Bahamian Coral Reefs Yield Evidence of a Brief Sea-Level Lowstand During the Last Interglacial

Home , Erosion surface, Inagua, San Salvador Island

Smith ScholarWorks

Geosciences: Faculty Publications Geosciences

1998

Brian White Smith College

H. Allen Curran Smith College, [email protected]

Mark A. Wilson The College of Wooster

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Recommended Citation White, Brian; Curran, H. Allen; and Wilson, Mark A., "Bahamian Coral Reefs Yield Evidence of a Brief Sea- Level Lowstand During the Last Interglacial" (1998). Geosciences: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/geo_facpubs/50

This Article has been accepted for inclusion in Geosciences: Faculty Publications by an authorized administrator of Smith ScholarWorks. For more information, please contact [email protected] BAHAMIAN CORAL REEFS YIELD EVIDENCE OF A BRIEF SEA-LEVEL LOWSTAND DURING THE LAST INTERGLACIAL

'Brian White, IH. Allen Curran, and 2Mark A. Wilson 'Department ofGeology, Smith College. Northampton. MA 01063 2Department of Geology, The College of Wooster. Wooster. OH44691

ABSTRACT: The growth of large, bank-barriercoral reefs on the Bahamian islands of Great Inagua and San Salvador during the last interglacialwas interruptedby at leastonemajor cycleof sea regressionandtransgression. Thefall of sea levelresulted in the development of a wave-cutplatformthat abradedearlySangamoncoralsin parts of the Devil's Pointreef on GreatInagua, andproducederosionalbreaks in thereefalsequenceselsewherein the Devil's Pointreef andin theCockburnTownreef on SanSalvador. Minorred calicheandplant trace fossils formed on earlier interglacialreefal rocks during the low stand. The erosional surfacessubsequently were bored by sponges and bivalves,encrustedby serpulids, and recolonizedby corals of youngerinterglacial age duringthe ensuingsea-levelrise. Theselater reefal depositsformthebase of a shallowing-upward sequencethat developedduringtherapidfallofsealevelthat marked theonsetofWisconsinan glacialconditions. Petrographicstudiesreveal a diageneticsequencethatsupportsthis sea-levelhistory. Preservationof pristine coralline aragonite, coupledwith advancesin U/Thage dating,allowtheseeventsin thehistoryof thereefs to be placed in a precisechronology. We use thesedata to show that there wasa timewindowof 1,500yearsor lessduringwhich theregression/transgression cycle occurred, andthat rates of sea-levelchange must have been very rapid. We compareour resultswith the GRIP ice-coredata, and show that the history of the Bahamian coral reefs indicates an episode of climate variability during the last interglacialgreater fuan any reported in what is widely believed to be the more stable climateof the Holocene interglacial.

INTRODUCTION reefs on the west side of San Salvador(Fig. 2) in order to determine theirgeologic history, particularly withrespectto Bank-barrier and patch coral reefs flourished on the sea-level events. These three fossil reefs are the largest Bahamian islands of San Salvador and Great Inagua (Fig. 1) currently documented from the Bahamas. Petrographic duringthelastinterglacial-variously knownassubstage 5eof studies havebeenusedto determine the diagenetic historyof the marine isotope scale, the Sangamon of North America, these reefs as it relates to the sea-level changes that have and theEemianof Europe. We haveconducted detailed field affected them. studiesof fossilreefs nearDevil's Pointon the westcoastof Greatlnagua,andof theCockburn TownandSuePointfossil Advances in the dating of aragonite using TIMS U/Th

Figure 1. Location of GreatInagua andSanSalvador islands. Bahamas. Ctll'bonotes & Evaporites, v. 13, no. I, 1998,p. 10-22. WI-llTE, CURRAN, ANDwn.SON

o 20 FIELD EVIDENCE I E="""d km Devil's Point FossilReef

Devil's Point A well-developed wave-eut platform withintheSangamonian sequence at Devil's Point (Fig. 3A) provides the most prominent fieldevidence fora fall and subsequent riseof sea levelduring the last interglacial. Corraded surfaces of large coral headsof Montastrea and Diploria fossilized in growth c-c' 21°N position (Fig.3B)suggestremoval ofseveraldecimeters ofthe coralheadtops. Otherfieldevidence showsthat the wave-eut platform developed during the Pleistocene, and that it is not a modern or Holocene feature. In places,the platform surface 500 1000m HaH has rhizomorphs, the fossilized traces of terrestrial plants, directly ontopofplaned-offcorals(Fig.3C). Elsewhere, coral rubblestone, collapsed but essentially in place coral debris (Curran and White 1985; White and Curran 1987),overlies the wave-cut surface (Fig. 30). These deposits commonly extendvertically intoa shallowing-upward sequence of strata deposited duringthe transition from marine subtidal to non marine eoliandeposition resulting fromthe regression caused by the onset of Wisconsinan glaciation (White and Curran 1987, 1995)(see also later discussion of profileB-B' Figure 6). Laterally, the sea-level fall is represented by an irregular Bamboo Point erosional surfaceuponwhicha well-preservedcoralpatchreef of Sangamon age is preserved (see laterdiscussion of profile Fernandez Bay C-C' Devil's PointFigure7). .

Figure 2. Location of the Devil's Point fossil reef on Great Cockburn Town FossilReef Inagua Island. and the Cockburn Town andSuePointfossil reefson SanSalvador Island. Although lacking a distinct wave-cut platform, many other features foundintheCockburn Townreefwereformedduring a sea-level lowstand that interrupted the growthof the reef. analyses, coupledwith the preservation of pristine aragonite Corals are preserved in growth position on an undulating in fossil corals from these reefs, allows us to create a erosional surfacethat isinterpreted as equivalentto thewave comprehensivechronology forthesequenceofeventsrevealed cut platform in the Devil's Point reef (Figs. 4A and 4B). by thedetailed fieldand petrographic work. Themethods and Coralsbeneath the erosional surface were truncated, in some resultsof theagedatingandtheoverallstratigraphic settingof casesincluding the lithophagid boringswithinthecorals(Fig. thesampledcorals werepresented in Chenet al. (1991). The 4C). Elsewhere the erosion surface itself was bored by presence of a wave-cut surface within parts of the Devil's sponges, and subsequently these borings were encrusted by Point reefal sequence also was noted. Subsequent detailed corals(Fig.40). field studieson Great Inaguaand San Salvador revealed the widespread occurrence of an erosional surface within the Fissures, erosional channels, and small caves formedduring reefalsequences. InthecaseoftheCockburn Townfossil reef, thesea-level lowstand, andtheycut through bothinsitucorals theselater studieswere facilitated by new exposures created and associated lithified subtidal sediments. Some of the during the construction of a marina. channels are wider in the lower part, thereby forming overhung cavities. These cavities and openings were Recent evidence from ice-eore data, pollen studies of lake subsequently filledby subtidal sediments (Figs.SA and 5B), sediments, and data from benthic foraminifera and stable which in some places have been removed by subsequent isotopes indicates that the last interglacial included one or erosion to reveal cavity floors, walls, and roofs that have more episodes of extreme climate fluctuation. This is a spongeand bivalveborings and serpulidencrustations (Fig. controversial conclusion, as the last interglacial has hitherto SC). Red paleosols overly the fissures and the infilling been considered to have had a stable and equable climate, subtidal calcarenites (Fig. 50). Following the physical based in large part on comparison with the Holocene stratigraphy of Carew and Mylroie (1995a) such paleosols (Broecker 1994; Dowdeswell and White 1995). We explore marktheboundary between thePleistoceneandHolocene, and this issuein the lightof our detailed sea-level history withits prove the Pleistocene age of thesefeatures. In somefissures precisely determined chronology. thered paleosol unconformably overliesan earliergeneration 11 BAHAMIAN CORAL REEFSYIELDEVIDENCE OF BRIEFSEA-LEVEL LOWSTAND

Figure 3. Geology atDevil'sPoinifossil reef, Greatlnagualsiand. A)General view ofthewave-cut platform thatformed during themid-Sangamonian sea-level lowstand. B) Planed-off coral heads exposed on thewave-cut surface. Arrowpointsto 10 em scale. C) Rhizomorphs (fossilized planttraces) on thewave-cut surface showing thatit is nota modern erosional feature. D) Coral rubblestone thatformsthebasal layer ofa shallowing-upward sequence which accumulated above thewave-cut surface during the second Sangamonian sea-level highstand. of redcalichethat linesfissurewallsand the adjacentfissure platform or erosional surfaceinterrupts the reefal sequence, filling subtidal calcarenites thatweredeposited in theinterval suggesting thatthisreef may havedeveloped only duringthe betweenthe two soil-forming episodes. This first generation later interglacial sea-level highstandfollowing the sea-level of paleosol/caliche provides additional evidence for a brief lowstand recordedat the Devil's Point an

Figure 4. Geology atCockburn Town fossilreef. SanSalvador Island.A)Diploria stri~osa fossilized in growth position onthe erosional surface that developed during the mid-Sangamonian sea-level lowstand. B) Montastrea aonularisfossilized in growth position on the erosional surface that developed during the mid-Sangamonian sea-level lowstand. C) Lithophagid borings into Montastrea annularis truncated by the erosion surface. D) Sponge borings (A) in the erosion surface partly covered by encrusting coral (B). Scale bar =4 cm.

may be absent from some of them. For example, the reefs URANIUMlTHORIUM DATING OF FOSSil. described byHalleyetat (1991) mayhaveformed onlyduring CORALS the younger Sangamon sea-level highstand. Such restricted duration maybe thecaseforpatchreefstoagreaterextentthan The Devil's Point Reef Wave-eut Platform for the moresubstantial bank/barrier reefs. Massspectrometric analysis of pristinecoral aragonite from theDevil' sPointfossil reefallowed preciseagedetermination of the reef (Chenet al. 1991). Two of the topographic and 13 BAHAMIAN CORAL REEFS YIELD EVIDENCE OF BRIEFSEA-LEVEL LOWSTAND

Figure 5. Geology at Cockburn Town fossil reef, San Salvador Island. A) Small. sediment-filled cave exhumed by marina construction work. Width offilledfissure is -50 em. B) Horizontal surface viewoffissure showing truncated Montastrea annularis and lithophagid boring (arrow). Fissure was subsequently infilled with marine calcarenites. Scale =15 em. C) Serpulid tubes encrusting thewallof an exhumed fissure. Tubes are- 2 mm in diameter. D) Partof afissure withpaleosol overlying thefissure fill of marine calcarenites. geologic profiles fromChenetal.,(1991) havebeenmodified fragments, which form the lower layer of a shallowing to incorporate morerecent field observations and to provide upward sequence. Such pristine preservation requires that important information regarding the timing of formation of corals be rapidly removed fromthe taphonomic environment the erosional surface. (Greenstein andMoffat1996). Thefollowing coral ageswere obtained from samples collected below the erosion surface: Profile B-B' (Figs. 2 and 6) is from the locality where the Acropora palmata, 1303±13 lea; Diploria strigosa. photographs showninFigure3weretaken. Herethewave-cut 125.4±1.7 lea; Montastrea annuiaris, 128.4±1.2 and surfaceis underlain by in situcorals, and is overlain by coral 124.9±2.l lea. One sample of Montastrea annularis from rubblestones of collapsed but unabraded and pristine coral abovethesurfacehasan ageof 123.8±1.5 lea. All coral ages 14 warrs, CURRAN, ANDwn..SON

meters 3

[I7l5 Medium oolitic calcarenite (eollanite) bJ:J with caliche dikes

r0J 4 Trough cross-bedded. coarse, shelly calcarenite tdJ with coral pebbles

2 D 3 Coarse, shelly calcarenite lh1J with polychaete burrows

p;:;;j 2 Coral rubblestone t@ with large mollusk shells

~ 1 Coral rubblestone, ~ in situ M.annularisand Dip/oria "'}.' 3

~. :- .....? ... ~ ~. ..v Erosion Sunaca ··0'" . ~,;HU)~(iW) . .'. . ~ - :11:"...... "";v_ ""

HTL 0 ff.m~(~y {mAg~tj -~~~0,

. '''J '. 130.3±1.3 ..~.' .. : ... '. . ·v.·· '.'~: : ~

o 10 15 20 25 meters B' B Figure 6. ProfileB-B', Devii's Pointfossil reef, GreatInagua Island. U-Th ages shown adjacentto sampled corals. A = Acroporacervicornis: M =Montastrea aMularis:D =Diploria striwsa. Modifiedfrom Chenet al. (1991).

meters Caliche Cap 3 G6J 2 In situ coral patch reaf

_ _ _ Erosionsurface

~ 1 A.cervicornis- dominated f;i:cij coral rubblestona 2

0 e:b .: ..t!' .

12 11 10 9 8 7 6 I; 3 2 ometers C Co Figure 7. Profile C-C', Devil's Pointfossil reef, GreatInaguaIsland. U-Th ages shown adjacentto sampled corals. A = Acropora cervicornis: M =Montastrea annularis; Ds =Diploria striwsa; Dc =D. clivQSa. ModifiedfromChenet al.(1991). and comprehensive documentation of relevant geochemical lea, and from above the surface the range is 123.8tl.5 to and analytical data are presentedin Chen et al. (1991). 122.lt1.3 lea.

ProfileC-C' (Figs. 2 and 7) illustrates a localitywherea fully The Cockburn Town Reef Erosional Event developed andextraordinarily well-preserved coralpatchreef overliestheerosionsurface. AnAcropora cervicornis sample ProfileC-C' (Fig.8) modified from our earlierwork on San from below the erosion surfacehas an age of 128.7tl.4 lea. Salvador (Curran and White 1985) illustrates the most Corals fromabovetheerosionsurfacegavethefollowing ages: complete set of data for establishing the age of the erosional Diploria strigosa, 123.8tl.1 ka; D. clivosa, 123.3t1.5 ka; surfacefoundintheCockburn Townreef. Thefollowing coral Montastrea annularis, 122.8t1.6 ka and 122.1t13 lea. ageswereobtainedfrom threesamplesof Acroporapalmata Combining data from the two profiles givesa range of coral collected belowthe erosionsurface: 132.6±1.3lea, 125.5t1.4 ages from belowtheerosionsurfaceof 1303t1.3to 124.9±2.1 lea, and 125.3t1.7 lea. A specimen of A. palmata collected 15 BAHAMIAN CORAL REEFS YIELDEVIDENCE OF BRIEFSEA-LEVEL LOWSTAND

, 3 Medoum o%llC caJcar. I. <_ I ) W1 callcl\."'kesand rtlilomo

50 45 40 35 30 25 20 15 10 5 o meters C' Seaward OIrecWn C

Figure 8. Profile C-C', Cockburn Townfossil reef, San Salvador Island. U-Th agesshown adjacent to sampled corals (all Acrqporapalmata). Modifiedfrom Chen et al. (1991).

Figure 9. Photomicrographs undercrossedpolarizers ofsamplesfromDevil'sPoinifossil,reef, GreatInaguaIsland. Allwidths of view=2 mm.A) Earlymarine high-Mg calcite andaragonite cementsfollowed bysparse non-marine sparrycalcite cement. B) Sparse early marine cements followed by abundant non-marine vadose sparry calcite cement. C) Early non-marine isopachous sparrycalcite cement followedby sparsemarine high-Mg calcite and aragonite cements. D) Sparse, earlynon marine isopachous sparry calcite cementfollowed by well-developed, isopachous marine high-Mg calcite and aragonite cements. from above theerosion surface hasaU-TIl ageof123.8±1.7lea PETROGRAPIllC EVIDENCE (Chen et aI. 1991, includes all analytical data). Thisgives a range of coral ages from below the erosion surface of Changing sea levels may expose nearshore reefs and 132.6±1,3 to 125.3±1.7 lea, and from above the surface a associated subtidal grainstone facies to a sequence of single ageof 123.8±1.7 lea. diagenetic environments, each of which may leave a distinctive imprint thatcreates a record of these changes in 16 WIDTE,CURRAN, ANDwaSON

---+-~-B c---t------'~-B

A--=:':==+:=-_I B ______L:o~I:~~c: -_-_-_+_-_-_: _ A C B A C C A A

134 132 130 128 126 124 122 120 ... Figure 10. Diagram showing the U-Th ages of corals in relation to the erosional surface thatformed during the mid Sangamonian sea-level lowstand. A =coralsfromB-B' Devil' sPointfossil reef; B =coralsfrom C-G' Devil'sPoitufossil reef; C =coralsfrom C-G' Cockburn Townfossil reef; dashed line represents theerosional surface. Horizontal scale in thousands of years before present. No vertical scale implied. Stratigraphic location of thedated corals is shown in Figures 6. 7, and8. environment Figure9A showsa grainstone witha sequence during the lowstand are exposedin close vertical proximity of earlier marine cements consisting of micritic high-Mg alongprofiles B-B' (Fig. 6) and C-C' (Fig. 7) at the Devil's calcite followed by aragonite. Also presentarelater sparry Point reef and C-C' (Fig. 8) at the Cockburn Town reef. calcite cements typical of meteoric, non-marine diagenetic Uranium-thorium ages of corals from these localities are environments in unburied Quaternary limestones. The shownin Figure 10. In attempting to evaluateall of our data grainstone shownin Figure9B hasa similaroverallsequence from a geological perspective we have tendedto follow the but the amountof earliermarinecementis muchless and the common practiceof focusing on the central tendency of the rockiscementedlargelybynon-marine calcsparwithapatchy agedata (see forexampleChenetal. 1991; CarewandMylroie distribution typicalof a vadosezone. Thecementsequence is 1995a; Eisenhauer et al. 1996). Our interpretations are reversedin the rock illustrated by Figure 9C, with the rock constrained by the knownrockrecord,and we areimpressed being largelycementedby an earliernon-marine isopachous by the relationships between the clustering of dates in calcsparwitha smallamountof later,irregularly distributed, relationship to their stratigraphic source. Failure to follow marine high-Mg calcite and subsequent aragonite. A more this multifaceted approach leads to geologically absurd completely developed sequence of later marine cements is conclusions, for example placing younger rocks beneathan illustratedinFigure9Dwhereearlyisopachous topatchynon erosional disconformity and older ones aboveit Following marine calcsparis followed by isopachous marine high-Mg these techniques the data from Figure 10, which represent calciteandaragonite. Usingthewell-established principles of situations wheredatedcoralsareinverticaljuxtaposition from cementstratigraphy (Meyers1974), wecandeducea sequence beneathandabovetheerosion surface,indieateatimewindow of diagenetic environments from marine to non-marine and for the regression-lowstand-transgression sequence in the thena returnto marine. This cementsequence indicates that rangeof 1.1 to 1.5lea. an interval of sea-level lowstand occurred during the diagenetic history of these rocks and that sea level fell far Ratesof Sea-levelChange enoughtoexposereefalandassociated rockstothefreshwater phreatic and vadoseenvironments. Sea level prior to the regression to the lowstand was a minimum of 4 m above present sea level and the ensuing DISCUSSION transgression raised sea level to 6 m above presentsealevel (Curran andWhite 1985). Duringthe lowstand, sea levelfell Timing of the Sea-levelExcursion to approximately presentsealevel (Curranet al, 1989). Thus a totalchangeof sealevelof 10m occurredduringthefalland A compelling bodyof fieldandpetrographic evidenceshows subsequent rise. To abrade the broad wave-cut platform that the development of coral reefs on Greatlnagua and San presentat the Devil's Point reef site and to remove several Salvador islands during the Sangamon interglacial was decimeters of coral rock duringthis abrasion, sealevel must interrupted by a fall of sea level that exposed the reefs and havebeen maintained at the lowstandlevel for a significant, associated sedimentary facies to non-marine conditions. Sea but unknown, partof this interval. Similarly, development of levelprior to thefall wasat least4 m higherthanthelowstand the variouserosioncavitiesand thin caliche liningsrequires level. SUbsequently, sealevelrosetoapproximately +6mand thatthe lowstand existedfor a significant fraction of the time the reefs flourished once again. interval. Table1showstheaveragerateofchangerequiredfor cycles of sea-level fall and rise of 1.1 and 1.5 ka duration Corals frombeneathand above theerosionsurfaceproduced assuming various periodsof lowstand. 17 BAHAMIAN CORAL REEFS YIELDEVIDENCE OF BRIEFSEA-LEVEL LOWSTAND

Table1.Calculated averageratesof sea-level changeforthemid-Sangamon fallandriseof sealevel,assuming various durations for the lowstand and usingtwo estimates (1100and 1500years) derived from dataof Figure10 for the length of the sea-level cycle.

Lowstand duration in yrs. 1100 yrs. - rnm/yr. 1,500 yrs .• nun/yr.

100 10.0 7.1 200 11.1 7.7 300 12.5 8.3 400 14.3 9.1 500 16.7 10.0 600 20.0 11.1 700 25.0 12.5 800 33.3 14.3 900 50.0 16.7 1000 100.0 20.0 1100 - 25.0 1200 - 33.3 1300 - 50.0 1400 - 100.0

Non-reefal Evidence for ClimaticInstabilityDuring the coolingduring the last interglacial thatcoincide with colder Sangamon periodsindicated by the GRIPice-coredata (Thouveny et aI. 1994). Seidenkrantz et aI. (1995) presented data on the The Bahamas are generally regarded as lacking tectonic abundance of benthic foraminiferal speciesfoundintwocores activity that would explain relative sea-level changes that ofmarineshelfsediments from Denmark. Theyinterpret their occur in tandem over the whole archipelago (Carew and data as indicating two cooling events during the last Mylroie 199580 b). Such changes must be due to absolute interglacial, thattheycorrelate withcolderintervals indicated changes in the volume of sea water, most likely caused by in the GRIP ice-core data. Furthermore, these authors changes in terrestrial ice volume. By comparison with the conclude that climatic change was rapid, on the scale of generally stable climates of the Holocene, it was widely decades or centuries. Recently, Seidenkrantz and Knudsen assumed thatthe last interglacial was also a period of stable (1997) presented a detailed analysis of benthic foraminifera climate. This stabilitywas in sharp contrastto the 100lea of fromoneof thesecoresthatsupports theirearlierconclusions. the last glacial period when relatively stableclimateperiods Thus,the knownsignatures of climatic instability duringthe lastingseveral milleniawere disrupted by abruptchanges to lastinterglacial extend from Greenland to Europeand are, in radically different climate states that occured within a few fact, believedto be global(Broecker 1994). decades (Broecker 1994). This viewof stableclimates during the last interglacial was calledintoquestionby ice-core data SangamonClimateInstabilityand Sea-level Changes whichshowedthatabruptchangesof temperature occurred in Greenland duringthattime(GRIPmembers 1993; Johnsen et Literature survey> Theevidence indicatesthatrapidchanges aI. 1995). of temperature of several degrees Celsius occurred on a decadal time scale during the last interglacial, producing a In a reviewof the Greenland ice-core data and the recordof variety of signatures recorded in several parts of the globe rapid climate changes, Dowdeswell and White (1995) (Broecker 1994). The question arises whether such comment that the terrestrial and marine records of the last temperature changes would also cause rapid changes in the interglacial give mixed signals. They concluded. somewhat volume of terrestrial ice and attendantrapid changes in sea cautiously, thatthe weightofevidence fromtheterrestrial and level. Based on strarigrsphic studiesof carbonate rocks on marine records suggests stable climate during the last Oahu, Hawaii, Sherman et aI. (1993) reported two distinct interglacial. Notall of the evidence supports thatconclusion, sea-level highstands during the last interglacial. However, however. In a brief summary, Tzedakis et aI. (1994) report theiragedatinghadwideerrorbarsthatfellwithin thegeneral that pollen data from Europe supportthe view thluclimatic agerangeof thelastinterglacial, butwerenotaccurate enough fluctuations occurred during the Eemian. More detailed tosubdivide it MorerecentstudiesbyMuhsandSzabo(1994) pollen data from annually laminated lake sediments in of uranium-series dating of the Waimanalo Formation on Germany, and peats from France, showed that the last Oahudo not supportthedoublesea-level highstand, and they interglacial climatewasmoreunstable than the Holocene, and concluded that Oahu and similar tectonically active Pacific that at times, winter temperatures reached levels similar to islands are unsuitable as reference pointsfordetermining last thosethat occurred during glacialperiods(Fieldet aI. 1994). interglacial highstands. Magnetic susceptibility, pollen, and organic carbon records from maar lakesthat formed in explosive volcanic cratersin Precisely dated corals from a core through fossil reefaI the Massif Central of France show two periods of rapid deposits in the Houtman Abrolhos islandsoff thetectonically 18 WI-llTE, CURRAN, ANDWILSON passive coast of Western Australia give some information exposures of some Bahamian fossil coralreefsandassociated about sea-level changes and elevations during the last facies, detailed field work. the preservation of pristinecoral interglacial (Eisenhauer et al. 1996). Sea level rose steadily aragonite, and breakthroughs in U/Ib age dating techniques from-4 m belowpresentaround 134ka, passedpresentlevel creates an opportunity to compare evidence of sea-level between 130and 127 ka, and reached a maximum of at least changes in the Bahamas, with climate changes recorded 3.3 m above presentat -124 lea. Sea levelfell belowpresent elsewhere fromthe lastinterglacial period. FigureII is based datumat -116lea. Eisenhaueret al. (1996) foundnoevidence on data presented in GRIP members (1993), and shows of intra-interglacial fall of sea level, but they state that the measured changesin CS180 of the portionof the Greenland ice resolution of their data would limit the duration of any such core that formed during the last interglacial, and the undetected regressions to 1.0 ka, or less. calculated temperature fluctuations basedon the isotopedata. Because theseice core data yield the best presently available Based on the island geology of Bermuda and The Bahamas, information about the duration and timing of climate Hearty and Kindler (1995) developed a chronology for sea fluctuations duringtheEemian,we usethisdiagram todiscuss level highstands for the past 1.2Ma. For the last interglacial, the results of our work on sea-level changes recorded in they proposed two highstands separated by a regression that Bahamian fossil coral reefs, in the context of temperature lasted from approximately 128 to 123 ka based on protosols fluctuations recorded in Greenland ice. that lie between the marine deposits of the two highstands. They referred to Chen et al. (1991)in pointing out that coral One of the main features of the ice-core record for the last reefs grew during the earlier oscillations. However, corals interglacial is rapid, and commonlysignificant, temperature presentlyin situ above presentsea levelalso are reported by fluctuations. However, somegeneraltrendscan bediscerned. Chenet al. (1991)from muchof the timeinterval represented Froma low of approximately 1O"C below average Holocene by the proposedlowstandof Heartyand Kindler(1995). In a temperatures around 142ka, a generalwarmingtrend brought more recent paper, Neumann and Hearty (1996) focused temperatures aboveHolocene levelsbyapproximately 133lea. mainlyon evidencefor a rapidriseand subsequent fall of sea These warmer conditions continued as a general trend of levelat theendof the lastinterglacial. However, theyinferred temperatures approximately 4"CaboveHolocenevaluesuntil thatsea level for most of the interglacial remained near 2 m 126 ka, when a rapid cooling began. This period of higher abovepresent,interrupted only by a short-lived. sea-level fall temperatures coincidesquitewell with the firststageof coral of about 1.5 m at approximately 125 lea. The timing of this growth in the Bahamian reefs whichextendedfrom 132-125 event appears to be constrained largely by data fromChen et lea. Rapid cooling events punctuated this early last al. (1991). Toexplainbioerodednotches at6 m abovepresent interglacial warm period,although the most severe of these sea level,Neumannand Hearty(1996) invoked a rapidriseof pre-datetheoldestcoralsyetfoundin the Bahamian reefsthat sea levelfrom+ 2 to + 6 m at theend of theinterglacial and a wehavestudied. A short-lived cold spell,sometimebetween highstand that lasted only a few hundred years. This 129 and 128 ka, brought temperatures approximately 2"C interpretation was questioned by Carew (1997) who stated below Holocene values. We have found no record in the thatthe postulated rate of notch formation is much too high Bahamian reefs of this cold spell, but it is possiblethatany and that the notches could not have been formed in a few effectsarebeyondthe resolution of fieldstudiesina complex hundredyears, and by Mylroie(1997) who reinterpreted the reeffacies. notches as the eroded remnants of flank margin caves. Regardless, such caves also would need much longer thana The ice-core data show a major fall in temperature of few hundred years to form. Our work on the fossil reefs indicates that sea level was at or close to the + 6 m level for /)180 several thousand years from about 124 ka to approximately ·30 +6 119lea. This providessufficienttimeforbothnotchand cave formation but obviatesthe need forinvoking a veryrapidrise in sea levelclose to the end of the interglacial. ---- -35

Interesting information is presented by Precht(1993)basedon studiesof reefalfaciesof theFalmouth Formation inJamaica. According to Precht (1993), the Falmouth Formation -40 comprises two shallowing-upward parasequences that represent reefal development during two separate sea-level 110 115 120 125 130 135 140 ka highstands of substage 5e. Uranium-series dating of aragonitic corals suggests that the lower parasequence Figure 11. Temperature data spanning the last interglacial corresponds to a sea-level highstand at 134-127 lea, and the based on oxygen isotope data from a Greenland ice core. upper sequenceto a highstand at 124-119 lea. Dashed line represents average Holocene temperature. Redrawnfrom data inGRIP members, 1993. Horizontal scale The evidence from thisstudy.-- The combination of excellent is non-linear. 19 BAHAMIAN CORAL REEFS YIELD EVIDENCE OF BRIEFSEA-LEVEL LOWSTAND approximately 9"C from 126 to 125 lea. just prior to the taphonomic studies of the Cockburn Town fossil reef by dramatic fall in sea level recorded in the Bahamian reefs Greenstein and Moffat (1996). beginning at approximately 125 lea. The most reasonable conclusion from these data is that this majorcooling led to TheGreenland ice-core data showone morewarmerepisode greatly increased snowfall and the rapid increase in the that occurred around 115 ka when temperatures reached accumulation of land-based ice. approximately 4"C above the Holocene average for a short interval, beforefalling to theglaciallevelsthatpersisted until Following 125lea. a slowwarming is indicated by theicecore the Holocene. No corals of this age are known from the data, but average Holocene temperature values were not Bahamian fossil reefsthatwehavesmdied, norareany corals reached untilapproximately 121 ka. Temperatures thenrose found encrusting the terminal shallowing-upward sequence quickly to approximately 4DC aboveHolocene levels, where sands. theyremained, with minorfluctuations, for a littlemorethan a millennium. In the fossil reefsdiscussed in this paper this CONCLUSIONS warming trendcoincides witha rapidrisein sealeveland re establishment of the coralreefsand the secondstageof their I. Bahamian coral reefs on San Salvador and Great Inagua growth, which lastedfrom 124-119 lea. Although the general islands developed during two stages within the last trends of warming climate, rising sea level,and regrowth of interglacial separated by a short-lived sea-level lowstand. coralsarecoincident, therearediscrepancies in themagnitude of theseevents. The ice-core data showthat temperatures in 2. The first stageof reef growthoccurredduring the interval Greenland remained belowHolocene values for muchof this 132-125 ka when Greenland ice-core data indicate that interval, whereas sea levelandaccompanying coralgrowthin temperamres were generally about 4"C higher than the the Bahamian reefs was higherthan presentlevels. average for the Holocene. Sea level during this first reef development phasewasat least4 m abovepresent The reason for the discordance between rapidly rising sea level and slowly rising temperatures in Greenland is not 3.According to ice-core data,temperatures fellapproximately known. However, somespeculations arepossible. Pollendata 9"C during the interval 126-125 lea. This corresponds to a from Germany and France show that the average mean rapidsea-level fallthat interrupted coralgrowthand led to a temperature of thecoldestmonth remained lowandrelatively period of erosion in the earlier reef, and to an episode of stableduring theIanerpartof thelastinterglacial beforerising freshwater diagenesis. Sea levelfell duringtheinterval 125 rapidly during the last 1000 years (Field et al. 1994). This 124 ka to about the current level, at rates probably corresponds more closely to the ice-core data than to the significantly in excess of 10mm per year. Bahamian sea-level history. Summer temperatures remained relatively highduring this period,suggesting a moreextreme 4. After 124lea. sea levelrose rapidlyto a maximum of 6 m seasonality thanispresently experienced. Therateofincrease above present, and coral reef growth was renewed. This in mean annual temperatures in the higher Iatitudes of the second phase of reef growth lasted from 124-119 lea. Northern Hemisphere may have lagged behind the average Although the ice-core data indicate a slow warming during global increase, perhaps due to the lack of penetration by this second interval of reef development, the extent of the relatively warm oceanic currents. In this scenario, the low warming seems insufficient to explain the magnitude and Northern Hemisphere temperatures are anomalous and the highrate of sea-level rise. The reasonfor this discrepancy is rapidly rising sea levels identified from the Bahamian reefs unknown. Warming inGreenland mayhavelagged theglobal are theglobalnorm. Hollin(1980) proposed that somepolar rate due to its isolation from warming ocean currents, perhaps icemasses couldpassacriticallimitandsurgeintotheoceans, due to the kinds of longitudinal shifts in ocean circulation and thus cause rapid sea-level rise. It is possible that such patterns described byJohnsen et al. (1995). eventsoccurred duringtheearlystages of the warming in the latter part of the last interglacial, and accelerated the rate of 5. A rapidcooling of approximately 9"C beginning at 119ka sea-level rise. coincides witha veryrapidfall in sea levelbeginning shortly after119lea. This fallwasduetoan earlyphaseoficevolume The ice-core data show that temperatures in Greenland fell increase that marked the beginning of the Wisconsinan approximately9"C in a5OQ-year spanbetween 119and 118ka glacial stage. Falling sea level led to the rapid burial of the to 5"C belowaverage Holocene levels. This timing corres coral reefs in regressive facies sands and to their excellent ponds to the record from Bahamian coral reefs with the preservation. youngest mown coralbeing119.9±1.4 lea. Theexcellent state ofpreservation of Bahamian coralreefshasbeen attributed to 6. A brief return to temperatures up to 4"C higher than rapidburialby the entombing sandsof a shallowing-upward Holocene values isindicated by icecoredataataround115lea. sequence resulting fromrapidsea-level drawdown beginning Ifthisledto a sea-level highstand higherthanpresentwehave at about 119lea (White et al. 1984; Whiteand Curran1995). detected no impactin the Devil's Pointand Cockburn Town This interpretation has been supported by the recent reefs. The ice-core datashowno other time between 115ka 20 WIDTE,CURRAN, ANDWILSON and the beginning of the Holocene when temperatures Geotimes, v, 39, p. 16-18. approached the Holocene average. CAREW, lL., 1997,Comment: Rapidsea-levelchangesattheclose of the last interglacial (substage 5e) recorded in Bahamian islandgeology: Geology, v. 25,p, 572-573. 7. Our data support the concept that the last interglacial CAREW,lL. andMYLROIE, lE., 1995a,Depositional modeland climate was subject to rapid and significant fluctuations in stratigraphy for the Quaternary geology of the Bahama temperature that had dramatic effects on sealevel, whichin Islands,inCurran, H.A. and White, B., 005., Terrestrialand turn affectedthe development of coralreefs. Shallow Marine Geology of the Bahamas and Bermuda: Geological Society ofAmerica Special Paper, v, 300,p. 5-32. 8. Markedchangesof sea levelappearto have separatedtwo CAREW, J.L. and MYLROIE, lE., 1995b, Quaternary tectonic intervals of several millenia duration when sea level was stabilityof the BahamianArchipelago: Evidencefrom fossil higher than presentand reef growthflourished. coral reefs and flank margin caves: Quaternary Science Reviews, v, 14,p. 145-153. CHEN, lH., CURRAN, HA, WIDTE, B., and WASSERBURG, 9. The frequency of sharptemperature fluctuations shownby GJ., 1991,Precisechronology of the last interglacial period: the ice-core data is greaterthan thatreflected in the observed 234U-UTh data from fossil coral reefs in the Bahamas: effectsin the coral reefs of sea-level changes. Whetherthis Geological Society ofAmericaBulletin, v, 103,p. 82-97. meanstherewerenoeffectsonsealevelorthereefs,orthatthe CURRAN, HA andWlllTE, B., 1985,The CockbmnTown fossil effects are too subtle for present methods of analysis to reef, in Curran, HA, ed., Pleistocene and Holocene discover, is unknown. carbonate environments on San Salvador Island, Bahamas, Geological Society of America, Orlando Annual Meeting 10.Basedon therecordof thelastinterglacial, therecan beno Field Trip Guidebook: Ft Lauderdale, Florida, CCFL assurance thatdramatictemperature fluctuations andchanges BahamianField Station, p. 95-120. CURRAN, HA., WIDTE, B., CHEN, IH., andWASSERBURG, inrelativesealevelwillnot occurduringtheremainder of the GJ., 1989, Comparative morphologic analysis and present interglacial. geochronology for the development and decline of two Pleistocene coral reefs, San Salvador and Great Inagua 11. As thechangesof sealevelduringthelastinterglacial most islands, in Mylroie, lE., ed., Proceedings of the Fourth likelyare due to changesin the volume of land-based ice, it is Symposium on the Geologyof the Bahamas: San Salvador, wrong to think of interglacials as being free of ice sheets. BahamianField Station, p. 107~1l7. Perhaps terminology misleads us and the terms glacials and DOWDESWElL, lA and WIDTE, lW.C., 1995,Greenland ice interglacials mightbe betterchangedto greaterglacials (i.e., core records and rapid climate change: Philosophical hyperglacials) and lesser glacials (i.e., hypoglacials) Transactions ofthe Royal Society ofLondon, A, v, 352, p. 359-371. respectively. EISENHAUER, A, ZHU, Z.R., COLLINS, L.B., WYRWOlL, K.H.,andEICHSTATTER, R., 1996,The lastInterglacialsea ACKNOWLEDGMENTS levelchange: new evidence from the Abrolhos islands, West Australia: GeologischeRundschau; v, 85, p, 606-614. We thank Don and Kathy Gerace for their long-term FIELD, M.H., HUNTLEY, B., and MOLLER, H., 1994, Eemian encouragement of our research in the Bahamas, and for their climate fluctuations observed in a European pollen record: kindness and assistance during our many visits to San Nature, v, 371,p. 779-783. Salvador. We also thank Dan and Nicole Suchy for their GREENSTEIN, BJ. and MOFFAT, H.A., 1996, Comparative untiring support since their arrival at the Bahamian Field taphonomy of modem and Pleistocene corals,San Salvador, Bahamas: Palaios, v, 11, p. 57-63. are to Station. We grateful JimmyNixon,HenryNixon,and GRIP MEMBERS, 1993, Climate instability during the last Carl Farquharson for their generous hospitality and interglacial periodrecordedin the GRIP ice core:Nature, v, invaluable assistance on Great Inagua. Dick Fish, Smith 364, p. 203-207. College, preparedprints for the photo figures with skill and HALLEY, R.B., MUHS, D.R., SHINN, E.A.. DILL, R.F., and patience. We thank Bill Precht for organizing this special KINDINGER, lL., 1991, A + 1.5 m reef terrace in the issueoncoralreefsand thethreeanonymous reviewers whose southern Exuma Islands, Bahamas: Geological Society of helpfulcomments led to improvements in thispaper. Grants America Abstracts withPrograms, v, 23, p. 40. to CurranandWhitefromthePetroleum Research Fundofthe HEARTY, PJ. and KINDLER, P., 1995, Sea-level highstand American Chemical Society and from the Committee on chronology from stable carbonate platforms (Bermuda and The Bahamas): Journal ofCoastalResearch, v, 11, p, 675 Faculty Compensation and Development of Smith College 689. partially supported field work for this study. Mark Wilson HOLLIN, IT., 1980,Climateand sea level in isotopestage 5: An thanks theAdministration andtheFacultyDevelopment Fund eastAntarctic icesurgeat about95,000B.P.?:Nature, v, 283, at The Collegeof Woosterfor theirsupportof hisresearch in p.629-633. The Bahamas. JOHNSEN, s.r, CLAUSEN, H.B., DANSGAARD, W., GUNDESTRUP, N.S.,HAMMER, C.U., and TAUBER,H., REFERENCES 1995,The Eem stableisotoperecord alongGRIPicecoreand its interpretation: QuaJernary Research, v, 43, p. 117-124. 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