 1997 .

measurements of SSTs, and assuming that bias re-mains constant. This latter procedure was followed

Ž .by Gagan et al. 1998 and is generally in goodagreement with the new in-situ measurements ob-

Žtained during the past several years Berkelmans and.Oliver, 1999 . Differences in environmental condi-

Ž .tions, such as between inshore e.g. Orpheus IslandŽ .and mid-outer reef e.g. Davies, and Myrmidon

regions of the GBR, probably affect coralmetabolism, and may also, in some cases, influencethe SrrCa calibration. The reason for these differ-ences is still not understood, and as a precaution,comparisons are made between fossil and moderncorals from the same or equivalent settings.

3. LI sea levels

LI coral reefs occur in a variety of tectonic set-Žtings; stable margins that may either be distal e.g.

.Western Australia or relatively proximal to the mas-Ž .sive glacial ice sheets e.g. Florida ; and uplifted

sites such as the Huon Peninsula, Barbados, Sumbaand Vanuato. The best indicators of the relativeheight of LI sea levels compared to present-day arethose sites that occur along stable continental mar-gins such as Western Australia. These sites are stillsubject to the effects of changing water load, which

Žmay result in a tilting of the continental margin e.g..Nakada and Lambeck, 1988 . The magnitude of this

Žeffect is dependent, inter-alia, on the rate of rise or.fall of sea level and can be up to several metres

depending on the rheology model that is assumedŽ .Lambeck and Nakada, 1992 . For the near-fieldsites, glacio-isostatic effects also become important.For example, LI coral reefs that grew along or nearthe east coast of north America, such as Florida,Bermuda Islands, and to a lesser extent, Bahamasand Barbados are subject to differing degrees ofglacio-isostatic effects from the once massive Lau-rentide ice sheet. Furthermore, many tropical sites,such as the Hawaii, Huon Peninsula and Vanuato,are in regions of high precipitation and, therefore,carbonate diagenesis is often more extreme. To bet-ter constrain LI sea levels, it is thus important toconsider data from a variety of sites where thesefactors have differing influences.


3.1. Western Australia: a stable far-field site

Fringing coral reefs that grow along stable conti-nental margins provide one of the best indicators ofthe relative height of LI sea levels compared to thepresent-day. However, even the so-called ‘stable’continental margins, such as the coastline of the

Fig. 5. Map showing the distribution of LI reefs along the coast ofWestern Australia. During the LI, the southern-most occurrence ofcoral reef is at Rottnest Island, offshore Perth, whereas thesouthern-most extent of modern corals is now ;500 km furthernorth in the Houtman–Abrolhos Islands. Inset shows the NingalooReef area.

early ProterozoicrArchean shield of Western Aus-Ž .tralian Fig. 5 , local tectonic and hydro-isostatic

effects, cannot be entirely ignored. This is particu-larly the case during glacial–interglacial transitions

Ž .where sea level changes are large 120–130 m , anddepending on the width and depth of the continentalshelf, there can be a substantial migration of thecoastline. In general, however, the relatively narrowWestern Australian coastline is considered to betectonically stable on the 105–106 year time-scale,with the possible exception of Cape Cuvier, a site of

Ž .localised uplift Denman and van de Graaff, 1977 .Ž .The Western Australian coastline Fig. 5 has

many exposed fossil reefs of LI age. The southern-most exposure occurs at Rottnest Island on Fair-

Ž .bridge Bluff Playford, 1988 located approximately10 km offshore of Perth. This reef is dominated bythe branching coral Acropora with coral heads beingmainly of the FaÕiidae family with Porites coralbeing notably absent. A narrow discontinuous stripof LI coral reef extends northwards along the WAcoast with the Cape Range–Ningaloo Reef site hav-ing extensive exposures alongside the modern fring-ing reef of Ningaloo, the latter that extends for morethan 100 km. A typical exposure of the fossil coralsin the Ningaloo reef area is shown in Fig. 6a. At thissite, the LI reef is 2–3 m above the Mean Low

Ž . Ž .Water Spring MLWS tide Fig. 6b . Here, thecoastline fringes an arid interior, and as a result, theexposed LI reef is exceptionally well preserved.Thus, coral reefs along the stable far-field coastlineof Western Australia provide some of the most reli-able constraints on the duration and height of LI sealevels.

The TIMS U-series ages for Western AustraliaŽcorals Stirling et al., 1995, 1998; Zhu et al., 1993;

.Eisenhauer et al., 1996 are summarised in Fig. 7a ina plot of initial d

234 U vs. age. In addition to theusual criteria for identifying diagenetic alteration,such as absence of detectable calcite or secondaryaragonite, a large proportion of corals have initiald

234 U compositions that are similar to modern sea-Ž 234 .water d Us149"2 . For this reason, a rela-

tively strict criteria of d234 Us150"5 has been

Ž .adopted following Stirling et al. 1995 to excludesamples that may have been only moderately ef-fected by diagenesis. It is emphasized that this doesnot exclude the possibility that some samples that


Fig. 6. Photograph showing the typical exposure of LI reefs atNingaloo, Western Australia, and below a schematic cross-sectionof the LI reef showing heights and typical occurrence of Poritesand Acropora corals.

fall within the ‘acceptable’ d234 U range are still

affected by diagenesis and, therefore, care is requiredŽwhen interpreting single data points see later discus-

.sion . In general, surface samples from the NingalooŽ . Žreef Fig. 5 are very well preserved Stirling et al.,

.1995, 1998 , whereas a large proportion of thoseŽ .reported from Abrolhos Zhu et al., 1993 have

relatively high d234 U values outside the ‘acceptable’

Žrange. Drill-core samples from both Ningaloo Stir-. Žling et al., 1998 and Abrolhos Eisenhauer et al.,

. 234 Ž1996 also generally have higher d U values Fig..7a . In Fig. 7a, it can be seen that regardless of the

exact d234 U criteria, there is a sharp cut-off in the

distribution of ages older than 129 ka, implying thatthis is a good upper limit to the initiation of coral

reef growth. The cut-off of younger ages is not aswell defined, ranging from 116–118 ka.

When the constraint of the elevation of the coralis combined with the U-series age, a more systematic

Ž .pattern emerges Fig. 7b . The maximum height ofLI sea level of 2–4 m is reached by 128"1 ka andappears to be relatively constant until 120"1 ka, atwhich time sea level started to decrease. Aside from

Ž .isostatic considerations see later , the derivation ofsea level heights from the coral reef elevations re-quires an estimate of the coral paleo-depth. Here,these depths were probably minimums as the coralheads were probably growing at least of 0.5–1 mbelow MLWS. This is indicated by the direction of

Ž . 234Fig. 7. a Plot of initial d U vs. U–Th age for LI corals fromŽ .along the Western Australian coastline Stirling et al., 1995, 1998

Ž .and Abrolhos Islands Zhu et al., 1993; Eisenhauer et al., 1996 .The horizontal shaded band shows the acceptance criteria ofd

234 Us150"5. Samples that lie within this band are shown withŽ .solid symbols. b Plot of sea level vs. age for Western Australian

Ž .samples. The two samples indicated with ? have marginal initiald

234 U values of 160. Apart from the drill-core, most samples arelocated 2–3 m above MLWS, indicating that LI sea levels were attimes )4 m above present-day heights.


the growth axes of the dated corals, which are verti-cal, rather than horizontal, the latter being character-istic of micro-atolls that have reached their growthlimiting positions at MLWS. For this reason, it isinferred that the height of the LI sea level high-standwas at least 4 m. Younger ages of 116–118 ka are

Ž .found at lower elevations Fig. 7b and, hence, areinterpreted as the regressive sea level phase, marking

Ž .the termination of the LI Stirling et al., 1998 .Although outside the d

234 U criteria, results for adrill-core sample from Abrolhos, with an age of132.5"1.8 ka at y3.3 m and d

234 Us160"7Ž .Eisenhauer et al., 1996 , and another from Ninga-loo, with an age of 134.6"0.5 ka at y7.4m and

234 Ž .d Us160"2 Stirling et al., 1998 , can be inter-preted as indicating a continuous and relatively grad-ual increase in sea level from ;y8 m at 135 ka to

Ž .q4 m at 128 ka Fig. 7b . While this maybe aŽreasonable inference based on this data alone Stir-

.ling et al., 1998 , it is dependent on only two sam-ples, both of which have marginal d

234 U values. InSection 3.2, it will be shown that a continual sealevel rise from ;134 to 128 ka is not consistentwith other data from tectonically uplifted sites thatindicate a much more rapid rise in sea level occurredat ;130 ka.

3.2. 234U–230Th TIMS ages from Bahamas andHawaii

LI coral reefs from Bahamas and Hawaii are alsorepresentative of stable sites, although the Hawaiian

Ž .Island of Oahu Szabo et al., 1994 is undergoing aŽ .slow uplift ;0.05 mrka due to the flexure of the

oceanic crust from the large volcanic edifices. TheU-series ages vs. the initial d

234 U for BahamasŽ . Ž .Chen et al., 1991 and Hawaii Szabo et al., 1994are compared with those from Western Australia inFig. 8a,b,c. There is generally an excellent agree-ment between the results from Bahamas and Western

Ž .Australia. At Bahamas Fig. 8b , samples that meetthe d

234 U criteria, fall predominantly in the rangefrom 120–128 ka, with only one sample having aslightly older age of 130.4"1.1 ka. Glacio-isostatic

Ž .models Lambeck and Nakada, 1992 indicate that atBahamas the LI high-stand is likely to be of appar-ently shorter duration compared to Western Aus-tralia, consistent with the absence of younger ages

234 Ž .Fig. 8. Plot of initial d U vs. U–Th age for LI corals from aŽ . Ž . ŽWestern Australia Stirling et al., 1995, 1998 , b Bahamas Chen

. Ž . Ž .et al., 1991 and c Hawaii Szabo et al., 1994 .

Ž .circa 116 ka to 120 ka . Alternatively the lack ofyounger ages at Bahamas may be due to incompletesampling or lack of coral growth due to a lessfavourable environment in this region at the end ofthe LI.

Ž .LI corals from Oahu, Hawaii Szabo et al., 1994that meet the d

234 U criteria, exhibit a much widerrange of generally younger ages of from 110.5"3.8ka to 128"1.3 ka with one sample having an older


Ž .age of 132.6"3.3 ka Fig. 8c . In some cases thegreater age spread can be attributed to large analyti-cal uncertainties, but there is still remains a clearoffset towards younger ages. Several sites also ex-hibit a range in ages that is not consistent with asingle episode of reef growth in contrast for exampleto the systematic clustering of ages documented at

ŽMangrove Bay in Western Australia Stirling et al.,.1998 . There is no obvious explanation for this

substantially younger range of ages, although factorsŽ .identified by Szabo et al. 1994 , such as the con-

glomeratic nature of many of the sites, combinedwith the large depth range occupied by Porites andPocillopora, may mean that some samples werederived from depths significantly below the present-day sea level. In addition, Porites corals generallyhave a greater susceptibility to subtle effects ofdiagenesis, which is difficult to remove and is not

234 Žalways manifested by high d U values or de-.tectable calcite . These diagenetic effects may give

rise to small offsets in U-series ages of severalŽ .thousand years see later discussion . Due to these

inconsistencies, the younger Oahu ages are not con-sidered further in the reconstruction of LI sea levels.

3.3. Huon Peninsula and Barbados: tectonically up-lifted coral terraces

Tectonically uplifted coral reefs are especiallyvaluable for studies of past sea levels as they areusually dominated by ‘catch-up’ reefs that follow therising phase of sea level and, thus, provide an oppor-tunity to determine the rate of sea level rise. Thecombination of tectonic uplift and oscillating sealevels, results in younger reefs progressively onlap-ping over older terraces, generally at lower eleva-tions, producing an inverted horizontal sequence ofterraces. The two best-studied uplifted sites of this

Ž .type are the Huon Peninsula Chappell, 1974 andŽ .Barbados Edwards et al., 1987 . At Barbados, the LI

Fig. 9. Cross-section of the Huon Peninsula coral terraces showing the on-lapping sequence of progressively younger terraces. TheAladdin’s Cave site provides a window into the lower section of the LI Reef VII, approximately 80 m below the crest of the LI Reef VII.


terrace occurs at a height of q50–q60 m, corre-sponding to an uplift rate of ;0.5 m per 103 years,whereas at Huon Peninsula, the crest of the LI ReefVII occurs at q200 m, increasing to q500 m in thesoutheast, corresponding to uplift rates from 2 to

3 Ž .;5 m per 10 years, respectively Fig. 9 . The veryhigh uplift rate at Huon Peninsula has produced anexceptionally well-exposed sequence of coral ter-races that preserves a quantitative record of sea level

Ž .rise events Chappell, 1974 . This was especiallyimportant during the penultimate deglaciation, as atmost other locations an oscillation in sea level wouldbe overlain by reef growth from ensuing relativelyhigh LI sea levels. Huon Peninsula is unique in thatprecursor sea level oscillations prior to the LI havebeen rapidly uplifted and, hence, still remain ex-

Ž .posed Esat et al., 1999 .The U-series ages, initial d

234 U values and rela-Žtive sea levels, determined for Barbados Edwards et

al., 1987; Bard et al., 1990a,b; Hamelin et al., 1991;. ŽGallup et al., 1994 and the Huon Peninsula Stein et

.al., 1993; Esat et al., 1999 , are shown in Fig. 10a,b.At Barbados, the main focus of studies has been on

Žthe reef crest site at Rendezvous Hill Edwards et al.,.1987; Gallup et al., 1994; Bard et al., 1990a,b ,

which gives acceptable ages from 116–127 ka, con-sistent with those from the stable sites. In addition,an age of 129.1"0.8 ka has been reported for a

Žsample ;30 m below the LI reef crest Gallup et.al., 1994 . At Huon Peninsula, there is a concentra-

tion of ages in the range from 130 to 135 ka andyounger group of ages from 118 to 114 ka. Thisincludes ages from the ‘Aladdin’s Cave’ site whichprovides a window through to the lower basal sec-

Žtion of the LI Reef VII Esat et al., 1999; McCulloch.et al., 1999 . Aladdin’s Cave occurs in the younger

Ž .overlying reef VI ;100 ka complex, ;80 mŽ .below the top of the main LI Reef VII Fig. 9 .

Corals from the Cave meeting the criteria of d234 U

Ž . 234s150"10 Fig. 10a , form a continuum of U–230 Ž .Th ages ranging from 133 to 128 ka Table 1 ,with one exceptionally large Porites, having a

Ž .slightly younger age of 127"1 ka AC-U11 . Thislatter age maybe too young by ;1–2 ka, as thereare other well-dated samples from this site have

Ž .slightly older ;2 ka ages. Furthermore, repeatŽ .analyses of different parts of AC-U11 Table 1

show a larger than anticipated dispersion, which may

Ž . 234Fig. 10. a Plot of initial d U vs. U–Th age for LI corals fromŽ .the Huon Peninsula Stein et al., 1993; Esat et al., 1999 and

ŽBarbados Edwards et al., 1986, 1987; Bard et al., 1990a,b; Gallup.et al., 1994 . The horizontal shaded band shows the acceptance234 Ž .criteria of d Us150"5. b Plot of sea level vs. age. Vertical

Ž .arrow shows data points from Huon Peninsula Table 1 , that ifstrictly interpreted, imply an oscillation in sea level prior to thecommencement of the LI.

be indicative of subtle disturbance. With these limita-tions in mind, it is concluded that corals from Al-addin’s Cave site indicate that sea level was fromy80 to y70 m, for the period of 133–129 ka.

The interval from 133 to 129 ka maybe analogousto the Younger Dryas episode as the relatively stablesea level suggests a pause in the glacial melting,possibly due to a return to partial glacial conditions.

Ž .This is supported by cooler y6"28C SSTs de-rived from both oxygen isotopic and high precisionSrrCa analyses of the Porites in Aladdin’s CaveŽ .McCulloch et al., 1999 that give temperatures of;21"18C, substantial cooler than present-day tem-

Žperatures for equatorial oceans from this region Mc-.Culloch et al., 1996 . Younger ages of 115 and 112

ka have also been found for several corals in Al-addin’s Cave. These were probably formed during


the regressive phase, corresponding to a sea level ofŽ .;y50 m y80q30 m uplift and, hence, provide

a lower age limit to the timing of termination of theŽ .LI Fig. 10b .

Ž .At Huon Peninsula Stein et al., 1993 , there isalso a limited group of older ages of 135"2 kaŽ .Table 1 that occur at a height of ;15–20 m belowthe crest of the LI Reef VII. These samples meet the

Table 1U–Th isotope ratios and 234 U–230 Th ages for pre-LI corals

232 238 234 234 230 238a b c c d eŽ . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Sample Th ppb U ppm d U 0 ‰ d U T ‰ Thr U act Age ka

Huon PeninsulaAladdin’s CaÕe

pAC-U11a-top 0.214 2.85 108"1 164"1 0.7706"0.0009 125.5"0.9pAC-U11b-top 0.285 2.85 100"3 142"5 0.7646"0.0026 125.7"0.9pAC-U11b-bot 0.294 2.88 110"1 158"2 0.7790"0.0008 127.6"0.9

PAC-U11a-top 3.35 101"6 145"9 0.7662"0.0077 125.6"2.6PAC-U11a-bot 3.27 100"7 144"9 0.7748"0.0081 128.5"2.8

AC-U10 0.076 2.40 106"2 155"3 0.7953"0.0008 133.7"1.0AC-U10a 2.90 106"8 155"11 0.7900"0.0076 131.7"2.7AC-U12 0.075 2.44 105"2 146"2 0.7315"0.0013 115.0"0.9p

AC-U13a 0.080 2.70 107"1 155"2 0.7845"0.0009 129.9"0.9p

AC-U13b 0.058 2.70 111"2 160"2 0.7840"0.0009 128.9"0.9AC-U14 0.036 2.83 106"2 153"2 0.7831"0.0012 129.9"1.0AC-U17 0.034 2.93 102"2 139"2 0.7021"0.0010 107.8"0.8AC-U18 0.009 2.51 102"2 140"2 0.7189"0.0029 112.2"1.0AC-U19 0.008 2.23 108"2 157"2 0.7852"0.0010 129.9"0.9ACU21 0.044 2.47 102"2 148"2 0.7826"0.0012 130.6"1.0AC-U24 0.033 2.66 103"2 150"2 0.7904"0.0014 133.0"1.0Sialum Reef VII

( )HP-23 VIIb 0.191 2.68 105"1 155"2 0.8036"0.0056 136.7"1.6Ž .Stein et al., 1993HP-23a 103"4 151"6 0.7963"0.0021 134.7"1.3HP-23b 106"4 154"5 0.7900"0.0018 131.9"1.2

( )HP-22 VIIb 107"5 157"7 0.8024"0.0032 135.8"1.9SIAL-M-3 115"6 169"8 0.8039"0.0031 134.0"1.9

Ž .Barbados Gallup et al., 1994UWI-2 0.162 3.163 109"1 158"2 0.7832"0.0024 129.1"0.8

Western AustraliaŽ .Ningaloo Reef drill core Stirling et al., 1998

Ž .CR-TB-D1 y7.4 m 0.390 2.43 110"1 160"2 0.80106"0.0010 134.6"0.5Ž .Abrolhos Eisenhauer et al., 1996

Ž .TBC-9 y3.3 m 0.351 3.94 110"5 160"7 0.795"0.004 132.5"1.8

aa and b indicate duplicate analyses on the same sample. Except where indicated, all samples are from the FaÕiidae family; thesuperscript p denotes Porites corals. Analyses on FaÕiidae corals are for the wall fractions only.

b 232 232 Ž .Error in Th is dominated by the uncertainty in the Th blank correction 5"2 pg .c 234 �wŽ234 238 . Ž234 238 . x 4 3 Ž234 238 .d Us Ur U r Ur U y1 =10 . Ur U is the atomic ratio at secular equilibrium and is equal to l rleq eq 238 234

y5 238 234 234 Ž .s5.472=10 where l and l are the decay constants for U and U, respectively. d U 0 is the measured value, the initial238 234234 Ž . 234 Ž . lTvalue is given by d U T sd U 0 e , where T is the age in years.234

dŽ230 238 . Ž230 238 . Ž 238 230 .Thr U s Thr U r l rl .acte230 w230 238 x ylT Ž 234 Ž . .Ž Ž ..Ž Žlyl.T .Th-ages are calculated iteratively using 1y Thr U se y d U 0 r1000 l r l yl 1ye where T is230 230 230 234 234230

the age in years and l is the decay constant for 230 Th. l s1.551=10y10 yeary1 ; l s2.835=10y6 yeary1 ; l s9.195=10y6230 238 234 230

y1 234 Ž .year . Ages in bold type are considered ‘‘reliable’’ and have d U T values overlapping the range 150"5‰.The 2s uncertainty inM234 Ž . Ž230 238 .the age is determined by propagating the 2s analytical contribution from the parameters d U 0 and Thr U .M act


criteria of d234 Us150"10, and if strictly inter-

preted, imply a rapid oscillation in sea level thatreached ;y20 m of the present-day heights at 135ka. Alternatively, the possibility that diagenetic ef-fects may have increased their age by 4–6 ka cannotbe discounted as this is a common pattern in high

234 Ž .d U samples Gallup et al., 1994 . These possibili-ties are discussed in Section 3.4, where all availableage constraints are considered.

3.4. Rise and fall of LI sea leÕels

Using the results from both stable and tectonicallyuplifted sites, it is now possible to derive a much

Žmore reliable sea level curve for the LI period Fig..11 . Stable sites indicate that sea level reached the

present-day heights by 129"1 ka, and from 128"1to 119"1 ka, sea level was at a height of severalmetres or greater above the present-day’s. At thetermination of the LI, there is some evidence, for

Fig. 11. Summary of LI sea levels using the combined constraintsŽ .from stable Western Australian, Bahamas and uplifted sites

Ž .Huon Peninsula and Barbados . Prior to 130 ka, two possiblescenarios are shown, with the robust constraint being that sea level

Ž .is required to pass through Aladdin’s Cave y80 m at 131"1ka. The solid line shows the preferred model, but an oscillation insea level prior to the LI is also suggested by some data points.Regardless, at 130"1 ka, global sea level rose extremely rapidlyto be at or several metres higher than present-day levels by 128ka. The extremely rapid rise in sea level, equivalent to ;30–50mrka is required to account for the overlap within analytical

Ž .uncertainties "1 ka of corals ages from the Aladdin’s Cave sitewith those from stable LI sites. The LI period persisted from;129 to ;118 ka, with sea levels generally being up to 4 mhigher than today. By ;119 ka, sea levels had again started todrop, reaching a level of ;y50 m by 112–115 ka, marking areturn to the more usual peri-glacial conditions.

Žexample, from notches in wave-cut platforms Neu-.mann and Hearty, 1996 , that sea levels may have

reached heights of q6–q7 m above present-dayMSLW. From 122 to 126 ka, LI sea levels were;q4 m above MSLW. LI sea levels started todecline at ;119 ka and at 115 ka and were proba-bly at least 40–50 m below the present heights.Based on these sea level constraints, the duration ofthe LI was 10 ka, covering the period from 129 to119 ka, a similar duration to that based on orbitally

18 Žtuned d O record of benthic foraminifera Broecker,.1998 .

The timing of the penultimate deglaciation is bestconstrained by ages from corals from the Aladdin’sCave site at Huon Peninsula, where sea level isconstrained at 80"10 m below the present-dayheights at 131"2 ka. The period defined by trans-

Ž .gressive Aladdin’s Cave corals 133–129 ka is in-ferred to be a quiescent interval of relatively slow

Ž .sea level rise 2 mrka with similarities to theŽ .younger LI Dryas period see later discussion . Al-

ternatively, it cannot be ruled out that some of thedispersion in U-series ages of the corals from Al-

Ž .addin’s Cave of from 128.9 to 133.7 ka Table 1may be due to carbonate diagenesis. It is consideredhighly unlikely, however, that the ages of all of thecorals analysed from this locality have been shiftedsystematically to younger ages. Thus, there is verygood evidence that at 131"2 ka, sea level was aty80 m, and immediately following this at ;130–129 ka, a rapid rise in sea level to the present-dayheights. This scenario is also consistent with a sam-

Ž .ple from Barbados Gallup et al., 1994 that gives anage of 129"1 ka at a height of ;y30 m. A veryrapid rise in sea level at 130"1 ka is thus requiredto satisfy the age constraints from both Huon Penin-

Ž .sula Aladdin’s Cave and Barbados, together withthose already discussed from the stable LI sites,which require sea level to be at present-day heightsby 129 ka.

The earlier pre-Aladdin’s Cave record of sea levelremains, however, much less well-constrained andmore difficult to interpret although there is now anaccumulating evidence for older ;135 ka ages from

Ž . Ž .Reef VIIbc Fig. 10b at Huon Peninsula Table 1 ,as well as from the drill-core in Western AustraliaŽ .Fig. 7b . Taken at face values, the older ages implyan oscillation in sea level that commenced prior to


140 ka, reaching heights from y10 to y20 mŽ .relative to present-day by 135"2 ka, followed bya fall in sea level of at least ;60 m at 131"2 kaŽ .to satisfy the Aladdin’s Cave constraint , and fi-nally, a rise again to present-day heights by 129 ka.Although there are a number of independent lines of

Ž .evidence that support this scenario Esat et al., 1999 ,Žthey are generally based on single observations Ta-

.ble 1 and, hence, caution is still required. In particu-lar, the possibility of subtle diagenetic effects, whichincreases the apparent age and are not always de-tected using the d

234 U criteria, cannot be excluded.The other obvious difficulty with such an oscillationin sea level is that while rapid rises can be accountedfor by rapid large-scale melting and associated icesheet instabilities associated for example with Hein-rich events, a rapid decrease in sea level requiresgrowth of ice sheets, the latter being precipitationrate limited. In summary, the Aladdin’s Cave con-straint on sea level at ;y80 m at 131"2 ka isconsidered to be very robust as it is based on a

Ž .number 6 of independent analyses of individualcorals from the same site, whereas the evidence for aprecursor rise in sea level at 133–135 ka, is not aswell-constrained.

3.5. Isostatic corrrections

The observations discussed so far are based onrelative sea levels, i.e. the height of sea level relativeto the present-day. Even in the far-field sites, such asWestern Australia, isostatic effects due to the adjust-ment of the lithosphere to changing ocean water loadcan be important, especially immediately following arapid rise in sea level as what occurred during the

Ž .penultimate deglaciation. Eustatic sea level esl isŽ .defined as Lambeck and Nakada, 1992 :

D s r =DŽ .esl ice ice vol

r r =ocean surface area .Ž .water

For a step-wise increase in esl, the response inrelative sea level at far field sites is registered as atransient overshoot that then decays exponentiallyŽ .Fig. 12a . The amount of overshoot depends on themantle rheology and the rate and magnitude of sealevel rise. Based on the calculations of Lambeck and

Ž .Nakada 1992 , with a standard mantle rheology, a

Ž .Fig. 12. a Diagram showing the effects of hydro-isostacy at afar-field ‘stable’ continental margin site such as the WesternAustralian. For a step-function increase in eustatic sea level, thereis an initial overshoot in the relative sea level that decays expo-nentially due to the coastal margin re-adjusting to the increasedwater load. Isostatic effects are thus more pronounced at the

Ž .commencement of the LI. b The observed relative sea levelcurve for Western Australia and the corrected eustatic curve.

transient overshoot or apparent sea level high-standof 2–3 m is anticipated at the commencement of theLI. Observations of sea level heights vs. age, how-ever, show the opposite trend of increasing ratherthan decreasing sea level towards the mid-latter partsof the LI. This can be explained by compensatingdecrease in esl as present-day sea levels is ap-proached. This would correspond, for example, todelayed melting of ice sheets as what occurred in theHolocene. Furthermore, if an oscillation in sea leveloccurred during the penultimate deglaciation, thiswould have the effect of extending the deglaciationperiod and would be expected to reduce the magni-tude of the overshoot. Quantitative modeling is re-quired to evaluate these effects.


4. LI SSTs

4.1. LI SSTs from Western Australia

Present-day climate along the Western Australiancoastlines is moderated by the warm Leeuwin cur-rent which flows from Indonesia, southwards alongthe Western Australian coast and then eastwardaround Cape Leeuwin into the Great Australian Bight.The winter SSTs vary according to a combination ofthe normal winter seasonal cooling, the timing ofonset of the Leeuwin current maximum flow and theabsolute warmth of the current itself. During the

Ž .summer months January–February , the flow of theLeeuwin current is at its minimum and, hence, itseffects are substantially reduced. For these reasons,the minimum and maximum SSTs at Ningaloo reefcan vary from year to year by as much as severaldegrees. The winter SSTs are generally warmer withrespect to the mean when the Leeuwin current maxi-mum flow arrives early in the season and moderatesthe effects of winter cooling. The Leeuwin current isat is warmest when the Western Pacific Warm Poolis also at its warmest.

Ž .Satellite 113.58E, 21.58S; Fig. 13a and SrrCaSST records are shown in Fig. 13b with the mid-outer

Ž .reef calibration from the GBR Figs. 3 and 4 beingutilised for the Ningaloo coral. In general, there is agood agreement between the modern Ningaloo coralSrrCa and satellite SSTs, particularly during thesummer periods, implying a consistent calibration.Satellite SST records, which extends from 1981 to1996, however, averages a large oceanic regime,which, compared to the inshore Ningaloo Reef re-gion, has a damping effect on the SST. This dampen-ing effect is most pronounced during the winter,where more extreme cooling occurs in the inshoreregion. Consequently, the coral SrrCa record pre-serves a near-shore, shallow, and hence, more rapidlychanging SST record which exhibits greater wintercooling.

The fossil coral core used in this study wascollected from an LI reef at Vlaming Head on the

Žnorthern tip of Cape Range, Western Australia Fig..5 . The exposed portion of the reef was 1.5 km long

and 20–40 m wide and is bounded by the modernŽsea level and Holocene aeolian dune deposits Fig.

.6a,b . The ;3 m diameter coral is in an upright

Ž .Fig. 13. a Satellite SSTs from the WA coastline showing theŽ .variability in SSTs over several decades. b Comparison of

satellite vs. SrrCa SSTs for a modern coral from Ningaloo Reefusing the same calibration from the mid-outer reefs of the GBRŽ .Fig. 5 . Cooler winter SSTs that are recorded from SrrCa ratiosin the modern coral are consistent with greater inshore cooling,which is not evident in the large grid-sized satellite data.

growth position, at an elevation 2.74 m above theMLWS, 15 m from the wave cut front edge of theterrace. A reliable date could not be obtained fromthis Porites coral but other species of coral withinthis terrace had an age span of 128–122 ka years BPŽ .Stirling et al., 1998 . XRD of this coral revealed noobvious secondary material. Mild leaching experi-ments undertaken using 0.1 M HCL, however, indi-cated the presence of secondary aragonite precipitatewhich has the effect of increasing SrrCa ratios and,hence, lowering SSTs. After a careful pre-cleaninginvolving a sequential acid leaching treatment, the LIcoral gave reproducible summer maximum tempera-tures ranging from 268C to 288C and winter mini-mums ranging from 238C to 208C with an annual

Ž .mean temperature of 248C Fig. 14 . This is similarto, but slightly cooler than the modern coral. The


Fig. 14. LI SSTs obtained from SrrCa ratios in Porites coralfrom Ningaloo Reef in Western Australia and for comparison ofSSTs from a modern Ningaloo coral.

period, for example, from 1987–1993, has summermaximums of up to 298C and winter minimums of218C. The good agreement between the LI and mod-ern coral SST records, in both the mean SSTs, aswell as seasonal amplitude, is consistent with the LISSTs at Ningaloo Reef being closely linked to theLeeuwin Current, as is the case today. Compellingevidence for a similar or only slightly cooler climateregime in the LI is the range of winter minimumtemperatures of 21"18C, compared to 22"18Ctoday, which is probably closely linked to the strengthand temperature of the Leeuwin current. Like themodern record, the LI SST coral record also showsdecadal variations, one period being very similar tothe present-day climate and the others, 0.58C and0.98C cooler.

4.2. LI SSTs from Huon Peninsula

At Huon Peninsula, a Porites coral near the backŽ .of the Reef VII lagoon VIIa above Sialum has been

analysed for SrrCa. It is compared with two moderncorals, one from the rear of an equivalent lagoonal

Ž .setting Walingai , and the other, from the modernlagoon entrance near Sialum. The SrrCa ratios forthe two modern corals encompass the variation ex-hibited by the LI coral. Although a detailed timeseries calibration of SrrCa vs. SST has not yet beenundertaken at Huon, the similarity in SrrCa ratiosbetween the modern and LI corals clearly impliesSSTs that are essentially identical in both the annualmean of ;298C and the seasonality ;"28C. Onthis basis, a modern calibration similar to inshoreOrpheus Island region of GBR has been derived of:Ž .10=SrrCa 1000s10.7–0.06T. Using this calibra-tion, the Porites coral from Aladdin’s Cave gives

Žtemperatures that are 6"28C cooler McCulloch et. Ž .al., 1999 than the present-day Fig. 15 . This is a

minimum difference as the mid-outer reef GBR cali-bration gives temperatures that are even cooler. Ifsignificant shifts occur in the SrrCa ratio of glacialseawater as has been modeled by Stoll and Schrag

Fig. 15. LI SSTs for a Porites coral from the LI Reef VIIa atHuon Peninsula. The SrrCa ratios, and hence, SSTs are indistin-guishable from the LI compared to modern corals. The Aladdin’s

Ž .Cave coral McCulloch et al., 1999 shows significantly coolerSSTs consistent with being formed during a period of low sea

Ž .level 80 m lower than today .


Ž .1998 , then the offset to cooler SSTs would bereduced from 68C to 48C.

5. Discussion

5.1. Comparison of LI and modern SST

There is a very close similarity between the LIand modern SSTs, at both Huon Peninsula and West-ern Australia. A LI Porites coral from WesternAustralia, located 2–3 m above present-day sea lev-els, has the same range of summer temperatures, and

Ž .only slightly cooler ;18C winter SSTs than themodern coral. This is at first sight, an unexpectedresult as sea levels were higher in the LI, implyingmore extensive melting of the ice sheets, and hence,presumably warmer conditions. Coupled ocean–

Žatmosphere climate models for the Eemian Montoya.et al., 1998 provides insights into these observa-

tions. As expected, the models show strong summerwarming of 4–58C in the centres of the NH conti-

Žnents e.g. North America, Central Europe, Middle.East and Siberia in direct response to increased NH

summer insolation and reduced albedo. For the sameŽ . Žperiod in the southern hemisphere SH i.e. SH

. Žwinter , the continental interiors South Africa, South.America and Australia show some mild warming

while SSTs are slightly cooler, the latter being con-sistent with observations from the LI coral fromWestern Australia. In the NH, winter temperaturesappear to be reduced in South Asia due to reduced

Ž .winter insolation. The SH summer SSTs are notstrongly affected due to the greater heat capacity ofthe southern oceans which dominate this region. Theobservations from both Huon Peninsula and WesternAustralia of similar summer and slightly cooler win-ter temperatures to present-day are thus generallyconsistent with LI climate models for the SH. Itwould clearly be of interest to test whether thewarming in the NH summer occurred as predicted bythe LI models.

One paradox that still remains unresolved, is thatduring the LI, corals extended their habitat to moresoutherly locations along the Western Australian

Ž .coast, such as the Rottnest Island Playford, 1988 ,offshore Perth. Although still contentious, this couldhave been in response to increased summer maxi-

mum rather than higher winter minimum SSTs. Stud-ies of coral growth rates at high latitudes in JapanŽ .Fallon et al., 1999 indicate that calcification occursmainly in the summer period with coral growthessentially ceasing below 188C. This suggests thatduring the summer months of the LI, the Leeuwincurrent was more intense, and hence, delivered alarger volume of warm water to the southern coast ofWestern Australia. A modern analogue may be thehigh latitude reefs of southern Japan, where coralcalcification occurs mainly in the summer months

Žbeing dependent on the warm Kushuro current Fal-.lon et al., 1999 .

5.2. Sea leÕels during the penultimate and lastdeglaciations

For the most recent deglaciation, U-series TIMSages for submerged coral terraces from BarbadosŽ . Ž .Bard et al., 1990a,b , Tahiti Bard et al., 1996 and

Ž .Huon Peninsula Edwards et al., 1993 have nowprovided a relatively detailed record of the rate

Ž .Holocene sea level rise Fig. 16 . Radiometric dat-ing, combined with studies of coral genera distribu-tions has been used to constrain the rate, magnitudeand timing of glacio-eustatic sea level changesŽ .Fleming et al., 1998 during the last deglaciation.Detailed studies of the elevations and ages ofdrowned A. palmata reefs from the Caribbean have

Ž .been interpreted Blanchon and Shaw, 1995 asshowing significant discontinuities in the rate of sea

Ž .level rise. These discontinuities at 14.2 H-1 , 11.5Ž .H-0 and 7.5 ka and have been interpreted by

Ž .Blanchon and Shaw 1995 as indicating catastrophicsea level rise events of )45 mmryear compared tolong-term average rates of ;10 mmryear. Theduration of the sea level rise events appears to berelatively short, -200"50 years, corresponding torises in sea level from 13.5 to ;6.5 m, respectivelyŽ .Blanchon and Shaw, 1995 . The results from HuonPeninsula are broadly consistent with those from

Ž .Barbados with a discontinuity at ;11–12 ka H-0 .The results, however, from Tahiti plot below bothHuon Peninsula and Barbados show a linear increasein sea level from ;14 to 9 ka, validating the older

Ž .H-1 event meltwater pulse A , but raising somedoubts about the magnitude of the younger H-O‘event’. At present, it is not clear whether the differ-


Ž .Fig. 16. a Sea-level curve for the last deglaciation based on234 230 Ž . 14 ŽU– Th solid symbols and corrected C ages open sym-

. Žbols for drowned corals from Barbados Bard et al., 1990a,b;. Ž .Fairbanks, 1989 , Tahiti Bard et al., 1996 and Huon Peninsula

Ž .Edwards et al., 1993 . Sea level is constrained to lie above thedata points. The vertical arrows show the occurrence of two majormeltwater pulses equivalent to the Heinrich events H-1 and H-0.These occurred at 14.2"0.1 and 11.5"0.1 ka and indicate pulsesrapidly increasing in sea level, equivalent to rates of )45 mrka,producing sea level rises of ;13.5"2 and 7.5"2 m over

Ž-290"50 and -160"50 years, respectively Blanchon and.Shaw, 1995 . The meltwater pulses bracket the Younger Dyras

Ž . Ž .YD episode shaded which occurred when sea-level was at aheight of ;y55 m. Summer insolation curves are shown at 658N

Ž .July and 658S January. b Equivalent sea level curve for thepenultimate deglaciation together with summer insolation curvesat 658N July and 658S January. The Aladdin’s Cave site indicatesa pause in sea level rise that maybe equivalent to either theYounger Dryas or the period immediately prior to H-1. The 70–80m rise in sea level that occurred at 130"1 ka is extremely rapid,equivalent to a meltwater pulse =2–3 larger than the combinedeffects of H-1 and H-0 Heinrich events of the last deglaciation.Vertical arrow shows the coral data that are older than those fromAladdin’s Cave suggestive of a possible precursor oscillation insea level prior to the LI. Solid line shows preferred sea leveltrajectory.

ent depth range of coral species in the Tahiti studyŽ .Bard et al., 1996; Montaggioni et al., 1997 is asignificant factor or if the magnitude of the H-0evidence is smaller than previously inferred. Despitethese uncertainties, meltwater pulses appear tobracket the Younger Dryas episode, and during this

Ž .interval ;14–11.5 ka , the pause in the rate of sealevel rise indicates a partial return to cooler glacialconditions.

The sea level curve for the penultimate deglacia-Žtion shows many similarities to the Holocene Figs.

.16, 17 except that sea level rose much more rapidly.During the penultimate deglaciation, the final 70–80m rise in sea level, occurred over an interval of 1–2

Ž .ka i.e. at 130"1 ka , whereas during the mostrecent deglaciation, the equivalent increase in sea

Fig. 17. Comparison of sea levels and the 658N July insolationcurves for the last and penultimate deglaciations. The upper andlower time scales refer to the Last Interglaciation and Holoceneperiods, respectively. For both periods of deglaciation, meltingcommenced when the 658N July insolation was close to the sameminima of ;420 Wrm2. The penultimate deglaciation wascharacterised by an extremely rapid rise in sea level resulting inthe LI highstand occurring ;2 ka before the NH insolationmaximum. In contrast, the most recent deglaciation had a muchslower final phase of sea level rise, with the high-stand not beingreached until ;3 ka after the NH insolation maximum. Thesedifferences are attributed to both the larger amplitude and morerapid rate of increase of NH insolation during the LI. Theexcellent overall correlation between NH forcing and sea levelvalidates the Milankovich theory and confirms the reliability oforbitally tuned time scales such as SPECMAP. The end of the LIperiod occurred at ;118 ka, at which time the NH summerinsolation had decreased to 425 Wrm2, the same as the present-day value.


Ž .level took ;7 ka i.e. from 14 to 7 ka . Thus, thepenultimate deglaciation is extremely rapid, equiva-lent to a meltwater pulse =2–3 larger than thecombined effects of H-1 and H-0 Heinrich events ofthe last deglaciation. It requires a rate of sea levelrise of ;50 mrka, similar to that for the H-1 orH-0 events, but continuing for 1000–2000 years,compared to ;300 years for H-1. This massivemeltwater pulse for the penultimate deglaciation mayhave resulted from a combination of both the higher

Žabsolute LI summer insolation Berger and Loutre,. 2 21991 of ;490 Wrm compared to ;470 Wrm

for the Holocene, a relative increase of ;40% onbase levels. There was also a much a more rapid rateof increase in NH insolation during the penultimatedeglaciation compared to the Holocene deglaciationŽ .Fig. 17 . By 129 ka, the end of the LI ‘Dryas’period, NH insolation was already greater than theHolocene maximum and still rapidly increasing andcould thus have triggered a single very large catas-trophic sea level rise event.

The very rapid rate of sea level rise inferred fromthe coral record for the penultimate deglaciation ismuch faster than that indicated by marine oxygen

Žisotope records. Marine oxygen isotope records e.g..Raymo, 1997; Broecker and Henderson, 1998 typi-

cally show that the complete penultimate deglacia-tion occurred over ;10 ka and that the final ;80m of sea level rise took at least 5–6 ka. The timingof the oxygen isotope records, however, depends onassumptions of constant deposition rates and theserecords are also susceptible to ‘‘smearing’’ from theeffects of bioturbidation. Furthermore, the phase re-lationships between ocean temperature and ice sheetmelting, both of which influence the oxygen isotoperecords, may not remain constant.

5.3. MilankoÕitch and SH forcing

From the well defined chronology of the penulti-mate deglaciation, together with that already existingfor the last deglaciation, it is now possible to directlycompare the forcing conditions that led to the initia-tion of deglaciation. In Fig. 17, the Holocene and LIsea level curves are shown plotted together. For boththe Holocene, as well as LI periods, deglaciationappears to have commenced when the 658N Julyinsolation curves are at or close to their minimum of

;420 Wrm2. This is a somewhat surprising obser-vation bearing in mind that ice-albedo effects arethought to cause significant lags in the response ofice sheets to insolation changes, and clearly impliesthat at the glacial maxima, ice sheets, particularlythose that are marine-based, were extremely unstableand highly vulnerable to collapse. Although there aresome differences between the last and penultimatedeglaciations, these are mainly attributable to differ-ences between the forcing conditions, that is, therelative magnitude and rate of increase of the NHsummer insolation. There is clearly a strong coher-ence between the last and penultimate deglaciationsand their respective NH summer insolation curves.This confirms the general validity of the Mi-lankovitch theory and shows that orbitally tuned

Žtime-scales such as SPECMAP e.g. Imbrie et al.,.1992 to first order, provide a reliable chronology for

much of the d18 O variations found in oceans. It is

apparent, however, that there is no simple relation-ship between the timing of the NH summer insola-tion maxima and the commencement of the sea levelhigh-stand. In the LI, the high-stand commenced;2 ka prior to the NH insolation maximum, whilein the Holocene, the high-stand occurred ;3 kalater. Thus, differences between insolation curvesneed to be taken into account if orbital tuned time-

Ž .scales are to be used for finer scale -"2 kachronologies.

There is also a growing body of evidence mainlyŽfrom Antarctic ice cores Blunier et al., 1998; Steig

.et al., 1998; White and Steig, 1998 that manyclimatic events appear to be initiated from processes

Žoriginating in the SH for a review, see Broecker and.Henderson, 1998 . This suggests that southern, rather

than NH summer insolation may also play an impor-tant role in the instigation of the earliest phase of

Ž .deglaciation Shaffer et al., 1996 . The peak of theŽ .SH summer insolation 658S January is reached at

137 and 20 ka, respectively, close to the time whenthe glacial conditions ended and the transitions fromglacial to interglacial conditions commenced. As-suming a lag in the ice sheet response of )4 ka,similar to that in the Holocene, then warming mayhave commenced as early as ;142 and 24 ka,respectively, when the SH summer insolations were

Ž .still increasing Fig. 16 . Changes in the SH insola-tion could thus have modulated global climate, possi-


Fig. 18. Schematic diagram illustrating the effect of asymmetricpeak summer heating on the Hadley circulation. Displacement of

Ž .the peak summer heating fo to the SH significantly strengthensthe NH winter Hadley circulation. At times of particularly highSH insolation, such as prior to the LI, this could be a mechanismto trigger deglaciation. Diagram modified from Lindzen and HouŽ .1988 .

bly by influencing the meridional or Hadley circula-Žtion rising from the tropics Lindzen and Hou, 1988;

.Hou and Lindzen, 1992; Lindzen and Pan, 1994 , asa consequence of relatively small displacements ofzonally averaged surface temperature maximum away

Ž .from the equator Fig. 18 . The Hadley circulation isŽthe main means of seasonal primarily into the win-

.ter hemisphere transport of heat from the tropics tothe poles. Relatively small displacements of the zon-ally averaged surface temperature maximum in sum-mer away from the equator can have a profoundinfluence on the heat fluxes of the winter hemi-

Ž .sphere. It has been shown Lindzen and Pan, 1994that as little as an 88S displacement of the summersurface temperature maximum will result in an ;=6increase in the intensity of the NH Hadley winter

Ž .cell Fig. 18 . During periods of high SH summerinsolation, the increased heat flux into the NH wintermay have led to the destabilisation and melting ofthe most vulnerable ice sheets that were at theirsouthern-most limits. Subsequent large-scale meltingand ice sheet collapse would then have been drivenby the increasing NH summer insolation. Southernhemisphere forcing may be especially important indriving ice sheet instabilities associated with theprecursor oscillation in LI sea level that reached amaximum at 135 ka, requiring sea level to havecommenced rising before 140 ka, when SH insola-

Ž .tion was still approaching its maximum Fig. 16b .

The LI sea level curve also enables constraints tobe placed on the insolation level at which there is areturn to the more usual glacial conditions. For theLI sea level, commenced falling at about 119 kapassing through the present-day heights at ;118 ka,at which time the 658N July insolation had decreasedto ;420 Wrm2. Interestingly, this is the same as

Ž .the present-day value Fig. 19a , suggesting that if

Ž .Fig. 19. a,b Plot showing the possible relationship between thelarger amplitude of the NH isolation and increased insolation dueto Greenhouse warming. Doubling of atmospheric CO is approx-2

imately equivalent to an insolation increase of ;4 Wrm2, but incontrast to orbitally induced changes in insolation, Greenhouseforcing effects both the summer and winter seasons, as well as thenorthern and SHs. Thus, relatively small increases in insolationfrom Greenhouse forcing may have relatively large effects. Globalsea levels for the LI were 2–4 m higher than modern levels, even

Ž .after allowance is made for transient isostatic effects Fig. 12 .Greenhouse forcing has increased NH summer insolation to simi-lar levels as those reached during the LI. This, combined withincreases in SH insolation, indicates that a rapid rise in modernsea levels of several metres due to melting of, for example, theWest Antarctic ice sheet, is possible. Thus, by analogy with the

Ž 2LI, increases in sea level of 2–4 m over short time scales 10.years is possible, although the timing of such event remains

uncertain. In the absence of Greenhouse forcing, the decreasinginsolation of the NH implies that the Earth would, within the next1–2 ka, re-enter glacial conditions.


the natural course of events had prevailed, then weŽ .would soon within 1–2 ka begin to return to glacial

conditions.

5.4. Implications for future sea leÕels in a green-house-modified Earth

An obvious question is how will greenhouse-in-duced warming modulate the ‘natural’ glacial cycleand what will be the impact on future sea levels?Comparisons with the LI provide some insights, butas already pointed out, the conditions that occurredduring the LI are not identical. Greenhouse forcingaffects both the summer and winter seasons, andmost importantly, greenhouse gases are distributedthroughout the northern and SHs. With these limita-tions in mind, it is still, nevertheless, useful to makesome comparisons. The difference between LI andHolocene maximum insolation for the 658N July isshown in Fig. 17 as DQNH and is equal to about 15Wrm2. This corresponds to a mean annual increaseof ;2 Wrm2 relative to the present-day valuesŽ . 2Montoya et al., 1998 , compared to an ;4 Wrm

Žincrease predicted from doubled CO Schneider,2.1994 , but the latter being a global average. Clearly,

the effects of greenhouse forcing are of a comparablemagnitude to the differences in insolation betweenthe Holocene and LI. If we consider the effects ofincreased NH summer insolation alone, then‘mankind’s greenhouse experiment’ has been con-ducted at an ideal time, with the present-day NHinsolation curve being close to its minimum. Thus,by analogy with the LI, an increase of several Wrm2

in NH insolation may have the arguably beneficialeffect of prolonging our current interglacial periodby several thousand years.

Of most concern, however, is the observation thatduring the LI, relative sea levels were at least 2–3 m,and at most times, up to 4 m higher than thepresent-day levels. Although still controversial, theperiod near the termination of the LI also appears to

Žbe more susceptible to instabilities Kukla et al.,1997; Neumann and Hearty, 1996; Stirling et al.,

. Ž .1998 characterised by even higher 6 m sea levels.There is a substantial uncertainty regarding the re-sponse of the two major ice sheets Greenland, and

Žespecially, Antarctic to global warming e.g. Bent-.ley, 1997 . The growth or decay of an ice sheet

represents a balance between the amount of snowfallaccumulated, and the extent of summer melting. Inaddition, there are strong feedbacks with, for exam-ple, initially small increases in sea level releasingpartially grounded ice sheets, which in turn results in

Ž .further increases in sea level Weertman, 1976 .During the LI lower summer, insolation in theAntarctic resulted in cooler mean annual tempera-

Ž .tures in this region Montoya et al., 1998 , whichmay have helped to stabilise the Antarctic ice sheet.This may mean that the higher LI sea levels were aresult of more extensive melting of the Greenland icesheet, although this could at most, only account for

Žseveral metres of additional sea level rise Fleming.et al., 1998 . It is thus likely that the West Antarctic

ice sheet still contributed to the higher LI sea levelspossibly as a result of rapid retreat of the grounding-

Ž .line Weertman, 1976 .The overall contribution of glacial melting to sea

level rise is presently thought to be relatively smallŽ .IPCC 95 , being ;10–15 cm in the next centuryŽ .Houghton et al., 1996 . The combined estimates ofcontributions from the thermal expansion of theoceans together with glacial melting give a total rateof sea level rise of ;5 mmryear or ;50 cm forthe next century. This latter estimate does not, how-ever, take into account the possibility of catastrophicmelting of glaciers and the release of grounded icesheets such as in the West Antarctic. By analogywith the LI, there is clearly the potential for at least apartial collapse of the West Antarctic ice sheet as aresult of the effects of Greenhouse forcing, combinedwith increased SH summer insolation. While thepotential now exists for greenhouse warming to trig-ger such a collapse, and hence, a very rapid sea levelrise of several metres over an ;50–100 year pe-riod, it is still uncertain as to when in the nextmillenium will this occur.

6. Conclusions

Using high precision TIMS analyses of 234 U–230 Th in corals from both stable and tectonicallyuplifted sites, it is now possible to put much moreprecise and reliable constraints on the rise and fall ofLI sea levels. A major period of deglaciation oc-curred at 130"1 ka, with present-day sea levels


being reached by 129"1 ka. From 128 to 119 ka,sea level was 2–4 m higher, but SrrCa ratios incorals record indicate that in the SH, SSTs weresimilar or possibly slightly cooler than today. Theseobservations are broadly consistent with the Mi-lankovitch theory, that is, the high insolation of theNH summer was mainly responsible for the largescale melting of the NH ice sheets which resulted ina total sea level rise of over 130 m compared to theglacial maximum.

During the penultimate deglaciation, there wasmuch more rapid and sustained increase in sea levelcompared to the last deglaciation. The final 80 m ofsea level rise took only ;2 ka during the penulti-mate deglaciation compared to ;7 ka for the lastdeglaciation, which is attributed to the greater magni-tude and more rapid increase in NH insolation thatoccurred at the initiation of the LI. This catastrophicsea level rise event that marked the initiation of theLI is a factor of =10–20 faster than the currentestimates of sea level rise due to global warming

Žwith business as usual scenarios Houghton et al.,.1996 . By analogy with the LI, the possibility of a

very rapid 2–4 m increase in sea level cannot bediscounted, especially if the Earth again steps into adeglaciation mode. We are perhaps fortunate that forthe past ;6000 years, we have lived in an inter-glacial period with unusually stable sea level, andsummer insolation in the NH, the main driver ofglacial–interglacial changes, decreasing steadily.

Acknowledgements

We are grateful to the reviewers, especially toProfessor Kurt Lambeck for the helpful commentsand suggestions. Malcolm McCulloch and Tezer Esat,like many former students and colleagues, are in-debted to Professor Wasserburg for his inspirationand the superb example he provided in the rigorouspursuit of scientific excellence.

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The coral record of last interglacial sea levels and sea surface temperatures