U-series and amino-acid racemization geochronology of Bermuda: Implications for eustatic sea-level fluctuation over the past 250,000 years

Palaeogeography, Palaeoclimatology, Palaeoecology, 44 (1983): 41--70

41

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

U-SERIES AND AMINO-ACID RACEMIZATION GEOCHRONOLOGY OF BERMUDA: IMPLICATIONS FOR EUSTATIC SEA-LEVEL FLUCTUATION OVER THE PAST 250,000 YEARS

RUSSELL S. H A R M O N ' , RICHARD M. MITTERER 2, NIVAT K R I A U S A K U L 2, LYNTON S. LAND 3, HENRY P. SCHWARCZ 4, PETER G A R R E T T s, GRAHAME J. LARSON ~, H. LEONARD VACHER 7 and MARK ROWE 8

'Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275 (U.S.A.) 2Program in Geosciences, University of Texas at Dallas, Richard, TX 75080 (U.S.A.) 3Department of Geological Sciences, University of Texas at Austin, Austin, TX 78 712 (U.S.A.) 4Department of Geology, McMaster University, Hamilton, Ont. L8S 4M1 (Canada) SBox 5115, Winslow, ME 04902 (U.S.A.) ~Department of Geology, Michigan State University, East Lansing, MI 48824 (U.S.A.) 7Department of Geology, Washington State University, Pullman, WA 99163 (U.S.A.) ~Public Works Department, Hamilton (Bermuda) (Received May 19, 1983)

ABSTRACT Harmon, R. S., Mitterer, R. M., Kriausakul, N., Land, L. S., Schwarcz, H. P., Garrett, P., Larson, G. J., Vacher, H. L. and Rowe, M., 1983. U-series and amino-acid racemization geochronology of Bermuda: Implications for eustatic sea-level fluctuation over the past 250,000 years. Palaeogeogr., Palaeoclimatol., Palaeoecol., 44: 41--70. Bermuda is a stable, mid-oceanic carbonate platform for which a particularly complete record of Late Pleistocene eustatic sea-level fluctuation has been reconstructed from a detailed study of geological field relationships combined with an extensive programme of U-series and amino-acid racemization geochronology. Only twice in the past 250,000 yr. has sea level in Bermuda been above its present level, once at approximately 200 k.y. when it stood at about + 2 m and most recently at 125 -+ 4 k.y. when it stood at 5 + 1 m. These times of interglacial high sea level are characterized by the development of patch reefs and marine calcarenites at elevations above present sea level. Episodes of lower sea stand onto the Bermuda platform at elevations higher than --20 m are observed within the two interglacial periods and are characterized by the deposition of eolianites. By contrast glacial periods are times of residual soil formation and deposition of speleothems in caves at elevations below present sea level. Excellent correlation is observed between the Bermuda glacio-eustatic sea-level record and other marine and terrestrial paleoclimate records. INTRODUCTION

At present, our understanding of Late Pleistocene sea-level history has been derived largely from tectonically emergent coastal areas where flights of 0031-0182/83/$03.00

© 1983 Elsevier Science Publishers B.V.

42 coral terraces, beach deposits, and sea cliffs are perched above present sea level in staircase fashion. By comparison, the paleosea-level record of stable carbonate platforms, coastal areas, and oceanic islands is much more complex, typically consisting of intricate and complex associations of b o t h terrestrial and marine deposits and erosional features which are vertically compressed into a zone within a few meters of present sea level. As a result, two different pictures of Late Pleistocene sea-level history have emerged from tectonic versus stable areas. In stable areas the general picture has been one of broad sea-level maxima with only the last interglacial high sea stand well defined. For example, Ku et al. (1974) observed that fossil corals associated with the geographically extensive Waimanalo shoreline at about + 7.6 m in Hawaii gave 23°Th/234U ages ranging from 137 + 11 to 112 + 6 k.y. with a mean of 122 + 7 k.y., and noted that erosional features at lower elevations were most likely formed during regressive phases of the Waimanalo high sea stand. A similar situation was observed to occur in the Bahamas where fossil corals associated with a + 5.6 m bioerosional notch, sea caves, and a + 4.6 m marine terrace give 23°Th/234U a mean age of 122 + 7 k.y. (Neumann and Moore, 1975). In neither area is there unequivocal evidence for older or younger major high stands of sea level. Because the ~ 1 2 5 k.y. high stand is recognized in other stable areas around the world as the last time the sea level was at or above its present position (Broecker and Thurber, 1965; Osmond et al., 1965; Thompson and Walton, 1972; Harmon et al., 1978; Szabo et al., 1978; Szabo, 1979), this event has been taken to be correlative with the last interglacial maximum as recorded b y Stage 5e of the deep-sea oxygen isotope record (Shackleton and Opdyke, 1973). This situation contrasts markedly with that observed in areas of active tectonism where discrete fluctuations of paleosea level within each major inter-glacial period over the past 300,000 k.y. are well defined (Mesolella et al., 1969; Konishi et al., 1974; Bloom et al., 1974) and where, in addition, general periods of interglacial high sea stand dating back to ~ 6 9 0 k.y. have been recognized (Chappel and Veeh, 1 9 7 8 ; B e n d e r et al., 1979). For example, high sea stands in Barbados are observed during the penultimate interglacial period at a b o u t 2 2 0 , 2 0 0 and 180 k.y. and during the last interglacial period at 125, 105 and 82 k.y. (Mesolella et al., 1969; Bender et al., 1979), whereas in New Guinea and several adjacent island areas relative high sea stands have been observed at a b o u t 220 and 180 k.y. and subsequently at ~ 1 5 , 0 0 0 to 2 0 , 0 0 0 year intervals from 140 k.y. to present (Bloom et al., 1974; Chappell, 1974; Chappell and Veeh, 1978). In b o t h areas the 125 k.y. event is correlated to the last interglacial high sea stand and its elevation fixed b y correlation to this same temporal feature in stable areas. Based u p o n this correlation, the paleosea-level elevations for other terraces in a sequence are estimated b y a knowledge of their age and some assumption a b o u t local uplift history. The usual procedure is to assume a constant uplift rate in an area, b u t this philosophy has been strongly criticized b y Stearns (1976), so that paleosealevel elevations reported for tectonic areas must be regarded as equivocal

43

because they contain components of both glacio-eustacy and tectonism that cannot be individually determined. The purpose of this paper is to demonstrate that it is possible through a combination of fortuitous geological circumstances, detailed fieldwork, and extensive geochronology to recognize multiple, major interglacial paleosealevel events in a tectonically stable area as well as to delineate minor fluctuations of sea level within individual interglacial periods. During the past five years we have been independently and collectively working in Bermuda (Fig.l) on detailed field mapping and interpretation of the Late Pleistocene stratigraphy (P.G., H.L.V., and M.R.--L.S.L. and G.J.L.), 23°Th/:34U geochronology of subaerially exposed marine deposits and cave travertines, as well as submerged stalagmites in presently flooded caves (R.S.H. and H.P.S.), and amino-acid racemization studies of terrestrial and marine molluscs from the eolianites, soils, and marine limestones (R.M.M. and N.K.). This report is a summary of our collective work and concensus of our thinking as regards the Late Pleistocene sea-level history of Bermuda. Detailed reports on the other aspects of our work are currently in preparation. Here we will demonstrate that the number and timing of sea-level events recognized in Bermuda over the past 250,000 yr. are entirely correlative with those documented in BERMUDA LOCALHIES I

Ft St Cotherme

790515-6

2 Convict Boy 3 Stokes Point (North) 4 Stokes Point (South)

N

I

5 Ferry Reo~h Coney Isiand (Nortn) Coney Island (West) 8 Blermonn {]uorry g Crystot Cove 10 Government Ouarry 11 Cable Boy Pomt 12 Wtnsor House 13 Watch HtH Pork 14 Spencer's Point

6 7

780525-2

PG-329 790515~ PG-32.4(~ 31'J

cAsrt~NAeeo~ 319.BH2 ,,o ,"~?~PG -322

5

(~)780521- 3 780524-14 780522 -6

PG-218

L•AEATPG-210(~ SOUND

~79o5~6-4

7

780522-3 780522-5 304

IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Saueos Hllt (North) Soucos Hill{Soulh} Devonshire Boy Hungry Boy Grape Bay Bird Island Bluok's Island Hawkins ]stand Horseshoe Bay Sugar Cone Point Boaz Island Spanish Potnt Blackwotch Pass Barker's Hill Flatt's Inlet

Fig.1. Outline map of Bermuda showing sample localities and other geographic features mentioned in the text.

44 Barbados and New Guinea. Because Bermuda is a stable area not subject to tenuous assumptions about the rate or constancy of tectonism or submergence, the paleosea-level record presented here reflects primarily the eustatic rise and fall of Late Pleistocene sea level in response to episodic Northern Hemisphere glaciation, and thus can be adopted as a working standard against which tectonic areas can tentatively be calibrated. Bermudian geology is schematically illustrated in Fig.2 and stratigraphic relationships summarized in Tables I and II. Here we will follow the stratigraphic terminology of Land et al. (1967). Critical localities have been restudied in detail and two critical sections reinterpreted as a consequence of our chronologic work. Throughout the discussion that follows it must be kept in mind the geological relationships in Bermuda are complex, consisting of overlapping and interfingering marine carbonates, eolianites, and residual soils that have undergone variable amounts o f marine and/or meteoric (vadose or phreatic) diagenesis. Because o f this: (1) rock units formed at a particular time m a y have a lithologic character which varies from place to place due to different depositional or diagenetic environments, and (2) similar lithologies ['~

eotionite

~

marine

biocalcarenite

Recent Boil Southampton Fm.(S) St. George's Bait Spencer's Point Fm (SP) Pembroke Fro. (P)

~

marine conglomerate land

Harrington Soil (H)

snails

r ~

cove with travertine and breakdown

~

soil

Devonshire Fm (D)

Shore Hills Boil Belmont Fm (B) ----?--unconformity (soit) ~11~ t Walsingham Fm (W)

$

s0oh

North

H

P

\/.

f/,

w/

' /

leve,

Fig.2. Semi-diagrammatic N--S cross-sectionof Bermuda illustratingstratigraphicrelations between lithologic units. The Walsingham Fm. has been projected into the line of section from stratigraphic relationships in Government Quarry. Each succeeding unit represents incomplete deposition on a complex, pre-existing topography. Revised from Land et al. (1967) as discussed in text.

45 TABLE I Stratigraphic column of Bermuda (as revised from Land et al., 1967) Modern dunes: intermittently active dunes in the area of Horseshoe Bay and Warwick Long Bay Southampton Fm. : poorly consolidated dunes with unconsolidated zones rich in land snails St. George's Soil: red residual soil of moderate development and only rare palmetto stump casts "Spencer's Point Fm. ": discontinuous patches of marine conglomerates along the south shore and harbour islands plastered in cliff pockets and recesses or on large breakdown blocks at elevations between +1 and +11 m Pembroke Fro. : extensive island-wide eolianites with landward dipping foreset beds which commonly extend down to and below sea level Harrington Fm. : a yellowish to reddish unconsolidated layer on the Devonshire or Belmont Fms. which has abundant land snails near the top and grades down into coarse, fossiliferous marine beds near the base Devonshire Fm. : intertidal marine conglomerates and calcarenites at elevations of + 2 to + 6 m on the south shore which contain Belmont Fm. clasts and a characteristic fauna of Citterium and Lucina as well as seaward-dipping sands along the north shore "Black Watch Pass Fro. ": pre-Devonshire eolianites underlying the seaward-dipping north shore sands Shore Hills Soil: a deep residual soil which has been largely stripped from the Belmont surface near sea level during Devonshire time and which contains abundant palmetto stump casts Belmont Fm.: a complex unit of shallow-water marine beach calcarenites underlain and overlain by eolianites Unconformity: a rarely seen reddened surface between the Walsingham and Belmont Fms. Walsingham Fro. : a complex of highly altered eolianites and red soils TABLE II Comparison of Bermudian stratigraphy according to Land et al. (1967), Vacher (1981), and this study Land et al. (1967)

Vacher (1981)

Southampton Fm. (eol) St. George's Soil , , PAGET Spencer's Point Fro. [mar) GROUP Pembroke Fro. (eol) Harrington Fm. (soil) Devonshire Fro. (mar) Shore Hills Soil Belmont Fro. (mar)

Southampton Fm. (eol) soil Rocky Bay Fm. Pembroke Mere. (eol) , Devonshire Mem. (mar)

soil BELMONT ~ Spittal Point Fm. GROUP soil Harris Bay Fro.

this study modern dunes Southampton Fm. (eol) St, George's Soil Spencer's Point Fm. Pembroke Fro. (eol) Harrington Fro. Devonshire Fro. (mar) Black Watch Pass Fro. (eol) Shore Hills Soil (eol) Belmont Fro. (mar) (eol)

Soil (?)

soil

soil (?)

Walsingham Fro. (eol)

WalsinghamFro.

(eol) Waisingham Fm. (soil) (eol)

46 may not have been formed at the same time. Thus, rock stratigraphic units (formations), as mapped in the field, are not necessarily time stratigraphic units. Often it is not possible solely on the basis of geologic relationships and lithologic criteria to identify even the relative stratigraphic position of a particular isolated outcrop, because, away from the coast, contacts may be obscure and any individual exposure of calcarenite, eolianite, or soil may be lithologically similar to and indistinguishable from another of very different absolute age. As a result, important insights into Bermudian stratigraphy have been achieved by the absolute dating we report here. This has enabled temporal correlation of isolated and problematic geologic sections to type sections and thereby the reconstruction of Bermuda's Late Pleistocene sealevel history. U-SERIES GEOCHRONOLOGY Naturally occurring disequilibrium within the 238U decay series is the most suitable technique for dating Pleistocene carbonate materials. Both corals and speleothems contain trace amounts of U and essentially no Th when they form. Thus, the ingrowth of 23°Th from the decay of its parent 234U with a half-life of 75,200 yr. provides a means of dating these carbonates over the time range ~350 k.y. to present, provided a sample has not been affected by open-system diagenetic alteration. The age determinations reported here were done by the 23°Th/234U method. U and Th isotopic activities were measured by alpha spectrometry in the East Kilbride and McMaster laboratories by isotope-dilution techniques using a common 228Th--23~U spike of known activity. The two laboratories are intercalibrated through their participation in the "U-Series Interlaboratory Comparison Project" (Harmon et al., 1979). The reported ages were calculated from background corrected :3°Th/234Uratios as described by Harmon et al. (1978), assuming that no 23°Th was initially present in the samples, and that each one had remained a closed system with respect to isotope migration since the time of its formation. We have confidence in these assumptions because: (1) two living corals analysed have negligible 23°Th/234U activity ratios (Table IV); (2) duplicate analyses of fossil corals axe equivalent within analytical uncertainty; (3) ages for fossil corals from the same stratigraphic unit are internally consistent, with the exception of one marine unit which may be a composite deposit; and (4) stalagmite ages are in correct stratigraphic order. Sample locations are shown in Fig.1 and sample descriptions given in Table III together with elevations relative to present mean sea level. U-series analytical data and 23°Th/234U ages for the Bermudian corals and speleothems analysed are given in Tables IV, V and VI as well as for the previously published data of Harmon et al. (1978, 1981). In this study six drowned stalagmites from depths of between --3 and --20 m below sea level in Crystal Cave, seven subaerially exposed travertine sequences at various localities, and 33

47 fossil corals from various localities around Bermuda were initially selected for analysis. Eleven of the corals were subsequently rejected when found to contain <95% aragonite because it can be demonstrated that the recrystallisation of aragonite to calcite in a coral causes loss of U and a non-systematic disruption of U and Th isotope relationships. All of the drowned stalagmites were pristine, palisade calcite showing no signs of diagenetic alteration or evidence of having been affected b y marine biologic activity during sea-water submergence. Fig.3 is a 234U/23sU--23°Th/234U isochron diagram of the data from Table IV through which the "closed system" assumption can be further considered, and the temporal evolution of the U-series geochemistry of Bermudian corals evaluated. In this diagram the model evolutions of 23°Th/234U and 234U/23sU ratios are shown for various initial 234U/238U ratios in a system initially free of 23°Th. Because a coral form containing only a few ppm of U of the same 234U/23sU ratio as the sea water in which it grew, its isotopic evolution should follow the decay curve for the appropriate 234U/23sU initial ratio as a function of time since the death of the organism provided the coral has not been disturbed b y isotope migration into or out o f the sample. The 23°Th/234U--234U/ 238U data for the fossil corals, the two modern samples from Table IV, are also shown in Fig.3. Although the two modern corals have 234U/238U ratios lower than the 1.15 + 0.2 value of the average ocean (Koide and Goldberg, 1965), it is apparent that the fossil corals, within analytical uncertainty, follow a decay curve of 1.09 + 0.02 which extrapolates back to the field of the t w o modern corals thus indicating that they have remained "closed" to the loss or gain of either U or Th from the local environment since the time of their formation. It is also clear from Fig.3 that the coral ages cluster into t w o temporal groups which coincide with the last t w o major periods of interglacial climate as recorded in both the marine (Shackleton and Opdyke, 1973) and terrestrial (Harmon et al., 1977; Kukla, 1978) paleoclimate records. The two oldest coral ages are b o t h in excess of 250 k.y. and suggestive of an early high stand in the Bermudian paleosea-level record. However, Bender et al. (1979) have shown that corals in excess of a b o u t 220 k.y. may be subject to some loss or gain of radionuclides, and thus may not be accurate chronometers despite their high aragonite content. The four coral ages which fall within the range 228 to 200 k.y. are all from marine deposits and conglomerates of Belmont stratigraphic age. Sample 780525-2 is especially important because it is an in situ Siderastrea from a vermetid patch reef at + 1 m on Boaz Island and thus firmly establishes a Belmont high sea stand within a few metres of this elevation. The youngest group of coral ages, excepting the modern ages for the two living and one fossil coral, fall within the period 134 to 83 k.y. Eleven corals of this group which are of unequivocal Devonshire stratigraphic age have a mean 23°Th/234U age of 125 + 4 k.y., whereas the other ten corals, from marine conglomerate storm deposits that had previously been called the "Spencer's Point Fm." b y Land et al. (1967), have ages of between 131 and 83 k.y. The distribution of these conglomerate ages

48 T A B L E III D e s c r i p t i o n o f B e r m u d i a n corals a n d s p e l e o t h e m s a n a l y s e d in this s t u d y ( t h e 8-figure n u m b e r in p a r e n t h e s i s are grid references, allowing l o c a t i o n o f sample sites t o w i t h i n 10 m) I. Corals collected b y R.S.H., L.S.L., G.J.L. a n d H.P.S. in 1978 780521-3 780522-3 780522-5 780522-6 780524-1 780524-14 780525-2

Oculina f r o m coarse m a r i n e s a n d infilling a + 3 m n o t c h in t h e B e l m o n t eolianite n o t c h at W a t c h Hill Park ( 3 8 3 3 - 7 6 2 7 ) Diploria f r o m a m a r i n e c o n g l o m e r a t e cliff plaster at + 2 m in t h e Saucos Hill S e c t i o n ( 3 6 4 4 - 7 5 1 6 ) Siderastrea f r o m an isolated m a r i n e c o n g l o m e r a t e b l o c k at + 3.5 m in t h e Saucos Hill s e c t i o n ( 3 6 7 0 - 7 5 3 3 ) Diploria f r o m m a r i n e c o n g l o m e r a t e cliff plaster o n B e l m o n t e o l i a n i t e at + 3 to + 4 m ( 3 6 7 7 - 7 5 4 1 ) Diploria f r o m a m a r i n e c o n g l o m e r a t e cliff plaster in + 3 m n o t c h in Walsingh a m eolianite at n o r t h S t o k e s P o i n t ( 4 1 0 6 - 8 2 8 2 ) Montastraea f r o m an isolated p a t c h of m a r i n e c o n g l o m e r a t e cliff plaster o n B e l m o n t b e a c h r o c k at + 1 m ( 3 6 9 9 - 7 5 5 3 ) Siderastrea f r o m in situ v e r m e t i d reef " b i o l e r " b l o c k at + 1 m o n B e l m o n t b e a c h r o c k o n Boaz Island ( 2 5 3 7 - 7 5 8 6 )

II. Corals collected b y P.G. a n d H.L.V. in 1 9 7 8 PG-4

Montastraea f r o m fresh m a r i n e c o n g l o m e r a t e b e n e a t h t h e H a r r i n g t o n Fm.

PG-210

Siderastrea f r o m b e a c h r o c k w h i c h is early p h a s e o f a b e a c h d u n e c o m p l e x

at 0 - - + 2 m at Sugar Cane P o i n t ( 2 4 8 0 - 7 5 4 0 )

PG-218 PG-220 PG-304 PG-320 PG-321 PG-322 PG-324 PG-328 PG-329

at + 6 m o n H a w k i n s Island ( 2 7 7 2 - 7 3 0 7 ) Oculina f r o m m a r i n e c o n g l o m e r a t e at 0 t o + 1 m o n t h e west s h o r e o f Bird Island ( 2 9 6 2 - 7 4 3 9 ) Siderastrea f r o m a m a r i n e c o n g l o m e r a t e p o c k e t infilling at + 6 m in Belm o n t eolianite o n t h e east e n d of B l u c k ' s Island ( 2 9 4 7 - 7 3 2 8 ) Oculina f r o m b e a c h r o c k o v e r l y i n g + 1 m B e l m o n t m a r i n e b e a c h at East H u n g r y Bay ( 3 4 3 5 - 7 3 4 9 ) Oculina f r o m + 1 m B e l m o n t m a r i n e u n i t at East G r a p e Bay ( 3 3 8 0 - 7 3 0 0 ) Montastraea f r o m m a r i n e c o n g l o m e r a t e infilling + 3 m n o t c h in B e l m o n t b e a c h r o c k at Winsor H o u s e ( 4 1 4 6 - 7 8 2 9 ) Oculina f r o m + 3 t o + 4 m b e a c h r o c k u n i t at Cable Bay P o i n t ( 4 1 2 1 - 7 8 4 6 ) Millepora f r o m m a r i n e c o n g l o m e r a t e o n wave c u t B e l m o n t e o l i a n i t e b e a c h at + 1 m o n C o n e y Island ( 3 8 4 7 - 8 1 0 1 ) Siderastrea f r o m + 1 m c o n g l o m e r a t e o n s o u t h side o f S t o k e s P o i n t -- stratigraphic r e l a t i o n s u n c l e a r ( 4 0 9 7 - 8 2 5 6 ) Porites f r o m b e a c h sand u n i t infilling a + 1 m n o t c h in B e l m o n t eolianite along Ferry Reach (3995-8221)

III. Corals collected b y R.S.H., L.S.L., R.M.M. a n d H.P.S. in 1979 790513-2 790515-6 790516-2 790516-3 790516-4

Diploria f r o m + 1 m in situ m a r i n e d e p o s i t o n B e l m o n t eolianite at n o r t h e n d o f C o n e y Island ( 3 8 6 0 - 8 1 2 5 ) Oculina f r o m m a r i n e c o n g l o m e r a t e d e p o s i t o n + 1 m wave c u t B e l m o n t m a r i n e b e a c h at F o r t St. C a t h e r i n e ( 4 2 4 5 - 8 4 6 5 ) Porites f r o m same site as P G - 3 2 9 Porites f r o m same site as P G - 3 2 8 Favia f r o m m a r i n e s a n d infilling + 3 to + 4 m n o t c h in B e l m o n t e o l i a n i t e at C o n v i c t Bay ( 4 3 1 0 - 8 3 4 6 )

49 T A B L E III (continued) IV. Speleothems collected by R.S.H. and H.P.S. in 1 9 7 3 , 1 9 7 5 , 1 9 7 7 , 1 9 7 8 , a n d 1 9 7 9 73018 73023 73036 73037 73039 75001 75005 77518 77519 77520 77522 77524 780827-11 780825-5

780825-6 79RGE

BH2-2

Flowstone sequence from Government Quarry Cave ( 3 9 5 0 - 7 9 0 5 ) at +7 m Flowstone sequence from a sub-vertical fracture cave at Wilkinson Quarry ( 3 8 5 0 - 8 0 5 0 ) at + 1 6 m Stalagmite f r o m - 8 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Broken stalactite from + 3 to + 4 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from --7 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite f r o m - - 8 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from --3 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from - - 1 5 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from - - 1 5 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from - - 1 5 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from - - 1 3 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Stalagmite from - - 2 0 m in Crystal Cave ( 3 8 7 0 - 8 0 1 0 ) Travertine on Belmont eolianite at Spencer's Point ( 3 7 2 0 - 7 5 6 0 ) Flowstone from breached sea cave in Walsingham eolianite at Watford Island ( 2 5 2 7 - 7 5 7 0 ) Stalagmite from relict cave at east side o f North Ireland Island cricket pitch (2691-7740) Intercalated sequence o f multiple flowstone; soil, and micritic limestone units present as an isolated erosional remnant o f a sea-breached cave at north Stokes Point ( 4 1 0 6 - 8 2 8 2 ) Flowstone sequence from Government Quarry Cave ( 3 9 5 0 - 7 9 0 5 ) at + 7 m

defines three distinct temporal groups (one from 131 to 118 k.y., a second from 108 to 97 k.y., and the third from 85 to 83 k.y.), which are broadly coincident with episodes of relatively high sea stand within the last interglacial period as recognized in Barbados (Mesolella et al., 1969) and New Guinea (Bloom et al., 1974). In addition, one coral (780522-3) from such a conglomerate deposit in the Spencer's Point section, that was lithologically and texturally indistinguishable from the last interglacial conglomerates, gave an age of < 2 k.y. A total of 30 23°Th/234U age
50 T A B L E IV U c o n t e n t s , i s o t o p e activity ratios, and calculated ages for n o r t h e r n and fossil corals f r o m Bermuda

Sample

Species

% Arag. U conc. F~3*UI

number

~3°Thl

V23°Th]Age

(ppm)

[_238U_J

L23-~-UJ

[_232T--~J(103 yr. B.P.)

GROUP 1

780522-3

Diploria Oculina Diploria

100 100 100

2.6 2.0 2.0

1.09 ± 0.02 1.08 ± 0.02 1.14 ± 0.02

0.01 0.01 0.02 ± 0.001

Oculina Oculina

100 100

2.4 2.9

1.08 ± 0.02 1.08 ± 0.01

0.54 ± 0.02 0.53 ± 0.02

Diploria Diploria Diploria Diploria Montastraea Siderastrea

100 100 100 100 98 100

3.1 2.9 3.1 2.9 3.0 2.5

1.11 1.09 1.11 1.11 1.11 1.05

± ± ± ± ± ±

0.02 0.01 0.02 0.02 0.03 0.01

0.60 ± 0.59 ± 0.61± 0.60 ± 0.62 ± 0.63 ±

0.03 0.01 0.06 0.03 0.02 0.02

>200 >200 196 >200 32 26

Diploria Montastraea Montastraea Diploria Diploria Diploria Diploria Diploria Diploria Millepora Favia Oculina Diploria Diploria Diploria Oculina Oculina Siderastrea Siderastrea

100 100 100 95 95 100 100 100 100 98 100 98 100 100 100 90 100 99 100

3.1 2.4 3.1 2.1 2.4 2.8 2.9 2.9 2.9 1.9 2.1 2.5 2.9 2.8 2.7 2.1 2.3 2.6 2.8

1.10 ± 1.06 ± 1.09 ± 1.09 ± 1.10 ± 1.11 ± 1.10 ± 1.08 ± 1.09 ± 1.11 ± 1.04 ± 1.10 ± 1.09 ± 1.11 ± 1.06 ± 1.07 ± 1.11± 1.07 ± 1.12 ±

0.02 0.02 0.01 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.02

0.67 ± 0.67 ± 0.67 ± 0.68 ± 0.68 ± 0.69 ± 0.70 ± 0.68 ± 0.69 ± 0.69 ± 0.69 ± 0.69 ± 0.68 ± 0.70 ± 0.70 ± 0.70 ± 0.70± 0.71± 0.72 ±

0.02 0.04 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.04 0.04 0.01 0.03 0.03 0.03 0.02 0.02 0.02

>200 70 >200 146 31 >200 >200 >200 >200 30 49 >200 175 >200 27 >200 57 >200 57

----

living coral living coral <2

GROUP 2 790515-6 PG-218

151 62

85 ± 6 83 ± 5

GROUP 3 75012*

780524-14 780522-5

9726

106 ± 6 108 ± 6

GROUP 4 75014* PG-321 PG-4 780522-6 75013* 75016* 75020* PG-324 790516-4 PG-322 75015* 790513-2 PG-304 780521-3 PG-220 PG-210

118 ± 6 118 ± I I 1185 6 121 ± 6 124 ± 6 124 ± 8 124 124 124 124

± ± ± ±

6 8 12 12

125 ± 6 127 127 127 131 134

± ± ± ± ±

9 9 6 7 8

GROUP 5 780525-2

Siderastrea

99

2.1

1.06 ± 0.01

0.84 ± 0.02

>200

790516-2 PG-329

Porites Porites

99 95

2.8 2.4

1.01 ± 0.02 1.08 ± 0.02

0.84 ± 0.02 0.88 ± 0.03

72 >200

+ 20 --16 + 20 208 - - 16 200

51 TABLE IV (continued) Sample number

Species

% Arag. U cone. (ppm)

L23--~J

~23'UI

p. 3°Th.~

780524-1

Diploria

100

3.0

1.10 +- 0.02

0.89 -+ 0.03

70

790516-3

Porites

99

3.2

1.04+- 0.02

0.89-+ 0.03

95

PG-320

Oculina

97

2.2

1.05 +- 0.03

0.92-+ 0.03

73

262

PG-328

Siderastrea

100

2.4

1.07 +- 0.06

0.96 + 0.04

>200

>300

r23°Thl Age L23=ThJ (103 yr. B.P.)

~3-'~U-UJ

+ 24 --18 + 3O 222 --21 228

GROUP 6 + 35 --27

*Data from Harmon et el. (1978).

1,24

~

>, o

),

120

o o"

o

o

~- * insitu coro[ in pot*ch reef I " ~ ¢

I o coro! i~ mori~e [1,00[ 0

T--'~

conglomerot e storm

deposit 0.1

L 0-2

ii 03

I J I 04

,J 0"5

06

07

08

09

10

230Th123/,U

Fig.3. 234U/~38U--~3°Th/234U isochron plot showing the coral data from this study and that of Harmon et el. (1978, 1981). See text for discussion.

Fifteen 23°Th/234U age determinations were made on a stalactite from Crystal Cave and on subaeriaUy exposed travertines from Biermann Quarry Cave, Government Quarry Cave, and a cave on Ireland Island as well as from sea-breached caves and solutionally-enlarged cavities at Watford Island, Spencer's Point, and north Stokes Point (Table VI). Ages range from > 3 5 0 k.y. for the-Watford Island travertine (780525-5) and for the base of a travertine sequence (79RGE) from a small, sea-breached and almost entirely eroded cave in Walsingham eolianite at north Stokes Point to 58 k.y. for a flowstone deposit ( 7 3 0 2 3 ) from a solutionally enlarged, sub-vertical fracture cave in Belmont eolianite at Biermann Quarry. Of importance are the two of these deposits that are associated with marine phenomena. Stalactite 7 3 0 3 7

52 TABLE V U c o n t e n t s , i s o t o p e activity ratios, and calculated ages for s u b m e r g e d s p e l e o t h e m s f r o m Crystal Cave, B e r m u d a

Sample number

F:s4Ul Distance Elevation U conc. L23-'~UJ above (m) (ppm)

~.2s°Th~ L23~-U--J

r~S°Th.~Age L~32ThJ (lO s yr.B.P.)

base

(cm) 77519:3 :4

14.5 2.0

--15

0.12 0.13

1.02 ± 0.02 1.03 ± 0.02

0.10 ± 0.02 0.16 ± 0.03

101 162

11 ± 2 16 ± 3

75003:4" :5

16.5 2.0

--11.5

0.55 0.67

1.81 ± 0.03 1.45 ± 0.03

0.08 ± 0.02 0.23 ± 0.04

26 100

10 ± 2 27 ± 4

77524:1 :2

9.0 1.5

--20

1.01 0.93

1.02 ± 0.03 1.08 ± 0.03

0.19 ± 0.05 0.26 ± 0.04

>200 69

23 ± 6 34 ± 6

75001:3"

2.0

--8

0.48

1.20 ± 0.01

0.30 ± 0.01

196

38 ± 2

77520:1"* :3 :2

23.5 14.0 1.0

--15

0.14 0.09 0.08

1.17 ± 0.03 1.05 ± 0.04 1.60 ± 0.02

0.09 ± 0.02 0.30 ± 0.05 0.67 ± 0.03

30 >200 >200

10 ± 2 39 ± 7 111± 9

77518:1

29.5

--15

0.09

1.03 ± 0.03

0.32 ± 0.03

26

43 ± 5

750()5:2 :1

20.0 2.0

--3

0.08 0.07

1.04 ± 0.04 1.11 ± 0.03

0.47 ± 0.03 0 . 6 1 ± 0.03

23 >200

68 ± 6 97 ± 9

77522**

12.0

--13

0.08

1.53 ± 0.02

0.75 ± 0.03

110

134 ± 11

75002:3* :4

11.0 2.0

--6

0.46 0.39

1.15 ± 0.02 1.17 ± 0.02

0.62 ± 0.03 0.65 ± 0.02

46 >200

102 ± 9 113 ± 12

75004:15"

81.0

---6.5

0.94

1.31 ± 0.02

0.06 ± 0.03

5

0.75 0.73 0.84 0.64 0.69

1.00 0.97 0.96 1.04 0.99

0.61 0.62 0.64 0.65 0.68

0.50

:14 :13 :12 :11 :10 73036:11"

79.5 59.5 34.0 22.0 1.5 51.0

:10 :12 :13 :9

47.0 29.5 17.0 4.0

73039:7* :8 :6

25.0 9.5 1.1

--8

--7

± ± ± ± +

2

+ 0.02 ± 0.02 ± 0.02 ± 0.03 ± 0.02

71 22 41 147 102

1.34 ± 0.02

0.65 ± 0.02

25

0.80 1.00 0.26 0.30

1.07 1.13 1.02 1.01

0.78 0.81 0.83 0.84

0.02 0.02 0.01 0.02

38 46 40 43

114 ± 9 hiatus 153 ± 9 168 ± 18 179 ± 14 195 ± 20

0.18 0.09 0.10

1.06 ± 0.02 1.10 ± 0.03 1.10 ± 0.02

0.76 ± 0.02 0.82 ± 0.04 0.83 ± 0.02

50 23 25

150 ± 16 178 ± 21 195 ± 19

± ± ± ±

0.02 0.02 0.02 0.03 0.02

6 ± hiatus 99 ± 104 ± 109 ± 114 ± 119 ±

0.02 0.01 0.01 0.02

± ± ± ±

* D a t a f r o m H a r m o n e t al. (1978); * * d a t a f r o m H a r m o n et al. (1981).

5 6 8 8 6

53

T A B L E VI U contents, isotope activity ratios, and calculated ages for subaerially exposed spelcothems from various localities around Bermuda Sample

Distance

number

above base

U eonc. F ~ l

Elevation (m)

(ppm)

Fm~l

L2,,u]

L23,u]

LmThJ

A,e (10 3 yr. B.P.)

(m) 73018*

3.0

+7

0.51

1 . 3 5 -+ 0 . 0 3

0.82 ± 0.03

>200

162 + 14

73023:9 :8

6.0 0.2

+16

0.37 0.29

1 . 2 2 -+ 0 . 0 2 1.21 + 0.03

0 . 4 2 +- 0 . 0 2 0.62 + 0,03

>200 >200

58 + 5 101 + 18

73037:8* :6

---

+4

0.40 1.10

1 . 1 6 -+ 0 . 0 2 1.09 + 0.02

0 . 6 6 +- 0 . 0 2 0 . 7 1 +- 0 . 0 3

>200 >200

1 1 0 +- 1 4 1 3 0 +- 11

780524-11

bulk

+3

0.43

1.08

0.39

156

780525-5

bulk

+2

0.21

1.03

1.07

98

+6

0.29

1.02 + 0.03

0 . 8 8 -+ 0 . 0 3

55

0.21

1 . 0 4 -+ 0 . 0 3

0 . 9 2 -+ 0 . 0 3

121

0.16

1 . 0 9 +- 0 . 0 4

0.49 + 0.02

19

780525:6-1 :6-2 79RGE:5

30.0

+0.5

5 3 +- 2 1 >350 + 32 --25 + 34 257 --26 226

68 + 5 hiatus

:4

30.0

0.10

1.10 + 0.05

0.87 + 0.04

>200

206

+ 26 21

--

hiatus

>200

183

29 --23 --

:7

20.0

0.12

0 . 9 7 +- 0 . 0 4

0 . 8 1 -+ 0 . 0 4

:3

0.2

0.12

0 . 9 9 -+ 0 . 0 4

1.03 + 0.04

35

BH2-2:l**

12.5

0.06

1 . 1 2 -+ 0 . 0 8

0.73 ± 0.06

6

120

0.10

1 . 2 8 +- 0 . 1 0

0 . 6 8 -+ 0 . 0 5

7

1 0 4 -+ 1 7

hiatus

:10"*

3.0

+8

>350 + 25 --22

*From Harmon et al. (1978); **corrected for detrital :3°Th. formed at + 3 to + 4 m in Crystal Cave and contains an interior layer of marine aragonite which records a high sea stand between ~ 130 and 110 k.y. (Harmon et al., 1978). The Stokes Point travertine sequence (79RGE) contains three intercalated marine deposits, the middle one of which records the Belmont high sea stand at about 200 k.y. and the upper one which records the Devonshire high sea stand at 125 k.y. The three other travertine sequences (73018, 73023, BH2), all of which occur at elevations above + 7 m, show no evidence of marine alteration or surface coatings, fresh-water overgrowths, or any other evidence that they have in any way been affected by postdepositional flooding by sea water or a meteoric phreatic lens. This is consistent with the observation that there is no sedimentological evidence for subwater table cementation of eolianites above the +6 m level associated with the Devonshire high sea stand. Preliminary results have also been obtained on the dating of aragonite from the eolianites by electron spin resonance (ESR) techniques. The intensity of the ESR signal corresponds to the number o f electrons, produced by

54 ionising radiation, which have been trapped at lattice defect sites in the aragonite. The intensity of the signal is an increasing function of age of the sample. For our Bermuda work this age dependence has been roughly calibrated b y analysis of eolianites of Belmont, Devonshire, and modern age. Ages of the first t w o of these eolianites were taken from the mean of the 23°Th/234U coral ages from the correlative marine deposits. Based on this calibration, the apparent ages for the tIarrington Formation and Pembroke Formation are approximately 105 and 95 k.y. respectively. These results are consistent with the ages proposed on the basis of the amino-acid racemization dating and geomorphic arguments developed in this paper. Further details of this ESR work will be presented elsewhere by Schwarcz and Karakostaoglou (in prep.). AMINO-ACID RACEMIZATION CHRONOLOGY

The use of amino-acid racemization reactions is a new and developing technique for determining both relative stratigraphic relationships and absolute ages. The m e t h o d is based u p o n interconversion of the L- and D-isomers of amino acids preserved within skeletal fossils (e.g. shells and bones). Originally, all amino acids in proteins of living organisms are present only in the L-form; however, u p o n death of an organism the protein in protected structures such as mollusc shells undergoes hydrolysis to free amino acids and the L-amino acids gradually convert to the D-form until an equilibrium between the t w o isomers is reached. The rate o f racemization is dependent on a number of variables, with the t w o most important ones being the time and temperature of the reaction. In theory it is possible from a knowledge of one o f these parameters, together with a detailed understanding of the reaction kinetics, to calculate the u n k n o w n variable. However, because o f the large uncertainty in estimates of diagenetic temperature, it is c o m m o n l y more practical to calculate ages for fossils based u p o n a calibration sample o f k n o w n age from the same region. From the age of the calibration sample a reaction rate constant for the region can be calculated for the racemization of any amino acid. The reaction rate constant can then be used to calculate the ages of other fossils in that region. In the absence of a calibration sample, the extent o f amino-acid racemization may still be used as a guide to the relative ages of fossils. Of the various possible amino acids, we have chosen to measure the degree of interconversion of isoleucine (fie) and alloisoleucine (aile) where the reaction is termed epimerization because these two amino acids are diastereoisomers rather than optical isomers. Details of preparative techniques and analytical procedures are given in Kriausakul and Mitterer (1980). The results are presented and discussed as the ratio of allisoleucine to isoleucine; the ratio becoming larger with increasing age of a fossil. Multiple chromatographic runs of the same sample hydrolysate are within +0.01 of the average aile/ile value while different specimens from the same o u t c r o p are generally within + 0.02 of the average value. Difference in preservation of fossils in an outcrop

55 may occasionally result in greater ratio differences. To minimize the uncertainties in the aile/ile value arising from such differences in preservation, multiple samples were analysed from each locality. The ratios for all samples are displayed in histogram form (Fig.4) to emphasize the clustering of ratios into discrete groups, with the average aile/ile value for each group given together with its stratigraphic and absolute age in Table VII. Multiple accretionary soils ("white soils") and eolianites record transgressions of eustatic sea level on to the Bermuda platform to elevations lower than that of present sea level. Mapping of such soils and eolianites is difficult because of the similar appearance and discontinuous nature of the outcrops. As a result, there are no stratigraphic means b y which the various temporal groups of accretionary softs and dunes can be unequivocally recognized in the field. However, soils and isolated pockets of unconsolidated dune sand c o m m o n l y contain well-preserved land snails (Poecilzonites sp.) on which the amino-acid racemization analyses were performed. As seen from Fig.4, aile/ ile ratios for about fifty land snails from various localities across Bermuda cluster into five distinct groups (L ~V in Table VII). Each group has its own characteristic aile/ile ratio, which thus permits categorisation and differentiation of different soils and eolianites b y this geochemical means. A relative chronology for the various gastropod-bearing units based u p o n clearly demonstrable stratigraphic relationships is given in Table VII. Given this information, it is possible to derive an absolute chronology for the sequence b y calibrating one or more of the soil or eolianite units with independent absolute age determinations. We have approached this problem 10 9 8 7 6

~s .= 4

3 2 t 0,25

M

o!3

0!35

0[4

0145

UII

o5

o!s5

0[6

M

o!65

o!?

o!Ts

Altoisoleucine/Isoteuc ine

,°5

,~0

44

44

Age(10 yr BP)

Fig.4. Histogram showing the distribution of land snail amino-acid racemization ages for eolianites and accretionary soils.

56 TABLE VII Alloisoleucine/isoleucine ratios for the land snail Poecilozonites from Late Pleistocene accretionary soils and eolianites from Bermuda and their probable age and stratigraphic equivalent Unit

Mean (aile/ile)

Relative stratigraphic age

Absolute age (k.y.)*

Stratigraphic equivalent

V IV III II I

0.38 0.46 0.51 0.58 0.71

post-Harrington post-Harrington post-Devonshire pre-Devonshire pre-Devonshire

87 105 120"* 134 184 ** *

Southampton Pembroke Harrington "Blackwatch Pass" Belmont

*Ages calculated using fossils from the Harrington Fm. as a "calibration" sample. **Calibration sample; kis o = 4.22 × 10 -~ yr. -~ is calculated from equation 3 of Mitterer (1975) assuming an age of 120 k.y. for the Harrington Fro. ***The age is a minimum as the aile/ile value is probably beyond the range of linear racemization kinetics.

in two ways: first b y 23°Th/234U dating of corals immediately underlying the Harrington Fm. and secondly b y 23°Th/234U dating of the Harrington Formation itself. The Harrington Formation, which overlies the well~lated marine Devonshire Formation and records the regression of the Devonshire sea, is an unaltered and uncemented calcarenite containing marine fossils at its base and terrestrial gastropods in its upper portions. Thus, the Harrington Formation (Unit III in Table VII) must be slightly younger than the 125 + 4 k.y. age of the Devonshire Formation, so that an age of 120 k.y. has been assigned to it based u p o n the absolute age of the Devonshire Formation and a bulk Useries analysis of the Harrington Formation itself which yielded a ~3°Th/234U age of 121 + 9 k.y. (U = 0.9 ppm, 238U/238U = 1.08 + 0.02, 23°Th/234U = 0.68 + 0.03, 23°Th/232Th = 70). In order to obtain absolute amino-acid racemization ages from this calibration, it is also necessary to have a knowledge o f the epimerization kinetics of Poecilozonites. Although the kinetics of epimerization of isoleucine in this system are not known, it is reasonable to assume linear, first-order kinetics as the aile/ile ratios for most samples are less than 0.6. Thus, the ages listed for the other four groups in Table VII have been calculated from the reaction rate constant, kiso, determined for the Harrington Formation (see Mitterer, 1975, for calculations) assuming linear racemization kinetics. Further confidence in this assumption comes from the good agreement b e t w e e n aminoacid ages for the younger, post-Devonshire eolianites and those assigned by Shackleton and O p d y k e (1973) to oxygen isotope Stages 5c and 5a of the deep-sea foraminiferal record (Fig.4). Thus, the Harrington Formation is characterized b y aile/ile values clustering a b o u t a mean o f 0.512 to which an age of 120 k.y. is assigned: kiso, the

57

reaction rate constant, is 4.22 X 10 -6 yr. -I. Group I is a composite of the two oldest, post-Walsingham eolian events, those of the Belmont Formation, for which amino-acid ages range from 194 to 174 k.y.; however, these are hkely minimum ages as the aile/ile value is probably b e y o n d the range of linear kinetics. An insufficient number of samples was analysed to definitively characterize individual sea-level events within Belmont time, but it is clear from the amino-acid racemization data that this time was a ~ 3 0 , 0 0 0 yr. period of general platform submergence. Group II is a substantially younger dune-building event most pronounced along the north shore between Blackwatch Pass and Barkers Hill. This unit, with an aile/ile ratio of 0.57 underlies the sublittoral deposits of the last interglacial, Devonshire high sea stand, and thus represents a pre-Devonshire transgression on to the Bermuda platform at about 134 k.y. Units IV and V are post-Harrington eolianites, best developed along the south shore and in western Bermuda. The aile/ile ratios for these t w o units are 0.46 and 0.38, giving respective ages of ~ 1 0 5 and ~ 87 k.y. indicating that t h e y represent transgressive phases of the last interglacial period during which sea level rose onto the platform, b u t remained well below modern sea level. With the exception of two analyses, which give ages of about 70 k.y. but may be spurious due to differences in preservation, no further eolian activity is recognized during the last glacial period between 70 k.y. and the formation of the modern south-shore dunes which were initiated with the Holocene flooding of the platform at a b o u t 9 k.y. GEOLOGY AND LATE PLEISTOCENE SEA-LEVEL HISTORY

The geology of Bermuda has undergone continuous revision since first described by Nelson (1837), Thompson (1873) and Verrill (1907). Sayles (1931) first recognized that glacial eustasy was responsible for the observed distribution of elevated marine limestones and eolianites. Recent studies have emphasized the glacio-eustatic cyclicity of Bermuda's exposed carbonate rocks (Bretz, 1960; Ruhe et al., 1961; Land et al., 1967; Vacher, 1973) and concentrated on establishing the details of Pleistocene eustasy (Land et al., 1967; Vacher, 1973; Harmon et al., 1978, 1981). Most investigators have assumed that Bermuda is relatively stable, and thus approximates a "tide gauge" against which absolute changes in sea level can be measured. Certainly there is no evidence to indicate that Bermuda has been subject to major tilting, faulting, uplift, or subsidence during the past I m.y. or that would suggest that it has been substantially affected by isostatic deformation. We do not k n o w the degree to which uplift or subsidence on the scale of a few meters may have occurred, b u t because we know of no current data b y which such minor tectonic motion might be assessed, we assume stability. Some aspects of Bermudian geology have always been clear and agreed u p o n b y virtually all investigators, whereas other aspects have always been controversial. Fig. 5 presents the Late Pleistocene sea-level curve as published

58

¢:

~

o

~

E

~

o

~=

I ~i~

~"~ I

I[I

~o

~.

I I

I~1 21

-8o

ft

-60 -40

,/I

..-.

I I

I

I

I

I I

I .

.

.

-20

--MSL

.

160

1/,,0 - - -

120

100

80

60

40

20

0

TIME (xlO 3 YEARS]

Fig.5. Late Pleistocene paleosea-level curve for Bermuda as published by Land et al. (1967). b y Land et al. (1967), and around which the discussion which follows will focus. Subsequent work b y Vacher (1973), Harmon et al. (1978, 1981) and this study has added considerable chronological information and reinterpreted some of the geological evidence u p o n which that first Bermuda paleosea-level curve was based. This summary presents a revised paleosea-level curve for the past 250,000 yr. (Fig.6) based upon the 2a°Th/234U and amino-acid racemization age data in Tables IV to VII, discusses the geological relationships upon which this curve is based, and draws a correlation with other Late Pleistocene paleosea level and paleoclimate records. This curve has been constructed solely on the basis of Bermudian geology and geochronology without reference to other areas. The curve may therefore be incomplete as not all Late Pleistocene sea-level fluctuations may have been recorded b y or correctly interpreted from the exposed geology. The oldest exposed carbonate rocks in Bermuda are the highly diagenetically altered and karstified, pure calcite eolianites of the Walsingham Fm. Both outcrop studies and recent mapping as well as diving in the sea-water flooded caves within the Walsingham Formation d o w n to depths of --25 to --30 m show this to be a composite unit comprised of at least three separate eolianite and intercalated red soils. No absolute age data are available on the Walsingham itself, b u t 23°Th/234U ages for travertines in sea-breached Walsingham caves at Watford Island {780825-5) and Stokes Point (79RGE :3) place a minimum age o f > 3 5 0 k.y. on the Walsingham Formation Extensive soil development on the unit, karstification, and cave development within the unit also suggest that the Walsingham is much older than the overlying Belmont Formation. Thus, we infer an Early to Middle Pleistocene age and conclude that the Walsingham records several episodes of platform submergence and exposure.

59 The next high sea stand recorded in Bermuda is possibly a pre-Belmont event which flooded the platform and deposited a sub-littoral conglomerate at Stokes Point somewhat before 300 k.y. Evidence for this event is based upon equivocal stratigraphic relationships and a single 23°Th/234U age determination on a reworked coral (PG-328) from a + 1 m marine conglomerate that also yielded a fresh coral of Belmont age (780516-3). Land et al. (1967) described an outcrop of marine conglomerate at Government Quarry that was plastered unconformably onto a Walsingham cliff at about + 28 m above sea level. This conglomerate, similar to the ~ u t h - s h o r e storm deposits, contained volcanic fragments, rounded Walsingham clasts, and red soil fragments. Recent quarrying operations have removed most of the outcrop, b u t a similar outcrop was recently located at + 9 m. Unfortunately, a coral obtained from it was only 80% aragonite and unsuitable for dating. This event may be correlative with the pre-Belmont highstand at Stokes Point, but both must be considered tenuous until additional cllronologic work firmly establishes the age of the deposits. Recent detailed geological mapping (Vacher et al., in prep.) also suggests a significant post-Walsingham, pre-Belmont eolian formation which, judging from intraformational soils, is probably a multi-phase unit. The most extensive marine deposits of Bermuda are those of the Belmont Fm. This unit characteristically exhibits a + 1 m bench throughout "older" Bermuda which is the result of beachrock cementation that occurred during or just after deposition (Land et al., 1967; Land, 1970). Initially, Land et al. (1967) interpreted the Belmont Fm. to record two episodes of higher than present sea level (Fig.5). The first at +28 m was based upon the problematic Government Quarry conglomerate discussed above. The second highstand is recorded b y the characteristic "type-Belmont" beachrock about which there is no controversy. A coral (780525-2) from an in situ patch reef deposit at +1 m in association with Belmont beachrock at Boaz Island gives a 23°Th/ 234U age of 200_16 +2o k.y. This establishes the marine Belmont high sea stand in Bermuda as correlative with the Kingsland terrace ("second high cliff") in Barbados (Bender et al., 1979) and reef terrace VIII in New Guinea (Chappell and Veeh, 1978); i.e. the penultimate interglacial period in the marine and terrestrial paleoclimate records (Shackleton and Opdyke, 1976; Harmon et al., 1977; Kukla, 1978). The Belmont Formation also consists of eolianites which both pre- and post
60 I

I

I

I

A-21 ",'7"

-1.1

II

Ill

IV

V

Vl

VII

VIII

E

W o

-

._

_

-

=

~

~

-

(~

.~. -

L~ w

x

.15

.10 x--7

7--,

.,5

lJ~

i

-

7-- ---'----7 oo

,,--?I'~* O/

E-

.~

?

/

°

0 ~ - s e a revel

.,.,, -5

-10

~ ~ /l

//

.¢ •

/ /

/ /

-~

I z, /

~, -15

'

t

,

""

-

-20

Bermuda

• :.

platform

"dl

.< ~ ~,

"

,

,

I

/

,

,

1

50

I

"

/ /

/

.

.

/

i

i

,

,

100

150 Age (103 y BP)

l

" - ;"

." ~

, ~"

;

f. -

;

.,

,

..

~

>' ;

7.~ ,

,C"

;

/I

1

Id

¢

" ,?,

; i

/

/ /

l,., l

~ ,

7 J

"

/z

,"

,"

./ ./ .

,, ~ ; ,~.;,,

5 , ? " ,

/ e

I

/

i

. , .,,

/

.?,~. ." ./

?

,~

/ ,/ i

l/

x/

;

z ~

"

/ -

/i

/

,, i

?.',/"~.,..

.

,

? I

200

,

i

, ,

/-,

250

61

225 k.y. which produced storm-type conglomerates at Stokes Point. Thus we envisage flooding of the Bermuda platform at: (1) about 230 to 220 k.y. which produced the early Belmont eolianites and some storm-type marine conglomerates at elevations just above sea level; (2) a b o u t 210 to 190 k.y. with sea level rising to +2 m (_+2 m) with the extensive +1 m Belmont beach deposits developed during the regression; and (3) some time after 200 k.y. to produce the late Belmont eolianites. By 195 k.y. stalagmite deposition is documented at --7 m (Table V) which indicates that the main Belmont sea had beg~n to fall shortly after reaching its ~ 2 0 0 k.y. maximum stand. Sea level remained below this level from a b o u t 195 to 150 k.y., and likely at an elevation below the platform for a majority of this time except during the interval when the late Belmont eolianites were deposited. It was during this ~ 4 0 , 0 0 0 yr. interval that the well-developed and geographically extensive Shore Hills Soil was formed. The next and most complex phase of Bermudian geology, as recognized both stratigraphically and chronologically, is the last interglacial period. Based upon the work of Harmon et al. (1978, 1981) and this study, several important modifications must be made to the earlier sea level curve of Land et al. (1967) as regards events over the period 145 to 75 k.y. The original interpretation of the Devonshire and Harrington Formations remains unchanged. The Devonshire Formation records a + 5 m (+ 1 m) high sea stand which produced a wave-cut notch in the Belmont Formation, deposited extensive sublittoral calcarenites, and flooded what are at present the air-filled portions of many caves. Marine aragonitic surface coatings developed on actively forming stalactites and stalagmites at elevations up to + 4 m. The Harrington Formation records the regression. Eleven 23°Th/2~4U dates on corals from Devonshire sites around Bermuda where stratigraphic relationships are clear range from 134 -+ 8 to 118 -+ 6 k.y. with a mean of 125 +- 4 k.y. (Table IV). Thus, the timing and elevation of the Devonshire high sea stand are b o t h equivalent to those observed from 125 to 120 k.y. in other stable areas such as Hawaii (Ku et al., 1974), the Bahamas (Neumann and Moore, 1975), Yucatan (Szabo et Fig.6. Late Pleistocene sea-level fluctuations for Bermuda based upon the U-series and amino-acid racemization ages as well as the geological reinterpretation discussed in the text. Filled circles denote corals from marine calca~enites or in situ patch reefs and open circles denote corals from storm-deposit conglomerates; associated vertical lines denote elevations of the coral bearing deposit. Ages for submerged stalagmites are given by the short vertical lines, the solid lines connecting these points denote continuous deposition over the indicated time interval, whereas dashed lines indicate a period of interrupted deposition, and question marks indicate that only top or bottom age was determined so that the full growth history is not known. The small dots in the upper part of the diagram indicate time,elevation relationships for 14C-dated peats as determined by Redfield (1967) and Neumann (1969). Diagonal crosses indicate age determinations for subaerially exposed travertines and stalactites. Diamonds indicate subaqueous overgrowths or submerged stalagmites; filled being freshwater and open being marine. Also shown is the deep-sea foraminiferal 180]160 curve of core V28-238 of Shackleton and Opdyke (1976), and the lithologi¢ units which correlate to the documented sea level events.

62 al., 1978), and the Florida Keys (Broecker and Thurber, 1965; Osmond et al., 1965) and also indicate temporal correlation with the Rendezvous Hill terrace ("first high cliff") in Barbados (Mesolella et al., 1969) and reef terrace VI in New Guinea (Bloom et al., 1974; Chappell, 1974). Prior to Devonshire time between ~150 and 130 k.y. we postulate, based on amino-acid racemization ages (Table VII), widespread eolian activity which implies platform submergence and subsequent reef development to provide a source of sand for the eolianites. This eolianite deposition is recorded in the ridge of coastal dunes which extends along the north shore from Flatts Inlet to Spanish Point, and was previously assigned by Land et al. (1967) to the Pembroke Formation. We have tentatively called these pre-Devonshire eolianites, dated at ~ 134 k.y. by amino-acid racemization, the "Black Watch Pass" unit. Cessation of stalagmite deposition at --7 m in Crystal Cave at ~150 k.y. (Fig.6) is also suggestive of platform submergence at this time. This pre-Devonshire event was relatively short-lived because stalagmite deposition is documented at --14 m at 135 k.y. (Table V). In fact sea level was likely below --50 m at this time as documented by speleothem deposition in a "Blue Hole" in the Bahamas (Gascoyne et al., 1979) and as suggested by oxygen-isotope data for deep-sea sediments (Shackleton and Opdyke, 1973) and corals (Fairbanks and Matthews, 1978). Following the Devonshire regression at ~120 k.y., sea level did not again rise to within --15 m of present level until the Holocene transgression. The renewal of stalagmite deposition at --6 m by 119 k.y. (Table V) indicates that the Devonshire sea stood at its maximum level for only a short time, certainly no more than 8000 to 10,000 yr. A 23°Th/~34U whole-rock analysis of the Harrington Formation from the Devonshire Bay type-section has yielded an age of 121 + 9 k.y. A fresh-water, subaqueous overgrowth on stalagmite 73036 at --6.5 which formed at ~115 k.y. (Table IV) may record a short stillstand during the post-Devonshire regression, but the lack of eolianite deposition by 110 k.y. suggests that sea level may have fallen off of the platform by this time. The remainder of the last interglacial period in Bermuda is a time of episodic dune building and soil formation resulting from periodic submergence and exposure of the platform. The eolianites of the Pembroke and Southampton Formations, which form much of the present topography of "younger" Bermuda, record this dune-building activity. Eighteen amino-acid racemization eolianite ages and five 23°Th/234U dates on storm-deposit conglomerate corals cluster into two groups at about 105 k.y. and 85 k.y. denoting the timing of the short-lived Pembroke and Southampton transgressions. These two post-Devonshire events, the first time recognized in a stable area in Bermuda, are correlated with the Ventnor and Worthing terraces in Barbados (MesoleUa et al., 1969) and reef tracts V and IV in New Guinea (Bloom et al., 1974). The deposition of stalagmites at elevations below present sea level was continuous throughout this time and one specimen (77520) was deposited at --15 m without interruption throughout the interval ~110 to 10 k.y.

63 (Table V), thus constraining the Pembroke and S o u t h a m p t o n high sea stands to elevations b e t w e e n - - 1 5 to --20 m (the platform margin). This is consistent with the fact that no marine facies of either unit has been observed and that the outcrop of b o t h eolianite units extends below present sea level. This sea-level history for the last interglacial period represents a major revision of the Land et al. (1967) version in two respects. The first concerns the high sea at stand approximately + 20 m, the Spencer's Formation postulated at 105 k.y., and the correlation of the south-shore conglomerates and the seaward 3 0 k.y. ~4C age previously cited b y Velastro et al. (1972). Thus the Southampton Formation records a platform-flooding event late in the last interglacial period which did n o t reach modern sea level. Although disagreements still exist a b o u t the ages of specific lithologic units at particular outcrops in Bermuda, none of the arguments known to us affect the revised sea-level curve we have presented in Fig.6. Given the complex and often discontinuous nature o f Bermudian geology, these are problems which will not be entirely resolved until every questionable unit has been dated.

64 The final sea-level event observed in the Bermuda record is the Holocene rise to present level as documented by Redfield (1967) and Neumann (1969) from 14C dating of peat deposits down to --25 m and our 23°Th/234U dating of drowned speleothems down to --15 m. Flooding of the platform occurred at about 9 k.y. and only reached its level at about 2 k.y. (Fig.6). Speleothem deposition down to --20 m and the absence of any post-Southampton eolianites indicate that the platform must have been subaerially exposed throughout the ~ 70,000 yr. interval of the Wisconsinan glacial period from about 80 to 10 k.y. P A L E O C L I M A T E IMPLICATIONS

The eustatic curves shown in Fig.6, derived from our investigation of Bermudian geology, suggests that sea level during the last two interglacial periods did not exceed present sea level by more than a few meters. With respect to the last interglacial high stand at 125 k.y., this view is in basic agreement with reported sea-level estimates from islands and coastlines in the Pacific, Indian and Atlantic Oceans (Broecker and Thurber, 1965; Osmond et al., 1965; Veeh, 1966; Thompson and Walton, 1972; Ku et al., 1974; Neumann and Moore, 1975; Szabo et al., 1978; Szabo, 1979; Cronin et al., 1981) where a high stand dating about this time has been estimated as being 1.5 to 9 m higher than present sea level (F.ig.7). High sea stands dating about 200 k.y. have also been recognized in New Guinea, Barbados and along the Atlantic Coastal Plain (MesoleUa et al., 1969; ChappeU, 1974; Bender et al., 1979; Cronin et al., 1981) and, as with the case in Bermuda, the evidence from these areas suggests that sea level during the penultimate interglacial likewise attained a maximum elevation only a few metres higher than present sea level (Fig.7). In addition, the paleosea-level curve presented in Fig.6 indicates that each interglacial high sea stand was relatively short-lived with a duration not exceeding 10,000 yr. This conclusion is supported by data from both New Guinea and Barbados (Mesolella et al., 1969; Bloom et al., 1974; Chappell and Veeh, 1978; Bender et al., 1979) where multiple radiometric dates from the same elevated reef complex associated with the interglacial high stands suggests that the culmination of the reef building episode lasted only a few thousand years. The implications thus drawn from the Bermudian eustatic record are: (1) that during the past 250 k.y. interglacial ice volumes and sea levels were approximately what they are at present; and (2) that the interglacial periods were characteristically short-lived events terminated by rapid expansion of continental ice. Perhaps the most significant aspect of the Bermudian eustatic paleosealevel curve, however, is the indicated rate of sea level lowering immediately following the last two interglacial high sea stands. For example, the curve suggests that during the first 5000 yr. following these high sea stands sea level dropped in excess of 12 to 14 meters. This value is significantly less

65

~--[ Tectan,c Areas -~Lg~ BARBADO~ NEW GLNEA d ! y BP) NesotelIa et 011969 Bloomet at 197~ Bender et s 1979 Chappei[ 1974

"--

. . . . . . BA~A~AS

HAWAII

Neumann& Moo~e !975

Ku et Q

!97~

9obte Areas YUCATAN Szobe et cl !9~

~ . . . . . . . FLORIDA KEYS BERmuDA MARNE x~,iN B'oecher ~Thurber ~55 lh~S Study)ISOTOPE % E ] < Osmon~ et o11969 (Shackletap & Opdyke 1973}
0 i

~-I[--* ~]il~

%

~orthmg ~ terroc e 100 -

~V

poshBevoosnlre eobamte

~

~'entnor ~terroce -

I

Rendezvous ~Hdl terrae~

4

wolmanelo Shoreline

postOevorshlre eohomte Oevonshlr~ Marine preOevonshlre eohonlte

153-

44 4 230_,i,~ ;

KendaU H I [ terrace

~V[I[--*

KmgsIGnd terrace ~

4,,---Aberdole j terrace

~:X0 ~

p0St Belmont

eO[lande ~Be[mont~ MGrlne pre Belmont eohar',te

?SOc

~ I Boarne re-race

~ROWa~S

,

-

J 4~

'err~:e 'b r e H ' L ~

9 'i

Fig.7. Suggested correlation between the Bermuda Late Pleistocene sea-level record presented here and other marine paleosea-level and paleoclimate records. See text for discussion.

than the reported 70 m drop which is inferred from tectonically rising New Guinea (Chappell, 1974; ChappeU and Veeh, 1978), and is also considerably less than the estimated 50 m drop proposed by Johnson and Andrews (1979) which is based on calculations from temperature-corrected, oxygen-isotope ratios for deep sea cores. However, a sea-level decline on the order of 25+ m in the 10,000 yr. period immediately following the last t w o interglacial high sea stands is not unreasonable when compared to the calculated drop of 19 m reported by Andrews and Mahoffy {1976) which is derived from a realistic glacier growth numerical model for the Laurentide Ice Sheet. It should be noted that rates of eustatic sea level rise and fall calculated from the curves in Fig.6 are based in part on dates for initiation of speleothem growth which may n o t occur until a substantial period of time after sea level (or an associated water table) has dropped below the point of deposition.

66 GLACIAL STRATIGRAPHIC CORRELATIONS

Because Pleistocene eustatic sea-level fluctuations are largely a product of the growth and disintegration of the continental ice sheets, there should exist a relationship between the continental Pleistocene glacial stratigraphic records of North America and the sea-level curve shown in Fig.6. This should be particularly true of the glacial--interglacial sequence of central North America which has traditionally been used as a standard for correlations throughout much of the continent (Flint, 1971). The subdivision of this record into glacials and interglacials are summarized by Willman and Frye (1970), represents the culmination of almost one hundred years of investigation. Unlike the basic subdivision of the European Pleistocene, which is based chiefly on morphology (Kukla, 1977), the subdivision of the North American Pleistocene is based exclusively on rock and soft stratigraphic units, with the notable exception of the eastern Great Lakes region, where radiocarbon age determinations extending back to approximately 60 k.y. have resulted in the development of an absolute age framework for the subdivision of the Wisconsinan glacial period (Dreimanis, 1976). On radiometric criteria alone it is evident that the Wisconsinan of central North America overlaps in time the last glacial period represented in the eustatic sea-level record from Bermuda. However, since the base of the Wisconsinan has nowhere been radiometrically dated, it is not altogether certain whether it corresponds to the beginning of the last or penultimate glacial events of the paleo-sea level record. Complicating matters further, there as yet have been no reliable rock stratigraphic correlation established between the pre-Wisconsinan record from central North America and the Atlantic Coastal region where sediments from Pleistocene marine transgressions are well preserved and in some areas have been radiometrically dated (Cronin, 1980). Absolute age control is also completely lacking for the Sangamon, IUinoian and Yarmouthian events of the continental record. Volcanic ash deposits, however, do occur in the Kansan and have yielded radiometric ages of about 600 k.y. (Izett et al., 1972). Other ash deposits occurring in the Nebraskan have also yielded ages spanning the period between 1.2 m.y. and 700 k.y. (Boellstorff, 1973). It is evident that any attempt to relate the continental record of glacials and interglacials from central North America to the Pleistocene eustatic record is fraught with difficulties. Chief among these are the lack of radiometric dates for the continental record between the period 600 and 60 k.y., and the absence of any rock stratigraphic correlations which could possibly tie together the coastal marine record with the continental record. Without these controls it is clear that any proposed relationship between the two records would be unreliable and undoubtedly useless. Some success, however, might be achieved if other Pleistocene records from elsewhere in North America are utilized and which are not beset with as many uncertainties. Harmon et al. (1977) demonstrated that the deposition of speleothems

67 within alpine caves in the Rocky and MacKenzie Mountains occurred during interglacial periods and that the timing of glacial periods in western North America were coincident with glacial periods of l SO-enrichment in the marine foraminiferal oxygen-isotope record and periods of low sea level in the marine glacio-eustatic sea-level record. However, it was not possible to directly tie this continental paleoclimate record directly to the local glacial stratigraphic record in any of the localities studied. Also, dated sequences of tills, tephra, and lake silts covering the entire Pleistocene have been reported for the area around Yellowstone National Park by Richmond (1976). Additionally, 23°Th/ 234U dates for travertines associated with glacial deposits in this area indicate that these deposits were formed during times of worldwide glaciation as defined by the marine oxygen-isotope and sea-level records (Schwarcz and Richmond, unpubl, data). There is also a unique sequence of Late Pleistocene drift in Baffin Island which includes interbedded marine sediments some of which have been dated {Miller et al., 1977). SUMMARY The overall picture o f Late Pleistocene sea-level fluctuation derived from our geologic and chronologic study of Bermudian geology is one of multiple episodes of platform submergence during interglacial periods {deposition of sublittoral calcarenites, eolianites, and storm conglomerates) separated by glacial periods of platform exposure (regolith paleosol development, and speleothem deposition at elevations below present sea level). Only twice in the past 250,000 yr. has sea level in Bermuda been above its present level; once at ~ 2 0 0 k.y. when it stood at about + 2 m and most recently at 125 k.y. when it stood at +5 + 1 m. Episodes of lower high sea stand onto the Bermuda platform at --20 m are observed within the penultimate interglacial period at ~ 2 2 0 k.y. and shortly after 200 k.y. and within the last interglacial period at between 150 and 130 k.y., ~ 1 0 5 k.y. and ~ 8 5 k.y. Our paleosealevel curve for the past 250,000 yr. (Fig.6) suggests that sea level rise and fall for individual events within each of the last three interglacial periods was rapid, on the order of 3.5 to 6 m m / y r . This record of Bermudian Late Pleistocene sea-level history is consistent with that from other marine records (Table VIII, Fig.7), although to our knowledge it is the first time that the penultimate interglacial high sea stand and the fine-structure of the last interglacial high sea stand have been recognized in a tectonically stable area. ACKNOWLEDGEMENTS Our fieldwork and geochronology in Bermuda has been supported by grants from the U.S. National Science Foundation (EAR 77-23026, EAR23408), the Natural Environment Research Council of Great Britain, and the National Research Council of Canada, and the Geology Foundation of the University of Texas. We would also like to thank the Bermuda Public Works Department and Bermuda Biological Station for support and assistance. Spe-

68 TABLE VIII Comparison of sea-level elevations in Bermuda, Barbados and New Guinea with those calculated for deep-sea core V28-238 over the past 250,000 years Time

Bermuda I

(k.y.) 82--85 100--105 120--125 135--145 ~ 180 ~ 200 ~ 200

Barbados 2 (coral)

--15 to --20 --15 t o - - 2 0 + 4 t o +6 above --20 above --20 +1 above --20

New Guinea 3 (mollusc)

--45 20, --47 --43 20, --28 +5 +10 below 50 ---22 ---12 . --32 --

.

(coral)

V28-238 *,4 (mollusc)

--20 ---14 --9 +8 --7 --8 t o - - 1 2 + 10 --20 to --30 -. . near -present

--11 --17 +11 --100 --71 20 --12

*Calculated for core V28-238 assuming --0.1%o change in 81sO per 10 m rise in sea level. 1This study, Harmon et al. (1978); 2Fairbanks and Matthews (1978); 3Bloom et al. (1974), Chappell (1974), Aharon et al. (1980); 4Shackleton and Opdyke (1976). c i a l t h a n k s a r e d u e t o W. S t e r r e r , B. S t o v e l l , M. E m m o t t , A . L a t h a m , T. Iliffe, a n d J. L a n g f o r a s s i s t a n c e w i t h f i e l d w o r k a n d s a m p l e c o l l e c t i o n as w e l l as R . K . M a t t h e w s , N. J. S h a c k l e t o n a n d C. N e u m a n n f o r c o n s t r u c t i v e d i s c u s s i o n a t v a r i o u s s t a g e s o f t h e p r o j e c t . T h e m a n u s c r i p t w a s t y p e d b y Mrs. E. Y u i l l a n d Miss J. M c L e a n a n d i l l u s t r a t i o n s d r a w n b y Mrs. J. B e n n e t t . B e r muda Biological Station Contribution Number 902. McMaster Isotope, Nuclear, and Geochemical Studies Contribution Number 119. REFERENCES Aharon, P., Chappell, J. and Compston, W., 1980. Stable isotope and sea level data from New Guinea supports Antarctic ice-surge theory of ice ages. Nature, 283: 649--651. Andrews, J. T. and Mahoffy, M. A. W., 1976. Growth rate of the Laurentide Ice Sheet and sea level lowering (with emphasis on the 115,000 B.P. sea level low). Quaternary Res., 6: 167--183. Bender, M. L., Fairbanks, R. G., Taylor, F. W., Matthews, R. K., Goddard, J. G. and Broecker, W. S., 1979. U-series dating o f the Pleistocene reef tracts of Barbados, West Indies. Geol. Soc. Am. Bull., 90: 577--594. Bloom, A. L., Broecker, W. S., Chappell, J. M. A., Matthews, R. K. and Mesolella, K. J., 1974. Quaternary Sea level fluctuations on a tectonic coast: New 23°Th/234U dates from the Huon Peninsula, New Guinea. Quaternary Res., 4: 184--205. Boellstoerff, J., 1973. Correlating and dating some older "Pleistocene" tills in the midcontinent. Geol. Soc. Am. Abstr., 5(4): 301. Bretz, J. H., 1960. Bermuda: A partially drowned, late nature Pleistocene karst. Geol. Soc. Am. Bull., 71: 1729--1754. Broecker, W. S. and Thurber, D. L., 1965. Uranium-series dating of corals and oolites from Bahaman and Florida Key limestones. Science, 149: 58--60. Chappell, J. M. A., 1974. Geology of coral terraces, Huon Peninsula, New Guinea: A case study of Quaternary tectonic movements and sea level changes. Geol. Soc. Am. Bull., 85: 553--570. Chappell, J. M. A. and Veeh, H. H., 1978. Late Quaternary tectonic movements and sea level changes at Timor and Atauro Islands. Geol. Soc. Am. Bull., 89 : 356--368.

69 Cronin, F. M., 1980. Biostratigraphic correlation of Pleistocene marine deposits and sea level, Atlantic Coastal Plain of the Southeastern United States. Quaternary Res., 13: 213--229. Cronin, F. M., Szabo, B. J., Ager, F. A., Hazel, J. E. and Owens, J. P., 1981. Quaternary climates and sea levels, U.S. Atlantic Coastal Plain. Science, in press. Dreimanis, A., 1976. Quaternary Stratigraphy of Southern Ontario. In: W. C. Mahaney (Editor), Quaternary Stratigraphy of North America. Dowden, Hutchinson and Ross, Stroudsburg, PA, pp. 51--64. Fairbanks, R. G. and Matthews, R. K., 1978. The marine oxygen isotope record in Pleistocene coral, Barbados, West Indies. Quaternary Res., 10: 181--196. Flint, R. F., 1971. Glacial and Quaternary Geology. Wiley, New York, NY, 892 pp. Gascoyne, M., Benjamin, G. J., Schwarcz, H. P. and Ford, D. C., 1979. Sea-level lowering during the Illinoian glaciation: Evidence from a Bahama "Blue Hole". Science, 205: 806--808. Harmon, R. S., Ford, D. C. and Schwarcz, H. P., 1977. Interglacial chronology of the Rocky and MacKenzie Mountains based upon 23°Th/~3'U dating of calcite speleothems. Can. J. Earth Sci., 14: 2543--2552. Harmon, R. S., Schwarcz, H. P. and Ford, D. C., 1978. Late Pleistocene sea level history of Bermuda. Quaternary Res., 9: 205--218. Harmon, R. S., Ku, T. L., Matthews, R. K. and Smart, P. L., 1979. Limits of U-series analysis: Phase 1 results of the Uranium-Series Intercomparison Project. Geology, 7: 405--4O9. Harmon, R. S., Land, L. S., Mitterer, R. M., Garrett, P., Schwarcz, H. P. and Larson, G. J., 1981. Bermuda sea level during the last interglacial: Nature, in press. Izett, G. A., Wilcox, R. E. and Borchardt, G. A., 1972. Correlation of volcanic ash bed in Pleistocene deposits near Mount Blanco, Texas, with the Quaje Pumice Bed of the Jemez Mountains, New Mexico. Quaternary Res., 2: 554--578. Johnson, R. G. and Andrews, J. T., 1979. Rapid ice~heet growth and initiation of the last glaciation. Quaternary Res., 12: 119--134. Koide, M. and Goldberg, E. C., 1965. U234/U :3s ratios in seawater. In: M. Sears (Editor), Progress in Oceanography. Pergamon, New York, NY, 3, pp. 172--178. Konishi, K., Omura, A. and Nakamichi, O., 1974. Radiometric coral ages and sea level records from the Late Quaternary reef complexes of the R y u k y u Islands. Proc. 2nd Int. Coral Reef Symp., 2, pp. 595--613. Kriausakul, N. and Mitterer, R. M., 1980. Comparison of isoleucine epimerization in a model dipeptide and fossil protein. Geochim. Cosmochim. Acta, 4 4 : 7 5 3 - - 7 5 8 , Ku, T.-L., Kimmel, M. A., Easton, W. H. and O'Neil, T. J., 1974. Eustatic sea level 120,000 years ago on Ohau, Hawaii. Science, 185: 959--962. Kukla, G. J., 1977. Pleistocene land--sea correlations. I. Europe. Earth-Sci. Rev., 13: 307--374. Kukla, G., 1978. The classical European glacial stages: correlation with deep-sea sediments. Trans. Nebr. Acad. Sci., 6: 57--93. Land, L. S., 1970. Phreatic versus vadose diagenesis of limestones: Evidence from a fossil water table. Sedimentology, 14: 175--185. Land, L. S., MacKenzie, F. T. and Gould, S. J., ]967. Pleistocene history of Bermuda. Geol. Soc. Am. Bull., 78: 993--1006. Mesolella, K. J., Matthews, R. K., Broecker, W. S. and Thurber, D. L., 1969. The astronomical theory of climatic change. Barbados data. J. Geol., 77: 250--274. Miller, G. H., Andrews, J. F. and Short, S. K., 1977. The last interglacial--glacial cycle, Clyde Foreland, Buffin Island, N.W.T.: Stratigraphy, biostratigraphy and chronology. Can. J. Earth Sci., 14: 2842--2857. Mitterer, R. M., 1975. Ages and diagenetic temperatures o f Pleistocene deposits of Florida based upon isoleucine epimerization in Mercenaria. Earth Planet. Sci. Lett., 28: 275-282.

70 Nelson, R. J., 1837. On the geology of the Bermudas. Trans. Geol. Soc. London, 5: 102-123. Neumann, A. C., 1969. Quaternary sea level data from Bermuda. In: Proc. 8th Int. Quaternary Assoc. Congress, pp. 228--229 (abstract). Neumann, A. C. and Moore, W. S., 1975. Sea level events and Pleistocene coral ages in the northern Bahamas. Quaternary Res., 5: 215--224. Osmond, K. J., Carpenter, J. R. and Windom, H., 1965. Th23°/U234U ages of the Pleistocene corals and oolites of Florida. J. Geophys. Res., 75: 469--479. Redfield, A. C., 1967. Postglacial change in sea level in the western north Atlantic Ocean. Science, 157: 687--692. Richmond, G. M., 1976. Pleistocene stratigraphy and chronology in the mountains of Western Wyoming. In: W. C. Mahaney (Editor), Quaternary Stratigraphy of North America. Dowden, Hutchinson and Ross, Stroudsburg, PA, pp. 353--379. Ruhe, R. V., Cady, J. G. and Gomez, R. S., 1961. Paleosols of Bermuda. Geol. Soc. Am. Bull., 72: 1121--1142. Sayles, R. W., 1931. Bermuda during the ice age. Proc. Am. Aead. Arts Sci., 66: 381--467. Shackleton, N. J. and Opdyke, N. D., 1973. Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: Oxygen isotope temperatures on a 10 s and 10 ~ year time scale. Quaternary Res., 3: 39--55. Shackleton, N. J. and Opdyke, N. D., 1976. Oxygen isotope and paleomagnetic stratigraphy of Pacific core V28-239 Late Miocene to Latest Pleistocene. Geol. Soc. Am. Mere., 145: 449--463. Stearns, C. E., 1976. Estimates of the position of sea level between 140,000 and 75,000 years ago. Quaternary Res., 6 : 445--450. Szabo, B. J., 1979. Uranium-series age o f coral reef growth on Rottnest Island, Western Australia. Mar. Geol., 29: 11--15. Szabo, B. J., Ward, W. C., Weidie, A. E. and Brady, M. J., 1978. Age and magnitude of the late Pleistocene sea-level rise on the eastern Yucatan Peninsula. Geology, 6 : 7 1 3 - - 7 1 5 . Thompson, C. W., 1873. Geological peculiarities o f the Bermudas. Nature, 13: 266--267. Thompson, J. and Walton, A., 1972. Redetermination of chronology of Aldabra Atoll by 23°Th/234U dating. Nature, 240: 145--146. Vacher, H. L., 1973. Coastal dunes of younger Bermuda. In: D. Coates (Editor), Coastal Geomorphology. State Univ. New York, Publ. Geomorphol., pp. 355--391. Vacher, H. L., 1981. Geology of Bermuda. In: W.E. Sterrer (Editor), A n a t o m y of an Oceanic Island, in prep. Veeh, H. H., 1966. Th23°/U23S and U234/U ~3s ages of Pleistocene high sea level stand. J. Geophys. Res., 71: 3379--3386. Velastro, S., Davis, E. M. and Varela, A. G., 1972. University o f Texas at Austin Radiocarbon dates IX. Radiocarbon, 14: 461--485. Verrill, A. E., 1907. The Bermuda Islands, Part IV: Geology and paleontology. Trans. Conn. Acad. Arts Sci., 12: 45--348. Willman, H. B. and Frye, J. C., 1970. Pleistocene Stratigraphy of Illinois: Ill. Geol. Surv. Bull., 9 4 : 1 4 pp.