Geol Rundsch (1996) 85 :606–614 Q Springer-Verlag 1996

ORIGINAL PAPER

A. Eisenhauer 7 Z. R. Zhu 7 L. B. CollinsK. H. Wyrwoll 7 R. Eichstätter

The Last Interglacial sea level change:

new evidence from the Abrolhos islands, West Australia

Received: 26 January 1996 / Accepted: 25 May 1996

A. Eisenhauer (Y)Geochemisches Institut, Goldschmidtstr. 1, D-37077 Göttingen,Germany

Z. R. Zhu 7 L. B. CollinsSchool of Applied Geology, Curtin University of Technology,Bentley, W.A., 6102 Australia

K. H. WyrwollDepartment of Geology and Geography, University of WesternAustralia, Nedlands, W.A., 6009, Australia

R. EichstätterHeidelberger Akademie der Wissenschaften, INF 366,D-69120 Heidelberg, Germany

Abstract U-series ages measured by thermal ionisationmass spectrometry (TIMS) are reported for a Last In-terglacial (LI) fossil coral core from the Turtle Bay,Houtman Abrolhos islands, western Australia. Thecore is 33.4 m long the top of which is approximately5 m a.p.s.l. (above present sea level). From the 232Thconcentrations and the reliability of the U-series ages,two sections in the core can be distinguished. Calcu-lated U/Th ages in core section I (3.3 m a.p.s.l to11 m b.p.s.l) vary between 124B1.7 ka BP(3.3 m a.p.s.l.) and 132.5B1.8 ka (4 m b.p.s.l., i.e. belowpresent sea level), and those of section II (11–23 m b.p.s.l.) between 140B3 and 214B5 ka BP, re-spectively. The ages of core section I are in almost per-fect chronological order, whereas for section II no clearage–depth relationship of the samples can be recog-nised. Further assessments based on the i234U(T) crite-ria reveal that none of the samples of core section IIgive reliable ages, whereas for core section I severalsamples can be considered to be moderately reliablewithin 2 ka. The data of the Turtle Bay core comple-ment and extend our previous work from the HoutmanAbrolhos showing that the sea level reached a height ofapproximately 4 m b.p.s.l at approximately 134 ka BPand a sea level highstand of at least 3.3 m a.p.s.l. at ap-proximately 124 ka BP. Sea level dropped below itspresent position at approximately 116 ka BP. Although

the new data are in general accord with the Milanko-vitch theory of climate change, a detailed comparisonreveals considerable differences between the Holoceneand LI sea level rise as monitored relative to the Hout-man Abrolhos islands. These observation apparentlyadd further evidence to the growing set of data that theLI sea level rise started earlier than recognised bySPECMAP chronology. A reconciliation of these con-tradictionary observations following the line of argu-ments presented by Crowley (1994) are discussed withrespect to the Milankovitch theory.

Key words Sea level 7 Last interglacial 7 Milankovitch 7TIMS-U/Th 7 Eem 7 Palaeoclimatology 7 Abrolhos 7Australia

Introduction

From the age–depth relationship of suitable coral coresamples chronological information and rates of sea lev-el rise can be gained which are important to reconstructpast climatic and environmental changes (Mesollela1967). In particular, the dating of fossil reefs can beused to determine the onset and timing of the majordeglaciations (e.g. termination I and II), which is im-portant for the understanding of the mechanisms driv-ing the glacial–interglacial cycles (Edwards et al. 1987a;Chen et al. 1991). Scientific efforts which were made tocompare the Holocene and Last Interglacial (LI) sealevel history with the Milankovitch theory of climate(SPECMAP; Imbrie 1984) showed that they are in gen-eral accord for the Holocene time period. However,concerning the Last Interglacial maximum (LIM) thereis a growing set of precise U/Th age data from coralreefs around the world which show that the LI periodstarted earlier and lasted longer than recognised fromthe SPECMAP time table (Stirling et al. 1995; Szabo etal. 1994). These findings support the observations fromthe Devils Hole calcite vein (Winograd et al. 1992) thatthe Milankovitch sea level relationship at termina-

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tion II are fundamentally different from those relatedto termination I. However, the number of reliable U/Thages which have pointed to the likelihood of an earlyhigh sea level prior to the LIM have been limited (Stirl-ing et al. 1995) and suffer to a large extent from prob-lems of diagenetic alteration and post-depositional mi-gration of U- and Th-isotopes (Stirling et al. 1995; Bardet al. 1991; Edwards et al. 1987a). The i234U(T) criterionto test the reliability of a calculated U/Th age is neces-sary, but may not be sufficient in all cases. Thus, thereis no self-consistent criterion available to test whetheror not a U/Th age can be considered reliable. In addi-tion, those studies which have provided reliable ageswere taken from scattered outcrops with restricted stra-tigraphic control.

In this study we present U/Th dating of coral coresamples from an LI coral core from West Wallabi Is-land (Houtman Abrolhos, western Australia) whichcomplement and extend our previous LI studies (Zhuet al. 1993). The goal of this study is to provide a strati-graphically controlled series of U/Th dates which al-lows a reconstruction of the sea level rise prior to theLIM relative to the western Australian margins.

Core location and sample material

The core analysed in this study was taken from the Tur-tle Bay reef which is located at the northern tip of theEast Wallabi Island, Wallabi Group (Fig. 1). The coreobtained on the Turtle Bay reef is 33.4 m long. TheTurtle Bay reef has an elevation of 5 m a.p.s.l. The ex-posed part of the coral reef framework is domal inshape and the highest corals in growth position occurca. 4 m a.p.s.l. Coral skeletons, consisting of a suite ofwell-developed, thick branching, platy and headcorals,including Acropora, Platygyra, Favites, and Goniopora,comprise 60–80% of the framework. The coral frames-tones are overlain by semi-lithified, coarse sand- togravel-sized skeletal grainstones, with a hard calcretesurface (ca. 0.5 m thick). The upper 1.4 m of the coreconsist of skeletal grainstone, as seen in outcrops. Thetop part of the grainstones is significantly calcretizedand well lithified. From the depths of 1.4–11 m, thecore consists of mainly coral framestones, with minorcoralline algal bindstones and skeletal grainstones fill-ing in coral intraskeletal cavities. In situ branching andplaty corals (mostly Acropora) form rigid frameworkand abundant material is available for radiometric dat-ing. The recovery of this part of the core is relativelyhigh, varying from 50 to 100%. From 11 m down to30 m, the lithofacies change into branching coral and/orrudstone facies. The corals there are mostly thinbranching species, and commonly encrusted and filledwith carbonate micrite, forming an open and porousframework. The recovery of this part is low, averagingca. 20%. At the depth of 30 m in the core exists anotherwell-lithified calcrete surface. Calcretization and me-

Fig. 1 The location of the Turtle Bay at the western end of EastWallabi Island

teoric diagenesis of the reefal limestones near the sur-face (ca. 1 m thick) are intense, indicating a long-timeexposure and formation of an unconformity. Below theunconformity is a suite of well-lithified skeletal grain-stone, coral framestone and coralline algal bindstone.Recovery of this part of the core is 100%, but the mate-rial is not suitable for TIMS dating because of the in-tensive diagenetic alteration.

Nine coral samples have been selected from the corefor mass-spectrometer dating. The samples consist ofthick branching and platy Acropora corals which werecarefully prepared to remove visible encrustations, bor-ings and infills of internal sediments and cements. Thecut blocks were ultrasonically cleaned twice, each timefor 5 min. The mineralogy of the samples was examinedby X-ray diffraction, and the petrology was investigatedusing thin sections. The XRD analysis shows that thecleaned samples consist of nearly pure aragonite (Ta-ble 1). Thin-section examination shows that the coralskeletons sampled and analysed have retained their ori-ginal fibrous crystal morphology without apparentchange of skeletal aragonite to calcite. Marine cemen-tation and internal sediment deposition in the preparedsamples are generally scarce.

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Table 1 U- and TH Concentrations and Isotope Ratios of the Turtle Bay Coral Core Samples

No. Samplelabel

Elevation(m)

232Th(ppb)

238U(ppm)

u234U(0)(‰)

u234U(T)(‰)

230Th/238Uactivity ratio

Ages(ka)

Aragonite(%)

123456789

1011121314

TBC4TBC5TBC582TBC6TBC7TBC8TBC9TBC13TBC14TBC20TBC19TBC15TBC16TBC1682

c 3.3c 3.3

c 1.0P 0.6P 1.7P 3.3P 6.0P11.0P13.1P16.8P19.6P23.0

0.340B0.0030.78 B0.030.369B0.0030.209B0.0010.383B0.0050.313B0.0030.351B0.0040.605B0.0020.678B0.0071.864B0.0066.22 B0.024.41 B0.034.6 B0.14.81 B0.06

3.639B0.0074.352B0.0054.349B0.0063.813B0.0054.133B0.0064.16 B0.013.945B0.0073.923B0.0073.342B0.0053.893B0.0063.769B0.0074.153B0.0082.829B0.0052.89 B0.01

100B4106B6104B3119B2107B3113B4110B5129B4143B3140B3153B489B397B3

106B6

142B6151B9149B4171B3155B4163B6160B7192B6247B5208B5281B7153B5154B5163B9

0.759B0.0040.764B0.0090.762B0.0070.784B0.0040.785B0.0070.787B0.0030.795B0.0040.834B0.0040.976B0.0100.844B0.0021.025B0.0040.916B0.0080.867B0.0140.847B0.014

124.0B1.7124.0B2.7124.0B2.0126.9B1.7130.2B2.2129.3B1.2132.5B1.8140 B3193 B8140 B3214 B5190 B5163 B6152 B5

97999997

100100999599989496

100100

NOTE: All given statistical errors are two standard deviations ofthe mean (2s). Column 2: This column shows the position of thecoral samples relative to the present height of the sea level. Allsamples are corrected for blanks and inherited 230Th associatedwith detrital 232Th assuming that the 232Th/238U atomic ratio is3.8. Column 5: The measured (234U/238U) activity ratios(u234U(0)) are presented as deviation per mil (‰) from the equil-

ibrium value. Column 6: The decay-corrected (234U/238U) activityratios (u234U(T)) are calculated from the given ages and withl234Up1.835710P6 l/a. Column 8: The ages are calculated usingthe equation: (230Th/238U)act. p 1Pexp(Pl230Th7T) c[u234U(0)] 7 0.001 7 [l230Th/(230ThPl234U)] 7 [1Pexp((l234UPl230Th)7T)]. Decay constants are: l234U: 2.835710P6 l/a; l238U:1.55125710P10 l/a; l230Th: 9.195710P6 l/a

U- and Th measurement results

U- and Th measurements were performed at the mass-spectrometer facility of the “Heidelberger Akademieder Wissenschaften” (Heidelberg, Germany). The pro-cedures for chemical separation and purification of Uand Th, as well as the dating procedures, are close tothose described by (Chen et al. 1986) and (Edwards etal. 1987a). All measured data and calculated U/Th agesare summarized in Table 1. In addition, the i234U(T) ra-tios and the calculated U/Th ages are presented inFigs. 2 and 3. Analysis of sample TBC5 and TBC16 isduplicated and the results agree well within the statisti-cal uncertainties. Eleven measurements of 234U/238Uatomic ratios in standard material 112A from the NewBrunswick Laboratories give a mean value of5.290B0.007710–5 corresponding to a i234U(0) ratio of–33.2B1.3‰. These values agree well with those pre-viously published by (Edwards et al. 1993).

Our data show that 238U concentrations vary be-tween 2.829B0.005 ppm (TBC16) and4.352B0.005 ppm (TBC5) in the core with a mean con-centration of 3.930 ppm. These values fit well into theframe of previously published 238U concentrations pro-vided by Holocene and LI corals from the Abrolhos is-lands (Eisenhauer et al. 1993; Zhu et al. 1993).

The 232Th concentrations of corals analysed varyover one order of magnitude from 0.209B0.001 ppb(TBC6) to 6.22B0.02 ppb (TBC19). The markedchange of the 232Th concentrations from low to high232Th concentrations occurs between 3 and 6 m b.p.s.lsubdividing this core in two depth sections. In section I(3.3 m a.p.s.l to 3 m b.p.s.l.) the 232Th concentrationsare lower than approximately 0.4 ppb (mean0.392 ppb), whereas in depth section II (below –11 m)

Fig. 2 The i234U(T) ratios for the Turtle Bay coral core. The dot-ted horizontal lines mark i234U(T) ratios corresponding to 140 and160‰ which represent upper and lower acceptance limits for thereliability of U-series ages. It can be seen that most samples ofsection I, except one, value plot into the frame of reliability,whereas most of the samples of section II plot outside the limits.Note that there is a general increase in the i234U(T) ratio whichmay be attributed to subtle and progressive diagenetic alterationof the fossil corals

the 232Th concentrations are generally twofold and upto one order of magnitude higher than in the sectionabove (mean 4.5 ppb).

It is generally accepted that the influence of diagen-etic alteration and/or post-depositional migration of U-and Th isotopes can be neglected when their i234U(T)

values are close to the value of modern seawater(149B2; c.f. Gallup et al. 1994). Corals with i234U(T)

values between 140 and 160‰ are considered to beslightly affected by diagenetic alteration resulting in aU/Th age bias of approximately 2 ka (Chen et al. 1991).

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Fig. 3 The calculated U-series ages of the Turtle Bay core areplotted as a function of the depth in core. The dotted horizontalline marks the position of the present sea level and the solid linemarks the transition from depth sections I to II. It can be seenthat the ages corresponding to section I are following the correctchronological order, whereas for depth section II no correct ordercan be recognized

From Fig. 2 it can be seen that all samples of depth sec-tion I, except one, matches this latter criterion, al-though there is a general increase in the i234U(T)valueswith depth for these samples and the first part of sec-tion II. This behaviour may be attributed to subtle (sec-tion I) and progressive (section II) diagenetic alterationof fossil corals with age (i.e. secondary addition of both234U and 230Th). In particular, the first four samples ofsection II show i234U(T) values between 190 and about250‰ indicating a high degree of chemical alterationand post-depositional migration of U- and Th isotopes.However, in the lower part of section II samples showi234U(T) values comparable to modern seawater indi-cating less diagenetic alteration and hence reliable U/Th ages.

The calculated U/Th ages (Fig. 3) corresponding todepth section I are increasing as a function of the depthin core. The youngest sample on the top of the coreshow an age of 124.0B1.7 ka BP (3.3 m a.p.s.l) and theoldest sample an age of 132.5B1.8 ka BP(3.3 m b.p.s.l.). In contrast, the U/Th ages of depth sec-tion II, varying between 214B5 ka and 140B3 ka BP,do not show any chronological order at all.

To further assess the reliability of our U/Th ages wefollow an approach of Szabo et al. (1994) to classify oursamples (criterion I). We consider category A sampleswith i234U(T) ratios in the range of 149B2‰ as to bereliable. Category B samples with i234U(T) ratios be-tween 140 and 160‰ as to be moderately reliable with-in 2000 years of the calculated U/Th age. Whereas,samples with i234U(T) larger than 160‰ or lower than140‰ are considered unreliable (see Fig. 2). A furtherlogically consistent criterion is the natural chronologi-cal order within a coral core where younger samplesare supposed to lie above older ones (criterion II).Such stratigraphic behaviour can be expected for coral

cores which consist of solid coral framework. However,we note that subtle and progessive alteration as we ob-serve it in our core may produce a positive age bias, butstill shows a correct chronological order. Thus, even thecorrect chronological order is only a necessary, but notsufficient, criterion for the reliability of our U/Thages.

Core sections dominated by coral rubble materialcoral pieces, which are displaced in space and time,may also be abundant. Although single ages of dis-placed rubble material can be considered reliable U/Thages, they do not follow the correct stratigraphic/chron-ological order and do not provide reliable informationconcerning sea level rise. Thus, U/Th ages scatteraround and chronological age reversals occur.

Following the approach of criterion I samples TBC4,5, 52, 7, 8, 9, 15 and 16 correspond to category B andsamples TBC6, 13, 14 19 and 20 belong to category C.Thus, five samples have to be rejected, whereas the restof our samples can apparently be considered reliable atleast within 2 ka. From Fig. 3 it can be seen that all re-liable sample ages corresponding to depth section I arein almost perfect chronological order. It is noteworthythat even sample TBC6 (marked with parentheses inFig. 3) would fit into the chronological frame, althoughit is considered unreliable according to criterion I. Thisobservation is in accordance with previous findings(Chen et al. 1991; Stirling et al. 1995) that U/Th agesmay give chronologically consistent ages, although thei234U(T) ratios do not fit into the accepted frame for thei234U(T) values. This indicates that the diagenetic ef-fects on U/Th ages of some samples are limited, andthat the i234U(T) criterion is necessary, but not suffi-cient, for all samples.

In depth section II only two samples (TBC16 andTBC15) show reliable ages, but they are chronologicalage reversals and are rejected according to criterion II.Thus, none of the samples of this section provide relia-ble U/Th ages according to either criteria I or II. Thisindicates open-system behaviour concerning chemicalalteration and post-depositional migration, which isalso indicated by the high 232Th concentrations presentin these samples. In addition, the low core recovery andthe abundant presence of coral rubble material point tothe likelihood of displaced material obscuring the cor-rect stratigraphic order.

The following discussions of the LI sea level rise isbased on the moderately reliable U/Th ages of sec-tion I. These data cannot place final constraints on thelimits of the LI period because there is still no criterionavailable to unambiguously verify the influence ofchemical alteration and/or post-depositional isotopemigration on the U/Th ages presented in this study.Thus, we consider our data as preliminary, pendingconfirmation by future work. However, given that thepresented U/Th ages are a good approximation of the“true” ages, they may be used to reconstruct the LI sealevel change relative to the western Australian mar-gins.

610

Discussion

Last interglacial sea level rise relative to western

Australia

In a previous publication (Zhu et al. 1993; Collins et al.1993) we presented several coral dates including somecoral core data from the Abrolhos islands (Fig. 4).From these data it was concluded that sea level risestarted at approximately 134 ka BP approximately4 m b.p.s.l. and reached a highstand of at least3 m a.p.s.l at approximately 124 ka. However, the timespan between 134 ka and 124 ka BP could not be re-solved with sufficient precision due to coral sampleswhich were affected by chemical alteration resulting inunreliable U/Th ages (Zhu et al. 1993). This lack of in-formation can now be filled with the new data from theTurtle Bay core because the U/Th ages of depth sec-tion I of the Turtle Bay core fit exactly into the narrowframe of ages provided by the Zhu et al. (1993) data.

The combined records show that the rising sea levelreached a position of 4 m b.p.s.l. at approximately134.3B1.3 ka BP. Reef growth continued with a rate ofapproximately 0.62B0.21 m/ka and passed the modernposition of the sea level between 127 ka and 130 ka BP.At approximately 124B1.7 ka BP the LI sea level high-stand of at least 3.3 m a.p.s.l. is reached and fell againbelow its present position after 115.6B0.9 ka BP Thus,sea level remained above the present position for ap-proximately 13B2 ka.

Here we note that corals are a conservative recorderof sea level change. This is because the “true” sea levelis always somewhere (centimetres to meters) above thegrowth position of the corals. The Caribbean coralAcropora palmata is considered to be the best sea levelindicator because this species is strongly restricted tothe uppermost 5 m of the water column. Most other Pa-cific Acropora may grow from very close to the sea lev-el up to 25 m below sea level. Therefore, in general,fossil coral reef cores do not provide precise constraintson the position of local sea level because of their con-siderable range of vertical growth. However, sea levelcurves derived from coral cores provide a minimum es-timate for the position of local sea level (see extensivediscussion in Eisenhauer et al. 1993). Transferring thisconsiderations onto our own data we can only concludethat the “true” sea level curve lies anywhere above theenvelope provided by our measured points. We furthernote that the coral core studied herein only recogniserising sea levels. Given that there were periods of fall-ing sea level at any time between 134 and 116 ka, theduration of such an event would correspond to onlysome several hundred to approximately 1000 years ac-cording to depth resolution of our samples.

We compare our dating results with observationsfrom scattered outcrops along the western Australiancoastline. The available U/Th data based on traditionalalpha counting technique: a review can be found in

Fig. 4 The Turtle Bay coral core data together with the Zhu et al.(1993) data. The dotted line marks the position of the modern sealevel. Both data sets complement and extend each other to a con-sistent LI sea level curve for West Australia. The combined re-cords show that sea level was at approximately 4 m b.p.s.l. at ap-proximately 134 ka and reached its highstand around 124 ka. Nofurther coral a.p.s.l. was found with ages younger than approxi-mately 116 ka

Murray-Wallace and Belperio (1989) and in Smart andRichards (1992) show that LI sea level was approxi-mately 5 m a.p.s.l at approximately 123B12 ka BP(Cape Range and Cape Cuvier; Veeh et al. 1979) and atapproximately 120B7 ka BP at the Pelsaert reef (Veehand France 1988). Sea level observation at Rottnest is-land (Szabo 1979) indicate an early sea level highstandat approximately 132B5 ka BP (2–3.2 m a.p.s.l.). Morerecent results from Leander Point and Rottnest Island,which are based on very precise U/Th TIMS measure-ments (Stirling et al. 1995), indicate that sea level wasapproximately 2.43 m a.p.s.l. at approximately127.3B1.0 (Rottnest Island) and approximately1.67 m a.p.s.l. at approximately 121.8B0.8 ka (LeanderPoint).

All of these observations show that the LI sea levelreached a highstand of approximately 1–5 m a.p.s.l. inthe time span between 127 and 122 ka. These observa-tions are in general accordance with our findings, al-though the Turtle Bay core record a somewhat longerhighstand extending from approximately 127 to approx-imately 116 ka. It is noteworthy that the onset of the LIsea level highstand is almost simultaneously recordedat the Turtle Bay (Abrolhos islands) and at RottnestIsland, although these sites are from far distant areasand controlled by distinctively different environmental(e.g. wave-energy regimes) and geological conditions.However, the simultaneous observations of a high sealevel at approximately 127 ka BP at both sites puts fur-ther constraints on the onset of the LI sea level rise.Furthermore, these consistent observations may beproof that differential vertical tectonic motion (uplift orsubsidence processes; tilting of shelf areas) are of minorto negligible importance along the West Australiancoastline.

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On a more global scale our observations are in gen-eral agreement with the results from the nearby IndianOcean (Montaggioni and Hoang 1988), Hawaii (Szabo1994), Barbados (Gallup et al. 1994) and the Bahamas(Chen et al. 1991). In particular, the sea level observa-tions from Hawaii and the Bahamas place strict limitson the LI sea level highstand. At Hawaii sea level wasapproximately 1–2 m above present sea level 131 ka BPand lasted until approximately 114 ka BP and at theBahamas islands corals growth a.p.s.l. started around130 ka and lasted until 120 ka. These observations fromoceanic islands of the “intermediate field” are in gener-al agreement with our observations from the Abrolhosislands, although the onset of the sea level highstandoccurs somewhat earlier at both “intermediate sites” .The end of the LI period is terminated somewhat ear-lier at the Bahamas and lasted somewhat longer at Ha-waii when compared with the Turtle Bay. From the Ha-waii and the Bahamas data it can be calculated thathigh sea level lasted for approximately 17 and 12 ka,respectively. Similar calculation for the Turtle Bay coredata give an estimate of approximately 13B2 ka, whichis in between the two ocean island observations.

In summary, we see that there is excellent generalagreement between sea level observations on a regionaland global scale. In particular, the time when the LI sealevel rise reached its highstand and its end is confirmedby independent data sets. The occurrence of an LIhighstand on the Abrolhos islands in phase with Rott-nest island indicate that there is no evidence for anyvertical tectonic movement. Thus far the coral datafrom the Abrolhos islands provide the most completeand detailed LI sea level record presently available forthe western Australian coastline.

Late Quaternary sea level and the Milankovitch theory

of climate

The observation of a sea level at early LI slightly belowthe present position indicate climatic conditions closeto the present one at approximately 134 ka BP (Zhu etal 1993). In addition, a sea level highstand of at least3.3 m a.p.s.l. as observed from the Turtle Bay and Rott-nest Island (Stirling et al. 1995) indicate full interglacialconditions already at 127 ka BP The timing of these ob-servations are in apparent contradiction to the chrono-logy of the marine stable oxygen isotope (Martinson etal. 1987) record and are challenging the Milankovitchtheory of climate change (Milankovitch 1941, Imbrie etal. 1984). In the following section we discuss the simi-larities and differences of the LI (this study) and Holo-cene sea level rise (Eisenhauer et al. 1993) as moni-tored from the Abrolhos islands with respect to the Mi-lankovitch theory and the Devils Hole observation(Winograd et al. 1992).

There is general agreement that the rise in sea levelprior to the Holocene (termination I) and LI maximum(termination II) is a direct consequence of global

Fig. 5 The solar radiation (Berger 1991) as a function of time be-tween 12 ka BP and the present. Arrows mark the position ofmaximum insolation and Holocene sea level highstand

warming triggered by the increase in solar radiation(Berger 1991; Berger 1978) and their positive feedbackmechanisms on Earth. Because of the response time ofthe continental ice sheets to global warming, sea levelchange is expected to show a delayed reaction corre-sponding to several thousand years. This is well docu-mented in the Holocene sea level records from Papua-New Guinea (Chappell and Polach 1991) and theAbrolhos islands (Eisenhauer et al. 1993) where sealevel was approximately 25 m b.p.s.l at both sites at so-lar radiation maximum approximately 10 ka BP, butpeaked around 6.5 ka BP, which is approximately 4 kaafter the Holocene solar radiation maximum (Fig. 5).However, the LI sea level record of the Abrolhos is-lands show that the sea level passed modern position atapproximately 127 ka (Fig. 4), which is 1000 years priorto the radiation maximum (expected at 126 ka BP;Fig. 6). In addition, a sea level almost at its modern po-sition around 134 ka is in contradiction to the solar ra-diation minimum which is predicted for the same time(Fig. 6). Assuming that the Holocene observationscould be transferred to the LI sea level rise, we wouldexpect to see the sea level maximum around 122 ka BPand approximately 25 m below its present position atapproximately 126 ka.

These contradictory observations can be transferredto a comparison between the LI sea level rise and themarine stable oxygen isotope record (Fig. 7) because itis tuned to the cyclic variations of the solar insolation(Martinson et al. 1987).The Holocene sea level recordand the generalised marine stable oxygen isotope re-cord of benthic foraminifera show consistent results.Coral data indicate a sea level of approximately120 m b.p.s.l. at approximately 17 ka BP (LGM; Fair-banks 1989) which correspond to a high i18O values(c0.83‰) of benthic foraminifera (Martinson et al.1987). Thus, low sea levels and high i18O values of thebenthic foraminifera indicat the same climatic condi-tions at the same time. In accordance with the solar ra-diation curve, the lowest i18O values (–0.76‰) are at-tached to an age of approximately 122 ka BP (full inter-

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Fig. 6 The solar radiation (Berger 1991) during the Last Intergla-cial period as a function of time. Vertical dotted lines mark the sealevel highstand as recorded from the Turtle Bay core. It can beseen that sea level reached its highstand slightly before the solarradiation maximum

Fig. 7 The generalised i18O record of marine foraminifera (Mar-tinson et al. 1987) together with the observations from the TurtleBay core. It can be seen that sea level highstand was reached pri-or to the climatic optimum, although full glacial conditions arepredicted from the marine i18O record

glacial conditions) according to the marine record andhighest values (c0.71‰) are attached to an age of ap-proximately 134 ka BP (full glacial conditions). Theseobservations are in contradiction to the LI as recordedfrom the Turtle Bay core which indicates almost full in-terglacial conditions at approximately 134 ka BP, be-cause sea level reached a position slightly below thepresent position of the sea level at that time. In addi-tion, the Turtle Bay core data already indicate high sealevels at approximately 127 ka BP, which is 5 ka priorto the climatic maximum as recorded by the marinecurve.

The Holocene reef grew at an average rate of ap-proximately 7.1 m/ka relative to the Abrolhos islands.Only within the uppermost depth range between 5 and0 m b.p.s.l. did the growth rate decrease to approxi-mately 3 m/ka (Eisenhauer et al. 1993). The Turtle Baycore data indicate that the LI reef grew at a rate of ap-proximately 0.62B0.21 m/ka between approximately

4 m b.p.s.l. and approximately 3 m a.p.s.l., which is afactor of 10–5 lower than the Holocene rates. From thisrate and given that the LI sea level amplitude is similarto the Holocene sea level amplitude of approximately120 m (Fairbanks 1989), it can be calculated that ap-proximately 200 ka would be required for a completeglacial/interglacial sea level change. Such a long timecorresponds to almost two glacial/interglacial cyclesand is not realistic. Thus, the rate of reef growth doesnot necessarily reflect sea level rise and was probablymuch faster than 0.62 m/ka prior to 134 ka. Assumingthat the Holocene growth rate of approximately 7–10 m/ka is close to the rate of sea level rise, then 13–18 ka are required for a sea level rise of 130 m. Thus,major melting of the continental ice sheets must havestarted around 147-152 ka BP, which is much earlierthan recognised by the i18O chronology. The low rateof sea level rise as recorded by the uppermost part ofthe Turtle Bay core may therefore just represent a peri-od of low reef growth independent of the LI sea levelchange. We note that we are aware of the problem thatthe rate of reef growth cannot easily be interpreted asto reflect sea level rise. Most reefs lag behind the sealevel rise and, in addition, reef growth also depends onhigh or low energy environments where it has beenshown that reefs grow a factor of two faster (Collins etal. 1993b).

The observation of an early start of the ice meltingand the long duration of the LI period is in general ac-cordance with the findings from the Devils Hole calcitevein (Winograd et al. 1992; Ludwig et al. 1992), whichpredict an early start of the LI sea level rise at around147 ka and a long duration of the LI period of approxi-mately 20 ka. Thus, these observations, in conjunctionwith similar observations from other coral reef sites(c.f. Szabo et al. 1994), point to the likelihood that theclimatic transitions at terminations I and II are differ-ent. It was pointed out that other than cyclic variationsof the solar insolation, such as change in atmosphericCO2, accelerating heat enhancement and/or changes inthermohaline circulation may also trigger climatechange prior to an interglacial (Crowley and Kim 1994;Steven et al. 1994). This apparently challenges the solarforcing of climate change and points to the necessitythat some revisions are required to reconcile contradic-tory observations and theoretical predictions. Certain-ly, on the whole the Milankovitch theory does not needa general revision, which is also clearly demonstratedby the Devils Hole chronological record being in excel-lent agreement with the marine record, except termina-tions II and V at approximately 128 ka and 420 ka BP(SPECMAP chronology), respectively. These two ter-minations may be exceptional because the glacial to in-terglacial amplitude in insolation is largest in these timeperiods. Thus, ice melting at glacial/interglacial transi-tions may follow slightly different processes becauselarge ice sheets tend to be unstable and react more sen-sitive to small changes in insolation prior to the termi-nation (Crowley 1994). Thus, it seems plausible to us

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that large amounts of ice were rapidly melted into thesea prior to the LI solar maximum.

Up to now there are no robust chronological con-straints on the marine i18O record. Thus, the sequenceof maxima and minima of the i18O record is tuned tothe solar insolation curve by peak matching assumingthat climatic optima are in phase or follow slightly upthe solar insolation curve represented by the July inso-lation at 657N only (Berger 1978). All other latitudessouth or north of 657N and months other than July arearbitrarily neglected. However, choosing the June inso-lation at 657N as insolation curve would shift the solarinsolation maximum to 128 ka approximately 2000years earlier relative to the July curve. Given the Juneinsolation curve the Holocene maximum insolation val-ue is already exceeded at approximately 128.5 ka. Bothages are closer to our age calculations and show thatSPECMAP tuning is very sensitive to the latitude. Inaddition, applying an energy balance model Crowleyand Kim (1994) showed that temperatures over criticalnorthern hemisphere latitudes were approximately2.5 7C warmer than present by approximately130 ka BP, and that warming exceeded the present val-ue by approximately 1–1.5 7C as early as 134 ka BP. Inparticular, this model showed that the land masses werewarmer than now at approximately 130 ka BP. Crowleyand Kim (1994) argue that this rapid warming prior tothe LIM insolation is related to the obliquity whichpeaked earlier than precession and may have triggereda rapid melting of the polar ice caps prior to the Julysolar insolation maximum at 657N.

Summary

The reconstruction of LI sea level change as monitoredin the Turtle Bay core is based on moderately reliableU/Th ages (reliable within 2 ka) and do not place finalconstraints on the limits of the LI period. Unfortunate-ly, there is still no criterion available to unambiguouslyverify the influence of chemical alteration and/or post-depositional isotope migration on U/Th ages. Thus, weconsider our data as preliminary, pending confirmationby future work. However, given that the U/Th ages ofthe Turtle Bay corals are a good aproximation of the“true” age, the discrepant observations concerning sealevel rise prior to the LI optimum and the predictionsof the i18O record require reconciliation. This can bedone by retuning the marine i18O record and by aslight revision of the forcing mechanism following thearguments of Crowley and Kim (1994). In particular, inour opinion it is evident that the orbital–climate inter-action requires a fuller understanding of the behaviourof extended continental ice sheets prior to solar warm-ing and of the influence of solar insolation at latitudesother than 657N.

Acknowledgement The critical comments of Prof. L. Montag-gioni and of one anonymous reviewer helped to improve the

manuscript. Funding for this study was provided by the“Deutsche Forschungsgemeinschaft” (DFG) grants Ma821/1–11and Ei272/1–3, and the BMBF (“Bundesministerium für Bildungund Forschung” , Paläoklimaprojekt). This is a contribution tothe PAGES core project of the International Geosphere Bio-sphere Program (IGBP).

References

Bard E, Fairbanks RG, Hamelin B, Zindler A, Hoang CT (1991)Uranium-234 anomalies in corals older than 150000 years.Geochim Cosmochim Acta 55 :2385–2390

Berger AL (1978) Long-term variations of caloric insolation re-sulting from the Earth’s orbital elements. Quaternary Res9 :139–167

Berger AL, Loutre MF (1991) Insolation values for the climate ofthe last 10 million years. Quaternary Sci Rev 10 :297–317

Chappell J, Polach H (1991) Post-glacial sea-level rise from a co-ral record at Huo Peninsula, Papua New Guinea. Nature349 :147–149

Chen JH, Edwards RL, Wasserburg GJ (1986) 238 U, 234 U and232Th in sea water. Earth Planet Sci Lett 80 :241–251

Chen JH, Curran HA, White B, Wasserburg GJ (1991) Precisechronology of the Last Interglacial period: 234U-, 230Th datafrom fossil coral reefs in the Bahamas. Geol Soc Am Bull103 :82–97

Collins LB, Zhu ZR, Wyrwoll K-H, Hatcher BG, Playford PE,Chen JH, Eisenhauer A, Wasserburg GJ (1993a) Late Quater-nary evolution of coral reefs on a cool-water carbonate mar-gin: the Abrolhos carbonate platforms, southwest Australia.Mar Geol 110 :203–212

Collins LB, Zhu ZR, Wyrwoll KH, Hatcher BG, Playford PE, Ei-senhauer A, Wasserburg GJ, Chen JH, Bonani G (1993b) Ho-locene growth history of a reef complex on a cool-water car-bonate margin: Easter Group of the Houtman Abrolhos, east-ern Indian Ocean. Mar Geol 115 :29–46

Crowley TJ (1994) Potential reconciliation of Devils Hole anddeep-sea Pleistocene chronologies. Palaeoceanography 9 1 :1–5

Crowley TJ, Kim K-Y (1994) Milankovitch forcing of the Last In-terglacial sea level. Science 265 :1566–1567

Edwards RL, Chen JH, Ku T-L, Wasserburg GJ (1987a) 238U-,234U-, 230Th-, 232Th-, systematics and the precise measure-ments of time over the past 500000 years. Earth Planet SciLett 81 :175–192

Edwards RL, Beck JW, Burr GS, Donahue DJ, Chappell JMA,Bloom AL, Druffel ERM, Taylor FW (1993) A large drop inatmospheric 14C/12C and reduced melting in the YoungerDryas, documented with 230Th ages of corals. Science260 :962–968

Eisenhauer A, Wasserburg GJ, Chen JH, Bonani G, Collins LB,Zhu ZR, Wyrwoll KH (1993) Holocene sea-level determina-tions relative to the Australian continent: U/Th (TIMS) and14C (AMS) dating of coral cores from the Abrolhos Islands.Earth Planet. Sci. Lett. 114 :529–547

Fairbanks RG (1989) A 17000-year glacio-eustatic sea level re-cord: influence of glacial melting rates on the Younger Dryasevent and deep ocean circulation. Nature 342 :637–642

Gallup CD, Edwards RL, Johnson RG (1994) The timing of highsea levels over the past 200000 years. Science 263 :796–800

Imbrie J, Hays JD, Martinson DG, McIntyre A, Mix AC, MroeyJJ, Pisias NG, Prell WL, Shackleton NJ (1984) The orbital the-ory of Pleistocene climate: support from a revised chronologyof the marine D18O record. In: Berger JIAL, Hays J, Kukla G,Satzmann B (eds) Milankovitch theory and climate. Reidel,Dordrecht, pp 269–305

Ludwig KR, Simmons KR, Szabo BJ, Winograd IJ, LandwehrJM, Riggs AC, Hoffman RJ (1992) Mass spectrometric 230Th-,234U-, 238U-dating of the Devils Hole calcite vein. Science258 :284–287

614

Martinson DG, Pisias GN, Hays JD, Imbrie J, Moore TC Jr,Shackleton NJ (1987) Age dating and the orbital theory of theIce Ages: development of a high-resolution 0 to 300000-yearchronostratigraphy. Quaternary Res 27 :1–29

Mesollela KJ (1967) Zonation of uplifted Pleistocene coral reefson Barbados, West Indies. Science 156 :638–649

Milankovitch M (1941) Kanon der Erdbestrahlung und sein Eis-zeitenproblem. Acad. R. Serbe (Belgrade), ed. Spec. 133 Sect.Sci. Math. Naturales, 633, (translated by the Israel Programfor Scientific Translations, Jerusalem, 1970)

Murray-Wallace CV, Belperio AP (1991) The Last Interglacialshoreline in Australia – a review. Quaternary Sci Rev 10 :441–461

Smart PL, Richards DA (1992) Age estimates for the Late Qua-ternary high sea-stands. Quaternary Sci Rev 11 :687–696

Steven RK, Giles GL, White JWC, Figge RA, Markgraf V, CiaisP, Kenny R (1994) Climate in the Pleistocene. Nature371 :111–112

Stirling CH, Esat TM, McCulloch MT, Lambeck K (1995) High-precision U-series dating of corals from western Australia andimplications for the timing and duration of the Last Intergla-cial. Earth Planet Sci Lett 135 :115–130

Szabo BJ (1979) Uranium-series age of coral reef growth in Rott-nest Island, western Australia. Mar Geol 29 :M11-M15

Szabo B, Ludwig KR, Muhs DR, Simmons KR (1994) Thorium-230 ages of corals and duration of the last interglacial sea-levelhigh stand on Oahu. Science 266 :93–96

Veeh HH, France RE (1988) Uranium-series ages of corals andco-existing phosphate deposits on Pelsaert Reef complex,Houtman-Abrolhos Islands. Quaternary Res 30 :204–209

Veeh HH, Schwebel D, Graaf WJE van de, Denham PD (1979)Uranium-series ages of coralline terrace deposits in westernAustralia. J Geol Soc Australia 26 :285–292

Winograd IJ, Coplen TB, Landwehr TB, Riggs JM, Ludwig AC,Szabo KR, Kolesar PT, Revesz KM (1992) Continuous500000-year climate record from vein calcite in Devils Hole,Nevada. Science 258 :284–287

Zhu ZR, Wyrwoll K-H, Collins LB, Chen JH, Wasserburg GJ,Eisenhauer A (1993) High precision U-series dating of LastInterglacial events by mass spectrometry: Houtman AbrolhosIslands, western Australia. Earth Planet Sci Lett 118 :281–293