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Quaternary Science Reviews 107 (2015) 182e196

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Quaternary Science Reviews

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Tropical tales of polar ice: evidence of Last Interglacial polar ice sheetretreat recorded by fossil reefs of the granitic Seychelles islands

Andrea Dutton a, b, *, Jody M. Webster c, Dan Zwartz d, Kurt Lambeck b,Barbara Wohlfarth e

a Department of Geological Sciences, University of Florida, Gainesville, FL, USAb Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australiac Geocoastal Research Group, School of Geosciences, University of Sydney, NSW, Australiad Antarctic Research Centre, Victoria University of Wellington, Wellington, New Zealande Department of Geological Sciences, Stockholm University, Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 17 June 2014Received in revised form21 October 2014Accepted 25 October 2014Available online

Keywords:Sea levelUeThMIS 5eCoralSeychelles

* Corresponding author. Department of Geological SGainesville, FL, USA.

E-mail address: adutton@ufl.edu (A. Dutton).

http://dx.doi.org/10.1016/j.quascirev.2014.10.0250277-3791/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In the search for a record of eustatic sea level change on glacialeinterglacial timescales, the Seychellesranks as one of the best places on the planet to study. Owing to its location with respect to the formermargins of Northern Hemisphere ice sheets that wax and wane on orbital cycles, the localdor rela-tivedsea level history is predicted to lie within a few meters of the globally averaged eustatic signalduring the Last Interglacial period. We have surveyed and dated Last Interglacial fossil corals to ascertainpeak sea level and hence infer maximum retreat of polar ice sheets during this time interval. We observea pattern of gradually rising sea level in the Seychelles between ~129 and 125 thousand years ago (ka),with peak eustatic sea level attained after 125 ka at 7.6 ± 1.7 m higher than present. After accounting forthermal expansion and loss of mountain glaciers, this sea-level budget would require ~5e8 m of polar icesheet contribution, relative to today's volume, of which only ~2 m came from the Greenland ice sheet.This result clearly identifies the Antarctic ice sheet as a significant source of melt water, most likelyderived from one of the unstable, marine-based sectors in the West and/or East Antarctic ice sheet.Furthermore, the establishment of a þ5.9 ± 1.7 m eustatic sea level position by 128.6 ± 0.8 ka wouldrequire that partial AIS collapse was coincident with the onset of the sea level highstand.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Understanding the coupling between surface temperature, at-mospheric CO2, and sea level is critical for assessing future changeon a warming planet. This can be achieved through model studiesof the interactions between atmosphere, oceans and ice sheetsunder global warming conditions or through examination of pale-oclimate and sea level records during interglacial periods similar tothose of today. The peak of the Last Interglacial (LIG) period, duringMarine Isotope Stage (MIS) 5e, is particularly important for suchstudies because the observational record is better preserved thanfor earlier interglacial periods. LIG sea level globally reached at leastseveral meters higher than present, with recent work convergingon a ~6e9 m estimate for peak sea level (Kopp et al., 2009; Dutton

ciences, University of Florida,

and Lambeck, 2012; Kopp et al., 2013) when temperatures at highlatitudes were a few degrees warmer than during the presentinterglacial (Clark and Huybers, 2009). This makes the intervalappropriate for testing models of the response of sea level toclimate forcing. In this paper we examine one part of this rela-tionship: the record of sea-level change during the LIG period.

The precise timing, duration, and peak elevation of the LIGhighstand continues to be debated. The timing and duration havebeen estimated using conventional (closed-system) UeTh ages offossil coral reefs at several sites, such as the Bahamas(~132e120 ka), Barbados (~132e120 ka), and Western Australia(~128e116 ka) (Chen et al., 1991; Stirling et al., 1998; Speed andCheng, 2004). Open-system model ages yield significantlyyounger age estimates in the Bahamas (~126e113 ka) and Barbados(~124e114 ka) but produce similar ages for the Western Australiadataset (~129e115 ka) (Thompson and Goldstein, 2005; Thompsonet al., 2011; O'Leary et al., 2013). Despite some discrepancy in theabsolute timing between these studies, the duration of the sea levelhighstand has been consistently estimated at ~10 to 12 ky.

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196 183

However, there is no clear consensus regarding the timing ofpeak sea level during this time period, the magnitude of the peak,and the stability of sea level. Interpretations range from: (1) a stablesea level at 3e4m above present (Stirling et al., 1998), (2) a bipartitesea level highstand with two prominent peaks separated by anephemeral drop in sea level (Chen et al., 1991), (3) a stable sea levelfollowed by a rapid sea-level rise (Blanchon et al., 2009; O'Learyet al., 2013), to (4) several oscillations (3e4 peaks) in sea level(Rohling et al., 2008; Thompson et al., 2011).

In summary, analysis of compiled sea level data from this timeperiod indicate that the magnitude and timing of the LIG sea levelhighstand are geographically variable (Kopp et al., 2009; Duttonand Lambeck, 2012). This variability reflects the combined uncer-tainty from relating the markers to mean sea level, establishing aconsistent chronology, estimating the effect of tectonic landmovements and glacial isostatic adjustments (GIA) on paleo-shoreline elevation. The last of these, the eartheocean response tothe variable loading of ice and water on glacialeinterglacial time-scales has deterministic global consequences on sea level and othergeodetic observables that can be modeled with a high degree ofconfidence given assumptions of several parameters (e.g., ice vol-umes, ice geometry, solid earth profile).

A key contribution from such geophysical models is the pre-diction that even at sites far from the former margins of theNorthern Hemisphere ice sheets, relative sea level (RSL) may recordsubstantial departures from the eustatic signal (Farrell and Clark,1976; Nakada and Lambeck, 1989; Milne and Mitrovica, 2008).Thus, in general, eustatic sea level (ESL) cannot be measureddirectly but, in the absence of tectonic contributions, can be infer-red by removing the non-eustatic components from field obser-vations through numerical modeling. GIA has several componentsthat can influence RSL histories including deformation of the solidearth and gravity field and rotational effects, all of which are adirect consequence of the changing loads of ice and water on gla-cialeinterglacial timescales. For this reason, past changes in ESL areeasier to estimate from far-field sites where such isostatic effectsare smaller.

Uncertainties in GIA predictions result from incompleteknowledge of both the ice history before and after the formation ofthe sea-level evidence and also the earth-response function ormantle rheology (Lambeck et al., 2012). But there are locationswhere GIA corrections are both small and insensitive to these

Fig. 1. (A) Map of the granitic Seychelles islands with modern reefs indicated by dashed linesmall black box near the north end of the Mascarene Ridge denotes the area of enlargemendiscussed in the text.

uncertainties (Milne and Mitrovica, 2008; Lambeck et al., 2010;Dutton and Lambeck, 2012). One such location is the Seychelleswhere earlier investigations have reported LIG limestone reefs upto þ10 m elevation (Baker, 1963; Veeh, 1966; Braithwaite, 1984;Montaggioni and Hoang, 1988; Israelson and Wohlfarth, 1999)and where the non-eustatic component is small (RSL is predicted tolie within a few meters of ESL during the LIG period) (Milne andMitrovica, 2008; Dutton and Lambeck, 2012; Hay et al., 2014).Finally, relative tectonic stability since the Eocene (Mart, 1988)gives this site the rare combination of tectonic stability and far-fieldlocation needed to more accurately constrain ESL history.

2. Geologic setting

The granitic Seychelles islands, also known as the Mahe-Praslingroup, lie ~4e5� south of the equator in the Indian Ocean on afragment of continental crust (Fig. 1). Emergent marine limestoneshave long been recognized as evidence for a previous sea levelhighstand in the area and were first UeTh dated to the LIG periodusing alpha counting (Veeh, 1966). Ten additional corals weresubsequently dated using UeTh thermal ionization mass spec-trometry (TIMS), but more than half of these samples were partiallyrecrystallized (4e35% calcite) (Israelson and Wohlfarth, 1999).Because recrystallization is often accompanied by open-systembehavior of the UeTh isotopes that leads to erroneous ages, thiscasts doubt on the validity of some of the previously reported ages.

3. Methods

We undertook detailed surveying, sedimentologic description,and sampling of outcrops that displayed in situ coralgal frameworkat several sites on both La Digue and Curieuse islands. We focusedon these two islands because despite intensive reconnaissance, weonly observed cemented coral rubble on the larger islands of Maheand Praslin and no in situ corals.

3.1. Surveying

We surveyed outcrops on Curieuse and La Digue islands(Table S1) using either a total station or an auto-level and graduatedstaff to establish elevations relative to a tidal datum. Herewe reportelevations for samples from five outcrops (Site 19A on Curieuse;

s. Inset shows the location of the regional map within the western Indian Ocean wheret. (B) Location of the modern fringing reef and fossil outcrop sites on La Digue that are

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Sites 4, 7, 8, and 11 on La Digue) and for one previously sampledoutcrop (Site 7A on La Diguedequivalent to Pte. Source d'Argentoutcrop of Israelson and Wohlfarth (1999)).

To establish elevations above sea level, we surveyed the positionof the sea surface at intervals spanning at least one high or low tideat the nearest beach to each outcrop. We obtained hourly mea-surements from the tide gauge at Point La Rue located near the endof the runway on the main island of Mah�e from the University ofHawaii Sea Level Center. The tide gauge records were compared toour observations and to the predicted tides at each site to relate ourmeasurements to an established tidal datum.

Tidal datums were obtained from the Admiralty TideTables (ATT) published by the UKHydrographic Office. Tidal datumslisted in the ATT are identical to those published in the tide tablesprovided by the Seychelles Maritime Safety Administration, andwere established using data from Port Victoria, Mah�e from 1986 to1989. Despite the recent trend of rising sea level in the Seychelles,themeanwater level during those years is within 0.1 m of the meanwater level during the year of our field campaign. Given that this iswithin the uncertainty of our elevation measurements (see below)the absolute value of the tidal datums relative to the chart referencedatum of the tide gauge have not been adjusted for modern sea-level rise. The height difference (offset) between mean low watersprings (MLWS) at La Digue and Curieuse islands relative to PortVictoria is 0.0 and �0.1 m, respectively, so the data from Curieusewere adjusted accordingly. Once this tidal reference frame wasestablished for our tidal measurements near each outcrop, theposition of our surveyed sample elevations was calculated relativeto the local datum. We used MLWS as our reference datum for themaximum height of the corals. Corals provide a minimum estimateof former sea level position depending on the paleodepth of thespecimen in question. Therefore, independent sedimentologicaland fossil reef biota assemblage observations were made toestablish paleodepth estimates at each outcrop.

After using a total station or optical level to survey at least onepoint on each outcrop, we used a measuring tape to determine thevertical distance from the survey point(s) to each sample. Surveypoints on the outcrop were directly tied to sea level observations atthe nearest beach. Elevations reported for corals refer to theelevation at the uppermost surface of the coral head in the outcropexposure relative to MLWS.

In addition to surveying outcrops that we sampled, we alsomeasured the elevations of other outcrops, including outcrops atPte. Source d'Argent (Site 7A) and Grande Anse (Site 4) on La Diguethat were previously sampled and dated by Israelson andWohlfarth (1999). Elevations reported in Israelson and Wohlfarth(1999) were estimated from a topographic contour map, not sur-veyed, so our objective was to attempt to place these samples intoan improved elevation framework. This was limited in some casesby our ability to pinpoint the exact sample locations in these out-crops and is reflected in the larger uncertainty that we ascribe tothe elevation measurements for samples from Site 7A. We wereable to survey Site 7A at Pte. Source d'Argent, but due to a heavylayer of black biofilm on the outcrop and blasting of the outcropthat has occurred since it was originally sampled, we place a largeruncertainty (0.5 m) on the elevation data here. Unfortunately, dueto heavy swells associated with the southeast trade winds, wewereunable to establish a tidal reference frame near Grand Anse (Site11), and are presently unable to determine an absolute elevation forthese samples.

The error on the elevation measurements was calculated bytabulating all the contributing sources of error and adding them inquadrature (Table S2). Tidal datums are only reported to one dec-imal place; hence the uncertainty in this number is taken as ±0.1 m. The uncertainty in the sea level measurements is calculated

by taking the average of the difference between the sum of thepredicted tidal elevation (taking into account the effect of weatherusing the offset between the predicted and measured tides at thetide gauge onMah�e) and the measured value. This provides us withthe relative sea level error component in Table S2. This calculationcaptures the degree towhichwewere able to track the relative tidalposition, but this does not account for a possible offset in absoluteelevation if, for example, we consistently overestimated the posi-tion of the sea surface in all of the readings due to wave set-up.Hence we add an additional 0.1 m uncertainty to account for sucha possible offset. This is reported as the absolute sea level errorcomponent in Table S2. Combining all of these contributing factorsyields an overall uncertainty of ±0.2 m.

The final source of uncertainty in the estimation of past sea levelposition is the paleodepth estimate, which ultimately dominatesthe total uncertainty in the elevation component of the sea levelreconstruction. Using facies analysis coupled with the taxonomyand growth morphology of the dated corals, and visual observationof the modern depth range of similar species andmorphologies, weinterpret these samples to have grown in very shallow water,probably close to MLWS. We estimate a maximum of ~2 m paleo-depth for the samples dated and reported here. All elevations arereported relative to MLWS since both modern and fossil corals maygrow as high as MLWS.

3.2. X-ray diffraction

Coral samples were screened for mineralogy using X-raydiffraction (XRD) to assess alteration through conversion ofaragonite to calcite. XRD measurements were carried out at theResearch School of Earth Sciences in The Australian National Uni-versity. We established a protocol following the procedure ofDelanghe et al. (2002) using in-house standards for pure aragonite(modern coral), high-Mg calcite, and calcite. Two separate cali-brations were defined for aragonite and high-Mg calcite. Ourreproducibility at the low end of the scale (�about 3% calcite) waspoor, possibly owing to inconsistencies in sample-loading tech-nique. For this reason, we applied a somewhat high screening cri-terion (<4% calcite) to our data because of the higher uncertainty inthe precise composition at the lower end of the scale. Thus a samplewith a reported % calcite of 3 or 4 may in fact be only 2% calcite, forexample. We also chose 4% as the cut off for accepting samplesbased on the observation that samples with significant calciterecrystallization (�4% calcite) are significantly affected in theirUeTh geochemistry and can cause stratigraphic inversions in suc-cessions of corals that are directly superimposed as well as incon-sistent reproducibility of ages for subsamples of these corals (seeTable 1 and Israelson and Wohlfarth, 1999).

3.3. UeTh dating

Samples that passed the initial XRD screening were subse-quently analyzed on a Neptune multi-collector inductively-coupledplasma mass spectrometer (MCeICP-MS) at The Australian Na-tional University to establish U-series ages. We dated 16 coralscollected on our expedition, plus an additional six samples ob-tained previously (Israelson and Wohlfarth, 1999). The 16 newsamples were dated in triplicate (3 separate subsamples of thesame coral) to assess intra-sample heterogeneity and the pre-existing six samples are represented by 1e2 new measurementsper sample to cross check previously published TIMS UeTh dates.

The methodology used to conduct the U-series dating followsthe procedure described in McCulloch & Mortimer (2008), but inthis case we used the decay constants of Cheng et al. (2000) for230Th and 234U. Sample pre-treatment involved hand drilling out

Table 1Sample description and elevation.

Sample ID Species Growth form Elevation (m)b,c Field observationsa

SY-09 Favites Submassive N.D. Site 11: Near the base of the outcrop; in situ coral framework composed 30 cmthick interval dominated by encrusting and submassive Faviids

SY-12 Goniastrea retiformis? Massive/columnar N.D. Site 11: shore parallel section; 70 cm from top from top of the sectionSY-22 Goniastrea retiformis? Massive 6.3 Site 4: Largest colony in the section of Outcrop A; capped by cm-scale crusts of

CAR (SY-21); likely very shallowSY-24 Cyphastrea Encrusting 6.9 Site 4: Part of in situ encrusting complex of corals (Montipora & Cyphastrea), that

overlies SY-21 (thick CAR) but is also overlain but more cm-scale CAR crusts;likely very shallow

SY-27 Goniastrea retiformis? Massive 5.3 Site 4: Largest colony in the section of Outcrop B, capped by 58-cm interval that isdominated by cm-scale crusts of CAR (SY-28), with abundant vermetids andtrapped coarse clastic grains; likely very shallow, intertidal

SY-36 Pavona/Leptastrea Encrusting 5.4 Site 4: Part of in situ encrusting complex of corals (Cyphastrea, Pavona/Leptastrea,with SY-37), that overlies SY-32 characterized by the thick crusts of fruticose CARand below that, directly plastering the granite wall, thick cm-scale crusts CARwith large and abundant vermetids; likely very shallow, intertidal

SY-37 Pavona/Leptastrea Encrusting 5.5 Site 4: Part of in situ encrusting complex of corals with SY-36SY-41 Favia? Submassive 5.9 Site 4: Near the top of Outcrop G, close to thick cm-scale crusts of CAR

that drape down from where they are attached to the granite wall; likelyvery shallow, intertidal.

SY-52 Goniastrea retiformis Massive 8.0 Site 7: Top of profile 3, likely very shallow (intertidal), stratigraphically just belowwell-developed interlayered, thick CAR with vermetid gastropods

SY-53 Acropora (humilis grp.?) Corymbosebranching

7.9 Site 7: Close to rubble deposit at the top of the profile 3 but likely in situ, likely veryshallow (intertidal), stratigraphically 10 cm below SY-52

SY-56 Goniastrea retiformis? Massive 7.4 Site 7: Part of in situ, coralgal framework from the top ~1.5 m of profile 3 that isdominated by massive and submassive Faviids, likely very shallow, intertidal

SY-63 Goniastrea retiformis? Submassive 3.7 Site 8: Near base of an in situ, ~50 cm submassive and massive complex of corals,protected cave, outcrop is landward facing, rather than seaward facing as with allother outcrops

SY-66 Stylophora Branching 5.8 Site 19A: Interval characterized by thick cm-scale, interlayered CAR between in situStylophora colonies, CAR also thickened on branch tips; likely very shallow

SY-68 Favites Massive 6.6 Site 19A: Large well developed colony, some interlayered CAR with cm-scale crustscapping part of the top of the coral; likely very shallow

SY-69 Favia Encrusting 6.2 Site 19A: Growing off CAR that is attached to the granite wall

a CAR ¼ coralline algae.b N.D. ¼ not determined.c Height of top of coral to mean low water springs, ±0.2 m.

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the thickest and visually best-preserved pieces of coralline arago-nite. Approximately 0.2 mg of coral was used for each analysis.Coral fragments were sonicated in water and ethanol prior todissolution in concentrated nitric acid. Samples were spiked withan in-house 233Ue229Th spike (U-4) that has been calibrated rela-tive to a secular equilibrium standard (HU-1). The remainder of thesample preparation and analysis follows that described inMcCulloch andMortimer (2008). We routinely ran a spiked HU-1 tomonitor accuracy and measured blanks to check for contamination.

3.4. Glacio-hydro-isostatic modeling

The RSL predictions are identical to those provided in Duttonand Lambeck (2012). The ice model used is that described inLambeck et al. (2006), which includes a similar glacial maximumice volume during MIS 6 (~140 ka) as for MIS 2 (~20 ka), but adifferent distribution of ice between North America and northernEurope. In particular the Eurasian ice sheet in MIS 6 is larger thanthat of MIS 2, based on evidence for a more geographically exten-sive ice sheet during that time (Svendsen et al., 2004). In this for-ward model, the ice volume during the LIG period is set equal tothat of the modern preindustrial ice sheets for the duration of thehighstand (the null hypothesis) and the predicted RSL in theSeychelles is associated with an uncertainty of ±1.5 m. The residualbetween the observed data and the GIA-predicted RSL was used toestimate the ESL signal. This approach to resolve ESL is only valid atfar-field sites where the non-eustatic component is small andsensitivity to model parameters is low. However, this calculationdoes not account for the GIA effects associated with any ice-volume

changes during the interglacial such as a rapid collapse of polar icesheets. The spatial pattern in magnitude of sea-level change that isexpected from a collapse of the GrIS, WAIS, or EAIS over a 1 to 3thousand-year timeframe has previously been modeled by Hayet al. (2014) who predict that in the Seychelles, polar ice-sheetcollapse will result in a ~10e20% overestimation of the actualeustatic sea-level rise. Hence this additional correction has beenmade to the RSL elevations to estimate ESL.

4. Results

4.1. Emergent limestone fabrics

Intense weathering has created dramatic rilled and flutedmorphologies of the granite bedrock on these islands (Johnson andBaarli, 2005) and has eroded away most of the LIG fossil reef, butwhere they are protected by granite overhangs, portions of the reefstill remain at elevations above present sea level. Many of these LIGlimestones are composed of cemented coral reef rubble emplacedin cracks and crevices that would have received limited light.Modern coral rubblewas observed in the supratidal zone, and likelyrepresents storm deposits (Montaggioni and Hoang, 1988). Hencetheir fossil rubble counterparts are not good markers for sea levelposition.

Where a suitable combination of access to light to allow coralgrowth and protective overhangs occur, in situ coralgal frameworkis preserved (Figs. 2e5). In all the in situ coral framework outcropsthat we examined, the massive (e.g., Goniastrea, Favites, Leptastrea)and robust branching (e.g., Acropora, Stylophora) coral facies

Fig. 2. Outcrop picture and location of samples for Site 4, ‘Inland Site’, La Digue. UeTh ages in bold, initial d234U values in parentheses, and sample numbers in upper right corner ofindividual sample photos. Ages shown are screened closed-system ages that are inverse variance-weighted means of measurements from the same coral. Star denotes location ofSite 4 on map in bottom right inset. The black rectangle shows the position of Outcrop A also shown in detail in Fig. 3.

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196186

dominate and are commonly encrusted with thick (at least severalcm) coralline algae that is variably associated with vermetid gas-tropods, indicating that these reefs closely tracked sea level as itrose. This observation is consistent with observations ofMontaggioni and Hoang (1988), who argue that the alternatingcoral-coralline algae facies in these limestones represents contin-uous transitions between the subtidal and intertidal zones. Thus,although we cite a conservative maximum estimate of 2 m for thepaleodepth of the corals we dated, it is likely that most of thesewere <1 m below MLWS.

New fossil coral samples collected on this expedition have beencatalogued and described within their stratigraphic and faciescontext. Detailed notes and elevations corresponding to each of thesamples dated, including a paleodepth interpretation, are con-tained in Table 2. Herewe provide brief summaries of the three key,representative outcrops that were dated for this study, Site 4 (theInland Site on La Digue), Site 7 (Anse Source d’Argent, La Digue),and Site 19A (Curieuse).

Site 4 is adjacent to one of the roads on La Digue (Fig. 2). This siteconsists of 7 coralgal framework-dominated outcrops (A-G) thathave accumulated in situ on the seaward facing side of a graniteoutcrop. Similar coralgal reef accumulations can be presentlyobserved on the sides of granite outcrops in the modern subtidalzone, very close to MLWS. These corals, both the fossils and theirmodern counterparts, are observed to nucleate growth directly onthe sub-vertical granitewalls. Detailed notes were taken to describethe succession of facies in each of these 7 outcrops comprising Site4. As an example, a profile sketch of Outcrop A is shown in Fig. 3.

The highest in situ coral from this site is an encrusting Cyphastrea, at7.5 ± 0.2 m.

Site 7, at Anse Source d'Argent, La Digue, is the largest and mostvertically extensive outcrop we identified (Fig. 4) located justinland and northwards of the outcrop (Site 7A) that was previouslystudied in detail by Israelson andWohlfarth (1999). Site 7 preserves~6 m of vertical exposure of in situ coralgal reef framework, butowing to less protection from rain and runoff at this site the surfaceof the outcrop is unfortunately heavily weathered as evidenced bycalcite recrystallization and karstic pitting on the surface. Thisoutcrop contains the highest in situ coral that we found, a massiveGoniastrea retiformis (?), at 8.0 ± 0.2 m. The succession is capped bya 25-cm layered coralline algae-vermetid gastropod facies thatformed in the uppermost subtidal to intertidal zone (Montaggioniand Hoang, 1988). The presence of the well-developed corallinealgae-vermetid gastropod facies immediately above the highestcoral at this site gives high confidence that the highest corals in thissection grew up to the base of the subtidal zone and thus can beconsidered to have negligible paleodepth.

Site 19A on Curieuse is one of a cluster of outcrops found on theshore of Baie Laraie, situated immediately northeast of the northerntermination of the old causeway for the turtle pond (Fig. 5). Despitenumerous outcrops at this locality, Site 19A is the only outcrop inthis area that preserves in situ coralgal framework growing off asub-vertical granite wall; the rest are cemented coral rubble. Theprevalence of coral rubble in this area may be the result of thesoutheast facing aspect of the outcrop, because of the heavy swellsassociated with the southeast trade winds in the Seychelles.

Fig. 3. Sketch of Site 4, Outcrop A, Inland Site, La Digue (See black rectangle in Fig. 2).Profile sketch looking southwest (from left to right in picture shown in Fig. 2). Themorphology of this outcrop is typical of many of the outcrops at this site as well as atothers. Similar in situ coralgal vertical buildups were also observed on the modern reef,confirming that these seemingly fragmentary outcrops are preserving primary growthmorphologies. CAR ¼ coralline algae.

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Though not vertically extensivedit only spans ~1.5 m inheightdthe outcrop preserves a clear succession of in situbranching (Stylophora sp.) to massive coral heads, most of whichare capped by coralline algae, and some coral rubble infill. Thehighest in situ coral from this outcrop is a massive Favites at6.6 ± 0.2 m.

4.2. UeTh dating results

We analyzed six corals that had been collected by Israelson andWohlfarth (1999) to perform a cross-calibration of the datingmethods and assess within-sample heterogeneity in age accuracy.Half of these corals had previously been dated, and the comparisonof the UeTh measurements with associated 2s errors is shown inTable 2. The UeTh ages from MCeICP-MS (this study) and TIMS(Israelson and Wohlfarth, 1999) demonstrate excellent reproduc-ibility of ages between the two laboratories for unrecrystallizedcorals. In particular, ages for two of the samples are the samewithinerror, but in the third case (sample 90/4) the ages do not agree. Thisthird sample has 4% calcite, an anomalously high 238U concentra-tion (4.5 ppm) in one of the subsamples, and high 232Th concen-tration in both subsamples (~40e70 ppb). These observationssuggest alteration and detrital contamination, so the disagreementin ages is likely to be a result of heterogeneous preservation withina single coral head rather than instrumental differences.

The new samples in this study were dated in triplicate to assessthe reproducibility of the measurements and within-sample

diagenetic trends (Table 3, Fig. 6, S1). Four corals display unusuallylarge intra-sample variability in ages (total age range of ~10e20 ky).Of these, onehadhigh levels of 232Th contamination and twodisplaytrends that could be consistent with U addition from contaminantsource with lower 234U/238U activity ratio than seawater (shown bythedata points that trend to the lower left corner in Figs. 6a andS1a).These corals were removed from further analysis.

One common means of assessing open-system diagenesis is toback calculate initial d234U values from the measured d234U andcalculated age, where d234U is the activity ratio of 234U/238U relativeto secular equilibrium reported in ‰. Initial d234U values that aresignificantly higher than modern seawater are used to screen outaltered samples on the basis that open-system behavior oftenproduces elevated d234U values associated with correspondinglyelevated ages (e.g., Gallup 1994). However, triplicate analyses ofsome coral specimens (e.g., SY-36 and SY-63) reveal identical agesfor the sub-samples despite intra-sample variability in initial d234Uvalues that ranges from 0 to 4 ‰ above modern seawater. Sampleswith more extreme initial d234U values, however, appear to havebeen affected by open-system alteration as many of these sampleshave other anomalous parameters (e.g., high 232Th, inconsistencywith other subsamples from same coral, etc.). We also note that themost anomalous initial d234U values are from corals sitting at thetops of outcrops, particularly at Sites 4 and 7, where the samples aremore exposed to rainfall and weathering (Fig. S2).

There are several notable observations of the UeTh data shownin Fig. 6. First, a higher proportion of the sample set collected byIsraelson and Wohlfarth (1999) sit above the seawater evolutioncurves (that is, they have higher initial d234U values) whencompared to the new set of samples analyzed in this study(Fig. 6b). We attribute this effect to the XRD screening in thepresent study that removed more of the altered specimens. Sec-ond, screening out all measurements where the 230Th/232Th ac-tivity ratio is <500 removes many of the outliers (Fig. 6a) andsamples that are inconsistent with other subsamples from thesame coral. This corresponds to 232Th concentrations >~12 ppb.Assuming a secular equilibrium input of detrital thorium (anduranium) from local granites and re-calculating corrected agesdoes not change the age of the corals within the reported 2s er-rors for the samples with 230Th/232Th activity ratios >500. Hencethis level of contamination does not affect the ages reported here.The third notable observation is that the dataset (as a whole,within an outcrop, or within a sample) does not display a diage-netic pattern that is consistent with the open-system Th-additionmechanism of Thompson et al. (2003). Some of the within-sampletrends lie exactly on closed-system isochrons, but none werefound to track open-system isochrons defined by continuous Thaddition (Fig. S1). This observation is consistent with other studiesthat have identified diagenetic trends resulting from other UeThexchange processes that are not always easily modeled (Scholzet al., 2007; Andersen et al., 2008). Furthermore, arrays pro-duced by subsamples from the same corals on an evolution dia-gram display different trajectories, indicating different modes ofdiagenesis for different coral heads.

Attempts at isochron-based age calculations of corals with sig-nificant quantities of detrital Th were unsuccessful owing to lowprobability of fit. That is, the scatter in the samples could not beexplained by a simple detrital Th mixing explanation. We note,however, that isochrons based on only three data points may not bemeaningful. The presence of 232Th was sometimes associated witholder ages, and other times associated with younger ages, and in acouple instances associated with identical ages to uncontaminatedsamples. However, those samples with the highest concentrationsof 232Th (low 230Th/232Th activity ratios) are generally outliers in

Fig. 4. Outcrop picture and location of samples for Site 7, Anse Source d'Argent, La Digue. UeTh ages in bold, initial d234U values in parentheses, and sample numbers in upper leftcorner of insets. Ages shown are unscreened closed-system ages that are inverse variance-weighted means of triplicate measurements. Karst dissolution on the vertical face of theoutcrop is evidence of more pervasive weathering of the limestone at this site because of its exposure. Star denotes location of Site 7 on map in bottom right inset.

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comparison to the dataset as a whole or in comparison to othersubsamples from the same coral.

4.3. Surveying results

The highest in situ coral we found is situated at þ8.0 ± 0.2 m(elevations are reported as meters above mean low watersprings) at the top of Site 7 on Anse Source d'Argent. The GIA

correction in the Seychelles predicted from modeling is time-dependent, ranging from �1.2 to �0.8 ± 1.5 m relative to theESL curve (Fig. 7b) (Dutton and Lambeck, 2012). This yields anestimated peak LIG ESL at þ8.8e9.2 m (±1.7 m) greater thanpresent given the possible range of the GIA correction andcombining the error from the GIA model and the elevationmeasurements. This is the same calculation that was made inDutton and Lambeck (2012) to arrive at the peak ESL estimate of

Fig. 5. (A) Outcrop photo of sample locations at Site 19A on Curieuse island. UeTh ages (closed-system, screened ages, see text) in bold, initial d234U values in parentheses, andsample numbers in upper right corner of insets. Dashed line on map of Curieuse represents position of stone retaining wall of old turtle pond. (B) Outcrop sketch of Site 19Ademonstrating stratigraphic relationship of corals, CAR, and coral rubble.

Table 2Comparison of MCeICP-MS and TIMS ages.

Sample % Calcite [230Th/238U] 2s [234U/238U] 2s Age (ka)a 2s Same age (within error)

TIMS Israelson and Wohlfarth (1999) 90/2 0 0.7676 0.0029 1.1100 0.0040 124.7 1.390/3 wall 0 0.7645 0.0041 1.1140 0.0040 123.0 1.390/3 whole 0 0.7645 0.0026 1.1130 0.0030 123.0 1.090/4 4 0.8037 0.0106 1.1130 0.0070 135.1 3.9

MCeICP-MS (this study) 90/2 0 0.7590 0.0018 1.1037 0.0011 123.6 0.690/3 0 0.7579 0.0019 1.1065 0.0011 122.6 0.690/4 4 0.7666 0.0019 1.1045 0.0009 125.7 0.6

D(TIMS-MC) 90/2 0 0.0086 0.0063 1.1 Yes90/3 0 0.0066 0.0070 0.4 Yes90/4 4 0.0371 0.0085 9.4 No

a Ages for the TIMS data have been recalculated using the decay constants for 230Th and 234U in Cheng et al. (2000) assuming a secular equilibrium spike calibration.

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9.0 m. However, performing the GIA correction in this way doesnot explicitly account for the magnitude of the sea level finger-print associated with a rapid loss of ice from one of the polar icesheets. Hay et al. (2014) calculated that collapse of the Greenlandice sheet (GrIS), WAIS, or East Antarctic ice sheet (EAIS) willmanifest in a local sea-level rise in the Seychelles that over-estimates the ESL by 10e20 %. Hence, accounting for thisfingerprint effect brings the GIA-corrected ESL estimate downfrom 9.0 m, as previously reported, to 7.6 ± 1.7 m.

5. Discussion

5.1. Tectonic stability

To assess tectonic stability over the timescales considered here,we searched for corals from previous interglacials at higher eleva-tions that might be evidence of uplift. No such deposits were foundor reported elsewhere in the literature. We also considered modern

GPS measurements, which are continuously recorded on the mainisland of Mah�e. However, the uncertainty on these measurementsis not sufficient to resolve rates of uplift on the order of a fewmeters over the past 125,000 years. The absence of Holocene sealevels above present is also indicative of vertical movements lessthan ~0.17 mm/year (based on 1 m in 6000 years). Hence, at thisstage there is no compelling evidence of uplift in the Seychelles.The response of topography and sea level to mantle flow (dynamictopography) becomes important on longer time scales (Mouchaet al., 2008; Rowley et al., 2013), but it has not yet been demon-strated to be significant at the meter level on time scales of 100,000years.

5.2. Sea level reconstruction

An important observation from this new dataset is the addi-tional insight offered by making measurements on three separatefragments from each coral. Some samples were rejected when the

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three subsamples from the same coral had widely variable UeThgeochemistry, and hence ages, despite having passed XRDscreening, having low or negligible detrital Th, and having initiald234U values consistent with modern seawater. In other words,some coral samples that pass all other diagenetic screening toolshave still experienced open-system behavior with respect to U andTh isotopes. The dataset from the LIG reef on the Yucatan peninsulais similar in this respect (Blanchon et al., 2009). Some of theYucatan corals that passed all screening criteria still had triplicateages that spanned ~10 ky within the same coral head. Likewise,some corals from Huon Peninsula and Barbados have been found tobehave as open systems with respect to U-series isotopes based onPaeTh data, but have an apparent unaltered signal based on initiald234U values that are close to that of modern seawater (Edwardset al., 1997). These observations, spanning a range of sites andages of corals, indicate that open-system behavior of U and Thisotopes can be difficult to detect by standard screening methodsand suggest that the practice of performing multiple measure-ments on each coral is important for assessing sample heteroge-neity stemming from diagenesis, particularly with older samples.

The quality of the UeTh data was assessed in multiple ways,including comparison of triplicate ages for each coral, assessmentof 232Th concentrations (as a proxy for detrital contamination) andd234U (as a proxy for open-system behavior). Slightly elevatedinitial d234U values in many samples did not appear to produceanomalous ages, which may indicate that ambient seawatercomposition at the time of growth was slightly higher and/orvariable due to the influence of chemical weathering of localgranites. Based on this observation, slightly elevated d234U valuesshould not necessarily be taken as evidence for diagenetic alter-ation. However, samples with more extreme initial d234U valuesare likely to be a consequence of open-system diagenesis based ontheir association with other anomalous geochemistry parametersand inconsistent age-stratigraphic relationships (see Section 4.2).This raises an important point regarding the assessment of coralUeTh age-elevation data. In general, it is not possible to use thepresence of age inversions in UeTh age-elevation data from coralreefs as a screening criteria to identify altered samples unlessdetailed spatial relationships of the samples within the reef areknown. This is because coral reefs grow in 3 dimensions and havecomplicated surface morphologies that allow ample opportunityfor younger corals to occupy spaces that are lower in elevationthan older corals within the same reef or terrace. However, in thiscase, because we are dealing with coral reef framework thataccreted vertically on the granite walls and because we have madedetailed observations of the spatial relationships and faciescontext of the samples, we are able to use the presence of ageinversions within an outcrop to identify samples that may haveundergone open-system diagenesis. Hence, the ability to viewsamples from multiple orientations in these outcrops versus adrill-core sample or a flat 2-D sampling surface allows us to makemore confident interpretations about stratigraphic relationshipsand in situ preservation.

We considered reliable ages to include samples with a230Th/232Th activity ratio >500, as discussed above and as shown inFig. 6a. Applying an additional screening criteria to remove allsamples with an initial d234U that is >5‰ away from that of modernseawater (147‰, Andersen et al., 2010) only removes one additionaldata point from Site 19A, and hence does not affect our in-terpretations of sea level behavior. We also tested a variety of tol-erances for detrital Th (232Th) content and initial d234U values toscreen out the geochemically altered samples, but regardless of thelimits we applied, two key observations remained consistent. First,the majority of reliable ages fall between 129 and 125 ka. Second,two outcrops (Site 4dthe inland site on La Digue, and Site 19A on

Curieuse) display in situ coral reef facies that suggest a gradual sealevel-rise that was interrupted by a single coral rubble layer in eachoutcrop. UeTh age data from these two outcrops support this faciesinterpretation as they both exhibit an age-elevation pattern ofgradual relative sea level-rise between ~129 and 125 ka with noobserved stratigraphic inversions (Figs. 2, 5 and 7a).

The corals from Sites 4 and 19A that demonstrate gradual sea-level rise from þ5.8 to 6.6 m between 129 and 125 ka are cappedwith coralgal-vermetid layers and hence are interpreted to closelytrack the position of sea level. The tight agreement in data betweenSites 4 and 19A, which are located on different islands and hencewere surveyed relative to independent tidal measurements, sup-ports our interpretation of extremely shallow paleodepth of thecorals in these outcrops as both record the same elevations throughtime. Both sequences also contain a rubble layer immediatelybelow the highest (youngest) coral in the outcrop, indicating a briefinterruption in reef accretion. However, coral rubble does not havea unique indicative meaning relative to the position to sea level, asit can accumulate in the supratidal or subtidal zone, so this layer isnot necessarily evidence for a change in sea level.

There is one coral (SY-56) from Site 7 on La Digue that passes thescreening criteria, but its elevation appears inconsistent with time-equivalent data from Sites 4 and 19A (Fig. 7). Site 7 had the worstpreservation in terms of mineralogy, and it is possible that despitepassing the XRD screening that we dated a portion of the coral thatwas more altered than the corresponding XRD subsample. Withoutbeing able to assess its age-elevation relationship with othersamples within this outcrop, it is difficult to make a more rigorousassessment of the reliability of this particular sample. Based on itsstratigraphic inconsistency with data from Sites 4 and 19A weconsider it more likely that SY-56 from Site 7 is altered than thealternate possibility that it recorded the true position of sea levelwhile corals from Sites 4 and 19A simultaneously tracked sea levelat 1.5 m paleodepth. There is also one coral from the south end ofAnse Source d'Argent on La Digue (Site 8) at a lower elevation thansimilar age corals from Sites 4 and 19A. This particular outcrop isdifferent than the other sampled outcrops in that it is attached onthe landward side of a granite boulder rather than seaward facinglike the rest of the outcrops we sampled. It is also in a protectedcave, so may represent a lower energy environment and a differentpaleodepth.

There are only two corals from the Israelson and Wohlfarth(1999) study that pass the screening criteria (Fig. 7a). These twocorals have identical ages (123.8 ± 0.5 ka) and occur at an elevationnear þ4 m. These two samples (90/1 and 90/2) are described asbeing capped by coralgal-vermetid layers (Israelson andWohlfarth,1999), and if they grew up to the intertidal zone may represent adrop in sea level from the þ6.6 ± 0.2 m attained at 125.1 ± 0.4 ka.This interpretation is viewed as tentative because the previouslysampled outcrop has since been partially destroyed by beach pathconstruction and we were unable to make direct comparisons ofthe reef facies between this and the recently collected samples.

Finally, the highest two in situ corals that we sampled (SY-52and -53 at Site 7) both have very elevated initial d234U values, low230Th/232Th activity ratios, and poor reproducibility in ages be-tween subsamples. Unlike Sites 4 and 19A, the large outcrop at Site7 on Anse Source d'Argent is not a thin, plastering outcrop on agranite wall, but has a larger 3-dimensional reef structure with ahorizontal extent of at least twometers before it abuts granite wallsor cemented reef rubble in the crevices behind. Because of this,during rainfall events water ponds on the surface of the outcrop,which explains the generally poorer preservation at thissitedparticularly toward the topdand is consistent with eogenickarst development visible in Fig. 4. The two subsamples of thehighest coral (SY-52, atþ8.0 m) that agree within error yield an age

Table 3MC-ICP-MS UeTh measurements and ages.

Location SampleID

cca

(%)

238U(ppm)

230Th(ppt)

232Th(ppb)

[230Th/232Th]b [230Th/238U] 2s [234U/238U]b 2s Age (ka) 2s d234Uic

(‰)2s Avg. age

(ka) ± 2sd

This expeditionSite 11 SY-09a 0 2.416 32.246 0.652 9273 0.8214 0.0026 1.1059 0.0010 143.0 1.0 158.6 1.0

SY-09b 0 2.568 32.042 0.447 13,433 0.7677 0.0025 1.1044 0.0010 126.0 0.8 149.1 1.0SY-09c 0 2.454 29.557 1.275 4344 0.7412 0.0015 1.1075 0.0006 117.6 0.5 149.9 0.6SY-12a 3.1 2.682 33.231 0.592 10,516 0.7656 0.0047 1.1142 0.0012 123.1 1.4 161.8 1.7 124.1 ± 0.5SY-12b 3.1 2.776 34.597 0.240 27,067 0.7700 0.0019 1.1140 0.0014 124.5 0.6 162.0 1.9SY-12c 3.1 2.734 33.935 0.267 23,846 0.7668 0.0027 1.1138 0.0008 123.6 0.8 161.4 1.2

Site 4 SY-22a 0 2.452 30.395 1.456 3913 0.7629 0.0016 1.1048 0.0010 124.5 0.5 149.0 1.0SY-22b 0 2.527 26.838 1.451 3468 0.6535 0.0016 1.1036 0.0007 96.1 0.4 136.0 0.7SY-22c 0 2.305 26.960 1.558 3243 0.7196 0.0020 1.1042 0.0012 112.4 0.6 143.1 1.2SY-24a 1.0 2.456 28.755 4.188 1287 0.7204 0.0019 1.1051 0.0012 112.5 0.6 144.4 1.2SY-24b 1.0 2.442 30.480 3.408 1676 0.7681 0.0016 1.1036 0.0010 126.3 0.6 148.1 1.0SY-24c 1.0 2.783 27.502 7.204 716 0.6080 0.0015 1.1036 0.0017 86.0 0.4 132.1 1.7SY-27a 0 2.247 28.105 0.511 10,301 0.7697 0.0017 1.1030 0.0008 127.0 0.6 147.4 0.8 127.3 ± 0.4SY-27b 0 2.234 28.100 0.668 7886 0.7738 0.0026 1.1028 0.0011 128.3 0.8 147.7 1.1SY-27c 0 2.290 28.635 0.680 7892 0.7694 0.0019 1.1024 0.0014 127.0 0.7 146.6 1.4SY-36a 2.9 4.546 56.616 4.921 2157 0.7694 0.0032 1.1042 0.0007 126.6 1.0 149.1 1.0 126.6 ± 0.5SY-36b 2.9 4.372 54.500 16.303 627 0.7701 0.0017 1.1050 0.0009 126.6 0.6 150.2 1.2SY-36c 2.9 4.403 54.791 42.681 241 0.7689 0.0014 1.1034 0.0008 126.6 0.5 147.9 1.0SY-37a 0 4.212 52.642 11.282 875 0.7722 0.0021 1.1041 0.0010 127.5 0.7 149.3 1.2 127.2 ± 0.5SY-37b 0 3.958 49.204 33.938 272 0.7680 0.0025 1.1045 0.0008 126.1 0.8 149.2 1.1SY-37c 0 4.030 50.308 10.753 877 0.7713 0.0023 1.1058 0.0008 126.8 0.7 151.5 1.1SY-41a 0.1 2.443 30.809 1.061 5441 0.7759 0.0063 1.1036 0.0010 128.7 2.0 149.1 1.0 125.8 ± 0.5SY-41b 0.1 2.551 31.323 12.323 476 0.7556 0.0019 1.1050 0.0012 122.3 0.6 148.4 1.2SY-41c 0.1 2.433 30.280 1.690 3358 0.7658 0.0015 1.1038 0.0009 125.6 0.5 148.0 0.9

Site 7 SY-52a 0.8 2.733 33.860 23.598 269 0.7623 0.0019 1.1107 0.0011 123.0 0.6 156.8 1.5SY-52b 0.8 2.553 31.534 21.068 281 0.7601 0.0021 1.1097 0.0007 122.5 0.6 155.1 0.7SY-52c 0.8 2.375 29.389 181.115 30 0.7614 0.0028 1.1241 0.0009 119.8 0.8 174.0 0.9SY-53a 1.5 3.311 41.550 122.917 63 0.7753 0.0037 1.1186 0.0010 124.9 1.1 168.8 1.3SY-53aa 1.5 3.753 46.817 93.963 93 0.7706 0.0018 1.1212 0.0008 123.0 0.5 171.6 1.0SY-53b 1.5 3.255 40.327 128.543 59 0.7655 0.0029 1.1318 0.0009 119.2 0.8 184.6 1.2SY-53bb 1.5 3.398 42.585 149.655 53 0.7742 0.0021 1.1278 0.0008 122.5 0.6 180.7 1.0SY-53c 1.5 3.235 39.622 159.206 47 0.7568 0.0033 1.1387 0.0012 115.5 0.9 192.2 1.6SY-53cc 1.5 3.840 34.410 6.342 1017 0.5536 0.0011 1.1322 0.0011 72.1 0.2 162.0 1.3SY-56a 2.8 2.426 40.847 90.243 85 1.0402 0.0021 1.1058 0.0011 169.3 2.6 226.4 2.1 127.3 ± 0.4SY-56b 2.8 2.789 34.785 2.136 3053 0.7705 0.0020 1.1056 0.0010 126.6 0.7 151.0 1.3SY-56c 2.8 2.683 33.552 2.198 2861 0.7728 0.0014 1.1044 0.0013 127.6 0.5 149.7 1.6

Site 8 SY-63a 0.9 2.921 36.439 2.931 2330 0.7709 0.0017 1.1035 0.0005 127.2 0.5 148.3 0.7 127.1 ± 0.3SY-63b 0.9 2.899 36.170 2.406 2818 0.7708 0.0012 1.1036 0.0009 127.2 0.4 148.3 1.2SY-63c 0.9 2.767 34.562 5.971 1085 0.7718 0.0018 1.1057 0.0008 127.0 0.6 151.3 1.1

Site 19A SY-66a 0.8 2.837 35.953 17.731 380 0.7797 0.0024 1.1038 0.0011 129.9 0.8 149.8 1.5 128.6 ± 0.8SY-66b 0.8 2.859 35.723 29.960 224 0.7688 0.0025 1.1039 0.0008 126.5 0.8 148.6 1.1SY-66c 0.8 2.749 34.689 9.492 685 0.7765 0.0023 1.1050 0.0010 128.6 0.8 151.0 1.0SY-68a 0 2.538 31.681 20.572 289 0.7680 0.0020 1.1021 0.0010 126.7 0.6 146.0 1.0 125.1 ± 0.4SY-68b 0 2.694 33.429 6.402 979 0.7635 0.0021 1.1040 0.0008 124.9 0.6 148.0 0.8SY-68c 0 2.501 31.087 1.713 3401 0.7648 0.0018 1.1039 0.0008 125.3 0.6 148.0 0.8SY-69a 1.0 2.287 28.746 5.879 917 0.7734 0.0013 1.1072 0.0010 127.1 0.5 153.6 1.0 126.6 ± 0.4SY-69b 1.0 2.227 27.843 11.952 437 0.7694 0.0017 1.1083 0.0013 125.6 0.6 154.5 1.3SY-69c 1.0 2.261 28.296 10.084 526 0.7699 0.0019 1.1075 0.0012 126.0 0.6 153.5 1.2

New Data from Expedition of Israelson and Wohlfarth (1999)Unknown SY-PRC-1 N.D. 3.286 40.995 2.331 3297 0.7677 0.0022 1.1108 0.0017 124.5 0.8 157.5 2.3 124.5 ± 0.8Site 7A SY-90/1a N.D. 2.537 30.026 58.834 96 0.7283 0.0025 1.1113 0.0014 113.4 0.7 153.4 1.8

SY-90/1b N.D. 2.281 28.376 3.493 1523 0.7656 0.0016 1.1112 0.0009 123.8 0.5 157.8 1.2 123.8 ± 0.5SY-90/2 0 2.583 31.858 0.335 17,842 0.7590 0.0018 1.1037 0.0011 123.6 0.6 147.1 1.4 123.8 ± 0.5SY-90/3 0 2.251 27.729 38.498 135 0.7579 0.0019 1.1065 0.0011 122.6 0.6 150.6 1.4SY-90/4 4 3.722 46.377 70.686 123 0.7666 0.0019 1.1045 0.0009 125.7 0.6 149.1 1.2

Site 19 SY-90/45a N.D. 2.636 32.133 4.933 1221 0.7500 0.0017 1.1033 0.0012 121.1 0.5 145.5 1.5SY-90/45b N.D. 2.781 34.283 1.551 4143 0.7587 0.0016 1.1035 0.0009 123.6 0.5 146.8 1.2

Previously Published Data from Expedition of Israelson and Wohlfarth (1999)Unknown 90/2 0 2.803 0.560 11,741 0.7676 0.0029 1.1100 0.0040 124.7 1.3 156.5 5.3 124.7 ± 1.3Site 7A 90/3 0 2.030 9.190 516 0.7645 0.0041 1.1140 0.0040 123.0 1.3 160.7 4.6 123.0 ± 1.3

90/3 0 2.338 95.420 57 0.7645 0.0026 1.1130 0.0030 123.0 1.0 160.0 4.090/4 4 4.488 44.660 247 0.8037 0.0106 1.1130 0.0070 135.1 3.9 165.5 9.590/5 4 2.818 0.280 23,891 0.7768 0.0038 1.1010 0.0080 129.7 2.3 146.0 11.090/6 9 2.526 0.530 11,340 0.7786 0.0035 1.1100 0.0050 128.0 1.6 157.9 6.790/7 7 2.495 1.190 5075 0.7921 0.0038 1.1140 0.0070 131.1 2.1 165.1 9.390/8 0 2.735 0.790 8171 0.7723 0.0074 1.1100 0.0090 126.1 3.1 157.0 12.0 126.1 ± 3.190/9 0 2.914 12.900 540 0.7821 0.0042 1.1090 0.0040 129.3 1.6 157.1 5.4 129.3 ± 1.690/10 35 2.494 127.020 47 0.7794 0.0082 1.1090 0.0060 128.5 2.9 156.7 8.090/47 9 2.932 24.150 287 0.7741 0.0038 1.1030 0.0050 128.3 1.7 148.0 6.7

N.D ¼ not determined.Underline font denotes corals withwide intra-sample age variation that otherwise pass screening criteria; italic font denotes 230Th/232Th activity ratios below 500 and calcite %of 4 or above; boldface font denotes samples passing screening criteria (see text).

a cc ¼ calcite.b Square brackets denote activity ratios.c Subscript ‘i’ denotes initial d234U value.d Average ages shown for each coral are inverse variance-weighted means of data passing the screening criteria (see text). All ages are calculated using the decay constants

of Cheng et al. (2000).

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196192

of 122.8 ± 0.4 ka, but because of poor preservation and very highinitial d234U values this age is not considered reliable. Hence thetiming of peak sea level remains uncertain, other than that it waslikely attained at some point after 125 ka.

5.3. Rates of sea-level change

Using the combined age-elevation data from Sites 4 and 19Awecalculate a RSL rise of 0.26 ± 0.04 m per thousand years (ky) that,when corrected for GIA, corresponds to a slightly lower rate in ESLrise (0.22 ± 0.04 m/ky). This is an order of magnitude lower thanthe ESL-rise rate of recent decades (Cazenave and Nerem, 2007) butis consistent with simulated rates of GrIS meltdestimated at

Fig. 6. (A) Evolution diagram showing activity ratios with 2s errors for individual UeTh meaevolution curve of seawater. Modern seawater is 147‰ (Andersen et al., 2010) and two othlines). Removing measurements with [230Th/232Th] activity ratios <500 (open symbols) remoof 147‰ predicted by the Thompson et al. (2003) model (gray straight lines) are shown as wfrom Israelson and Wohlfarth (1999) superimposed.

0.26e0.45 m/ky at 128 ka where the reference experiment con-tributes 0.36 m/ky at 128 ka (Helsen et al., 2013). This raises thepossibility that sea level-rise observed across this 4-ky time inter-val in the Seychelles may be largely or even entirely attributed tomelting of the GrIS. Furthermore, the difference in elevation be-tween the oldest coral at Site 19A (5.8 m; 128.6 ± 0.8 ka) and thehighest in situ coral at Site 7 (8.0 m; age undetermined) is 2.2 m. Ifthe gradual sea-level rise is due to GrIS melt and we apply theestimate from Hay et al. (2014) that the Seychelles would over-estimate the ESL change associated with GrIS collapse by 9e13%depending on the timescale of the collapse, then the elevationdifference in these corals corresponds to an ESL rise of 1.9e2.0 m.This is a good match for the total volume of ice melt expected from

surements of subsamples from corals collected for this study. Corals should plot on theer seawater evolution curves are shown for reference (150‰ and 152‰) (black curvedves the most of the outliers. Open-system isochrons for an initial seawater compositionell as the closed-system isochrons (black dashed lines). (B) Same as in (A), but with data

Fig. 7. (A) UeTh age-elevation data for coral samples plotted using screened closed-system ages (see text). Coral ages are plotted as inverse variance-weighted means ofreplicate measurements for each coral with 2s errors; elevation error is smaller thanthe size of the symbol for most samples. Elevation of highest coral represented withdashed line. Observed RSL history in the Seychelles denoted with gray line; possibleRSL history denoted with gray dashed line. The site on Curieuse (19A) and the inlandsite on La Digue (4) both demonstrate a history of gradual sea level rise. Samples fromSite 7A are from Israelson and Wohlfarth (1999). (B) GIA model prediction forSeychelles RSL using a forward model where ESL is held constant at 0 m as in Duttonand Lambeck (2012). (C) RSL data are converted to ESL data using GIA model (in panelB) and GIA effects from rapid ice sheet collapse (Hay et al., 2014). Data are shown witherror boxes that include 2s errors from age measurements as in (A) and combinederrors from measured elevation and GIA corrections. Dashed crossed lines representelevation of highest in situ coral head that did not produce a reliable age owing to highdetrital Th and elevated initial d234U values.

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196 193

the GrIS during the LIG period according to a number of studies, andrecently estimated at 2 m using evidence from the NEEM ice core(Dahl-Jensen et al., 2012) (Fig. 8).

Other estimates of rates of sea-level change during the LIGperiod (Rohling et al., 2008; Kopp et al., 2009; Thompson et al.,2011; Kopp et al., 2013) focus on or include rates of sea-levelchange during ephemeral sea level oscillations within the LIG sealevel highstand, so are not directly comparable to the rate ofgradual sea-level rise that we calculate here. The gradual sea-levelrise we have documented in the Seychelles between 129 and 125 kaand that may have continued until reaching the peak sea level, mayhave been punctuated by brief oscillations in sea level character-ized by higher rates of sea-level change estimated from 1 to 7 m/ky(Rohling et al., 2008; Thompson et al., 2011; Kopp et al., 2013). Asnoted above, the Seychelles data is at least consistent with anephemeral drop in sea level at 123.8 ± 0.4 ka which is within errorof the timing of the first sea level oscillation reported in theBahamas by Thompson et al. (2011) using either the open-systemmodel or the closed-system screening approach to interpretingthe Bahamian UeTh ages. We also note the presence of a layer ofcoral rubble just below the highest coral dated in the sequences atSites 4 and 19A, which is also consistent with, but not necessarilyindicative of, a brief drop in sea level at ~125 to 126 ka. These ob-servations provide tantalizing possible evidence of sea level oscil-lations that punctuate the overall rising sea level in the Seychelles,but are not considered conclusive evidence for such events.

5.4. Eustatic sea level and polar ice sheet contribution

As described in Section 4.3, we have made a two-step correctionto the RSL data to translate them into an estimate of ESL position.The Seychelles data indicate that ESL had already reached5.9 ± 1.7 m by 128.6 ± 0.8 ka, which is within error of the timingwhen ESL exceeds modern sea level according to either the open-system or closed-system ages of the data from Western Australia(Stirling et al., 1998; Dutton and Lambeck, 2012; O'Leary et al.,2013). Together, these imply rapid collapse of a polar ice sheetwithin 1e2 ky at the onset of the LIG period. Because recent esti-mates of total ice melt from the GrIS range from ~2 to 4 m (Fig. 8),the more likely candidate to explain such a rapid rise is a marine-based portion of the Antarctic ice sheet (AIS). The WAIS couldcontribute up to a 5-m ESL rise (Gomez et al., 2010) with possibleadditional contributions from adjacent sectors of the EAIS (Pingreeet al., 2011), although the marine-based portion of the WAIS thatwould be most susceptible to collapse would contribute a ESL riseof 3.3e3.4 m (Bamber et al., 2009; Fretwell et al., 2013). Althoughthe WAIS is often invoked as the most susceptible sector to rapidcollapse, marine-based portions of the EAIS may be equally proneto collapse (Fretwell et al., 2013; Mengel and Levermann, 2014). Ourobservations in the Seychelles indicate that ESL then continues torise until after 125 ka, possibly punctuated by ephemeral sea leveloscillations that occur over ~1e3 ky time intervals (Rohling et al.,2008; Thompson et al., 2011). This additional, more gradual sea-level rise presumably stems from GrIS melt, if our interpretationof partial AIS collapse early in the LIG period is correct. The timingof peak sea level and the ensuing sea level regression associatedwith glacial inception is not resolved by the Seychelles dataset.

A critically important observation to note from our dataset interms of implications for ice-sheet response is that even early in theinterglacial, ESL reached 5.9 ± 1.7 m above present sea level, whichwe have attributed to rapid retreat from an unstable sector(s)within the AIS. This scenario is at odds with other studies that havesuggested a collapse of the WAIS near the end of the LIG high-standdimmediately prior to glacial inception (Hearty et al., 2007;Blanchon et al., 2009). O'Leary et al. (2013) also suggest a rapid

Fig. 8. Estimates of the contribution of the GrIS to global mean sea level during the LIG period. Most likely range shown (black bars) and most likely value or best estimate withinrange (circles). Right axes display corresponding magnitude of Seychelles RSL signal for GrIS collapse assuming instantaneous collapse (DT ¼ 0 ka), or over 3 thousand years(DT ¼ 3 ka). Thick black bar represents height difference (2.2 m) between corals that grew at the beginning of the sea level highstand and at the peak sea level in the Seychelles. Thestudies numbered along the x-axis are: (1) Cuffey and Marshall (2000); (2) Huybrechts (2002); (3) Tarasov and Peltier (2003); (4) L'homme et al. (2005); (5) Otto-Bliesner et al.(2006); (6) Oerlemans et al. (2006); (7) Robinson et al. (2011); (8) Colville et al. (2011); (9) Fyke et al. (2011); (10) Born and Nisancioglu (2012); (11) Quiquet et al. (2013); (12) Dahl-Jensen et al. (2012); (13) Helsen et al. (2013); (14) Stone et al. (2013). Note that the most recent studies (8e14) bound the range to between 0.6 and 3.5 m (dashed lines).

Table 4Eustatic sea-level budget for peak LIG sea level.

Source of sea-levelrise

Estimate ofcontribution (m)

Likelyrange (m)

Source

Thermal expansion 0.4 0.1e0.7 McKay et al. (2011)Mountain glaciers 0.6 0.53e0.67 Radi�c and Hock

(2010)Greenland ice sheet

(GrIS)2.0 Up to 3.5 See Fig. 8

Antarctic ice sheet(AIS)

4.6 Up to 5.0 WAIS e Bamber et al.(2009); Gomez et al.(2010)

Up to 4.7 EAIS e Pingree et al.(2011)

Total 7.6 5.9e9.3 This study

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196194

rise in sea level at the end of the LIG highstand, though they do notattribute it to a specific source. Resolving this discrepancy in thetiming of collapse within the AIS is of course essential to under-standing the climatic context of this dynamic ice sheet response. Tothis end, we note that the timing of partial AIS collapse that weinfer from the Seychelles dataset (128.6 ± 0.8 ka) is coincident withthe timing of rapid increases recorded in Antarctic temperature,CO2 (peak is ~128.9e128.4 ka), CH4 (sharp rise from 128.9 to128.6 ka), and an abrupt shift in the deuterium excess value (at128.5 ka) in the EPICA Dome C ice core (Masson-Delmotte et al.,2010).

The ~4 m rapid rise that occurs at ~129 ka in Western Australiaat the onset of the highstand is also consistent with the interpre-tation of an early WAIS or EAIS collapse (see data compilations inDutton and Lambeck, 2012; O'Leary et al., 2013). The relatively flatRSL trend from ~129 to 120 ka in Western Australia is consistentwith a gradually rising ESL (see Fig. 2d in Dutton and Lambeck,2012). Both of these observations are consistent with the evolu-tion of sea level we have documented in the Seychelles.

The Seychelles ESL estimate of peak sea level at þ7.6 ± 1.7 m(5.9e9.3 m) agrees with an independent analysis that LIG ESLextremely likely (95% probability) exceeded 6.4 m but is unlikely(33% probability) to have exceeded 8.8 m (Kopp et al., 2013). In anearlier analysis of LIG fossil reefs from Western Australia, peak ESLwas estimated at 5.5 ± 1.5 m (4.0e7.0 m) (Dutton and Lambeck,2012). This assessment should be revised downward to5.0 ± 1.5 m (3.5e6.5 m) based on the expected sea level fingerprintof polar ice sheet collapse (Hay et al., 2014). A more recent studyhas suggested peak sea level inWestern Australia much higher, to alevel of ~9 m, based on the elevations of corals preserved along theQuobba ridge (O'Leary et al., 2013). Both of these estimates fromWestern Australia overlap with the Seychelles estimateof þ5.9e9.3 m, albeit at different ends of the range.

To explain the components that contributed to peak LIG sealevels wemust rely on sources of information beyond the coral reefdata. It has been estimated that thermal expansion may havecontributed 0.4 m of sea-level rise (McKay et al., 2011) with anadditional 0.6 m from loss of mountain glaciers (Radi�c and Hock,2010). To achieve the Seychelles estimate of þ7.6 ± 1.7 m wouldthen require a total of 6.6 ± 1.7 m from polar ice sheet retreat

(Table 4). If the GrIS only contributed 2 m, this would require anadditional 4.6 ± 1.7 m from the AIS, an amount that is compatiblewith complete WAIS retreat. However, the Seychelles data cannotresolve which sector of the AIS may have collapsed. To establish thesource of AIS contributions to LIG sea-level rise (WAIS or EAIS)requires either near-field data that is sensitive to nearby ice sheetchanges or far-field data of sufficient precision to discern a finger-print of the ice sheet collapse. The fingerprinting approachmay be achallenge given that the difference in the signal expected for theWAIS versus the EAIS may be too small relative to the precision ofthe field observations, and that the signal may reflect a combina-tion of ice-loss from both the WAIS and the EAIS, making it difficultto separate the two. Observational data on the former extent of theAIS or offshore geochemical sedimentary evidence of increasederosion or iceberg discharge would be valuable tools to assesswhich sector of the AIS experienced mass loss.

6. Conclusions

Vertical successions of in situ fossil corals from the Seychellesrecord a gradual sea-level rise between ~129 and 125 ka at anaverage eustatic sea-level rise rate of about 0.22 ± 0.4 m/ky (mm/yr). An intervening layer of coral rubble just before 125 ka in twooutcrops indicates that this gradual rise may have been brieflyinterrupted, but the meaning of this rubble layer is still open to

A. Dutton et al. / Quaternary Science Reviews 107 (2015) 182e196 195

interpretation. Significantly, RSL reached at least þ6.8 ± 0.2 m(corresponding ESL: þ5.9 ± 1.7 m) by 128.6 ± 0.8 ka, at thebeginning of the sea level highstand. This implies mass loss frompolar ice-sheets early in the interglacial, coincident with the timingof rapid changes in several climate parameters in the EPICA Dome Cice core (Masson-Delmotte et al., 2010). Given the propensity ofcoupled climate-ice sheet models of the GrIS to predict progressivemelting until ~121e122 ka at rates consistent with the gradual sea-level rise observed in the Seychelles, we suggest that loss of amarine-based sector of the AIS may have been triggered near theonset of the highstand to explain the elevation of sea levelsobserved.

Current ice sheet models do not predict loss of the WAIS duringMIS 5e, though one is predicted during MIS 5c and MIS 7 owing tostrong austral summer insolation in these intervals that warmsSouthern Ocean surface waters and eventually leads to WAISretreat (Pollard and DeConto, 2009). There is a clear need to betterunderstand the past history of the WAIS as well as vulnerablesectors of the EAIS during previous interglacial periods that drawsupon observational data as well as modeling (Bentley, 2010;Joughin and Alley, 2011). In the context of present warming inthe upper kilometer of the circumpolar Southern Ocean (Mayewskiet al., 2009) and rising sea levels, are we poised for another partialcollapse of the AIS? A suite of recent analyses suggests that thisprocess may have already begun (Favier et al., 2014; Joughin et al.,2014; Mouginot et al., 2014). Our observations in the Seychellesimply that this event was triggered early in the LIG period, thoughdecisive field evidence from the Antarctic region is still lacking.

Acknowledgments

We thank citizens and authorities in the Seychelles who facili-tated our fieldwork, including P. Samson at PetroSeychelles, theSeychelles National Parks Authority, the Ministry of Environmentand Energy, and the Seychelles Bureau of Standards. We also thankG. Mortimer, K. Holland, and M. Pfahl for assisting in samplepreparation and analysis, and P. Woodworth, P. Caldwell, and S.Woodroffe for discussions on tidal data. This work was supportedby the Australian Research Council (ARC DP0773019 to K.L.) and theNational Science Foundation (Award #1155495 to A.D.) and theFondazione Internazionale Balzan.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quascirev.2014.10.025.

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